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A Fundamental Study of Interface Effects in HgCdTe Materials and Devices
by
Jing Zhang BSc, MSc
This thesis is presented for the degree of
Doctor of Philosophy
of
The University of Western Australia
School of Electrical, Electronic and Computer Engineering
The University of Western Australia
2015
Declaration Declaration of Published Work Appearing in this Thesis
This thesis contains published work and/or work prepared for publication, which
has been co-authored. The bibliographic information of the published works and the
details of contribution of the multiple authors to each publication are set out following
this declaration, pages 6 to 8.
Signature: (Candidate)
Jing Zhang
Signature: (Supervisor)
Professor Gilberto A. Umana-Membreno
Signature: (Supervisor)
Professor Jarek Antoszewski
Signature: (Supervisor)
Professor John M. Dell
Signature: (Supervisor)
Winthrop Professor Lorenzo Faraone
i
Abstract
The semiconductor-passivating layer interfaces, as well as the dielectric properties of
the passivating layers, play important and very often dominant roles in determining
HgCdTe device performance. With a narrow bandgap, HgCdTe infrared detectors are
strongly influenced by the quality of the passivation layer(s). The surface band
bending is often of the order of the bandgap energy for HgCdTe materials, even those
used for short-wave and mid-wave infrared detection, and can easily accumulate,
deplete, or invert the surface, drastically affecting device performance. The situation
is worse for long and very long wave infrared detectors. Surface recombination
processes can be enhanced in narrow bandgap materials like HgCdTe, and become the
dominant loss mechanism for photo-generated excess carriers. High-quality
photodiode detectors are limited by generation-recombination within the depletion
region, tunnelling through the depletion region and surface/interface effects. Surface
leakage is another surface-related current mechanism. The 1/f noise is surface related,
and is associated with surface charge tunnelling into and out of the passivation
interface. For ‘n-type/Barrier/n-type’ (nBn) heterostructure HgCdTe detectors, the
absorber is covered with the barrier which consists the passivation layer itself, yet
surface related phenomena impact greatly on the performance of nBn detectors.
Surface passivation technology can greatly improve the HgCdTe/insulator interface,
leading to a reduction of 1/f noise and generation-recombination noise, and an
increase of responsivity and detectivity of HgCdTe IR detectors. Understanding the
fundamental properties of interface states in narrow bandgap semiconductors is
essential to the systematic development of techniques to ameliorate their effects and
improve device performance.
The surface and interface chemistry of II-VI compounds has not been as extensively
studied in the open literature as that of III-V compounds, and there is a lack of
consensus on key questions related to II-VI surfaces and interfaces. Passivation
techniques for HgCdTe have been developed using empirical approaches over a long
time, with detailed information about surface conditioning, passivation material
properties, deposition conditions and annealing processes often retained as proprietary
knowledge. The absence of published work on a rigorous physical understanding of
ii
interfaces in narrow bandgap materials is the motivation for this work, and also the
challenge.
In this thesis the interface effects in HgCdTe materials and devices have been
investigated, concentrating on two passivant materials: CdTe and silicon nitride. The
surface and interface effects in molecular beam epitaxy (MBE) low-temperature
grown CdTe passivated HgCdTe structures have been studied, employing
photoconductive devices and gated photodiode devices. In order to determine the
effectiveness of this low-temperature deposited CdTe passivating film,
photoconductors were utilised to investigate the effectiveness of the passivation by
comparing photoresponsivity between devices with and without sidewall CdTe
passivation. Surface recombination simulations of the photodetectors were performed
to understand the behaviour of the passivation and estimate the surface recombination
velocity at the interfaces of CdTe passivated surfaces. This is a new and effective way
of estimating surface recombination velocities. The gated HgCdTe photodiode,
passivated by MBE low-temperature grown CdTe was used as a tool to investigate
passivation properties and performance, allowing the band bending at the surface to
be controlled by varying bias through the gate. This allowed the magnitudes of dark
current and dynamic resistance to be manipulated by changing the conditions at the
passivant/semiconductor interface in the photodiode, and therefore change the
dominant surface recombination mechanism.
The capabilities of low-temperature processing, good surface insulation and
hydrogenated films make SiNx a suitable choice for passivating HgCdTe. In this thesis
studies have been carried out to investigate SiNx thin films for surface passivation of
HgCdTe epitaxial layers without the need for a CdTe intermediate capping layer.
Conventionally, high-quality SiNx films for surface passivation layers are deposited at
temperatures in the range 200 °C to 750 °C. These temperatures are much higher than
the maximum allowed for HgCdTe processing temperature (typically < 120 °C) that
can be used without a Hg overpressure to prevent dissociation of the HgCdTe.
Inductively-coupled plasma-enhanced chemical vapour deposition (ICPECVD)
systems with a high-density plasma source offer the ability to deposit relatively high
quality SiNx films using a minimal thermal budget. SiNx films in this thesis were
deposited at low temperatures (80 °C - 130 °C) employing a Sentech SI500D
ICPECVD system with a high-density and low ion energy plasma source [1, 2]. The
iii
low ion energy of the plasma source enables the SiNx film to be deposited on the
HgCdTe without significant surface damage. Prior to SiNx films being deposited on
HgCdTe, a series of SiNx films were firstly deposited on CdTe/GaAs and Si substrates
under different deposition conditions to examine the influence of ICP power,
deposition temperature, and NH3/SiH4 flow ratio on the properties of SiNx films
themselves.
The SiNx/HgCdTe metal-insulator-semiconductor structures were utilised as a tool in
studying the interface between SiNx and HgCdTe. Interface trap density, Dit, was
considered as the measure in evaluating surface passivation performance and in
correlating passivation quality with other film properties. The SiNx/n-Hg0.68Cd0.32Te
interface characteristics were investigated employing capacitance-voltage and
conductance-frequency measurements, and the corresponding Dit were extracted from
the high-frequency and low-frequency capacitance-voltage characteristics, and also by
the conductance method. Analysis of the SiNx/n-Hg0.68Cd0.32Te MIS structures
indicated that Si-rich SiNx film deposited at 100 °C by ICPECVD exhibit electrical
characteristics suitable for surface passivation of HgCdTe-based devices. That is,
interface trap densities in the range of mid-1010 cm-2eV-1, and fixed negative interface
charge densities of ~ 1011 cm-2 [1, 2]. In addition, the relationship between different
bond concentrations in the SiNx and surface passivation performance has been
explored using infrared absorbance spectra. The Si-H and N-H bond concentrations
were found to be directly correlated with passivation performance, such that SiNx
films with a combination of high [Si-H] and low [N-H] bond concentrations were
found to be suitable as electrical passivation layers on HgCdTe. This could be a useful
criterion for optimising the passivation quality of SiNx films for HgCdTe-based
devices.
iv
For Jayden and Yingliang
v
Acknowledgements
I would like to express my gratitude to the numerous people who have contributed to
this thesis. First and foremost, I would like to express my gratitude to my supervisors,
Prof. Gilberto A. Umana-Membreno, Prof. Jarek Antoszewski, Prof. John Dell, and
Prof. Laurie Faraone for their guidance and supervision throughout my Ph.D. study.
Without their encouragement and profound knowledge in HgCdTe, this research
would not have been possible. I feel especially grateful how supportive they have
been after the birth of my child.
It has been such an enjoyable and unforgettable experience being a member of the
Microelectronics Research Group (MRG), headed by Prof. Lorenzo Faraone. The
facilities, funding and travel opportunities afforded by the group have given enormous
support throughout the thesis. I am thankful to all group members who have warmly
helped me on my research and also on personal life. I have enjoyed working with all
of my fellow Ph.D. candidates and staff members in the group. Particularly, I would
like to thank Gordon and Ryan for their kind training and assistance to me on the
nanofabrication facilities and MBE. I would also like to thank Richie, Gordon, Imtiaz,
Renjie, Wen and Jarek for their great efforts in running, updating and maintaining the
MBE system. Special thanks to Ms. Sabine Betts and Ms. Karen Kader, for their
warm support over the years in making MRG group a big happy family.
I would like to thank for the financial supports during my study from International
Postgraduate Research Scholarship, the Samaha Research Scholarship,
Microelectronics Research Group, School of Electrical, Electronic and Computer
Engineering, and the University of Western Australia.
I acknowledge the Australian Department of Innovation, Industry, Science and
Research and the International Science Linkages (ISL) for support during the study. I
also acknowledge the support from the Australian Research Council, Western
Australian Node of the Australian National Fabrication Facility, and the Office of
Science of the WA State Government. I would also like to thank the Centre for
Microscopy, Characterisation and Analysis, the University of Western Australia, for
the support with SEM and XRD. I also thank the School of Chemistry and
Biochemistry for XRD.
vi
Let me reserve my final appreciation to my family. I am grateful for the love and
support that my sister, father and mother have provided over the years. Special thanks
to my husband, Yingliang, whose sincerest love has always given me strength in
overcoming difficulties. I am very proud of my son, Jayden, who joined the family
during my Ph.D. study. He has given our family huge pleasure and kept me energised
throughout the final stages of my study.
vii
Contents
Contents
Declaration..................................................................................................................... i
Abstract ......................................................................................................................... ii
Acknowledgements ..................................................................................................... vi
Contents ........................................................................................................................ ii
List of Figures ............................................................................................................... ii
List of Tables ................................................................................................................ ii
1 Introduction ........................................................................................................... 1
1.1 Infrared detection technologies .................................................................. 1
1.2 Material interface limitations to photon detector performance .................. 2
1.3 Research objectives and significance ......................................................... 3
1.4 Thesis structure .......................................................................................... 4
1.5 Publications arising from this thesis .......................................................... 6
2 HgCdTe Passivation Technologies ....................................................................... 9
2.1 HgCdTe as an infrared detector material ................................................... 9
2.1.1 HgCdTe device architectures ........................................................... 10
2.1.2 Measures of device performance ..................................................... 16
2.2 Surface and interface issues with HgCdTe .............................................. 18
2.3 Surface passivation materials and technologies ....................................... 20
2.3.1 Passivation materials ........................................................................ 20
2.3.2 CdTe passivation .............................................................................. 22
2.3.3 ZnS passivation ................................................................................ 22
2.3.4 SiNx passivation ............................................................................... 23
2.3.5 Dual-layer passivation ..................................................................... 24
ii
Contents
2.4 Surface preparation for HgCdTe and CdTe ............................................. 25
2.5 Modification of interface trap density ...................................................... 26
2.6 Summary .................................................................................................. 29
3 Material Characterisation .................................................................................. 30
3.1 Introduction .............................................................................................. 30
3.2 Physical Characterisation ......................................................................... 30
3.2.1 Microscopy ...................................................................................... 30
3.2.2 X-ray diffraction .............................................................................. 32
3.2.3 Reflection high energy electron diffraction ..................................... 34
3.2.4 Energy dispersive X-ray analysis..................................................... 36
3.2.5 Spectroscopic ellipsometry .............................................................. 37
3.2.6 Optical reflection/transmission for structural and compositional characterisation ............................................................................................ 38
3.2.7 Optical reflection/transmission for bonding and detailed characterisation of thin films ....................................................................... 40
3.3 Electrical Characterisation ....................................................................... 40
3.3.1 Magneto-transport measurements .................................................... 40
3.3.2 Current-voltage measurements ........................................................ 44
3.3.3 Capacitance-voltage and capacitance-frequency measurements ..... 46
3.4 Performance of ICPECVD SiNx passivation on other semiconductors .................................................................................................... 49
3.4.1 Surface passivation by hydrogenated silicon nitride ....................... 50
3.4.2 Experimental setup and design ........................................................ 51
3.4.3 Investigation on SiNx film stability over time ................................. 56
3.4.4 Investigation on SiNx film properties influenced by NH3/SiH4 flow ratio ...................................................................................................... 64
3.5 Summary .................................................................................................. 91
4 Surface and Interface Effects in CdTe/HgCdTe Structures ........................... 93
4.1 Introduction .............................................................................................. 93
4.1 Sidewall effects in photoconductive devices ........................................... 93
4.1.1 Experimental procedures ................................................................. 93
4.1.2 Surface and interface recombination in photoconductive devices ... 95
iii
Contents
4.2 Interface effects in ZnS/CdTe/HgCdTe gated photodiodes ..................... 98
4.2.1 Gated photodiode fabrication process .............................................. 99
4.2.2 Dark current as a function of gate bias .......................................... 101
4.3 Summary ................................................................................................ 107
5 Interface Effects in Metal/SiNx/HgCdTe Structures ...................................... 108
5.1 Fabrication of the MIS structures .......................................................... 108
5.2 Study of interface trap density at the SiNx/HgCdTe interface ............... 110
5.2.1 Capacitance-voltage measurements on MIS structures ................. 110
5.2.2 Interface trap density extracted by quasi-static method ................. 116
5.2.3 Conductance-frequency measurements on MIS structures ............ 118
5.2.4 Interface trap density extracted by conductance method ............... 122
5.3 Relationship between SiNx passivation performance and thin film bond concentrations ........................................................................................... 124
5.4 Summary ................................................................................................ 130
6 Conclusions and Future Work ......................................................................... 131
6.1 Summary and Conclusions .................................................................... 131
6.2 Recommendations for future work ........................................................ 134
References ................................................................................................................. 138
Appendix A: HgCdTe Properties ........................................................................... 164
A.1 Bandgap ................................................................................................. 164
A.2 Lattice constant ...................................................................................... 166
A.3 Intrinsic carrier concentration ................................................................ 167
A.4 Mobility.................................................................................................. 167
Appendix B: Deposition Parameters Concerning High-temperature Deposited SiNx Films ................................................................................................................. 169
iv
List of Figures
List of Figures
Figure 2.1 Device cross section of HgCdTe photoconductor and its schematic band
diagram ........................................................................................................................ 10
Figure 2.2 Device cross sections and their schematic energy band diagrams of (a) n+-
on-p planar homojunction and (b) P+-on-n mesa heterojunction photodiodes. ........... 12
Figure 2.3 Device cross section of a HgCdTe nBn detector and its schematic energy
band diagram under bias. ............................................................................................. 15
Figure 2.4 Possible charge centres for a semiconductor surface passivated with a
dielectric, and the resultant interface states and surface band-bending. ...................... 19
Figure 3.1 SEM micrographs corresponding to (a) and (b) 60 nm-thick CdTe film on
HgCdTe, and (c) 300 nm-thick CdTe on HgCdTe. The CdTe layer was deposited in
an MBE system. ........................................................................................................... 31
Figure 3.2 Double crystal X-ray diffraction spectra of MBE grown CdTe layer on
GaAs substrate. ............................................................................................................ 33
Figure 3.3 Double crystal X-ray diffraction spectra of MBE grown HgCdTe layers on
CdZnTe substrate (n-HgCdTe/n+-HgCdTe/CdZnTe). ................................................. 33
Figure 3.4 X-ray diffraction spectra of (a) the LPE HgCdTe before CdTe growth and
(b) the MBE grown CdTe (on LPE HgCdTe). ............................................................. 34
Figure 3.5 The RHEED patterns recorded for HgCdTe sample CMCT042. (a) During
substrate thermal cleaning; (b) Toward the end of thermal cleaning; (c) At the start of
growth of n-HgCdTe layer (x = 0.4); (d) Toward the end of the growth of n-HgCdTe
(x = 0.4) layer; (e) At the start of growth of the MWIR absorber layer (x = 0.316); (f)
Toward the end of the growth of absorber layer. ......................................................... 36
Figure 3.6 (a) Refractive index, n, and (b) extinction coefficient, k, measured by
ellipsometry for a 11.18 μm-thick HgCdTe/CdZnTe sample numbered MCT223
(x = 0.281). ................................................................................................................... 37
Figure 3.7 FTIR transmission spectra for MCT223 (x = 0.281, depilayer = 11.18 μm)
with the air background being subtracted. ................................................................... 38
Figure 3.8 FTIR transmission spectra for MCT225 (x = 0.375, depilayer = 8.9 μm)
before and after wafer annealing, with the annealing being carried out in a saturated
Hg atmosphere at 235 °C for 24 hours. ....................................................................... 39
Figure 3.9 Image of the centre part of a fabricated Greek Cross van der Pauw structure
on HgCdTe taken under an optical microscope. .......................................................... 42
ii
List of Figures
Figure 3.10 Comparison of the electron conductivity - electron mobility spectra
measured before and after MBE CdTe growth. ........................................................... 42
Figure 3.11 Plots showing the electron mobility spectrum measured after the vacancy
filling anneal at liquid nitrogen temperature for (a) MCT231 (x = 0.388, depilayer =
6.4 μm) and (b) MCT240 (x = 0.347, depilayer = 5.23 μm). ........................................... 43
Figure 3.12 A typical current-voltage characteristic of Au/Cr/SiNx/Si MIS structure.45
Figure 3.13 Resistivity variation of D4-100C SiNx film over a period of four months.
The MIS structures were left to age in laboratory atmosphere. ................................... 45
Figure 3.14 The SiNx/Si MIS structure measured at 298 K with variable sweep ranges
from -15 V to 15 V (innermost pair of curves), -20 V to 20 V, -24 V to 24 V and -
30 V to 30 V (outermost pair of curves). ..................................................................... 48
Figure 3.15 The SiNx/Si MIS structure measured at 298 K (red solid line) and 77 K
(blue dashed line) with sweep ranges from -20 V to 20 V. ......................................... 48
Figure 3.16 Deposition rates of silicon nitride films on silicon substrate as a function
of ICP power at a substrate temperature of 80 °C and 100 °C. ................................... 57
Figure 3.17 Deposition rates of silicon nitride films on CdTe/GaAs substrate as a
function of ICP power at a substrate temperature of 80 °C and 100 °C. ..................... 57
Figure 3.18 A typical IR absorbance spectra of low-temperature (80 °C - 100 °C)
deposited silicon nitride film deposited by Sentech SI500D system. .......................... 60
Figure 3.19 The IR absorbance spectra of the as-deposited silicon nitride films by six
different recipes on CdTe/GaAs substrate. .................................................................. 61
Figure 3.20 The IR absorbance spectra of the silicon nitride films by six different
recipes on CdTe/GaAs substrate after six-months exposure to a laboratory atmosphere.
...................................................................................................................................... 62
Figure 3.21 The IR absorbance spectra of the C5-SiNx film on CdTe/GaAs substrate
monitored over a six month time frame. The films were allowed to age in laboratory
atmosphere. .................................................................................................................. 63
Figure 3.22 Measured refractive index at a wavelength of 632.8 nm and [N]/[Si] ratio
estimated by EDS, as a function of NH3/SiH4 flow ratio for samples deposited at
80 °C and 100 °C. ........................................................................................................ 67
Figure 3.23 Measured refractive index at a wavelength of 632.8 nm and [N]/[Si] ratio
estimated by EDS, as a function of substrate temperature........................................... 67
Figure 3.24 Plot illustrating the relationship between n and x for ICPECVD SiNx
deposited at 80 °C -100 °C with varied NH3/SiH4 flow ratio. ..................................... 70
iii
List of Figures
Figure 3.25 [N]/[Si] ratio as a function of NH3/SiH4 flow ratio for ICPECVD SiNx
deposited at 80 °C -100 °C with varied NH3/SiH4 flow ratio. ..................................... 71
Figure 3.26 Plot of film refractive index, n632.8nm, of ICPECVD SiNx deposited at
80 °C -100 °C as a function of the SiH4/NH3 gas ratio................................................ 72
Figure 3.27 The change in film deposition rate versus NH3/SiH4 flow ratio for silicon
nitride films deposited at 80 °C, 90 °C and 100 °C with a fixed SiH4 gas flow of
6.9 sccm. ...................................................................................................................... 74
Figure 3.28 IR absorption coefficient spectra of SiNx films deposited by ICPECVD at
various NH3 flow rates and a fixed SiH4 flow rate at 80 °C. ....................................... 76
Figure 3.29 IR absorption coefficient spectra of SiNx films deposited by ICPECVD at
various NH3 flow rates and a fixed SiH4 flow rate at 100 °C. ..................................... 77
Figure 3.30 Plots showing the Si-H stretching peak shifting to higher frequency as a
function of (a) film composition [N]/[Si] and (b) NH3/SiH4 flow ratio increases. ..... 78
Figure 3.31 Plots showing the main absorption coefficient peak shifting to higher
frequency as a function of (a) film composition [N]/[Si] and (b) NH3/SiH4 flow ratio.
...................................................................................................................................... 79
Figure 3.32 The absorption coefficient as a function of wavenumber for sample B1-
NH8-80C as an illustration of the fitted absorption bands (dashed line) in the range
from 450 cm-1 to 1400 cm-1 with four different Gaussian distributions. ...................... 81
Figure 3.33 [N-H] and [Si-H] bond concentration as a function of film composition
[N]/[Si] and NH3/SiH4 flow ratio. ................................................................................ 86
Figure 3.34 The ratio of H bonded to N over that bonded to Si, [N-H]/[Si-H], as a
function of NH3/SiH4 flow ratio, refractive index and film composition [N]/[Si]. ..... 87
Figure 3.35 The fraction of [N-H] and [Si-H] as a function of SiNx film composition
[N]/[Si]. The indicated temperatures refer to the substrate temperature during
deposition. .................................................................................................................... 88
Figure 3.36 The atomic densities of [Si], [N] and [H] as a function of SiNx film
composition [N]/[Si] and NH3/SiH4 flow ratio. The indicated temperatures refer to the
substrate temperature during deposition. ..................................................................... 89
Figure 3.37 Film density, ρ, as a function of (a) SiNx film composition [N]/[Si] and
(b) NH3/SiH4 flow ratio. The indicated temperatures refer to the substrate temperature
during deposition. ........................................................................................................ 90
Figure 4.1 Schematic of the photoconductive devices showing location of the
unpassivated sidewall surfaces. The passivating CdTe film is approximately 200 nm
iv
List of Figures
thick. (a) Fully passivated structure; (b) Partially passivated structure with no CdTe
film on sidewalls. ......................................................................................................... 95
Figure 4.2 Measured and modelled normalised spectral photoresponse of
Hg0.71Cd0.29Te photoconductive devices, measured at a field of 10 V/cm at 80 K. The
low field minimises the effect of sweepout so that the response should be most
sensitive to surface recombination velocity. ................................................................ 96
Figure 4.3 Simulated photoresponse ratio of all surfaces passivated devices (RF) and
partially passivated devices (RP) versus recombination velocity of the top
CdTe/Hg0.71Cd0.29Te interface (sTop) at 80 K, with recombination velocity of the
unpassivated surfaces sWall = 1×104 cm/s. .................................................................... 97
Figure 4.4 A photomicrograph and cross section of the completed gated photodiodes.
(a) Photo of fabricated gated photodiodes and (b) cross-sectional view. .................. 100
Figure 4.5 Diagrams illustrating the effects of n-type region band-bending on a n-on-p
junction. The gate voltage, Vg, is referenced to the p-type HgCdTe. a) p-type surface
in accumulation; b) Vg = Vfb in p-type, c) p-type surface in depletion or weak
inversion; d) p-type surface in inversion and field-induced junction breakdown occurs
in the p-type region under the gate. ........................................................................... 103
Figure 4.6 Measured dark current at 77 K in a cryostat with a cold shield in the
absence of photocurrent for gated photodiodes with (a) a diameter of 300 μm and (b)
360 μm. The gate voltage is referenced to the p-type substrate. The seven curves from
top to bottom are for various gate biases from -1.5 V to 1.5 V in 0.5 V steps. ......... 104
Figure 4.7 Measured dynamic resistance-area product at 77 K in a cryostat with a cold
shield in the absence of photocurrent for gated photodiodes with a diameter of (a)
300 μm and (b) 360 μm. The seven curves from bottom to top are for varying gate
bias from -1.5 V to 1.5 V in 0.5 V steps. ................................................................... 105
Figure 5.1 C-V curves measured at 1 MHz with the three different sweeping voltage
ranges for each of the four MIS sample for the range of ± 2 V, ± 4 V and ± 6 V ..... 111
Figure 5.2 Illustration on the definitions of ∆V+, ∆V– and ∆VH used in the C-V
analysis. ...................................................................................................................... 113
Figure 5.3 Flat band voltage as a function of (a) substrate temperature, and (b)
[N]/[Si], for the three MIS samples of D1-80C, D1-90C, and D1-100C. ................. 115
Figure 5.4 The change in (a) hysteresis widths VH and (b) slow interface charge
densities as a function of bias extremes for the four MIS structures. ........................ 115
v
List of Figures
Figure 5.5 Comparison of the interface trap densities (Dit) of the SiNx/n-
Hg0.68Cd0.32Te MIS structures extracted by the quasi-static method as a function of the
energy from mid-gap at 77 K. .................................................................................... 117
Figure 5.6 Measured capacitance – log ω characteristics at various biases for the four
MIS structures at 77 K. .............................................................................................. 120
Figure 5.7 Measured and fitted Gp/ω versus log ω characteristics at various gate biases
for the four MIS structures at 77 K. ........................................................................... 121
Figure 5.8 Comparison of the interface trap densities, Dit, of all the SiNx/n-
Hg0.68Cd0.32Te MIS structures extracted by the conductance method as a function of
the energy from mid-gap at 77 K. .............................................................................. 123
Figure 5.9 Comparison of the time constant (τit) of all the SiNx/n-Hg0.68Cd0.32Te MIS
structures extracted by the conductance method as a function of the energy from mid-
gap at 77 K. ................................................................................................................ 123
Figure 5.10 Electron capture cross section as a function of energy for the SiNx/n-
Hg0.68Cd0.32Te MIS structures at 77 K. ...................................................................... 124
Figure 5.11 The IR absorbance spectra of the reference silicon nitride films on Si
substrate under four deposition conditions for the MIS structures. ........................... 126
Figure 5.12 Relationship between Dit and [Si-H] and [N-H] bond concentrations. .. 128
Figure 5.13 Dit at mid-gapwith the variations of (a) [H], [N-H], [Si-H] (a) and (b) [Si-
H]/[N-H]. ................................................................................................................... 129
Figure A1.1 Schematics of energy bandgap of (a) HgTe. (b) HgTe-CdTe transition
(zero bandgap). (c) CdTe. The Г6 and Г8 point refer to the electron band and
light/heavy hole band, respectively. ........................................................................... 165
Figure A1.2 Bandgap of Hg1−xCdxTe as a function of cadmium composition, x. ..... 165
Figure A1.3 Cut-off wavelength of Hg1−xCdxTe as a function of cadmium
composition, x. ........................................................................................................... 166
Figure A1.4 Lattice constant of Hg1−xCdxTe as a function of cadmium composition, x.
.................................................................................................................................... 166
Figure A1.5 Intrinsic carrier concentration of Hg1-xCdxTe as a function of x for
T = 77 K, 150 K, 200 K and 300 K. ........................................................................... 167
Figure A1.6 Electron mobility in Hg1-xCdxTe as a function of temperature for varying
mole fraction, x. ......................................................................................................... 168
vi
List of Tables
List of Tables
Table 3.1 Summary of extracted electron transport parameters for HgCdTe sample
MCT225 before and after the CdTe passivation .......................................................... 43
Table 3.2 Data extracted from C-V analysis on a SiNx/Si MIS capacitor measured at
298 K and 77 K ............................................................................................................ 47
Table 3.3 Summary of deposition conditions of SiNx film on CdTe/GaAs and Si
substrate in the Sentech SI 500D system ..................................................................... 52
Table 3.4 Silicon nitride film deposition procedures used for sample D1-SiNx in the
Sentech SI500D ICPECVD system ............................................................................. 54
Table 3.5 Summary of MBE grown CdTe on GaAs substrate .................................... 56
Table 3.6 Absorption bands observed in the as-deposited SiNx samples .................... 59
Table 3.7 Summary on ICPECVD SiNx/Si wafers with varied SiH4/NH3 ratio and
temperature .................................................................................................................. 66
Table 3.8 IR absorption spectra analysis and calculations for bond and atom
concentrations on SiNx/Si wafers deposited under varied NH3/SiH4 flow ratios at
temperatures between 80 °C - 100 °C .......................................................................... 82
Table 3.9 Summary of bond and atom concentrations calculated for SiNx/Si wafers
deposited under varied NH3/SiH4 flow ratios at temperatures between 80 °C - 100 °C
...................................................................................................................................... 84
Table 5.1 Summary of SiNx/HgCdTe MIS samples with silicon nitride film deposited
under different conditions .......................................................................................... 109
Table 5.2 Summary on flat band voltage, fixed charge density, slow interface trapped
charge density and interface trap density extracted for the four SiNx/HgCdTe MIS
samples for a bias sweep range of ± 2 V ................................................................... 114
Table 5.3 Hysteresis widths VH in the high-frequency C-V curves versus bias
extremes for the four SiNx/HgCdTe MIS structures .................................................. 114
Table 5.4 Summary on bond and atomic concentrations calculated for SiNx/Si
reference wafers of the SiNx/HgCdTe MIS structures ............................................... 127
Table 5.5 Summary of results from C-V and IR absorbance analysis on the four
SiNx/HgCdTe MIS structures ..................................................................................... 128
ii
Chapter 1 Introduction
1 Introduction
1.1 Infrared detection technologies
The infrared (IR) electromagnetic spectrum spans wavelengths from 1 μm to 100 μm.
IR radiation was first discovered by Sir William Herschel in 1800, who built a crude
monochromator that used a thermometer as a detector to measure the distribution of
energy in sunlight [3]. He saw temperature fluctuations where there was no visible
light, which led him to discover an invisible light spectrum, associated with heat
radiation, at wavelengths longer than those of visible light.
Infrared detectors can be classified as either thermal detectors or photon detectors.
The principle of thermal detectors is that there is a measurable change in the electrical
characteristics of the material due to a temperature change after the absorption of IR
radiation. In contrast, photon detectors measure directly generated charged carriers
resulting from the absorption of photons. Thermal and photon detectors have different
dependencies of detectivities on wavelength and temperature [3, 4]. Thermal detectors
are favoured at the very long wavelength IR (VLWIR: 14 - 30 µm) region, whereas
photon detectors are favoured at IR regions of shorter wavelength, such as long-
wavelength IR (LWIR: 8 - 14 µm), due to speed of response, the influence of
fundamentally different types of noise, i.e. generation-recombination (GR) noise in
photon detectors and temperature fluctuation noise in thermal detectors. Generally,
higher operating temperature requirements needed to attain background limited noise
performance (when the noise is limited by the random arrival of photons on the
detector) favour thermal detectors over photon detectors. However, this performance
is speed dependent, and thermal detectors generally exhibit significantly slower
response for background limited performance compared to photon detectors.
Photon detectors can be intrinsic, extrinsic, free-carrier detectors or quantum
detectors, depending on the excitation process that induces a carrier concentration
variation on absorption of a photon in the detector material – i.e. intrinsically
(interband transition), extrinsically (impurity to band transition), or by free carrier
absorption (intraband transition). Intrinsic infrared photon detectors typically have
high optical absorption coefficient, high quantum efficiency, and low thermal
generation rate. The most commonly used intrinsic detectors are photoconductors and
1
Chapter 1 Introduction
photodiodes. The main disadvantages of extrinsic detectors are the significant cooling
requirements, and higher levels of noise.
Hg1-xCdxTe is a widely used material for infrared photon detectors and sensors, and is
of significant importance for IR detection in defence and security, mineral
exploration, environmental monitoring, and biological and chemical sensing for
commercial and defence related uses. Its cut-off wavelength, λc, can be tuned by
changing the mole fraction x of CdTe to HgTe to be in the short-wavelength IR
(SWIR: 0.75 - 3 µm), mid-wavelength IR (MWIR: 3 - 5 µm), and long-wavelength IR
(LWIR: 8 - 14 µm) spectral regions [5], regions in which the atmosphere exhibits low
optical absorption. More information on the cut-off wavelength variation with x-value
and temperature is presented in Appendix A.1. Performance advantages offered by
HgCdTe-based photodetectors include higher sensitivity at a given operating
temperature, fast response times, tuneable cut-off wavelength, and suitability for
integration into focal-plane array configurations. These advantages have been very
well documented in the literature [6, 7].
1.2 Material interface limitations to photon detector performance
Surface (when a solid is in contact with vacuum or the gas phase) and interface (when
the solid is in contact with another solid) effects can dominate the performance of
semiconductor devices. Interfaces present between passivation, insulation and contact
layers and at heterostructures. The semiconductor-passivating layer interfaces, as well
as the dielectric properties of the passivating layers, play important and very often
dominant roles in determining performance of all semiconducting devices. With a
narrow bandgap, HgCdTe IR detectors are particularly sensitive to interface effects,
and are strongly influenced by the quality of the passivation layer(s). For Hg1-xCdxTe
with x-value between 0.2 and 0.3, the band gap at 77 K varies between 0.1 eV and
0.25 eV. Hence, the surface potential band bending can easily be of the order of the
band-gap energy, and easily accumulate, deplete, or invert the surface, thus
significantly affecting device performance [8].
For high-quality HgCdTe, the performance of photoconductive detectors is found to
be limited by the carrier recombination at the surface, which determines the effective
minority carrier lifetime. Surface recombination processes can be enhanced in narrow
2
Chapter 1 Introduction
bandgap material detectors like HgCdTe, and can become the dominant loss
mechanism for photo-generated excess carriers [9]. Surface passivation for HgCdTe
photoconductors is critical in achieving high responsivity, especially for devices with
small areas [10, 11].
Sensitivity of high-quality photodiodes, typically exhibiting high zero-bias dynamic
resistance-effective junction area product (R0A), are usually limited by generation-
recombination within the depletion region, tunnelling through the depletion region
and surface/interface effects [12]. In addition to generation-recombination at the
surface/interface and within surface channels, surface leakage is another surface-
related current mechanism. The noise in HgCdTe photodiodes is observed to vary
with bias and temperature [13]. The 1/f noise is believed to be dominantly surface
related, and is associated with surface charge tunnelling into and out of the
semiconductor/passivation layer interface [14, 15]. Surface passivation technology
can greatly improve the HgCdTe/insulator interface, leading to a reduction of 1/f and
g–r noise, and an increase of responsivity and detectivity.
For ‘n-type/Barrier/n-type’ (nBn) heterostructure HgCdTe detectors in which the
absorber is covered with the barrier that consists of the passivation layer itself, it is
expected that passivation is less critical than for other detector structures. However,
several reports have shown that surface passivation plays a significant role in
improving the performance of nBn detectors [16-18]. HgCdTe nBn structures show
potential to outperform HgCdTe photodiodes and reach background-limited infrared
performance (BLIP) at ~ 207 K for MWIR wavelengths [16], while experimental
results have shown dark currents two orders of magnitude higher than expected,
which can be attributed to surface leakage currents associated with etching induced
defects [17], indicating the passivation process needs to be improved.
1.3 Research objectives and significance
The surface and interface chemistry of II-VI compounds has not been as extensively
studied as that of III-V compounds, and there is a lack of consensus on key questions
related to II-VI surfaces and interfaces. Passivation techniques for HgCdTe have long
been researched, however, detailed information on surface conditioning, passivation
material, deposition conditions and annealing processes are often retained as
3
Chapter 1 Introduction
proprietary knowledge, which has led to a lack of detailed publications. All this adds
to the difficulties and challenges associated with this research project.
The principal objectives of this thesis are to investigate interface effects in HgCdTe
materials and devices, encompassing the following:
Characterise molecular beam epitaxy (MBE) low-temperature grown CdTe
passivation films and their passivation effects using photoconductive devices;
Modelling of surface and interface recombination effects in photoconductive
devices;
Investigate changes in narrow-bandgap HgCdTe photodiode performance
resulting from band bending at the surface;
Develop, optimise and characterise low-temperature (80 °C - 100 °C) deposited
SiNx films for passivating HgCdTe without the need for a CdTe capping layer;
Evaluate SiNx passivation quality by comparing the interface trap densities at the
SiNx/HgCdTe interface for SiNx films deposited under varying deposition
conditions utilising SiNx/n-HgCdTe metal-insulator-semiconductor (MIS)
structures;
Correlate performance of SiNx as a passivation layer on HgCdTe with bond
configurations of SiNx thin films.
1.4 Thesis structure
The organisation of this thesis, exclusive of this introductory chapter, is as follows:
Chapter 2 stresses the importance of surface passivation for HgCdTe detectors in
terms of the device architecture and detector performance, and reviews the available
materials and technologies for HgCdTe passivation. The passivation materials and
technologies used in this thesis are presented. Surface preparation treatment, that is
crucial to HgCdTe and CdTe prior to the passivation, is also reviewed. The possible
approaches to ameliorate the effects of interface states, including annealing,
hydrogenation, and modification of stress in the layers, are discussed.
Chapter 3 covers the techniques used for material characterisation related to this
thesis, divided into two categories - physical and electrical characterisations. The
4
Chapter 1 Introduction
purpose for these characterisations is discussed, and a selection of experimental
results presented. In addition, in order to determine the suitable deposition conditions
of SiNx passivation film for HgCdTe, a series of low-temperature (80 °C - 130 °C)
ICPECVD SiNx films were deposited on other semiconductors - CdTe/GaAs and Si
substrates, under different deposition conditions to investigate the influence of ICP
power, deposition temperature, and NH3/SiH4 flow ratio on properties of the
deposited SiNx films.
Chapter 4 studies the surface and interface effects in CdTe/HgCdTe devices,
including photoconductive devices and gated photodiode devices. The CdTe
passivation used was a low-temperature MBE grown film. In order to determine the
effectiveness of this low-temperature deposited CdTe passivating film,
photoconductors were utilised to investigate the influence of the passivation by
comparing photoresponsivity between devices with and without sidewall CdTe
passivation. Surface recombination simulations of the photodetectors were performed
to understand the behaviour of the passivation and estimate the surface recombination
velocity at the interfaces of CdTe passivated surfaces. The gated photodiode was used
as a tool to investigate device performance, allowing the band bending at the surface
to be controlled by varying the bias applied to the gate. This allowed the magnitudes
of dark current and dynamic resistance to be manipulated at the surface of the
photodiode, and therefore change the dominant surface recombination mechanism.
Chapter 5 presents work on SiNx thin films for surface passivation of HgCdTe
epitaxial layers without the need for a CdTe capping layer. The SiNx/HgCdTe MIS
structures were utilised as a tool in studying the interface between SiNx and HgCdTe,
and the interface trap density, Dit, was extracted and examined by analysing high-
frequency and low-frequency capacitance-voltage data, as well as by the conductance
method. The correlation between different bond concentrations in the passivation
layer and surface passivation performance was then studied. The Si-H and N-H bond
concentrations (i.e. [Si-H] and [N-H]) were found to be directly correlated with
passivation performance, such that SiNx films with a combination of high [Si-H] and
low [N-H] being considered to be suitable as electrical passivation layers on HgCdTe.
This could be a useful criteria for optimising the passivation quality of SiNx films for
HgCdTe-based devices.
5
Chapter 1 Introduction
Conclusions of this thesis and suggestions for further work are discussed in Chapter 6.
1.5 Publications arising from this thesis
Refereed Journal Publications
[1] J. Zhang, G. A. Umana-Membreno, R. Gu, W. Lei, J. Antoszewski, J. M. Dell,
and L. Faraone, ‘Investigation of ICPECVD Silicon Nitride Films for HgCdTe
Surface Passivation’, Journal of Electronic Materials, vol. 44 (9), pp. 2990-3001,
2015. (Presented at the The U.S. Workshop on the physics and chemistry of II-VI
materials, 2014)
The percentage contribution of each author is as follows:
J. Zhang 80 %, All, except -
G. A. Umana-Membreno Supervisor
R. Gu 10 % MBE growth and technical discussions
W. Lei 10 % MBE growth and technical discussions
J. Antoszewski Supervisor
J.M. Dell Supervisor
L. Faraone Supervisor
[2] J. Zhang, G. K. O. Tsen, J. Antoszewski, J. M. Dell, L. Faraone, and W. D. Hu,
‘A Study of Sidewall Effects in HgCdTe Photoconductors Passivated with MBE-
Grown CdTe’, Journal of Electronic Materials, vol. 39, pp. 1019-1022, 2010.
(Presented at the The U.S. Workshop on the physics and chemistry of II-VI
materials, 2009)
The percentage contribution of each author is as follows:
J. Zhang 85 %, All, except -
G. K. O. Tsen 10 % MBE growth and technical discussions
J. Antoszewski Supervisor
J.M. Dell Supervisor
L. Faraone Supervisor
W. D. Hu 5 % Technical discussions
6
Chapter 1 Introduction
[3] W. Hu, X. Chen, Z. Ye, J. Zhang, F. Yin, C. Lin, Z. Li, and W. Lu, ‘Accurate
Simulation of Temperature-Dependent Dark Current in HgCdTe Infrared
Detectors Assisted by Analytical Modeling’, Journal of Electronic Materials, vol.
39, pp. 981-985, 2010. (Presented at the The U.S. Workshop on the physics and
chemistry of II-VI materials, 2009)
The percentage contribution of this author is as follows:
J. Zhang 10 %, Oral presentation at The US Workshop and technical discussions.
Refereed Conference Proceedings
[1] J. Zhang, G. A. Umana-Membreno, R. Gu, W. Lei, J. Antoszewski, J. M. Dell, L.
Faraone, ‘Characterisation of SiNx-HgCdTe Interface in Metal-Insulator-
Semiconductor Structure’, 2014 Conference on Optoelectronic and
Microelectronic Materials and Devices, pp.64-66, 2014. (Presented at Conference
on Optoelectronic and Microelectronic Materials and Devices, 2014)
The percentage contribution of each author is as follows:
J. Zhang 80 %, All, except -
G. A. Umana-Membreno Supervisor
R. Gu 10 % MBE growth and technical discussions
W. Lei 10 % MBE growth and technical discussions
J. Antoszewski Supervisor
J.M. Dell Supervisor
L. Faraone Supervisor
[2] J. Zhang, R. J. Westerhout, G. K. O. Tsen, J. Antoszewski, Y. Yang, J. M. Dell,
and L. Faraone, ‘Sidewall effects of MBE grown CdTe for MWIR HgCdTe
photoconductors,’ 2008 Conference on Optoelectronic and Microelectronic
Materials and Devices, pp. 82-85, 2008. (Presented at Conference on
Optoelectronic and Microelectronic Materials and Devices, 2008)
7
Chapter 1 Introduction
The percentage contribution of each author is as follows:
J. Zhang 75 %, All, except -
R. J. Westerhout 10 % Technical discussions
G. K. O. Tsen 10 % MBE growth and technical discussions
J. Antoszewski Supervisor
Y. Yang 5 % SEM assistance and technical discussions
J.M. Dell Supervisor
L. Faraone Supervisor
[3] G. K. O. Tsen, J. Zhang, C. A. Musca, J. M. Dell, J. Antoszewski, and L. Faraone,
‘Various annealing methods for activation of arsenic in Molecular Beam Epitaxy
grown HgCdTe,’ 2008 Conference on Optoelectronic and Microelectronic
Materials and Devices, pp. 125-128, 2008.
The percentage contribution of this author is as follows:
J. Zhang 20 %, MBE growth/annealing assistance, and technical discussions.
8
Chapter 2 HgCdTe Passivation Technologies
2 HgCdTe Passivation Technologies
The importance of surface passivation of HgCdTe detectors will be further discussed
in this chapter from the device architectures and detector performance point of view.
Various passivation materials and technologies are subsequently reviewed. Surface
treatments, being crucial to preparation of HgCdTe and CdTe before formation of the
passivation, are also reviewed.
2.1 HgCdTe as an infrared detector material
The discovery of Hg1-xCdxTe (MCT) by Lawson’s group was first published by them
in 1958, and the publication has been recognised as the earliest known reference to
HgCdTe [19]. HgCdTe is a solid solution of the two binary alloys, CdTe and HgTe.
Several important properties of HgCdTe have rendered it the material-of-choice in the
field of high-performance IR detection. Firstly, it is a direct bandgap semiconductor,
leading to large photon absorption coefficient and high quantum efficiency. It is
suitable for short-wave infrared (SWIR, wavelengths between 0.75 µm and 3 µm),
mid-wave infrared (MWIR, wavelengths between 3 μm and 5 μm), and long-wave
infrared (LWIR, wavelengths between 8 μm and 14 μm) detection, due to its
adjustable direct energy bandgap by tuning the Hg/Cd ratio. The lattice constant
across the entire composition range changes by only 0.3 %, making multilayer
crystalline growth possible. HgCdTe has a small electron effective mass, high
electron mobility (50,000 cm2V-1s-1 at 80 K in Hg0.7Cd0.3Te at 77 K) and hence the
potential for very fast response time. A more detailed description of the properties of
HgCdTe can be found in Appendix A.
Epitaxy techniques such as molecular beam epitaxy (MBE) and metal-organic
chemical vapour deposition (MOCVD) have made available many new bandgap-
engineered materials and device structures. MBE facilitates the growth of
multilayered HgCdTe heterostructures with abrupt changes in alloy mole-fraction, x,
between layers. Bandgap engineered structures can produce infrared sensors with
performance well above that obtained from devices fabricated using a single-layer of
material [20]. Additionally, research results indicate that for some structures
compositionally graded CdTe/Hg1-xCdxTe interfaces for passivation and subsequent
9
Chapter 2 HgCdTe Passivation Technologies
annealing can improve lifetime and surface recombination velocity characteristics [21,
22].
2.1.1 HgCdTe device architectures
A variety of HgCdTe device architectures have been developed since the 1970’s, with
the common ones described below. The work on surface passivation was initially
pioneered by Societe Anonyme de Telecommunication (SAT) [23, 24], and
passivation has evolved to be of great importance ever since in these device
architectures.
1) Photoconductors:
Photoconductive devices, as the first generation of HgCdTe devices, entered
production owing to reproducible bulk growth techniques and surface passivation by
anodic oxide. They are fabricated by applying metal electrodes to pure n-type material
(Figure 2.1), and are generally limited to linear arrays with typically fewer than 200
elements [25]. Passivation was found to be a critical step in the fabrication of
photoconductive detectors, especially for small-area devices, and the recombination
of photo-generated carriers at the surfaces/interfaces was found to have a direct
Figure 2.1 Device cross section of HgCdTe photoconductor and its schematic band diagram
hv
CdZnTe substrate
n-HgCdTe
Surface passivation
Contact
CdZnTe substrate
hv
n-HgCdTe
Ec
EF
Ev
hole
electron
10
Chapter 2 HgCdTe Passivation Technologies
impact on the performance of detectors [10, 11]. Anodic oxide is an effective
passivation for n-type HgCdTe photoconductive detectors due to its large positive
fixed charge that accumulates the n-type HgCdTe surface and creates a surface field
which drives the minority carrier holes away from the interface, thus separating them
from electrons [9]. Consequently, the carrier lifetime is increased resulting in higher
performance detectors. However, the large positive fixed charge renders anodic oxide
inappropriate for devices that incorporate pn-junctions.
2) Photodiodes:
A variety of HgCdTe photodiode configurations have been proposed, including mesa,
planar and lateral n-p, n+-n-p, p-n, n+-p homojunction and heterojunction structures
[26, 27]. The realization of HgCdTe photodiodes usually relies on the two most
important junction architectures based on n-on-p planar homojunctions or P-on-n
mesa heterojunctions (Figure 2.2) [3, 28]. The p-n junctions can be formed by various
techniques including Hg in- and out-diffusion, impurity diffusion, ion implantation,
electron bombardment, doping during growth, and a variety of other more esoteric
methods [29]. The desired p-type doping can be controlled by the density of acceptor-
like Hg vacancies. Arsenic is also a very useful p-type dopant with stability in the
lattice, low activation energy, and ability to control concentration over a wide range
from 1015 cm−3 to 1018 cm−3 [30]. The n-type doping can be produced by Al, Be, In
and B ion implantation into vacancy doped p-type material [29]. Indium is also
frequently used as a well-controlled n-type dopant due to its high solubility and
moderate diffusion.
n+-HgCdTe
p-HgCdTe
CdZnTe substrate
11
Chapter 2 HgCdTe Passivation Technologies
(a) n+-on-p homojunction photodiode
(b) P+-on-n heterojunction photodiode
Figure 2.2 Device cross sections and their schematic energy band diagrams of (a) n+-on-p planar homojunction and (b) P+-on-n mesa heterojunction photodiodes.
P+-HgCdTe
CdZnTe substrate
n-HgCdTe
CdZnTe Substrate
p-Hg1-xCdxTe base layer
n-Hg1-xCdxTe
cap layer
EF hv
Ec
Ev holes
electrons
hv
CdZnTe Substrate
Ec
EF Ev
x < y
P-Hg1-yCdyTe wider bandgap
cap layer
n-Hg1-xCdxTe base layer
holes
electrons
12
Chapter 2 HgCdTe Passivation Technologies
Surface passivation is a key technology for reducing surface recombination and
improving the performance of photodiode devices [25]. Commonly CdTe or CdZnTe
are used, deposited by MBE, metal organic chemical vapour deposition (MOCVD),
sputtering or e-beam evaporation.
Homojunction devices suffer from significant surface-related issues, where the excess
thermal generation at the surface results in increased dark current and recombination,
which reduces the photocurrent. Heterojunctions such as N+-p-p+ and P+-n-n+ with
heavily doped contact regions (‘+’ denotes high doping, and the capital letters for
wider bandgap) have demonstrated improved performance over homojunction devices
(such as n-p, n+-p, p+-n) [3]. For heterojunction mesa diodes, the passivation process
can be difficult to control, especially for small-area devices typically used for focal
plane arrays for imaging applications (such devices can be less than 100 µm2 [31,
32]). Double layer heterostructures formed by implantation in planar photodiodes
have less stringent requirements for surface passivation since the junction interface is
buried and protected [27, 33].
The most common open literature architectures used in HgCdTe photodiode
applications by the major focal plane array (FPA) manufactures are listed below [3,
26]:
The n+-on-p planar diode pioneered by SAT [34] has been the most widely
developed and used by Sofradir. It is based on ion implantation into acceptor-
doped p-type liquid phase epitaxy (LPE) HgCdTe grown by Te-solution slider.
The n+-n--p planar diodes used by Rockwell generally have boron implanted
junctions into Hg-vacancy doped p-type HgCdTe with MOCVD CdTe as a buffer
layer and ZnS for surface passivation. To overcome the problem with
hybridisation, Rockwell’s Producible Alternative to CdTe for Epitaxy (PACE)
technology provides intrinsic detector arrays with BLIP performance and a
satisfactory yield.
The p-on-n double-layer planar (buried) heterostructure (DLPH) diode developed
by Rockwell has a wide-bandgap cap layer covering a narrow-bandgap absorber,
which overcomes the sidewall problem [33]. The DLPH structure uses arsenic
implantation into indium-doped N-n or N-n-N material grown by MBE on
CdZnTe substrates.
13
Chapter 2 HgCdTe Passivation Technologies
The fabrication technology of mesa-etched photodiodes is based on etching
trenches between devices that define the individual (mesa) diodes. In the p-on-n
double-layer heterojunction (DLHJ) diodes, the n-type absorber layer is
commonly doped with indium sandwiched between the CdZnTe substrate and the
highly arsenic-doped, wider-gap p-type region. One of the critical steps is the use
of CdTe passivant to reduce surface currents, especially for small area devices
found in infrared focal plane detector arrays, preventing surface accumulation or
inversion [35].
The technology of the n+-n−-p vertically integrated photodiode (VIP) used by DRS
Infrared Technologies, also referred to as a high-density vertically integrated
photodiode (HDVIP), is based on Te-solution LPE grown HgCdTe on CdZnTe
substrate followed by the formation of a planar, ion implanted n-on-p junction
[36-38]. The diodes are epoxy hybridized directly to the read out integrated
circuits (ROICs) on 100 mm Si wafers. After epitaxial growth, the substrate is
removed and the HgCdTe layer is passivated on both surfaces. Backside
passivation and frontside passivation with inter-diffused layers of CdTe are
considered critical for high performance and yield. Since the diffusion length in
the absorbing region is typically longer than its thickness, any carriers generated
in the base region can be collected, giving rise to the photocurrent. This effect has
been used in a lateral collection device with a small central contact, called a
‘loophole’ device [39, 40]. For the n-p loophole diodes fabricated by GEC-
Marconi Infra-Red (GMIRL), n-type islands are formed by ion beam milling in a
p-type Hg-vacancy-doped layer grown by Te-solution LPE on a CdZnTe
substrate, and diodes are epoxied onto the silicon ROIC wafer [40].
The impact ionisation that can occur in the high-field region of an avalanche
photodiode multiplies the number of photo excited carriers by the avalanche gain,
leading to increased signal level. The alloy composition of the avalanche gain
layer was tuned to achieve both efficient absorption and low excess-noise
multiplication. A high quality surface passivation process, commonly by CdTe, is
essential in achieving high performance avalanche photodiodes [41, 42].
14
Chapter 2 HgCdTe Passivation Technologies
3) nBn (n-type/Barrier/n-type) detectors
There has been growing interest in the ‘n-type/Barrier/n-type’ (nBn) heterostructure
HgCdTe detectors, which consist of an n-type absorbing layer, a wide bandgap barrier
layer, and a thin n-type layer for contact [16-18, 43]. This structure allows for
efficient collection of minority carrier holes whilst creating an efficient barrier to
block majority electrons in the conduction band [44]. Figure 2.3 illustrates the nBn
detector and its energy band diagram. By optimising the band structure, it is, in
principle, possible for the device to operate near flat band conditions. Therefore,
Shockley-Reed-Hall (SRH) generation-recombination processes associated with
depletion regions can be eliminated and noise current can be reduced [16-18, 45-47].
The absorber of an nBn detector is covered with the barrier which also acts as the
passivation layer, thus, it may appear that passivation is less critical than in other
device structures. However, work has shown that surface passivation still plays a
significant role in determining the performance of nBn detectors [16-18]. The
HgCdTe nBn structure shows potential to outperform HgCdTe photodiodes and
achieve BLIP at ~ 207 K (for ≈cλ 6 μm) [16]. However, experimental results showed
dark currents that were two orders of magnitude higher than can be attributed to
surface leakage currents associated with etching induced defects alone [17], thus
indicating that the passivation process needs to be improved.
Figure 2.3 Device cross section of a HgCdTe nBn detector and its schematic energy band diagram under bias.
barrier
hv
n-absorber - n+
+
electron
hole ∆Ec
thermal GR
∆Ev Ec
Ev
15
Chapter 2 HgCdTe Passivation Technologies
2.1.2 Measures of device performance
In this section, the main figures of merit for quantifying HgCdTe detector
performance are listed, and the impact of surface passivation on device performance
is discussed.
1) Voltage responsivity:
The voltage responsivity is a commonly used figure of merit for photoconductive
detectors. When a n-type photoconductor is biased by a constant current, the voltage
responsivity Rλ (V/W) can be expressed as [48]
( )
=
lEqr
hcR e
Dτζµhλ
λ V/W (2.1)
where λ is the wavelength, h is Planck’s constant, c is speed of light, η is the quantum
efficiency usually defined as the number of electron-hole pairs generated per incident
photon, q is the electron charge, rD is the detector resistance, and l is the interelectrode
length. The expression in brackets in Eq. (2.1) is referred to as the photoconductive
gain with E the DC bias electric field, μe the electron mobility, τ the excess carrier
lifetime, and ζ represents the reduction in effective carrier lifetime due to
recombination at the contacts.
The voltage responsivity of a photoconductive detector is limited by carrier
recombination at the surface, which affects the minority carrier lifetime. Because
surface recombination processes can be enhanced in narrow bandgap material
detectors, it can become the dominant loss mechanisms for photo-generated excess
carriers [9]. Good surface passivation reduces the minority carrier recombination,
resulting in an increase in the minority carrier lifetime and hence the voltage
responsivity [10, 11]. However, strongly accumulated surfaces that can be found in an
anodic oxide passivated devices can result in a shunting path that reduces the detector
resistance and hence responsivity.
2) Current responsivity, detectivity and noise
The spectral current responsivity is a widely used figure-of-merit for photodiode
16
Chapter 2 HgCdTe Passivation Technologies
detectors that represents the number of carriers collected at the contacts for one photo-
generated electron-hole pair. The spectral current responsivity is defined as [12]:
( ) gqhc
WAR λhλ =/ (2.2)
where q is electron charge, and g is the photoelectric gain.
NEP (noise equivalent power) is the incident optical power necessary to produce an
output signal equivalent to the internally generated output noise. The detectivity, D, is
the reciprocal of NEP. As NEP and detectivity are sensitive to the size of the detector
and sampling rate, specific detectivity, D*, that is normalized to the detector area and
sampling rate becomes used more frequently [49]. D* is a primary figure-of-merit for
infrared photodetectors, given by [12]:
( )NEP
fAD
2/10D=∗ (2.3)
where A0 is the optical area of the detector, and ∆f is the frequency bandwidth. The
commonly discussed noise sources for IR detectors include generation-recombination
noise, 1/f noise, Johnson-Nyquist noise, shot noise, and photon noise.
At sufficiently low temperature, the detector thermal noise is negligible, and the
detectivity is limited by randomness in the photon arrival conversion process. For IR
detectors used in imaging, this is referred to a BLIP operation. As the device
temperature increases, the detector thermal noise increases approximately
exponentially, and the detectivity therefore decreases exponentially [12]. The 1/f noise
is found to be dominantly surface related and is associated with surface charge
tunnelling into and out of traps at the semiconductor/passivation layer interface [14,
15].
Surface passivation technology can greatly improve the HgCdTe/insulator interface,
leading to a reduction of both 1/f noise and generation-recombination noise, and
results in an increase of responsivity and detectivity. As an example, Lin et al.
reported on a Hg0.8Cd0.2Te photoconductive detector with a stacked ZnS/photo-
enhanced native oxide passivation that showed higher performance than one
passivated with a single ZnS layer [50]. The stacked passivation had a high quality
17
Chapter 2 HgCdTe Passivation Technologies
and stable native-oxide/HgCdTe interface with a low surface state density, a near flat-
band condition, and low leakage current. As a result, the device with such passivation
exhibits an improved noise power spectral density, lower effective surface trap
density and higher detectivity than detectors passivated with only ZnS.
3) R0A product
The R0A product is a commonly used figure of merit for HgCdTe photodiode
detectors, and is the produce of the dynamic resistance at zero bias and the effective
junction area. R0A should be independent of the area of the junction, and represents
the variation of the current density caused by a small variation of the voltage at zero
bias [51]. For photovoltaic devices, R0A product is derived from the diffusion current
and generation current combined in parallel at zero junction bias, and it can expressed
as [52]:
1
0
1
00
−
=
−
=
∂∂
+∂∂
+∂∂
=
∂∂
=bb V
GRhe
V VJ
VJ
VJ
VJAR (2.4)
where J = I /A is the current density, and I is the I-V characteristic of the diode. Je and
Jh are the diffusion current densities of the minority carriers at the edge of the
depletion region in the p-type and n-type materials, respectively. JGR is the current
density due to generation and recombination in the space charge region. The R0A
product is directly proportional to signal-to-noise ratio (SNR). A high R0A
corresponds to high SNR. High-quality photodiodes with high R0A product are
considered to be limited by generation-recombination within the depletion region,
tunnelling through the depletion region and surface/interface effects [12]. Hence,
reducing surface-related current mechanisms can result in an increase in R0A, when
appropriate surface passivation is applied.
2.2 Surface and interface issues with HgCdTe
The performance of HgCdTe detectors is often dominated by the properties of the
interface between the HgCdTe surface and any overlaying passivation layers.
Compared with the bulk, the interface contains a higher defect density. Within the
passivation layer at the interface between the passivation and semiconductor, fixed
charges and interface traps can cause band-banding that depends on the passivation
18
Chapter 2 HgCdTe Passivation Technologies
film. For Hg1-xCdxTe with x ≈ 0.2 - 0.3, the bandgap at 77 K varies between 0.1 and
0.25 eV. Hence, the surface potential band bending is often of the order of the
bandgap energy, and can easily accumulate, deplete, or invert the surface. For
example, for HgCdTe photodiodes, passivation of the p-HgCdTe can induce surface
depletion and even surface inversion resulting in surface shunting to adjacent diodes,
while fixed negative charge can accumulate the surface of p-HgCdTe, resulting in
higher electric field across the depletion region and increased tunnelling at the surface
with the N+ contact [14].
Figure 2.4 Possible charge centres for a semiconductor surface passivated with a dielectric, and the resultant interface states and surface band-bending.
Ec
EF Ev
Empty
Filled
Interface Trap States
+
+ +
+
+ + + + +
+-_ +
+-
+
x x x x
x x x x
Fixed Charge
Mobile Ionic Charge
Interface Trapped Charge
HgCdTe Transition Region
Dielectric Passivation
Dielectric Trapped Charge
+ +
19
Chapter 2 HgCdTe Passivation Technologies
The characteristics of HgCdTe devices are strongly influenced by the properties of
their surfaces and interfaces. While a review on passivation in narrow-gap II-VI
materials has been given by Nemirovsky et al. [53], there have been several other
reviews of passivation of HgCdTe [8, 9, 53-57], and a number of issues with HgCdTe
have added to the difficulties in studying or improving the properties of HgCdTe
surfaces/interfaces [58]. Firstly, because of the weak bonding of Hg in the lattice,
HgCdTe is sensitive to physical and chemical treatments. Cleaving, lapping, wet
etching, and passivation deposition can all lead to surface damage or a change in
stoichiometry. For example, surface wet etching/cleaning can make the surface Te-
rich [58]. After being exposed to air, the surface oxidises to form TeO2 at the surface.
The process history will affect the density of interface states and surface/interface
recombination. This has led to inconsistencies in the literature regarding HgCdTe
passivation results. The weak bonding of Hg in the lattice means that thermal
instability is an issue for HgCdTe unless special precautions are taken, without which
the processing temperature of HgCdTe cannot exceed 90 °C - 100 °C. As a result, any
passivation approach that involves high-temperature deposition is generally
incompatible with good device performance.
Electrical defect sites associated with the passivation can be attributed to dangling
bonds at the interface, impurities, antisites or interstitial atoms, possibly ionised, at the
interface and within the passivation layer. At the interface these defects introduce
additional energy levels within the bandgap and are called surface states or trapping
sites that act as generation-recombination centres, increasing dark currents and
reducing carrier lifetime. If located just within the passivation layer, electrons can
tunnel in and out of these sites, thus demonstrating the characteristics of a slow trap.
In addition to trapping sites, a passivation layer will contain fixed charge, which may
be distributed throughout the passivation layer [59, 60]. Figure 2.4 illustrates a typical
semiconductor passivation system, showing the possible charge centres in the
passivating dielectric and at the interface.
2.3 Surface passivation materials and technologies
2.3.1 Passivation materials
The passivation film must be thick enough to provide adequate environmental
protection and electrical insulation between the metal interconnect and the underlying
20
Chapter 2 HgCdTe Passivation Technologies
substrate. A relatively thick layer may be achieved by deposition of a dielectric
material, either as a capping layer for the native passivation layer or directly onto the
HgCdTe [60].
Passivation technologies for HgCdTe were initiated with anodic oxide growth. The
large positive fixed charge in anodic oxide can be advantageous for n-type MCT
photoconductive detectors. However, the positive charge can invert the surface of p-
type MCT [9]. Other attempts to develop a native passivation include anodic sulfide,
plasma oxide, photochemical oxide and anodic fluor-oxide, often combined with
deposited films such as ZnS, SiN, CdTe, and CdZnTe [60]. Plasma and
photochemically grown oxides offer improved fixed charge levels, and native
sulphides and fluorides may be grown with an even lower fixed charge, providing a
suitable passivation layer for p-n junctions. The drawback of all native layers is the
limited thickness (a few hundred nanometers) to which the layer may be grown, and
their lack of thermal, mechanical and chemical stability.
CdTe has eventually emerged as the leading passivation technology for HgCdTe
devices. Some devices employ dual layer passivation, for example, CdTe and ZnS [2,
34], CdTe and SiNx, and SiOx and SiNx. The CdTe layer provides a good passivation
of the surface, whilst the ZnS provides a higher resistivity insulator, improves the
stability of the CdTe, and can also be used as an anti-reflection (AR) coating.
Passivants for HgCdTe can be classified into three groups summarised below. The
commonly used passivants are a single layer or a combination of the passivants.
1) Native films
• Native oxides, such as anodic oxide, plasma oxide, chemical oxide;
photochemical oxide [8, 53, 54, 56];
• anodic sulfide [8, 61, 62];
• anodic fluor-oxide [63, 64];
2) Deposited films
• ZnS [65];
• CdZnTe [30, 66];
• CdTe [30, 67];
• SiOx [68, 69];
• SiNx [11, 70-72];
• Polymers, such as SU-8 [73].
21
Chapter 2 HgCdTe Passivation Technologies
2.3.2 CdTe passivation
There has been an intensive research effort on CdTe and heterojunction-based
passivation over the last 10 years. High quality CdTe is known to have high
resistivity, be transparent in the IR region, is nearly lattice-matched to the important
HgCdTe compositions (lattice mismatch < 0.3%), and is chemically compatible with
HgCdTe. CdTe passivation typically results in good interface properties, low fixed
charge, and low interface trap densities [58]. Drawbacks of using CdTe in the
passivation process for HgCdTe include the lack of a selective etch between CdTe
and HgCdTe, and a relatively high processing temperatures of > 200 °C required to
deposit high quality stoichiometric CdTe films.
Epitaxy techniques, like MBE and metal-organic chemical vapour deposition
(MOCVD), enable the in-situ growth of high-quality CdTe on top of HgCdTe in a
single run. In-situ growth reduces the likelihood of contamination and would be the
ideal method of CdTe deposition [74]. The grading at the CdTe/HgCdTe interface is
found to be beneficial for surface passivation [53], as it shifts the HgCdTe surface
into the wider gap CdTe region, which gives higher thermal stability.
2.3.3 ZnS passivation
ZnS has been a popular choice among passivation materials, especial being used as a
dual layer on top of CdTe. The disadvantages include its hygroscopic nature and
inconsistent interface properties caused by contamination during the deposition
process. The distribution of fixed charge, usually negative, can also cause non-
uniformity of device performance [75, 76].
Research has been reported on the study and improvement of its long-term stability.
For example, Zhang et al. [77] characterised interface electrical characteristics of
ZnS/CdTe passivation films on HgCdTe by C-V characteristics of MIS test structures
(x-value = 0.22, 0.25). MIS devices with a ZnS/CdTe dual layer were shown to be
superior to devices with just ZnS. For the MIS structure with anodic oxide/ZnS/CdTe
triple layer films, the surface fixed charge density was too high (1.59×1012 cm-2) for
good passivation of photodiodes.
22
Chapter 2 HgCdTe Passivation Technologies
2.3.4 SiNx passivation
SiNx has been characterised as having high dielectric quality and relatively low
deposition temperature (depending on the deposition technique), which is desirable
for HgCdTe passivation. It is more moisture resistant than ZnS or SiOx, and acts as an
effective barrier to external contaminants. It has excellent film uniformity and high
resistivity. Tuneable near-zero mechanical stress can be obtained by modifying the
deposition conditions.
Most of the published work on SiNx as a passivant has been on silicon. Initial papers
on plasma deposited silicon nitride are by Sinha et al. [78] and Kern and Rosler [79]
in the late 1970’s. Later, more work was carried out on PECVD deposited
hydrogenated films (SixNy:Hz or α-SiNx:H) [80], hereafter referred to as SiNx.
Schorner and Hezel have reported that the density of interface states Dit of SiNx/Si is
much higher than that of the SiO2/Si interface [81]. Various studies have been
undertaken in order to improve the hydrogenated SiNx passivation quality and to
investigate the influence of film composition on electrical and mechanical
properties [82-85]. A more detailed literature review on the deposition parameters
concerning high-temperature deposited SiNx films is in Appendix B. High-
temperature processes are not applicable to HgCdTe as detailed earlier.
There are two different views on the structural properties of as-deposited SiNx films in
the literature, yet the issue still remains unresolved. Some argue that these materials
are made up of phases. The material may be composed of a mixture of amorphous Si
and silicon nitride [86, 87] or a two-phase mixture of a highly structured Si3N4-like
phase with a low level of defects and low H content, and a highly disordered phase
with a high concentration of Si-H and N-H defect states and high H content [88, 89].
Some others have adopted the random bonding model, in which there are five possible
Si tetrahedrons Si-SiNnN4-n (n = 0, 1, 2, 3 or 4) in the films, excluding H. The
probability for the occurrence of each tetrahedron is determined statistically [90].
Information in the open literature on SiNx as a HgCdTe passivant is limited. Two
papers from Fujitsu Laboratories [70, 71] reported electron cyclotron resonance
plasma chemical vapour deposited (ECR PCVD) SiNx passivant for Hg0.7Cd0.3Te n+p
diodes. The maximum temperature was 95 °C during the deposition. Their
23
Chapter 2 HgCdTe Passivation Technologies
measurements of flat band voltage shifts after exposure to humidity verified that SiNx
is more moisture resistant than conventional ZnS passivant. Another paper explored
using SiNx for CdZnTe surface passivation [91], and found that sputtered SiNx can be
used to passivate the CdZnTe surface; however, by itself it appears to provide only
modest improvements in surface resistivity; a combination of oxygen plasma and SiNx
coating greatly improved the surface resistivity with likely long-term stability.
Westerhout et al.’s work on PECVD SiNx passivated HgCdTe photodiodes has shown
that there is an increase in the dynamic resistance and decrease in the leakage current
[92], compared to photodiodes passivated by ZnS alone [72].
The atomic hydrogen in the SiNx film can neutralise defects in the interface and the
bulk, which could be enhanced by high-temperature annealing or thermal treatment to
help with diffusion and releasing hydrogen from the Si-H and N-H bonds located in
the SiNx film into the interface and the bulk [82]. For semiconductors or substrates
with high defect densities, such as HgCdTe, hydrogen is expected to work effectively
when diffusing into the bulk material even without any high temperature process step
[93, 94]. More details of related work on SiNx passivation on HgCdTe are discussed
in Chapter 5.
2.3.5 Dual-layer passivation
To improve the resistivity and stability of CdTe passivation, the application of a dual-
layer passivation has been reported, and has been shown to improve the performance
of HgCdTe photodiodes alluded to in the previous sections. The additional ZnS or
SiNx layer improves the insulation characteristics and protects the underlying CdTe
from environmental factors [92, 95-97].
When using CdTe as a passivant, its high index of refraction, which is similar to
HgCdTe, makes it unsuitable as an antireflection (AR) coating. However, with an
additional film of ZnS or SiNx, both with good adhesion to HgCdTe, an AR coating
can be achieved for front-illuminated devices. .
Plasma or evaporated deposited SiO2 and SiOx have shown good insulation and
interface properties [98, 99], however, their porosity can lead to poor moisture
resistance, similar to that of ZnS. For example, absorption of H2O in SiO2 passivated
devices has been shown to induce flatband voltage shifts [99].
24
Chapter 2 HgCdTe Passivation Technologies
Westerhout et al. used PECVD SiNx/CdTe dual-layer passivation on gated diodes
[92], and they demonstrated that SiNx can be used as a high resistivity, stable
replacement for ZnS to passivate plasma-type converted HgCdTe photodiodes.
2.4 Surface preparation for HgCdTe and CdTe
Surface cleaning and conditioning is an essential step before any passivation process,
in order that the passivant can be deposited or grown with minimal fixed charge and
interface states. The specific pre-passivation surface treatment used is dependent on
the passivation process, and, in general, the surface is cleaned via wet processes. The
commonly used etchants include Br2/methanol (1-10%), Br2/HBr (1-5%),
Br2/HBr/Ethylene glycol (in various ratios), K2Cr2O7:HNO3:H2O(4 gm:10 ml:20 ml),
photo electrochemical etching in 1M HClO4, or in 1M KCl [100].
One of the advantages of wet etching over dry etching is that it generates lower
structural and electrical damage to the material. Wet etching of HgCdTe nowadays is
generally not used for device processing, and is limited to surface preparation because
of its poorly controlled etch rates and isotropic etching behaviour. Nearly all the
etchants listed below leave the HgCdTe surface Te rich, which leads to increased
surface leakage across p-n junctions that degrades diode characteristics. The Te rich
layer can be controlled by quenching the etching using methanol [20]. The removal of
the Te-rich layer, Te oxides, and HgTe-rich layers can be achieved by soaking in a
10% KCN solution that regains surface stoichiometry without removal of further
HgCdTe [100, 101].
For dual-layer passivation, surface cleaning and conditioning of the CdTe layer is
important. Since CdTe has been widely used in solar cells and room-temperature X-
ray and gamma ray detectors, as well as IR HgCdTe detectors, there is a significant
body of literature on wet etching of CdTe. Almost all of the etchants used for CdTe
are transferable to HgCdTe. Reported etching solutions include [102-104]:
• HCl or H2SO4
• HNO3
• NaOH
• 10% NaOH + Na2SO4
• 2 HF, 1 HNO3, 1 CH3COOH
25
Chapter 2 HgCdTe Passivation Technologies
• Br2 + CH3OH (e.g. 0.5% Br2 in CH3OH)
• 50 HNO3 + 10 CH3COOH + 1 HCl + 18 H2SO4
• 7g K2Cr2O7 + 3g H2SO4
• 0.5% Br2 + 10 mg AgNO3 + CH3OH
• 2 H2O2, 3 HF, 1(or 2) H2O
• 2 HNO3, 2 HCl, H2O
• 10 ml HNO3, 20 ml H2O, 4 g (or 2 g) K2CrO7
• HNO3/H3PO4 (NP) (used for solar cells) [105, 106]
• H2O2 + HBr + ethylene glycol(EG) [107]
The surface cleaning and conditioning solutions for HgCdTe and CdTe used in this
thesis were all based on a Br2/methanol (0.05-1 %) wet etch, with different ratios used
according to the specific purpose of the processing.
2.5 Modification of interface trap density
There are a number of possible approaches to ameliorate the effects of interface traps,
including annealing, hydrogenation, and modification of stress in the passivation
layers.
Epitaxy techniques, like MBE and metal-organic chemical vapour deposition
(MOCVD), enable the in-situ growth of high-quality CdTe on HgCdTe. In-situ grown
CdTe layers have been characterised as having low fixed charge density [74]. The
electrical properties of the interface can be significantly influenced by the CdTe
growth temperature, pre-treatment and post-treatments [108]. Thermal annealing in a
Cd/Hg atmosphere has been recognised as a crucial step in passivation. Annealing
leads to the formation of compositional grading across the CdTe/HgCdTe interface,
and the reduction in surface recombination velocity (SRV) from 2 × 104 cm·s-1 to ~
3000 cm/s at 77 K [109]. Diodes fabricated using the compositionally graded layer
were reported to have one order of magnitude higher R0A value in comparison to
those passivated by an abrupt interface between CdTe and HgCdTe [110].
Although annealing at elevated temperature helps to create a graded interface layer
due to interdiffusion across the CdTe/HgCdTe interface, electrical and photoelectrical
properties of the HgCdTe active layer may also be changed. A combination of
26
Chapter 2 HgCdTe Passivation Technologies
annealing at 350 °C for 1 hour followed by annealing at 125 °C for 24 hours was
proposed in [111], which resulted in a stable interface as shown by long-term
annealing for two weeks. The CdTe passivation film used in this thesis was grown by
MBE at 100 °C and followed by in-situ annealing at 180 °C in the growth chamber to
achieve a compositionally graded CdTe/HgCdTe interface.
Photon-induced interface modification was reported by Agnihotri et al. [108], which
includes pre-annealing the HgCdTe wafers under ultraviolet photon excitation and in
a Hg environment, as well as the post-deposition annealing of CdTe/HgCdTe under
photon excitation. Pre-annealing of 2 hours and post-annealing of 3 hours were found
to give the lowest fixed charge and interface trap density. This work postulates that
excited Hg atoms and hydrogen radicals are formed by direct collisions in the vapor
phase, which then passivate the wafer surface, reduce the native oxides, and remove
water molecules on the wafer surface.
For the case of ZnS as a passivant, ammonium sulfidation treatment ((NH4)Sx) of the
HgCdTe substrate was reported to be a way of modification of interface traps [112].
Sulfidation results in a decrease in the concentration of contaminants originating from
the native oxide-covered HgCdTe substrates. The fixed charge and slow trap density
were found to be between 2 and 7 times lower in the treated samples compared to
untreated MIS structures.
Most hydrogenation studies of semiconductors have been carried out on GaAs and Si
materials, and has led to the development of the technical application of hydrogenated
amorphous silicon [113]. However, little work has been performed on II-VI
compounds. The principle interest in hydrogenated passivants is the ability to
passivate the electrical activity of dangling or defective bonds.
Importantly, and of less positive impact, hydrogen can also significantly change the
electrical and optical properties of the bulk materials, including passivation of shallow
acceptor and donor impurities in several technologically important semiconductors
[114]. Hg vacancies have been shown to be effectively passivated by atomic
hydrogen. Hydrogen injection and passivation of the residual impurities are also
observed in Hg0.8Cd0.2Te boiled in water [115, 116]. The work in this thesis also
observed the surface and bulk passivation effects induced by ICPECVD hydrogenated
SiNx. Magneto-transport measurements were carried out before and after SiNx
passivation on HgCdTe. Comparing the results analysed using quantitative mobility
27
Chapter 2 HgCdTe Passivation Technologies
spectrum analysis (QMSA), there is an increase in electron mobility in HgCdTe after
being passivated by SiNx, which can be explained by hydrogen passivation of
dangling bonds during the ICPECVD process. This is discussed further in Section
3.3.5.
As the thermal stability of hydrogen passivation, most of the defects and impurities in
Si, such as Au, Cu, Fe and Zn, that are passivated by reaction with atomic hydrogen
were found to be regenerated by post-hydrogenation annealing ( > 400 °C) [117].
Similarly, Hirayama and Tatsumi and Iyer et al. have observed remarkable stability of
the hydrogen-passivated surface when being supplied with atomic hydrogen at
temperatures below 400 °C [118, 119], whereas they have found that the hydrogen
passivation was quickly lost at temperatures higher than 400 °C. Also, because of the
greater degree of lattice relaxation associated with deep levels, the passivation of deep
level centres was found to be much more thermally stable than shallow level
passivation [117].
Exposing a ZnS passivation layer to H2/CH4 RIE plasma has been shown to reduce
the fixed charge density, while the interface trap density is unchanged [120]. It was
found that the LWIR photodiode leakage current was reduced after the treatment, and
the junction properties also were improved with hydrogenation [121].
Lattice mismatch between passivation and epilayer and/or between substrate and
epilayers of different mole-fractions can create a variety of distortions and defects in
HgCdTe, affecting the performance of infrared detectors. Therefore, modification of
stress in the layers can result in modification of interface traps. White et al. have
observed an increase in trap-assisted tunneling at the junction surface perimeter,
which is associated with the increased lattice stress [122]. The interdiffusion of Zn
and Hg, due to the deposition of ZnS on HgCdTe and post-baking in a vacuum, has
led to lattice stress due to the smaller lattice constant of ZnTe. CdTe passivation as the
standard approach in infrared detector technology is associated with its nearly lattice
match with HgCdTe, resulting in minimal stress [9]. As to the effects of interfacial
lattice mismatch between HgCdTe epilayers, Sugiura et al. [123] observed that
HgCdTe epilayers can be easily affected by lattice mismatch of less than ± 0.1%. In
addition, they observed that the mismatched HgCdTe epilayers in tension tend to
deteriorate more easily than those in compression, which is associated with the
28
Chapter 2 HgCdTe Passivation Technologies
asymmetric dislocation distribution due to excess mercury vacancies, the sign of
strain and lattice structure.
2.6 Summary
Surface passivation is becoming a key technology for reducing surface recombination
and improving the performance of HgCdTe detectors. This chapter reviewed the
importance of surface passivation for HgCdTe detectors in terms of device
architectures and performance. Passivation materials and surface treatments for
HgCdTe were also reviewed. Lastly, this chapter presented surface passivation
materials and technologies used in this thesis.
29
Chapter 3 Material Characterisation
3 Material Characterisation
3.1 Introduction
Characterisation techniques that have been used to investigate the CdTe/HgCdTe
interface include material characterisation probes, optical characterisation studies and
electrical characterisation. There are a number of material analysis techniques that are
applicable to HgCdTe, including high-field low-temperature magneto-optic studies
[124-128], scanning capacitance micro-probe measurements [129], photo-reflectance
and other modulated optical pump-probe techniques [130, 131], reciprocal-space X-
ray topographic imaging measurements [20, 132], magneto-transport studies based on
quantitative mobility spectrum analysis (QMSA) or high-resolution mobility spectrum
analysis (HR-MSA) [133-136], spatial lifetime mapping [137, 138], and scanning
photoluminescence [129, 137, 139].
The characterisation techniques employed in this thesis are discussed below in two
categories: physical characterisation and electrical characterisation. Some of the
techniques were performed as in-situ monitoring characterisations within the MBE
growth chamber, whereas others were undertaken as ex-situ characterisations. The in-
situ diagnostic techniques used to monitor the growth of HgCdTe in the MBE
chamber related to this thesis include reflection high energy electron diffraction
(RHEED) and spectroscopic ellipsometry. The other two commonly used in-situ
diagnostic techniques in the growth of HgCdTe, beam flux monitoring and residual
gas analysis (RGA), will not be discussed here.
3.2 Physical Characterisation
3.2.1 Microscopy
In this thesis, optical microscopy, as an ex-situ characterisation technique, has been
used to inspect defects and morphology. As to the growth of MBE HgCdTe in the
thesis, the Cd0.96Zn0.04Te substrates with a (211)B orientation from the Nippon Mining
and Materials were used. The Cd0.96Zn0.04Te substrate is nearly lattice matched to
Hg0.78Cd0.22Te that helps to minimise the misfit dislocation formations [57]. The CdTe
cell temperature of 525 °C to 540 °C, Te cell temperature of 310 °C to 325 °C and
30
Chapter 3 Material Characterisation
substrate temperature of 185 °C were used, with the beam equivalent pressures (BEP)
of approximately 10-6 Torr. The LPE HgCdTe wafers used in the thesis were
purchased from the Epitech Company, and have a (111) orientation and x value of 0.3.
Figure 3.1 SEM micrographs corresponding to (a) and (b) 60 nm-thick CdTe film on HgCdTe, and (c) 300 nm-thick CdTe on HgCdTe. The CdTe layer was deposited in an MBE system.
(a)
(b)
(c)
200 nm
100 nm
200 nm
31
Chapter 3 Material Characterisation
MBE grown samples undergo inspection by microscopy after unloading from the
MBE growth system and after wafer processing for defect assessment. The Nomarski
contrast imaging mode is particularly useful as it offers high resolution and clarity
[140], providing easily observed information on surface planarity and macro-defects.
Crosshatched patterns are generally observed on the HgCdTe surface even when
samples are grown close to MBE optimal conditions, which is due to strain induced
by lattice mismatch. The crosshatch pattern on the (211)B surface comprises of three
sets of lines in the ]312[ , ]132[ and ]101[ directions [141].
Scanning electron microscopy (SEM) has also been used for imaging to examine
surface morphology in more detail. For example, the low-temperature MBE grown
CdTe passivation films used in this thesis were studied using SEM to investigate their
surface morphology. Smooth grains with flat surfaces were observed for both 60 nm-
thick and 300 nm-thick CdTe films (Figure 3.1 (a) and (c)). An SEM micrograph of
observed surface defects is shown in Figure 3.1 (b).
3.2.2 X-ray diffraction
Double crystal X-ray diffraction (DCXRD) was used to characterise the crystalline
quality of MBE grown CdTe and HgCdTe materials, utilising a Panalytical Empyrean
X-ray diffractometer. The rocking curve measurements allow the crystal perfection
and strain to be characterised. The full-width-at-half maximum (FWHM) of the
DCXRD rocking curve peak was used as a metric of the crystalline quality of the
MBE grown layers. Depending on the MBE growth conditions, HgCdTe samples
used in this thesis typically have a FWHM between 70 - 120 arcsecs, which is an
indication of good single-crystal material [142]. Figure 3.2 shows the DCXRD spectra
of a CdTe layers on a GaAs substrates, grown at 275 °C by MBE, indicating a FWHM
of 66 arcsecs. The samples were used for the SiNx stability studies in Section 3.4.
Figure 3.3 shows the DCXRD spectra of a MBE grown HgCdTe layers on CdZnTe
substrate (n-HgCdTe/n+-HgCdTe/CdZnTe) with a FWHM of 73 arcsecs. These
samples were used for MIS structure fabrication and capacitance-voltage analysis in
Chapter 5.
32
Chapter 3 Material Characterisation
Figure 3.2 Double crystal X-ray diffraction spectra of MBE grown CdTe layer on GaAs substrate.
Figure 3.3 Double crystal X-ray diffraction spectra of MBE grown HgCdTe layers on CdZnTe substrate (n-HgCdTe/n+-HgCdTe/CdZnTe).
37.95 38 38.05 38.1 38.15 38.20
200
400
600
800
ω
Cou
nts
(degrees)
36.1 36.15 36.2 36.25 36.3 36.350
500
1000
1500
2000
2500
ω
Cou
nts
(degrees)
33
Chapter 3 Material Characterisation
Figure 3.4 X-ray diffraction spectra of (a) the LPE HgCdTe before CdTe growth and (b) the MBE grown CdTe (on LPE HgCdTe).
The CdTe passivation films used in Chapter 4 were grown by MBE at a low
temperature of 100 °C, followed by in-situ annealing at 180 °C in the growth chamber
to achieve a compositionally graded CdTe/HgCdTe interface. Because of the 100 °C
growth temperature, the films are found to be polycrystalline. The θ-2θ XRD spectra
were collected before and after CdTe growth. The crystallographic studies of the low-
temperature grown and in-situ annealed CdTe films were carried out by XRD using
Cu Kα radiation at room temperature. Figure 3.4 (a) shows the XRD pattern of the
HgCdTe before CdTe growth, which indicates that structure of LPE grown HgCdTe is
composed of HgCdTe (111) as the preferred orientation. Figure 3.4 (b) shows the
low-temperature deposited CdTe passivation film to be polycrystalline with (100) and
possibly (111) orientations.
3.2.3 Reflection high energy electron diffraction
Reflection high energy electron diffraction (RHEED) is a very useful surface analysis
technique, and its compatibility with the MBE HgCdTe growth process allows the in-
situ monitoring of the diffraction pattern in order to extract information on the atomic
arrangement of atoms near the crystal surface. The technique has the potential to
10 20 30 40 50 60 70 800
5
10
15C
ount
s (x
104 )
10 20 30 40 50 60 70 800
2
4
2θ
Cou
nts
(x10
2 )
(a)
(b)
LPE HgCdTe
200nm MBE CdTe filmCdTe(111)
CdTe(400)
HgCdTe(111)
CdTe(511)
(degrees)
34
Chapter 3 Material Characterisation
allow real-time adjustment of the growth condition during the actual growth for some
material composition and crystal orientation. The main parts of a RHEED system are
an electron gun, deflection plates, the crystalline film to be analysed, and a phosphor
screen with a shutter. When the electron beam is directed onto the sample at grazing
incidence, an interaction with the surface atoms, which diffract the electrons in a
pattern, may be observed on the phosphor screen.
In this thesis, RHEED patterns were monitored periodically during the MBE growth,
so that in addition to the flatness of the substrate, the condition of the buffer layer and
HgCdTe layers could also be monitored. In contrast to the growth on (100) substrates,
HgCdTe growth on (211)B substrates do not show observable RHEED intensity
oscillations, which are determined by the surface atomic flatness [143].
An example of the recorded RHEED patterns is shown in Figure 3.5 for HgCdTe
sample CMCT042, which was later fabricated into MIS structures in order to
investigate SiNx/HgCdTe interface, as detailed in Chapter 5. Figure 3.5 (a) is the
pattern seen during the thermal cleaning of the (211) B CdZnTe substrate, which has a
Te-rich surface due to the etch in a Br/methanol solution used for surface conditioning.
During the thermal cleaning process, which heats the sample to an elevated
temperature of 285 °C - 290 °C for 20 minutes under ultra-high vacuum, the excess
Te becomes mobile on the CdZnTe surface, giving the RHEED pattern the
appearance of an array of bright spots. During thermal cleaning, the spotty pattern
gradually turns into a streaky one, as shown in Figure 3.5 (b), indicating a flat,
crystalline surface.
The first HgCdTe layer grown on the CdZnTe substrate is a 2 μm-thick indium-doped
n-type layer with an x-value of 0.4, which serves both as a low resistance contacting
layer and as a confinement layer to confine photogenerated minority carriers in the
absorber layer. The RHEED patterns monitoring the above n-HgCdTe growth are
shown in Figure 3.5 (c) and Figure 3.5 (d). The pattern with clear and elongated
streaks is an indication of a flat and single crystal surface morphology. Then the
5 μm-thick absorber layer (x = 0.316) was grown, with RHEED patterns shown in
Figure 3.5 (e) and Figure 3.5 (f), still with elongated streaks, showing the absorber
layer has an atomically flat surface morphology.
35
Chapter 3 Material Characterisation
Figure 3.5 The RHEED patterns recorded for HgCdTe sample CMCT042. (a) During substrate thermal cleaning; (b) Toward the end of thermal cleaning; (c) At the start of growth of n-HgCdTe layer (x = 0.4); (d) Toward the end of the growth of n-HgCdTe (x = 0.4) layer; (e) At the start of growth of the MWIR absorber layer (x = 0.316); (f) Toward the end of the growth of absorber layer.
3.2.4 Energy dispersive X-ray analysis
Energy dispersive spectroscopy (EDS) is an electron beam analysis procedure for
examination of the stoichiometry of SiNx films. The FEI Verios SEM system is
equipped with an Oxford energy dispersive X-ray spectrometer with an 80 mm2
silicon drift detector installed. EDS is a technique for quantitative elemental analysis
of a sample by measuring the spectrum of characteristic X-rays emitted from a sample.
To stimulate the emission of characteristic X-rays from a sample, a high-energy beam
of electrons focused onto the sample under study. The number and energy of X-rays
photons emitted from the sample can be measured by an energy-dispersive
spectrometer.
The SiNx/Si samples under study in Section 3.4 had corresponding SiNx films
deposited on GaAs substrates as reference samples for EDS using a beam energy of
5 keV in order to estimate stoichiometry of SiNx films, i.e. [N]/[Si] atomic ratio.
(b) (a)
(e) (d) (f)
(c) 10 mm 10 mm 10 mm
10 mm 10 mm 10 mm
36
Chapter 3 Material Characterisation
GaAs was chosen as the substrate to prevent interference from a Si substrate that
would occur due to the relatively thin SiNx films used. Also, EDS was applied on the
SiNx/GaAs reference samples of SiNx/HgCdTe to estimate the stoichiometry of SiNx
films on HgCdTe, in order to avoid any electron beam damage to the HgCdTe wafers.
3.2.5 Spectroscopic ellipsometry
Spectroscopic ellipsometry measures the change in polarization as light reflects or
transmits from a material structure as a function of optical wavelength. By comparing
the measured wavelength-dependent dielectric function with a pre-stored library file,
film thickness and composition can be extracted. It is also capable of characterising
crystallinity, surface/interface roughness, doping concentration, and other material
properties associated with a change in optical response [144].
Figure 3.6 (a) Refractive index, n, and (b) extinction coefficient, k, measured by ellipsometry for a 11.18 μm-thick HgCdTe/CdZnTe sample numbered MCT223 (x = 0.281).
5 6 7 8 9 103.15
3.2
3.25
3.3
Wavelength (µm)
n
5 6 7 8 9 100.05
0.1
0.15
Wavelength (µm)
k
(a)
(b)
37
Chapter 3 Material Characterisation
The ability to perform in-situ ellipsometry during MBE growth allows real-time
adjustment of growth conditions to improve film quality. While in-situ capability is
now available in our MBE system, during the growth of the samples related to this
thesis the in-situ ellipsometry had not been commissioned, so refractive index and
extinction coefficient of grown HgCdTe layers was measured by ex-situ ellipsometry,
with an example shown in Figure 3.6. Such a thick layer of 11.18 μm was designed
for the sample to be measured by ellipsometry with an increased accuracy in
modelling.
3.2.6 Optical reflection/transmission for structural and compositional characterisation
Transmission measurements were undertaken using Fourier transform infrared (FTIR)
spectroscopy to determine the HgCdTe compositions (x-value) and layer thicknesses.
FTIR absorbance spectra from the SiNx films were used and monitored to examine the
film stability over a six-month time frame. The results of these studies are discussed
in detail in Section 3.4 and Chapter 5. The bonding configurations are discussed in
Section 3.4.4.3.
Figure 3.7 FTIR transmission spectra for MCT223 (x = 0.281, depilayer = 11.18 μm) with the air background being subtracted. Solid line: measured FTIR transmittance; dotted line: modelled transmittance curve.
500 1000 1500 2000 2500 30000
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Wavenumber (cm-1)
Tran
smis
sion
ModelMeasured
38
Chapter 3 Material Characterisation
Figure 3.8 FTIR transmission spectra for MCT225 (x = 0.375, depilayer = 8.9 μm) before and after wafer annealing, with the annealing being carried out in a saturated Hg atmosphere at 235 °C for 24 hours. Dotted line: measured transmittance before annealing; Solid line: transmittance after annealing.
Figure 3.7 shows the room-temperature transmission spectra of HgCdTe sample
MCT223 that was characterised by ellipsometry in Section 3.2.2. The abrupt cut-off
indicates the MBE grown layer has good compositional uniformity. The x-value and
film thickness can be extracted from the transmittance curve. The cut-off is a function
of the bandgap and is dependent on x-value. The fringes in the curve show the
interference effects of the infrared radiation reflected between the sample surface and
the epilayers/substrate layer interface, allowing film thickness to be calculated from
the periodicity of the fringes. The extracted x-value for HgCdTe sample MCT223 is
0.281, with a thickness of 11.18 μm. The estimated error in determining the epilayer
thickness and its x-value is ± 0.1 %. The measured transmission is shown in
Figure 3.7 as the solid line and the modelled results as the dotted line. The FTIR
transmission spectra for HgCdTe sample MCT225 before and after a vacancy-filling
anneal are shown in Figure 3.8, with the annealing being carried out in a saturated Hg
atmosphere at 235 °C for 24 hours. There was only a slight change in the mole
fraction indicated after wafer annealing.
1000 2000 3000 40000
0.1
0.2
0.3
0.4
0.5
0.6
Wavenumber (cm-1)
Tran
smis
sion
After annealingBefore annealing
39
Chapter 3 Material Characterisation
3.2.7 Optical reflection/transmission for bonding and detailed characterisation of thin films
An indirect estimate of the [N]/[Si] ratio in thin films of SiNx can be obtained based
on relations between [N]/[Si] and parameters that are [N]/[Si]-sensitive that have a
monotonic variation with [N]/[Si] and are easily measurable. In this thesis, refractive
indices determined by ex-situ spectroscopic ellipsometry have been used to achieve
this [145, 146]. Ex-situ ellipsometry has been carried out by using a Filmetric
spectroscopic thin-film analyser. After data fitting, refractive index was firstly
acquired in the range from 380 nm to 1050 nm, then the refractive index at a single
wavelength (usually at 632.8 nm, n632.8nm) was used for the estimation of film
stoichiometry. A single n value of n632.8nm is useful when comparing the properties of
SiNx films with literature and to other materials, such as SiO2.[147] The relationship
between [N]/[Si] ratio and refractive index are investigated in Section 3.4.
3.3 Electrical Characterisation
3.3.1 Magneto-transport measurements
Hall-effect and resistivity measurements are employed to study carrier transport in
semiconductor materials to gain knowledge of carrier concentrations and mobility,
thus providing information that can be correlated to material quality and device
performance [148]. The principle of the Hall effect can be explained by considering a
slab of conducting material through which a uniform current density flows under the
presence of an applied magnetic field applied in the direction perpendicular to the
current flow [149, 150]. In this session, Greek Cross Van der Pauw test structures
[151] were utilised for the Hall measurements. Figure 3.9 illustrates a
photolithographically defined Greek Cross Van der Pauw structure fabricated on
HgCdTe.
For the Hall-effect measurements, Hall and resistivity voltages were measured as
function of magnetic field intensity [152, 153] in a narrow-gap 2-Tesla electromagnet
Hall-effect measurement system at UWA. In this thesis, the high-resolution mobility
spectrum analysis (HR-MSA) algorithm developed by researchers at the University of
Western Australia was used to analyse the measured magnetic field dependent
resistivity and Hall coeffient, since HR-MSA has demonstrated to be less sensitive to
40
Chapter 3 Material Characterisation
moderate noise levels in the measured data, and with improved robustness and
resolution than commercially available mobility spectrum analysis algorithms [154,
155]. The HR-MSA algorithm, although slow computationally, has the ability to
resolve closely spaced mobility peaks and provides accurate information on the
mobility distribution of each carrier species [1, 2, 133, 135, 156]. Mobility spectrum
analysis of the variable magnetic field Hall data yields a conductivity spectrum as a
function of mobility for the individual hole and electron carrier species present in the
sample, with each conductivity peak representing an individual carrier distribution.
From each conductivity peak an average mobility and carrier concentration can be
obtained, thus enabling the discrimination of carrier transport parameters in samples
exhibiting multiple carrier species [20, 134, 157]. It is important to note that, since all
the samples studied were n-type, only electron mobility spectra are discussed and
presented in this session, since no significant contribution from holes was found in the
HR-MSA extracted mobility spectrum characteristics.
In order to evaluate the effect of the CdTe passivation film, grown by MBE as at
100 °C and in-situ annealed at 180 °C, on the transport parameters of HgCdTe
epitaxial layers, magneto-transport measurements were carried out on the HgCdTe
wafer labelled MCT225 (x = 0.375, epilayer thickness depilayer = 8.9 μm) using the
following procedure:
1. As-grown wafer was annealed under saturated Hg atmosphere at 235 °C for
24 hours. The wafer was then diced into two pieces.
2. After the vacancy filling anneal, one wafer piece was processed into a mesa
isolated Van der Pauw [158] structure with a Greek Cross topology.
3. Magneto-transport measurements were performed on the processed Van der Pauw
structures at liquid nitrogen temperature. The results of the analysis of these
magnetic field-dependent resistivity and Hall-effect data extracted using HR-MSA
are illustrated in Figure 3.10.
4. The surface of the second half of the annealed MCT225 sample was etched in 0.1%
Br2:Methanol solution, and immediately loaded into the MBE chamber where the
CdTe passivation layer (~ 100 nm thick) was deposited. Following CdTe
passivation, the sample was processed into a Van der Pauw structure. For ohmic
contact formation, the CdTe passivation layer was etched in a 1% Br:HBr solution
41
Chapter 3 Material Characterisation
prior to Au/Cr metal deposition. Note that the sidewalls of the structure were left
unpassivated.
5. Magneto-transport measurements were then performed at liquid nitrogen
temperature on the processed CdTe-passivated Van der Pauw structure from Step 4.
Again, the measured data was analysed using HR-MSA. The extracted electron
mobility spectra are presented in Figure 3.10, for comparison with the unpassivated
structure. The extracted transport parameters are summarised in Table 3.1.
Figure 3.9 Image of the centre part of a fabricated Greek Cross van der Pauw structure on HgCdTe taken under an optical microscope.
Figure 3.10 Comparison of the electron conductivity - electron mobility spectra measured before and after MBE CdTe growth.
400 μm
103 104 1050
1
2
x 10-4
Mobility (cm2 V-1 s-1)
She
et C
ondu
ctiv
ity (Ω
/squ
are)
-1
HgCdTeCdTe/HgCdTe
3
42
Chapter 3 Material Characterisation
Table 3.1 Summary of extracted electron transport parameters for HgCdTe sample MCT225 before and after the CdTe passivation
Peak 1 (Bulk) Peak 2 (substrate interface) Peak 3 (surface) Total sheet
conductivity
µb (cm2/V⋅s) Nb (cm-2) σe µsi (cm2/V⋅s) Nsi (cm-2) σsi µs (cm2/V⋅s) Ns (cm-2) σs σxx (0) (Ω/square)-1)
Unpassivated 25597 1.27×1011 0.62 9801 1.14e×1011 0.21 4331 2.01e×1011 0.17 8.47×10-4 Passivated 20176 7.41×1010 0.66 7307 5.75×1010 0.19 2522 1.24×1011 0.14 3.67×10-4
µ (cm2/V⋅s): electron mobility; N (cm-2): electron concentration; σ : conductivity percentage; σxx (0) (Ω/square)-1): total sheet conductivity
Figure 3.11 Plots showing the electron mobility spectrum measured after the vacancy filling anneal at liquid nitrogen temperature for (a) MCT231 (x = 0.388, depilayer = 6.4 μm) and (b) MCT240 (x = 0.347, depilayer = 5.23 μm).
103 104 1050
0.2
0.4
0.6
0.8
1 x 10-4
Mobility (cm2 V-1 s-1)
She
et C
ondu
ctiv
ity (Ω
/squ
are)
-1
(a)
103 104 1050
0.2
0.4
0.6
0.8
1 x 10-4
Mobility (cm2 V-1 s-1)S
heet
Con
duct
ivity
(Ω/s
quar
e)-1
(b)
43
Chapter 3 M
aterial Characterisation
43
Chapter 3 Material Characterisation
From Figure 3.10 and Table 3.1, it is evident that the conductivity of both before and
after CdTe passivation is due to three distinct electron peaks which can be attributed
to the electrons in the bulk of the HgCdTe epilayer (highest mobility), electrons
associated with the substrate/epilayer interface, and the surface or the CdTe/HgCdTe
interface (lowest mobility) [135, 155, 156, 159]. However, the most striking features
of the results presented in Figure 3.10 are: (i) a significant decrease in the total
conductivity of the CdTe passivated sample, and (ii) the reduction in the average
electron mobility for all electron peaks, and (iii) a slight reduction in total
conductivity component associated with the lowest electron peak after CdTe
passivation. While the latter is likely an indication of the effect of surface passivation,
the former two characteristics are most likely an effect of compensation resulting
from Hg-vacancy formation during the in-situ annealing step at 180 °C following the
deposition of CdTe.
Measurements of the electron mobility spectra on other wafers, such as sample
MCT231 (Figure 3.11 (a)) and MCT240 (Figure 3.11 (b)), from the same MBE
growth campaign indicate that after the vacancy filling anneal on MBE grown
HgCdTe wafers, the samples exhibit comparable values of mobility as presented in
the literature [160].
3.3.2 Current-voltage measurements
In the work presented in this thesis, current-voltage (I-V) measurements were carried
out to study directly the insulating properties of the passivant and to investigate the
passivation/semiconductor interface through measurement of the I-V characteristics of
gated diodes as a function of gate bias. All measurements were performed using an
HP4156A semiconductor parameter analyser. In particular, leakage current of the
Au/Cr/SiNx/Si MIS capacitors to evaluate the insulating properties of SiNx; and the
dark I-V versus gated bias measurements were used for on the ZnS/CdTe/HgCdTe
gated diodes to investigate the influence of surface band-bending on the diode
characteristics. This latter will be discussed in more detail in Section 4.3.
Leakage currents were measured at 300 K on circular, 500 μm diameter
Au/Cr/SiNx/Si MIS capacitor structures. Silicon wafer cleaning and buffered oxide
etch were conducted just before SiNx film deposition. The I-V measurements were
44
Chapter 3 Material Characterisation
Figure 3.12 A typical current-voltage characteristic of Au/Cr/SiNx/Si MIS structure.
Figure 3.13 Resistivity variation of D4-100C SiNx film over a period of four months. The MIS structures were left to age in laboratory atmosphere.
0 1 2 3 42
4
6
8
10 x 1010
Months since fabrication
Res
istiv
ity (Ω
cm)
-10 -5 0 5 10-1.5
-1
-0.5
0
0.5
1
1.5 x 10-11
Voltage (V)
Cur
rent
(A)
sweeping from -10V to 10Vsweeping back from 10V to -10V
45
Chapter 3 Material Characterisation
carried out using a voltage sweep from -10 V to 10 V, and back to -10 V, with an
example plotted in Figure 3.12. It can be seen that the I-V characteristics within the
sweep range is essentially linear, indicating a resistive leakage mechanism. Leakage
current can be caused by traps and defects in the hydrogenated SiNx film and at the
SiNx/HgCdTe interface [161]. Traps in the SiNx film and at the interface can be
attributed to dangling bonds, as well as Si-H and N-H bonds in the forbidden
gap [162]. Low-temperature deposited SiNx films can exhibit higher trap densities
[163], therefore the optimisation of film deposition conditions is crucial.
An effective resistivity of the samples was calculated by averaging the calculated
resistivity points (R = dV/dI) in the linear part of the I-V characteristics. The
thickness of the insulating layer used in the resistivity calculations was measured with
a Dektak 150 stylus surface profilometer. Additionally, leakage measurements were
taken over a four month period after fabrication to determine the stability of the SiNx.
Figure 3.13 shows the resistivity of the B4-SiNx film over a four month period, with
the MIS devices left to age in laboratory atmosphere. A gradual decrease in the film
resistivity over time is clearly evident, although the resistivity remains > 1010 Ω cm,
even after four months. Moisture in the air may have contributed to this degradation
of the SiNx films [2].
3.3.3 Capacitance-voltage and capacitance-frequency measurements
Analysing the capacitance-voltage (C-V) characteristics of a MIS structure can
provide information on the insulator thickness, semiconductor doping concentration,
fixed charge within the passivation film and interface trap density. Fixed charge is
thought to be due to structural defects very close to the passivation-semiconductor
interface, which are not in electrical communication with the semiconductor. Interface
traps are attributed to defects or impurities acting as donor or acceptor sites
introducing additional energy levels within the semiconductor bandgap at the
interface. In addition, there are mobile ionic charges within the insulator, and charge
trapped in the bulk of the insulator as well. The electrical properties of the
SiNx/HgCdTe interface can be evaluated from the standard MOS theory based on a
MOS structure [164].
Metal-SiNx-HgCdTe MIS structures allow investigation of the interface between SiNx
and HgCdTe, including the interface trap density, Dit, and were used in this thesis as
46
Chapter 3 Material Characterisation
the primary tool to evaluate surface passivation performance and to correlate
passivation quality with other film properties. To investigate the insulating properties
of the SiNx passivation films, capacitors for test purposes were firstly fabricated on Si
substrates. All C-V measurements were taken with an HP4284A precision LCR meter
with a small-signal frequency of 1 MHz.
As an example of the C-V characteristics and analysis, Figure 3.14 shows the high-
frequency C-V curves measured at 298 K with variable sweep ranges from -15 V to
15 V and then back to -15 V, -20 V to 20 V then back to -20 V, -24 V to 24 V then
back to -24 V and -30 V to 30 V then back to -30 V. The SiNx was deposited by
ICPECVD at 100 °C with a thickness of 192 nm, using the deposition condition of
D4-100C (conditions are detailed in Table 5.1). Figure 3.15 shows another batch of
SiNx/Si MIS devices measured at 298 K (solid line) and at the typical IR
photodetector operating temperature of 80 K (dashed line), also using the same
deposition condition of D4-100C, with a thickness of 220 nm. The C-V analysis and
extracted information are listed in Table 3.2. The interface trap density, Dit, was
extracted by Terman’s method by analysing the high-frequency C-V data measured at
1 MHz [165]. The interface trap density, Dit, can also be extracted from the high-
frequency and low-frequency capacitance-voltage characteristics, and also by the
conductance method, as detailed in Chapter 5.
Table 3.2 Data extracted from C-V analysis on a SiNx/Si MIS capacitor measured at 298 K and 77 K
T = 298 K T = 80 K
Flat band voltage (V) -8.32 -5.47
Fixed charge density ( × 1012 cm-2) 1.51 0.99
Hysteresis width (V) 3.98 3.03
Slow interface states ( × 1011 cm-2) 7.55 5.67
Interface trap density Dit ( × 1012 eV-1cm-2) 3.48 1.74
47
Chapter 3 Material Characterisation
Figure 3.14 The SiNx/Si MIS structure measured at 298 K with variable sweep ranges from -15 V to 15 V (innermost pair of curves), -20 V to 20 V, -24 V to 24 V and -30 V to 30 V (outermost pair of curves). The D4-100C SiNx (deposition conditions detailed in Table 3.3) was deposited by ICPECVD at 100 °C with a thickness of 192 nm.
Figure 3.15 The SiNx/Si MIS structure measured at 298 K (red solid line) and 77 K (blue dashed line) with sweep ranges from -20 V to 20 V. The D4-100C SiNx (deposition conditions detailed in Table 3.3) was deposited at 100 °C with a thickness of 220 nm.
-20 -10 0 10 200
0.5
1
1.5
2
2.5
3
3.5 x 10-8
Applied voltage (V)
Cap
acita
nce/
Are
a (F
/cm
2 )
298 K
80 K
-30 -20 -10 0 100
1
2
3
4 x 10-8
Applied voltage (V)
Cap
acita
nce/
Are
a (F
/cm
2 )
48
Chapter 3 Material Characterisation
3.4 Performance of ICPECVD SiNx passivation on other semiconductors
Epitaxially grown CdTe/HgCdTe heterojunctions have been a leading surface
passivation choice for HgCdTe, however, it is not always feasible to have in-situ
grown CdTe on HgCdTe, due to the requirement of additional processing steps before
the CdTe growth. Also, the relatively high growth temperatures (> 200 °C) required to
deposit stoichiometric CdTe films is well above the desired processing temperature
for HgCdTe. Silicon nitride films of high dielectric quality deposited at low
temperatures are attractive for surface passivation of HgCdTe devices. Hydrogenated
SiNx has demonstrated its capability in passivating the surface, and can improve the
bulk material by hydrogen passivation of defect centers as a consequence of hydrogen
incorporation during the plasma process [84, 166, 167]. The presence of CdTe on
HgCdTe may hinder the diffusion of hydrogen atoms and hence the effect of
passivation. There are few reported studies on SiNx as a passivation layer for HgCdTe
and related compounds. This section aims to develop, optimise and characterise low-
temperature (80 °C - 100 °C) deposited SiNx films for passivating HgCdTe without
the CdTe layer in between [1].
Conventionally, high quality SiNx films for surface passivation layers are typically
deposited at temperatures in the 200 °C - 750 °C range, much higher than the
maximum allowed HgCdTe processing temperature (< 120 °C). The ECR-plasma
CVD process features low-temperature deposition of SiNx films suitable for HgCdTe
passivation [70, 71], however, ECR-plasma CVD systems can suffer from uniformity
limitations [168, 169]. Inductively-coupled plasma-enhanced chemical vapour
deposition (ICPECVD) offers the ability to deposit high quality SiNx films at
temperatures as low as 80 °C [169-171]. Low-temperature (80 °C - 130 °C) SiNx
films deposited in ICPECVD SI500D (SENTECH Instruments GmbH) with a high-
density and low ion energy plasma source have been reported to be characterised by a
low etch rate in wet-chemical etchants, minimal damage to substrate surface during
deposition, low stress and high breakdown voltage [163]. The SiNx films under study
in the thesis were all deposited in the Sentech SI500D ICPECVD system. The low ion
energy of the plasma source in the ICPECVD systems enables SiNx films to be
deposited on HgCdTe without significant surface damage.
49
Chapter 3 Material Characterisation
3.4.1 Surface passivation by hydrogenated silicon nitride
Under amino-saturated conditions, the deposition of SiNx from NH3+SiH4 gas mixture
may be explained as [172]
242Plasma
34 4H )Si(NH 4NH SiH + →+ (3.1)
Then followed by a surface condensation reaction
↑+→ 343Heat
42 8NH NSi )3Si(NH (3.2)
Note that the surface condensation process is facilitated by high temperature and slow
deposition rate. Condensation to stoichiometric Si3N4 cannot be achieved even at a
temperature as high as 530 °C [172]. Hydrogenated SiNx films passivate the bulk and
surface of the semiconductor and minimise surface recombination rate , as discussed
below [93, 173]:
Chemical passivation of the semiconductor surface is achieved by the chemical
bonding of silicon, nitrogen and hydrogen atoms to the atoms at the interface. At the
surface of a semiconductor, there exist unsaturated bonds, referred to as dangling
bonds, which may introduce energy levels within the bandgap of the semiconductor.
Also, surface states can result from dislocations, or chemical residues and metallic
depositions on the surface [83, 172, 174-178]. Chemical passivation terminates the
dangling bonds at the interface, lowers the density of interface states and reduces the
SRH recombination rate.
The polarity of the fixed charge in SiNx films was reported to be negative on HgCdTe
substrates [70]. This has been confirmed in the thesis and is opposite to the one when
deposited on silicon substrates as shown in Section 3.3.7 and in the literature [179,
180].
50
Chapter 3 Material Characterisation
3.4.2 Experimental setup and design
3.4.2.1 Features of the ICPECVD system employed for surface passivation
The SiNx films employed in this chapter were deposited employing a high-density
PECVD system - SI500D (SENTECH Instruments GmbH). The PECVD system
features a planar triple spiral antenna inductively coupled plasma (ICP) source, a He-
cooled substrate electrode and a high-vacuum chamber.
The ICP source is driven by a 13.56 MHz generator. The high-density plasma makes
possible the deposition of high-quality silicon nitride films at much lower temperature
than conventional PECVD [169-171, 181]. The low ion energy of the plasma source
in the ICPECVD systems enables SiNx films to be deposited on HgCdTe without
significant surface damage. The controlled helium gas flow, maintaining a pressure of
1000 Pa to the wafer backside, is for effective wafer temperature control. The high-
vacuum system has a turbo pump and a rotary pump, which are designed for the low
pressure and high flow requirements of the deposition process. During the deposition,
an automatic throttle valve maintains the chamber pressure independent of the gas
flows, and mass flow controllers (MFC) provide precise flow rates. Aiming for higher
hydrogen content in the films, and thus possibly better surface passivation, the SiNx
films employed in this thesis were all deposited employing SiH4+NH3+Ar+He gas
mixtures. Additional H2 could be added into the gas mixtures during deposition to
increase the hydrogen content in the film for future work.
3.4.2.2 Procedures for the development of SiNx deposition conditions for HgCdTe surface passivation
In order to determine ICPECVD SiNx deposition conditions suitable for surface
passivation of HgCdTe, the following procedures were used:
1. A series of low-temperature (80 °C - 100 °C) deposited SiNx films were firstly
deposited on CdTe/GaAs and Si substrates under different deposition
conditions, as discussed in Section 3.4.3. The Si substrates were employed as
reference samples. The influence of ICP power on the quality of the deposited
SiNx films and long-term stability was assessed through the IR absorbance and
film insulating quality. The deposition conditions employed in step 1 are
detailed in Table 3.3.
51
Table 3.3 Summary of deposition conditions of SiNx film on CdTe/GaAs and Si substrate in the Sentech SI 500D system
Sample number Substrate Substrate
temperature (°C)
ICP RF
power (W)
Gas flow
rates (sccm) Film thickness (nm)
Deposition
rate (nm/min)
1 C1-SiNx CdTe/GaAs
80 300 FlowI* -240 (surface 240 nm
under CdTe level) -
A1-SiNx(C1 reference) Si 141 17.230
2 C2-SiNx CdTe/GaAs
80 350 FlowI* 216 26.395
A2-SiNx (C2 reference) Si 153 18.697
3 C3-SiNx CdTe/GaAs
80 450 FlowI* 259 31.650
A3-SiNx (C3 reference) Si 179 21.874
4 D1-SiNx CdTe/GaAs
100 350 FlowI* 194 23.707
B1-SiNx (D1 reference) Si 134 16.375
5 D4-SiNx CdTe/GaAs
100 350 FlowII** 202 24.684
B4-SiNx (D4 reference) Si 192 23.462
6 C4-SiNx CdTe/GaAs
100 450 FlowI* 219 26.762
A4-SiNx (C4 reference) Si 161 19.674
7 C5-SiNx CdTe/GaAs
100 600 FlowI* 166 20.285
A5-SiNx (C5 reference) Si 193 23.585
8 A6-SiNx Si 125 350 FlowI* 126 15.397 *FlowI (sccm): SiH4 6.9, NH3 10.3, Ar 120, He 131.1, SiH4/NH3 = 0.670; NH3/ SiH4 = 1.493
**FlowII (sccm): SiH4 7.5, NH3 9.7, Ar 120, He 131.1, SiH4/NH3 = 0.773; NH3/ SiH4 = 1.293
52
Chapter 3 M
aterial Characterisation
52
Chapter 3 Material Characterisation
2. A series of low-temperature (80 °C - 100 °C) deposited SiNx films were
deposited on Si substrates to investigate the influence of SiH4/NH3 on film
refractive index, film composition, deposition rate and bond/atom densities, as
discussed in Section 3.4.4. The deposition conditions employed in Step 2 are
listed in Table 3.3.
3. SiNx films under four different deposition conditions were then employed on
HgCdTe substrates for capacitance-voltage and conductance-frequency analysis,
as discussed in Section 3.4.1. The deposition conditions for SiNx films in the
four MIS structures are detailed in Table 3.3. For samples labelled D1-80C, D1-
90C and D1-100C, the SiNx films were deposited by varying only the
temperature of the substrate, with samples D1-100C and D4-100C differing
only in the SiH4/NH3 ratio employed for a preliminary assessment of the effect
of N/Si ratio on the quality of surface passivation. The densities of interface
states, Dit, were compared, as it has been considered the key-parameter when
comparing passivation quality [175, 179, 182, 183].
4. Bond density calculations were carried out on the SiNx/Si reference wafers,
where more detailed literature is available, in an attempt to correlate the bond
density with interface states extracted from Step 3. The calculation of the bond/
atom concentrations and their correlation with Dit are summarised in Table 5.4
and Table 5.5. The [Si-H] and [N-H] bond densities in the SiNx film were
considered as indicators of passivation quality.
Surface cleaning and conditioning have been found to be an indispensable part of
surface passivation, in order that the film can be deposited with minimal fixed charge
and interface traps. In this thesis, organic cleaning followed by buffered oxide etch
(BOE) were implemented for Si substrates before silicon nitride deposition, as the
standard RCA clean [184] is not transferable to HgCdTe and CdTe substrates. For
HgCdTe and CdTe substrates, the cleaning procedures used were organic cleaning,
HCl etching followed by Br/Methnol etching, with the substrate left in running DI
water before loading into the ICPECVD system ready for film deposition.
The film deposition procedure in ICPECVD for sample D1-SiNx can be found in
Table 3.4, which gives as an example to show the steps in the film growth. The other
SiNx films in this chapter followed the same procedure.
53
Chapter 3 Material Characterisation
Table 3.4 Silicon nitride film deposition procedures used for sample D1-SiNx in the Sentech SI500D ICPECVD system
Procedure Comments 1 Plasma cleaning of chamber
using gas mixture of CF4 and O2, prior to SiNx film deposition
This step is crucial to have reproducible film properties in different runs.
In general, 20 mins to 40 mins is considered enough to remove any deposited film from the previous run. The duration can be extended depending on the chamber status.
2 Ventilate chamber with N2 3 Load wafer together with its
reference wafer(s) Remember to load reference wafers for film characterisations. The commonly used reference wafers for HgCdTe are as below:
SiNx/CdTe/GaAs for measurements of FTIR in order to check film stability over time. Some obvious damage to CdTe substrate by ion bombardment could also be seen by FTIR if the absorbance fringes of CdTe film changes after the deposition;
SiNx/Si for measurements of refractive index, film thickness, etch rate, FTIR, C-V and I-V;
SiNx/Au/Cr/Si for leakage test; SiNx/GaAs for EDS purpose to estimate film
composition, [N]/[Si].
4 Evacuate chamber 5 Insert wafer(s) and pump the
deposition chamber to high vacuum
6 Waiting period 25 sec 7 He Pressure for wafer
backside cooling to 1000.0 Pa
8 Waiting period 20 sec 9 Increase electrode temperature
to 100°C
54
Chapter 3 Material Characterisation
10 Waiting period of 900 seconds for HgCdTe substrates after the set-point temperature is reached
The longer waiting period of 900 sec is due to the relatively poor thermal conductivity of HgCdTe and CdZnTe, in order to stabilise the surface temperature. A waiting period of 200 seconds is commonly used for silicon and GaAs substrates.
11 Waiting period 10 sec 12 Start gas
sources MFC 1 for NH3 = 10.3 sccm
Purge gas of N2 must be open before or at the starting point of this step for safety purpose when using the gas of SiH4. MFC 6 for Ar =
120 sccm MFC 7 for SiH4 = 6.9 sccm MFC 8 for He = 131.1 sccm
13 Waiting period 15 sec 14 Set 10 Pa for pressure reactor 15 Waiting period 45 sec 16 ICP power source of 350W on Starting of film deposition 17 Deposition duration 491 sec Aiming at ~ 200 nm film 18 ICP power source off End of film deposition 19 Stop gas
sources MFC 1 for NH3 = 0 sccm
MFC 7 for SiH4 = 0 sccm
MFC 8 for He = 0 sccm
20 Waiting period 45 sec Stop purging N2 gas after this step. 21 Set 0 Pa for pressure reactor 22 Stop gas of MFC 6 for
Ar = 0 sccm
23 Pump the deposition chamber to high vacuum
24 Waiting period 30 sec 25 Retract wafer 26 Ventilate chamber with N2
and collect sample(s)
27 Pump the chamber to vacuum or perform plasma cleaning of chamber
55
Chapter 3 Material Characterisation
3.4.3 Investigation on SiNx film stability over time
In order to determine the ICPECVD SiNx deposition conditions suitable for surface
passivation of HgCdTe, a series of low-temperature (80 °C-100 °C) deposited SiNx
films were deposited on CdTe/GaAs and Si reference substrates under different
deposition conditions. The deposition conditions employed are detailed in Table 3.3,
where the SiNx films with sample number starting with ‘A’ or ‘B’ were deposited on
Si reference substrates, the SiNx films with sample number starting with ‘C’ were on
CdTe/GaAs substrate GACT004, and ‘D’ on CdTe/GaAs substrate GACT006.
Regarding the CdTe/GaAs substrates used in the thesis, the CdTe epilayers with a
thickness of approximately 6 μm were grown in a Riber-32 MBE system on GaAs
substrates at a substrate temperatures of between 270 °C to 285 °C. The information
for the MBE grown CdTe wafers is listed in Table 3.5. During the growth, the
background pressure was 2 × 10-9 Torr, the CdTe flux was 1 × 10-6 Torr and Te flux
was 1.5 × 10-6 Torr, and the growth rate at the above mentioned flux rate was
approximately 1 μm/hour. The films on Si substrates were employed as reference
samples. The Si substrate used was 300 μm-thick phosphorous-doped n-type single
crystal (100) silicon with a resistance of 1-20 Ω⋅cm.
Except for C1-SiNx, the other seven batches of SiNx films demonstrated good film
thickness uniformity. The surface of sample C1-SiNx had a roughened and dull
appearance. The films’ thickness, determined by mechanical step profiler Dektak, was
in the range of 141 nm to 259 nm, as shown in Table 3.3. The duration of each
deposition was 491 seconds that can result in ~ 200 nm-thick SiNx film using
deposition conditions of D4-100C.
Table 3.5 Summary of MBE grown CdTe on GaAs substrate
CdTe/GaAs Sample No.
GaAs substrate lot
No.
Expected Thickness
(μm)
Growth Temperature
(°C)
FWHM from XRD
(arcsec) GACT004 23133-18-4 6 270 170
GACT006 23133-18-6 6 275 66
56
Chapter 3 Material Characterisation
Figure 3.16 Deposition rates of silicon nitride films on silicon substrate as a function of ICP power at a substrate temperature of 80 °C and 100 °C.
Figure 3.17 Deposition rates of silicon nitride films on CdTe/GaAs substrate as a function of ICP power at a substrate temperature of 80 °C and 100 °C.
300 350 400 450 500 550 60016
18
20
22
24
26
28
30
32
ICP Power (W)
Dep
ositi
on R
ate
(nm
/min
)
80oC100oC
300 350 400 450 500 550 60016
18
20
22
24
26
28
30
32
ICP Power (W)
Dep
ositi
on R
ate
(nm
/min
)
80oC100oC
57
Chapter 3 Material Characterisation
Figure 3.16 and Figure 3.17 plot the change of deposition rates as a function of ICP
power for samples listed in Table 3.3. The deposition rates were found to increase
with an increase in ICP power, except for sample C5-SiNx and C1-SiNx. The
deposition rate of C5-SiNx was found to be much lower than expected, which could be
correlated with its porosity, as discussed in the Section 3.4.4, and as indicated by the
observed absorbance curves monitored over a six-month time frame.
Regarding sample C1-SiNx, the CdTe surface had a roughened and dull appearance
after the SiNx deposition process. The surface was found to be etched or reacted with
the substrate, since the underlying CdTe surface was found to be 240 nm lower than
previously found by Dektak. No Si-N related band observed in the SiNx film. The
mechanism of CdTe surface etching is unclear. One possibility is the reaction of CdTe
and NH3 during the film deposition expressed in Eq. (3.3) [185]:
34NH CdTe + −+ ++ e2Te)Cd(NH 243 (3.3)
Regarding the BOE etch rate of the SiNx film, a piece of the B4-SiNx sample was
etched in BOE for 20 minutes, and the BOE etch rate was found to be approximately
4 nm/min, which is relatively low, but similar to what was reported by the
manufacture of SENTECH Instruments [186]. Such a low etch rate is an indication of
good SiNx quality [163].
3.4.3.1 Examination of film stability through infrared absorbance
The influence of ICP power on the quality of the deposited SiNx films was assessed
through IR absorbance of the films. The IR absorbance of each film was measured on
the day of the deposition and was monitored during the following six month period.
The films were allowed to age in laboratory atmosphere.
A typical IR absorbance spectrum of low-temperature (80 °C - 100 °C) deposited
silicon nitride film deposited by the ICPECVD system is shown in Figure 3.18.
Table 3.6 identifies the physical cause behind the observed absorption bands in
Figure 3.18 [183, 187, 188]. The band of Si-O-Si stretching cannot be seen in any of
the as-deposited films, and only appeared in sample C5-SiNx after aging in
atmosphere.
58
Chapter 3 Material Characterisation
Table 3.6 Absorption bands observed in the as-deposited SiNx samples
Absorption bands
Frequency of
absorbance peak (cm-1)
1 Si-N sym. Stretching (Si-N (s)) 480
2 Si-H wag-rocking (Si-H (w-r)) 650
3 Si-N asym. Stretching (Si-N (s)) 840
4 Si-O stretching (Si-O (s)) 1080
5 N-H rocking (N-H (r)) 1180
6 Si-H stretching (Si-H (s)) 2185
7 N-H stretching (N-H (s)) 3350
8 O-H2 stretching (O-H2 (s)) 3640
The absorbance spectrum of each of the six films on CdTe/GaAs substrates was
measured on the day of the deposition (Figure 3.19) and monitored again over a six-
month time frame (Figure 3.20). The SiNx/CdTe/GaAs sample labelled C5-SiNx
deposited using ICP power of 600 W appeared to be porous and more susceptible to
oxidation under conventional ambient conditions, with the Si-O-Si stretching peak
appearing after deposition at 1080 cm-1, as shown in Figure 3.20. The evolution of
absorbance of C5-SiNx with time is shown in Figure 3.21. This Si-O-Si oxidation
peak has been reported to become evident when SiNx films undergo oxidation in air,
as reported by Liao et al. [2], Chang et al. [3] and Westerhout et al. [4]. H2O in the air
(moisture) has been found to be responsible for the oxidation of the SiNx films [2]. It
is noted that although the SiNx/Si reference samples showed good stability over six
months, the results from SiNx/CdTe/GaAs samples suggest that high ICP power
conditions are not suitable for CdTe or HgCdTe substrates. Deposition conditions C2,
C3, C4, D1 and D4 showed good long-term stability in terms of the IR absorbance
peaks associated with exposure to O2 and H2O in the atmosphere.
59
Chapter 3 Material Characterisation
Figure 3.18 A typical IR absorbance spectra of low-temperature (80 °C - 100 °C) deposited silicon nitride film deposited by Sentech SI500D system.
60
Chapter 3 M
aterial Characterisation
5001000150020002500300035004000Wavenumber (cm-1)
Abs
orba
nce
(a.u
.)
O-H2(s) N-H(s) Si-H(s) N-H(r)
Si-N asym.(s)
Si-H (w-r)
Si-N Sym.(s)
60
Chapter 3 Material Characterisation
Figure 3.19 The IR absorbance spectra of the as-deposited silicon nitride films by six different recipes on CdTe/GaAs substrate.
5001000150020002500300035004000Wavenumber (cm-1)
Abs
orba
nce
(a.u
.)
D4-SiNx
D1-SiNx
C5-SiNx
C4-SiNx
C3-SiNx
C2-SiNx
Si-N-Si(s)
61
Chapter 3 M
aterial Characterisation
61
Chapter 3 Material Characterisation
Figure 3.20 The IR absorbance spectra of the silicon nitride films by six different recipes on CdTe/GaAs substrate after six-months exposure to a laboratory atmosphere.
5001000150020002500300035004000Wavenumber (cm-1)
Abs
orba
nce
(a.u
.)
D4-SiNx
D1-SiNx
C5-SiNx
C4-SiNx
C3-SiNx
C2-SiNx
Si-N-Si(s)
Si-O-Si(s)
62
Chapter 3 M
aterial Characterisation
62
Chapter 3 Material Characterisation
Figure 3.21 The IR absorbance spectra of the C5-SiNx film on CdTe/GaAs substrate monitored over a six month time frame. The films were allowed to age in laboratory atmosphere. Text on the left indicate the time after deposition that the measurement was taken.
5001000150020002500300035004000Wavenumber (cm-1)
Abs
orba
nce
(a.u
.)
as-deposited
2 days
4 days
1 week
2 weeks
4 weeks
2 months
3 months
4 months
6 months
Si-O-Si(s)Si-N-Si(s)
63
Chapter 3 M
aterial Characterisation
63
Chapter 3 Material Characterisation
3.4.3.2 Examination of film stability through insulating properties
To investigate the insulating properties of the SiNx passivation films, MIS capacitors
were fabricated first on SiNx/Si wafers, using films that were deposited using
condition of D4-SiNx. The capacitance-voltage measurements were carried out, and
fixed charge density and density of interface states were extracted, which have been
discussed on Section 3.3.7. The polarity of fixed charge was positive for the
Au/Cr/SiNx/Si MIS capacitor, which is in agreement with literature [179, 180].
Leakage currents were measured on Au/Cr/SiNx/n-Si MIS capacitor structures at
298 K on the day of fabrication and were monitored within a four-month period of
time after storage in a laboratory atmosphere. The resistivity of the SiNx films on Si
substrates was calculated from the I-V characteristics. Although there was a gradual
decrease in the film resistivity from 9.24 × 1014 Ω⋅cm to 2.61 × 1014 Ω⋅cm over the
four-month period, the MIS devices retained good stability in terms of film
resistivity (Figure 3.12).
3.4.4 Investigation on SiNx film properties influenced by NH3/SiH4 flow ratio
The flow ratio of NH3/SiH4 is a crucial parameter in determining film properties and
passivation performance, influencing film stoichiometry and semiconductor interface
quality [174, 175, 179, 180, 189, 190]. In this section, the influence of NH3/SiH4 flow
ratio on SiNx film properties is explored.
3.4.4.1 Influence of NH3/SiH4 flow ratio on film refractive index, composition and deposition rate
The stoichiometry of the silicon nitride film is defined by the [N]/[Si] ratio, x. The
value of [N]/[Si] in hydrogenated amorphous silicon nitride films can be measured by
direct measurements involving massive-particle detection, such as Rutherford
backscattering spectroscopy (RBS) and elastic-recoil-detection (ERD). The value can
also be extracted by electronic probing methods, such as, energy dispersive
spectroscopy (EDS), Auger electron spectroscopy (AES), X-ray photoelectron
spectroscopy (XPS) and electron microprobe analysis (EPMA). There are indirect
estimations based on relations between x and physical parameters that are x-sensitive,
64
Chapter 3 Material Characterisation
having a monotonic variation with x and that are easily measurable, such as measured
refractive indices at a given wavelength [145, 146]. In this section, the refractive
index, composition and deposition rate of SiNx under study are discussed, and their
relationship was explored.
To investigate the influence of NH3/SiH4 flow ratio on film properties, a series of
films were deposited at low temperatures (80 °C - 100 °C) on Si substrates with
varying NH3 flow rates and a fixed SiH4 flow rate. The deposition conditions
employed are listed in Table 3.7. The Si substrates used were 300 μm-thick
phosphorous-doped n-type (100) silicon with a resistivity of 1-20 Ω⋅cm. The films
deposited on GaAs substrates were employed as reference samples for EDS purpose
for an estimation of [N]/[Si]. The flow rate of SiH4 was kept the same at 6.9 sccm,
and the various flow rates of NH3 used were 12.4, 10.3, 8.2 and 6.1 sccm for films
deposited at 80 °C and 100 °C. The set of four samples of D1-80°C-Si reference, D1-
90°C-Si reference, D1-100°C-Si reference, and D4-1 0°C-Si re&erence are the Si
reference wafers for the SiNx/HgCdTe MIS structures discussed in Chapter 5.
Measured refractive indexes for each sample disted in Table 3.7 are all given at a
wavelength of 632.8 nm. As an estimate of the film compo3ition, [N]/[Si], the EDS
spectra werE obtained using a beam energy of 5 keV on the SiNx/GaAs instead of on
SiNx/Si wafers in order to avoid interference from the Si substrate due to the relatively
thin film. Except for sample D1-NH12-80°C, the composition of all the other samples
under study are below x = 1.33 (Si-rich). The ICPECVD system was found to form
Si-rich films much more readily than Ni-rich ones, which is likely to be due to the fact
that Si-H and Si-Si bonds can be formed more easily than the N bonds [161].
Measurad refractive index at 632.8 nm, n632.8nm, and [N]/[Si], as a function of
SiH4/NH3 flow rate ratio for samples deposited at 80 °C and 100 °C are 0lotted in
Figure 3.22. Within the investigated range of the SiH4/NH3 flow rate ratio from 0.56
to 1.13, n increases with SiH4/NH3 flow rate ratio for samples deposited at 80 °C and
100 °C, with x following the opposite trend.
Measured n and [N]/[Si] as a function of substrate temperature for the samples with
NH3 of 10.3 sccm and SiH4 of 6.9 sccm are plotted in Figure 3.23. It can be seen that
n632.8nm increases with an increase of substrate temperature and [N]/[Si] decreases
65
Chapter 3 Material Characterisation
Table 3.7 Summary on ICPECVD SiNx/Si wafers with varied SiH4/NH3 ratio and temperature*
SiNx/Si wafer ID Temperature (°C)
NH3 flow rate
(sccm)
SiH4 flow rate
(sccm)
NH3/ SiH4 ratio
Deposition duration
(sec)
SiNx thickness
(nm) Deposition rate (nm/min) n632.8nm [N]/[Si]
D1-NH12-100°C 100 12.4 6.9 1.80 700 227 19.46 1.90 1.24 D1-NH10-100°C
(i.e. D1-100°C-Si reference) 100 10.3 6.9 1.49 700 239 20.49 1.93 1.02
D1-NH8-100°C 100 8.2 6.9 1.19 700 220 18.86 2.12 0.98 D1-NH6-100°C 100 6.1 6.9 0.88 1147 220 11.51 2.41 0.61
D1-NH12-80°C 80 12.4 6.9 1.80 700 254 21.77 1.80 1.37 D1-NH10-80°C
(i.e. D1-80°C-Si reference) 80 10.3 6.9 1.49 700 268 22.97 1.86 1.17
D1-NH8-80°C 80 8.2 6.9 1.19 700 236 20.23 1.97 1.11 D1-NH6-80°C 80 6.1 6.9 0.88 700 151 12.94 2.30 0.64
D1-NH10-80°C (i.e. D1-80°C-Si reference) 80 10.3 6.9 1.49 700 268 22.97 1.86 1.17
D1-90°C-Si reference 90 10.3 6.9 1.49 700 252 21.60 1.90 1.07 D1-NH10-100°C
(i.e. D1-100°C-Si reference) 100 10.3 6.9 1.49 700 239 20.49 1.93 1.02
D4-100°C-Si reference 100 9.7 7.5 1.29 491 202 24.68 1.94 0.90
*Other deposition parameters are the same for all the samples listed in the above table, that is ICP RF power of 350 W, chamber pressure of
10 Pa, Ar flow rate of 120 sccm, and He flow rate of 131.1 sccm.
66
Chapter 3 M
aterial Characterisation
66
Chapter 3 Material Characterisation
Figure 3.22 Measured refractive index at a wavelength of 632.8 nm and [N]/[Si] ratio estimated by EDS, as a function of NH3/SiH4 flow ratio for samples deposited at 80 °C and 100 °C. Solid lines –: n632.8 nm; dashed lines – –: [N]/[Si]; : for samples deposited at 80 °C; : samples deposited at 100 °C.
Figure 3.23 Measured refractive index at a wavelength of 632.8 nm and [N]/[Si] ratio estimated by EDS, as a function of substrate temperature. Solid lines –: n632.8 nm; dashed lines – –: [N]/[Si].
80 85 90 95 1001.86
1.87
1.88
1.89
1.9
1.91
1.92
1.93
1.94
n (λ
= 6
32.8
nm
)
Substrate temperature (oC)80 85 90 95 100
1.02
1.04
1.06
1.08
1.1
1.12
1.14
1.16
1.18
[N]/[
Si]
0.8 1 1.2 1.4 1.6 1.81.8
2
2.2
2.4
2.6
n ( λ
= 6
32.8
nm
)
NH3/SiH4
0.8 1 1.2 1.4 1.6 1.80.6
0.8
1
1.2
1.4
0.8 1 1.2 1.4 1.6 1.80.6
0.8
1
1.2
1.4
[N]/[
Si]
67
Chapter 3 Material Characterisation
with increasing temperature, in agreement with the trends reported in the literature
[93, 170, 172].
The five different expressions found in the literature for the relationship of [N]/[Si]
and n632.8nm [146, 175, 183, 191, 192] are discussed as below. The expressions for
[Si]/[N] (i.e. 1/x) and n, includingl several linear ones, are shown in Figure 3.24 (a),
whereas the expressions for [N]/[S)] and n are shown in Figure 3.24 (b).
Makino [193 developed an expression for n in the Si-rich films deposited by
LPCVD, assuming that the films are a bonding-density-weighted linear combination
of NSi3N4 and NSi, ignoring the small amounts of hydrogen bonding of NN-H and NSi-H.
The n to x relation was expressed as
4/31)2)(4/3(
43
xnnxn
n SiNSiSi
+
−+= (3.4)
Bustarret et al. [146] applied Makino’s bonding-density-weighted linear combination
assumption [193] to the case of PECVD α- SiNx:H films, and gave the relation
between n and x as:
4/31)2)(4/3( :: 43
xnnxn
n HSiNSiHSi
+
−+= −−− ααα (3.5)
or
5.03.3
34
234
][][
43:
:
−−
=−+−
==−−
−
nn
nnnnn
SiNx
NSiaHSia
HSia (3.6)
where the refractive index of 3.3: =− HSian is for HSia :− , and 9.143=− NSian is for
nearly stoichiometric 43NSia − nitride film.
The above equation was then modified by Mäckel and Lüdemann [183] as
5.03.3
43
243
][][
43:
:
−−
=−+−
==−−
−
nn
nnnnn
SiNx
NSiaHSia
HSia (3.7)
Claassen et al. [193] gave an expression between n and x by applying an empirical fit
to their data for three different process gases of SiH4+NH3+N2, SiH4+NH3+Ar,
68
Chapter 3 Material Characterisation
and SiH4+NH3+H2, as given below
39.17.0
−=
nx (3.8)
Dauwe et al. [192] also gave an empirical fit to experimental data as the following
35.174.0
−=
nx
(3.9)
In addition, Lelievre et al. [175] gave an empirical fit for their SiNx:H film by LF-
PECVD (Low Frequency PECVD) shown below
22.161.0
−=
nx (3.10)
The values of n in the fit used are at a wavelength of 605 nm, as opposed to 623.8 nm
in the other n and x expressions discussed in this chapter. However, this expression is
listed and plotted together with other expressions in Figure 3.24 to illustrate the trend.
A linear least-squares fit to the measured data of this thesis listed in Table 3.7
between n and 1/x was found to be
37.161.0+=
xn (3.11)
where the norm of residuals is 0.19564.
or
37.161.0
−=
nx (3.12)
It is interesting to note that the linear least-squares fitted line is parallel to what is
given by Lelievre et al. [175]. The lack of data points in the N-rich range decreases
the accuracy of the linear fit, which will require more experimental work on low-
temperaure SiNx in the future. It can also be seen from Figure 3.24 (b) that Si-rich
SiNx is more sensitive to variations in processing conditions than stoichiometric Si3N4.
Small changes in the gas ratio were found to give relatively large variations in the
refractive index and mechanical stress [194]. Gardeniers et al. have noted that
changing the processing gas flow rate ratio can result in relatively large variations in
SiNx refractive index and mechanical stress as compared to those of stoichiometric
Si3N4 [194]. The sensitivity of Si-rich SiNx could be advantageous since it makes it
possible to adjust the properties of the Si-rich SiNx by moderately adjusting
69
Chapter 3 Material Characterisation
(a)
(b)
Figure 3.24 Plot illustrating the relationship between n and x for ICPECVD SiNx deposited at 80 °C -100 °C with varied NH3/SiH4 flow ratio.
0.5 1 1.5 2 2.5 31
1.5
2
2.5
3
3.5
4
[Si]/[N]
n (λ
= 6
32.8
nm
)
T = 80oCT = 90oCT = 100oCBased on Dauwe et al.Based on Claasen et al.Based on Mackel et al.Based on Lelievre et al.Based on Bustarret et al.Linear fit of data points
0.5 1 1.5 21
1.5
2
2.5
3
3.5
4
4.5
5
[N]/[Si]
n (λ
= 6
32.8
nm
)
T = 80oCT = 90oCT = 100oCBased on Dauwe et al.Based on Claasen et al.Based on Mackel et al.Based on Lelievre et al.Based on Bustarret et al.Linear fit of data points
70
Chapter 3 Material Characterisation
(a)
(b)
Figure 3.25 [N]/[Si] ratio as a function of NH3/SiH4 flow ratio for ICPECVD SiNx deposited at 80 °C -100 °C with varied NH3/SiH4 flow ratio. (a) The line is a linear least-squares fit to the measured data as [N]/[Si] = 0.68 × NH3/SiH4 + 0.09. (b) log-log scale plot of [N]/[Si] versus NH3/SiH4 flow ratio. The lines have slopes of β = 1/2 and 1.
0.5 1 1.5 2
0.4
0.6
0.8
1
1.2
1.4
1.6
NH3/SiH4
[N]/[
Si]
T = 80oCT = 90oCT = 100oC
flow ratio
0.5 1.0 2.00.4
0.6
0.8
1.0
2.0
NH3/SiH4 ratio
[N]/[
Si] r
atio
T = 80oCT = 90oCT = 100oCβ = 1/2β = 1
flow ratio
71
Chapter 3 Material Characterisation
Figure 3.26 Plot of film refractive index, n632.8nm, of ICPECVD SiNx deposited at 80 °C -100 °C as a function of the SiH4/NH3 gas ratio. The line is a linear least-squares fit to the measured data.
deposition parameters and hence [N]/[Si] and n632.8nm to match a certain application
requirement.
Figure 3.25 and Figure 3.26 illustrate the relationship between film composition
[N]/[Si] and refractive index n632.8nm of ICPECVD SiNx deposited at 80 °C - 100 °C as
a function of the gas flow ratio. It can be seen that [N]/[Si] increases with the
NH3/SiH4 flow ratio nearly linearly, with the linear least-squares fit as [N]/[Si] = 0.68
× NH3/SiH4 + 0.09, while n632.8nm increases with the SiH4/NH3 ratio with the linear
least-squares fit as n632.8nm = 0.91 × SiH4/NH3 + 1.30, which is also an indication of
the linear relationship between [Si]/[N] and n632.8nm as expressed in Eq (3.11) and
Eq (3.12). The dependence of the silicon nitride composition on the gas ratio could be
explained by the kinetics of the dissociation processes and the free radical sticking
coefficient values [195].
The linear part of the log-log plot of Figure 3.25 (b) yields a square-root dependence
(β ≈ 1/2) for the [N]/[Si] when the NH3/SiH4 flow ratio is in the 1.2 to 1.8 range,
where [N]/[Si] = (NH3/SiH4) β + a0. The dependence becomes stronger (β ≈ 1) at the
0.2 0.4 0.6 0.8 1 1.2 1.41.4
1.6
1.8
2
2.2
2.4
2.6
SiH4/NH3 flow ratio
n (λ
= 6
32.8
nm
)
T = 80oCT = 90oCT = 100oC
72
Chapter 3 Material Characterisation
low gas ratio of 0.88, which is in agreement with the behaviour of PECVD SiNx
reported by Bustarret et al. [146] and Giorgis et al. [196].
The two regions with different values of β were found to be associated with the
polyamine concentration change in the plasma [146, 172]. The (SiH4+NH3) plasma
contains mainly two species, the disilane Si2H6 and the aminosilane Si[(NH2)]m. Si2H6
is formed from SiHm (m < 4) radicals, whereas Si[(NH2)]m is formed from a
combination of SiHm (m < 4) and NHm (m < 3) radicals. The PECVD parameters
determine the ratio and nature of the above mentioned species which, in turn,
determine the composition of the film [197].
The first region with a slope of β ≈ 1/2 in Figure 3.25 (b) is considered a characteristic
of the dissociation-controlled equilibrium between polyaminosilanes (Si[NH2]m,
m = 3, 4) and silicon and nitrogen atoms absorbed at the surface [146]. Disilane Si2H6
dominates the contribution to growth in the second region with β ≈ 1. The minimum
NH3 percentage required to reach a transition to aminosilanes formation is associated
with the conditions under which a change in β is observed [196], as seen in
Figure 3.25 (b).
In addition to film composition, film deposition rate has also been found to be an
indicator of polyamine concentration in the plasma [161, 172, 195, 196]. Figure 3.27
illustrates the change in film deposition rates versus NH3/SiH4 flow ratio for films
deposited at 80 °C, 90 °C and 100 °C. The trend of deposition rate versus NH3/SiH4
flow ratio is non-linear. A linear dependence of the deposition rate on the SiH4 flow
rate has been reported [163, 198], and complex deposition rate variations as a function
of NH3/SiH4 flow ratio have also been reported [161, 196, 197, 199, 200].
The relationship between both [N]/[Si] and deposition rate with NH3/SiH4 flow ratio
are in agreement with those reported by Giorgis with undiluted gas mixtures of
NH3+SiH4 [196]. At very low NH3 flow rate and NH3/SiH4 flow ratio, the
composition of the plasma is very poor in NHm (m = 1, 2) free radicals, as the NH3
dissociation rate is much lower than the SiH4 one [172]. Disilane Si2H6 rises and
Si[NH2]3 is suppressed in the plasma composition. Si and SiHm (m = 1, 2, 3) free
radicals dominate the contribution to growth, and silicon atoms are expected to bond
to more than one hydrogen atom. For NH3/SiH4 flow ratios from 0.88 to 1.19 in
Figure 3.27, there was a dramatic increase in the deposition rate, in the [N]/[Si] ratio
73
Chapter 3 Material Characterisation
(Figure 3.25 (b)), and also in the area under the Si-H (s) peak (~ 2180 cm-1) for films
deposited at 80 °C and 100 °C (Figure 3.28 and Figure 3.29), possibly suggesting a
change in the dominant radicals from disilane to polyaminosilanes.
With the increase of NH3 flow rate and NH3/SiH4 flow ratio, the concentration of NHn
free radicals in the plasma (mainly Si[NH2]3), increases and that of Si2H6 reduces,
resulting in an increase of [N]/[Si] and [N-H]/[Si-H] [172]. In addition, with
increasing of NH3 flow rate, the chamber pressure increases, contributing to an
increase in deposition rate and enhanced disilane elimination [172].
From the data in Figure 3.27, either the conversion efficiency reaches a maximum at
an NH3/SiH4 flow ratio of around 1.49 or the competing absorption-desorption surface
reactions impede the growth rate of films deposited at NH3/SiH4 flow ratios ≳ 1.49
[200], thus resulting is a drop in deposition rate at a NH3/SiH4 flow ratio of 1.80 for
films deposited at 80 °C and 100 °C. At an NH3/SiH4 flow ratio of 1.80, films will be
deposited with Si[NH2]3 as the principal film precursors and suppressed Si2H6 in the
plasma resulting in little or no Si-H bonding, as confirmed by the decrease of Si-H (s)
peaks ( ~ 660 cm-1 and 2180 cm-1) and increase of N-H peaks (1180 cm-1 and
3200 cm-1) in the IR absorption curves.
Figure 3.27 The change in film deposition rate versus NH3/SiH4 flow ratio for silicon nitride films deposited at 80 °C, 90 °C and 100 °C with a fixed SiH4 gas flow of 6.9 sccm.
0.5 1 1.5 210
15
20
25
NH3/SiH4 ratio
Dep
ositi
on ra
te (n
m/m
in)
T = 80oCT = 90oCT = 100oC
flow ratio
74
Chapter 3 Material Characterisation
3.4.4.2 Influence of NH3/SiH4 flow ratio on film IR absorbance
Figure 3.28 and Figure 3.29 plot the IR absorption coefficient spectra of SiNx films
deposited by ICPECVD at varied NH3 flow rates and a fixed SiH4 flow rate at 80 °C
and 100 °C, respectively. The silicon substrate absorption was subtracted and a
baseline correction procedure was applied to the spectra. The data analysis on the IR
absorption spectra is listed in Table 3.8, together with the calculations of bond and
atomic densities. The general trends in Figure 3.28 and Figure 3.29 are that the area of
N-H bonding related peaks increase with NH3 flow rate and NH3/SiH4 flow ratio,
whereas the area of Si-H peaks decrease with NH3 flow rate and NH3/SiH4 flow ratio
for films deposited at 80 °C and 100 °C. This is in agreement with the characteristics
reported by Knolle and Osenbach [201]. The line in Figure 3.30 (a) is the least-
squares fit to the measured Si-H stretching peak frequency versus [N]/[Si] of the
films, expressed as
×=− 163)( 1cmυ [N]/[Si] 2054+ (3.13)
Similarly, the line in Figure 3.30 (b) is the least-squares fit to the measured Si-H
stretching peak frequency versus NH3/SiH4 flow ratio:
×=− 126)( 1cmυ NH3/SiH4 2048+ (3.14)
The Si-H (w-r) vibration was also found to shift to higher frequency with higher
[N]/[Si] and higher NH3/SiH4 flow ratio, similar to the Si-H (s) vibration. The Si-H
(w-r) peak for higher N content films deposited with higher NH3/SiH4 flow ratios of
1.80 and 1.49 are less obvious in the IR absorption spectra than for the Si-rich films,
because there is less amorphous Si in the film.
In addition, the N-H (r) vibration at ~ 2180 cm-1 was found to shift to higher
frequency with higher [N]/[Si] and higher NH3/SiH4 flow ratio, similar to the
behaviour of the Si-H (s) and Si-H (w-r) peaks. The main absorption coefficient peak
also shifts to higher frequency with an increase of [N]/[Si] (Figure 3.31 (a)) and
NH3/SiH4 flow ratio (Figure 3.31 (b)).
75
Chapter 3 Material Characterisation
Figure 3.28 IR absorption coefficient spectra of SiNx films deposited by ICPECVD at various NH3 flow rates and a fixed SiH4 flow rate at 80 °C.
5001000150020002500300035000
0.5
1
1.5
2
2.5 x 104
Wavenumber (cm-1)
Abs
orpt
ion
Coe
ffici
ent (
cm-1
)
D1-NH6-80CD1-NH8-80CD1-NH10-80CD1-NH12-80C
Si-N (sym.s)Si-H (w-r)
Si-N (asym. s)
N-H (r)Si-H (s) N-H (s)
76
Chapter 3 M
aterial Characterisation
76
Chapter 3 Material Characterisation
Figure 3.29 IR absorption coefficient spectra of SiNx films deposited by ICPECVD at various NH3 flow rates and a fixed SiH4 flow rate at 100 °C.
5001000150020002500300035000
0.5
1
1.5
2
2.5 x 104
Wavenumber (cm-1)
Abs
orpt
ion
Coe
ffici
ent (
cm-1
)
D1-NH6-100CD1-NH8-100CD1-NH10-100CD1-NH12-100C
Si-N (sym.s)
Si-H (w-r)
Si-N (asym. s)
N-H (r)Si-H (s) N-H (s)
77
Chapter 3 M
aterial Characterisation
77
Chapter 3 Material Characterisation
(a)
(b)
Figure 3.30 Plots showing the Si-H stretching peak shifting to higher frequency as a function of (a) film composition [N]/[Si] and (b) NH3/SiH4 flow ratio increases. The least-squares fitted lines in (a) and (b) are 2054]/[][163)( 1 +×=− SiNcmυ , and 2048/126)( 43
1 +×=− SiHNHcmυ , respectively.
0.5 1 1.52100
2150
2200
2250
2300
[N]/[Si]
[Si-H
] pea
k fre
quen
cy (c
m-1
) T = 80oCT = 90oCT = 100oC
0.5 1 1.5 22100
2150
2200
2250
2300
NH3/SiH4 ratio
[Si-H
] pea
k fre
quen
cy (c
m-1
) T = 80oCT = 90oCT = 100oC
flow ratio
78
Chapter 3 Material Characterisation
(a)
(b)
Figure 3.31 Plots showing the main absorption coefficient peak shifting to higher frequency as a function of (a) film composition [N]/[Si] and (b) NH3/SiH4 flow ratio.
0.5 1 1.5820
830
840
850
860
870
[N]/[Si]
Pea
k fre
quen
cy (c
m-1
)
T = 80oCT = 90oCT = 100oC
0.5 1 1.5 2820
830
840
850
860
870
NH3/SiH4 ratio
[Si-N
] Pea
k fre
quen
cy (c
m-1
) T = 80oCT = 90oCT = 100oC
flow ratio
79
Chapter 3 Material Characterisation
3.4.4.3 Calculations of bond and atom concentrations
The concentration of Si-N, N-H and Si-H bonds in the layers can be extracted from
the IR absorption band areas in the optical absorption coefficient curve, by the method
introduced by Lanford and Rand, using the expression [80, 183, 187]:.
∫−=− ννα dYXKYX )()(][ (3.15)
where ][ YX − is the concentration of YX − bonds, )( YXK − is the IR absorption
cross section, and the band area is the integral of optical absorption coefficient α over
the band in question.
The absorption coefficient α can be calculated from the absorbance, A, obtained from
the FTIR measurements. After correction for reflection losses, α may be obtained
using the following equation [202-204]:
dA303.2)( =να (3.16)
where d is the sample thickness in cm.
The bands used in the calculation are the Si-N (stretching) band near 840 cm-1, the Si-
H (stretching) band near 2185 cm-1, the N-H (rocking) band near 1180 cm-1, and the
N-H (stretching) band near 3350 cm-1. As to the IR absorption cross section for the
vibration of Si-H (s), the values used are proportional to the film refractive index,
n, [205]
K [Si-H] = 161058.2 ××n cm-1 (3.17)
The values of absorption cross section for other vibrations [80, 187, 205] are listed in
Table 3.8.
Spectra deconvolution is a conventional technique in IR absorption analysis [146, 187,
205]. Figure 3.32 shows the absorption coefficient as a function of wavenumber for
one of the samples, B1-NH8-80C, as an illustration of the spectra deconvolution.
80
Chapter 3 Material Characterisation
Figure 3.32 The absorption coefficient as a function of wavenumber for sample B1-NH8-80C as an illustration of the fitted absorption bands (dashed line) in the range from 450 cm-1 to 1400 cm-1 with four different Gaussian distributions. The solid line shows the absorption coefficient acquired from measured film absorbance and thickness.
The solid line shows the absorption coefficient being acquired from measured film
absorbance and thickness. The spectra in the range extending from 600 cm-1 to
1400 cm-1 can be decomposed into four Gaussian distributions [187] (dashed lines in
Figure 3.32), centred near 1180 cm-1, 950 cm-1, 840 cm-1 and 650 cm-1 that are
considered as a N-H rocking mode, two Si-N asymmetric stretching modes, and a Si-
H wag-rocking mode.
There are three other absorption bands outside the figure displayed range of 600 cm-1
to 1400 cm-1 in Figure 3.28 and Figure 3.29, which are N-H stretching centred near
3350 cm-1, Si-H stretching near 2185 cm-1, and Si-N sym. stretching near 480 cm-1.
The calculated bond concentrations of Si-N, Si-H and N-H are shown in Table 3.9.
Si-Si bonds are easily formed in Si-rich SiNx films. In contrast with Si-Si bond
formation in Si-rich films, N-N bonds are rarely formed, even in N-rich films [206].
6008001000120014000
0.5
1
1.5
2x 104
Wavenumber (cm-1)
Abs
orpt
ion
Coe
ffici
ent (
cm-1
)
81
Chapter 3 Material Characterisation
Table 3.8 IR absorption spectra analysis and calculations for bond and atom concentrations on SiNx/Si wafers deposited under varied NH3/SiH4
flow ratios at temperatures between 80 °C - 100 °C
Si-N N-H Si-H
Sample SiH4/ NH3
n [N]/[Si] fmax (cm-1)
α (peak) (cm-1)
f (peak) (cm-1)
α (peak) (cm-1)
∫ αdf (cm-2)
K (1019 cm-2)
f (peak) (cm-1)
α (peak) (cm-1)
∫ αdf (cm-2)
K (1019 cm-2)
f (peak) (cm-1)
α (peak) (cm-1)
∫ αdf (cm-2)
K (1019 cm-2)
D1-NH12-100°C 0.56 1.9 1.24 861.6 20742.6 928.7 12462.0 2.93×106 2.07 3341.0 2546.0 4.58×105 8.20 2274.0 1240.0 4.61×105 4.85
837.7 12180.0 1.82×106 1.82 1181.1 2632.6 4.37×105 2.07 660.0 164.0 1.22×104
495.0 811.0 4.98×104
D1-NH10-100°C 0.67 1.93 1.02 842.4 21327.9 950.4 9266.8 2.04×106 2.07 3325.0 1650.0 3.51×105 8.20 2250.0 1200.0 3.19×105 4.98
(i.e. D1-100°C- Si reference)
831.6 17327.0 2.69×106 1.82 1185.1 1541.9 2.13×105 2.07 660.0 399.0 2.97×104
484.3 1391.9 9.92×104
D1-NH8-100°C 0.84 2.12 0.98 832.8 22531.6 909.1 11759.0 3.05×106 2.07 3290.0 1302.0 5.54×105 8.20 2221.0 2560.0 7.09×105 5.62
821.8 13869.0 1.98×106 1.82 1156.0 356.0 6.82×104 2.07 649.0 440.0 4.03×104
480.0 656.0 3.69×104
D1-NH6-100°C 1.13 2.41 0.62 833.6 20390.5 917.9 12481.0 2.92×106 2.07 2151.3 2885.0 5.28×105 6.40 814.9 12716.0 1.87×106 1.82 1175.0 80.0 5.11×103 2.07 635.6 1058.2 8.32×104
82
Chapter 3 M
aterial Characterisation
82
Chapter 3 Material Characterisation
D1-NH12-80°C 0.56 1.8 1.37 852.1 18681.5 941.7 8608.9 2.20×106 2.07 3325.0 1894.0 3.43×105 8.20 2270.0 701.0 1.93×105 4.64
840.2 13140.0 2.06×106 1.82 1197.0 2070.0 2.86×105 2.07 661.0 157.0 2.61×104
497.0 697.0 4.31×104
D1-NH10-80°C 0.67 1.86 1.17 837.8 19324.7 927.4 9120.1 2.35×106 2.07 3325.0 1699.0 3.53×105 8.20 2226.0 1506.0 3.53×105 4.80
(i.e. D1-80°C- Si reference)
831.5 13086.0 1.94×106 1.82 1192.4 1161.0 1.68×105 2.07 666.9 349.6 2.59×104
485.5 765.0 4.49×104
D1-NH8-80°C 0.84 1.97 1.11 836.6 21162.4 903.7 12754.0 3.39×106 2.07 3330.0 874.0 1.67×105 8.20 2210.0 1410.0 3.00×105 5.08
821.1 11192.0 1.54×106 1.82 1181.5 819.0 1.13×105 2.07 643.0 449.0 3.25×104
490.0 1101.0 7.79×104
D1-NH6-80°C 1.13 2.3 0.64 829.8 18598.8 912.4 10952.0 2.49×106 2.07 3323.0 555.0 1.13×105 8.20 2145.9 2520.0 4.56×105 6.34
812.6 11969.0 1.71×106 1.82 2.07 627.4 1305.2 1.65×105
D1-90°C- Si reference
0.67 1.9 1.07 841.6 20306.5 930.5 9535.1 2.48×106 2.07 3330.0 1690.0 3.60×105 8.20 2223.0 1050.0 1.90×105 4.90
833.3 13837.0 2.10×106 1.82 1194.4 1402.0 1.99×105 2.07 672.4 236.9 7.33×103
D4-100°C- Si reference
0.77 1.94 0.9 834.6 20575.5 906.3 11488.0 2.87×106 2.07 3308.0 1280.0 3.27×105 8.20 2217.0 1496.0 3.50×105 5.01
820.8 12189.0 1.70×106 1.82 1180.0 790.0 9.25×104 2.07 660.0 400.0 2.98×104
480 1100 6.19×104
83
C
hapter 3 Material C
haracterisation
83
Chapter 3 Material Characterisation
Table 3.9 Summary of bond and atom concentrations calculated for SiNx/Si wafers deposited under varied NH3/SiH4 flow ratios at temperatures between 80 °C - 100 °C
Sample SiH4/ NH3
NH3/ SiH4
n632.8nm [N]/[Si] [Si-N] (1022 cm-3)
[N-H] (1022 cm-3)
[Si-H] (1022 cm-3)
[Si-Si] (1022 cm-3)
[Si] (1022 cm-3)
[N] (1022 cm-3)
[H] (1022 cm-3)
Total bond
density (1022 cm-3)
Film density (g/cm3)
D1-NH12-100°C 0.56 1.80 1.90 1.24 9.37 4.66 2.23 1.74 3.77 4.68 6.89 18.01 2.96
D1-NH10-100°C 0.67 1.49 1.93 1.02 9.11 3.32 1.59 2.78 4.06 4.14 4.91 16.80 2.94
D1-NH8-100°C 0.84 1.19 2.12 0.98 9.92 4.69 3.98 2.99 4.97 4.87 8.67 21.57 3.59
D1-NH6-100°C 1.13 0.88 2.41 0.62 9.44 0.01 3.38 3.75 5.08 3.15 3.39 16.58 3.16
D1-NH12-80°C 0.56 1.80 1.80 1.37 8.29 3.40 0.90 1.10 2.85 3.90 4.30 13.69 2.31
D1-NH10-80°C 0.67 1.49 1.86 1.17 8.40 3.24 1.69 1.59 3.32 3.88 4.93 14.92 2.53
D1-NH8-80°C 0.84 1.19 1.97 1.11 9.83 1.61 1.52 1.19 3.43 3.81 3.13 14.15 2.54
D1-NH6-80°C 1.13 0.88 2.30 0.64 8.26 0.93 2.89 3.99 4.79 3.06 3.82 16.08 3.01 D1-90°C- Si
reference 0.67 1.49 1.90 1.07 8.95 3.36 0.93 2.73 3.84 4.10 4.29 15.97 2.81
D4-100°C- Si reference 0.77 1.29 1.94 0.90 9.03 2.87 1.75 3.43 4.41 3.97 4.63 17.09 3.06
84
Chapter 3 M
aterial Characterisation
84
Chapter 3 Material Characterisation
Assuming there are no N-N or H-H bonds in the films, the atom concentration of
N(Si), N(N) and N(H) can be calculated using the following expressions [183, 187,
207]:
[Si] = ([Si-N] + [Si-H]) / 4 + [Si-Si] / 2 (3.18 a)
[N] = ([N-H] + [Si-N]) / 3 (3.18 b)
[H] = [N-H] + [Si-H] (3.18 c)
and
=ρ mSi [Si] + mN [N] + mH [H] (3.19)
where ρ is the film density [208]. mSi, mN and mH are the atomic masses of Si, N and
H, respectively.
The Si-Si bond density in Eq. (7.5) can be obtained from Eq. (7.1) and Eq. (7.7) as
[Si-Si] = 2 × [N] / x – ([Si-N] + [Si-H]) / 2 (3.20)
The total bond density is a sum of [Si-N], [N-H], [Si-H] and [Si-Si] as
Ntotal = [Si-N] + [N-H] + [Si-H] + [Si-Si] (3.21)
The film densities calculated by Eq. (3.19) are from 2.31 g/cm3 to 3.59 g/cm3, as
listed in Table 3.9, which are within the range of previous values reported for PECVD
silicon nitride [80, 145, 172, 191, 208]. The calculated bond/atom densities and film
densities are summarised in Table 3.9.
3.4.4.4 Influence of NH3/SiH4 flow ratio on bond and atom concentrations
The influence of the NH3/SiH4 flow ratio on the bond and atom concentrations is
investigated in this section. With the increase of NH3/SiH4 flow ratio, NHn free
radicals, mainly Si[NH2]3, increases and Si2H6 reduces in the plasma composition
[172], hence the ratio of H bonded to N over that bonded to Si, [N-H]/[Si-H] [191,
209], and [N]/[Si] [210] are expected to increase with increasing NH3/SiH4 flow ratio,
resulting in refractive index, n632.8nm, decreasing with the NH3/SiH4 flow ratio [206,
211], as shown in Figure 3.33 and Figure 3.34.
85
Chapter 3 Material Characterisation
Figure 3.33 [N-H] and [Si-H] bond concentration as a function of film composition [N]/[Si] and NH3/SiH4 flow ratio. The indicated temperatures refer to the substrate temperature during deposition.
0.4 0.6 0.8 1 1.2 1.4 1.60
1
2
3
4
5
[N]/[Si]
[N-H
] bon
d de
nsity
( x
1022
cm
-3)
T = 80oCT = 90oCT = 100oC
0.4 0.6 0.8 1 1.2 1.4 1.60.5
1
1.5
2
2.5
3
3.5
4
[N]/[Si][S
i-H] b
ond
dens
ity (
x 10
22 c
m-3
)
T = 80oCT = 90oCT = 100oC
0.5 1 1.5 20
1
2
3
4
5
NH3/SiH4 ratio
[N-H
] bon
d de
nsity
( x
1022
cm
-3)
T = 80oCT = 90oCT = 100oC
flow ratio
0.5 1 1.5 20
1
2
3
4
5
NH3/SiH4 ratio
[Si-H
] bon
d de
nsity
( x
1022
cm
-3)
T = 80oCT = 90oCT = 100oC
flow ratio
86
Chapter 3 Material Characterisation
(a) (b)
(c)
Figure 3.34 The ratio of H bonded to N over that bonded to Si, [N-H]/[Si-H], as a function of NH3/SiH4 flow ratio, refractive index and film composition [N]/[Si]. The line in (a) is a linear least-squares fit to the data points, as [N-H]/[Si-H] = 3.25 × NH3/SiH4 - 2.62. The indicated temperatures refer to the substrate temperature during deposition.
0.4 0.6 0.8 1 1.2 1.4 1.60
1
2
3
4
[N]/[Si]
[N-H
]/[S
i-H]
T = 80oCT = 90oCT = 100oC
1.8 2 2.2 2.4 2.60
1
2
3
4
n (λ = 632.8 nm)
[N-H
]/[S
i-H]
T = 80oCT = 90oCT = 100oC
0.5 1 1.5 20
1
2
3
4
NH3/SiH4 ratio
[N-H
]/[S
i-H] r
atio
T = 80oCT = 90oCT = 100oC
flow ratio
87
Chapter 3 Material Characterisation
(a)
(b)
Figure 3.35 The fraction of [N-H] and [Si-H] as a function of SiNx film composition [N]/[Si]. The indicated temperatures refer to the substrate temperature during deposition.
0.7 0.8 0.9 1 1.1 1.2 1.320
40
60
80
100
[N]/[Si]
[Si-H
]/([S
i-H]+
[N-H
]) %
T = 80oCT = 90oCT = 100oC
0.7 0.8 0.9 1 1.1 1.2 1.30
20
40
60
80
100
[N]/[Si]
[N-H
]/([S
i-H]+
[N-H
]) %
T = 80oCT = 90oCT = 100oC
88
Chapter 3 Material Characterisation
Figure 3.36 The atomic densities of [Si], [N] and [H] as a function of SiNx film composition [N]/[Si] and NH3/SiH4 flow ratio. The indicated temperatures refer to the substrate temperature during deposition.
0.4 0.6 0.8 1 1.2 1.4 1.62.5
3
3.5
4
4.5
5
5.5
[N]/[Si]
[Si]
bond
den
sity
( x
1022
cm
-3) T = 80oC
T = 90oCT = 100oC
0.4 0.6 0.8 1 1.2 1.4 1.63
3.5
4
4.5
5
[N]/[Si]
[N] b
ond
dens
ity (
x 10
22 c
m-3
)
T = 80oCT = 90oCT = 100oC
0.5 1 1.50.25
0.3
0.35
0.4
0.45
0.5
[N]/[Si]
[H] b
ond
dens
ity (
x 10
2 2 c
m-3
)
T = 80oCT = 90oCT = 100oC
0.5 1 1.5 23
3.5
4
4.5
5
NH3/SiH4 ratio
[N] b
ond
dens
ity (x
1022
cm
-3)
T = 80oCT = 90oCT = 100oC
0.5 1 1.5 2
2.5
3
3.5
4
4.5
5
5.5
NH3/SiH4 ratio
[Si]
bond
den
sity
(x10
22 c
m-3
)
T = 80oCT = 90oCT = 100oC
0.5 1 1.5 2
0.25
0.3
0.35
0.4
0.45
0.5
NH3/SiH4 ratio
[H] b
ond
dens
ity (x
1022
cm-3
)
T = 80oCT = 90oCT = 100oC
flow ratio
flow ratio
flow ratio
89
Chapter 3 Material Characterisation
(a)
(b)
Figure 3.37 Film density, ρ, as a function of (a) SiNx film composition [N]/[Si] and (b) NH3/SiH4 flow ratio. The indicated temperatures refer to the substrate temperature during deposition.
0.4 0.6 0.8 1 1.2 1.4 1.62
2.5
3
3.5
4
[N]/[Si]
ρ (g
/cm
3 )
T = 80oCT = 90oCT = 100oC
0.5 1 1.5 22
2.5
3
3.5
4
NH3/SiH4 ratio
Film
den
sity
, ρ (g
/cm
3 )
T = 80oCT = 90oCT = 100oC
flow ratio
90
Chapter 3 Material Characterisation
Figure 3.35 plots the fraction of [N-H] and [Si-H] as a function of SiNx film
composition [N]/[Si]. As expected, Figure 3.35 shows that at lower [N]/[Si], almost
all the hydrogen atoms are bonded to Si, and with an increase in [N]/[Si] there is an
increase of hydrogen atoms being bonded to N and an decrease of H bonded to Si.
The atom densities of [Si], [N] and [H] as a function of SiNx film composition
[N]/[Si] and NH3/SiH4 flow ratio are shown in Figure 3.36. The general trends are that
[Si] decreases with NH3/SiH4 flow ratio and [N]/[Si], which is as expected, while both
[N] and [H] increase with NH3/SiH4 flow ratio and [N]/[Si].
Film densities as a function of SiNx film composition [N]/[Si] and NH3/SiH4 flow
ratio are shown in Figure 3.37. The film density was seen to increase with elevated
substrate deposition temperature, and the defect density is expected to decrease with
increasing film density [212]. The trend of film density with either [N]/[Si] or
NH3/SiH4 flow ratio is non-linear, since it first increases with [N]/[Si] and then
decrease, which is in agreement with the results reported by Claassen et al. [191, 210].
3.5 Summary
This chapter has described the various experimental techniques used for material
characterisation related to this thesis, divided into two categories - in-situ monitoring
characterisation and ex-situ characterisation. Some selected experimental results have
been included, and the purposes for which these material characterisation techniques
were used to extract device parameters were given for each procedure.
Lastly, in order to determine the suitable deposition conditions of SiNx passivation
film for HgCdTe, a series of low-temperature (80 °C - 130 °C) ICPECVD SiNx films
were deposited on other semiconductors - CdTe/GaAs and Si substrates, under
different deposition conditions to investigate the influence of ICP power, deposition
temperature, and NH3/SiH4 flow ratio on properties of the SiNx films. The influence
of ICP power on the quality of the deposited SiNx films was assessed through the
long-term IR absorbance of the films, which determined that SiNx films deposited at a
high ICP power of 600 W appeared to be porous and more susceptible to oxidation.
Regarding the influence of NH3/SiH4 flow ratio on SiNx film properties, such as
refractive index, film composition, deposition rate, IR absorbance and bonding
91
Chapter 3 Material Characterisation
configuration, a series of SiNx films were deposited on Si substrates with a fixed SiH4
flow rate and various NH3 flow rates. The method used for the calculations of bond
and atom concentrations was introduced based on the band areas in the optical
absorption coefficient curve, and thus the influence of NH3/SiH4 flow ratio on bond
and atom concentrations was studied.
92
Chapter 4 Surface and Interface Effects in CdTe/HgCdTe Structures
4 Surface and Interface Effects in CdTe/HgCdTe Structures
4.1 Introduction
Following the establishment of reproducible bulk growth techniques and anodic oxide
surface passivation technology, HgCdTe photoconductive detectors entered
production in the late 1970’s as first-generation IR imaging systems [30]. As a result
of these developments, very detailed understanding of photoconductor behaviour has
been established including surface recombination, carrier sweepout effects, backside
shunting effects etc.. Because of the detailed understanding and the relative simplicity
of the structure, photoconductors are useful tool for characterisation of various
physical phenomena in HgCdTe materials. In particular, photoconductors allow very
detailed information to be extracted about minority carrier dynamics that is equally
applicable to other photodetector structures.
In this chapter, photoconductive devices are used as a tool in examining the
effectiveness of low-temperature deposited CdTe passivating films by comparing the
photoresponsivity between devices with and without sidewall CdTe passivation [213].
In addition, these results are augmented using gated HgCdTe photodiode structures,
passivated by the same MBE low-temperature grown CdTe, allowing band bending at
the surface to be controlled by varying the gate bias.
4.1 Sidewall effects in photoconductive devices
The responsivity of a photodetector is dependent on the bulk minority carrier lifetime
and the surface recombination velocities at each the surfaces (front surface, back
surface and sidewalls). By comparing devices with and without sidewall passivation,
the surface recombination at CdTe passivated surfaces can be determined.
4.1.1 Experimental procedures
The experiments were carried out using the following procedure:
Four wafers were cleaved from the same Epitech MWIR n-HgCdTe material (x =
0.29), that has a 13.5 μm-thick vacancy-doped absorber grown by LPE. Figure 4.1
summarises the process flow for each of the wafers. Wafer 1 was first etched using a
93
Chapter 4 Surface and Interface Effects in CdTe/HgCdTe Structures
standard wet etchant (1% Br2/HBr) to form mesa structures that will later be
processed into photoconductors. Then Wafer 1, Wafer 2 and Wafer 3 were
passivated with CdTe by MBE in the same run. To minimise fixed charge and
interface states in the CdTe passivating layer, surface cleaning and conditioning of the
HgCdTe layer before CdTe deposition is essential [100]. A Br2/methanol wet etch
was used after an organic surface clean to reveal a fresh HgCdTe surface. After this
surface conditioning, an approximately 200 nm-thick layer of CdTe was deposited by
MBE in the same run on all three samples. The CdTe passivation film was grown by
MBE at 100 °C. During the growth, the CdTe as measured by beam equivalent
pressure flux was 1 × 10-6 Torr and Te as measured by beam equivalent pressure flux
was 1.5 × 10-6 Torr, which were recorded by a Bayard-Alpert vacuum gauge that
could be rotated into the growth position. The background pressure was 2 × 10-9 Torr.
The resulting growth rate was approximately 10 nm/min. Following growth, an in-situ
anneal at 180 °C in the MBE growth chamber was done to achieve a compositionally
graded CdTe/HgCdTe interface.
Wafer 2 was etched using 1% Br2/HBr to form mesa structures that will later by
processed into photoconductors. It is necessary for the photoconductor structures on
Wafer 2 to have their top surfaces passivated with CdTe in order to reduce total
surface recombination so that the carrier lifetime was not too low, and to ensure that
photoresponsivity measurements could be meaningfully compared with the ones on
Wafer 1. The photoconductor structures on both Wafer 1 and Wafer 2 were 240 μm
× 200 μm × 13.5 μm in dimension.
To reveal the contact regions, both Wafer 1 and Wafer 2 were then patterned by
photolithography followed by a Br2/HBr wet etch. Lastly, the two wafers were
patterned by photolithography again for contact formation, and metal contacts were
formed by thermally evaporating 5 nm of Cr and 200 nm of Au. After metal liftoff in
acetone, photoconductors on Wafer 1 were resulted with both sidewall and surface
passivation, whereas the ones on Wafer 2 with only top surface passivation.
Wafer 3 and Wafer 4 were characterised by XRD in order to examine their X-ray
diffraction spectra before (Wafer 4) and after CdTe passivation (Wafer 3), as
previously detailed in Section 3.2.2.
94
Chapter 4 Surface and Interface Effects in CdTe/HgCdTe Structures
Figure 4.1 Schematic of the photoconductive devices showing location of the unpassivated sidewall surfaces. The passivating CdTe film is approximately 200 nm thick. (a) Fully passivated structure; (b) Partially passivated structure with no CdTe film on sidewalls.
4.1.2 Surface and interface recombination in photoconductive devices
The performance of a photoconductive detector is directly impacted by carrier
recombination at the surface, which affects the effective minority carrier lifetime.
Surface recombination processes in narrow bandgap detectors can become the
dominant loss mechanism for photo-generated excess carriers [9]. Surface passivation
greatly reduces the recombination of photo-generated carriers at the surface/interface,
resulting in an increase in the effective minority carrier lifetime and hence the voltage
responsivity [10, 11].
The responsivity of an infrared detector is defined as the output signal of the detector
divided by the input photon power. When a photoconductor device being exposed to a
monochromatic, modulated source, its responsivity (Volts/Watt) can be written
95
Chapter 4 Surface and Interface Effects in CdTe/HgCdTe Structures
as [214]
d
s
IAVR =λ (4.1)
where Vs (Volts r.m.s.) is the signal output, I (Watts/cm2) is the intensity of the
source, Ad is the size of the optically sensitive area of the detector.
The spectral photoresponsivity of the photoconductors described in the previous
section were measured using a system based on an Optronics Laboratories spectral
radiometer measurement system. During the photoresponsivity measurement, the
source of infrared radiation was calibrated firstly, so that the photoconductor devices
were subjected to a known intensity of infrared radiation at a known wavelength, and
was chopped at a relatively low frequency of 1 kHz. The alternating signal output
from the photoconductor, Vs, was measured using a lock-in technique at a field of
10 V/cm with the sample being held at 80 K in a cryostat with a cold shield. This
relatively low field minimised the effect of sweepout, so that the response should be
most sensitive to surface recombination velocity. As expected, it was found that the
photoconductors with all surfaces passivated have significantly higher responsivity
than those partially passivated, as shown in Figure 4.2.
Figure 4.2 Measured and modelled normalised spectral photoresponse of Hg0.71Cd0.29Te photoconductive devices, measured at a field of 10 V/cm at 80 K. The low field minimises the effect of sweepout so that the response should be most sensitive to surface recombination velocity.
4 4.5 5 5.5 60
0.5
1
1.5
Wavelength(µm)
Nor
mal
ised
Pho
tore
spon
se
with sidewall passivation (experimental data)without sidewall passivation (experimental data)with sidewall passivation (simulation)without sidewall passivation (simulation)
96
Chapter 4 Surface and Interface Effects in CdTe/HgCdTe Structures
Figure 4.3 Simulated photoresponse ratio of all surfaces passivated devices (RF) and partially passivated devices (RP) versus recombination velocity of the top CdTe/Hg0.71Cd0.29Te interface (sTop) at 80 K, with recombination velocity of the unpassivated surfaces sWall = 1×104 cm/s.
In a passivation layer, the fixed charge and interface traps can result in band-bending
in addition to recombination, all of which are dependent on the properties of the
passivating film. In order to evaluate the effectiveness of the sidewall passivation and
estimate the surface recombination velocity of CdTe passivated surfaces, surface
recombination simulations of the photodetectors were performed using Synopsys
TCAD device simulator Synopsis Sentaurus Device [215]. The Sentaurus fitting
parameters of trap ionisation energy value of 0.7Eg was used based on some
experimental evidence that Hg vacancies leave a trap at this level [216, 217], and the
interface trap density, Dit, value of 8 × 1012 cm-2eV-1 has resulted in satisfactory fit to
the experimental data.
An estimate of the surface recombination velocity at the CdTe/HgCdTe interface was
obtained in the following manner: First, the structure with unpassivated sidewalls was
modelled using an ideal interface (sTop = 0 cm/s) for the top CdTe passivated surface.
The HgCdTe/substrate interface was modelled with a recombination velocity of
100 cm/s, using results from previous studies on similar structures [218]. The
unpassivated sidewall surfaces were modelled as having sWall = 104 cm/s. Values
larger than this had little effect on the responsivity. Then the all surfaces passivated
structure and partially passivated structures were modelled by increasing the
102 103 1042
4
6
8
10
12
Surface recombination velocity (cms-1)
RF/R
P
97
Chapter 4 Surface and Interface Effects in CdTe/HgCdTe Structures
CdTe/HgCdTe sTop values until the ratio of peak responsivity of the all surfaces
passivated and partially passivated structures (RF /RP) matched the experimental
results (see Figure 4.2). The best fit value for sTop was 200 cm/s, as shown in Figure
4.3, which is more than an order of magnitude lower than the one at the unpassivated
surface sWall. Very high values of the surface recombination velocity of the
unpassivated HgCdTe of > 1 × 105 cm/s have been reported in literature, and, after
passivation, obvious drop with at least an order of magnitude in the interface
recombination velocities were observed [9, 219]. The interface recombination
velocities at the CdTe/HgCdTe interfaces were reported to be much lower than that at
the ZnS/HgCdTe interfaces, and two order of magnitude lower than that of freshly
etched surfaces [220]. The sharp decreases in photoresponsivity in the theoretical
curves were not observed in the experimental data, possibly because of field effect
[221, 222], Stark effect [223-225], Franz-Keldysh effect [223, 225, 226] and graded
band structure [227].
4.2 Interface effects in ZnS/CdTe/HgCdTe gated photodiodes
HgCdTe photodiodes are one of the most widely used devices for IR detection, since
they provide low power, high sensitivity detection, faster response times, and
improved uniform spatial response in focal plane arrays (FPA). A gated photodiode is
a photodiode structure that incorporates a metal - insulator - semiconductor gate
across the region where the pn junction intersects the surface, and can be used to
investigate how the surface band-bending affects the diode characteristics by applying
a gate voltage. The fact that the band-bending at the surface is localised to the region
under the gate and is controlled by the gate voltage has made the gated diode structure
useful in the study of surface related effects.
The device structure used in this thesis is planar, with a reactive ion etching (RIE)
induced n-on-p homojunction [6, 228, 229]. MBE grown CdTe is used as the first
passivation layer on HgCdTe and the second layer of ZnS serves as a higher
resistivity insulating layer in the gated photodiode. This section investigates the
properties of CdTe passivation by observing and interpreting the change in
photodiode performance when influenced by surface band bending.
98
Chapter 4 Surface and Interface Effects in CdTe/HgCdTe Structures
4.2.1 Gated photodiode fabrication process
Surface cleaning of the semiconductor prior to passivation formation is crucial to
device performance. In this thesis, the LPE grown p-type MWIR HgCdTe (x = 0.31,
depilayer = 13.5 μm) wafer was initially cleaned by soaking in successive baths of
trichloroethylene, acetone, and methanol at 70 °C, followed by a light etch in
Br/methanol solution for surface conditioning before CdTe deposition. The sample
was then kept in running deionised water until being dried just prior to loading into
the MBE chamber for deposition of the CdTe passivation.
An approximately 100 nm-thick CdTe passivation film was grown by MBE at 100 °C,
followed by an in-situ anneal at 180 °C in the MBE growth chamber to achieve a
compositionally graded CdTe/HgCdTe interface, as detailed in Section in 4.1.1.
The third step in the fabrication process is the conversion of regions of the p-HgCdTe
to n-type. The devices fabricated in this thesis were based on a RIE plasma-based p-
to-n type conversion process [6, 228, 229], which is a simpler technique than
traditional ion implantation and ion milling methods. CdTe is used as a surface
passivant and as a mask for the p-to-n type conversion process. Windows for type
conversion were defined photolithographically, and the CdTe was etched in a 1%
Br/HBr solution. The photoresist was then stripped and the samples organically
cleaned before being exposed to a hydrogen-methane plasma in a Plasma Technology
RIE 80 system for two minutes, with RIE power of 120 W, H2 of 54 sccm, CH4 of
10 sccm, and a chamber pressure of 100 mTorr. The entire semiconductor area
beneath the circular RIE type converted n-type region is expected to be uniformly
type converted from p to n type with a junction depth of approximately 1.5 μm [230].
ZnS (approximately 200 nm-thick) was then deposited in a thermal evaporator under a
vacuum of 1×10−6 Torr. The wafer was heated under vacuum prior to deposition,
starting at a temperature of 50 °C, and reaching approximately 70 °C by the end of the
ZnS deposition. The ZnS deposition rate is controlled at a low level of ~ 0.02 nm/s in
order to achieve a denser and higher quality film [92].
The wafer was then patterned by photolithography and etched to reveal the type
converted region, and the ZnS was wet etched in 2:1 HCl: distilled and deionised
water solution. To reveal the shared common p-type contact regions, the wafer was
99
Chapter 4 Surface and Interface Effects in CdTe/HgCdTe Structures
patterned by photolithography again, and the ZnS was etched in 2:1 HCl: distilled and
deionised water solution, followed by the etching of CdTe in 1% Br/HBr.
Lastly, for contact formation, the wafer was patterned by photolithography with a
single photolithographic mask for both n and p contacts, and metal contacts to the p
and n type regions of HgCdTe were formed by thermally evaporating 5 nm of Cr and
200 nm of Au. After metal liftoff in acetone, all photodiodes share a common p-type
contact region, and have individual contacts for the n-type regions. A
photomicrograph and cross section of the completed devices are shown in Figure 4.4.
(a)
(b)
Figure 4.4 A photomicrograph and cross section of the completed gated photodiodes. (a) Photo of fabricated gated photodiodes and (b) cross-sectional view.
Junction Bias
ZnS
p-HgCdTe
contact
CdZnTe
CdTe
contact
n
300 μm
100
Chapter 4 Surface and Interface Effects in CdTe/HgCdTe Structures
4.2.2 Dark current as a function of gate bias
Unlike SiNx passivation, ZnS/CdTe is unsuitable for standard C-V analysis, because of
excessive leakage currents through the passivation layer. To evaluate ZnS/CdTe as a
passivation layer, gate bias swept diode dark current measurements have been used.
This section describes the results of dark current measurements on the gated diodes
used to investigate CdTe passivation performance. Varying the gate bias, Vg, allows
the magnitudes of dark current and dynamic resistance to be manipulated at the surface
of the photodiode, and determination of the dominant generation-recombination
mechanisms.
4.2.2.1 Dark current mechanism
Dark currents in HgCdTe photodiodes primarily include diffusion, generation-
recombination (GR), band-to-band tunnelling (BTB), trap-assisted tunnelling (TAT),
shunt and surface currents [231, 232]. Each dark current component can be treated as a
resistance in parallel with the junction, expressed as:
surfaceshuntTATBTBGRdiff RRRRRRR1111111
+++++= (4.2)
Diffusion current is a fundamental mechanism in p-n junction photodiodes due to the
random thermal generation of carriers within a minority carrier diffusion length of the
depletion region edges. In MWIR HgCdTe photodiodes, diffusion current dominates at
higher temperatures (> 150 K), whereas it is less than other current components at
77 K in the reverse bias region, and dominant in the forward bias region.
SRH type generation-recombination centres in the space charge region can contribute
significantly to the dark current, with impurities or defects within the depletion region
acting as GR centres producing GR current within the diode [233]. GR current can also
occur when impurities or defects are not involved, such as radiative and Auger
recombination [234].
BTB and TAT current components affect the performance of photodiodes greatly due
to the narrow bandgap of HgCdTe, and they can be influenced by depletion width,
bandgap, trap energy level and trap density [235, 236]. BTB current occurs when
carriers tunnel from the valence band to the conduction band across the depletion
region. A decrease in depletion width can lead to an increase of BTB current due to the
decreased width of barrier to tunnelling. Narrowing of the depletion width at the
101
Chapter 4 Surface and Interface Effects in CdTe/HgCdTe Structures
semiconductor surface can also lead to an increase in the BTB component. TAT occurs
via impurities or defects located within the depletion region when carriers travel from
the valence band to the conduction band. Increased trap density leads to the increase in
TAT current.
Shunt current component is considered to be associated with the formation of localised
defects in the vicinity of the junction at the semiconductor surface. Shunt current can
occur when the surface is placed in inversion due to incorrect passivation of the surface
[52]. Surface current component is associated with incorrect termination of bonds at
the surface of the HgCdTe or charges in the passivation layer.
4.2.2.2 Dark current measurement of gated diodes
The field induced junction at the surface will be modified with any change of charge
density in the passivation and at the interface. As shown in Figure 4.5 (a), the p-type
surface is in accumulation when Vg < Vfb. Narrowing of the depletion region of the of
the p-n junction on the p-side caused by accumulation will result in an increase of
electric field across the junction, which can increase the TAT current component,
since TAT is sensitive to changes in the electric field across the junction.
Figure 4.5 (b) illustrates the p-type surface in the flatband condition when Vg = Vfb
(~ -2.7 V), where narrowing of the depletion width of the p-n junction on the n-side
can still be observed. As shown in Figure 4.5 (c), a depletion region will form under
the gate when increasing the gate voltage, which decreases the TAT current at the
surface, since the field induced junction reduces the junction electric field at the
surface. An increase in the RdA is expected to be observed. The p-type surface will be
in inversion and field-induced junction breakdown will occur in the p-type region
under the gate, when Vg is increased beyond the threshold voltage (~ 2 V).
Dark current measurements were made with a HP 4156A semiconductor parameter
analyser. The dark currents were measured on two fabricated gated diodes at 77 K in a
cryostat with a cold shield to prevent generation of photocurrent. One diode had a
diameter of 300 μm and the other 360 μm, as shown in Figure 4.6. For a junction bias
of -50 mV, which is typically used to minimise dark current in HgCdTe photodiodes,
the dark current was observed to decrease by three orders of magnitude as Vg was
increased from -1.5 V to 1.5 V. Dark current was GR limited and then diffusion
102
Chapter 4 Surface and Interface Effects in CdTe/HgCdTe Structures
Figure 4.5 Diagrams illustrating the effects of n-type region band-bending on a n-on-p junction. The gate voltage, Vg, is referenced to the p-type HgCdTe. a) p-type surface in accumulation; b) Vg = Vfb in p-type, c) p-type surface in depletion or weak inversion; d) p-type surface in inversion and field-induced junction breakdown occurs in the p-type region under the gate.
(a)
CdZnTe
Vg
n p
(b)
Vg
n p
CdZnTe
(c)
Vg
n p
CdZnTe
(d)
Vg
n p
CdZnTe
103
Chapter 4 Surface and Interface Effects in CdTe/HgCdTe Structures
(a) d = 300 μm
(b) d = 360 μm
Figure 4.6 Measured dark current at 77 K in a cryostat with a cold shield in the absence of photocurrent for gated photodiodes with (a) a diameter of 300 μm and (b) 360 μm. The gate voltage is referenced to the p-type substrate. The seven curves from top to bottom are for various gate biases from -1.5 V to 1.5 V in 0.5 V steps.
-0.4 -0.2 010-12
10-10
10-8
10-6
10-4
Junction Bias (V)
Cur
rent
(A)
Vg = - 1.5 V
Vg = 1.5 V
-0.4 -0.2 0 0.210-12
10-10
10-8
10-6
10-4
Junction Bias (V)
Cur
rent
(A)
Vg = - 1.5 V
Vg = 1.5 V
104
Chapter 4 Surface and Interface Effects in CdTe/HgCdTe Structures
(a) d = 300 μm
(b) d = 360 μm
Figure 4.7 Measured dynamic resistance-area product at 77 K in a cryostat with a cold shield in the absence of photocurrent for gated photodiodes with a diameter of (a) 300 μm and (b) 360 μm. The seven curves from bottom to top are for varying gate bias from -1.5 V to 1.5 V in 0.5 V steps.
-0.4 -0.2 0100
102
104
106
Junction Bias (V)
Dyn
amic
Res
ista
nce
(Ω.c
m2 )
Vg = 1.5 V
Vg = - 1.5 V
-0.4 -0.2 0100
102
104
106
Junction Bias (V)
Dyn
amic
Res
ista
nce
(Ω.c
m2 )
Vg = 1.5 V
Vg = - 1.5 V
105
Chapter 4 Surface and Interface Effects in CdTe/HgCdTe Structures
limited when being forward biased at the junction. The increase of current in the GR
limited region can be caused by a widening of the depletion region formed under the
gate in the n-type region. A slightly reverse junction biased region is dominated by
TAT in the Vg sweeping range in Figure 4.6 (-1.5 V to 1.5 V), and would be by BTB
tunnelling with even greater negative gate bias (> -4 V). This is due to a narrowing of
the junction depletion region at the surface caused by accumulation of the p-type
surface. The tunnelling currents can often dominate the dark current characteristics in
gated diodes due to the decrease in the depletion width at the surface caused by band-
bending. The BTB and TAT can occur at much lower reverse junction biases in gated
diodes than with an accumulated surface in comparison to what would occur in non-
gated diodes.
Within a given range of Vg in Figure 4.6, the p-type surface is depleted, and a wider
depletion region will be formed under the gate by increasing Vg up to ~ 2 V, which
decreases the TAT current at the surface [72, 92], as illustrated in Figure 4.5 (c).
Recombination at the surface will be increased due to the increased volume of
depletion in the p-type, resulting in a current increase. Similar to the gated diode
results presented by Westerhout et al. [237], there was no obvious hysteresis observed
in the I-V or RdA curves when Vg was swept from accumulation to depletion and swept
back.
Shown in Figure 4.7 is the measured dynamic resistance-area product, RdA, for the
two photodiodes at 77 K in a cryostat with a cold shield in the absence of
photocurrent for gated photodiodes. The gate voltage Vg is referenced to the p-type
HgCdTe substrate. By increasing Vg from 0 V to 1.5V, RdA improved by
approximately an order of magnitude, whereas RdA dropped by an order of magnitude
when decreasing Vg to -1.5 V. As Vg is increased from -1.5 V to 1.5 V, the depletion
region at the p-type surface is continually widened within this gate bias regime, which
reduces the TAT current, and results in a monotonic increase in RdA. The relatively
low values of dynamic resistance of the diodes may be due to negative charges within
the passivation layer that can accumulate the p-type HgCdTe surface. The
performance of the photodiodes was observed to be improved by varying the gate
bias, also indicating a non-ideal passivated surface which has increased the surface
current component at zero bias [237].
106
Chapter 4 Surface and Interface Effects in CdTe/HgCdTe Structures
4.3 Summary
In this chapter, surface and interface effects were studied in CdTe/HgCdTe devices,
including photoconductive devices and gated photodiode devices. The CdTe
passivation used was MBE low-temperature grown film. In order to determine the
effectiveness of this low-temperature deposited CdTe passivating film,
photoconductors were utilised to investigate the passivation effect of MBE grown
CdTe films, by comparing photoresponsivity between devices with and without
sidewall CdTe passivation. Surface recombination simulations of the photodetectors
were performed to evaluate the effectiveness of the sidewall passivation and estimate
the surface recombination velocity of CdTe passivated surfaces. The gated photodiode
was used as a tool to investigate device performance, and the band bending at the
surface can be influenced by varying the gate bias. This allows the magnitude of dark
current and dynamic resistance to be manipulated at the surface of the photodiode,
which also changes the dominant surface GR mechanisms. Gated diode results indicate
that positive charge may be trapped in CdTe/ZnS. This is a way to evaluate the
CdTe/ZnS passivation.
107
Chapter 5 Interface Effects in Metal/SiNx/HgCdTe Structures
5 Interface Effects in Metal/SiNx/HgCdTe Structures
The capabilities of low-temperature processing, good surface insulation and
hydrogenated films make SiNx a suitable choice for passivating HgCdTe. The MIS
structure has long been utilised as a tool in studying the interface between the
insulator and semiconductor [164, 238], and the interface state density Dit has been
considered as the key parameter in evaluating surface passivation performance and in
correlating passivation quality with other film properties [174, 175, 179, 182, 183]. In
this chapter, SiNx films were deposited under different conditions, and then the
characteristics of SiNx/n-Hg0.68Cd0.32Te MIS structures were determined, with the
interface trap density Dit examined by both quasi-static and conductance methods.
Also, work in this chapter has also been carried out on the correlation of film
passivation performance with film bond densities. The concentration of silicon-
hydrogen bonds, [Si–H], in silicon nitride based passivation has been regularly
considered as a measure of surface passivation quality, since a higher [Si–H] implies a
higher probability that hydrogen terminates any dangling bonds at the interface. In
Section 5.3, the [Si-H] and [N-H] bond densities in the SiNx/Si films will be discussed
as indicators for passivation quality [1].
Given the limitations on the current literature on SiNx/HgCdTe interface effects, it is
difficult to formulate a physical model relating the optical and electrical properties of
SiNx to its passivation performance for HgCdTe. However, based on the literature on
SiNx/Si, the work in this chapter hopes to provide some insight into the issue by
demonstrating the interaction of film bond density with the interface trap density Dit
using SiNx/HgCdTe MIS structures, and also the impact of deposition parameters on
the interface.
5.1 Fabrication of the MIS structures
The HgCdTe epilayers employed were grown at UWA in a Riber-32 MBE facility. A
layer of 2 μm-thick Hg0.6Cd0.4Te was grown on CdZnTe substrate followed by a
5 μm-thick Hg0.68Cd0.32Te layer. The HgCdTe wafer was then diced into four pieces,
and on each piece SiNx was deposited under different conditions. Circular gate
contacts of 5 nm Cr and then 200 nm of Au were deposited onto the SiNx films and
defined photolithographically.
108
Chapter 5 Interface Effects in Metal/SiNx/HgCdTe Structures
(a) D1-80C (b) D1-90C
(c) D1-100C (d) D4-100C
Figure 5.1 C-V curves measured at 1 MHz with the three different sweeping voltage ranges for each of the four MIS sample for the range of ± 2 V (circle ‘o’), ± 4 V (plus ‘+’) and ± 6 V (square ‘’). Each C-V measurement was swept from surface inversion to accumulation (dashed lines) and then swept back to inversion (solid lines).
-6 -4 -2 0 2 4 60.7
0.75
0.8
0.85
0.9
0.95
1
Applied voltage (V)
Chf
/ C
ox
-6 -4 -2 0 2 4 60.8
0.85
0.9
0.95
1
Applied voltage (V)
Chf
/ C
ox
-6 -4 -2 0 2 4 60.7
0.75
0.8
0.85
0.9
0.95
1
Applied voltage (V)
Chf
/ C
ox
-6 -4 -2 0 2 4 60.6
0.7
0.8
0.9
1
Applied voltage (V)
Chf
/ C
ox
111
Chapter 5 Interface Effects in Metal/SiNx/HgCdTe Structures
Flat band voltage, fixed charge density and slow interface trap density were extracted
using standard MIS theory [164, 238], summarised in Table 3.3 and Table 3.9. The
discussion on extracted Dit will be presented in the following sections. The polarities
of fixed charge density were found to be negative for all the four samples. A positive
fixed charge in SiNx film on a n-HgCdTe substrate would make the HgCdTe surface
accumulated at zero-bias, which is advantageous for n-type HgCdTe photoconductive
detectors due to the field-effect surface passivation, whereas a low negative charge
could be beneficial for n+-p photo-diodes. A high negative fixed charge will invert the
n-HgCdTe surface, and this inverted region of p-HgCdTe, separated from the n-
HgCdTe by a depletion region, could lead to a decrease in recombination rate, and
thus leads to a field-effect passivation. In Figure 5.1, the high frequency capacitance
was seen to increase slightly in inversion for increasingly negative gate bias, which is
an indication of an inverted semiconductor surface at zero bias.
Figure 5.2 defines the terms of ∆V+, ∆V–, ∆VH and Vfb used in the C-V analysis. ∆V+
is a measure of negative charge trapping, whereas the corresponding ∆V– is a measure
of positive charge trapping. ∆VH is the sum of ∆V+ and ∆V–. The asymmetrical ∆V+
and ∆V– is an indication of the asymmetrical electron and hole trapping characteristics
of the defects [241]. All the four samples indicate higher electron trapping at he
interface compared hole trapping, with the low-temperature deposited sample D1-80C
exhibiting the highest hole trapping characteristics, and the defects at the interface can
trap either electrons or holes.
It can be seen in Figure 5.1 that the Vfb and ∆V– barely change when voltage sweeping
from negative to positive. But when sweeping from positive back to negative voltage,
Vfb shifted to positive voltages by a significant amount, resulting in a change of ∆V+
for different sweep conditions. This phenomenon becomes more obvious with
increased sweep bias extremes from Vmax (solid C-V curve in Figure 5.2) to Vmax'
(dashed C-V curve in Figure 5.2), similar to what have been observed in anodic
oxide/HgCdTe [242] and ZnS/HgCdTe [243] MIS structures.
112
Chapter 5 Interface Effects in Metal/SiNx/HgCdTe Structures
Figure 5.2 Illustration on the definitions of ∆V+, ∆V– and ∆VH used in the C-V analysis.
It is well-known that a shift in Vfb is caused by the charge exchange between the
semiconductor and the slow traps due to the relative movement of the Fermi levels
with the band edges [164]. The experimental C-V curves indicate that the charge
exchange takes place mainly when the n-HgCdTe surfaces were accumulated. Under
positive bias, the slow traps in SiNx are filled with the electrons accumulated at the
HgCdTe surface, thus exhibiting a higher negative charge and, hence, a positive Vfb
shift. With an increase in the voltage extreme Vmax, the number of electrons captured
by the traps increases, resulting in a greater Vfb shift to more positive voltages.
A summary on Vfb, fixed charge density (Qf), ∆VH, slow interface trapped charge
density (Qit) and Dit extracted from the C-V characteristics of the four SiNx/HgCdTe
MIS samples is given in Table 5.2. The relationship between Vfb and substrate
temperature, [N]/[Si] and n632.8nm for the MIS samples of D1-80C, D1-90C, and D1-
100C, with SiNx deposited at the same condition except for substrate temperature, is
plotted in Figure 5.3. It can be seen that the flat band voltages Vfb decrease with the
increase of substrate temperature and film refractive index n632.8nm, and increase with
[N]/[Si]. The values of VH and Qit at various Vmax for the four MIS samples are listed
in Table 5.3, which are illustrated in Figure 5.4.
CFB
∆V_ ∆V+
∆VH'
∆VH
Vmax Vmax' -Vmax' -Vmax 0 VG
C
113
Chapter 5 Interface Effects in Metal/SiNx/HgCdTe Structures
Table 5.2 Summary on flat band voltage, fixed charge density, slow interface trapped charge density and interface trap density extracted for the four
SiNx/HgCdTe MIS samples for a bias sweep range of ± 2 V
D1-80C D1-90C D1-100C D4-100C
[N]/[Si] 1.17 1.07 1.02 0.90
n632.8nm 1.86 1.90 1.93 1.94
Flat band voltage Vfb (V) 0.98 0.86 0.71 0.76
Fixed charge density Qf (×10 10 cm-2) -12.86 -8.53 -9.42 -11.48
Hysteresis width ∆VH (V) 0.19 0.08 0.208 0.207
Slow interface charge density Qit (×10 10 cm-2) 3.17 1.03 3.87 4.33
Interface trap density Dit_min (×10 10 eV-1cm-2) 5.07 17.47 11.78 4.01
Interface trap density Dit (×10 10 eV-1cm-2) at mid-gap 13.65 50.38 17.56 10.57
Table 5.3 Hysteresis widths VH in the high-frequency C-V curves versus bias extremes for the four SiNx/HgCdTe MIS structures
Hysteresis width VH (V) Slow interface trap density (× 1010 cm-2)
Sample name D1-80C
D1-90C
D1-100C
D4-100C
D1-80C
D1-90C
D1-100C
D4-100C
± 2 V 0.19 0.08 0.208 0.207 3.17 1.03 3.87 4.33
± 4 V 0.84 0.21 0.48 0.46 14.23 2.80 9.01 9.70
± 6 V 1.75 0.36 0.80 0.73 29.81 4.82 15.08 15.57
114
Chapter 5 Interface Effects in Metal/SiNx/HgCdTe Structures
(a) (b)
Figure 5.3 Flat band voltage as a function of (a) substrate temperature, and (b) [N]/[Si], for the three MIS samples of D1-80C, D1-90C, and D1-100C.
(a) (b)
Figure 5.4 The change in (a) hysteresis widths VH and (b) slow interface charge densities as a function of bias extremes for the four MIS structures. The absolute values of the negative slow interface trap density are shown in the plot.
2 3 4 5 60
0.5
1
1.5
2
Vmax (V)
Hyst
eres
is wi
dth
(V)
D1-80CD1-90CD1-100CD4-100C
± 2 3 4 5 6
0
5
10
15
20
25
30
Vmax (V)Slo
w in
terfa
ce c
harg
e de
nsity
(x10
10cm
-2)
D1-80CD1-90CD1-100CD4-100C
±
0.7 0.75 0.8 0.85 0.9 0.95 11.02
1.04
1.06
1.08
1.1
1.12
1.14
1.16
1.18
[N]/[
Si]
Vfb (V)0.7 0.75 0.8 0.85 0.9 0.95 1
1.86
1.87
1.88
1.89
1.9
1.91
1.92
1.93
1.94
n (λ
= 6
32.8
nm
)
0.7 0.75 0.8 0.85 0.9 0.95 11.02
1.04
1.06
1.08
1.1
1.12
1.14
1.16
1.18
[N]/[
Si]
Vfb (V)0.7 0.75 0.8 0.85 0.9 0.95 1
1.86
1.87
1.88
1.89
1.9
1.91
1.92
1.93
1.94
n (λ
= 6
32.8
nm
)
80 85 90 95 1000.7
0.8
0.9
1
Substrate temperature (oC)
Vfb
(V)
115
Chapter 5 Interface Effects in Metal/SiNx/HgCdTe Structures
5.2.2 Interface trap density extracted by quasi-static method
The quasi-static C-V method [164, 244, 245] was employed to extract the density of
fast interface trap Dit from the high-frequency and low-frequency C-V data in the bias
sweep range of ±2V. The basic theory of the quasi-static C-V method was developed
by Berglund [245], which can be carried out by comparing a low-frequency (lf) C-V
with a high-frequency (hf) C-V. The high-frequency C-V curve should be measured at
a frequency high enough so that interface traps can be assumed not to respond,
whereas the low-frequency C-V is where interface traps and minority carrier inversion
charges are able to respond to the probe frequency used in the measurement. The low-
frequency capacitance Clf in depletion-inversion can be expressed as 1
11−
+
+=itSox
lf CCCC (5.1)
where
itit DqC 2= (5.2)
and
hfox
hfoxS CC
CCC
−= (5.3)
therefore,
−−
−=
−
−=
oxhf
oxhf
oxlf
oxlfoxS
lfox
lfoxit CC
CCCC
CCqC
CCC
CCq
D/1
//1
/122 (5.4)
where Cox is oxide capacitance and CS is semiconductor capacitance. Replacing CS in
Eq. (5.4) with Eq. (5.3) eliminates the uncertainty in the calculation of CS. A summary
of the flat band voltage, fixed charge density, slow interface charge density and
interface trap density extracted for the four SiNx/HgCdTe MIS samples is shown in
Table 3.9. The Dit characteristics for the four MIS capacitors as a function of surface
potential at the SiNx/HgCdTe interface are shown in Figure 5.5.
116
Chapter 5 Interface Effects in Metal/SiNx/HgCdTe Structures
Figure 5.5 Comparison of the interface trap densities (Dit) of the SiNx/n-Hg0.68Cd0.32Te MIS structures extracted by the quasi-static method as a function of the energy from mid-gap at 77 K.
Figure 5.5 shows on a logarithmic scale the distribution of the interface trap density
over energy relative to mid-gap energy Ei. The Dit characteristics manifest a typical
U-shaped distribution over the bandgap, with sample D4-100C indicating lower Dit
over most of bandgap range. The interface trap densities were found to increase
strongly toward the band edges. All the four Dit curves were found to have similar
mid-gap values Dit-midgap but differ from each other when the surface potential moves
towards either the valance or conduction band edges. When comparing sample D4-
100C with sample D1-100C, in which all deposition conditions were the same except
for the NH3/SiH4 flow ratio, it is evident that D4-100C with a higher NH3/SiH4 flow
ratio and a more Si-rich film shows better passivation quality. This indicates that the
deposition conditions corresponding to sample D4-100C, which was deposited under
Si-rich conditions, gives the best result and can be employed to passivate HgCdTe
based devices without the need for a CdTe capping layer. The SiNx deposition
conditions for sample D4-100C were found to result in a SiNx/HgCdTe MIS structure
characterised by a negative fixed trap density of -1.2 × 1011 cm-2, a slow interface trap
density of 4.3 × 1010 cm-2, and interface trap density Dit of 4.0 × 1010 eV-1cm-2. These
-0.1 -0.05 0 0.05 0.11010
1011
1012
1013
1014
E-Ei (eV)
Dit
(cm
-2eV
-1)
D1-80CD1-90CD1-100CD4-100C
117
Chapter 5 Interface Effects in Metal/SiNx/HgCdTe Structures
results are a significant improvement on the best reported ECR-PCVD deposited SiNx
films on HgCdTe, which indicated a negative fixed charge density of -1.4 × 1011 cm-2
and an interface trap density Dit of 1 × 1011 eV-1cm-2 [70]. Thus ICPECVD SiNx films
deposited at relatively low temperatures (80 °C - 100 °C) have significant potential as
surface passivation films for HgCdTe-based devices.
5.2.3 Conductance-frequency measurements on MIS structures
The measurements in this section aim to extract Dit by the conductance method, which
was proposed by Nicollian and Goetzberger in 1976 [238]. They observed the
capacitance decreases with increasing frequency, which depends on the relaxation
time of the interface states and frequency of the signal. The conductance method is
based on measuring the equivalent parallel conductance GP of the MIS capacitor as a
function of bias and frequency, f. The change in conductance represents the loss
mechanism due to interface trap capture and emission of carriers, from which Dit can
be extracted [164, 238, 246].
The simplified equivalent circuit used in the conductance method consists of the oxide
capacitance Cox, semiconductor capacitance CS and the interface trap capacitance Cit.
The lossy process of capture-emission of carriers by Dit is represented by Rit, and the
MIS-C has an interface trap time constant τit = RitCit. The Cp and Gp are given by [164,
238, 246]
2)(1 it
itSP
CCCωτ+
+= (5.5)
2)(1 it
ititP DqGωτ
ωτω +
= (5.6)
where itit DqC 2= , and fπω 2= . When assuming a continuous distribution of
interface traps within a few kT/q above and below the Fermi level, Gp/ω is expressed
as
])(1ln[2
2it
it
itP qDG ωτωτω
+= (5.7)
118
Chapter 5 Interface Effects in Metal/SiNx/HgCdTe Structures
Assuming negligible series resistance, the relationship between Gp/ω and measured
capacitance Cm, oxide capacitance Cox and conductance Gm is
222
2
)( moxm
oxmP
CCGCGG−+
=ωω
ω (5.8)
The value of 5.2=itωτ is found by solving 0)(/)/( =∂∂ nPG ωτω [238]. The Gp/ω
curves pass through a maximum at 5.2=itωτ with the values of
max4029.01
≈ω
Pit
Gq
D (5.9)
In order to apply the conductance method to the analysis of the MIS structures,
capacitance- and conductance - frequency measurements were implemented at 77 K
with the frequency ramping from 1 kHz to 1 MHz. The measured capacitance - log ω
characteristics at various biases for the four MIS structures at 77 K are shown in
Figure 5.6. At lower frequency, all the interface traps are able to respond to the
applied signal, so the interface trap capacitance is in parallel with the depletion
capacitance, resulting in a higher value of measured capacitance as shown in
Figure 5.6. As the frequency is increased to an intermediate level, only part of the
interface traps are able to respond to the applied signal, resulting in a decrease of the
measured capacitance. When the sweeping frequency is high enough, none of the
interface traps contribute to the measured capacitance, since the interface trap time
constant is too long to permit charge move in and out of the interface traps in response
to an applied signal [164, 247].
The measured and fitted Gp/ω – log ω characteristics for various biases for the four
MIS structures at 77 K are shown in Figure 5.7. Except for sample D1-90C , the fit to
the Gp/ω - log ω characteristics suggests the presence of a continuous distribution of
interface traps [164].
119
Chapter 5 Interface Effects in Metal/SiNx/HgCdTe Structures
(a) D1-80C (b) D1-90C
(c) D1-100C (d) D4-100C
Figure 5.6 Measured capacitance – log ω characteristics at various biases for the four MIS structures at 77 K.
3.5 4 4.5 5 5.5 6 6.51.5
2
2.5
3
3.5 x 10-8
log ω (Hz)
Cap
acita
nce/
Are
a (F
/cm
2 )
-2.0-1.6-1.2-1.1-0.9
Bias (V)
4 4.5 5 5.5 6 6.51.5
2
2.5
3
3.5x 10-8
log ω (Hz)C
apac
itanc
e/A
rea
(F/c
m2 )
-2.0-1.6-1.2-1.0
Bias (V)
3.5 4 4.5 5 5.5 6 6.51.5
2
2.5
3
3.5x 10-8
log ω (Hz)
Cap
acita
nce/
Are
a (F
/cm
2 )
-2.0-1.6-1.2-1.1
Bias (V)
3.5 4 4.5 5 5.5 6 6.51.5
2
2.5
3
3.5 x 10-8
log ω (Hz)
Cap
acita
nce/
Are
a (F
/cm
2 )
-2.0-1.6-1.2-1.0-0.9-0.8-0.7-0.6-0.5
Bias (V)
120
Chapter 5 Interface Effects in Metal/SiNx/HgCdTe Structures
(a) D1-80C (b) D1-90C
(c) D1-100C (d) D4-100C
Figure 5.7 Measured and fitted Gp/ω versus log ω characteristics at various gate biases for the four MIS structures at 77 K. (a)-(d) have the same scale in the plots. Dots: measured data points, lines: fitted curves.
3.5 4 4.5 5 5.5 6 6.50
1
2
3
4
5
x 10-9
log ω (Hz)
GP/ω
(F/c
m2 )
-2.0-1.6-1.2-1.0-0.9-0.8-0.7-0.6-0.5
Bias
3.5 4 4.5 5 5.5 6 6.50
1
2
3
4
x 10-9
log ω (Hz)
GP/ω
(F/c
m2 )
-2.0-1.6-1.2-1.0
Bias
3.5 4.0 4.5 5.0 5.5 6.0 6.50
0.5
1
1.5
x 10-9
log ω (Hz)G
P/ω
(F/c
m2 )
-2.0-1.6-1.2-1.1
Bias
3.5 4.0 4.5 5.0 5.5 6.0 6.50
0.5
1
1.5
2
2.5
3
3.5 x 10-9
log ω (Hz)
GP/ω
(F/c
m2 )
-2.0-1.6-1.2-1.1-0.9
Bias
121
Chapter 5 Interface Effects in Metal/SiNx/HgCdTe Structures
5.2.4 Interface trap density extracted by conductance method
Based on the conductance method [164, 238, 246], interface trap densities, Dit, for
each of the four MIS structures were extracted, as plotted in Figure 5.8. It can be seen
that Dit decreases with interface energy towards mid-gap. Except for sample D1-90C
that exhibits an anomalous non-continuous distribution of interface trap energy levels
[164], samples D4-100C and D1-80C were found to have lower levels of interface
traps than sample D1-100C. There can be seen differences between the Dit levels
shown in Figures 5.5 and Figures 5.8. The disagreement in the extracted interface trap
densities between the two methods is likely to be due to the influence of leakage
current or series resistance.
The time constant, τit , of the four SiNx/n-Hg0.68Cd0.32Te MIS structures extracted by
the conductance method at 77 K are shown in Figure 5.9. As seen from Figure 5.9, τit
increased with the interface energy from the valance band towards mid-gap.
The majority carrier capture cross section nσ of the interface trap can be calculated
from itτ by the equation [238, 248]
−=
kTq
NVs
Ditthn
φτ
σ exp1 (5.10)
where the thermal velocity thV (cm/s) of electrons and holes can be expressed using
××
××=
∗
∗
2/1
2/1
)/3(
)/3(
h
eth mTk
mTkV (5.11)
where ∗em and ∗
hm are effective mass of electrons and holes, respectively,
corresponding to 101084.7 ×=thV cm/s and 91018.3 × cm/s for electrons and holes at
77 K.
The electron capture cross section, nσ , as a function of energy for the MIS structures
are shown in Figure 5.10. The four curves of nσ were found to decrease with
interface energy towards mid-gap. Further experiments on SiNx/HgCdTe MIS
samples with PECVD SiNx deposited under different conditions combined with
theoretical modelling are required in order to better understand the nature of the
interface defects.
for electrons
for holes
122
Chapter 5 Interface Effects in Metal/SiNx/HgCdTe Structures
Figure 5.8 Comparison of the interface trap densities, Dit, of all the SiNx/n-Hg0.68Cd0.32Te MIS structures extracted by the conductance method as a function of the energy from mid-gap at 77 K.
Figure 5.9 Comparison of the time constant (τit) of all the SiNx/n-Hg0.68Cd0.32Te MIS structures extracted by the conductance method as a function of the energy from mid-gap at 77 K.
-0.1 -0.05 0 0.05 0.1
1010
1011
E-Ei (eV)
Dit
(eV
-1cm
-2)
D1-80CD1-90CD1-100CD4-100C
-0.1 -0.05 0 0.0510-6
10-5
10-4
10-3
10-2
E-Ei (eV)
τ it (s
)
D1-80CD1-90CD1-100CD4-100C
123
Chapter 5 Interface Effects in Metal/SiNx/HgCdTe Structures
Figure 5.10 Electron capture cross section as a function of energy for the SiNx/n-Hg0.68Cd0.32Te MIS structures at 77 K.
5.3 Relationship between SiNx passivation performance and thin film bond concentrations
The IR absorption coefficient curves of the four SiNx /Si reference wafer are shown in
Figure 5.11, with the three inserts zooming in on a specific absorption band. It can be
observed that samples D4-100C and D1-80C with lower levels of Dit, exhibit higher
Si-H (s) peaks at the wavelength of ~ 2200 cm-1 and lower N-H (r) peaks at
~ 1180 cm-1 compared to samples D1-90C and D1-100C.
Bond density calculations were carried out on the SiNx/Si reference wafers, where
more detailed literature is available for silicon substrates, in an attempt to correlate the
bond density with the level of interface traps. The [Si-H] bond density has been
considered as a measure of passivation quality, and good passivation could be
achieved if the SiNx has a higher density of [Si-H] [183]. The sources of hydrogen
incorporation used in the ICPECVD are the SiH4 and NH3 process gases.
A summary on bond and atom concentrations calculated for SiNx/Si reference wafers
of the SiNx/HgCdTe MIS structures were given in Table 3.9, and results from C-V
and IR absorbance analysis on the four SiNx/HgCdTe MIS structures were listed in
Table 3.8. Figure 5.12 plots the relationship between Dit and [Si-H] and [N-H] bond
concentrations, which have been identified as a measure of the quality of interface
-0.14 -0.12 -0.1 -0.08 -0.0610-20
10-18
10-16
10-14
10-12
E-Ei (eV)
σ n (c
m2 )
D1-80CD1-90CD1-100CD4-100C
124
Chapter 5 Interface Effects in Metal/SiNx/HgCdTe Structures
passivation for the SiNx/HgCdTe MIS samples. The variations of [H], [N-H], [Si-H]
and [Si-H]/[N-H] with Dit at mid-gap are shown in Figure 5.13. An approximately
linear relationship is observed between the Dit taken at mid-gap and [Si-H] bond
density of the SiNx film. Sample D4-100C was found to have the highest [Si-H] bond
density among the four wafers, with D1-80C the second, which can be directly
correlated with the observed lower densities of interface traps.
Following the Robertson and Powell model [249], the origin of the interface traps is
assumed to be associated with dangling bonds at the insulator/semiconductor
interface. A higher percentage of [H] bonded with Si can effectively terminate the
dangling bonds, resulting in a decease in Dit. Mäckel and Lüdemann have investigated
the reaction pathway of the passivation of dangling bonds, and shown that hydrogen
plays a fundamental role in the formation of Si-H and =Si-H2 bonds, and found that
the addition of H2 gas to the plasma enhances the passivation of dangling bonds and,
hence, the quality of surface passivation [183].
Generally, within a certain range of [Si-H], Dit is expected to decrease with any
increase in [Si-H] and decrease in [N-H]. This observation could be useful for
optimising the passivation quality of SiNx films. The decrease of Dit with increasing
[Si-H] and the increase of Dit with increasing [N-H] indicates that the formation of
hydrogen bonds at the interface plays an important role in surface passivation [183,
239]. In addition, a correlation has been reported between an increased fixed charge
density Qf and an enhanced [Si-H] bond concentration [250], hence sample D1-80C
and D4-100C with higher level of negative charge can possibly be explained by their
higher level of [Si-H].
Dit at mid-gap has shown to decrease linearly with the increasing [Si-H] bond density
in Figure 5.13, however, Si dangling bonds were reported to produce a near mid-gap
trap state in SiNx with increasing of [Si-H] [249, 251]. Also, SiNx films with non-
detectable Si-H absorption band were found to have improved electrical properties
[172]. Therefore, to some extent, a high [Si-H] could become disadvantageous. More
work needs to be done in investigating the upper limit of [Si-H], where the film
quality and its surface passivation performance starts to decrease or stops increasing
with further increases in [Si-H] bond concentration.
125
Chapter 5 Interface Effects in Metal/SiNx/HgCdTe Structures
Figure 5.11 The IR absorbance spectra of the reference silicon nitride films on Si substrate under four deposition conditions for the MIS structures.
5001000150020002500300035000
0.5
1
1.5
2
2.5 x 104
Wavenumber (cm-1)
Abs
orpt
ion
Coe
ffici
ent (
cm-1
)
D1-80CD1-90CD1-100CD4-100C
Si-N (sym.s)Si-H (w-r)
Si-N (asym. s)
N-H (r)Si-H (s) N-H (s)
11001150120012500
500
1000
1500
2000
2500
3000
3500
Wavenumber (cm-1)
Abs
orpt
ion
Coe
ffici
ent (
cm-1
)
200021002200230024000
500
1000
1500
2000
Wavenumber (cm-1)
Abs
orpt
ion
Coe
ffici
ent (
cm-1
)
320032503300335034003450400
600
800
1000
1200
1400
1600
1800
Wavenumber (cm-1)
Abs
orpt
ion
Coe
ffici
ent (
cm-1
)
126
Chapter 5 Interface Effects in M
etal/SiNx /H
gCdTe Structures
126
Chapter 5 Interface Effects in Metal/SiNx/HgCdTe Structures
Table 5.4 Summary on bond and atomic concentrations calculated for SiNx/Si reference wafers of the SiNx/HgCdTe MIS structures
Sample NH3/ SiH4
SiH4/ NH3
n632.8nm [N]/[Si] [Si-N] (1022 cm-3)
[N-H] (1022 cm-3)
[Si-H] (1022 cm-3)
[Si-Si] (1022 cm-3)
[Si] (1022 cm-3)
[N] (1022 cm-3)
[H] (1022 cm-3)
Total bond
density (1022 cm-3)
Film density (g/cm3)
D1-80°C 1.49 0.67 1.86 1.17 8.40 3.24 1.69 1.59 3.32 3.88 4.93 14.92 2.53
D1-90°C 1.49 0.67 1.90 1.07 8.95 3.36 0.93 2.73 3.84 4.10 4.29 15.97 2.81
D1-100°C 1.49 0.67 1.93 1.02 9.11 3.32 1.59 2.78 4.06 4.14 4.91 16.80 2.94
D4-100°C 1.29 0.77 1.94 0.90 9.03 2.87 1.75 3.43 4.41 3.97 4.63 17.09 3.06
127
Chapter 5 Interface Effects in M
etal/SiNx /H
gCdTe Structures
127
Chapter 5 Interface Effects in Metal/SiNx/HgCdTe Structures
Table 5.5 Summary of results from C-V and IR absorbance analysis on the four SiNx/HgCdTe MIS structures
D1-80C D1-90C D1-100C D4-100C
[N]/[Si] 1.17 1.07 1.02 0.90
n632.8nm 1.86 1.90 1.93 1.94
Flat band voltage Vfb (V) 0.98 0.86 0.71 0.76
Fixed charge density Qf (×10 10 cm-2) -12.86 -8.53 -9.42 -11.48
Hysteresis width VH (V) 0.19 0.08 0.208 0.207
Slow interface charge density Qit (×10 10 cm-2) 3.17 1.03 3.87 4.33
Interface state density Dit_min (×10 10 eV-1cm-2) 5.07 17.47 11.78 4.01
Interface state density Dit (×10 10 eV-1cm-2) at mid-gap 13.65 50.38 17.56 10.57
[H] (1022 cm -3) 0.41 0.35 0.37 0.36
[Si-H] (1022 cm -3) 1.69 0.93 1.59 1.75
[N-H] (1022 cm -3) 3.24 3.36 3.32 2.87
[Si-H]/[N-H] 0.52 0.28 0.48 0.61
Figure 5.12 Relationship between Dit and [Si-H] and [N-H] bond concentrations.
0 10 20 30 40 50 600.5
1
1.5
2
2.5
3
3.5
Dit ( x 1010 eV-1cm-2)
Bon
d de
nsity
( x
1022
cm
-3)
Dit minimum vs [N-H]
Dit at mid-gap vs [N-H]
Dit minimum vs [Si-H]
Dit at mid-gap vs [Si-H]
128
Chapter 5 Interface Effects in Metal/SiNx/HgCdTe Structures
(a)
(b)
Figure 5.13 Dit at mid-gapwith the variations of (a) [H], [N-H], [Si-H] (a) and (b) [Si-H]/[N-H].
0 1 2 3 4 50
10
20
30
40
50
60
Bond density ( x 1022 cm-3)
Dit-
mid
-gap
( x
1010
cm
-2eV
- 1)
[H][N-H][Si-H]
0.2 0.3 0.4 0.5 0.60
10
20
30
40
50
60
Dit-
mid
-gap
( x
1010
cm
-2eV
-1)
[Si-H]/[N-H] ratio
129
Chapter 5 Interface Effects in Metal/SiNx/HgCdTe Structures
5.4 Summary
In this chapter, work is reported on SiNx thin films for surface passivation of HgCdTe
epitaxial layers without the need for a CdTe capping layer, with SiNx/HgCdTe MIS
structures being utilised as a tool during the study.
The interface trap density, Dit, was extracted and examined by analysing high-
frequency and low-frequency C-V data, as well as by the conductance method. Dit
was considered as the dominant measure in evaluating surface passivation
performance and in correlating passivation quality with other film properties.
Analysis of the SiNx/n-Hg0.68Cd0.32Te MIS structures indicated that Si-rich SiNx films
deposited at 100 °C exhibit electrical characteristics suitable for surface passivation of
HgCdTe-based devices, with interface trap densities in the range of mid-1010 cm-2eV-1,
and fixed negative interface charge densities of ~ 1011 cm-2.
The relationship between different bond concentrations in the SiNx film and surface
passivation performance was also presented using the method presented in
Section 3.4. The Si-H and N-H bond concentrations were found to be directly
correlated with passivation performance, such that SiNx films with a combination of
high [Si-H] and low [N-H] bond concentrations found to be suitable as electrical
passivation layers on HgCdTe.
130
Chapter 6 Conclusions and Future Work
6 Conclusions and Future Work
6.1 Summary and Conclusions
The principal objectives of this thesis have been to study interface effects in HgCdTe
materials and devices. In the process of achieving the objectives outlined in Chapter
1, the main results achieved and conclusions drawn are as follows.
A series of in-situ monitoring characterisations and ex-situ characterisations on
HgCdTe and its passivation materials have been conducted in order to examine the
properties of HgCdTe, its passivants of CdTe and SiNx, and their interfaces of
CdTe/HgCdTe and SiNx/HgCdTe. In particular, some of the characterisations were
able to be conducted and analysed either before or after passivation, such as RHEED,
HR-MSA and XRD, to investigate the influence of passivation.
The effectiveness of MBE low-temperature grown CdTe passivating film was studied
using photoconductive devices. As expected, the HgCdTe photoconductors with their
surface all surfaces passivated with CdTe show significantly higher photoresponsivity
than those without sidewall passivation, which indicates the effectiveness of low-
temperature MBE grown CdTe as a passivation layer in reducing surface
recombination velocity. Characterisation of the responsivity differences between the
photoconductors with and without the sidewall CdTe passivation offers a potential
method to measure the interface/surface recombination velocity. This has been
demonstrated by extracting the value of surface recombination velocity using a
commercial device modelling package (Synopsis Sentaurus) to fit responsivity data
for all surfaces passivated and partially passivated devices.
Low-temperature MBE grown CdTe passivated HgCdTe gated photodiodes were
employed as a tool to investigate device performance, with the band bending at the
surface being influenced by gate bias. By increasing the gate bias from 0 to 1.5 V, the
RdA improved by an order of magnitude, whereas RdA was reduced by an order of
magnitude when decreasing the gate bias to -1.5 V. As the gate bias was increased
from -1.5 V to 1.5 V, the p-type surface depletion region widened within this gate bias
regime and TAT current was reduced, resulting in a monotonic increase in RdA. This
131
Chapter 6 Conclusions and Future Work
was due to the field induced junction formed under the gate, which widened the
depletion area and increased the barrier to tunnelling at the surface.
In this thesis work has been carried out to investigate SiNx thin films for surface
passivation of HgCdTe epitaxial layers without the need for a CdTe capping layer.
Low-temperature (80 °C - 130 °C) deposited SiNx films in this thesis were deposited
employing the Sentech SI500D ICPECVD system with a high-density and low ion
energy plasma source. The low ion energy of the plasma source enables the SiNx film
to be deposited on the HgCdTe without significant surface damage.
In order to investigate suitable SiNx film deposition conditions for HgCdTe surface
passivation, the influence of ICP power on the quality of the deposited SiNx films was
assessed through the IR absorbance of the films. The absorbance spectrum of each
film was measured on the day of the deposition and was regularly monitored over a
six month period, with the films exposed to a standard laboratory atmosphere. The
results indicate that the SiNx/CdTe/GaAs sample C5-SiNx deposited using a relatively
high ICP power of 600 W appeared to be porous and more susceptible to oxidation
under conventional ambient conditions, with the presence of the Si-O stretching peak
appearing near 1080 cm-1. It is noted that deposition conditions C2, C3, C4, D1 and
D4 have demonstrated excellent long-term stability in terms of the IR absorbance
peaks associated with exposure to O2 and H2O in a laboratory atmosphere, with no
evidence of the Si-O oxidation peak in the IR spectra.
The influence of NH3/SiH4 flow ratio on SiNx film properties, such as refractive
index, film composition, deposition rate, IR absorbance and bonding configuration,
were investigated on Si substrates before being applied on HgCdTe substrates. A
series of films were deposited with a fixed SiH4 flow rate of 6.9 sccm and various
NH3 flow rates of 12.4, 10.3, 8.2 and 6.1 sccm at low substrate temperatures (80 °C-
100 °C). Within the investigated range of NH3/SiH4 flow ratio from 0.88 to 1.80, the
[N]/[Si] ratio was found to decrease and n632.8nm to increase with the NH3/SiH4 flow
ratio. The dependence of the SiNx film composition on the NH3/SiH4 flow ratio is as
expected, and can be readily explained by the kinetics of dissociation processes and
the free radical sticking coefficient values. The film density ρ was seen to increase
with elevated deposition temperature, and the defect density is expected to decrease
with increasing film density. The trends of deposition rate and film density with
NH3/SiH4 flow ratio were found to be non-linear. The general trends in bonding
132
Chapter 6 Conclusions and Future Work
configuration are that [N-H], [N] and [H] all increase with NH3/SiH4 flow ratio
whereas [Si-H] and [Si] decrease with NH3/SiH4 flow ratio.
Film composition, deposition rate and IR absorbance can be correlated with the
polyamine concentration in the plasma. For a NH3/SiH4 flow ratio in the range of 0.88
to 1.19, a dramatic increase in deposition rate and [N]/[Si] ratio was observed,
suggesting a change in the dominant radical from disilane to polyaminosilanes. In
terms of film IR absorbance, when the NH3/SiH4 flow ratio increased from 0.88 to
1.19, a significant drop in the area of the Si-H (stretching) peak (~ 2180 cm-1) was
observed, also suggesting a change in the dominant radicals from disilane to
polyaminosilanes. Films with a high NH3/SiH4 flow ratio of 1.80 contained little or no
Si-H bonding and enhanced N-H bonding in the IR absorption spectra, suggesting that
Si[NH2]3 was the principal film precursor, with suppressed Si2H6 in the plasma during
film deposition.
SiNx/n-Hg0.68Cd0.32Te MIS structures were utilised as a tool to study the interface
between SiNx and HgCdTe, and the interface trap density Dit was considered as the
primary parameter to evaluate surface passivation performance and in correlating
passivation quality with other film properties. SiNx films were deposited on HgCdTe
wafers under different conditions, and characterisation of the SiNx/n-Hg0.68Cd0.32Te
MIS structures was carried out to extract Dit by analysing high-frequency and low-
frequency C-V data and by the conductance method. The observed Dit characteristics
manifest a U-shaped distribution over the HgCdTe bandgap, with sample D4-100C
indicating the lowest Dit over most of the bandgap range. This indicates that the
deposition conditions corresponding to sample D4-100C, which was deposited under
Si-rich conditions with a lower NH3/SiH4 flow ratio, provides the best results and can
be employed to passivate HgCdTe based devices without the need for a CdTe capping
layer. The SiNx deposition conditions for sample D4-100C were found to result in a
SiNx/HgCdTe MIS structure characterised by a negative fixed charge density of -1.2 ×
1011 cm-2, a slow interface trap density of 1.6 × 1011 cm-2, and a minimum fast
interface trap density Dit of 4 × 1010 cm-2eV-1. These results represent a significant
improvement on the best reported ECR-PCVD deposited SiNx films on HgCdTe,
which indicated a negative fixed charge density of -1.4 × 1011 cm-2 and an interface
trap density Dit of 1 × 1011 cm-2eV-1. Thus, the ICPECVD SiNx films deposited at
133
Chapter 6 Conclusions and Future Work
relatively low temperatures (80 °C - 100 °C) in the thesis have been shown to have
significant potential as surface passivation films for HgCdTe-based devices.
The correlation between bond concentration and surface passivation performance has
been studied. [Si-H], [N-H] and [Si-H]/[N-H] were identified as potential measures of
surface passivation performance at the SiNx/HgCdTe interface. An approximately
linear relationship is observed between the Dit taken at mid-gap and [Si-H] bond
density of the SiNx film. SiNx films with high [Si-H] and low [N-H] bond
concentrations have been identified as suitable electrical passivation layers. This
could be a useful criteria for optimising the passivation quality of SiNx films for
HgCdTe-based devices. The decrease of Dit with increasing [Si-H] and the increase of
Dit with increasing [N-H] indicates that the formation of hydrogen bonds at the
interface plays an important role in surface passivation.
6.2 Recommendations for future work
In relation to the issues with HgCdTe passivation and the study of interface effects in
this thesis, the following suggestions are noted as requiring further investigation:
1. The nearly lattice-matched CdTe surface passivation for HgCdTe and the grading
at the CdTe/HgCdTe interface can be beneficial for device performance;
however, the additional layer of CdTe may hinder the diffusion of hydrogen
during the PECVD SiNx deposition in a dual-layer passivation process. Thus,
additional work needs to be undertaken on a comparison of the passivation
performance of dual-layer passivation of SiNx/CdTe/HgCdTe with single-layer
passivation of SiNx/HgCdTe and CdTe/HgCdTe.
The HR-MSA technique has shown a decrease of bulk electron mobility after
MBE CdTe passivation deposited at ~ 100 °C compared to prior to
passivation, suggesting that the low-temperature MBE CdTe growth and post
in-situ annealing conditions can be improved.
More work needs to be carried out on passivating nBn detectors and other
photodetector devices to compare the passivation performance of low-
temperature MBE grown CdTe ( ~ 100 °C) and low-temperature SiNx
( < 100 °C).
134
Chapter 6 Conclusions and Future Work
2. In addition to the bond density analysis from IR absorbance, it would be
worthwhile to correlate the SiNx passivation quality with the analysis via the
following:
transport measurements - in particular, the HR-MSA can provide information
not only on the surface passivation but also on bulk passivation, which could
be a powerful tool in examining hydrogenation related processes;
Photoluminescence measurements;
transient photoconductive decay measurements;
surface recombination velocity - the effective surface recombination velocity
was found to depend strongly on Dit, irrespective of the varied process
parameters, and it depends primarily on Dit rather than the insulator
charge [240];
nano-indentation - the SiNx film densities have been calculated by IR
absorbance analysis in the thesis, which need to be correlated with the results
from nano-indentation;
XPS measurements
3. The mechanisms of SiNx hydrogenated from PECVD have been extensively
studied on silicon substrates, yet there are issues that still remain unclear. There
has been little published work on the mechanisms of hydrogenated SiNx
deposited on HgCdTe and their interface effects. This area is worthy of further
investigation.
4. As to the passivation performance and its correlation with interface traps, more
work could be carried out on the aspects listed below, based on the results from
the thesis:
Dit at midgap has been observed to decrease approximately linearly with
increasing [Si-H] bond density, however, Si dangling bonds were reported to
produce a near midgap trap states in SiNx with increasing [Si-H] [249, 251].
Also, SiNx films with non-detectable Si-H absorption band were found to
have improved electrical properties [172]. Therefore, to some extend, a high
[Si-H] may eventually become a disadvantage. More work could be done in
investigating the upper limit of [Si-H], where the quality of the thin film and
135
Chapter 6 Conclusions and Future Work
its surface passivation performance start to decrease or stop increasing with
an increase of the [Si-H] bond concentration.
With a series of SiNx films with different [N]/[Si] ratio deposited on HgCdTe
wafers, work could be carried out to reveal whether there is any correlation
between film composition and passivation performance, since the literature
has indicated different views on this issue for silicon substrates, and no
relevant work has been published on HgCdTe substrates as yet.
It is possible to attribute the dominant type of interface traps for SiNx
samples deposited at different substrate temperatures by their capture cross
section characteristics, by using small-pulse DLTS [252]. Further
measurements and analysis on SiNx/HgCdTe MIS structures with PECVD
SiNx deposited under various conditions, combined with theoretical
modelling, would be useful in order to explore the nature of the interface
traps at the interface.
5. In addition to the conductance method, quasi-static C-V under triangular voltage
sweep with varied sweep rates could be useful in studying the interface traps.
Under varied sweep rates, interface states will show different characteristics when
being filled and emptied. Modelling of quasi-static C-V on the MIS capacitors
can be performed by using the simulation tool of Synopsis Sentaurus.
6. The deposition parameters for SiNx passivation on HgCdTe could be optimised
through a statistical experimental design [199] and/or a central composition
experiments (CCE) [240]. With the aid of genetic algorithm (GA) optimised
generalized regression neural network (GRNN) [199], the effect of deposition
parameters on the quality of surface and/or bulk HgCdTe passivation could be
characterized systematically, and hence be optimised.
Defect passivation of dangling bonds can be enhanced by the addition of a
further hydrogen source to the plasma gas [183]. Additional H2 could be
added to increase the hydrogen content in the SiNx film. The composition of
the film would be affected by the additional H2, as well as n632.8nm and
[N]/[Si].
As to the modulation of fixed charge in the passivant, gamma irradiation on
the SiNx film has been found to have an impact on the fixed charge
density [253], which could be a possible method in manipulating the fixed
136
Chapter 6 Conclusions and Future Work
charge density in the film and, hence, improving passivation quality. Work
on dual-layer passivation of SiNx/SiO2/HgCdTe has reported that different
signs and values of the fixed charge are possible [254], which could be
another possible direction.
The effect of thickness of SiNx passivant could also be examined.
7. The work presented in thesis has focused on SiNx surface passivation for MWIR
n-HgCdTe, and more work could be carried out on CdTe/p-HgCdTe, SiNx/p-
HgCdTe, and on HgCdTe layers with different mole fractions.
137
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163
Appendix A: HgCdTe Properties
Appendix A: HgCdTe Properties
A.1 Bandgap
CdTe is a semiconductor with a relatively wide energy bandgap of 1490 meV at room
temperature, and HgTe is a semimetal with an inverted bandgap of -144 meV.
Hg1−xCdxTe, as a compound semiconductor, has an energy bandgap, Eg, that lies
between these two extremes of bandgap. The conduction band minimum and valence
band maximum are located at the Г-point of the Brillouin zone [255], making
HgCdTe a direct bandgap semiconductor, as shown in Figure A1.1. This inherent
property leads to a large photon absorption co-efficient and a high quantum efficiency
of HgCdTe.
One of the most widely used bandgap energy, Eg (eV), expressions for HgCdTe is a
function of the mole fraction, x, and temperature T (K), derived empirically by
Hansen et al., which is expressed as [256]
( ) 324 832081021103559313020 x. x. x) - T( . x . . x,TEg +−×++−= − (A1.1
)
where Eq.(A1.1) is plotted in Figure A1.2. It can be seen that at 77K Eg varies from
approximately -0.3 eV to 1.6 eV when adjusting x from 0 to 1. The relationship
between Eg and the cut-off wavelength of the material, λc, is given by:
g g c E
.Ehc λ 2451
== (A1.2)
Figure A1.3 shows the relationship between HgCdTe cut-off wavelength and x using
Eq.(A1.1 ) and Eq.(A1.2), which illustrates one of the advantages of HgCdTe - its cut-
off wavelength being adjustable by varying x from the region of shortwave infrared
(SWIR) (0.75 to 3 μm) to mid-wave infrared (MWIR) (3 to 5 μm) and long-wave
infrared (LWIR) (8 to 14 μm).
164
Appendix A: HgCdTe Properties
Figure A1.1 Schematics of energy bandgap of (a) HgTe. (b) HgTe-CdTe transition (zero bandgap). (c) CdTe. The Г6 and Г8 point refer to the electron band and light/heavy hole band, respectively.
Figure A1.2 Bandgap of Hg1−xCdxTe as a function of cadmium composition, x.
0 0.2 0.4 0.6 0.8 1-0.5
0
0.5
1
1.5
2
Cadmium composition (x)
Ener
gy g
ap (e
V)
77 K200K300K
EnergyΓ8
e
Γ6 lh
Γ8 hh
EnergyΓ6
e
Γ8 hh
Γ8 lh
Γ8 hh
Γ6 e
Γ8 lh
Energy
(a) (b) (c)
165
Appendix A: HgCdTe Properties
Figure A1.3 Cut-off wavelength of Hg1−xCdxTe as a function of cadmium composition, x.
A.2 Lattice constant The lattice mismatch between CdTe and HgTe is very small, and the lattice constant
across the entire composition range changes by only 0.3%, making multilayer
crystalline growth possible. The relationship between lattice constant of Hg1−xCdxTe,
a (Å), and mole fraction, x, given by Higgins et al., is expressed as [257]
x - x x a 32 0057.00168.00084.04614.6 ++= (A1.3)
which is plotted in Figure A1.4.
Figure A1.4 Lattice constant of Hg1−xCdxTe as a function of cadmium composition, x.
0.2 0.3 0.4 0.5 0.6 0.7 0.80
2
4
6
8
10
12
14
16
Cadmium composition (x)
Cut
-off
wav
elen
gth
(µm
) 77 K200K300K
LWIR
MWIR
0 0.2 0.4 0.6 0.8 16.44
6.45
6.46
6.47
6.48
6.49
6.5
Cadmium composition (x)
Latti
ce c
onst
ant (
A)
o
166
Appendix A: HgCdTe Properties
A.3 Intrinsic carrier concentration The most widely used expression for intrinsic carrier concentration, ni, of HgCdTe is
an empirical expression given by Hansen and Schmit as [258]:
( ) ( ) )2
exp10001364.0001753.082.3585.5 234314
kTE
(TExTTx x, Tn g//gi −××××−+−=
(A1.4)
which is valid for 0.16 < x < 0.7 and 50 K < T < 359 K. k is the Boltzmann constant
(eV K−1). The change of ni as a function of x for varying temperatures is illustrated in
Figure 3.5. At a given temperature, ni decreases with increasing x (that is, with
increasing bandgap).
Figure A1.5 Intrinsic carrier concentration of Hg1-xCdxTe as a function of x for T = 77 K, 150 K, 200 K and 300 K.
A.4 Mobility The mobility of electrons and holes in HgCdTe vary with mole fraction and
temperature. The empirical expression for the electron mobility in HgCdTe with
0.2 < x < 0.6 is given by Rosbeck et al. as [259]:
ae Zb
2
8109×=µ (A1.5a)
where 6.02.0
=
xa (A1.5b)
0.1 0.2 0.3 0.4 0.5 0.61010
1012
1014
1016
1018
Cadmium composition (x)
Intri
nsic
car
rier c
once
ntra
tion
(cm
-3)
77 K 150 K200 K
300 K
167
Appendix A: HgCdTe Properties
75.02.0
=
xb (A1.5c)
and
>
≤−−
×=
KTT
KTTZ
50
50352600
1018.107.2
5
(A1.5d)
The electron mobility as a function of temperature (T > 50 K) for varying x is plotted
in Figure A1.6 using Eq.(A1.5).
Figure A1.6 Electron mobility in Hg1-xCdxTe as a function of temperature for varying mole fraction, x.
The mobility of holes in HgCdTe is approximately two orders of magnitude lower
than that of the electrons, due to the material’s small electron effective mass, being
expressed as:
eh µµ 01.0= (A1.6)
A more detailed description of the hole mobility is presented in the work by Yadava
et al. [260].
50 100 150 200 250 300103
104
105
106
Temperature (T)
Ele
ctro
n m
obilit
y (c
m-2
V-1
s-1
)
x = 0.20
0.25
0.300.350.40
168
Appendix B: Deposition Parameters Concerning High-temperature Deposited SiNx Films
Appendix B: Deposition Parameters Concerning High-
temperature Deposited SiNx Films
The properties of SiNx and its passivation quality can be affected by the gas reactants
used [261, 262], the mode of PECVD reactor [84, 167, 263], and specific details of
the deposition conditions [174, 175, 179].
There are several chemistries available for plasma deposition of SiNx, the gas
reactants utilised in deposition have a significant effect on the quality of surface
passivation and film properties [261, 262]. For plasma enhanced deposited silicon
nitride, films are typically deposited using SiH4 and other reactant gases, such as NH3
and/or N2. Different plasma chemistries result in different radicals responsible for the
film deposition and hence film properties [172, 212, 264]. The hydrogen content
incorporated in silicon nitride films decreases significantly if N2 is employed rather
than NH3 [262, 265].
Secondly, the quality of surface passivation and film properties were found to be
strongly affected by the mode of PECVD [84, 167, 263]. SiNx films grown using
direct high-frequency or remote PECVD have lower surface recombination velocity
than those using low-frequency direct PECVD. The low-frequency systems (in the 10
- 500 kHz range) tend to produce comparatively poorer and unstable passivation [266].
The SiNx films discussed in this chapter were deposited in a high-frequency
(13.56 MHz) direct ICPECVD system.
In addition, deposition conditions have an influence over film properties and the
quality of surface passivation; hence deposition conditions need to be tuned to achieve
specific film characteristics such as film composition, film refractive index and
hydrogen concentration.
Deposition temperature is a crucial parameter, and may be independent of other
factors [190, 263, 266, 267]. For SiNx films deposited on Si, Lauinger reported a
substrate temperature of around 350 °C as the optimum surface passivation, which is
independent of the deposition technique. Cuevas et al. reported the deposition
temperature to be the dominant parameter in achieving low surface recombination
velocity for silicon-based solar cells, with SiNx films deposited at around 400 °C
169
Appendix B: Deposition Parameters Concerning High-temperature Deposited SiNx Films
providing a high quality surface passivation [263]. The difference in the reported
optimum temperature is possibly due to the difference between the hot-plate
temperature and wafer temperature. For example, a hot-plate temperature of 400 °C
leads to a wafer temperature of around 350 °C if the wafer is only bottom
heated [190].
The ratio of NH3/SiH4 gas flow rate has been found to be another crucial parameter in
determining film properties and passivation performance. Varying the NH3/SiH4 flow
ratio will result in changes in film stoichiometry and passivation performance. In
general, Si-rich SiNx films can be obtained by reducing the NH3/SiH4 flow ratio,
whereas N-rich films will result from increasing the ratio [183]. The influence of SiNx
stoichiometry on the interface quality with the semiconductor has been the subject of
extensive studies, although different trends can be seen in the literature when using
different modes of PECVD, gas reactants, deposition temperatures, surface treatment
and post-annealing steps [174, 175, 179, 180, 189, 190].
Generally speaking, in terms of effective surface recombination velocities at the
SiNx/Si interface, as-deposited Si-rich SiNx films provide a superior surface passivant
in comparison to as-deposited N-rich films if there is no high temperature processing
involved, whereas N-rich films tend to give the best properties after high temperature
annealing step(s) resulting in denser and thermally more stable films than Si-rich SiNx
films [175, 207]. After the high-temperature treatment, N-rich hydrogenated films are
considered to be denser and thermally more stable than the Si-rich ones.
In the literature, Si-rich SiNx with n > 2.3 was reported by Lauinger et al. [167, 239,
266] to have the best surface passivation using either a remote or high-frequency
(13.56 MHz) PECVD system. The gas reactants used in the PECVD were pure SiH4
and NH3 at a temperature of around 375 °C. Soppe et al. obtained their optimum
surface passivation with Si-rich films with refractive indices between 2.3 to 2.4
deposited at temperatures between 350 °C and 400 °C, using gas ratios of NH3/SiH4 ≈
1.1 and N2/SiH4 ≈ 1.3, respectively. Silicon nitride deposited with NH3 and SiH4 gas
reactants with a refractive index of about 2.1 showed further improvement after
thermal annealing, while films with refractive index of 2.3 appear to be very stable
under thermal annealing [93]. The silicon-rich SiNx films (n > 3) reported by Mäckel
170
Appendix B: Deposition Parameters Concerning High-temperature Deposited SiNx Films
and Lüdemann show higher [Si-H] bond density, and hence better passivation of the
dangling bonds at the silicon surface [183].
Stoichiometric Si3N4 films (n ≈ 1.95) were reported by Schmidt and Kerr to give the
best surface passivation for silicon using 4.5 % SiH4 in N2 and NH3 in parallel-plate
reactor (Oxford Plasma Technology, Plasmalab 80+) [268]. Sanjoh et al. [174] also
found near stoichiometric films to give the best surface passivation for silicon. The
films were deposited at 300 °C, using a conventional, capacitively-coupled plasma
CVD system with a 13.56 MHz RF generator using SiH4+N2+NH3 mixture. A rapid
drop in Dit with increasing x up to 1.33 (stoichiometric film) was observed. For the
case of N-rich films (x > 1.33), they observed a slight increase in the minimum Dit.
The decrease in excess silicon or nitrogen atoms from a non-stoichiometric SiNx film
was found to be effective for the reduction of Dit. In addition, there are other
publications from different research groups that have shown that SiNx films with
n633nm between 1.95 (stoichiometric film) and 2.4 (Si-rich film) deposited at around
400 °C can give high quality surface passivation for solar cells [93, 167, 190, 269].
Ghosh et al. observed that N-rich nitride films (x = 1.63) gave the best surface
passivation with the lowest Dit [179]. The defect density caused by silicon dangling
bonds was noted to increase with increasing silicon content in the films. As nitrogen
atoms can terminate silicon dangling bonds, the minimum Dit was found to decrease
when the nitrogen content increased, and increased with increasing silicon content.
Basa et al. reported that the minimum interface state density decreased with an
increase in [N]/[Si] by analysing C-V characteristics. The films were deposited at
250 °C using SiH4+N2+NH3 mixture in a capacitively coupled parallel-plate
commercial PECVD system [182].
171