understanding the growth of iii-v nanowires incorporated ...686916/s4315954_final... · iii-v...
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Understanding the Growth of III-V Nanowires Incorporated
with In by Molecular Beam Epitaxy
Chen Zhou
Bachelor of Engineering
A thesis submitted for the degree of Doctor of Philosophy at
The University of Queensland in 2017
School of Mechanical and Mining Engineering
I
Abstract
III-V semiconductor nanowires have attracted a great attention due to their peculiar
electronic and optical properties. Among them, GaAs semiconductor nanowires, with direct
band gap, high carrier mobilities (than Si) and the heterojunction formation with other III-V
semiconductor nanowires, such as InAs and AlGaAs, have potential for application in
future nanoscale devices. To realize such optoelectronic devices fabrication, the III-V
nanowires growth must be under control to enable their effective integration into devices,
which can be achieved by Au catalyzed epitaxial nanowires growth under the vapor-liquid-
solid (VLS) mechanism.
Compared with binary III-V nanowires, ternary nanowires have been attracting an
increasing interest as they allow the tunability of desired band gap for different applications
by modulating the composition fraction of their alloys. In particular, InGaAs nanowires
have been demonstrated to be technologically important for a wide range of applications,
attributed to their tunable band gap ranging from near-infrared to infrared regions by tuning
the InGaAs alloy composition, which makes them constitute the ideal material system for
numerous optoelectronic devices, such as light-emitting diodes and nanolasers.
Accordingly, the motivation to investigate ternary InGaAs nanowires system and its related
binary GaAs nanowires system is triggered for their optoelectronic applications in the
future.
In this thesis, binary GaAs nanowires system was first studied. With the help of Au
nanoparticles, GaAs nanowires were grown on GaAs {111}B substrates. By modulating the
growth parameters, such as growth temperature, V/III ratio and growth duration, the
morphological and structural characteristics of grown GaAs nanowires were investigated.
Through extensive electron microscopy investigations, we found that by lowering the
group-V flux to the low V/III ratio, nanowire growth is As-limited and thermodynamically
controlled, leading to the slow nanowire growth and defect-free wurtzite structured grown
nanowires. On the other hand, we found that by prolonging the growth duration, the
structural quality of GaAs nanowires was significantly enhanced from defected to defect-
free wurtzite structure.
Moreover, the growth behavior and compositional characteristics of ternary InxGa1-xAs
nanowires were studied by tuning the In concentration from 50 to 85 at.%. With extensive
cross-sectional study of the grown nanowires by advanced electron microscopy, we found
II
that when x = 0.5, core-multishell structure spontaneously formed at the nanowire bottoms
and Ga concentration in nanowire cores increases towards the nanowire tops; and when x
= 0.85, InGaAs nanowires formed the core-shell structure, with the In-rich core and the
Ga-enriched shell, in which the composition of the cores and shells were varied at varied
nanowire regions. The fundamental reasons behind these new phenomena were
investigated and the corresponding growth mechanism was unveiled.
III
Declaration by author
This thesis is composed of my original work, and contains no material previously published
or written by another person except where due reference has been made in the text. I
have clearly stated the contribution by others to jointly-authored works that I have included
in my thesis.
I have clearly stated the contribution of others to my thesis as a whole, including statistical
assistance, survey design, data analysis, significant technical procedures, professional
editorial advice, and any other original research work used or reported in my thesis. The
content of my thesis is the result of work I have carried out since the commencement of
my research higher degree candidature and does not include a substantial part of work
that has been submitted to qualify for the award of any other degree or diploma in any
university or other tertiary institution. I have clearly stated which parts of my thesis, if any,
have been submitted to qualify for another award.
I acknowledge that an electronic copy of my thesis must be lodged with the University
Library and, subject to the policy and procedures of The University of Queensland, the
thesis be made available for research and study in accordance with the Copyright Act
1968 unless a period of embargo has been approved by the Dean of the Graduate School.
I acknowledge that copyright of all material contained in my thesis resides with the
copyright holder(s) of that material. Where appropriate I have obtained copyright
permission from the copyright holder to reproduce material in this thesis.
IV
Publications during candidature
Peer-viewed papers:
1. Zhou, C.; Zheng, K.; Liao, Z.; Chen, P.; Lu, W.; Zou, J., Phase Purification of GaAs
Nanowires by Prolonging the Growth Duration in MBE. J. Mater. Chem. C 2017, 5,
5257-5262.
2. Zhou, C.; Zheng, K.; Lu, Z.; Zhang, Z.; Liao, Z.; Chen, P.; Lu, W.; Zou, J., Quality
Control of GaAs Nanowire Structures by Limiting as Flux in Molecular Beam
Epitaxy. J. Phys. Chem. C 2015, 119, 20721-20727.
3. Wei, D.-D.; Shi, S.-X.; Zhou, C.; Zhang, X.-T.; Chen, P.-P.; Xie, J.-T.; Tian, F.; Zou,
J. Formation of GaAs/GaSb Core-Shell Heterostructured Nanowires Grown by
Molecular-Beam Epitaxy. Crystals 2017, 7, 94. 4. Lu, Z.; Zhang, Z.; Chen, P.; Shi, S.; Yao, L.; Zhou, C.; Zhou, X.; Zou, J.; Lu, W.;
Bismuth-induced Phase Control of GaAs Nanowires Grown by Molecular Beam
Epitaxy. Appl. Phys. Lett. 2014, 105, 162102.
5. Shi, S.; Lu, Z.; Zhang, Z.; Zhou C.; Chen, P.; Zou, J.; Lu W.; Morphology and
Microstructure of InAs Nanowires on GaAs Substrate Grown by Molecular Beam
Epitaxy. Chin. Phys. Lett. 2014, 31 (9): 098101.
V
Publications included in this thesis
Zhou, C.; Zheng, K.; Lu, Z.; Zhang, Z.; Liao, Z.; Chen, P.; Lu, W.; Zou, J., Quality Control
of GaAs Nanowire Structures by Limiting as Flux in Molecular Beam Epitaxy. J. Phys.
Chem. C 2015, 119, 20721-20727. – incorporated as Chapter 4.
Contributor Statement of contribution
Chen Zhou (Candidate) Carried out characterization (80%)
Carried out data analyses (60%)
Wrote the paper (60%)
Kun Zheng Wrote and edited paper (10%)
Supervised the project (20%)
Zhengyu Lu Carried out sample synthesis (50%)
Zhi Zhang Carried out data analyses (10%)
Zhiming Liao Carried out characterization (20%)
Pingping Chen Carried out sample synthesis (50%)
Designed experiments (40%)
Supervised the project (10%)
Wei Lu Designed experiments (20%)
Supervised the project (10%)
Jin Zou Designed experiments (40%)
Supervised the project (60%)
Carried out data analyses (30%)
Wrote and edited paper (30%)
VI
Zhou, C.; Zheng, K.; Liao, Z.; Chen, P.; Lu, W.; Zou, J., Phase Purification of GaAs
Nanowires by Prolonging the Growth Duration in MBE. J. Mater. Chem. C 2017, 5, 5257-
5262. – incorporated as Chapter 5.
Contributor Statement of contribution
Chen Zhou (Candidate) Designed experiments (20%)
Carried out characterization (80%)
Carried out data analyses (60%)
Wrote the paper (60%)
Kun Zheng Carried out data analyses (10%)
Wrote and edited paper (10%)
Supervised the project (20%)
Zhiming Liao Carried out characterization (20%)
Pingping Chen Carried out sample synthesis (80%)
Designed experiments (40%)
Supervised the project (10%)
Wei Lu Designed experiments (10%)
Carried out sample synthesis (20%)
Supervised the project (10%)
Jin Zou Designed experiments (30%)
Supervised the project (60%)
Carried out data analyses (30%)
Wrote and edited paper (30%)
VII
Contributions by others to the thesis
No contributions by others.
Statement of parts of the thesis submitted to qualify for the award of another degree
None
VIII
Acknowledgements
First, I would like to express my sincere gratitude for my principle supervisor Prof. Jin
Zou at The University of Queensland, who gave me the chance to carry out this PhD
research project as an undergraduate student then. He always passes positive attitude to
us PhD students, no matter what problems we faced during the research. Apart from the
strong research skills, the most important thing is that he trained us to build a strong mind,
so that we can have inner peace in facing any difficulties in life. Besides, I would also like
to thank my co-supervisor, Prof. Kun Zheng, a visiting professor from Beijing University of
Technology. He has a strong logic thinking ability so that he can cut to the chase very
quickly, which helps me a lot when I was struggling in experimental data analyses. My
other co-supervisors, Prof. Ping-Ping Chen and Prof. Wei Lu at Shanghai Institute of
Technical Physics, also gave me a lot of support in designing experiment and carrying out
sample syntheses. Without their help and support, this PhD research project cannot be
fulfilled.
Second, I would like to acknowledge all my loverly colleagues: Dr. Zhi Zhang, Dr Hongyi
(Justin) Xu, Dr Guang Han, Dr Wen Sun, Dr Zhi-Gang Chen, Dr Lihua Wang, Dr Lei Yang,
Dr Min Hong, Dr Yichao Zou, Ms Mun Teng (Abby) Soo, Mr Zhiming Liao, Mr Liqing
Huang, Mr Han Gao, Mr Xiang Sun, Mr Van Nguyen, Mr Xiaolei Shi, Mr Weidi Liu, Ms
Yuzhe Yang and Mr Haining Luo in our research group at The University of Queensland.
Thank you all for your kind share of your research experience and skills.
Besides, thanks the Centre of Microscopy and Microanalysis (CMM) at The University of
Queensland for providing those fabulous facilities to support our experiments. Thanks all
the CMM staff for your patient training and technical support.
Last but not least, thanks all my dear families and friends who are always there for me
through thick and thin. Your company and selfless love are the power that keeps me
moving forward.
IX
Keywords
GaAs nanowires, InGaAs nanowires, vapor-liquid-solid mechanism, vapor-solid
mechanism, growth duration, structural quality, phase segregation, core-shell, MBE
Australian and New Zealand Standard Research Classifications (ANZSRC)
ANZSRC code: 100712, Nanoscale Characterisation, 60%
ANZSRC code: 100706, Nanofabrication, Growth and Self Assembly, 30%
ANZSRC code: 091203, Compound Semiconductors, 10%
Fields of Research (FoR) Classification
FoR code: 0912, Materials Engineering, 50%
FoR code: 1007, Nanotechnology, 50%
X
Table of Contents
Chapter 1 Introduction ......................................................................................................... 1
1.1 Background ................................................................................................................ 1
1.2 Objectives and scope ................................................................................................. 2
1.3 Thesis outline ............................................................................................................. 3
Chapter 2 Literature Review ................................................................................................ 5
2.1 Introduction ................................................................................................................. 5
2.2 Synthesis of nanowires ............................................................................................... 6
2.2.1 Vapor-liquid-solid mechanism .............................................................................. 7
2.2.2 Vapor-solid-solid mechanism ............................................................................. 13
2.2.3 Vapor-solid mechanism ...................................................................................... 15
2.3 Morphology of III-V nanowires .................................................................................. 16
2.3.2 Effect of catalysts ............................................................................................... 17
2.3.2 Effect of nanowire density .................................................................................. 21
2.3.2 Effect of growth temperature .............................................................................. 23
2.3.3 Effect of V/III ratio ............................................................................................... 26
2.3.4 Effect of growth duration .................................................................................... 31
2.4 Structural characteristics of III-V nanowires.............................................................. 31
2.4.1 Crystal structure of III-V nanowires .................................................................... 31
2.4.2 Side facets of III-V nanowires ............................................................................. 33
2.4.3 Effect of catalysts diameter ................................................................................ 34
2.4.4 Effect of growth direction .................................................................................... 37
2.4.5 Effect of growth parameters ............................................................................... 39
2.5 Ternary III-V nanowires ............................................................................................ 43
2.5.1 Growth behavior ................................................................................................. 44
2.5.2 Compositional characteristics ............................................................................. 45
2.6 Unclear issues .......................................................................................................... 48
2.6.1 About GaAs nanowires ....................................................................................... 48
2.6.2 About InGaAs nanowires .................................................................................... 48
Chapter 3 Methodology...................................................................................................... 65
3.1 Introduction ............................................................................................................... 65
3.2 Epitaxial growth ........................................................................................................ 66
XI
3.2.1 Molecular beam epitaxy ..................................................................................... 66
3.2.2 Growth procedures ............................................................................................. 69
3.3 Scanning electron microscopy .................................................................................. 70
3.4 Transmission electron microscopy ........................................................................... 76
3.4.1 Imaging mode ..................................................................................................... 77
3.4.2 Diffraction mode ................................................................................................. 80
3.4.3 Scanning mode .................................................................................................. 81
3.4.4 Analytical mode .................................................................................................. 82
3.5 Sample preparation .................................................................................................. 84
3.5.1 SEM sample preparation .................................................................................... 84
3.5.2 TEM sample preparation .................................................................................... 85
3.5.3 Cross-sectional sample preparation ................................................................... 85
Chapter 4 Effect of V/III ratio on binary GaAs nanowires ................................................... 91
4.1 Introduction ............................................................................................................... 91
4.2 Quality control of GaAs nanowire structures by limiting As flux in MBE ................... 92
Chapter 5 Effect of growth time on binary GaAs nanowires ............................................. 109
5.1 Introduction ............................................................................................................. 109
5.2 Phase purification of GaAs nanowires by prolonging growth duration in MBE ....... 110
Chapter 6 Effect of growth time on ternary InGaAs nanowires ........................................ 129
6.1 Introduction ............................................................................................................. 129
6.2 Unexpected formation of hierarchical structure in ternary InGaAs nanowires ........ 130
Chapter 7 Composition characteristics of ternary InGaAs nanowires .............................. 150
7.1 Introduction ............................................................................................................. 150
7.2 Composition gradient ternary InGaAs nanowires caused by strain relaxation ........ 151
Chapter 8 Conclusions and recommendations ................................................................ 168
8.1 Conclusions ............................................................................................................ 168
8.2 Recommendations .................................................................................................. 170
XII
List of Figures
Figure 2.1 (a) Schematic phase diagram is between the catalyst material (Au) and growth
material (Si) with red arrows showing the growth process. (b) Schematic illustration of VLS
growth mechanism of Si whisker from reaction of SiCl4 and H2 vapor phases. Black arrows
indicate the impingement of the vapor Si atoms to the catalyst, nanowires sidewall and
substrate and the diffusion of the atoms.
Figure 2.2 (a-c) TEM images show the top regions of the three grown GaAs nanowires
samples. (d-f) Corresponding electron diffraction patterns taken from the nanowires-
catalysts interfaces marked in (a-c).
Figure 2.3 Au-Ga phase diagram.
Figure 2.4 The model of novel VLS growth of a real nanowire with an elongated droplet on
the top.
Figure 2.5 Environmental TEM images (a) showing the Si nanowires growth with
increased growth duration, in which the corresponding ledge flow and growth direction are
illustrated in (b-d). (e) Plot of step edge position versus growth time.
Figure 2.6 Schematic of VSS nanowires growth. (a) Transport of the gas source to the
nanowire surface. (b) Atoms are delivered to the growth interface by surface or bulk
diffusion processes. (c) Atoms are incorporated at the growth interface leading to the
growth.
Figure 2.7 (a) Top-view SEM image of InAs nanowires arrays on a patterned substrate.
(b-c) Zoom-in 45°-tilted SEM images of grown nanowires.
Figure 2.8 (a,c,e,g) High-resolution TEM images of GaAs NWs tips grown with increased
As/Ga ratio. On the left, relevant diffractogram are shown, indicating the local crystal
structure. (b,d,f,h) Typical scanning TEM images from the nanowires tips (in false color to
enhance the detail visibility).
Figure 2.9 Schematic representations and the corresponding observed nanowires
(indicated by arrows) grown by the VLS mechanism with Ga catalysts at the tip of the
nanowires.
Figure 2.10 30°-tilted SEM images of Au-catalyzed GaAs nanowires with Au film thickness
of 0.5, 1, 2, 3, 5 nm, respectively. The inserts are top-view SEM images of corresponding
Au film after annealing.
XIII
Figure 2.11 (a) Radius dependencies of normalized growth rate of nanowires at different
growth temperature with fixed flux rate. (b) Diameter-dependent growth rates of GaAs
nanowires at different V/III ratios.
Figure 2.12 (a) 30°-tilted SEM images of nanowires grown at (a) 500 °C and V/III ratio of
24; (f) 580°C and V/III ratio of 1.2; and (k) 590°C and V/III ratio of 0.8. (b,g,l)
Corresponding TEM images taken from nanowires tops. Corresponding (c,h,m) HRTEM
images, (d,i,h) enlarged TEM images taken form the catalyst-nanowire interfaces, (e,j)
selective area electron diffraction (SAED) patterns and (o) Fast Fourier Transform (marked
in (b,g,l)).
Figure 2.13 (a) Overview and (b) high-resolution TEM images of Zn-doped InP nanowires.
(c) Schematic growth model for periodic twinning in nanowires (the contract angle between
droplet and nanowires is marked).
Figure 2.14 (a) 30°-tilted and (b) 45°-tilted SEM images of GaP nanotrees with 8.9
branches per tree on average. Top-view SEM images of (c) three branches directions and
(d) six branches directions.
Figure 2.15 SEM images of nanowires with their spacing varying from (a) 400 nm, to (b)
500 nm and (c) 1000 nm.
Figure 2.16 Schematic model of nanowires growth in different regimes. (a) Competitive
regime, d < λs, where the distance of neighbouring nanowires d is within the diffusion
length along the substrate λs. (b) Synergetic regime, λs < d < λg (diffusion length of Ga in
vapour), where the substrate collection areas become fully independent and wire-to-wire
interaction in gas phase occurs. (c) Independent regime, d > λg > λs.
Figure 2.17 45°-tilted SEM images of CBE-grown GaAs nanowires at different
temperatures of (a) 480°C, (b) 500°C, (c) 515°C, (d) 535°C and (e) 560°C. (f) Schematic
growth model illustrating nanowires axial growth and radial growth.
Figure 2.18 30°-tilted SEM images of GaAs grown by MOCVD at the varied growth
temperatures. The V/III ratio was 46 for all samples. Scale bars are 1 μm.
Figure 2.19 Surface density and growth rate of self-catalyzed GaAs nanowires as a
function of Ga flux (left panel) and As flux (right panel).
Figure 2.20 Top-view and side-view SEM images of GaAs nanowires grown under varied
V/III ratios (as described by Table 2.1).
Figure 2.21 Growth rates of InAs nanowires versus (a) In-flux for a fixed As-flux, (b) As-
flux with a fixed In-flux.
XIV
Figure 2.22 (a) Axial and radial nanowires growth rate and (b) tapering parameters of
GaAs nanowires with varied V/III ratio or III flow. (c) 40°-tilted SEM images of InAs
nanowires grown under varied V/III ratios. Scale bars are 1 µm.
Figure 2.23 45°-tilted SEM images of GaAs nanowires grown on GaAs {111}B substrates
for (a) 3 min, (b) 10 min and (c) 30 min at a growth temperature of 525°C and a V/III ratio
of ~1.5 in MBE.
Figure 2.24 Crystal structures of GaAs (Ga in purple and As in green). (a) Crystal
structure of zinc blende phase with unit cell on top and projection along <110> on bottom.
(b) Crystal structure of wurtzite phase with 2 2 2 cells on top and projection along
<110> on bottom.
Figure 2.25 SEM images and schematic diagram of a typical GaAs nanowire with rotated
truncated triangular cross section.
Figure 2.26 TEM images viewed along the <110> zone axis of InAs nanowires with
different diameters grown at 420°C. The fraction of wurtzite phase decreases as the
diameter increases: (a) 100%, (b) 97%, (c) 86% and (d) 15% wurtzite. The arrows mark
the planar defects of the grown nanowires.
Figure 2.27 SEM images of InAs nanowires grown at (a) a high V/III ratio of 54, tilled-view
and (b) a low V/III ratio of 15, top-view. (c) 35°-tilted SEM images of inclined nanowires in
(b). (d) Corresponding sketch of inclined-nanowires side-facets. TEM images of (a) a
typical inclined nanowire, and the nanowire top regions viewed along (f) [ ] and (g) [ 0]
zone-axis. (h) EDS spectra taken from the catalyst and nanowire. (i,j) Corresponding
SAED patterns. (k) High-resolution TEM images taken from the nanowire middle region.
Figure 2.28 TEM images of InAs nanowires grown along < >B or < > direction. (a-
d) High magnified TEM images of nanowires grown at varied temperatures and V/III ratios.
The insets are corresponding High-resolution TEM images. (e-h) Corresponding SAED
patterns.
Figure 2.29 Map of crystal structure of InP nanowires in the range of V/III ratio 44 to 700
and growth temperature of 400 to 510°C.
Figure 2.30 TEM images of a GaAs nanowire (a) at initial growth and (b) after nanowire
cooling. The inset in (a) shows the interface between two phases. (c) TEM images of InAs
nanowire superlattices, defined by 60 periods of alternating zinc blende and wurtzite
structure, viewed along the <110> zone axis.
XV
Figure 2.31 Band gaps and corresponding light wavelength λ for selected binary
semiconductors plotted as a function of lattice parameter.
Figure 2.32 45°-tilted SEM images of InGaAs nanowires grown under varied temperature
and V/III ratios, and corresponding STEM HADDF images of cross sections and their EDS
elements maps. Scale bars are 500 nm.
Figure 2.33 (a,d) TEM images of InGaAs nanowires grown at the same V/III and In/Ga
ratio but varied In flux. (b,e) EDS spectra taken from nanowires and catalysts as marked in
(a,d). (c,f) Corresponding TEM images taken from nanowires bottoms and schematic
illustration of nanowires structural and cross-sectional models.
Figure 3.1 Schematic illustration of a top-view of a typical growth chamber in the MBE
system, which is equipped with the RHEED monitor.
Figure 3.2 RHEED patterns (<110> azimuth) and SEM images of (100)-oriented GaP
substrates at (a) 350°C, 550°C and 600°C, respectively.
Figure 3.3 Signals generated when a high-energy beam of electrons interacts with a thin
sample.
Figure 3.4 Schematic illustrations of SEM layout and function (http://www.ammrf.org.
au/myscope/sem/practice/principles/layout.php).
Figure 3.5 Schematic illustration of interaction volume created by electron beam on the
sample surface with electron singles generated from different depths.
Figure 3.6 Schematic illustration of the “edge effect” on the SE yields.
(© http://laser.phys.ualberta.ca/~egerton/SEM/sem.htm)
Figure 3.7 (a) BSE and (b) SE images of a grain of sand that contains Si and Ti.
(©http://www.ammrf.org.au/myscope/sem/practice/principles/imagegeneration.php)
Figure 3.8 Ray diagram of a TEM in (a) imaging mode, creating bright field TEM images
on the viewing screen and (b) diffraction mode, the diffraction pattern from the sample
captured on the viewing screen.
Figure 3.9 (a) BF and (b) DF TEM image of a polycrystalline thin film of Bi. The insets are
the diffraction pattern taken from the one of single crystalline Bi. (c) Schematic illustration
of atomic planes bending at the edge dislocations. At position P, the angle equals to the
Bragg angle of the incidence electron beam. (d) A BF TEM image of cobalt mental.
Figure 3.10 Two-beam imaging condition and many-beam imaging condition and the
corresponding TEM images.
Figure 3.11 Schematic illustration of the interference of the electron beam waves and the
principles of the electron diffraction pattern.
XVI
(http://www.ammrf.org.au/myscope/tem/background/concepts/imagegeneration/diffraction/
beam/)
Figure 3.12 Schematic illustrations of (a) Z-contrast technique in a STEM and (b) the
HAADF detector setting for the Z-contrast imaging.
Figure 3.13 (a) HAADF STEM image of the interface between GaAs and Bi2Te3 viewed
from the respective <110> and <210> zone-axis. (b) Intensity profile of the atom column
marked in (a).
Figure 3.14 (a) Atomic resolution Z-contrast STEM image of Ba1.7Ca2.4Y0.9Fe5O13 film and
corresponding (b) Ba, (c) Ca, (d) Y, (e) Fe and (f) their overlap EDS maps.
Figure 3.15 (a,b) Images of the resin block after baking. The nanowires sample is at the
container bottom. (c) Image of the nanowires sample after the neighbouring resin being
cleaned. (d) Image of the resin after the substrate being removed. Red arrows mark the
sample.
Figure 3.16 Schematic illustration of nanowires cross sections preparation.
Figure 3.17 (a) Image of the prepared “sandwich” sample with the real sample in the
middle. (b) Schematic illustration of the sample adherence to the holder. (c) Sample
loading on the tripot. (d) The sample interface after polishing and (e) attached to the Cu
grid.
XVII
List of Tables
Table 2.1 Growth conditions for GaAs nanowires grown by MBE.
Table 2.2 Common side-facets in wurtzite and zinc blende structured nanowires.
Table 2.3 Surface energy of sidewall facets in both zinc blende and wurtzite phases.
Table 3.1 Comparison of MBE and MOCVD
XVIII
List of Abbreviations
VLS: vapor-liquid-solid
VS: vapor-solid
VSS: vapor-solid-solid
MBE: molecular beam epitaxy
CBE: chemical beam epitaxy
CVD: chemical vapor deposition
MOVCD: metal-organic chemical vapor deposition
SEM: scanning electron microscopy
FE-SEM: field-emission scanning electron microscopy
SE: secondary electrons
BSE: backscattered electrons
TEM: transmission electron microscopy
SAED: selected area electron diffraction
HRTEM: high-resolution transmission electron microscopy
BF: bright-field
DF: dark-field
STEM: scanning transmission electron microscopy
HADDF: high-angular annular dark field
EDS: energy dispersive X-ray spectroscopy
CCD: charge-coupled device
RHEED: reflection high energy electron diffraction
FIB: focused ion beam
1
Chapter 1
Introduction
1.1 Background
With an increasing demand of nanoscale electronic, optoelectronic and electromechanical
devices, semiconductor nanowire has become one of the most potential and promising
materials due to its peculiar electronic and optical properties.1,2 Among them, III-V
semiconductor nanowires, with direct band gap, high carrier mobility and high degree of
2
stoichiometry, have received great attention.3-5 To explore and exploit the properties in
nanowires more effectively and efficiently, realizing the controllable growth of the
nanowires and understanding their structural characteristics have become a rapid growing
trend of research.6-8
GaAs semiconductors, with wide direct band gap (~1.4 eV), high electron mobility (~
8500 cm-2V-1s-1) and large absorption coefficient, have been extensively studied during the
last decade.9,10 Many devices have been demonstrated using these semiconductor
nanowires, such as field-effect transistors,3 photodetectors11 and solar cells.12 One of the
most popular techniques for nanowires growth is the vapour-liquid-solid (VLS) nanowires
growth catalyzed by Au nanoparticles in molecular beam epitaxy (MBE).13-17 However,
compared with other growth technique like metal-organic chemical vapour deposition
(MOCVD), the growth behaviour and structural quality of GaAs nanowires in MBE has not
been comprehensively understood and precisely controlled.
Compared with binary III-V nanowires, ternary III-V nanowires have been attracting
an increasing interest as they allow the tunability of desired band gap for varied
applications by modulating the composition fraction of their alloys.18-19 For example,
InGaAs nanowires have demonstrated to be technologically important for a wide range of
applications, attributed to their tunable band gap ranging from the near-infrared region to
the infrared region by tuning the InGaAs alloy composition,20 which makes it constitute the
ideal material system for numerous optoelectronic devices, such as light-emitting diodes21
and nanolasers.22 However, with spontaneous nanowire lateral growth on the sidewall via
the vapor-solid mechanism, the composition of the InGaAs nanowires can be varied due to
the phase segregation.23 Therefore, understanding the chemical characteristics of the
InGaAs nanowires is critical for achieving uniform nanowires.
1.2 Objectives and scope
This thesis aims to study the growth of III-V semiconductor nanowires (mainly GaAs)
incorporated with In grown on GaAs {111}B substrate MBE. By performing advanced
electron microscopy investigations on grown nanowires, their physical and chemical
characteristics would be revealed.
To have a better understanding of multinary epitaxial nanowires growth, especially
the GaAs/InGaAs nanowires system, binary GaAs nanowires are first studied. By tuning
the growth parameters, such as growth temperature, V/III ratio and growth time, the growth
3
behaviour and mechanism of Au-assisted GaAs nanowires will be carefully investigated
and the nanowires quality should be under control.
Based on the study of GaAs nanowires, the growth behaviour of hetero-epitaxial
InGaAs nanowires growth on GaAs {111}B substrates should be better understood. In
particular, to obtain the uniform ternary nanowires, the chemical characteristics of grown
III-V nanowires will be investigated with varied In/Ga ratios. In particular, InGaAs
nanowires growth with high In/Ga ratios of 50 at.% and 85 at.% were investigated, which
have narrower band gaps and it is possible to extend the cut-off wavelength up to about
2.6 µm.
1.3 Thesis outline
This thesis includes eight chapters in all.
Chapter 1 presents the introduction of this thesis.
Chapter 2 is the critical literature review related to the main focus of this thesis, which
includes the review of (1) semiconductor nanowire growth mechanisms, such as VLS,
vapor-solid-solid (VSS) and vapor-solid (VS) mechanism; influence of growth parameters
such as growth temperature, V/III ratio and growth time on (2) binary nanowires
morphology and (3) their structural characteristics; (4) ternary nanowires growth, such as
III-III-V (e.g. AlGaAs, InGaAs) and III-V-V (e.g. GaAsP) nanowires, and their compositional
characteristics.
In chapter 3, methodology involved in this project will be introduced. First, the MBE
system will be described in details theoretically, which is the growth system for all
nanowires samples in this thesis. Then, analyses techniques such as scanning electron
microscopy (SEM), transmission electron microscopy (TEM) and energy-dispersive X-ray
spectroscopy (EDS) will be described.
Chapters 4 and 5 study the growth behavior and structural quality of the binary GaAs
nanowires. Chapter 4 depicts the quality control of GaAs nanowires by tuning the V/III ratio
and Chapter 5 shows the growth behavior and quality purification of GaAs nanowires by
prolonging the growth duration.
Based on the study of binary GaAs nanowires, the related ternary InGaAs nanowires
growth with varied In concentrations (In/(In+Ga) ratios) are studied and correspondingly
results are shown in chapters 6 and 7.
4
Chapter 6 shows the growth of hierarchical structure in InGaAs nanowires with the
increased growth duration. The nominal In concentration for InGaAs nanowires growth is
50 at.%.
Chapter 7 demonstrates the composition gradient InGaAs nanowires induced by
strain relaxation with nominal In concentration of 85 at.%.
Finally, chapter 8 summarizes the conclusions of this PhD thesis and the contributions
for future research work based on this thesis.
5
Chapter 2
Literature Review
2.1 Introduction
One-dimensional semiconductor nanostructures, such as nanowires, nanotube, nanobelts
and nanorods, whose diameter normally less than 100 nm, can exhibit extraordinary
properties compared with their counterpart bulk materials. For example, with the size
shrinking below a critical value, quantum size effect and quantum tunnelling effect can be
6
significant, which is the foundation of future microelectronic devices, such as solar cells,24
light-emitting diodes25 and transistors.26
Among various semiconductor materials, III-V semiconductor materials show the
advantages of narrow and direct band-gaps, high electron mobility and possibilities to form
heterostructures with other III-V materials. However, the lattice mismatch between two
different III-V materials can induce strain into the heterostructures, which will be
detrimental to the crystal quality of the heterostructures and their corresponding optical
properties.27 Nevertheless, this misfit strain can be largely released in the one-dimensional
III-V nanowires heterostructures that have large aspect ratios, which enable the high
crystallographic and optical qualities of the heterostructures.28 Therefore, the III-V
semiconductor nanowires and their heterostructures have attracted an increasing
attention. Moreover, based on the study of binary nanowires, the physical properties of
ternary nanowires and their growth behaviour have been further investigated, which broad
the applications of the III-V semiconductor nanowires.
In this chapter, the strategies of fabricating III-V semiconductor nanowires will be first
illustrated. The crystalline nanowires growth principles and their varied growth behaviour
(such as morphology and growth rate) under varied growth parameters (growth
temperature, V/III ratios and growth duration) will be described. Moreover, the research
status of the specific materials system (GaAs, InGaAs) will be depicted, in which the
objectives of this PhD project will be secured.
2.2 Synthesis of nanowires
The approaches of fabricating III-V nanowires can be divided into two categories: top-
down and bottom-up.29 Normally, top-down methods involve lithography, such as electron
beam lithography or focused ion beam milling and etching techniques, which are not likely
to form nanostructure with smaller scale size and high quality.30 Instead, bottom-up
methods can offer opportunities to eliminate such limitations. It consists of two main sub-
categories: template-directed and free-standing.
For template-directed growth, the success of it is to fabricate a suitable template, for
example V-grooves, step edges and mental membrane.31 This method can offer a great
chance to form nanostructure in uniform length and diameter. On the other hand, the over
dependent on the template can cause some problems, such as structural impurity and lack
of effective templates.
7
Free-standing growth, the most popular way to synthesize nanowires, relies most on
the anisotropy of growth rate. After nucleation at a certain point, nanowires elongate at one
direction with the highest growth rate, resulting in the one-dimensional shape. Within this
method, several growth mechanisms can be associated such as VLS growth, VSS growth
and VS growth.
2.2.1 Vapor-liquid-solid mechanism
One of the most popular mechanisms for growing nanoscale one-dimensional structures is
the VLS mechanism, which was first proposed by Wagner and Ellis in 1964 as an
explanation for silicon whiskers growth in vapor ambient assisted by gold liquid droplets on
a silicon substrate.32 It can be described in three stages as shown in Figure 2.1.33 At stage
I, nano-sized Au particles are formed as solid by depositing a thin film of Au on the
substrate or using aerosol Au nanoparticles directly. After the growth temperature is
achieved at around 800K, above the eutectic point, the Au catalyst begins to absorb vapor
Si atoms from the ambient to form liquid alloy. When the Si concentration is high enough in
the Au-Si alloy to cross the second liquidus line, Si starts to nucleate and form the liquid-
solid interface. Until the liquid droplet reaches the supersaturation level, it starts to
precipitate out Si and nuclei of Si grow in the form of nanostructure while the catalyst still
keeps liquid. As illustrated from this process, the VLS growth mechanism means that three
phases are coexisting during the nanostructure growth: the vapor phase of SiCl4 which
ends up with Si, the liquid phase referring to the Au-Si alloy at the top and the solid phase
of the grown nanostructure between the interface of the droplet and the substrate. This
theory was supported by Wu and Yang,17 who reported the VLS growth of Ge nanowires
with real time monitoring and identified these three stages by using in situ transmission
electron microscopy (in situ TEM).
8
Figure 2.1 (a) Schematic phase diagram is between the catalyst material (Au) and growth
material (Si) with red arrows showing the growth process. (b) Schematic illustration of VLS
growth mechanism of Si whisker from reaction of SiCl4 and H2 vapor phases. Black arrows
indicate the impingement of the vapor Si atoms to the catalyst, nanowires sidewall and
substrate and the diffusion of the atoms.33
During the last decade, VLS growth mechanism has been widely applied on the
growth of III-V semiconductor nanowires. For example, the study of Au-assisted GaAs
nanowire on (111)B substrate via VLS growth was conducted by Harmand et al34 in 2005.
Three samples were grown at the same conditions except for the cooling down process,
which ended up with three different phases of post-catalysts at room temperature: the
hexagonal Au7Ga2 phase (Figure 2.2d), the nearly pure Au of face-centred-cubic phase
(Figure 2.2e) and the orthorhombic AuGa phase (Figure 2.2f) (it is noted that the catalysts
9
size in three samples varies, which may cause the varied phases of the catalysts).
According to the Au-Ga phase diagram (Figure 2.3)35 and the growth temperature of
590°C that is above the eutectic point (~339°C), Au catalysts shall be at liquid state during
the nanowires growth. Besides, the VLS mechanism also was adopted to promote self-
catalysed or group III-assisted III-V nanowire growth, during which the presence of group-
III droplets (either Ga or In) can be witnessed on the top of corresponding nanowires.36,37
Figure 2.2 (a-c) TEM images show the top regions of the three grown GaAs nanowires
samples. (d-f) Corresponding electron diffraction patterns taken from the nanowires-
catalysts interfaces marked in (a-c).34
10
Figure 2.3 Au-Ga phase diagram.35
In the VLS growth process, catalyst plays an important role not only in lowering the
reaction energy but also inducing the growth of nanowires at the desired shape and size.
Therefore, the requirements for the catalyst particles can be critical and crucial.
To begin with, it is necessary for the catalyst to form a liquid alloy with the crystal
material at the growth temperature. In VLS growth, the most widely used catalyst particle
is Au, since it can form eutectic alloy with most of semiconductor materials at low
temperatures, such as Au/Ga: 339°C,38 Au/Sb: 360°C,39 Au/Bi: 241°C, Au/In: 454°C.35
11
However, no trace of As can be found in the Au catalyst.40,41 It is of great importance to
analyse the composition concentration in the catalyst, since nanowires are grown via the
nanowires-catalysts interface, the catalysts composition can provide tracks of the specific
catalysts and nanowires status during the growth. Normally, to form a eutectic alloy, the
growth temperature should be above the eutectic point to meet the thermodynamic
conditions for crystallization. However, Kodambaka’s group42 realized Au-assisted Ge
nanowires growth below the eutectic temperature. By using in situ microscopy, they found
the Au-Ge alloy can stay liquid with the dependence on Ge2H6 pressure as well, which can
be ascribed to the kinetic enrichment of the alloy particle.
In addition, the interface energies of vapor-liquid, vapor-solid and liquid-solid should
be well examined before choosing catalyst as they play a key part in determining the
catalyst shape.43 Recently, a new mode of VLS nanowire growth was reported by
Dubrovskii et al,14 who theoretically studied nanowires growth with catalysts in a varied
wetting case. Unlike the standard VLS growth, the non-spherical droplet sits on the corner
of the sidewall and buries the growth interface as shown in Figure 2.4. This elongated
droplet can be observed during self-catalyzed GaAs nanowires growth. As a consequence,
the nucleation on the triple phase line can be supressed and the structural purity would be
affected.
Figure 2.4 The model of novel VLS growth of a real nanowire with an elongated droplet on
the top.14
12
The VLS growth mechanism can be applied with many growth techniques especially
for epitaxial growth, such as chemical vapour deposition (CVD),44,45 MOCVD,46,47 chemical
beam epitaxy (CBE)48 and MBE.49-51 Among these technique, MBE is one of the most
promising and popular technique for III-V nanowire growth as it can guarantee high purity
and atomic accuracy of the material structure. For VLS-grown nanowires by MBE,
essential prerequisites should be satisfied.
From the thermodynamics point of view, the driving force for crystals growth is the
chemical potential difference (supersaturation) between the depositing materials in the
growing crystal and in vapor phase, which can be defined as under
the same temperature.52 Under certain conditions, the supersaturation level is catalyst size
dependent as reported to grow MgO nanowire by Takeshi et al, in which the VLS growth is
no longer available when the catalyst size is above a critical value.53 Additionally, based on
the Gibbs-Thomson effect, the supersaturation can be expressed as a function of catalyst
diameter :
0 4 / d (2.1)
Where is the effective difference between the chemical potential of depositing material
in the nanowire and in the vapour phase, is the same difference at a plane boundary
, is the specific free energy of the nanowire surface, and is the atomic volume of
the depositing material.52
From the kinetics point of view, two aspects should be under concern for crystalline
nanowires growth: (1) the presence of energy barrier for growing crystal and (2) the
identification of growth rate-determining steps. For the first part, the energy barrier for
growth process is more significant at relatively low temperature. However, with the help of
catalysts, the activation barrier can be lowered thus the growth rate would be enhanced.
For the second part, unlike MOCVD, the absorption of vapour source does not need the
thermal decomposition of precursors in MBE system. Thus, several steps of the growth
process can be drawn: (1) mass transport of growing material in the vapour phase; (2)
atom diffusion through the liquid droplet to the growth interface; (3) formation of the crystal
lattice at the liquid-solid interface; (4) atom diffusion along sidewall and substrate to the
nanowire-catalyst interface.52,54 Theoretically, the growth rate of nanowire is reported to
be dependent on supersaturation as (where refers to the kinetic
coefficient) and its diameter. Besides, Dubrovskii et al55 promoted a theoretical model
describing the dependence of the nanowire growth rate on its lateral dimension. It has
13
been demonstrated that the nanowire growth rate increases with the enlarged nanowire
diameter under higher growth temperature due to the Gibbs-Thomson effect, in which
nanowires growth rate is determined by the direct impingement of growth species to the
droplets and nanowires sidewalls. However, with the decreased temperature, the
contribution of atom diffusion becomes a determining factor for nanowire growth, which
minimizes this tendency.
2.2.2 Vapor-solid-solid mechanism
Since the VLS nanowires growth happens when the growth temperature is higher than the
eutectic point, the catalysts are in the liquid state. However, when the growth temperature
is lower or similar with the eutectic temperature, the catalysts are solid or quasi-solid,
which induce nanowires growth under the VSS growth mechanism. This mechanism was
first reported by Kamins et al in 2001,45 when they grew Ti-catalyzed Si nanowires by CVD
and claimed that the growth process is analogous to the traditional VLS growth
mechanism except for the solid catalysts. Hofmann et al56 monitored the VSS growth of Si
nanowires using Pd as catalysts with in situ observation for the first time, in which
nanowires growth was induced by the ledge propagation. As noted from Figure 2.5, the
growth-recorded TEM images show the ledge formation and layer by layer growth during
Si nanowires growth. Since the eutectic temperature Pd2Si alloy is 810°C that is higher
than the growth temperature, catalysts remain solid during nanowires growth, which shows
a faceted cuboid shape rather than the hemisphere shape, suggesting that nanowires
growth is under the VSS mechanism rather than the VLS mechanism. In addition, mental
catalysts like Al,57 Cu58 and Ni59 have been reported to promote VSS growth of Si and Ge
nanowires effectively as well. For III-V nanowire system, the VSS growth mechanism was
proposed by Persson et al48 in growing Au-catalyzed GaAs nanowires by CBE. By
combining in situ heating experiment and EDS measurement, the Au catalysts were
proved in solid state during nanowires growth. Indeed, the in situ observation of nanowires
growth is a powerful tool to confirm the nanowires growth and catalysts status. On the
other hand, the alloy phase diagram is another possible approach for ensure the catalysts
status during nanowires growth, although it is theoretically estimated under the
thermodynamics equilibrium conditions, in which the kinetics factors such as the growth
species flux and pressure should also be taken into consideration.
14
Figure 2.5 Environmental TEM images (a) showing the Si nanowires growth with
increased growth duration, in which the corresponding ledge flow and growth direction are
illustrated in (b-d). (e) Plot of step edge position versus growth time.56
Figure 2.6 Schematic of VSS nanowires growth. (a) Transport of the gas source to the
nanowire surface. (b) Atoms are delivered to the growth interface by surface or bulk
diffusion processes. (c) Atoms are incorporated at the growth interface leading to the
growth.60
15
The VSS nanowires growth process can be divided into four steps as illustrated in
Figure 2.6:60 (i) transport of growing material in vapour phase; (ii) gas atoms stick and
deposit on the catalyst surface and nanowires sidewalls; (iii) diffusion in the solid catalyst
particle; (iv) incorporation of depositing material into the growing crystal via the growth
interface (nanowire-catalyst interface). As can be noticed, the difference in mass transport
from VLS is that atoms have to go through solid catalyst to the growth interface. However,
this step cannot be distinguished as a growth-rate determination in all cases, because it
still depends on the diffusivity of atoms in the catalysts.
2.2.3 Vapor-solid mechanism
For III-V nanowires growing on Si substrates, the use of mental catalysts such as Au can
raise concerns about contaminations during the VLS growth. For instance, it has been
demonstrated that Au can induce deep level traps in Si which can affect the performance
of the nanowire-based electronics, by potentially reducing the minority carrier life time and
increasing the generation-recombination of electron-hole pairs.61 On this basis, catalyst-
free nanowires growth was proposed, which can be realized by patterned growth or
selective area growth.62 It has been demonstrated during InAs nanowires growth on Si
(111) substrate, patterned regions ( , Figure 2.7a) were prepared by electron-
beam lithography and wet chemical etching, on which a SiO2 film was uses as masking
materials.63 The patterns opening diameters were set as 60 nm and the pitch ranges from
400 nm to 800 nm, in which the InAs nanowires were grown on these opening with defined
diameter (60 nm) and controlled positions (Figure 2.7b,c). Such growth process not
involved any seed particles during growth is under the VS mechanism, which has been
demonstrated in a wide materials systems, ranging from oxides nanowires such as ZnO, to
semiconductor compound nanowires such as GaN.64,65 In addition, Ambrosini et al
demonstrated that in Ga-catalyzed GaAs nanowires growth in MBE, nanowires growth can
be modulated from the VLS mode to the VS mode.66 By tuning the As/Ga ratio, they
observed that the Ga droplet was consumed with the increased As/Ga ratio and nanowires
axial growth continues through the VS mechanism (Figure 2.8), in which nanowires tips
have varied morphology and crystal structures. This phenomenon can also be found
during InAs nanowires growth on bare Si (111) substrate.67
16
Figure 2.7 (a) Top-view SEM image of InAs nanowires arrays on a patterned substrate.
(b-c) Zoom-in 45°-tilted SEM images of grown nanowires.63
Figure 2.8 (a,c,e,g) High-resolution TEM images of GaAs NWs tips grown with increased
As/Ga ratio. On the left, relevant diffractogram are shown, indicating the local crystal
structure. (b,d,f,h) Typical scanning TEM images from the nanowires tips (in false color to
enhance the detail visibility).66
2.3 Morphology of III-V nanowires
It has been demonstrated that during nanowires growth, nanowires axial growth is
governed by the catalysts under the VLS growth, in which nanowires diameter depends on
the catalysts size. While, epitaxial nanowire shell growth can happen on the nanowires
sidewall, which is under the VS growth. Therefore, the nanowires morphology is
determined by the combination of nanowire axial and later growth. It has been
demonstrated that the nanowires morphology is closely linked to their possible
applications.68 For example, it has been established that nanoneddle structures with
17
ultrasharp tips are of great importance for the strong electric field improvement, which
points the way to the sharper sensors.69 On the other hand, nanowires are expected to
form uniform and controllable size, and the precise controlled growth sites for integration
into the devices. Accordingly, template directed methods have been taken advantage of.
For instance, Si nanowires70 and uniformly aligned ZnO nanowires71 in a large scale are
synthesized in a mental membrane with the help of Au. However, the entire dependence of
an available template can also act as limitation and hindrance.
2.3.2 Effect of catalysts
Catalysts size Since the nanowires diameter is determined by the catalysts size, the
selective of catalysts material can be a key factor for the nanowires morphology. It has
been demonstrated that during the Ga-catalyzed GaN nanowires growth, the Ga droplets
varies in lateral dimensions with varied growth condition, resulted in varied nanowires
morphology (Figure 2.9).72 Besides, it has also been demonstrated that by controlling the
Au film thickness, the Au catalysts size can be tuned after annealing, which results in
grown nanowires with varied diameters.73 Through quantitative study of the effect of Au
film thickness on the Au catalysts diameter and the grown nanowires, it was found that
with thicker the Au film, Au catalyst are bigger, which promote nanowires growth with
larger diameter (see Figure 2.10). This suggests that nanowires pre-growth process can
play an important role in modulating nanowires morphology.
Figure 2.9 Schematic representations and the corresponding observed nanowires
(indicated by arrows) grown by the VLS mechanism with Ga catalysts at the tip of the
nanowires.72
18
Figure 2.10 30°-tilted SEM images of Au-catalyzed GaAs nanowires with Au film thickness
of 0.5, 1, 2, 3, 5 nm, respectively. The inserts are top-view SEM images of corresponding
Au film after annealing.73
As mentioned previously, the diameter of nanowire can play an important role in
determining the supersaturation in catalyst, due to the classical Gibbs-Thompson effect.
With the increased nanowire diameter, the value of chemical potential difference
(supersaturation) increases, resulting in higher the nanowires growth rate. However, this
only happens within a certain range of the nanowires diameter. With further increasing the
nanowires diameter, the nanowires growth rate decreases, due to the diminished Gibbs-
Thompson effect. Additionally, As reported by Dubrovskii et al,74 there exists a critical
range of diameter for nanowires growth with a certain window of growth temperature,
below which nanowire cannot be promoted. On the other hand, the dependence of
nanowire growth rate on its diameter can be largely affected by growth temperature and
V/III ratio as plotted by Figure 2.11.74--76
Figure 2.11 (a) Radius dependencies of normalized growth rate of nanowires at different
growth temperature with fixed flux rate. (b) Diameter-dependent growth rates of GaAs
nanowires at different V/III ratios.74-76
19
Catalysts materials Additionally, it has been demonstrated that during the Pd-
catalyzed GaAs nanowires growth, the varied Ga concentration in the growth environment
leads to the varied Ga concentration in the Pd catalysts.77 As a consequence, the
nanowire-catalyst interfacial energy can be varied, causing the varied nanowire-catalyst
interface and thus the varied growth orientations and morphology (see Figure. 2.12a). This
nanowires morphology dependence of the catalyst-nanowire interfacial energy was applied
to modulate nanowires morphology. It has been demonstrated that in InP nanowires
growth doped with Zn, twinned zinc-blende superlattice structure was formed with zigzag
nanowires sidewall (see the high-resolution TEM (HRTEM) image in Figure 2.13c), which
can be resulted from the liquid-solid contact angle caused by the Zn dopants.78
Consequently, the solid-liquid and liquid-vapor surface energy were altered. Without Zn,
such periodic structure was disappeared.
Figure 2.12 (a) 30°-tilted SEM images of nanowires grown at (a) 500 °C and V/III ratio of
24; (f) 580°C and V/III ratio of 1.2; and (k) 590°C and V/III ratio of 0.8. (b,g,l)
Corresponding TEM images taken from nanowires tops. Corresponding (c,h,m) HRTEM
images, (d,i,h) enlarged TEM images taken form the catalyst-nanowire interfaces, (e,j)
selective area electron diffraction (SAED) patterns and (o) Fast Fourier Transform (marked
in (b,g,l)).77
20
Figure 2.13 (a) Overview and (b) high-resolution TEM images of Zn-doped InP nanowires.
(c) Schematic growth model for periodic twinning in nanowires (the contract angle between
droplet and nanowires is marked).78
Catalysts position On the other hand, by controlling the dispersion sites of the
catalysts, the nanowires morphology can be controlled. Dick et al successfully grew the
branched GaP and InP nanotrees in MOCVD.79 First, the Au-assisted nanowires was
initially grown under the VLS growth as a trunk. By spreading Au droplets on the grown
nanowire trunk though aerosol deposition method, the nano-branches are grown.79 The
diameter and number of these nano-branches can be controlled by tuning the
corresponding diameter and density of Au particles (Figure 2.14). This controllable
branched nanostructure may offer possibility in mimicking photosynthesis nanotrees.
Besides, although the unexpected nanowires kinking morphology can be observed in
III-V nanowires growth, particularly in the axial hetero-structured nanowires, due to the
varied interfacial energy of two different materials with catalysts, the control of such
morphology can also enable the possible nanowire-based applications.80
21
Figure 2.14 (a) 30°-tilted and (b) 45°-tilted SEM images of GaP nanotrees with 8.9
branches per tree on average. Top-view SEM images of (c) three branches directions and
(d) six branches directions.79
2.3.2 Effect of nanowire density
It has been demonstrated that nanowires morphology can be largely affected by the
nanowires density, i.e. the wire-to-wire spacing.22 As noted in Figure 2.15, with increasing
the nanowires spacing, nanowires grow shorter, suggesting that nanowires growth rate
decreases with the decreased nanowires density. Accordingly, a growth model was
proposed, in which, when nanowires spacing is small, nanowires compete with their
neighboured nanowires for the growth materials (see Figure 2.16a), which is under the
competitive growth mode. In this case, the nanowires growth rate is dependent on their
ability to absorb growth materials, which can be determined by the catalysts size. While, at
a relative large nanowires spacing, the material collection area on the substrate is not
overlapped but nanowires are still close, so that nanowires growth rate can be increased
by the interaction with neighboured nanowires via atoms diffusion (Figure 2.16b). On the
22
other hand, with further increasing the nanowires spacing, nanowires growth is isolated,
which is under the independent growth regime (Figure 2.16c).81 Overall, it is noted that the
nanowires grow faster with relatively small spacing, which can be ascribed to the catalytic
growth precursor decomposition in MOVCD. This process is accelerated with the
increased catalysts number per unit area. Therefore, nanowires growth with narrow
spacing can have more growth species, which leads to faster nanowires growth. However,
this tendency has been demonstrated to be related to the nanowires growth temperature
by Joyce et al.82 At higher temperature, nanowires diffusion length is longer, in which the
density dependence of nanowires growth is more significant: strong competition occurs in
high nanowires density, leading to the short nanowires, while, nanowires height increases
in the low-density nanowires growth. On the other hand, when the growth temperature is
low, atoms diffusivity becomes low, leading to the decreased competition between
neighboured nanowires, so that the nanowires length is similar in both high-density and
low-density growth. This dependence on nanowires density is largely dependent on the
atoms diffusion length on {111}B substrates.82
On the other hand, the synergetic effect of narrow nanowires spacing on the
nanowires growth is varied with nanowires patterned growth by CBE, in which nanowires
grow shorter under the small nanowires spacing that leads to the growth materials
competition.83
Figure 2.15 SEM images of nanowires with their spacing varying from (a) 400 nm, to (b)
500 nm and (c) 1000 nm.82
23
Figure 2.16 Schematic model of nanowires growth in different regimes. (a) Competitive
regime, d < λs, where the distance of neighbouring nanowires d is within the diffusion
length along the substrate λs. (b) Synergetic regime, λs < d < λg (diffusion length of Ga in
vapour), where the substrate collection areas become fully independent and wire-to-wire
interaction in gas phase occurs. (c) Independent regime, d > λg > λs.82
2.3.2 Effect of growth temperature
It has been demonstrated that nanowires can only successfully grow within a certain range
of temperature. This temperature growth window depends on varied III-V materials and the
other growth parameters, mainly V/III ratios.84 It has been established that for Au-assisted
GaAs nanowires grown by MBE, when the growth temperature is below 339°C (Figure
2.3), the lowest eutectic point of the possible Au-Ga alloy, the catalyst is solidified and
nanowires fail to grow.85-87 As noted from Figure 2.17, GaAs nanowires growth
morphology is varied with varied growth temperature: at low growth temperature,
24
nanowires show tapering morphology, while, with increasing the growth temperature, the
tapering decreases and nanowires have uniform diameters along the growth directions.
The tapering degree can be evaluated by the ratio of nanowires axial growth to their lateral
growth (see Figure 2.17f). Besides, nanowires growth rate also shows dependence on the
growth temperature: with a certain range of the growth temperature, nanowires grow faster
with the increased temperature; while, with further increased temperature, nanowires grow
shorter and two-dimensional layer growth on the substrates becomes significant (Figure
2.17d,e).48
In addition, it is noted from Figure 2.17b, nanowires have the pencil shape
morphology, which can be particularly observed in GaAs nanowires growth in MBE87 and
other III-V nanowires systems, such as InAs88 and GaN.89 This interesting morphology
endows nanowires with straight sidewalls rather than the inclined high-index planes.
However, the growth mechanism of the pencil-shape morphology is still controversial,
which has been demonstrated as layer-by-layer growth, similar with the epitaxial thin film
growth, or the step-by-step growth.90 On the other hand, the tapering morphology is
commonly observed during III-V nanowires growth.91,92 The growth mechanism of this
morphology is established mostly depends on the theoretical mass diffusion model,93 in
which growth temperature plays an important role in affecting the atoms diffusivity. For
short nanowires, when atoms diffusion length is larger than the nanowire length, atoms
can diffuse to the catalysts along nanowire sidewall and participate in nanowires axial
growth. When the sidewall instead of nanowire length exceeds the diffusion length, atoms
tend to stay at incorporating into the catalyst. Consequently, nanowires lateral growth
increases, causing nanowire tapering.
25
Figure 2.17 45°-tilted SEM images of CBE-grown GaAs nanowires at different
temperatures of (a) 480°C, (b) 500°C, (c) 515°C, (d) 535°C and (e) 560°C. (f) Schematic
growth model illustrating nanowires axial growth and radial growth.48
On the other hand, in III-V nanowires growth by MOCVD, such as GaAs or InAs, the
effect of growth temperature on nanowires growth is varied. As demonstrated by Joyce et
al,94 nanowires grown above 400°C epitaxial and straight aligned, while, nanowires grown
at a lower temperature show kinking and other irregular morphology (see Figure 2.18).
Since nanowires axial growth is driven by the catalysts under the VLS growth mechanism,
it was thought that under the relative low growth temperatures, Au catalysts are not
completely in the liquid state, so that the catalysts cannot well induce perfect crystal
growth. Besides, it can be seen that above 450°C, nanowires axial growth decreases,
which can be results from the onsets of nanowires lateral growth and two-dimensional
growth on the substrate under the high temperature. These two-dimensional growths
compete with the one-dimensional nanowires axial growth, leading to the mass transport
limitation for the nanowire axial growth and hence the short grown nanowires. In terms of
nanowires morphology, nanowires tapering is minimized at the relatively low growth
temperature, in which nanowires lateral growth is not significant.
26
Figure 2.18 30°-tilted SEM images of GaAs grown by MOCVD at the varied growth
temperatures. The V/III ratio was 46 for all samples. Scale bars are 1 μm.94
2.3.3 Effect of V/III ratio
Another key growth parameter for III-V nanowires growth is V/III ratio, the beam equivalent
ratio of group-III to group-V flux rate in MBE, which is a scale of quantity of gas atoms per
unit time. The V/III ratio, together with respective III and V flux rate can exert a great
impact on the nanowire growth rate and morphology.
It has been demonstrated in self-catalyzed GaAs nanowires growth by MBE,
nanowires growth rate mainly depends on the As flux rate rather than the Ga flux (Figure
2.19).95 This is because the group-III Ga is the catalysts material, nanowires growth
depends on the rate of the combination rate of Ga and As and their incorporation to the
lattice.96,97 On the other hand, for Au-catalyzed GaAs nanowires grown under varied V/III
ratios (see Table 2.1), with increasing the V/III ratio by increasing the V flux, nanowires
grow longer (Figure 2.20a-c).98 Besides, nanowires have tapering morphology under the
low V/III ratio, while nanowires have uniform diameter along their whole growth direction
under the high V/III ratio. However, it is noted that under the low two-dimensional growth
rate (samples H and J in Figure 2.20), increasing the V/III ratio have a minor impact on
nanowires growth rate and results in similar nanowires length.
27
Figure 2.19 Surface density and growth rate of self-catalyzed GaAs nanowires as a
function of Ga flux (left panel) and As flux (right panel).95
Table 2.1 Growth conditions for GaAs nanowires grown by MBE.98
Sample Temperature (°C) V/III ratio Two-dimensional growth rate (nm s-1)
E 600 1.1 0.28
F 600 1.7 0.28
G 600 2.3 0.28
H 600 2.3 0.14
I 600 2.3 0.07
J 600 4.6 0.14
28
Figure 2.20 Top-view and side-view SEM images of GaAs nanowires grown under varied
V/III ratios (as described by Table 2.1).98
Besides, for Au-catalyzed InAs nanowires grown by MBE, by increasing In flux with a
constant As flux, nanowires grow faster due to the increase of In atoms supply for
nanowires axial growth. However, with further increased In flux, nanowires growth
reduced, possibly owing to the decreased V/III ratio, in which As is relatively insufficient for
nanowires growth, leading to the desorption of III atoms (Figure 2.21a). By estimating the
corresponding V/III ratio, the peak of nanowire growth rate occurs at the V/III ratio of ~15,
and nanowire growth rate decreases with reduced V/III ratio. On the other hand, by
increasing As flux with a constant In flux, nanowires growth rate keeps increasing (Figure
2.21b). It is noted that this trend is obtained at the V/III ratio ranging from 357 to 21, which
is consistent with that in Figure 2.21a.99
29
Figure 2.21 Growth rates of InAs nanowires versus (a) In-flux for a fixed As-flux, (b) As-
flux with a fixed In-flux.99
In MOCVD growth, the V/III also has a distinct effect on the growth rate and
morphology of III-V nanowires (Figure 2.22). It can be seen that by increasing the V/III
ratio via increasing the V flux, nanowires axial growth slightly increases while their lateral
growth increases more significantly. On the other hand, by keeping the same V/III ratio
while increasing the III and V flux, both nanowires axial and lateral growth increase (Figure
2.22a).100 This trend suggests that nanowires axial growth is governed by both mass
transport and growth kinetics, while nanowires lateral growth is mostly kinetics limited.
Besides, under a constant V/III ratio, with increasing the III and V flux, nanowires tapering
30
reduces, which can be ascribed to the increased III atoms for nanowires axial growth, so
that the nanowires aspect ratio increases (Figure 2.22b). In addition, similar dependence
of InAs nanowires growth on the V/III ratio has been demonstrated. From Figure 2.22c,
with increasing the V/III ratio, nanowires growth in both axial and lateral directions
increases, owing to the minimized activation energy for nanowires growth at relatively high
V/III ratios. The nanowires axial growth rate reaches highest at a critical V/III ratio. When
the V/III ratio exceeds this value, the axial rate drops dramatically and the radial growth
dominates, resulting in severe nanowires tapering. With even higher V/III ratio as 93 and
above, nanostructure shows island-like and irregular shape. This may be due to the high
As pressure, the diffusion of group-III can be diminished, which promotes three
dimensional growth.101 This impact of V/III ration on nanowires growth rate and
morphology is also consistent with that during InP102 and InAs94 nanowires growth.
Figure 2.22 (a) Axial and radial nanowires growth rate and (b) tapering parameters of
GaAs nanowires with varied V/III ratio or III flow. (c) 40°-tilted SEM images of InAs
nanowires grown under varied V/III ratios. Scale bars are 1 µm.100,101
31
2.3.4 Effect of growth duration
Growth duration is a basic growth parameter for nanowires growth. By examining
nanowires growth at different stages of the growth, nanowires growth status and the
related growth mechanism can be unveiled. It has been demonstrated in GaAs nanowires
growth by MBE, with increased growth duration, nanowires morphology transits from the
rod-shape to the pencil-shape (Figure 2.23), which can be ascribed to the increased
nanowires later growth.92
On the other hand, it has been demonstrated during the MOVCD growth of GaAs
nanowires, the nanowires length can be controlled by the growth duration. It was found
that nanowires length is proportional to growth duration, which shows a linear relationship.
This tendency was also observed under the varied growth temperatures. However, it
should be noted that the maximum experimental data is obtained under the growth time of
120s, which is short compared with normal nanowire growth durations.75
Figure 2.23 45°-tilted SEM images of GaAs nanowires grown on GaAs {111}B substrates
for (a) 3 min, (b) 10 min and (c) 30 min at a growth temperature of 525°C and a V/III ratio
of ~1.5 in MBE.92
2.4 Structural characteristics of III-V nanowires
2.4.1 Crystal structure of III-V nanowires
Most III-V semiconductors, including the III-As, normally adopt cubic zinc blende crystal
structure. However, for III-V nanowires, hexagonal wurtzite structure can also be found,
32
ranging from GaAs nanowires by MBE to GaP nanowires by MOCVD.103 The only
difference of these two structures is the stacking sequence in the close-packed directions.
As illustrated in Figure 2.24, for zinc blende phase, the stacking sequence is described as
ABCABC… along the <111> direction, compared with ABAB…along <0001> direction for
wurtzite phase. Furthermore, zinc blende structure is polar in nature owning two different
<111> directions, i.e. <111>A and <111>B. Among them, <111>B is the preferential growth
direction in nanowire growth, due to the lower surface energy.104 The interruption of the
stacking sequence can lead to defects or different phase. Often in zinc blende structure,
the stacking sequence of ABCACB... can be found, where the plane A is called a twin
plane. The polytypism of wurtzite-zinc blende phase can be normally found in
semiconductor nanowire with varied growth conditions. Besides, the 4H and 6H polytype
have been recently observed in a few cases in III-V nanowires.105,106 The stacking
sequences observed from TEM are noted as ABCBABCB…and ABCBACABCBAC…
respectively.
Figure 2.24 Crystal structures of GaAs (Ga in purple and As in green). (a) Crystal
structure of zinc blende phase with unit cell on top and projection along <110> on bottom.
(b) Crystal structure of wurtzite phase with 2 2 2 cells on top and projection along
<110> on bottom.
33
2.4.2 Side facets of III-V nanowires
From SEM and HRTEM study of III-V nanowires, Dick et al103 reported that {1120} and
{1100} facets can be normally found in wurtzite structure, while, {112} and {110} facets are
common in zinc blende phase. As described in Table 2.2,107 the atomic planes are
projected along the growth directions, normally [111]B direction in zinc blende structure and
[0001] direction in wurtzite structure. Both of them have hexagonal shape with symmetry in
cross section. Two types of nanowire side facets can be noticed: {110}-type parallel to the
cleavage plane in III-V compounds and {112}-type with a rotation of 30° to (110) planes. It
is noteworthy that only {112} facets in zinc blende is polar and nano-faceting, which can be
divided into {112}A and {112}B planes. A plane is defined as terminating with the group V
atoms which can rescue three dangling bonds. While, B plane ends with group III atoms
that can only rescue one or two broken bonds. Such polarity can result in different
chemical and physical behaviour of non-equivalent {112} sidewalls.108 With {112}A growing
faster, the asymmetric truncated hexagonal cross section can be observed in
nanowires.109 As can be seen from Figure 2.25, a number of overlapping truncated
triangles are organized in orientations rotated with 60°. Theoretically, nanowires tend to
keep their side facets with lowest energy as shown in Table 2.3.110-112 However, it is also
related to the growth parameters. As reported by Plante et al,113 the side facets of GaAs
nanowires are intermixed by both type {110} and {112} planes and even nanowires grow
with no side facets when the growth temperature and V/III increase.
Table 2.2 Common side-facets in wurtzite and zinc blende structured nanowires.107
34
Figure 2.25 SEM images and schematic diagram of a typical GaAs nanowire with rotated
truncated triangular cross section.109
Table 2.3 Surface energy of sidewall facets in both zinc blende and wurtzite phases.
Facet type Surface energy (J/m2)110,111 Surface energy (meV/Å2)112
(112) 1.73 1.04 25/23/21
(110) 1.50 0.86 21/19/16
( ) 1.30 0.69 30/28/29
) 1.50 0.73 22/20/17
2.4.3 Effect of catalysts diameter
Catalysts diameter is a key factor that not only impact nanowires growth rate and
morphology but also the crystal structure of nanowires. It has been well demonstrated that
small nanowires diameter favours wurtzite structure while large sized nanowires tend to
form zinc blende structure. For instance, Johansson et al114 have demonstrated that by
controlling the diameter of nanowire, the phase purity of InAs nanowires can be
modulated. As described in Figure 2.26, nanowires with small diameters have the pure
wurtzite structure, while, with the increasing of nanowire diameter, the nanowires sections
of pure wurtzite phase decreases and the density of stacking faults increases. When the
diameter becomes large, there is a gradual change from pure wurtzite phase to a mixed
twining zinc blende phase. This catalysts diameter dependence of crystal structure has
been confirmed by nanowires growth under varied growth temperature. In addition,
Shtrikman et al115,116 have observed pure wurtzite phase in ultrathin GaAs and InAs
nanowires (~10 nm).
35
Figure 2.26 TEM images viewed along the ⟨110⟩ zone axis of InAs nanowires with
different diameters grown at 420 °C. The fraction of wurtzite phase decreases as the
diameter increases: (a) 100%, (b) 97%, (c) 86% and (d) 15% wurtzite. The arrows mark
the planar defects of the grown nanowires.114
Besides, as Hoang et al reported,117 MBE grown GaAs nanowires with small diameters
(30-40 nm) showed wurtzite phase with high density of stacking faults, whereas pure zinc
blende structure was obtained with diameter at around 60 nm.
The purity of nanowires phase with varied diameters can be ascribed to the nanowires
nucleation status at the nanowire-catalyst interface. It has been demonstrated that the
Gibbs free energy for nanowires nucleation is reversely related to the supersaturation of
the catalysts during nanowires growth ( ), which is the chemical potential of the Au-In
particle and the nanowire (take Au-catalyzed InAs nanowires growth for an
example). The supersaturation of the catalysts is given by114
(2.2)
where is Boltzmann’s constant, is the absolute temperature, is the atomic
36
fraction of In in Au-In alloy particle with radius and the solubility of In, is the surface
energy of the particle, and is the atomic volume of indium in the particle. According to
the Equation 2.2, the catalysts supersaturation increases with the decreased nanowire
diameter. Based on the theoretical modelling and calculation of the nucleation energy of
the wurtzite and zinc blende phases, high catalysts supersaturation growth favours
wurtzite structured nanowires, while low supersaturation growth favours zinc blende
structured nanowires, which is in good agreement with the experimental observations that
small catalysts induce pure nanowires structure. As demonstrated by Zhang et al,118
during InAs nanowires growth in MBE, small Au catalysts absorb more In during nanowires
growth, due to the high droplet surface tension, and the grown nanowires structure is
defect-free wurtzite structure. While, large Au catalysts have low In concentration during
the growth, which results in defected wurtzite structure. This founding shows a good
agreement with the theoretical predictions, verifying the strong dependence of catalysts
diameter on the III-V nanowires crystal structures and their quality. Besides, it has been
demonstrated that for a given supersaturation, the diameter crossover of wurtzite and zinc
blende structure is smaller, below which no nanowires can grow.
On the other hand, it has been established that the side-facets or sidewall of
nanowires grown on (111)B substrate can be related to their diameter. As reported by
Chen et al,119 during GaAsP/GaAs heterostructured nanowires growth, nanowires sidewall
varies with varied nanowires diameters. They found that when nanowires diameter is
smaller than 200 nm, {110} nanowires side facets formed, which have low surface energy.
While, wider nanowires form {112} side facets. Since thin nanowires have large surface to
volume ratio, they prefer to adopt low surface energy-surfaces.
Besides, Li et al have found periodic faceting in <111> grown Si nanowires, which
have six {112} sidewalls.120 This facet period was demonstrated to be dependent on the
nanowires diameter,121 in which the facet size decreases with the increased nanowires
diameter, suggesting that under a constant length, wider nanowires have more facets
change. The corresponding theoretical estimation indicates that the nanowires sidewall
surface energy increases with the additional formation energy of edges and apexes at
nanowires sidewall. Therefore, for thin nanowires preferring to keep low-energy surfaces, it
is energy favourable for them to form a small number of faceted surfaces on the sidewall.
37
2.4.4 Effect of growth direction
In general, III-V nanowires tend to grow in preferential crystal directions such as <111>,
<110>, <001> and <112>, which have relatively low surface energy. With varied growth
directions, nanowires structural quality and facets formation would be varied. It has been
well demonstrated that III-V nanowires tend to grow in <111> directions regardless of
substrate orientations, owing to the lower surface energy.104 For example, <111>B growth
direction has been found for InP nanowires grown on (110), (111)A and (111)B
substrates,122 and GaAs nanowires grown on (111)B and (100) substrates.123 However,
this growth behaviour can cause some drawbacks, for example, the stacking faults
formation. Krishnamachari et al found that, in InP nanowires growth on InP(001) substrate,
both <001>- and <111>B-grown nanowires were observed, in which nanowires grown
along <001> direction were perfect zinc blende structured, whereas the <111>B-grown
nanowires have planar defects along the whole nanowire.124 This is because the formation
energy difference between wurtzite and zinc blende structures along <111>B direction is
small, so that the mix phase of these two different structures are easy to form. Besides,
they also found that the nanowires side-facets are varied with varied growth orientations.
The <001>-grown nanowires have rectangular shaped cross sections, indicating the four
{110} side facets, which is consistent with the <001>-grown InAs nanowires by MBE.
While, the <111>B grown nanowires normally adopt hexagon shaped cross sections, in
which the side facets depends on their crystal structure and growth conditions.
On the other hand, for Au-catalyzed III-V nanowires grown along <111>B diretcions,
their crystal structure are mainly wurtzite structure, which is varied from those grown in
MOCVD. However, zinc-blende structured III-V nanowires can be obtained in non-<111>B
nanowires growth by using non-{111} substrates. For example, Shtrikman observed that
<001>-GaAs nanowires were grown successfully on the (311)B substrates, which have
pure zinc-blende structure. In addition, Zhang in our group recently reported his work of
InAs nanowires growth on GaAs {111}B substrate by MBE.125 In his work, nanowires were
grown at varied V/III ratios, in which both vertical and inclined nanowires were observed
under the low V/III ratio growth. Through SEM investigations, two types of nanowires
directions were confirmed as <111> and <110> directions (Figure 2.27). The <110> grown
nanowires has an elongated hexagonal cross section with four {111} and two {110} side-
facets. Further TEM investigations show that the <110>-grown InAs nanowires have pure
38
Figure 2.27 SEM images of InAs nanowires grown at (a) a high V/III ratio of 54, tilled-view
and (b) a low V/III ratio of 15, top-view. (c) 35°-tilted SEM images of inclined nanowires in
(b). (d) Corresponding sketch of inclined-nanowires side-facets. TEM images of (a) a
typical inclined nanowire, and the nanowire top regions viewed along (f) [ ] and (g) [ ]
zone-axis. (h) EDS spectra taken from the catalyst and nanowire. (i,j) Corresponding
SAED patterns. (k) High-resolution TEM images taken from the nanowire middle region.125
zinc-blende structure along their growth direction, and the the nanowire-catalyst interface
is (111)B surface, which is energy favourable to keep the low interfacial energy of the
nanowire-catalyst interface. While, the <111>B grown InAs nanowires have the wurtzite
structure. The crystal structure difference in the InAs nanowires growth along varied
directions was clarified by the nucleation energy difference at the nanowire-catalysts
interface for nanowires growth. It was descried that the nucleation barrier is droplet surface
energy dependent, which can be determined by the contact angle between droplet and
nanowire side facet. This contact angle is small in the inclined nanowires growth, which
gives rise to a lower nucleation energy, suggesting that the zinc-blended structured <110>-
nanowires are energy favourable to grow.
39
2.4.5 Effect of growth parameters
Faced with the fact that the dominated <111> nanowires growth could cause planar
defects formation, researchers are searching other ways to achieve high purity III-V
nanowires. From the theoretical point of view, Glas et al108 built a growth model about the
preference of wurtzite and zinc blende structure formation in III-V nanowires.94 They
pointed out that the fundamental reasons behind the phase difference of nanowires is the
supersaturation in the catalyst droplets during the growth, which would affect the
nucleation of both wurtzite and zinc-blende structure. This theory has laid a solid
foundation for later research study on the structure perfection of III-V nanowires. So far, it
has been well demonstrated that by tuning the growth parameters, such as growth
temperature, V/III ratio, group III or group V flux during III-V nanowires growth, nanowires
structural quality can be under control.
In the MOCVD growth of III-V nanowires, Joyce et al systematically investigated the
impacts of growth temperature and V/III ratio on nanowires quality and obtained pure
wurtzite and zinc blende crystal structured nanowires.126 It has been demonstrated during
the growth of InAs nanowires, by tuning the growth temperature and V/III ratio, nanowires
structure was tuned from pure wurtzite structure to pure zinc blende structure (Figure
2.28). Under the growth with low temperature and high V/III ratio, pure wurtzite structured
InAs nanowires formed. While, under the growth of high temperature and low V/III ratio,
zinc blende structured nanowires were grown. Between these two ends, nanowires show a
trend of phase transition with formation of stacking faults or twin defects. The structure
dependence of the catalyst supersaturation can be impacted by the growth temperature
and V/III ratio. According to the nanowire nucleation model, high growth temperature and
high III concentration in the catalysts give rise to the high catalysts supersaturation, which
leads to the wurtzite structure. While, under a low temperature or high V/III ratio, the
nucleation and growth of zinc blende nanowires are more energetically favourable, due to
the low-energy surface construction of zinc blende surface.
Similar phenomena were also observed when they grow GaAs nanowires under the
varied temperature and V/III ratio.127 Besides, it was found that GaAs nanowires grown
under a relatively low temperature can secure high quality, in which however nanowires
morphology may be irregular and their growth direction can be inclined. To solve this
problem, a two-temperature growth strategy was proposed, in which nanowires nucleation
40
Figure 2.28 TEM images of InAs nanowires grown along < >B or < > direction. (a-
d) High magnified TEM images of nanowires grown at varied temperatures and V/III ratios.
The insets are corresponding High-resolution TEM images. (e-h) Corresponding SAED
patterns.126
at a high temperature while growth at a low temperature. By comparing with the single-
temperature, this strategy works effectively in achieving the high-quality GaAs nanowires.
In addition, the impacts of temperature and V/III ratio on the nanowire structure have
also been demonstrated during the InP nanowires growth by MOCVD.103,128 As depicted
by Paiman et al,129 the InP nanowires favour zinc blende crystal structure under low
growth temperature and a wide range of V/III ratios from 44 to 350. While, wurtzite
structure formed at the high temperature above 485°C and the V/III ratio between 44 and
700 (see Figure 2.29). These results show that high V flux and high temperature promotes
the wurtzite structured nanowires growth, due to the high catalysts supersaturation, which
is consistent with previous study of III-V nanowires growth in MOCVD.
On the other hand, during the MBE growth of III-V nanowires, such as GaAs and
InAs, the influence of growth parameters on the nanowires structure is varied. In the self-
catalyzed GaAs nanowires growth, nanowires form the pure wurtzite structure with the
decreased V/III ratio. While, under the high V/III ratio growth, nanowires adopt zinc blende
41
Figure 2.29 Map of crystal structure of InP nanowires in the range of V/III ratio 44 to 700
and growth temperature of 400 to 510°C.129
structure, in which the twin defects decreases with the increased As flux.130,131 Besides, it
has been demonstrated in the InAs nanowires growth by MBE, nanowires grown at a low
V/III ratio have the defect-free wurtzite structure, while stacking faults were formed in
nanowires with the increased V/III ratio.125 This can be ascribed to the catalysts
supersaturation, which increases with the increased III in the catalysts during nanowires
growth. Additionally, Dubrovskii et al presented an interesting phenomenon in the Au-
catalyzed GaAs nanowires growth, by applying a two-temperature growth.132 They found
that nanowires grown at low temperature is pure wurtzite structure, while with the
increased growth temperature, nanowires form the zinc blende structured sections.
However, instead of a sharp interface between them, a faulted wurtzite/zinc-blende mix
structured nanowire section of around 30 nm can be observed. This observation is varied
from those in the MOCVD growth, which can be ascribed to the small nanowires diameters
and the impact of surface energy.
In Au-assisted III-V nanowire growth by MBE, few work has reported pure zinc
blende structured nanowires grown along <111> directions.133 It is common for zinc blende
phase being observed in the nanowire at very beginning and very end of the nanowires
growth, where the low III concentration leads to the low catalysts supersaturation which
favours zinc blende structure formation.108,115 As shown in Figure 2.30a, at the initial stage
of nanowire growth, the flux of Ga and As started to be pumped in the growth environment.
42
Figure 2.30 TEM images of a GaAs nanowire (a) at initial growth and (b) after nanowire
cooling. The inset in (a) shows the interface between two phases. (c) TEM images of InAs
nanowire superlattices, defined by 60 periods of alternating zinc blende and wurtzite
structure, viewed along the <110> zone axis.108, 134
After nucleation, the insufficient source supply causes low catalysts supersaturation,
leading to zinc blende phase formation. During the cooling down stage (see Fig 2.30b), the
Ga source is usually switched off. The maintenance of As would consume Ga residual in
the chamber and catalysts particles, forming a so-called “neck” region. The structure of
this region normally exhibits zinc blende structure. By utilizing this feature, Dick et al
demonstrated that by applying this flux interruption via the group-V switched on and off,
InAs nanowire superlattices were grown, which has 60 periods of alternating wurtzite and
zinc blende phases.134
In addition, it has been established that the variation in Ga or As flux would result in
different nanowire side-facets. Under the growth of a constant Ga flux, with increasing of
As flux, nanowires side-facets changed from {1100} planes to {2110} planes. While,
nanowires grown under a constant As flux, by changing the Ga flux, their side-facets did
not change and kept the {2110} planes.135 This phenomenon indicates that nanowire side-
facets formation may be more sensitive to the change of As flux. Besides, as presented by
Jiang et al for understanding the true shape of GaAs nanowires grown in MOCVD,136 they
did the cross-sectional study of the grown zinc blende structured GaAs nanowires, and
found that the newly grown nanowires actually have the high-index side-facets, and with
nanowires lateral growth, those high-energy side-facets transit to the low energy {112}
planes. They also found that the nanowires side-facets are temperature dependent. By
increasing the growth temperature during the later stage of the growth, nanowires side-
facets transit from {112} facets to the lower-energy {110} facets.
43
2.5 Ternary III-V nanowires
Compared with binary nanowires systems, ternary III-V semiconductor nanowires provide
large possibilities in composition tunability of the nanowires, which in turn, brings in
possibilities of band structure engineering and the design of novel nanowire-based
devices.137-140 For example, InGaAs nanowires system, as one of important ternary III-V
nanowires systems, has been attract an increasing interest. Comparing with GaAs,
InGaAs compound has higher electron mobility, lattice match with InP which is a
technologically important material for optoelectronic devices.141 The band gap of InGaAs
can be tuned between 0.34 (InAs) and 1.42 eV (GaAs) by tuning the chemical composition
of InGaAs compound alloy (see Figure 2.31).142-147 This band-gap range is between the
infrared and near-infrared regions, making ternary InGaAs nanowires as building blocks for
possible optoelectronic devices, such as light-emitting diodes,148 nanolasers149 and
photodetector.150
Figure 2.31 Band gaps and corresponding light wavelength λ for selected binary
semiconductors plotted as a function of lattice parameter.142-147
Basically, there are two types of ternary III-V nanowires: III-III-V nanowire, such as
AlGaAs139,151 and InGaAs,152-154 and III-V-V nanowires, such as GaAsP.155-157 In particular,
for Au-catalyzed III-III-V nanowires, both group-III elements can be alloyed with Au
catalysts, and incorporate to the lattice by alloyed with group-V at the catalyst-nanowire
44
interface, which is under the VLS growth mode.118,158 Therefore, the nanowires
composition depends on the rate of III atoms bonded with V or the interfacial energy of III-
V at the catalyst-nanowire interface. On the other hand, nanowires lateral growth can
simultaneously take place at nanowires sidewall, which is under the VS growth mode. This
can cause phase segregation in the ternary III-V nanowires, which form the core-shell
structures and leads to the complicated composition distribution in the nanowires.159,160
The composition of grown nanowires can be varied from the nominal III/III ratio, which
makes the composition of ternary III-V nanowires difficult to control.
2.5.1 Growth behavior
Similar with binary nanowires morphology, cylinder shape and tapered morphology can be
observed in ternary III-V nanowires, which are determined by nanowires lateral growth.132
Additionally, nanowires morphology is also catalyst-size dependent. For example, in
MOCVD growth of InGaAs nanowires, nanowires catalyzed by small droplets have uniform
diameters, while, nanowires have tapering morphology when their catalysts are larger.
Besides, Kim et al demonstrated that during InGaAs nanowires growth, nanowires tapering
degree, the ratio of nanowires axial growth to lateral growth, increases with the increased
nominal In concentration.22,152
On the other hand, the growth rate of ternary III-V nanowires can also be impacted by
the growth parameters, such as growth temperature and V/III ratio, catalysts diameters
and nanowires density. It has been demonstrated that, during InGaAs ternary nanowires
growth under a Ga-enriched environment (Ga/In = 7/3), with increasing the V/III to a critical
value below 10, nanowires axial growth increases. While, with further increasing the V/III
ratio beyond the critical value, nanowires axial growth decreases.161 This can be ascribed
to the Gibbs-Thomson effect, in which As can be easily desorbed from the catalysts due to
the large curvature of small catalysts. Therefore, increasing the V/III ratio enable more As
bonding with group-III atoms, leading to the increased nanowires growth. With the further
increased V/III ratio, nanowires growth is III-controlled, in which high V/III ratio causes less
III available for nanowires growth. Additionally, a relatively low temperature can reduce
InGaAs nanowires tapering, which is consistent with that in binary GaAs and InAs
nanowires growth. This can be attributed to the reduced possibility of overcoming the
kinetic barrier for nanowires lateral growth, or the reduced atoms diffusivity, so that the
atom incorporation to the nanowires sidewalls is decreased.
45
Besides, in the MBE growth of InGaP nanowires, it was found that nanowires with
significant later growth have pencil-shape morphology, while, nanowires with less lateral
growth have tapering morphology. The nanowires later growth was found to increase with
decreased group-III flux. Additionally, nanowires axial growth rate increases with the
increased III flux.160
On the other hand, axial heterostructured nanowires containing ternary III-V
nanowires, such as InGaP with InAs,162 InGaP with InP,163 GaInP with GaP164 and
GaAsSb with GaAs,105 were grown straight and had sharp interfaces. However, the sharp
interfaces and straight growth directions of heterostructured nanowires with ternary III-V
nanowires can be only in one direction, such InGaAs on top of InAs, not in the growth of
InGaAs on top of the GaAs.165 This fact can be ascribed to the growth thermodynamics. If
the interface energy between Au catalysts and nanowire A is lower than nanowire B,
straight nanowires can be grown for A growing on top of B, but nanowires can be inclined
or kinked if B grows on top of A to minimize the system energy.166-168
2.5.2 Compositional characteristics
Impact of nanowires diameter It has been demonstrated that for ternary III-V nanowires,
their composition can be morphology dependent. As reported by Hou et al, InGaAs
nanowires with uniform diameters along the nanowire length have uniform composition
distributions. However, the nanowires composition can be deviated from the nominal
composition.20 Moreover, during the ternary InGaP nanowires growth with varied catalysts
diameters, nanowires grown by small catalysts have lower In concentration, which may be
due to the Gibbs-Thompson effect. This In incorporation limitation was eliminated with
increasing the catalysts diameter or the group-III flux, in which In supersaturation in the
catalysts can be increased, leading to the increased In incorporation in the catalysts.160
Impact of growth parameters Similar with binary nanowires, ternary nanowires
morphology can be also determined by growth parameters. During the ternary InGaAs
nanowires grown at high V/III ratio by MOCVD, with increasing the growth temperature,
nanowires lateral growth increases, leading to the increased nanowires tapering.82 While,
under the growth of low V/III ratio, increasing the growth temperature would result in faster
nanowires growth, and further the increased temperature would also ends up with
nanowires tapering. On the other hand, under the growth of low temperature, increasing
46
Figure 2.32 45°-tilted SEM images of InGaAs nanowires grown under varied temperature
and V/III ratios, and corresponding STEM (scanning TEM) HADDF images of cross
sections and their EDS elements maps. Scale bars are 500 nm.168
the V/III ratio would leads to the faster nanowires axial growth; while under the growth of
high temperature, increasing the V/III ratio would leads to the increased nanowires
tapering and decreased nanowires axial growth. Compositional measurements of the
cross sections of the nanowires indicate that the nanowires with uniform diameters along
the nanowires have uniform compositions; while, tapered nanowires grown at low
temperature have core-shell structure, due to phase segregation, but tapered nanowires
grown at high temperature have uniform compositions (Figure 2.32), possibility due to the
increased atoms interdiffusion at higher temperature.169
Apart from the impact of growth temperature and V/III ratio on III-V ternary nanowires
chemical composition, the III/III or V/V ratio can also exert an impact on nanowires growth
and chemical compositions. Guo et al have demonstrated that for Ga-rich InGaAs
nanowires growth under the low In/Ga ratios, nanowires composition can be
modulated.23,170 Nanowires were grown at the same V/III ratio (As/(In+Ga)) of 44 and
In/Ga ratio of 1/19, but the varied In flux rate of 7.13 × 10-7 mol/min for sample A, while
8.20 × 10-6 mol/min for sample B. From EDS analyses on both samples, it was found that
only In was contained in the post-growth catalysts, and In concentrations in the nanowire
bottoms are higher than those in the nanowires tops. Besides, nanowires show Moiré
47
Figure 2.33 (a,d) TEM images of InGaAs nanowires grown at the same V/III and In/Ga
ratio but varied In flux. (b,e) EDS spectra taken from nanowires and catalysts as marked in
(a,d). (c,f) Corresponding TEM images taken from nanowires bottoms and schematic
illustration of nanowires structural and cross-sectional models.23
contrast in the nanowires bottoms, suggesting the formation of core-shell structure, which
was verified by further cross-sectional study of grown nanowires. By comparison of two
samples, it was noted that In concentration in the catalysts was lower in sample A, in
which no In was contained in nanowires cores. Consequently, nanowires in sample A
formed the core-shell structure, with GaAs core and In-enriched InGaAs shell; while
nanowires in sample B formed the core-shell structure with Ga-enriched nanowire cores
and In-enriched shells. These results suggested that there is an energy barrier for In
concentration in the catalysts to incorporate to the nanowires. Based on the first-principle
calculations, it was verified that the thermodynamics affinity of Au-In is higher than that of
Au-Ga. Therefore, Au catalysts prefer to keep In while expel Ga to the nanowires. When
nanowires were grown at the In-limited growth environment, In supersaturation in the
catalysts is low, so that In cannot precipitate out from catalysts and incorporate to the
nanowires. However, with increasing the In flux rate, more In can be absorbed by catalysts,
leading to the higher In supersaturation in the catalysts, and thus In incorporation to the
nanowire cores.
Impact of planar layer growth During the one-dimensional nanowires growth, two-
dimensional planar layer simultaneously occurs on the substrate.171-173 It has been
48
demonstrated that during ternary GaAsP nanowires growth on the GaAs substrates, the
group-V incorporation to the GaAsP planar layer was varied from the nominal As/P
ratio,174,175 due to requirement of misfit strain minimization caused by the lattice mismatch,
which in turn leading to the nanowires composition deviated from the nominal
composition.176-178 Therefore, the co-growth of nanowires and planar layer can result in the
uncontrollable nanowires composition.
2.6 Unclear issues
2.6.1 About GaAs nanowires
In order to have a better understanding the growth of ternary InGaAs nanowires or
InGaAs/GaAs heterostructured nanowires, binary GaAs nanowires should be first
investigated. From current study, the morphology of GaAs nanowires can be under control
by modulating the growth parameters, such as growth temperature and V/III ratio.
However, there are still a few issues need to be clarified.
Zinc-blended structure has not been achieved in GaAs nanowires grown along
<111>B directions in MBE.
A more comprehensive study of GaAs nanowires growth by tuning the respective
group-III and group-V flux.
The growth mechanism of pencil-shape GaAs nanowires.
2.6.2 About InGaAs nanowires
So far, although ternary InGaAs nanowires were successfully grown on the GaAs
substrates, the nominal In concentration was low, possibility to achieve the lattice match
between nanowires and substrate to ensure the straight grown nanowires. Therefore, the
growth of In-rich InGaAs nanowires on GaAs substrates needs to be achieved.
Besides, since ternary nanowires can also have lateral growth, core-shell structures
can spontaneously form in the nanowires, nanowires with uniform composition are hard to
achieve. Apart from that, planar layer growth also simultaneously takes place on the
substrate, in which the composition can be varied from the nominal composition. This in
turn, can lead to the even more complex nanowires compositional characteristics. In this
49
regard, the investigation of InGaAs nanowires chemical compositions and growth behavior
under varied growth parameters, particularly the In/Ga ratio, is necessary to achieve the
uniform ternary InGaAs nanowires.
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Chapter 3
Methodology
3.1 Introduction
In this thesis, III-V semiconductor nanowires are first grown by molecular beam epitaxy
and then investigated by advanced electron microscopy, which is the core to this research.
Specifically, the morphology of the grown nanowires is studied by SEM and their structural
and chemical characteristics are investigated by TEM and EDS, respectively. This chapter
66
will give detailed introductions on these important experimental techniques and their
working principles, as well as the related sample preparation methods.
3.2 Epitaxial growth
3.2.1 Molecular beam epitaxy
Molecular beam epitaxy is a physical deposition method for growing single crystals,
especially thin epitaxial film such as oxides, mentals and semiconductors. It was invented
in the late 1960s and first applied to the growth of compound semiconductors. It has been
employed in manufacturing environment since 1984 to fabricate GaAs laser diodes1 and to
make monolithic microwave integrated circuits and high-power hetero-junction bipolar
transistors since 1986.2 Until now, MBE is still widely used in the manufacture of
semiconductor devices, such as lasers and transistors, which can be further used in such
applications as cellular phones, radar system, solar cell and display devices.3-5
MBE essentially involves highly controlled evaporation in an ultrahigh vacuum (~10-8
Torr) system, which enables compound semiconductor materials with great precision and
high purity. For example, in the process of depositing thin film by solid source MBE,
elements such as Ga and As are heated in separated effusion cells until they sublime (as
illustrated in Figure 3.1).6 The reaction of one or more evaporated beams of atoms and
molecules with the single crystal substrate yields the desired epitaxial film. The term
“beam” means that evaporated source do not interact with each other until they arrive at
the substrate. It has also been demonstrated that by applying MBE, it is of great possibility
to grow “superlattice” structures, for example, the alternate thin layers of GaAs and
AlGaAs with abrupt interfaces.7 It has been developed as a popular method to grow
semiconductor nanowires with precise control of their composition during growth.
RHEED Nowadays, MBE systems have been equipped with an electron gun and
fluorescent screen for the display of reflection high energy electron diffraction (RHEED)
patterns, which is often used for monitoring the structure and composition of epitaxial film
during the crystal growth. In RHEED system, an electron gun is set up to generate the
electron beam that strikes the sample surface with a small angle (note that the electron
beam is directly incident on the substrate without any physically interference with the
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deposition source). The incident electrons interact with the sample and a part of them
diffracted and interfere at specific angles to form diffraction patterns.
During the thin film epitaxial growth, normally both spotted and streaked patterns can
be observed: spots often appear as a consequence of three-dimensional volume
diffraction like islands (Figure 3.2a), while streaked patterns occur when smooth layer
surface formed (Figure 3.2c).8 In nanowires growth, this technique is often used to monitor
the quality of the buffer layer that deposit directly on the substrate and the crystallization of
the grown nanowires.
Figure 3.1 Schematic illustration of a top-view of a typical growth chamber in the MBE
system, which is equipped with the RHEED monitor.6
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Figure 3.2 RHEED patterns (<110> azimuth) and SEM images of (100)-oriented GaP
substrates at (a) 350°C , 550°C and 600°C, respectively.8
Apart from that, several parameters in MBE can also be independently adjusted and
monitored to enhance the crystal quality, such as:
Flux rate: flux is determined as the flow rate of source per unit area. In MBE, the
flux rate determines the amount of sources reaching the substrate and is
measured as the beam equivalent pressure (Torr). The flux rate of sources such
as Ga and As can be tuned by the their respective source temperature
depending on the heating methods. The respective flux of group-III and group-V
determine the V/III ratio, which is a key factor for nanowire growth. Normally, for
III-V nanowires growth, group-V (normally As) is excessive to prevent deposited
material evaporation and to ensure a stable crystal growth.
Substrate temperature: since the crystal growth directly deposited on the
substrate surface, the substrate temperature approximately equals to the growth
temperature, which determines the thermodynamic conditions for crystal growth
and affect the diffusivity of the impinging atoms.
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Table 3.1 Comparison of MBE and MOCVD
MBE MOCVD
Vaccum conditions high (10-5 10-8 Torr) low
Growth species elements source metalorganic precursors
Growth temperature low high
Growth rate slow: layer by layer growth quick
In situ monitoring yes: RHEED no
Growth control precise (at atomic level) rough
Growth state physical process, non-equilibrium
not thermodynamically favorable
Chemical process,
thermodynamically favorable
III-V nanowires
product structure mainly wurtzite wurtzite and zinc blende
3.2.2 Growth procedures
All nanowires investigated in this thesis were grown by solid-source Riber 32 MBE system
with a RHEED monitor. For instance, the process of growing GaAs nanowires is described
as below in details.
In the pre-growth stage, the growth chamber in MBE system was cooled down by
liquid nitrogen until the vacuum condition reaches as high as 10-5 Torr. Then, the GaAs
substrate was transferred to the buffer chamber, where the degassing and deoxidization
took place under As flux for about 10 minutes. During this essential process, surface
contaminants can be effectively desorbed. After that, a GaAs buffer layer was deposited
on the substrate to achieved atomic flat surface, which would be secured by the RHEED
monitoring. Then, a thin film of Au was deposited on the smooth GaAs buffer layer.
Next, the substrate with Au film was sent back to the growth chamber, in which
annealing took place at 550°C, above the eutectic point, to form the Au nanoparticles
acting as catalysts. The function of the Au catalysts is not only by minimizing the activation
energy for nanowire growth but also can attract source materials especially the group-III to
realize crystallization at the nanowire-catalyst interface.
After annealing for 5-10 min, the substrate temperature dropped down to the
preferential temperature for nanowire growth (normally higher than the eutectic
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temperature). Meanwhile, the group-III flux (Ga) was introduced to the growth chamber to
initiate the GaAs nanowires growth. With varied growth recipes, the growth parameters,
such as growth temperature, V/III ratios and growth duration, can be controlled accordingly.
When the GaAs nanowires growth was terminated, the valves of effusion cells were closed
and the grown nanowires sample was cooling down under the As protection until the
temperature decreased to 300°C. After the substrate temperature was dropped close to
the room temperature, the grown nanowire sample was removed from the growth chamber.
3.3 Scanning electron microscopy
An electron microscope uses a beam of accelerated electrons (rather than photons) to
illuminate the sample and generates a highly magnified image. With the wavelength
100,000 times shorter than that of the visible light photons, the electron microscope can
have a greater resolution power than an optical microscope, being able to reveal more
information about smaller objects in details. For example, electron microscopies, such as
SEM and TEM, have become essential and irreplaceable tools to characterize the
morphology, structure and composition of the crystal sample, in particular the
nanostructures.
In electron microscopy, the electron beam interacting with the sample can generate a
number of signals as described in Figure 3.3.9 If the sample is sufficiently thin, the electron
beam can go through it thoroughly without any hindrance and interaction, acting as a
transmitted beam. Most of these singles can be detected in different type of TEM modes,
such as imaging, diffraction and spectroscopy mode. It is important to note that not all the
incident electron beams keep their original directions. Instead, some of them are scattered,
either elastically or inelastically. When the incident electrons are elastically scattered, they
do not lose any energy but only changed their incident orientation. On the contrary, when
the coming electrons are inelastically scattered, they have energy loss which can be
transferred to the atoms on the sample.
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Figure 3.3 Signals generated when a high-energy beam of electrons interacts with a thin
sample.9
As one of the three types of electron microscopies, SEM uses a focused beam of
electrons to scan the sample surface and create signals for imaging.10 SEMs can magnify
an object from 10 times up to 300,000 times with high resolution better than 1 nm, which is
inaccessible by light microscopy. Sample can be observed in high vacuum, low vacuum or
wet conditions. SEM can detect singles generated from the sample surface, including
secondary electrons, backscattered electrons and characteristic X-rays. As can be seen
from Figure 3.4, different singles come from different depths of the sample. With higher
accelerating voltage, the electron beam penetration is greater and the interaction volume
increases. Electron scattered from the sample surface is actually made of atoms, which
has two major parts resulting in two main singles: secondary electrons (SE) and
backscattered electrons (BSE). Primary beam “knocks out” out-shells electrons from
sample atoms forming low energy secondary electrons. With higher energy, electron beam
accelerates towards the positive nucleus and leaves as backscattered electrons. In terms
of X-ray production, continuum and characteristic X-ray are included. Continuum X-ray is
produced by the inelastic scattering process of the primary electron beam and the sample
atoms. Most of them are low energy X-rays, which are heavily absorbed at the low energy
end of the spectrum. On the other hand, characteristic X-rays are emitted when outer shell
electrons fill in the inner shell vacancy and release the excess energy. The inner shell
ionisation caused by the incident electron beam leaves some empty states. To keep the
system stable, electrons from out shells relax to fill the empty state in the inner shell and
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release the extra energy in the form of x-ray photons at the same time. Each electron jump
has a unique or characteristic energy that represents the patent atoms. Since different
elements have different electronic structure, these characteristic x-rays can be used to
determine the element composition of the sample.
Figure 3.4 schematically illustrated the working principles of the SEM. After electrons
being accelerated, it is first confined by the condenser lens to travel in a desired direction
and get converged at the same time. Before the beam flares out again, it is converged
back again by the objective lens and down onto the sample. Condenser and objective lens
are both electromagnetic, making electron beam travel spiralling down the column.
Besides, they both have an aperture sitting well below them. The condenser aperture
controls the probe size by demagnifying the electron beam from the gun. While the
objective aperture focuses electron beam into a smaller probe to control the focus of the
final image. By applying magnetic deflection coils, the electron beam is scanned the
Figure 3.4 Schematic illustrations of SEM layout and function. (http://www.ammrf.org.
au/myscope/sem/practice/principles/layout.php)
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sample surface and generated the electric signals that can be collected by the electron
detectors. These signals are transferred and magnified and images are built up by these
scanned signals reflected on the screen. It should be noted that the specimen can be tilt
by a small angle to observe more detailed information of the sample and obtain much
more to lower the signal/noise ratio and thus secure higher-resolution SEM images.
Unlike in TEM, the electron accelerating voltage in SEM is much lower, ranging from
1 to 50 kV, and the sample is thick enough so that electron beam cannot penetrate
through the sample. The main electron signals generated by the incident electron beam
are secondary electrons (SE) and backscattered electrons (BSE).
SE imaging Secondary electrons are produced by inelastic scattering of the incident
electrons from the sample atom electron clouds, which usually have low energy (0-50 eV).
Most secondary electrons are reabsorbed by the sample. Figure 3.5 shows the schematic
illustration of the interaction between incident electron beam with the sample. As noted,
different signals generated from the different depths from the sample surface, bring in
different information about the sample. The interaction volume in the sample depends on
the energy of the incident electrons (accelerating voltage applied). It is noted that most of
secondary electron signals emit near the sample surface (at a depth from 5 to 50 nm),
bring in surface information or tomography of the sample. The signal intensity mainly
depends on the sample surface topography. As shown in Figure 3.6, the variation of
electron signals brings in contrast in SEM images due to the “edge effect”. Edges are
normally brighter in the SE SEM image because of a larger surface area for electrons to
escape from, which generate more signals that can be collected by the detector. By
utilizing this edge effect, the topography of the sample can be well investigated.
Normally, high accelerating voltage, small spot size and short working distance can
endow SEM images with high resolution. With relatively higher accelerating voltage, more
electrons can be generated out, leading to the enhanced signal/noise ratio and thus the
higher resolution of the image. Besides, by narrowing the spot size, those scattered
electrons can be excluded, in which the noise is decreased and the fine electron beam can
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Figure 3.5 Schematic illustration of interaction volume created by electron beam on the
sample surface with electron singles generated from different depths.
Figure 3.6 Schematic illustration of the “edge effect” on the SE yields. (©
http://laser.phys.ualberta.ca/~egerton/SEM/sem.htm)
carry on more fine sample details. On the other hand, depth of field also plays an import
role in SEM imaging. Although for SEM image with a small depth of field, the main features
of the samples can be revealed, the whole quality of the SEM images is poor, since the
features beyond the depth of field can be blur. Therefore, extend the depth of the field in
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the SEM image is essential to reveal the comprehensive information contained in the
sample. The larger can be simply achieved by increasing the working the distance, which
would sacrifice part of the resolution. Therefore, the high resolution SEM image can be
obtained under the balance of the accelerating voltage, spot size and working distance,
which also depends on sensitivity and conductivity of sample itself.
BSE imaging Backscattered electrons (BSEs) are the electrons scattering elastically
from sample nucleus. With large interaction volume from higher energy beam, the BSE
signals can provide chemical composition information that cannot be achieved by SE. The
intensity of signal generated depends on the atomic number and results in contrast in the
BSE images. The higher the average atomic number, more primary electrons are
scattered back out of the sample and generate stronger signals. The intensity of the signal
brings in sample information, particularly the element weight. Therefore, the contrast
shown in the BSE image can tell the rough distribution of heavy or light elements.
Figure 3.7 (a) BSE and (b) SE images of a grain of sand that contains Si and Ti.
(©http://www.ammrf.org.au/myscope/sem/practice/principles/imagegeneration.php)
The difference of SE and BSE images can be determined when both imaging modes
are applied on the same sample. Figure 3.7a shows a SE image of grains of
a mixture mineral sand, which indicate the grains morphology and size, and the smooth
surface of the grains. While, Figure 3.7b is the corresponding BSE image of the sand, in
which strong contrast can be witnessed: the brighter grain suggests the Ti grain while the
dark contrast indicates the Si grain.
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X-ray microanalysis In SEM, X-rays are generated though the whole interaction
volume (Figure 3.4). Therefore, the typical spatial resolution for X-ray microanalysis in
SEM is up to a few microns. The electron accelerating voltage in SEM is typical 5-30 keV,
way lower than the conventional TEM, which can largely affect the X-ray intensities (X-ray
peaks). Since the sample composition is determined based on the ratio of the X-ray peaks,
the sample must be stable, flat and void free to ensure the accuracy of the X-ray detection.
The low X-ray signals in SEM also brings in several limitations: light elements (Z < 11)
cannot be analysed and the composition of small grains with order of nanometres cannot
be guaranteed.
In this thesis, the morphology of nanowires, such as nanowires length, diameter,
growth direction and side facets, are investigated by the field emission SEM, mainly JEOL
JSM-7001F and JEOL JSM-7800F SEM, which are equipped with field emission gun that
can generate electron beam with narrower diameter and higher current density than
conventional thermionic emitters and they are normally set up with SE, BSE or X-ray
detectors.
3.4 Transmission electron microscopy
As mentioned previously, TEM is one of the most powerful and comprehensive techniques
for characterizations of nanoscale materials and devices. Apart from the TEM imaging
mode, a TEM can appear in other forms and functions, such as HRTEM, STEM, EDS and
electron energy loss spectroscopy.
In conventional TEM, an ultrahigh energy electron beam transmitting through an
ultra-thin sample, normally less than 100 nanometres, creates images in several different
forms. Figure 3.8 illustrates ray diagram of a typical conventional TEM. From the top down,
by connecting the electron gun to a high voltage between 100-300 kV, the gun begins to
emit electrons into the vacuum column. In the column, the electromagnetic lenses help to
shape the electron beam. Specifically, the emitted electron beam is paralleled by the
condenser lens before it penetrates the thin sample and further scattered. The transmitted
and scattered electrons pass through the objective lens and form two planes: the image
plane and the back focal plane, resulting in two types of modes: imaging mode and
diffraction mode. At the bottom of the column, it is a viewing screen with phosphor
coatings to visualize both TEM images and diffraction patterns.11
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3.4.1 Imaging mode
In the imaging mode, the number of transmitted electrons depends on the mass of atoms
in the sample or the thickness of the sample. With the various intensity of electron beam
reaching the screen, the contrast can be seen in the TEM image. After the electron beam
being focused by the objective lens, the intermediate lens projects the image of sample at
the image plane, which can be seen on the fluorescent viewing screen and recorded by a
charge-coupled device (CCD) camera (Figure 3.8a). By choosing the size of objective
aperture, the contrast of the image can be adjusted.
Diffraction-contrast imaging The contrast witnessed in TEM images is due to the
scattering of electron beam by the thin sample. The electron wave interacted with sample
can change its amplitude and phase, leading to the amplitude contrast and phase contrast
Figure 3.8 Ray diagram of a TEM in (a) imaging mode, creating bright field TEM images
on the viewing screen and (b) diffraction mode, the diffraction pattern from the sample
captured on the viewing screen.11
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in TEM images, respectively. Diffraction contrast is a special form of amplitude contrast, in
which electron scattering at Bragg angles. Since the crystal defects rotate the defecting
planes near the defects and the intensity of the diffracted beam depends on the deviation
factor, the diffraction contrast imaging is used to determine the crystal defects in the
sample. As noted in the inset of Figure 3.9a, if the aperture selects the centre spot, bright-
field (BF) TEM image is formed, while if it select the diffracted spot (Figure 3.9b inset), a
dark-field (DF) image is generated. The dark contrast in BF image and the bright contrast
in DF image indicate the bend contour areas. This is due to the fact that when the atomic
Figure 3.9 (a) BF and (b) DF TEM image of a polycrystalline thin film of Bi. The insets are
the diffraction pattern taken from the one of single crystalline Bi. (c) Schematic illustration
of atomic planes bending at the edge dislocations. At position P, the angle equals to the
Bragg angle of the incidence electron beam. (d) A BF TEM image of cobalt mental.
planes bend and then cause the electron beam incidence angle approximate to the local
Bragg angle, electron would largely diffracted (see Figure 3.9c) and these diffracted
electrons will be absorbed at the TEM objective aperture, leading to the dark in the TEM
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image. By applying this characteristic, the dislocations and stacking faults in the sample
can be determined. Figure 3.9d is a BF TEM image of the cobalt mental, in which the
dislocations are witnessed as dark curves while the stacking faults are seen as dark
straight stacking lines.12
Phase-contrast imaging In TEM, phase contrast imaging normally refers to high-
resolution TEM. A phase contrast image aims to see the atomic structure of the sample
and thus requires a very thin sample (normally thinner than 30 nm) and the aperture
selecting more than two beam of the diffraction pattern.13 Figure 3.10 compares the
difference of on-axis TEM images under two-beam imaging condition and many-beam
Figure 3.10 Two-beam imaging condition and many-beam imaging condition and the
corresponding TEM images.
imaging condition respectively, in which two -beam imaging form the image with lattice
fringe only along a certain direction, while a real structure can be reflected under many-
beam imaging condition. By applying the phase contrast imaging technique, crystal
structure and growth orientation of the sample can be determined and their dislocations
and stacking defects can be revealed. Therefore, more beam collected increases the
resolution of the HRTEM image, which is normally obtained at the low-index zone axis.
HRTEM is a common technique used to characterize the structure of nanowires, which is
one of the main techniques applied in this thesis.
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3.4.2 Diffraction mode
As described in Figure 3.8b, when the electron beam lines parallel to zone axis of the
crystalline sample, the objective lens take the electron beam scattered from the sample
and then disperse them to generate diffraction pattern in the back-focal plane. Readjusting
the intermediate lens for the imaging mode, the back-focal plane acts as an objective
plane this time and the diffraction pattern can be further projected onto the CCD. However,
such a strong beam could saturate the CCD camera, so we normally select a specific area
by inserting an aperture above the sample to get a typical diffraction pattern called SAED
pattern, which can be seen on the viewing screen. The space where the electron
diffraction pattern forms is called “real space” and the diffraction spots form when the
interference of the electron beam scattered beam form two adjacent planes that meets the
Bragg’s law = 2dsin B (λ is the electron beam wavelength, d is the lattice spacing of the
Figure 3.11 Schematic illustration of the interference of the electron beam waves and the
principles of the electron diffraction pattern. (http://www.ammrf.org.au/myscope/tem/
background/concepts/imagegeneration/diffraction/beam/)
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crystalline sample, 2 B is the beam diffracted angle, Figure 3.11).14 it is noted that for the
unscattered electrons, they directly travel through O and produce a bright centre spot. The
distance between the diffracted spot G and the centre spot O is inversely proportional to
the lattice spacing of the sample.
Therefore, by measuring the distance between the diffraction spots, the crystalline
information of the sample can be studied. However, it is noted that this distance depends
not only to the scattered angle of the electron beam, but also the camera length, which is
the length between the sample and the place that the diffraction pattern is recorded.
Additionally, the diffraction pattern can also be applied to investigate the crystal structure
and quality of the sample.
3.4.3 Scanning mode
It has been demonstrated in early days that STEM can image heavy and light atoms,
which generates signals collected by the high-angular annular dark field (HAADF) that are
sensitive to the electron scattering by one atom or one column of atoms (Figure 3.12a).15
In terms of STEM, it is more like the way that SEM works: by adjusting the scan coil, the
electron beam is scanned on the sample. The signals are generated at a point on the
sample and a corresponding one displayed at an equivalent point on the viewing screen.
The electron detector used in the STEM mode is like an aperture we used in the TEM
mode. So when we insert the BF detector, direct-beam signals are captured and further
processed to become a BF STEM image. If scattered electrons (with scattered angle
larger than 50 mrads, Figure 3.12b) are captured by the detector with shifting to the
diffraction mode, we can end up with the DF STEM images. As a result, HAADF images
can be created, which is used to determine the distribution of elements depending on the
contrast. In HAADF images, material with higher atomic number is usually brighter than
the light one. By controlling the probe size that determines the resolution of STEM image,
high resolution HAADF STEM image can be obtained. Figure 3.13a describes the atomic-
resolution HAADF image of the interface between GaAs and Bi2Te3, in which the brighter
contrast of the right-hand side indicate the heavier element material of Bi2Te3.16 By
correlating the intensity profile of atom columns across the interface, the exact atoms
position can be secured (Figure 3.13b).
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Figure 3.12 Schematic illustrations of (a) Z-contrast technique in a STEM and (b) the
HAADF detector setting for the Z-contrast imaging.
Figure 3.13 (a) HAADF STEM image of the interface between GaAs and Bi2Te3 viewed
from the respective <110> and <210> zone-axis. (b) Intensity profile of the atom column
marked in (a).16
3.4.4 Analytical mode
EDS is a quantitative and qualitative analytical technique that can provide information
of chemical characteristics of the sample. Unlike SEM sample, TEM samples are usually
very thin, therefore, there is less electron beam spreading as it passes though the sample
and the spatial resolution of X-ray microanalysis is nanometres order. Two types of X-rays
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Figure 3.14 (a) Atomic resolution Z-contrast STEM image of Ba1.7Ca2.4Y0.9Fe5O13 film and
corresponding (b) Ba, (c) Ca, (d) Y, (e) Fe and (f) their overlap EDS maps.17
are generated by the interaction of the primary electron beam with nucleus of the sample:
the background X-rays; and Characteristics X-rays. These X-rays are detected by Ener
gy Dispersive detector which transfer the signals and shows the spectrum of intensity
versus X-ray energy under the STEM mode. Specific peaks in the spectrum results from
the Characteristics X-rays that can be used to analyses the sample compositions. In
addition, X-ray line scan profile provides the intensity of elements distributed along a
certain direction. By comparing the intensities of different elements of the sample, the
qualitative composition of the sample can be determined. Moreover, X-ray mapping can
provides image of elements distributions of the whole area of the sample. This technique
also enable atom-by-atom mapping at high resolution HAADDF STEM imaging (Figure
3.14), in which a very small probe size is selected.17
In this report, the grown nanowires are characterized by Philips Tecnai F20 FEG-
S/TEM (operated at 200 kV). A double-tilt holder was used to load the TEM sample, which
allows 0° α-tilt and 30° β-tilt to get the nanowire to the low-index zone-axis <110> to
identify the crystal structure, twin plane and stacking faults of the nanowires. Beside,
selected electron diffraction pattern, HRTEM and EDS techniques are also applied to
confirm the nanowires quality and the chemical characteristics of the nanowires.
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Main features of Philips Tecnai F20:
Schottky type field emission source
Accelerating voltage 200 kV
Airlock system
Minimum probe size 1 nm
4k 4k One-View complementary metal–oxide–semiconductor camera with in-situ
software
Gatan image filter with a 2k 2k slow scan CCD
BF/DF TEM; BF/DF/HAADF STEM
Twin-lens objective pole-piece
Single-tilt holder, double-tilt holder, cryo-sample holders (-183°C), double-tilt (±30°)
low background (Be) EDS holder, high temperature double-tilt (±30°) holder
3.5 Sample preparation
3.5.1 SEM sample preparation
For non-conductive samples, a coating of carbon or gold or platinum is essential for SEM
characterizations to prevent sample charging. The coating materials is based on what
technique to use, for example, metal coating may affect the X-ray signals with the signals
generated by the sample itself. Besides, depending on the varied SEM setting, all samples
should be cut in an appropriate size to fill in the sample chamber and mounted on the
sample stub.
As III-V nanowires are semiconductor, the conductive adhesive is usually used in
case of sample damages by the confinement of electrons. However, the conductive
adhesive contains carbon and sample also may have been contaminated. Therefore, it is
necessary to put the sample into the vacuum oven baking for 7-8 hours at 70°C. After that,
all samples are sent to plasma cleaning for about 10 minutes to further remove the
contaminants. In this thesis, since all the nanowires were grown on the GaAs {111}B
substrates that have {110} cleavage plane, cutting the sample along the cleavage plan is
one of the methods to determine the side-facets of the grown nanowires during SEM
investigation. When the SEM session is finished, samples should be kept in the vacuum
container.
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3.5.2 TEM sample preparation
For individual nanowires being investigated by TEM, a small piece of nanowire sample (a
few millimetre wide) is cut and put in a small container, in which a few ml ethanol is filled to
cover the sample and then the container is sealed. After that, the container with nanowire
sample is put into the ultrasonicator to take a bath for around 20 minutes to shake the
nanowires off the substrate. Then, a plastic pasteur pipette is used to disperse the ethanol
onto a 3mm 3mm holy carbon film coated copper grid, which was put on a filter paper on
a glass board to ensure the nanowires are well dispersed on the Cu grid.
Apart from the individual TEM sample, the nanowires cross sections and the cross
section of nanowires/substrate are also prepared for TEM investigations in this thesis,
which is detailedly described as follows.
3.5.3 Cross-sectional sample preparation
Nanowires cross-sections To investigate the side-facets of nanowires or chemical
characteristics of the nanowires, particular for the core-shell nanowires, the cross-sectional
sample of individual nanowires shall be prepared. In this thesis, we use the Leica Ultracut
UC6 to section our grown nanowires. This state-of-the-art ultramicrotone is utilized for
cutting ultrathin sections of both biological and physical sciences samples.
Before the sectioning, a small piece of substrate with nanowire grown is cut and put
into a cylinder or cuboid container, in which the surface with nanowires are faced up. Then,
liquid epoxy resin is filled into the container until full and the container is transferred into
the oven and baked at 60° for more than 24 h. As can be seen in Figure 3.14a,b, the resin
block is solidified after baking and the nanowires sample keeps at the container bottom.
After that, the container is removed and the resin block is well located on the holder to
make sure it is locked and stable. Then, a razor blade is used to trim the resin block and
remove the resin near the sample (Figure 3.15c). When the sample is exposed and
isolated, the substrate can be easily removed and leave the nanowires in the resin.
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Figure 3.15 (a,b) Images of the resin block after baking. The nanowires sample is at the
container bottom. (c) Image of the nanowires sample after the neighbouring resin being
cleaned. (d) Image of the resin after the substrate being removed. Red arrows mark the
sample.
A bright and smooth surface is usually obtained after this process (refer Figure 3.15d).
After that, the exposed surface is further trimmed to a square or rectangle shape and
sectioned by the diamond knife. As schematically illustrated in Figure 3.16, after the resin
block is trimmed well, the sample holder is connected into the microtome, in which the
holder can be both manually and electrically driven to move up and down. Meanwhile, a
small “boat” filled with distilled water and with a diamond knife at the front is set up as
close to the resin surface. By setting the boat advance distance, the thin slices of the resin
are continuously cut, in which the thickness of the slices depends on the knife advance
distance (the distance is set 30-50 nm in this thesis) and the nanowires cross sections are
contained in the resin slice (step 3). These cut resin slices are floating on the water and a
Cu grid was used as a carrier to collect these slices for TEM investigations.
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Figure 3.16 Schematic illustration of nanowires cross sections preparation.
Nanowire/substrate cross sections Investigating the interface between the grown crystal
and the substrate is a common technique in two-dimensional thin film or zero-dimensional
quantum dots growth.18,19 Similarly, this technique can also be applied in one-dimensional
nanowires growth.20 It has been well demonstrated that the nanowires/substrate cross
section can give a strong proof for investigate the nanowires initial growth behaviour on
the substrate, particularly for the hetero-epitaxial growth. Figure 3.17 shows a rough
process for preparing the nanowires/substrate cross sections for TEM investigations. To
begin with, a nanowires sample is cut into a 1cm 3mm rectangle and two similar-size Si
plates are also prepared. These three plates are glued together as a “sandwich” with the
middle plate being the sample (Figure 3.17a). This sandwich plate is further glued
vertically and tight on the sample holder (Figure 3.17b). After baking the sandwich plate
and ensure its stable on the the holder, the holder is transferred and fixed on the tripot and
tilted to a small angle with respect to the horizontal (Figure 3.17c). By manually polishing
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Figure 3.17 (a) Image of the prepared “sandwich” sample with the real sample in the
middle. (b) Schematic illustration of the sample adherence to the holder. (c) Sample
loading on the tripot. (d) The sample interface after polishing and (e) attached to the Cu
grid.
the sample until the interface is thin though and then attached the sample to the Cu grid
for TEM invetigations (Figure 3.17d,e).
Another popular technique to prepare nanowire/substrate cross section sample for
TEM investigations is focus ion beam (FIB).21,22 The main benefits of FIB is site selective,
due to its ability to image the sample surface by using an integrated SEM, and it can be
applied in a wide range of materials, such as semiconductors, metals, ceramics and
polymers.23 Similar with electron beam for SEM, ion beam was used in FIB, in which
secondary electrons can also be detected to generate the image of the sample. Normally,
FIB uses Ga as an ion source, which has a low melting point and can be focused into a
very narrow probe (<10 nm in diameter). The primary ion penetration depth is ~20 nm for
25 keV Ga+.
The nanowires sample’s cross section was prepared as follows: the sample was first
cut along the cleavage plane, and took an ultrasonic bath in ethanol to shake off
nanowires. Then the cleaned sample was mounted horizontally in the chamber, in which
the electron beam was used to deposit a ~200 nm thick and 10-20 μm long of Pt layer on
the interest area of the sample surface, which is parallel to one of the samples cleavage
plane. Large trenches are sputtered on either side of interest area using a high Ga beam
89
current, so that the selected area was isolated, and further milling the Pt layer down to the
100 nm thick. This thin lamellar sample was lifted out and attached on the Cu grid for TEM
investigations.
References
1. Mataki, H.; Tanaka, H., Mass Production of Laser Diodes by MBE. Microelectronics
Journal 1994, 25, 619-630.
2. Izumi, S.; Kouji, Y.; Hayafuji, N., Multiwafer Gas Source Molecular Beam Epitaxial
System for Production Technology. J. Vac. Sci. Technol., B 1999, 17, 1011-1016.
3. Tsang, W. T., Extremely Low Threshold (AlGa)As Modified Multiquantum Well
Heterostructure Lasers Grown by Molecular‐Beam Epitaxy. Appl. Phys. Lett. 1981, 39,
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4. Patton, G. L.; Iyer, S. S.; Delage, S. L.; Tiwari, S.; Stork, J. M. C., Silicon-Germanium
Base Heterojunction Bipolar Transistors by Molecular Beam Epitaxy. IEEE Electron Device
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5. Jackrel, D. B.; Bank, S. R.; Yuen, H. B.; Wistey, M. A.; Jr., J. S. H.; Ptak, A. J.;
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Solar Cells by Molecular Beam Epitaxy. J. Appl. Phys. 2007, 101, 114916.
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7. Cho, A. Y., Growth of Periodic Structures by the Molecular‐Beam Method. Appl. Phys.
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8. Cho, A. Y.; Arthur, J. R., Molecular Beam Epitaxy. Prog. Solid State Chem. 1975, 10,
157-191.
9. Williams, D. B.; Carter, C. B., Transmission Electron Microscopy: A Textbook for
Materials Science. Springer 2009.
10. Joy, D. C., Scanning Electron Microscopy. In Mater Sci Tech-Lond, Wiley-VCH Verlag
GmbH & Co. KGaA: 2006.
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Imaging with an Aberration-Corrected Transmission Electron Microscope. Ultramicroscopy
2002, 92, 233-242.
14. Kacher, J.; Landon, C.; Adams, B. L.; Fullwood, D., Bragg's Law Diffraction
Simulations for Electron Backscatter Diffraction Analysis. Ultramicroscopy 2009, 109,
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N.; Murfitt, M. F.; Own, C. S.; Szilagyi, Z. S.; Oxley, M. P.; Pantelides, S. T.; Pennycook, S.
J., Atom-by-Atom Structural and Chemical Analysis by Annular Dark-Field Electron
Microscopy. Nature 2010, 464, 571-574.
16. Dycus, J. H.; White, R. M.; Pierce, J. M.; Venkatasubramanian, R.; LeBeau, J. M.,
Atomic Scale Structure and Chemistry of Bi2Te3/GaAs Interfaces Grown by Metallorganic
Van Der Waals Epitaxy. Appl. Phys. Lett. 2013, 102, 081601.
17. Sayers, R.; Flack, N. L. O.; Alaria, J.; Chater, P. A.; Palgrave, R. G.; McMitchell, S. R.
C.; Romani, S.; Ramasse, Q. M.; Pennycook, T. J.; Rosseinsky, M. J., Epitaxial Growth
and Enhanced Conductivity of an It-Sofc Cathode Based on a Complex Perovskite
Superstructure with Six Distinct Cation Sites. Chem. Sci. 2013, 4, 2403-2412.
18. Liao, X.; Zou, J.; Cockayne, D.; Wan, J.; Jiang, Z.; Jin, G.; Wang, K., Alloying,
Elemental Enrichment, and Interdiffusion During the Growth of Ge(Si)/Si(001) Quantum
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19. Liao, X.; Zou, J.; Cockayne, D.; Leon, R.; Lobo, C., Indium Segregation and
Enrichment in Coherent InxGa1-XAs/GaAs Quantum Dots. Phys. Rev. Lett. 1999, 82, 5148-
5151.
20. Liao, Z.-M.; Chen, Z.-G.; Lu, Z.-Y.; Xu, H.-Y.; Guo, Y.-N.; Sun, W.; Zhang, Z.; Yang, L.;
Chen, P.-P.; Lu, W.; Zou, J., Au Impact on GaAs Epitaxial Growth on GaAs (111)B
Substrates in Molecular Beam Epitaxy. Appl. Phys. Lett. 2013, 102, 063106.
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22. Phaneuf, M. W., Applications of Focused Ion Beam Microscopy to Materials Science
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Chapter 4
Effect of V/III ratio on binary GaAs nanowires
4.1 Introduction
In this chapter, the influence of V/III ratio on the growth behavior and structural quality of
GaAs nanowires is investigated. In particular, by tuning the As flux in MBE, GaAs
nanowires were grown at varied V/III ratios. Correponsing results were published on The
Journal of Physical Chemistry C (http://pubs.acs.org/doi/abs/10.1021/acs.jpcc.5b05606)
and intergrated in the Section 4.2 of this chapter.
92
4.2 Quality control of GaAs nanowire structures by limiting As flux in
MBE
In this study, we demonstrate that by merely limiting the As flux, the growth behavior and
structural quality of Au-catalyzed GaAs nanowires can be modulated in molecular beam
epitaxy. With decreasing the As flux through lowering the V/III ratio, GaAs nanowire growth
is found to be slow and defect-free wurtzite structured GaAs nanowires can be obtained
regardless of catalyst sizes. While, in the As-enrich environment (such as at relatively high
V/III ratio), thinner nanowires can grow longer with fewer planar defects. Based on our
extensive morphological, structural and compositional investigations, it is found that GaAs
nanowires grown under an As-limited status can lead to a thermodynamically controlled
growth process, while, when the nanowires grown under a relative high V/III ratio, a typical
kinetically dominated process is observed. This study provides a new insight for controlling
the structural quality of III-V nanowires by tuning the group-V flux.
Introduction
Development of III-V semiconductor nanowires has attracted great attention in the past
decade due to their intrinsic and superior properties,1,2 which makes them of interest for
applications in electronic and optoelectonic devices, such as solar cells,3 light-emitting
diodes4 and field-effect transistors.5 With higher electron mobility than Si,6 wide direct
bandgap (~1.4 eV)7 and possibilities to fabricate hetero-structured nanowires for band
structure engineering, such as GaAs/AlGaAs,8 and GaAs/GaAs(Sb),9 GaAs nanowire is
regarded as one of the essential components for a vast variety of future applications,
particularly in the near infrared region.7
To ensure the premium properties of III-V nanowire based devices, realization of
precise control in nanowire morphology, uniformity and structural quality becomes priority.
Normally, III-V nanowires are grown with metal catalysts (such as Au), which follows the v
VLS growth mechanism.10 MBE11 is one of the most effective techniques for growing
epitaxial III-V nanowires, during which growth parameters such as temperature,12 V/III
ratio,13-15 the absolute pressure of the group-III and group-V species15 can be well
controlled for varying the crystal structures and their structural qualities. Specifically, GaAs
nanowires have been grown by MBE in the past decade,16-18 with focus on optimizing the
93
nanowire structures. For example, Bauer et al.19 have studied the structural quality of self-
catalyzed GaAs nanowires. Also, several research groups have been dedicated to achieve
Au-catalyzed GaAs nanowires with defect-free structure by using different strategies, such
as lowering the As flux rate,20 controlling two-dimensional growth rate by tuning Ga flux
rate21 and lowering the growth rate of nanowires.22 It is well known that Au-catalyzed GaAs
nanowire growth by MBE depends on the different kinetics of Ga and As.23 It has been well
documented that increasing the flux rates of Ga or As can both lead to higher growth
rates.24,25 Also, the preferred formation of crystalline structure is attributed profoundly to
the supersaturation level in the catalysts,26 which is closely related to partial pressure of
Ga and As.27,28 However, few experimental studies in MBE have thoroughly explored the
impact of the group-V flux on the structural quality of GaAs nanowires.12,15,20 Therefore,
the role of As played in nanowire growth and structure control is still unclear in MBE, which
needs further clarification.
In this study, we explore the impact of the As flux on the growth behavior and
structural quality of Au-catalyzed GaAs nanowires grown by MBE. By tuning the absolute
As flux rate while keeping the Ga flux as a constant, we designed two nanowire growth
with different V/III ratios. Through carefully investigating the morphological and structural
features of grown nanowires, we found that nanowires grow slowly in the As-limited
condition (low V/III ratio), and generate low energy {100} facets with defect-free wurtzite
structure. However, in the As-enrich environment, nanowires show entirely different growth
behavior and structural characteristics. The fundamental reasons for such difference are
investigated and two growth mechanisms are proposed.
Experimental section
GaAs nanowires were grown on GaAs {111}B substrates using Au catalysts in a Riber 32
MBE system. Before nanowire growth, the GaAs {111}B substrates were degassed and
deoxidized to remove any contaminants in the growth chamber. In a typical growth, a
GaAs buffer layers with a thickness of 200 nm was grown on a pre-cleaned GaAs
substrate at 580°C to ensure that nanowires can grow on the atomically flat surface. The
substrate was then transferred to the preparation chamber and a ~1 nm thick Au thin film
was deposited directly on the buffer layer surface. After that, the Au coated substrate was
transferred back to the growth chamber. With annealing at 550°C for 5 min under the As
source protection, solid Au nanoparticles were generated on the GaAs substrate acting as
94
catalysts.18,29,30 The substrate temperature was then dropped to 500°C, where the growth
of GaAs nanowires was initiated by introducing Ga source to the growth chamber with a
flux rate of 1.2 10-7 Torr and adjusting flux rate of As source to be around1.2 10-6 Torr
and 2.4 10-6 Torr respectively, to achieve the V/III ratios of 10 and 20. The V/III ratio was
calculated by the As/Ga beam equivalent pressure ratio. After 60 min of nanowire growth,
the growth was terminated by switching off the Ga source while keeping the As source
until the system was cooled naturally to the room temperature.
The morphological characteristics of grown GaAs nanowires were investigated SEM
(JEOL 7800F, operated at an accelerating voltage of 5 kV) and their structural and
chemical characteristics were investigated by TEM (Philips Tecnai F20 equipped with EDS
for compositional analysis). SEM specimen for side-view investigations were cleaved
along the {110} cleavage plane31 of GaAs {111}B substrates. Individual nanowires for TEM
investigation were prepared through ultrasonicating the nanowire samples in ethanol and
dispersion individual nanowires onto the Cu grids coated with the holey carbon films.
Results and discussion
Figure 1a,b is typical plane-view SEM images taken from both samples, in which “white”
dots with similar densities are seen in all two samples, suggesting that nanowires might
grow vertically on the substrates. The insets of each SEM image reveal the hexagonal
faceted features of these “white dots”, indicating faceted side-walls of grown nanowires. It
is of interest to note that the sizes of “white dots” are varied in both cases, which should be
attributed to the annealing process. During which, the Au thin film breaks up and
aggregates into nanoparticles with different sizes, with small particles dissolving and
redepositing onto large ones due to the Ostwald ripening,32 leading to a significant large of
size distribution. In fact, such a phenomenon has been also observed by other groups
studying Au-catalyzed GaAs nanowires.15,33 To understand the growth status of these
white dots, side-view SEM investigations were performed and the results are shown in
Figure 1c,d correspondingly, in which nanowires were vertically grown on the substrates,
and the lengths of grown nanowires can be statistically determined. As can be seen,
grown nanowires in the sample with the V/III ratio of 10 are relatively short (less than 1 µm
95
Figure 1 SEM images of GaAs nanowires grown on GaAs {111}B substrate under different
V/III ratios. (a,b) Top-view SEM images of GaAs nanowires with the V/III ratios of 10 and
20, respectively. The inserts are high-magnified SEM images showing the side-facets of
nanowires. (c,d) Corresponding side-view SEM images with the insert in (d) being a
slightly tilted SEM image. All scale bars are 200 nm.
), and they have almost the same height. In contrast, in the sample with the V/III ratio of
20, grown nanowires have diverse heights with a tendency of thinner nanowires growing
longer, as indicated in the inset of Figure 1d. In both cases, no significant nanowire
tapering was observed, except some tapering were found at the bottom of the nanowires
caused by the lateral growth.34 It should be noted that the lateral dimensions of grown
nanowires vary and the observed variations fit well with the size distribution estimated from
the plan-view SEM images.
To understand the structural characteristics of grown nanowires, TEM investigations
were employed. Figure 2a,b is BF TEM images of a typical thin nanowire (~25 nm in
diameter) and a thick one (~65 nm in diameter) both taken from the sample with the V/III
ratio of 10. Figure 2c,d is the corresponding HRTEM images showing that nanowires have
the wurtzite structure, which were taken along the <110> zone axis. Interestingly, these
nanowires are defect-free along their entire lengths, irrespective of their catalysts’ sizes.
96
Figure 2 (a,b) BF TEM images of typical thin and thick nanowires from the sample grown
with the V/III ratio of 10. (c,d) Corresponding HRTEM images from both nanowires. (e,f)
BF TEM images of typical thin and thick nanowires from the sample grown with the V/III
ratio of 20. (g h) Corresponding HRTEM images.
This conclusion was further confirmed by our extensive investigations over a half dozen of
thin and thick nanowires. Similar TEM investigations were carried out for the sample grown
under the V/III ratio of 20, and the typical results are shown in Figure 2e–h. As presented
in Figure 2e,f, both thin and thick nanowires contain planar defects along their growth
directions. Figure 2g,h is the corresponding HRTEM images, from which planar defects
can be confirmed at the atomic level. Furthermore, our extensive HRTEM investigations
indicate that, in the sample grown under the V/III ratio of 20, the thicker the nanowires, the
higher the density of the planar defects.
97
It has been well documented that the catalyst composition and status are critical to
the growth of III-V nanowires,35,36 so that it is necessary to determine Ga concentration in
the catalysts during the nanowire growth. Accordingly, EDS analysis was performed
statistically on the catalysts of nanowires with different lateral dimensions in both samples.
Since EDS can only reveal the chemical composition in the post-growth catalysts and
since, during the cooling process of GaAs nanowire growth, the Ga can be expelled out
from the catalysts, we need to include the amount of Ga lost from the catalysts during the
cooling process. It has been well documented that, a “neck” region can be naturally formed
in grown nanowires during their cooling process,26,37 where the Ga in the catalysts can be
precipitated out in the catalyst-nanowire interface and combine with As to form a “neck”
section of nanowires underneath the catalysts, when the Ga source is switched off. In this
regard, the Ga presented in the “neck” region needs to be determined. Figure 3a shows a
TEM image of a top section of a typical nanowire, in which the nature of wurtzite structured
nanowire and a zinc-blende structured “neck” section can be seen. This structural feature
provides an opportunity to estimate the amount of Ga being expelled out from the catalyst
during the cooling process. In this study, we only focus on investigating the tendency of
the relationship between the Ga concentration in the catalysts and the catalyst size (in turn
the lateral dimension of the nanowires), so that we can simplify the estimation model by
assuming that the “neck” region has a cylinder shape and the catalyst has a hemisphere
shape, as illustrated in Figure 3b.
Since the total number of Ga atoms (NGa) in the catalyst during nanowire growth is
the sum of the number of Ga atoms (NN) in the “neck” section (zinc-blende structured
GaAs) and their number (NC) in the post-growth catalyst detected by EDS, that is, NGa =
NN + NC. Consequently, the Ga concentration during growth can be estimated as (NN +
NC)/ (NN + NC + NAu)38 with NAu being the number of Au atoms in the catalyst.
Since the “neck” region is zinc-blende structured GaAs, which contains 4 Ga atoms
in a unit cell, NN can be obtained by:
NN = 4VN/ , (1)
where is the volume of a unit cell of the zinc-blende structured GaAs, and VN is
the volume of the cylinder shaped “neck” region, which, according to Figure 3b, can be
determined by
2H (L being the lateral dimension of the nanowire and H the height
of the “neck” region).
98
To determine NC, we firstly determined the crystal structure of the catalysts. Figure 3c
is a selected area electron diffraction pattern taken from the interfacial region between a
catalyst and its underlying nanowire, in which two sets of electron diffraction patterns can
be seen. Using the diffraction spots of the GaAs nanowire as a reference, the diffraction
pattern of the catalyst can be determined as a [ 12] zone-axis of a face-centered cubic
structure with a lattice parameter of aC ≈ 0.41 nm. This is the Au crystal structure,
suggesting that the catalysts adopt the Au crystal structure. Accordingly, the volume of
each unit cell ( ) can be determined as . Based on the facts that (1) each unit
cell contains 4 atoms and (2) the total volume of catalyst is
(L/2 denotes the
radius of the hemispheric catalyst, as illustrated in Figure 3b), NC can be expressed as
, (2)
where CGa is the determined Ga atomic concentration in the post-growth catalysts by EDS.
Similarly, NAu can be determined using Equation 2, by replacing CGa with CAu. Figure 3d
shows an example of EDS profile taken from a typical catalyst, in which both Au and Ga
peaks are seen (noted that the observed Cu peaks are due to the Cu TEM grid). With our
careful compositional investigations over a dozen of nanowires, Ga concentration in the
post-growth catalysts is generally less than 10 at.%, which is smaller than the Ga solubility
in Au according to the Au-Ga phase diagram.39 Therefore, the catalysts adopt the Au
crystal structures.
Based on this approach, we can determine the Ga concentrations in the catalysts
during nanowire growth. Consequently, we examined a dozen of nanowires with different
diameters for each sample and the results are displayed in Figure 3e. As can be seen, a
clear tendency is shown as the thinner the nanowire, the higher the Ga concentration in
the catalyst during nanowire growth, which is consistent with previous studies.36,40
Interestingly, this tendency is seen to be more remarkably for the high V/III ratio, indicated
by the pink arrow in Figure 3e, possibly owing to the strong competition of Ga in catalysts
caused by its lower partial pressure in the growth chamber.
To understand our observations, two questions need to be answered. (1) Why, at a
relative high V/III ratio, nanowire lengths are varied and they contain planar defects with a
tendency of thinner nanowires being longer and having less planar defects? (2) Why are
nanowires grown at a low V/III ratio short, but have the same height and defect-free
structure?
99
To answer the first question, we note that, based on the classic theory of crystal
growth, the growth rate of nanowire ( ) can be expressed as:41,42
⁄ (3)
where is the Boltzmann constant, is the growth temperature andis the chemical
potential difference between the catalyst and nanowire nucleus, also known as
supersaturation in the catalyst, and can be presented as:27
=
, (4)
Figure 3 (a) A HRTEM image showing the top region of a typical GaAs nanowire grown at
the V/III ratio of 20. (b) A schematic model showing the nanowire top containing a
hemisphere catalyst, a cylinder disk-shaped “neck” region and the nanowire section. (c)
SAED pattern taken from catalyst/nanowire interface confirming the crystal structure of the
catalyst. (d) EDS spectra taken from the catalyst. (e) Estimated Ga concentration in the
catalysts as a function of catalysts size (dimension of nanowires) during nanowire growth.
100
where and are the Ga concentrations in the catalyst during nanowire growth
and at equilibrium with the GaAs crystal,27 respectively. Due to the low As solubility in Au,
As in the vapor would incorporate into the nanowire through the triple phase line,26 thus
and here indicate the As concentrations in the vapor. According to Equation 4, increases
with increasing , suggesting that the more Ga in the catalysts, the higher the chemical
potential difference,28,36,40,43 which in turn promotes fast nanowire growth according to
Equation 3.44 In this regard, with high Ga concentrations, small catalysts can induce
longer nanowires due to their high chemical potential difference and hence high growth
rate, resulting in the length variation of nanowires with different catalyst sizes.
It is has been well documented that high supersaturation in the catalyst can promote
the formation of wurtzite structure whereas low supersaturation is favored for the
generation of zinc-blende structure.26 Therefore, thinner nanowires with higher Ga
concentrations in the catalysts and hence with high catalyst supersaturation tend to
nucleate in wurtzite structure with better structural quality. In addition, from energy point of
view, the total free energy of nanowires, mainly including the surface energy and the
cohesive energy inside nanowires, tends to be minimized during nanowire growth. For
thinner nanowires, owing to the large surface-to-volume ratio, the surface energy plays a
crucial role in the total energy. In this situation, for thinner nanowires, wurtzite structure is
more energetically favorable due to their lower lateral surface energy compared to zinc-
blende structure, as suggested by the theoretical prediction.45,46 However, when the
diameter exceeds a critical value, the cohesive energy becomes dominating, which is
lower for the zinc-blende structure than for the wurtzite structure.33,47 As a result, for the
thicker nanowires, the difference of total free energy between wurtzite and zinc-blende
structured nanowires can be reduced, leading to the easy introduction of planar defects.
To answer the second question, we noted that, the group-III concentration in the
catalyst depends on the group-III partial pressure in the neighboring vapor,13,43 which is
determined by the direct absorption of group-III atoms by the catalyst and surface diffusion
of group-III atoms to the catalyst.48 It has been demonstrated that the diffusion length of
group III increases with decreasing the V/III ratio, resulting in higher group-III concentration
in the catalyst.49 During the MBE growth, only the growth species were introduced in the
growth chamber, hence the V/III ratio can be used to determine the partial pressure of
each growth species. Therefore, with the lower V/III ratio, the Ga partial pressure in the
growth chamber should be relatively increased in our study. Under this situation, almost all
Au catalysts can absorb more Ga regardless of their original sizes, resulting in a high
101
supersaturation in the catalysts. According to Equation 3, the increased catalyst
supersaturation should give rise to a high axial growth rate of nanowires. However, this is
not consistent with our observations, suggesting that the Ga concentration in the catalysts
is not the dominating factor for nanowire growth in our case. Therefore, we consider the
role of As during our nanowire growth under a low V/III ratio situation. Due to lowered As
partial pressure under a low V/III ratio, the chances of As atoms arriving at the growth front
and incorporating with Ga into the nanowire would be weakened,48 leading to a reduced
axial growth rate of nanowires.44 On this basis, we anticipate that the As flux can be
regarded as a limiting factor for our nanowire growth. Even though the Ga concentration in
catalysts varied from size to size, nanowires can still reach the same height due to the
limited As available in the vapor. Moreover, the dramatically slow growth of nanowires at
the low V/III ratio can be considered as a near equilibrium process,50 where the formation
of low energy {1100} facets (refer to Figure 1a)51 and the formation of perfect crystal
structure are favorable.
So far, we have achieved defect-free GaAs nanowires due to their slowed growth
rate under a low V/III ratio of 10, grown at a temperature of 500°C. Since, at a higher
temperature, As diffusion can be enhanced, which may enhance the nanowire growth, it is
necessary to clarify this point. For this reason, we repeat the GaAs nanowire growth with
the low V/III ratio of 10, but increasing the growth temperature to 550°C. Figure 4 shows
the electron microscopy characterization of this new designed sample. Figure 4a is the
side-view SEM image of the grown nanowires. Similar to its counterparts grown at 500°C,
many short nanowires with diverse nanowire sizes are grown on the substrate. Figure 4b
is a BF TEM image showing perfect crystal structure, and Figure 4c is a HRTEM image
showing the wurtzite crystal structure. Our extensive TEM investigations indicate that all
these nanowires are indeed defect-free wurtzite structured GaAs nanowires, suggesting
that under such an As limited environment, increasing their diffusibility is not sufficient to
promote the growth rate, which in turn benefit to adopt perfect crystal structure. This result
is consistent with the trend found in our previous study12 where the low V/III ratio promoted
defect-free crystal structure, although the V/III ratio was as low as 4. It is of interest to note
that, in our previous study,12 both higher Ga and As flux were used, in which As is no
longer as a limiting factor under the V/III ratio of 10. This conclusion suggests that the
absolute As flux should be as minimized in order for As becoming a limiting factor for
growing defect-free nanowires.
102
Figure 4 (a) Side–view SEM image of GaAs nanowires grown at 550°C with a V/III ratio of
10. (b) BF TEM image of a typical nanowire. (c) Corresponding HRTEM image showing
the structural characteristics.
Based on our comprehensive investigations and analysis outlined above, we believe
that our nanowires were grown under different mechanisms, controlled by the absolute As
flux. For a relatively low Ga flux of 1.2 10-7 Torr, Figure 5a illustrates the growth of GaAs
nanowires under a low V/III ratio and in turn a high Ga partial pressure in the vapor.
Therefore, all catalysts are able to absorb sufficient Ga to reach the supersaturation level
for nanowire nucleation and growth. With As being the limited factor for the nanowire
growth in this case, nanowires can only grow slowly and arrive at almost the same height
eventually. The slow growth of nanowires can be regarded as a thermodynamic process,
which results in the defect-free crystal structure. On the other hand, Figure 5b depicts
nanowire growth under a high V/III ratio, where Ga plays a dominated role for nanowire
growth in the As-enriched ambient. With competition of Ga, smaller catalysts are able to
absorb more Ga and would nucleate fast and promote longer nanowires with fewer planar
defects due to the higher supersaturation. As a consequence, nanowire growth under a
high V/III ratio can be regarded as a kinetic process.52
103
Figure 5 Schematic illustration of GaAs nanowires grown under varied V/III ratios. (a) As-
limited growth. (b) As-rich growth. The cuboid in red represents a three-dimensional
nucleus at the triple phase boundary and the stacking faults are marked as black lines in
the nanowire body.
Conclusions
In conclusion, we have demonstrated that the As flux can play an important role in the
growth behavior and structural quality of GaAs nanowires grown in MBE, and two different
growth mechanisms can be found under different V/III ratios when a low Ga flux is used.
The slow growth of GaAs nanowires under a low V/III ratio is As-limited, bringing in
uniform length and perfect crystal phase as a result of a thermodynamic process. In
contrast, under a high V/III ratio, Ga should be dominating factor for the GaAs nanowire
growth, which can be classified as a kinetic process. This study verifies the impact of the
As flux on the GaAs nanowires growth and realizes the controllable growth of defect-free
nanowires by limiting the overall As in growth chamber. Therefore, this study provides a
new approach for achieving phase perfection of III-V nanowires in MBE.
104
Author Information
Chen Zhou,† Kun Zheng,*,‡,§ Zhenyu Lu,∥ Zhi Zhang,† Zhiming Liao,† Pingping Chen,∥ Wei
Lu,∥ and Jin Zou*,†,‡
†Materials Engineering, ‡Centre for Microscopy and Microanalysis, and §Australian Institute
for Bioengineering and Nanotechnology, The University of Queensland, St. Lucia,
Queensland 4072, Australia
∥National Laboratory of Infrared Physics, Shanghai Institute of Technical Physics, Chinese
Academy of Science, 500 Yu-Tian oad, Shanghai 200083, People’s epublic of China
Corresponding Authors
*E-mail: [email protected].
*E-mail: [email protected].
References
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109
Chapter 5
Effect of growth time on binary GaAs nanowires
5.1 Introduction
In this chapter, the impact of growth duration on the growth rate and structural quality of
binary GaAs nanowires were investigated. By growing nanowires for 1h, 1.5h and 2h
respectively under the same growth temperature of 500°C, we found that nanowires grew
faster and their structural qualities were enhanced with longer the growth duration. This
tendency was further confirmed by repeating the growth under 400°C, in which nanowires
were grown for 20min and 60min. This study demonstrates that the growth duration can
110
play a critical role in semiconductor nanowires growth and modulating their structural
quality. Correponsing results were published on Journal of Materials Chemistry C
(http://pubs.rsc.org/en/content/articlepdf/2017/tc/c6tc05209f) and intergrated in the Section
5.2 of this chapter.
5.2 Phase purification of GaAs nanowires by prolonging the growth
duration in MBE
In this study, we demonstrated that by merely prolonging the growth duration, the growth
behavior and phase purity of GaAs nanowires can be manipulated in molecular beam
epitaxy. Through careful morphological, structural and chemical characterizations of grown
nanowires, it was found that under the group-III dominated regime, by increasing the
growth duration, the Ga concentration in the catalysts increases due to formation of
nanowire shoulders, which provides sites for Ga diffusion towards the catalysts. This fact
leads to the enhancement of the nanowires growth and improved phase purity. In
particular, single-phase nanowire sections have been observed with prolonged nanowire
growth duration. This study provides a new insight into the role of growth duration that can
play in III-V nanowires growth and a possible approach for nanowire phases engineering,
which is critical for nanowires applications.
Introduction
Semiconductor nanowires, with the built-in one dimensional configuration enabling
quantum confinement,1 show many interesting properties in mechanical,2 thermal,3
electronic4 and optical areas,5 which are not available in their bulk counterparts. However,
these properties can be affected by the phase purity of nanowires.2-4 In particular, for the
case of III-V semiconductor nanowires, lattice defects, such as twins/stacking faults are
commonly observed, which can be considered as alternating layers of wurtzite and zinc-
blende phases,6 leading to the multi-phase nanowires that show different properties from
the single-phase nanowires.7 For example, Chen et al.8 showed that the Young’s modulus
of multi-phase GaAs nanowires was increased compared with that of the single-phase
nanowires, due to the varied bonding configuration. Additionally, Han et al.7 demonstrated
the relationship between the phase purity and electronic transport properties of GaAs
111
nanowires, in which multi-phase nanowires exhibit n-type conductivity while single-phase
nanowires show p-type conductivity. Therefore, understanding the impact factors and
realizing manipulation of nanowires phase purity and defects are critical for nanowires
applications with desired properties.
As one of the important III-V semiconductor nanowires, GaAs nanowires have been
investigated extensively,9 predominately due to their advantages in high electron mobilities
(compared with Si nanowires),10 direct band gap11 and heterojunction formation with other
III-V semiconductor nanowires, such as InAs12 and AlGaAs,13 making them great potential
for applications in future devices. To date, one of the most popular approaches for
fabricating III-V nanowires is the vapor-liquid-solid growth mechanism using Au
nanoparticles as catalysts to induce axial nanowire growth, which was first described by
Wagner and Ellis in 1960s.14 Over the years, many theoretical models and numerous
experiments have studied the growth behavior and phase engineering of III-V nanowires
by tuning the basic growth parameters, such as the growth temperature15 and the V/III
ratios.16 For example, it has been established that a relative low growth temperature can
promote high-quality nanowires because of the thermodynamic controlled growth.17,18 In
addition, for nanowires grown at III-dominated regime, their growth rate and phase purity
can be manipulated by tuning the group-III vapor pressure.19 Particularly, for GaAs
nanowires grown in MBE, the axial growth and phase purity are driven by the catalyst
supersaturation,20 which is determined by the Ga partial pressure in the vapor and thus the
Ga concentration in the catalysts during nanowire growth.21
On the other hand, it has been demonstrated that the average III-V nanowire length
increases linearly within a certain period of growth duration.22,23 However, in these studies,
nanowires were grown at relatively low V/III ratios, which is under V-dominated regime, in
which the nanowire growth is determined by the group-V vapor pressure in the
environment.21 Additionally, very little study so far was reported on the impact of the
growth duration on nanowire phase purity, which is a key factor for the grown nanowires.
In this study, we present the impact of the growth duration on GaAs nanowires grown
under a relative high V/III ratio of ~20 in MBE. Through extensive morphological, structural
and chemical investigations on the grown nanowires, we found an extraordinary nanowire
behavior: with increasing the growth duration, nanowires grow faster and have improved
structural qualities. More notably, the nanowire phase purity has been significantly
improved from multi-phase to single-phase structures during prolonged nanowire growth,
where the Ga concentration in the catalysts is found to be remarkably enhanced. The
112
fundamental reasons behind these extraordinary phenomena were carefully investigated
by electron microscopy, from which the observed extraordinary nanowire behavior was
clarified.
Experimental section
The epitaxial growth of Au catalyzed GaAs nanowires was carried out on GaAs {111}B
substrates in a Riber 32 MBE system. Degassing and deoxidization of GaAs {111}B
substrates were performed before nanowire growth to remove any contaminants in the
growth chamber. To ensure that nanowires can grow on the atomically flat surface, GaAs
buffer layers were firstly deposited on the pre-cleaned GaAs substrates at 580°C. The
substrates were then transferred to the preparation chamber and Au thin films with a
thickness of ~1 nm were deposited directly on the buffer layer surfaces. After that, the Au
coated substrates were transferred back to the growth chamber, where they were
annealed at 550°C for 5 min under the As source protection and solid Au nanoparticles
were formed to act as catalysts. The substrate temperature was then dropped to 500°C, at
which the growth of GaAs nanowires was initiated by introducing Ga source to the growth
chamber with a beam equivalent pressure of 1.9 10-7 Torr and a V/III ratio of ~20. In this
study, GaAs nanowires were grown with different growth durations, namely 1h, 1.5h and
2h. Then, the nanowire growth was terminated by switching off the Ga source while
keeping the As source until the system was cooled to 300°C.
The morphological characteristics of grown GaAs nanowires were investigated using
field emission scanning electron microscopy (FE-SEM, JEOL 7800F, operated at an
accelerating voltage of 5kV) and their structural and chemical characteristics were
investigated by TEM (Philips Tecnai F20 equipped with EDS for compositional analysis).
SEM specimen for both tilted-view and side-view investigations were cleaved along the
{110} cleavage plane of GaAs {111}B substrates. Individual nanowires for TEM observation
were prepared through ultrasonicating the nanowire samples in ethanol and dispersion
individual nanowires onto the Cu grids coated with holey carbon films.
Results and discussion
Figure 1a-c is typical SEM images taken from the samples in which GaAs nanowires were
grown respectively for 1h, 1.5h and 2h (tilted 30° with regard to the surface normal), and
113
Figure 1 (a-c) 30°-tilted SEM images of GaAs nanowires grown for 1h, 1.5h and 2h,
respectively. (d-f) Corresponding side-view SEM images. The inset in (d) plots the
nanowires length as a function of the growth duration. All scale bars are 1µm.
shows that the grown nanowires in three samples have a similar density and the majority
nanowires are straightly aligned. It is of interest to note that the nanowire diameter is
relatively uniform for 1h growth, while nanowires grown for 1.5h and 2h exhibit shoulder
morphology, resulting from nanowire lateral growth.24 To better understand the nanowire
growth status, side-view SEM images were taken from three samples, respectively
(described in Figure 1d-f), where most of them grow perpendicularly to the substrate
surface, that is, along the <111>B direction. More notably, the average length of 1h grown
nanowires is ~1.5 µm, while 1.5h and 2h grown nanowires have ~3 µm and ~5.5 µm in
length, respectively. These results were confirmed by our extensive SEM investigations of
different areas in each sample. It is of interest to note that, through compared the average
lengths of nanowires grown under different durations (see Fig. 1d inset), we can clearly
witness the tendency that the nanowire average growth rate is accelerated at the later
stage of the nanowire growth.
To understand the structural characteristics of grown GaAs nanowires, extensive
TEM investigations were performed over a dozen nanowires for nanowires grown for
different durations. Figure 2a is a BF TEM image of a typical GaAs nanowire grown for 1h,
114
in which the nanowire with a uniform diameter can be observed. Figure 2b,c is the
corresponding <11 2 0> zone-axis HRTEM images taken from the frames in Figure 2a,
showing the phase purity of the grown nanowire. As can be seen, planar defects are
randomly distributed along the entire nanowire, with the defect density counted as
~240/μm. The inset in Figure 2b is the corresponding SAED pattern taken from the
nanowire bottom, confirming the wurtzite structured nanowire. Notably, pronounced
streaks along the 000l* diffraction spots can be seen in the SAED pattern, reflecting a
high-density of planar defects lying on the {0001} planes – in an excellent agreement with
the HRTEM image. On the other hand, Figure 2d is a BF TEM image of a typical nanowire
grown for 1.5h, which has a longer length when compared with nanowires grown for 1h.
Figure 2e,f is the corresponding <11 2 0> zone-axis HRTEM images taken respectively
from bottom and top regions of the nanowire. Similar to the case of nanowires grown for
1h, a large density of planar defects is witnessed in the nanowire bottom region, echoed
by the diffraction streaks along the 000l* diffraction spots shown in the corresponding
SAED pattern (see the inset in Figure 2e). However, it can be noted that the defects
density in the nanowire top region decreases notably to ~ 133/μm. Similarly, Figure 2g
shows a BF TEM image of a typical nanowire grown for 2h, and Figure 2h is the
corresponding HRTEM image taken from the nanowire bottom region, showing a high
density of planar defects (verified by the inset of corresponding SAED pattern), which is
consistent with other two samples, indicating that at the beginning of the nanowire growth,
planar defects exist indeed. Surprisingly, Figure 2i shows a section of defect-free nanowire
in the nanowire top region, suggesting that phase purity of the nanowire has been
significantly improved at the late stage of nanowire growth (verified by the inset of
corresponding SAED pattern). These observations are further confirmed by our extensive
TEM investigations from more than a dozen of nanowires for each sample. These
experimental results indicate that the phase purity of nanowires can be improved with
increasing the growth duration.
115
Figure 2 BF TEM images showing the typical GaAs nanowire grown for (a) 1h, (d) 1.5h
and (g) 2h. Corresponding HRTEM images taken from the bottom and top regions of the
nanowires grown for (b,c) 1h, (e,f) 1.5h and (h,i) 2h. The insets in (b,e,h,i) are
corresponding <11 2 0> zone-axis SAED pattern showing the phase purity of the nanowire
regions.
Since the growth behavior of III-V nanowires can be strongly affected by the catalyst
composition,25 particularly the group-III concentration,26 statistical EDS analysis was
performed on the post-growth catalysts of grown GaAs nanowires. Figure 3a-c is typical
BF TEM images taken from typical nanowire tips in nanowires grown for 1h, 1.5h and 2h,
respectively. Figure 3d-f is their corresponding EDS spectra taken from their catalysts, in
which a very small amount of Ga atoms were left in these catalysts after nanowire growth
116
(noted the Cu peaks come from the Cu grids used to support nanowires for TEM
investigations). This conclusion can be confirmed by extensive EDS analyses from over a
dozen post-growth catalysts of nanowires grown under different growth durations. To
determine the true Ga concentrations in the catalysts during the nanowire growth, we need
to take account the Ga atoms expelled out during the cooling stage (the necking region)
and those left in the post-growth catalysts.21 As can be seen from Figure 3a-c, the neck
region (the zinc-blende structured necking region below the catalyst) in Figure 3c is larger
than those in Figure 3a,b (confirmed by the their insets of corresponding HRTEM images),
suggesting that more Ga atoms were expelled out from the catalysts of nanowires grown
for 2h during the cooling process.21 Our statistic estimations suggest that, at the end of
nanowire growth, the Ga concentration in the catalysts are ~16 at.% (for 1h grown
nanowires), ~22 at.% (for 1.5h grown nanowires) and ~37 at.% (for 2h grown nanowires),
respectively (Figure S1). These results indicate that the Ga concentrations in the catalysts
vary during the nanowire growth - increasing with prolonged nanowires growth.
Figure 3 (a-c) High-magnified BF TEM images of the typical tip region in GaAs nanowires
grown for 1h, 1.5h and 2h, respectively. The yellow arrows mark the length of necking
regions. The insets are corresponding HRTEM images showing the structure of the
necking region. (d-f) Corresponding EDS spectrums taken from the catalysts.
117
Based on our detailed TEM investigations, the growth behavior of GaAs nanowires
can be summarized as follows. For the first hour of nanowire growth, nanowires are short
and have multi-phase structure; while, with increasing the growth duration, nanowires are
longer and their phase purities have been improved, which were significantly enhanced
during further prolonged nanowire growth. Particularly, for nanowires grown for 2h, single-
phase nanowire section can be found in the final growth stage. To understand this
tendency, we noted from our previous study21 that for GaAs nanowires grown in a Ga-
dominated growth regime (note that in this study, V/III ratio is ~20), catalysts containing
higher Ga concentration, and hence the resultant high supersaturation, enhance the axial
nanowires growth and wurtzite phase purity. It has also been well demonstrated that Ga in
the Au catalysts can affect the catalysts surface energy, leading to the phase purification
of GaAs nanowires.27 Therefore, to understand the differences of nanowire growth rate
and phase purity in our different growths, it is necessary to understand why Ga
concentrations in the catalysts increase with increasing the growth duration.
It has been generally acknowledged that in III-V nanowire growth, the size of catalysts
may vary the III concentration in catalysts due to their different surface tensions.21,28
Therefore, to understand the impact of the catalyst size on the catalyst composition and
the subsequent nanowire growth, we statistically measured the catalyst diameters in
nanowires grown under different durations. Figure 4 summaries our measurements and
shows that the distributions of catalyst sizes in three cases are similar (30-50 nm),
verifying the identical pre-growth (Au film deposition and annealing process). Based on
this part of the study, the impact of catalyst size on the nanowire growth can be eliminated.
On the other hand, it should be noted that the temperature difference between the hot
substrate and catalyst with longer the nanowire length (less than 10 µm) is minor;29
therefore, the temperature influence on the Ga concentration in the catalysts during
nanowire growth can be negligible.
118
Figure 4 Histograms statistically illustrate the catalysts diameters of GaAs nanowires
grown for 1h, 1.5h and 2h, respectively.
To further understand why Ga concentrations in the catalysts increase with increasing
the nanowire growth duration, , we note that Ga in the catalysts can be collected via three
ways: 23 directly absorption by catalysts, substrate diffusion and nanowires sidewall
diffusion. Since Ga absorption by catalysts is similar in the three samples which were
grown under the same V/III ratio, and Ga diffusion length is relatively short on GaAs {111}B
surface,30 the Ga concentration in catalysts is mainly determined by its sidewall diffusion.
On this point, we note that with increasing the growth duration, nanowires formed the
shoulder morphology. It has been theoretically demonstrated that these shoulders can be
considered as numerous atomic step sites (shown in Figure 5a) that increase with larger
shoulder diameter, where atoms can be collected in these sites and further diffuse towards
the catalysts.23 Since the incident angle (α) of Ga flux in the MBE system is less than 90°
(see Figure 5b), Ga atoms are not only collected by the nanowire sidewall (perpendicular
to the substrate surface) but also collected by the step sites (parallel to the substrate
surface, proportional to cosα),23 the fraction of Ga atoms reaching the catalysts can be
increased with increasing the step sites, which is in excellent agreement with our
experimental results. To comprehensively understand the growth mechanism of GaAs
nanowires with increasing the growth duration and their phase evolution, schematic growth
models were proposed as described in Figure 5c. As can be noted, nanowires grown for
1h have relatively uniform diameter, while, with increasing the growth duration to 1.5h, the
formed shoulders provide step sites for Ga atoms collection and assisting their further
diffusion towards the catalysts, and thus more Ga atoms can reach the catalysts (note:
119
under the relative high V/III ratio of 20 in this study, As diffusion is not a limiting factor for
nanowires growth). However, since the nanowires lateral growth occurs with increasing the
growth duration, we anticipate that the competition between Ga being consumed by
nanowires lateral growth and Ga being collected by the step sites is significant at this
stage (Figure S2). Consequently, Ga concentration in the catalysts remains relatively
stable during nanowire growth from 1h to 1.5h. Nevertheless, with further prolonging the
growth duration to 2h, the thicker shoulders provide more step sites, and leading more Ga
Figure 5 (a) HRTEM image of a typical shoulder area as marked in the inset. The red line
describes the shoulder with atomic steps. (b) Sketch of the typical shoulder and the
incident angle of Ga flux in the MBE system. (c) Schematic growth models illustrate GaAs
nanowires growth with increasing the growth time and corresponding Ga concentration in
the catalysts during the growth.
120
atoms to diffuse to the catalysts and in turn increasing the Ga concentration in the
catalysts. As a consequence, nanowires axial growth rate increase and their phase purity
are significantly improved. This impact of growth duration on nanowire growth was further
confirmed when we repeated the growth with varied growth parameters (see Figure S3-
S4). The multi-phase GaAs nanowires have been demonstrated to have a type II band
alignment and unique photoluminescence behavior,31,32 which shows advantages in
photovoltaics applications.33
Conclusion
In conclusion, we showed that growth duration can play a critical role in manipulating the
growth behavior and phase purity of GaAs nanowires grown in MBE. By increasing the
growth duration, the Ga concentration in the catalysts increases which in turn increases
the nanowire growth rate, caused by shoulder assistance sites for Ga diffusion towards the
catalysts. These lead to the growth of high phase-purity GaAs nanowires and the unique
configuration of multi-phase to single-phase structure opens up possibilities for band
structure alignment in nanowires. This study proposes a new mechanism for
understanding the growth behavior and structural characteristics of GaAs nanowires
through varying the growth duration, which may be universally applicable in other III-V
nanowires systems.
Author Information
Chen Zhou,† Kun Zheng,*, ♩, ¶ Zhi-Ming Liao,† Ping-Ping Chen,§ Wei Lu,§ and Jin Zou*, †, ♩
†Materials Engineering, ♩Centre for Microscopy and Microanalysis, ¶Australian Institute for
Bioengineering and Nanotechnology, The University of Queensland, St. Lucia,
Queensland 4072, Australia
§National Laboratory of Infrared Physics, Shanghai Institute of Technical Physics, Chinese
Academy of Science, 500 Yu-Tian oad, Shanghai 200083, People’s epublic of China
Corresponding Author
*Email: [email protected],
*Email: [email protected].
121
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Supporting information
It has been well demonstrated in our previous study1 that Ga concentration in the catalysts
during GaAs nanowires growth is the sum of Ga atoms (NN) in the “neck” and Ga atoms
(NC) in the post-growth catalyst detected by EDS: NGa = NN + NC. Since there is almost no
Ga left in post-catalysts of three samples (Figure 3d-f), NC ~ 0. Therefore, Ga
concentration during nanowires growth is estimated as NN / (NN + NAu) with NAu being the
number of Au atoms in the catalyst.
Since the “neck” region is zinc-blende structured GaAs, which contains 4 Ga atoms
in a unit cell, NN can be obtained by:
NN = 4VN/ , (1)
where is the volume of a unit cell of the zinc-blende structured GaAs, and VN is
the volume of the cylinder shaped “neck” region, which, according to Figure S1, can be
determined by
2H (L being the lateral dimension of the nanowire and H the height
of the “neck” region). Similarly, NAu can also be determined with Equation 1, by replacing
VN with catalyst volume
(L/2 denotes the radius of the hemispheric catalyst, as
illustrated in Figure 1Sb); and with unit cell volume of Au ( ) with face-
centered cubic structure: (aC ≈ 0.41 nm, the lattice parameter).
Figure S1 (a) A HRTEM image showing the top region of a typical GaAs nanowire grown
for 2 h. (b) A schematic model showing the nanowire top containing a hemisphere catalyst,
a cylinder disk-shaped “neck” region and the nanowire section.
125
Figure S2 shows the statistical measurements of GaAs nanowires lengths and
corresponding lateral dimension at nanowires bottom regions with the growth durations. As
can be noted, the average length of 1h grown nanowires is ~1.5 µm, while 1.5h and 2h
grown nanowires have ~3 µm and ~5.5 µm in length respectively, suggesting that
nanowire axial growth increases with the growth duration. The corresponding nanowires
diameters at the bottom regions are ~60 nm, 110 nm and 140 nm, suggesting the
nanowire lateral dimension increases with increasing the growth duration as well.
Figure S2 Plot of nanowires lengths and diameters in the bottom regions with the growth
durations.
To further confirm the impact of growth duration on nanowire growth behavior and phase
purity, we repeated the growth with different growth parameters (growth temperature of
400°C, a thinner Au film deposition (1100°C, 10s), growth duration of 20min and 60min,
respectively). (Figure S3-S5)
Figure S3a,b shows the typical side-view images of the grown sample, suggesting the
similar nanowire density and the major nanowire growth orientation is perpendicular to the
substrate surface. The average length of nanowires grown for 20min is ~500 nm, while
nanowires grown for 0min are as ~2 μm high. Additionally, it can be noted that the
diameter of 20min grown nanowires is uniform along the nanowire (see the inset in Figure
S3a), while nanowires grown for 60min formed the shoulders.
126
Figure S3 (a,b) Typical side-view SEM images of GaAs nanowires grown for 20min and
60min at 400°C. The inset in (a) is corresponding 30°-tilted SEM image.
Figure S4 shows the TEM analyses of the 20min-grown nanowires sample. Figure S4a
shows BF TEM image of a typical grown nanowire and Figure S4b,c is corresponding
HRTEM images of the nanowire top and bottom regions, indicating the defected structure
along the whole nanowire, which is confirmed by the our statistical analysis of this
nanowire sample, as shown by the BF TEM image (Figure S4d) of another typical grown
nanowire and the corresponding HRTEM image (Figure S4e) from the nanowire top.
Figure S4f shows a typical EDS point analysis profile taken from the catalyst and a table
summarizing the catalysts composition containing 5-8 at.% Ga of nanowires shown in
Figure S4a,b.
127
Figure S4 BF (a) and (b,c) HRTEM images of a typical 20min-grown GaAs nanowire and
its top and bottom regions. BF (d) and (e) HRTEM images of another typical nanowire and
its top region. (f) EDS point analysis profile taken from a typical catalyst and the table
show the catalysts composition of (a,d).
Figure S5 shows the TEM analyses of the 60min-grown nanowire sample. Figure S5a is
the BF TEM image of a typical nanowire, which has shoulder morphology. Figure S5b,c is
corresponding HRTEM images of the nanowire top and bottom regions, where the bottom
region is defected as the 20min-grown nanowire is, but the top region is defect-free. This
phenomenon is further confirmed by our statistical analyses on this sample, as typically
depicted in Figure S5d,e. Figure S5f shows a typical EDS point analysis profile taken
from the catalyst and a table summarizing the catalysts composition containing 14-17 at.%
Ga of nanowires shown in Figure S5a,b, which is higher than the Ga concentration in the
catalysts of the 20min grown nanowires.
128
Figure S5 BF (a) and (b,c) HRTEM images of a typical 60min-grown GaAs nanowire and
its top and bottom regions. BF (d) and (e) HRTEM images of another typical nanowire and
its top region. (f) EDS point analysis profile taken from a typical catalyst and the table
show the catalysts composition of (a,d).
References
(1) Zhou, C.; Zheng, K.; Lu, Z.; Zhang, Z.; Liao, Z.; Chen, P.; Lu, W.; Zou, J., Quality
Control of GaAs Nanowire Structures by Limiting As Flux in Molecular Beam Epitaxy. J.
Phys. Chem. C 2015, 119, 20721-20727.
129
Chapter 6
Effect of growth time on ternary InGaAs nanowires
6.1 Introduction
In this chapter, the growth of Au-catalyzed ternary InGaAs nanowires on GaAs {111}B
substrates was studied. The nominal In concentration in group-III (In/(In+Ga)) for InGaAs
nanowires growth is 50 at.%. By extensive cross-sectional study of nanowires tip, middle
and bottom regions which have typical varied chemical characteristics, we found that
grown nanowire formed the extraordinary hierarchical structure. By repeating nanowires
growth with shorter growth durations, the growth mechanism of such unique structured
130
nanowires was unveiled. A manuscript has been drafted based on our careful observations
and investigations, and was descried in Section 6.2 of this chapter.
6.2 Unexpected formation of hierarchical structure in ternary InGaAs
nanowires
The optoelectronic application of semiconductor nanowires largely depends on their
nanostructures and related chemical characteristics. Although the spontaneously formed
core-shell structure in ternary nanowires makes them often unpredictable, such versatile
growth with varied chemical characteristics may open up opportunities for widening their
applications. In this study, we present extraordinary phenomena observed during ternary
InGaAs nanowires growth by molecular beam epitaxy. It was unexpectedly found that
nanowires spontaneously formed the hierarchical structure during the growth: pure core
structure at nanowires tip, core-shell structure at middle and core-multishell structure at
bottom regions. By careful electron microscopy investigations on nanowires growth at the
early stages, the growth mechanism of such unique hierarchical structure was unveiled.
This study provides new insights for ternary III-V nanowires growth with the hope of
assistance in designing new nanomaterials and nanostructures for future optoelectronic
devices.
Introduction
One-dimensional semiconductor nanowires, due to their small dimensions and peculiar
properties,1 are the building blocks for nanoelectronic,2 catalysis3 and energy-conversion
devices.4 The rational design of nanowire structures for applications in nanotechnology
requires an improved understanding of their phase behavior in nanoscale systems.
However, the predictable growth of compound nanowires, particularly ternary and more
complex nanowires, is more difficult than their counterparts of elemental Si or Ge
nanowires, due to the complexity of ternary and quaternary phase diagrams.5 In this
regard, systematic and detailed investigations of semiconductor compound nanowires with
a wide range of multicomponent are required to gain a full understanding of one-
dimensional nanowire systems.
131
Ternary III-V nanowires, as an important class of one-dimensional nanomaterials,
have attracted a great attention, due to their band gaps tunability by tuning the
composition of nanowires.6-8 Among them, ternary InGaAs nanowires, with tunable band
gaps ranging from the near-infrared to the infrared region (0.34 1.42 eV),7 can find wide
applications in optoelectronics.9,10 The epitaxial growth of ternary III-V nanowires is
typically catalyzed by Au nanoparticles in MBE or metal-organic chemical vapor deposition
systems, in which the nanowire growth direction and quality can be secured for device
integrations.11,12 However, in these grown systems, complex nanostructures, such as core-
shell structure, can be formed spontaneously during the growth of a wide range of ternary
III-V nanowires, such as In/AlGaAs,6,13,14 GaAsN/P/Sb15-17 and InAsSb18 nanowires,
resulted from phase segregations. The structural and chemical variations in these ternary
nanowire systems have made them unpredictable, which in turn, however, may breed
diverse and exciting opportunities to create new nanomaterials and nanostructures in the
future. Therefore, understanding the versatile growth of ternary III-V nanowires has the
potential to answer fundamental questions about one-dimensional systems, which is
crucial to pursue towards understanding fundamental properties of nanostructures and
designing nanostructured materials with desired properties.
In this study, we present a new class of nanostructures with increasing complexity
formed in ternary InGaAs nanowires. It was found that during the “one-pot” growth,
nanowires spontaneously formed the hierarchical structure respective core, core-shell
and core-multishell structure at the nanowire tip, middle and bottom regions. This unique
configuration enables interfacial states and quantum effects in the nanowires, which would
exhibit peculiar physical properties and could also be obtained via the “step-by-step”
growth.19 With careful investigation through advanced electron microscopy, the growth and
evolution of this complex nanostructure were detailedly unveiled. This study, for the first
time, report the high complexity of nanostructure in ternary III-V nanowires via the “one-
pot” growth, which provides valuable insights in the field of ternary semiconductor
nanowires and possibilities for creating new nanomaterials and nanostructures via an
energy-saving and low-cost approach..
Experimental section
Au-catalyzed InGaAs nanowires were epitaxially grown on GaAs {111}B substrates in a
Riber 32 MBE system. After a GaAs {111}B substrate was degassed and deoxidized to
132
remove any contaminants in the growth chamber, a GaAs buffer layer was grown on the
substrate at 580°C to secure an atomically flat surface for nanowire growth. Subsequently,
the substrate was transferred to the preparation chamber, in which an Au thin film with a
thickness of ~1 nm was deposited on the top of the GaAs buffer layer. The Au-coated
substrate was then transferred back to the growth chamber, and annealed at 550°C for 5
min under As ambient (As source was switched on throughout the nanowire growth), in
which the Au thin film aggregates into nanoparticles.20,21 After annealing, the substrate
temperature was dropped to 400°C, where In and Ga vapor sources were introduced to
induce the nanowire growth, with the same beam equivalent pressure of ~2.2 10-7 Torr
to achieve the nominal In concentration (In/(In+Ga)) of ~50%, and As vapor pressure of
~4.4 10-6 Torr to achieve a V/III (As/(In+Ga)) ratio of ~10. The nanowire growth was
terminated after 60 min and cooled down in the As ambient naturally until 300°C.
The morphology, structure and chemical composition of grown nanowires were
investigated using FE-SEM (JEOL-7800F, operated at 5 kV) and TEM (Philips Tecnai-F20
operated at 200 kV equipped with EDS for compositional analysis), respectively. Individual
nanowires for TEM investigations were prepared by taking an ultrasonic bath in ethanol
and dispersing individual nanowires onto the Cu mesh grids coated with holey carbon
films. The cross-sections of grown nanowires for TEM investigations were prepared by first
embedding the nanowire sample in epoxy resins and then sectioning by an ultramicrotome
(Leica EM UC6). The sectioning was performed from nanowire bottoms to tops, and each
sectioned slice was parallel to the substrate surface. The individual thin slices from
different regions of nanowires were collected and transferred onto the Cu grids for TEM
investigations. On the other hand, to clarify the growth of the planar layer, the cross-
section of nanowires with their substrate was also prepared by using FIB (FEI-SCIOS),
after the grown nanowires were shook off. The prepared lamellar cross-section specimen
was then attached to the Cu lift-out grid for TEM investigations.
Results and discussion
Figure 1a is a top-view SEM image of grown nanowires, showing that most of the
nanowires were grown vertically with respect to their substrate, and its inset showing the
nanowire has the hexagonal cross-section. Figure 1b is the corresponding side-view SEM
image taken along the <110> direction, from which we further verify that the majority of
nanowires are grown perpendicular to the substrate surface, and their average length is ~2
133
µm. Additionally, grown nanowires have the pencil shape, similar to those binary GaAs
nanowires with the wurtzite structure,22-24 caused by their lateral growth,25 and pyramid
bases (marked by arrows) resulted from atoms surface diffusion.9,26 Figure 1c is a BF)TEM
image of a typical nanowire, and Figure 1d is the corresponding TEM image taken from
the nanowire tip, showing the catalyst size of ~25 nm. Figure 1e is a high-resolution HR-
TEM image taken from the middle region of the nanowire and the inset is the
corresponding SAED pattern, indicating that the nanowire has the wurtzite structure with
no observable lattice defects and its growth direction is along the [000 ] direction. Our
extensive TEM investigations from over a dozen nanowires confirmed that our nanowires
are defect-free. The combination of SEM and TEM investigations indicates that the side-
facets of our nanowires are {11 0}. Figure 1f is a high-magnified TEM image taken from
the typical nanowire bottom region, in which the strain contrast can be witnessed,
suggesting the formation of core-shell structure in the nanowire. To further understand the
compositional characteristics of grown nanowires, corresponding EDS analyses were
performed. Figure 1g shows respectively EDS spectra taken from the typical catalyst and
nanowire tip region (refer to Figure 1d), in which no Ga is found in the catalyst while the In
concentration is measured as ~26 at.% (point 1); while the Ga concentration at the
nanowire tip is measured as ~72 at.% of group III elements (point 2), higher than the
nominal Ga concentration (50 at.% of group III elements), which is consistent with previous
study that Au has a stronger affinity with In,13,27-31 leading to Ga being expelled out from
the catalysts and form the Ga-enriched nanowire tip.
To understand the chemical characteristics of our nanowires along their lateral
direction, extensive TEM investigations were further performed on the nanowire cross-
sections taken from different regions. Figure 2a shows a BF TEM image of a typical
nanowire and Figure 2b-d is BF TEM images of typical cross-sections taken from nanowire
tip, middle and bottom regions, respectively (marked in Figure 2a, note the middle region
is located in the pencil body but close to the nanowires tip, refer Figure S1, Supporting
Information). In Figure 2b, the nanowire cross-section has a lateral dimension of ~27 nm,
close to the catalyst size. Furthermore, the relative uniform contrast indicates that no
nanowire shell is found in this region; while both cross-sections at the nanowire middle and
bottom regions have a similar lateral dimension of ~120 nm, indicating existence of
134
Figure 1 Typical (a) top-view and (b) side-view SEM images taken from grown nanowires.
The inset in (a) is the zoom-in top-view SEM image. The arrow in (b) shows the nanowire
base. (c) BF TEM image of a typical nanowire. (d-f) Corresponding TEM/HRTEM images
taken from nanowire tip, middle and bottom regions respectively, as marked in (c). The
inset in (e) is corresponding <11 0> zone-axis SAED pattern. (g) Typical EDS spectra
taken from the catalyst and nanowire, as marked in (d).
nanowire lateral growth. Figure 2e-g shows corresponding EDS line-scan profiles of the
cross-sections taken from the respective regions (note that no As peaks were shown due
to its constant composition in the nanowires). The uniform element distributions along the
line-scan across the cross-section at the nanowire tip confirm the nanowire core structure;
while the In intensity profile over the cross-section from the nanowire middle region
135
suggests a core-shell structure with In-poor core and In-enriched shell (Figure 2f); and
surprisingly, three In peaks are witnessed in the EDS line-scan profile of the cross-section
at the nanowire bottom region, suggesting the In-enriched outer-shell, In-poor inner-shell
and In-enriched core. This extraordinary finding is further verified by EDS elements maps
of the cross-section from this region (Figure 2g). These experimental findings form
different regions are further confirmed by our statistical EDS analyses of over a dozen
Figure 2 (a) BF TEM image of a typical InGaAs nanowire. (b-d) BF TEM images of typical
cross-sections at nanowires tip, middle and bottom regions, respectively, as marked in (a).
(e-g) Corresponding EDS line-scan profile of the cross-sections. (h) Measured In
concentrations at different sections and different regions from the nanowire cross-sections
(based on statistic EDS analyses at varied positions - typically marked in (b-d)). (i)
Schematic illustration of the grown hierarchical structure.
136
cross-sections from each respective region. Figure 2h summaries the In concentrations
measured in different regions and different sections. Accordingly, Figure 2i illustrates the
composition profile found in our InGaAs nanowires. Based on the obtained In
concentrations found in the nanowire cores at different regions, we found that the In
concentration in the nanowire cores decreases (Ga concentration increases) along the
nanowires from bottoms to tops.
To understand how the hierarchical structure forms, particularly the core-multishell
structure, we examine nanowire status in the early stage of the nanowire growth.
Accordingly, we repeated the nanowire growth by shortening the growth duration to 15 min
and keep same for the other growth parameters. Figure 3a shows a side-view SEM image
of nanowires grown for 15 min, in which the nanowires have their average length of ~500
nm. Figure 3b,c is BF TEM images taken from a typical nanowire and a typical cross-
section at the nanowire bottom (mark in Figure 3b), both show nanowire diameter of ~60
nm. Figure 3d is the corresponding EDS line scan and Figure 3e is element maps, both
taken from the nanowire cross-section, indicating a nanowire core-shell structure with In-
enriched core and In-poor shell. This is further verified by our statistical compositional
measurements of over a dozen cross-sections at nanowire bottoms (Figure 3f), in which
the In concentration is measured as 41 ± 3 at.% in nanowire cores while 18 ± 3 at.% in the
nanowire shells. These values are similar to the compositions at the nanowire cores and
their inner-shells at nanowire bottoms grown for the 60 min. Further TEM study was also
performed at cross-sections from nanowire middle regions, showing similar chemical
characteristics (Figure S2, Supporting Information). At the nanowire tips, Figure 3g shows
a corresponding EDS spectrum and indicates the In concentration of ~40 at.% in the
nanowire core. These results suggest that nanowires spontaneously form the core-shell
structure and the cores maintain a constant composition during their first 15 min growth.
Moreover, by comparing the chemical characteristics of the nanowire bottoms in two
growth durations (15 min and 60 min), the nanowire outer-shells found in the nanowires
grown for 60 min must be formed during the late stage of the nanowire growth.
137
Figure 3 Typical (a) side-view SEM and (b) BF TEM images of the InGaAs nanowire
grown for 15 min. (c) BF TEM image of the cross section from the nanowire bottom region
marked in (b). (d) Typical EDS line scan profile, (e) EDS elements maps and (f)
compositional measurements (marked in (c)) of the cross section. (g) Typical EDS
spectrum taken from the nanowire top (mark in (b)).
By correlating the experimental results shown above, we note the following facts.
With short growth duration, nanowires form a core-shell structure and the composition of
the nanowire cores remains as a constant. With increasing the growth duration, (1) In-
enriched outer-shells form in the nanowire bottom regions, resulting in nanowire core-
multishell structure, and (2) the Ga concentration in the nanowire cores increases from
nanowire bottoms to their tops. As a consequence, a nanowire hierarchical structure is
formed.
138
To understand the formation of this extraordinary hierarchical nanowire structure, we
note that, during ternary III-V nanowire growth, their composition can be affected by the
simultaneous two-dimensional layer growth on the substrate.15 In this regard, it is
necessary to clarify the nature of the planar layer directly grown on the substrate.
Accordingly, TEM investigations were performed on the cross-sections of two nanowire
samples to characterize the grown planar layers. Figure 4a,c is cross-section TEM images
taken respectively from the nanowire sample grown for 60 min and 15 min, in which the
planar layer grown for 60 min contains larger convex areas than those in the planar layer
grown for 15 min (see the inset in Figure 4c). By correlating with the SEM images (e.g.
Figure 1b), these convex areas can be assigned as the nanowire bases.26 Figure 4b,d is
the corresponding EDS spectra taken from locations of planar layers (P1, P3) and typical
nanowire bases (P2, P4), showing different In concentrations of 36 ± 2 at.% at P1 and 57
± 2 at.% at P2, and the In concentrations of 33 ± 2 at.% at P3 and 36 ±2 at.% at P4 (refer
the respective insets). This suggests that the In concentrations in the nanowire bases are
higher than those in the planar layers, which is further confirmed by the typical EDS line
scans marked in Figure 4a,c. The comparison of planar layers in two cases indicates that
(1) with the short growth duration, the grown InGaAs planar layer and nanowire bases are
Ga-enriched (compared with the nominal composition of Ga:In = 50:50) which should be
energetically favorable by reducing the lattice mismatch with the GaAs substrates;32 and
(2) with the prolonged growth duration, the nanowire bases grow significantly, and become
comparatively In-enriched InGaAs bases. It has been demonstrated that nanowires base
growth occurs at the early stage of the nanowire growth,26 which is determined mainly by
two factors: mass transport (depending upon atom diffusion on the substrate surface)23
and growth energetics (depending upon energy minimization of the growth).33 It has been
well documented that, for a In(Ga)As layer epitaxially grown on a GaAs substrate, the
misfit strain gradually builds up, which can be eventually relaxed through the generation of
misfit dislocations34,35 or island growth through mass transport.36,37 Therefore, in the case
of growing InGaAs nanowires on GaAs substrates, the nanowire bases are formed through
mass transport and result in the strain relaxation between the planar layer and the
substrate. Due to the lower surface energy of InAs than that of GaAs,38,39 the nanowire
bases can accommodate more In, leading to the In-enriched nanowire bases, particularly
at the later stage of the nanowire growth.
139
Figure 4 (a,c) TEM image of the cross-sections taken from the nanowires samples grown
for 60 min and 15 min, respectively. The insets are typical EDS line scan profiles in (a,c)
and enlarged TEM image of a typical nanowire base in (c). (b,d) Corresponding EDS
spectra and the insets of In concentration measurements taken from the cross-sections as
marked in (a,c).
Accordingly, our extraordinary observations can be explained by answering the
following two questions: (1) Why the core-multishell structure is formed and (2) why the
group III composition of nanowire cores varies, both when the nanowire growth duration is
prolonged.
To answer the first question, we note that the spontaneously formed core-shell
structure in ternary III-V nanowires can be ascribed to the phase segregation, in which
catalysts induce the nanowire core growth via the VLS growth mode, while the nanowire
shell growth results from nanowire lateral growth under the VS growth mode.14,15 It has
been documented that, during III-V nanowire shell growth, their composition depends on
the chance of group-III atoms bonding with group-V atoms on the nanowire sidewalls.40 In
this study, the fact is that the In-poor inner shell at the nanowire bottoms was formed
140
during the early stage of the nanowire growth (15 min), in which the grown nanowire bases
and planar layer are comparatively Ga-rich when compared with the nominal group-III
concentration (Ga:In=50:50). Therefore, excessive Ga atoms on the substrate surface can
diffuse to the nanowire sidewalls, and in turn incorporating with As atoms to form Ga-
enriched nanowire shells. Additionally, since the Ga-As bond energy is stronger than In-
As, the Ga-As bonding should be relatively stable from decomposition.41 On the other
hand, during the later stage of the nanowire growth, the fact of In-enriched nanowire bases
indicates that relatively more In atoms can diffuse from the substrate surface to the
nanowire sidewalls. As a result, the In concentration on the nanowire sidewalls should be
increased, which increases their chances to bond with As atoms. Consequently, more In
atoms can incorporate to the nanowire sidewalls and form the In-enriched outer shells.
To answer the second question, we also note that the In concentration in nanowire
cores are lower than the nominal In concentration (Ga:In = 50:50), which is consistent with
previous observation that Au catalysts prefer to affiliate In while expel Ga after the
supersaturation of group-III concentration in the catalysts is reached, leading to the high
Ga concentration in the nanowire cores.13,30,42,43 Furthermore, the fact that, during the later
stage of the nanowire growth, the growth of In-enriched InGaAs nanowire bases may
result in less In available for the nanowire core growth.15 Due to less In impingement on
the catalysts surface, less In can then incorporate to the catalysts, leading to the
decreased In concentration but increased Ga concentration in catalysts, so as in the
nanowire cores at later stage of the nanowire growth. Therefore, compositional varied
nanowire cores are achieved..
To understand the growth mechanism of our hierarchical InGaAs nanowires, we
further investigate nanowires grown for 5 min to clarify when and why the pencil-shape of
our nanowires are formed. Figure 5a shows side-view SEM images of nanowires grown for
different durations, in which the pencil-shape morphology can be witnessed at early stage
of the nanowire growth (5 min) and such morphology was maintained with prolonging the
growth duration. This suggests that nanowires prefer to maintain the vertical {11 0}
sidewalls (Figure S2, Supporting Information) rather than the tapered morphology caused
by the lateral growth.25 Since the sidewalls of tapered nanowires are no longer to be the
{11 0} sidewalls for the wurtzite structure or the {110} sidewalls for the zinc-blende
structure, the fact that our nanowires remain {11 0} sidewalls suggests that the growth of
the nanowires was controlled by thermodynamics as the {11 0} planes have low surface
141
energy.44 According to our observations of nanowires growth at early stages, we propose a
growth model45 to illustrate the formation of our hierarchical nanowire structure in Figure
5b. It should be noted that, during the early stage of the nanowire growth, nanowires form
an initial core-shell structure along the nanowires (except the tip region below the
catalysts) with an In-enriched core and an In-poor shell. Simultaneously, nanowire bases
and the planar layer were grown due to atom deposition on the substrate surface and
adatom diffusion. With prolonging the growth duration, due to the strain relaxation between
the InGaAs planar layer and the GaAs substrate, adatom diffusion on the substrate
surface increases, particularly for In atoms with higher mobility,9,13 leading to the an
increased In concentration in the nanowire bases. Consequently, more In adatoms can
diffuse to the nanowire sidewalls, resulting in the growth of (1) In-enriched outer shell on
the initially formed core-shell nanowire structure (forming the core-multishell structure at
nanowire bottoms), and (2) the In-enriched shell on the newly grown nanowire core
(forming the core-shell structure at nanowire middle regions). As a result, the observed
hierarchical core-multishell nanowires are formed.
Figure 5 (a) Side-view SEM images of InGaAs nanowires grown with varied durations. (b)
Schematic growth model describes the hierarchical structure formation in grown InGaAs
nanowires with prolonged growth duration.
142
We anticipate that our extraordinary hierarchical structured nanowires have great
potential in versatile optoelectronic applications. Such unique nanostructure can affect
carriers’ transports in the nanowire and thus selectively realize quantum confinement in
individual nanowires. In particular, the core-multishell structure at nanowire bottoms may
confine carriers in the nanowire cores and outer-shells, acting as a quantum nanotube. In
fact, the core-shell nanostructure integrated photo-detector and emitters have shown
prominently amplified spontaneous emissions.46 For this reason, the findings observed in
this study suggest the potential usefulness of our unique hierarchical structured nanowires
as optoelectronic components and pave a pathway for developing new quantum structures
for boarding their applications in advanced devices.
Conclusion
In conclusion, we demonstrated an extraordinary phenomenon of a hierarchical structure
formed in the ternary InGaAs nanowire. By tracing the nanowires growth at early stages,
we found that this complex structure was actually formed during the later stage of the
nanowire growth, in which the composition of InGaAs planar layer that simultaneously
grown on the GaAs substrate was changed, due to the requirements of stain relaxation
between the planar layer and substrate, and the energy minimized growth of the planar
layer. This fact in turn, affects the nanowires composition and induces the hierarchical
structured nanowire. The formation of such extraordinary hierarchical structured nanowire
is expected to show unique optoelectronic properties and thus could lead to wider
applications in nanoscale optoelectronic and quantum communications systems. This
study provides a new insight for ternary III-V nanowires growth and sheds light on the
fundamental understanding extended to the general one-dimensional nanomaterials for
optoelectronic and quantum applications.
Author Information
Chen Zhou,† Kun Zheng,*, ♩, ¶ Ping-Ping Chen,§ Wei Lu,§ and Jin Zou*, †, ♩
†Materials Engineering, ♩Centre for Microscopy and Microanalysis, ¶Australian Institute for
Bioengineering and Nanotechnology, The University of Queensland, St. Lucia,
Queensland 4072, Australia
143
§National Laboratory of Infrared Physics, Shanghai Institute of Technical Physics, Chinese
Academy of Science, 500 Yu-Tian oad, Shanghai 200083, People’s epublic of China
Corresponding Authors
*E-mail: [email protected].
*E-mail: [email protected].
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Supporting Information
Figure S1a shows the side-view SEM image of the 60 min grown nanowires and Figure
S1b,c is the TEM images of the cross sections taken from the nanowires middle and
bottom regions, respectively. Since the cross sections were cut from nanowires bottom to
top with a step of ~ 50 nm, the small-size cross sections in Figure S1 indicate that they are
from the nanowires top (with lower height), while the larger-size cross sections indicate
that they are from the nanowire middle regions (as shown in Figure S1a).
Figure S1 (a) Side-view SEM image of the 60 min grown InGaAs nanowires. Typical TEM
images of the cross sections taken from the nanowires (b) middle and (c) bottom regions
marked in (a).
Figure S2a is a typical TEM image of the pencil-shape nanowire top and Figure S2b-d is
typical TEM images of the cross sections taken from the varied regions of the pencil top
with lateral dimensions of ~27 nm, 70 nm and 100 nm respectively (marked in Figure S2a).
Figure S2e-g is corresponding EDS line scan profiles of the cross sections, in which
nanowires keeps the core-shell structure of In-poor core and In-enriched shell until there is
no shell growth. This result is consistent with the core-shell structure observed at
nanowires middle regions (see Figure 2f). Figure S2h-i is corresponding [0001] zone-axis
SAED pattern, indicating that nanowires keep the {11 0} side-facets during the growth.
Figure S2j shows the corresponding composition measurements of the cores composition
taken from the cross sections, indicating that the In concentration ranging from 32 ± 2 at.%,
30 ± 2 at.% to 28 ± 2 at.% with the decreased diameter of the cross section. On the other
149
hand, Figure S2k is a typical TEM image of the 15 min grown nanowire and Figure S2m is
TEM images of the cross sections taken from the nanowires middle and top regions,
respectively. The corresponding EDS elements maps (Figure S2n) confirm that the grown
nanowires form core-shell structure with the In-enriched core and In-poor shell along the
nanowire (expect the region close to the catalyst). Figure S2l is corresponding [0001]
zone-axis SAED patterns, indicating the {11 0} side-facets of the nanowires.
Figure S2 (a) Typical TEM image of the 60 min grown nanowire top. (b-d) Typical TEM
images of the cross sections taken from the pencil-shape top as marked in (a). (e-g)
Corresponding EDS line scan profiles and (h-i) SAED patterns. (j) Compositional
measurements of the cores compositions marked in (b-d). (k) Typical TEM image of the 15
min grown nanowire. (m) Typical TEM images of the cross sections taken from the middle
and top regions marked in (k). Corresponding (n) EDS elements maps and (l) SAED
patterns.
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Chapter 7
Composition characteristics of ternary InGaAs nanowires
7.1 Introduction
In this chapter, Au-catalzyed ternary InGaAs nanowires were grown with a very high
norminal In concentration (In/(In+Ga)) of 85 at.%. By extensive cross-sectional study of
grown nanowires, it was found that nanowires formed the core-shell structure with In-
enriched cores, and the composition in nanowire cores and shells were varied at varied
nanowires regions. This new phenomenon of composition gradient of nanowire cores and
shells is demonstrated and clarified in Section 7.2 of this chapter, as a drafted manuscript.
151
7.2 Composition gradient ternary InGaAs nanowires caused by strain
relaxation
Understanding the compositional distribution of heteroepitaxially grown ternary nanowires
has been pursued for their controllable growth due to their potential applications with
desired band gaps. In this study, through growing ternary InGaAs nanowires with high In
concentration on GaAs {111}B substrates in molecular beam epitaxy, we demonstrate the
extraordinary core-shell structured ternary InGaAs nanowires with In-rich cores and Ga-
enriched shells. Furthermore, both nanowire cores and shells showed composition
gradient. Our detailed structural and chemical characterizations using electron microscopy
suggest that In-rich nanowire cores are formed due to the Ga-limited growth environment,
caused by the competition with the spontaneous InGaAs planar layer growth on the
substrate that consumes more Ga. Moreover, the composition gradient in the nanowires
cores is a result of strain relaxation with increased shell thickness and the limited Ga
diffusion length with long nanowires. This new finding of compositional gradient ternary
nanowires provides possibilities for continuous band-gap tuning and carrier confinement in
individual nanowires, which may pave a way for novel optoelectronic applications.
Introduction
III-V nanowires have become versatile building blocks for future electronic and
optoelectronic devices due to their unique physical and optical properties.1,2 In particular,
ternary III-V nanowires, although less studied than their binary counterparts, have been
attracting an increasing interest as they allow for the tunability of desired band gap for
different applications by modulating composition fraction of their alloys. Among them,
InGaAs nanowires have demonstrated to be technologically important for a wide range of
applications, attributed to their tunable band gap ranging from the near-infrared region to
the infrared region, which makes them constitute the ideal material system for numerous
optoelectronic devices, such as light-emitting diodes3 and nanolasers.4 In addition, arising
from the tunable band gap, the electric properties of InGaAs nanowires can be modulated
intentionally as well, particularly to maintain superior electron mobility and thus enhance
performance in electronic devices.5 Such band-gap tunability can be realized by tuning the
InGaAs alloy composition. For example, by tuning the In composition x from 0 to 1, the
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band gap of InxGa1-xAs can be lowered from ~1.42 to 0.34 eV,5 and the electron mobility
can be enhanced from ~1000 to 6000 cm2 V-1 s-1.6 The intrinsically lower band gap of In-
rich InGaAs than Ga-rich InGaAs or GaAs would realize carrier confinement when it forms
heterostructures with other III-V materials of larger band gap7 and its high electron mobility
makes it possible for applications in high electron mobility transistors8 and high-efficiency
photodetectors.9 Therefore, InGaAs nanowires with high In concentration are highly
desirable for performance and application of future nanoscale devices.
On the other hand, the capability of growing uniform ternary III-V nanowires on the
substrate with large lattice mismatch is a prerequisite for practical applications by the
combination of nanowire growth with semiconductor integration techniques. Within this
process, one of the critical factors to evaluate nanowire optoelectronic performance is the
composition control. However, due to the competition of the ternary nanowire and the
planar layer heteroepitaxial growth on the substrate, the composition of the grown
nanowire can be deviated from the nominal composition designs.10 On the other hand, for
Au-catalyzed ternary III-V nanowires grown by the VLS mechanism,11 the unexpected
lateral growth on nanowire sidewalls leads to nanowire tapering and phase
segregation,12,13 which makes the composition of ternary nanowires more complex. For
example, it has been discovered that the core-shell structure forms spontaneously in Au-
catalyzed ternary nanowires because of phase separation, such as AlGaAs,14 GaAsP15
and InGaAs16 nanowires. As for InGaAs ternary nanowires involving two group-III
elements that can be alloyed with Au, the stronger affinity of Au-In than that of Au-Ga
leads to In atoms staying in the catalysts rather than incorporating into the nanowires.17,18
Consequently, Ga-enriched cores and In-enriched shells were normally observed in
InGaAs nanowires grown under low or intermediate In/(In+Ga) ratios.19-21 So far, no study
has been reported for the nanowire formation of the core-shell composition configuration
under very high In/Ga ratios, so that the complete understanding of the InGaAs nanowires
growth mechanisms remains unclear.
In this study, we demonstrate the growth and the compositional characteristics of Au-
catalyzed InGaAs nanowires on GaAs {111}B substrates under a high In/(In+Ga) ratio of
~0.85 in molecular beam epitaxy (MBE). Through detailed electron microscopy
investigations on the grown InGaAs nanowires and their cross sections from different
regions, we found that InGaAs nanowires have the average In concentration of more than
80 at.% and they spontaneously formed In-rich cores and Ga-enriched shells. More
interestingly, we found that, during nanowire growth, the compositional difference between
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the core and the shell decreased towards nanowires bottom regions and the Ga
concentration in the nanowire core decreases along the nanowire growth direction, which
can be derived from the enhancing strain relaxation with the increasing shell thickness
towards the nanowire bottom and the limited Ga diffusion length along the nanowires. The
fundamental reasons behind these new phenomena were investigated and the growth
mechanism of our ternary InGaAs nanowires was clarified.
Experimental section
Epitaxial InGaAs nanowires were grown on GaAs {111}B substrates using Au as catalysts
in a Riber 32 MBE system. After a GaAs {111}B substrate was degassed and deoxidized to
remove any contaminants in the growth chamber, GaAs buffer layer was grown on the
substrate at 580°C to ensure atomically flat surface for the nanowire growth.
Subsequently, the substrate was transferred to the preparation chamber, where a Au thin
film was deposited directly on the top of the GaAs buffer layer. The Au-coated substrate
was then transferred back to the growth chamber and annealed at 550°C for 5 min under
As ambient (As source was switched on throughout the nanowire growth), in which the Au
thin film aggregates into nanoparticles. After annealing, the substrate temperature was
dropped to 400°C, where In and Ga vapor sources were introduced to induce the nanowire
growth with the respective beam equivalent pressures of ~2.2 10-7 and ~4 10-8 Torr
(so the nominal In concentration (In/(In+Ga)) of ~85%), and a V/III (As/(In+Ga)) ratio of
~15. The nanowire growth was terminated after 60 min and cooled down in the As ambient
naturally until 300°C.
The morphological, structural and chemical characteristics of grown InGaAs
nanowires were investigated using FE-SEM (JEOL 7800F) and TEM (Philips Tecnai F20
equipped EDS for compositional analysis), respectively. Individual nanowires for TEM
investigations were prepared through ultrasonicating the nanowire sample in ethanol and
dispersing individual nanowires onto the Cu mesh grids coated with holey carbon films.
The cross sections from different regions of grown nanowires for TEM investigations were
prepared by first embedding the nanowire sample in epoxy resins and then sectioning by
an ultramicrotome (Leica EM UC6). The nanowire sample was well located to ensure that
the sectioning was initiated from the substrate towards the nanowire top and each
sectioned slice was parallel to the substrate surface. The individual thin slices from
different sections of nanowires were collected and transferred onto the Cu grids for TEM
154
investigations. On the other hand, to clarify the growth of planar layer, the cross section of
nanowire sample with substrate was also prepared for TEM investigation, using FIB (FEI-
SCIOS) with Ga ion source after the sample was ultrasonicated to remove the grown
nanowires and coated with a Pt layer. This lamellar cross-section sample was then
attached to the Cu lift-out grid for TEM investigations.
Results and discussion
Figure 1a is a side-view SEM image of grown InGaAs nanowires, showing that they have a
high density with an average length of ~8 µm. Figure 1b is a BF TEM image taken from a
typical InGaAs nanowire, in which slight tapering caused by nanowire lateral growth can
be witnessed. The insets are the respective SAED pattern and HRTEM image taken from
the nanowire body (along the <11 2 0> zone-axis), indicating that the nanowire has the
high-quality wurtzite crystal structure. Figure 1c-e is corresponding high-magnified TEM
images taken from the nanowire top, middle and bottom regions (as marked in Figure 1b),
respectively. It is of interest to note that Moiré fringes can be observed in both nanowires
middle and bottom regions, suggesting that the grown nanowires might have a core-shell
structure, where the core and shell have different chemical composition and thus different
lattice parameters, leading to the strain contrast.22 To understand the composition
distribution along the nanowires, EDS point analyses were performed along the nanowire
and the results are shown in Figure 1f. As can be noted, no Ga was found in the post-
growth catalyst while the In concentration can be estimated as ~30 at.% (point 1). Since
Au has a stronger affinity with In over Ga,16 and Ga, if there is any in the Au catalysts, can
be expelled from the catalysts during the cooling process, and if so, a necking region
should be formed.23 However, no necking region is seen below the catalyst (refer to Figure
1c), suggesting that no Ga was absorbed in the catalyst, at least in the later stage of the
nanowire growth. Nevertheless, a small amount of Ga can be observed in the nanowire
top region (see point 2), suggesting that Ga atoms indeed participate in the nanowire core
growth, possibly via the triple phase line as As does. On the other hand, it is of interest to
note that the Ga concentration in the nanowire increases towards the bottom region
(points 3 and 4). Our extensive EDS investigations over a dozen of nanowires supports
this trend of compositional variation along nanowires, but exact Ga concentrations
155
Figure 1 Typical (a) side-view SEM image and (b) BF TEM image of the grown InGaAs
nanowire. (c-e) High-magnified TEM images of the nanowire top, middle and bottom
regions as marked by the colored rectangle in (b), respectively. (f) Typical EDS point
analysis profiles taken from the catalyst towards the nanowire bottom region as marked in
(c-e).
measured for different nanowires are slightly different, within the range between 25–30
at.% Ga at the bottom of the nanowires (see statistic analysis below).
Since our nanowires may have a core-shell structure, it is necessary to clarify the
structural and chemical characteristics of nanowire cores and shells at the different
regions. Figure 2a shows a BF TEM image of a typical cross section taken from a
nanowire bottom region, which has a hexagon shape and a lateral dimension of ~80 nm
(equivalent to mark A in Figure 1b). As can be noted, the cross section indeed has the
core-shell structure, in which the core diameter can be found as ~30 nm, similar to the
156
catalyst size, verifying that the growth of the nanowire core was directly induced by the
catalyst under the VLS mechanism.11 On the other hand, the nanowire shell has ~25 nm in
thickness, due to the lateral nanowire growth via the VS mechanism.13 Figure 2b is the
corresponding SAED pattern of the cross section viewed from the [0001] zone-axis,
indicating the {1120} side-facets of the grown nanowires, both for the nanowire core and
the shell. To understand the compositional variations between the core and the shell, EDS
mappings were performed, and Figure 2c,d shows the measured Ga and In EDS maps,
from which the In-enriched core and Ga-enriched shell can be clearly witnessed. Figure 2e
shows EDS point analysis profiles taken from the nanowire core and shell, indicating that
Ga concentration is ~15 at.% in the core while ~25 at.% in the shell. These observations
are verified by our extensive TEM investigations over a dozen of nanowire cross sections
(see detailed statistic analysis later).
Figure 2 (a) BF TEM image of a typical cross section from the InGaAs nanowire
bottom region. The inset is the plot of Ga concentrations at different positions of the
cross section as denoted by the orange circles, which is based on the EDS point
analyses. (b) Corresponding SAED pattern viewed along the [0001] zone-axis. (c,d)
EDS Ga and In maps of the cross section, respectively. (e) Typical EDS point
analysis profiles taken from the nanowire core and the shell, respectively.
Similar TEM characterizations were employed on the cross sections of nanowire
middle regions. Figure 3a is a BF TEM image taken from the cross section from a typical
nanowire middle region (with an overall diameter of ~53 nm – equivalent to mark B in
157
Figure 1b), showing that the nanowire has also maintained the core-shell structure in this
region with the core diameter of ~30 nm but a much thinner shell. Figure 3b is the
corresponding SAED pattern taken from Figure 3a, indicating that the core and the shell
have kept the same {1120} side-facets in the nanowire middle region. Figure 3c,d
illustrates the typical results of Ga and In EDS maps over the cross section respectively,
indicating the In-enriched core and the Ga-enriched shell, where the elements segregation
is similar to that in the nanowire bottom region. Figure 3e shows the typical result of
composition profile taken from the nanowire core and shell, showing that Ga concentration
is ~8 at.% in the core while ~35 at.% in the shell, different from those in the nanowire
bottom region.
Figure 3 (a) BF TEM image of a typical cross section from the InGaAs nanowire
middle region. The inset is the plot of Ga concentrations at different positions of the
cross section as denoted by the orange circles, which is based on the EDS point
analyses. (b) Corresponding [0001] zone-axis SAED pattern. (c,d) The
corresponding EDS Ga and In maps, respectively. (e) Typical EDS point analysis
profiles taken from the nanowire core and shell, respectively.
To further examine the compositional characteristics of the nanowire top region,
cross-sections of nanowire tops were investigated by TEM. Figure 4a shows an HRTEM
image of a hexagonal cross section in the typical nanowire top region with the lateral
dimension of ~30 nm (equivalent to mark C in Figure 1b), which is close to the catalyst
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Figure 4 (a) BF TEM image of the typical cross section from the InGaAs nanowire top
region. The inset is corresponding EDS line scan profile of the cross section. (b)
Corresponding [0001] zone-axis SAED pattern. (c) Typical EDS point analysis profile of
the cross section marked in (a).
size, implying that there is no shell at the nanowire top region. This can be confirmed by
the corresponding EDS line scan across the cross section, showing the uniform element
distribution (see the inset in Figure 4a, note that only Ga-K and In-L peaks were extracted
in this study, as the As concentration is considered as constant in nanowires). Figure 4b is
the corresponding SAED pattern. By correlating Figure 4a,b, the side-facets of the
nanowire core can be determined as six {1120} facets. Figure 4c shows the
Gaconcentration of ~5 at.%, which is lower than those in the nanowire middle and bottom
regions, and lower than the nominal Ga concentration.
Since the compositions of the nanowire cores and shells are both varied along the
nanowires, to quantify such variations, EDS measurements were carried out on the cross
sections from different regions of over a dozen nanowires. Figure 5a-c shows the
respective schematic models of the cross sections from the nanowire top to bottom
regions, demonstrating the elements segregation in each region. Figure 5d-f is the
histograms showing measured Ga concentrations in nanowire cores and shells for
different nanowires and different regions. As can be seen, in the nanowire top region, the
159
Ga concentration is 4-6 at.% in nanowire cores (much lower than that in the nominal 15
at.%), which is in excellent agreement with the EDS analysis shown above; in the
nanowire middle region, the Ga concentration is 6-8 at.% in nanowire cores and 30-35
at.% in the shells; and in the nanowire bottom region, the Ga concentration is 12-16 at.%
in the cores and 25-30 at.% in the shells.
The statistic results outlined above confirm the In-rich nanowire cores and Ga-
enriched shells and suggest that, along the nanowires from bottom to top, the Ga
concentration decreases in the core while increases in the shell. Since there is more Ga in
the nanowire shells than that in the nanowire cores, the difference of the Ga concentration
Figure 5 (a-c) Schematic illustration of the cross-section models from nanowire top to
bottom regions. (d-e) Corresponding histograms illustrate the Ga concentrations in
nanowire cores and shells of a number of nanowires studied, with the dashed line
indicating the averaged Ga concentrations.
160
between the nanowire cores and shells is decreased towards the nanowire bottom. To
understand these extraordinary observations, two issues need to be addressed: (1) Why
the grown InGaAs nanowires have the In-rich core and the Ga-enriched shell, and (2) why
the axial composition gradient formed in the core and the shell during nanowire growth
with the tendency of the difference of Ga concentration between the nanowire cores and
shells decreasing towards the nanowire bottom.
To address the first issue, we note that, during nanowire growth, the planar layer
growth on the substrate simultaneously takes place, competing with nanowires for the
growing materials. Particularly, for III-V ternary nanowires grown on the binary substrate,
such a competition may impact the consequent compositional distribution, and, in turn,
lead to compositional variations in the grown nanowires.10 Therefore, to understand the
role of planar layer growth that may affect the InGaAs nanowire compositions during their
growth, we investigated the composition of the planar layer grown on the substrate. Figure
6a is a typical BF cross-section TEM image, in which the planar layer grown on the
substrate is shown, in which several “hills” can be recognized and they are corresponding
to the nanowire bottoms. Figure 6b shows details of a “hill” region; and the inset is
corresponding EDS line-scan across the planar layer, indicating the uniformity of the Ga
composition across the grown planar layer. Figure 6c shows the EDS point analyses of
different regions, from which no Ga is found in the Pt coated layer (point 1), the Ga
concentration in the planar layer is determined as ~30 at.% (point 2), and the GaAs
substrate has their atomic ratio of ~1:1 (point 3). The comparison of these three EDS point
analyses suggests no Ga contamination during the preparation of cross-section TEM
specimen, so that our measured Ga concentration in the planar layer reflects the true
value, which is significantly higher than the nominal Ga concentration (~15 at.%) for the
nanowire growth. In fact, it is energetically favorable for the planar layer containing more
Ga (when competing with nanowire growth) as less misfit strain is introduced into the
system.24 Nevertheless, there is still a significant lattice mismatch existed between the
planar layer and the substrate, as verified by the split diffraction spots in the SAED pattern
taken at the interface between the substrate and the planar layer (the inset in Figure 6b).
Consequently, under the high In/Ga ratio growth environment, the enhanced Ga
incorporation in the planar layer growth will cause less Ga for the nanowire growth, and
consequently leads to less Ga to incorporate in the nanowire cores through the triple
phase line. As a result, the Ga concentration found in the nanowire cores is much less
than the nominal concentration. On the other hand, because the nanowire shells are
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Figure 6 (a) BF TEM image showing the typical cross-section of the nanowire sample. (b)
High-magnified TEM image demonstrating the enlarged area of the cross section. The
insets are respective EDS line profile of the planar layer and the [011] zone-axis SAED
pattern taken from the area marked by the dashed square. (c) EDS point analysis profiles
taken from the cross-section sample as marked in (a).
grown under the VS mechanism, the incorporation of In and Ga into the nanowire sidewall
depends on their mobility and bonding ability with As. Since the bonding energy of In-As is
weaker than that of Ga-As,25 it is energy favorable for Ga atoms to be bonded with As
atoms during the nanowire shell growth. Additionally, since Ga atoms are less mobile than
In atoms and have shorter diffusion length,17 more Ga atoms diffusing from the substrate
and nanowire sidewall would accumulate increasingly towards the nanowire bottom region
and incorporate to the sidewall, leading to the Ga-enriched shell, which is in good
agreement with our observations that Ga concentration decreases along the nanowire
from bottom to top and so does the shell thickness.
Regarding the second issue, it has been found that the In concentration in nanowire
cores increases linearly during InGaP nanowire growth, due to the longer In diffusion
length compared with Ga.26 However, in this study, the Ga concentrations in nanowire
cores decrease slowly from nanowire middle to top regions, suggesting that the shorter Ga
diffusion length may be one of the reasons for the compositional variations in nanowire
162
cores. On the other hand, as can be noted that similar to the planar heteroepitaxy, the
lattice mismatch does exist at the interface between the nanowire cores and shells with
different compositions. It has been well demonstrated that the misfit strain can facilitate the
mass transport through the hetero-interface, and lead to the strain relaxation.27 This fact
can be further enhanced with increasing the temperature and increasing the shell
thickness. For example, it has been demonstrated that, by annealing the grown
Ge/Si multilayer sample at higher temperature, the varied compositions become
uniform due to the strain relaxation between the layers realized through the
increased atoms diffusivity at high temperature.28 On the other hand, as reported for
the InAs/GaAs core-shell nanowire growth, when the GaAs shell thickness exceeds
a critical value, the radial strain relaxation takes place and the residual strain in
nanowires decreases with increasing the shell thickness.29 In this study, with
increasing the shell thickness towards the nanowire bottom that has the longest
annealing time, atoms interdiffusion is increasingly strengthened towards this
region, resulted in the decreased compositional difference between nanowire cores
and shells. This is supported by the fact that the grown nanowire has more
significant phase segregation in the middle region, where less strain has been
released and the composition difference is more significant near the core/shell
interface, confirming that less atoms interdiffusion takes place at the core/shell
interface in the nanowire middle region than that in the nanowire bottom (see the
insets in Figures 2a and 3a).
Based on our experimental results and discussion above, a growth model of InGaAs
nanowires with a high In concentration is proposed. Figure 7a illustrates the nanowire core
growth, where the Ga-enriched planar layer growth causes less Ga to participate nanowire
growth and leads to the nanowire core with the In concentration higher than its nominal.
With further growth, nanowire lateral growth takes place and forms the Ga-enriched shell,
causing the lattice mismatch and thus the misfit strain between nanowire cores and shells.
Such a misfit strain is gradually released to minimize the system energy via the atoms
interdiffusion between nanowire cores and shells (as shown in Figure 7b). With the axial
nanowire growth, nanowire shells grow laterally, so that the atoms interdiffusion is
enhanced towards the nanowire bottom region (see Figure 7c), resulting in the decreased
composition difference between the nanowire cores and the shells. Additionally, the Ga
concentration in the core decreases while the In concentration increases during nanowire
163
Figure 7 Schematic diagram illustrating the growth of core-shell structured InGaAs
nanowires. (a) Growth of nanowire cores and the simultaneous planar layer on the
substrate. (b) Nanowire shell growth and the atoms interdiffusion between the core and
shell (marked by the colored arrows). (c) Nanowire shell thickness increases with further
growth, leading to the enhanced atoms interdiffusion between the core and shell. The long
dashed arrows indicate the atoms diffusion lengths.
growth, in which the longer In diffusion length than Ga should also be taken into
consideration.
It is expected that our grown core-shell structured InGaAs nanowires have potential
in optoelectronic applications in the near-infrared region. The varied composition of core
and shell enable them with varied band gaps, in which the low-bandgap In-enriched
InGaAs core would confine carriers and the large-bandgap Ga-enriched shell would act as
a passivation layer.30 Accordingly, the nanowire high intrinsic surface state density would
be suppressed, which would possibly enhanced the performance of nanowire-based
devices.
164
Conclusions
In conclusion, we demonstrated the composition variations in ternary InGaAs nanowires
grown under the high In/Ga ratio by MBE. Our detailed electron microscopy analysis
confirms the spontaneous formation of core-shell structure in grown nanowires. With the
competition of the epitaxial planar layer growth, nanowire growth is Ga-limited, leading to
the In-rich nanowire cores and Ga-enriched shells. Additionally, with increasing the shell
thickness towards nanowire bottoms, the enhanced strain relaxation leads to the
decreased compositional difference between nanowire cores and shells, and further
resulted in the Ga concentration in the core decreasing during nanowire growth. Such
unique compositional variations in nanowire cores and shells can realize carrier
confinement towards the nanowire core and in the top region with narrow band-gaps. This
study shows that the compositional characteristics of InGaAs nanowires with high In
concentration, and highlight the important role of strain relaxation played in the
compositional distribution of ternary nanowires, which is of great significance for
composition and band-structure engineering in ternary III-V nanowire system.
Author Information
Chen Zhou,† Kun Zheng,*, ♩, ¶ Ping-Ping Chen,§ Wei Lu,§ and Jin Zou*, †, ♩
†Materials Engineering, ♩Centre for Microscopy and Microanalysis, ¶Australian Institute for
Bioengineering and Nanotechnology, The University of Queensland, St. Lucia,
Queensland 4072, Australia
§National Laboratory of Infrared Physics, Shanghai Institute of Technical Physics, Chinese
Academy of Science, 500 Yu-Tian oad, Shanghai 200083, People’s epublic of China
Corresponding Authors
*E-mail: [email protected].
*E-mail: [email protected].
165
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Chapter 8
Conclusions and recommendations
8.1 Conclusions
In this PhD thesis, we demonstrate our new findings in the study of binary GaAs nanowires
and ternary InGaAs nanowires growth. By extensive investigations on the morphological,
strutrual and chemical characterizations of grown nanowires, the growth mechanism
behind these new finding is unveiled.
169
In the binary GaAs nanowires growth, we found that:
By tuning the As flux in the growth environment, the growth rate and structural
quality of GaAs nanowires can be modulated. Under the high As flux, nanowires
gowth is Ga-limited, in which thin nanowires grow longer and have defect-free
wurtzite structure, while thick nanowires are short and have defected structure.
In this case, nanowires growth is kinetics controlled. Under the low As flux,
nanowires growth is As-limited, in which nanowires are short and their structure
are defect-free. In this case, nanowires growth is thermodynamics controlled.
Growth duration can play a critical role in the nanowires growth rate and
structural quality. With short growth duration, nanowires diameters are uniform
and planar defects are randomly distributed along the nanowires. With
prolonged growth duration, nanowires grow much longer and form the shoulder
morphology, and their structure becomes defect-free at top regions. This
phemonon can ascribed to the increased Ga supersaturation in the catalysts
resulted from increased Ga diffusion from the shoulder surface.
In the ternary InGaAs nanowires growth, we found that:
Growth duration can also play a role in the core-shell strucutrue formation of
ternary nanowires. With the nominal In concentration of 50 at.% in the group-III,
nanowires formed the core-multishell structure at their bottoms, which was
formed during the later stage of the nanowires growth. This phenomenon was
attributed to the composition variation of planar layer on the substrate, in which
the strain relaxation between the InGaAs layer and GaAs substrate occurs.
With the nominal In concentration of 85 at.% in the group-III, nanowires formed
the core-shell structure, with In-rich cores and Ga-enriched shells. The
composition in nanowire cores and shells were varied at varied from nanowire
bottoms to tops. This composition gradient can be attributed to the atoms
interdiffusion during nanowires growth.
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8.2 Recommendations
In this PhD thesis, the growth behaviour and strcutral quality of GaAs nanowires have
been studied by varying the group-V flux and the growth duration, which provides new
insights for binary III-V nanowires growth. Particularly, nanowires formed half defected and
half defect-free sections with the prolonged growth duration, in which the related
optoelectronic properties can be varied at varied nanowire sections.
On the other hand, the structural and compositional characterisics of ternary InGaAs
nanowires grown with varied In concentrations have been demonstrated. Based on this
study, a more systematic study of tenary InGaAs nanowires should be achieved. The
careful investigations of phase segregation induced core-shell or core-multishell structure
formed in ternary InGaAs nanowires provides a new understanding of complex one-
dimensional nanomaterials system. Besides, it is interesting to invesitugate the
correspongding optoelectronic properties of these unique nanowire structures.