spie proceedings [spie optical engineering + applications - san diego, ca (sunday 10 august 2008)]...

Layered III-VI Chalcogenide Semiconductor Crystals for Radiation Detectors

Krishna C. Mandala*, Alket Mertiria, Gary W. Pabsta, Ronald G. Roya, Y. Cuib, P. Battacharyab, M. Grozab, A. Burgerb, Adam M. Conwayc, Rebecca J. Nikolicc, Art J. Nelsonc, and Stephen A. Paynec

a EIC Laboratories, Inc., 111 Downey Street, Norwood, MA 02062, USA b Department of Physics, Fisk University, Nashville, TN 37208, USA

c Lawrence Livermore National Laboratory, Livermore, CA 94550, USA

ABSTRACT The layered anisotropic chalcogenide semiconductors GaSe and GaTe single crystals have been grown by a modified vertical Bridgman technique using high purity Ga (7N) and in-house zone refined (ZR) precursor materials (Se and Te). The crystals harvested from ingots of up to 10 cm length and up to 2” diameter, have been characterized by measuring resistivity through current-voltage (I-V) characteristics and bulk carrier concentration and mobility through Hall effect measurements. Micro-hardness, infrared microscopy, etching characteristics, low-temperature photoluminescence (PL) and contact resistivity studies have also been performed to further characterize the grown crystals.

Keywords: Layered Semiconductors, Chalcogenides, GaSe, GaTe, Radiation Detectors

1. INTRODUCTION There is currently a great need for ‘direct read-out’ semiconductor detectors which can characterize, monitor and identify isotopes of radioactive materials.1-2 Presently, semiconductor radiation detectors serve a crucial role in detecting illicit nuclear weapons and radiological dispersal devices (RDDs) by virtue of their ability to distinguish isotopes.3 The best performers in this arena are germanium (Ge) detectors4-5 which have a resolution of 0.2%, although they require cryogenic cooling to function properly (<110K). Consequently, there has been an enormous effort to develop room temperature (RT) alternative to Ge. The effort has yielded substantive progress in CdTe and CdZnTe (CZT) detectors.6-9 However, the poor yield from crystal boules and prohibitive cost have impeded the widespread deployment of these devices. Therefore, there remains a need for RT detectors with <0.5% resolution at 662 keV that can be grown in large areas at high yield and at relatively low cost.

Layered chalcogenide semiconductors are promising candidates to satisfy these requirements. In particular, the relatively unstudied GaSe and GaTe are of high interest. They are anisotropic binary III-VI compounds with a layered structure that have the potential for producing a nearly ideal nuclear detector that can operate at and above RT with low electronic noise. GaSe and GaTe are very attractive owing to their wide bandgaps (2.0 eV, 1.7 eV at 300K) which are in the optimal range and would allow an intrinsic resistivity of more than 1011 Ω⋅cm yielding very low noise detectors. Additionally, GaSe has a large nonlinear optical coefficient (d22=75 pm/V), high charge carrier mobilities and is easily cleaved for detector fabrication.10-17 The physicochemical properties of these materials are amenable to conventional material processing procedures and crystal growth at moderate temperatures. Furthermore, they are very stable chemically. High resolution RT detectors fabricated from either GaSe or GaTe would find wide range of applications in Homeland security, nuclear non-proliferation, medical imaging, and nuclear physics research.

We have grown both doped and undoped GaSe and GaTe single crystals. These crystals, grown using a modified vertical Bridgman method, have been characterized thoroughly through structural, optical, electrical and opto-electronic charge transport measurements.

* Send correspondence to K.C.M.: e-mail: [email protected]. Telephone: 1 (781) 769-9450

Hard X-Ray, Gamma-Ray, and Neutron Detector Physics X, edited by Arnold Burger, Larry A. Franks, Ralph B. James,Proc. of SPIE Vol. 7079, 70790O, (2008) · 0277-786X/08/$18 · doi: 10.1117/12.796235

Proc. of SPIE Vol. 7079 70790O-1

Opto-electronic properties of semiconductor crystals are highly dependent on the presence of residual impurities, since they influence the charge carrier transport properties. Thus to obtain the highest quality of the grown crystals, it is imperative that the highest purity precursor materials should be utilized (≥7N for Ga, Se and Te). Our approach is to employ zone refining to achieve these purity levels, verified by glow discharge mass spectrometry (GDMS). The zone refining method is based on the fact that most impurities distribute differently in solid and liquid phases at equilibrium. If a section of solid material is melted (the ‘zone’) and passed slowly through the material, this will result in the re-distribution of impurities in the remaining solid. If this process is repeated many times (multi-pass zone refining), the resulting solid may become extremely pure at one end.

Crystal growth is achieved by a modified vertical Bridgman method. One of the primary difficulties in GaTe and GaSe single crystal growth is their low thermal conductivity along the c-axis (1.4W/mK for GaTe and 0.37W/mK for GaSe) close to their melting temperature. These values are much lower than other semiconductors grown by the Bridgman method, e.g., Ge: 16W/mK and CdTe: 2.91W/mK. Also, their thermal conductivities are anisotropic (e.g., the thermal conductivities perpendicular to c-axis for GaTe is 8.7W/mK and for GaSe is about 2.9W/mK). That means the radial conduction in the solid is more efficient for heat removal from the growth interface relative to axial conduction in GaTe and GaSe crystal growth. Anisotropy and liquid/solid conductivity ratios are expected to strongly influence the interface shape, which in turn affects twinning and other defects. Furthermore, the Prandtl numbers for GaTe and GaSe are ~3.2 and ~2.8 respectively, which are much larger than other semiconductors (e.g. Ge: 0.007 and CdTe: 0.406). In GaTe and GaSe Bridgman growth, melt flow and heat transfers are strongly coupled. So, it is expected that any disturbance on melt flow from the pulling rate and/or rotation rate will significantly affect the temperature distribution, and consequently, the interface shape. It is therefore extremely important to properly control the melt flow, growth interface, and solute transport during crystal growth. To further understand the procedures required to grow this material, we used a previously reported on state-of-the-art computer model, MASTRAPP, to model the heat and mass transfer in the Bridgman growth system.12, 15 This also helps in predicting stress distributions in the as-grown crystals. EIC’s crystal growth systems are computer controlled and based on the growth parameters derived from in-house advanced computer modeling and simulation.

Achieving high resistivity (i.e. free charge carrier densities on the order of 106 cm-3) is a particularly challenging aspect of developing RT semiconductor detectors. In order to achieve high resistivity, it may be necessary to use compensating dopants such as Ge or Sn in GaTe as well as post-growth annealing.16 Indium doping has also been used in GaSe to enhance the charge transport properties and to improve the mechanical hardness of the crystals.14-15, 17

EIC uses several high temperature furnaces, round the clock computer operated, multi-zone furnaces to grow crystals up to 4” diameter (Figure 1). Silicon carbide heating elements are used as the high temperature heat source in these furnaces. These high temperature heating elements give EIC the unique capability to grow single crystal materials up to 1550oC. There are several separate controlled hot zones in each furnace to give the ability to easily customize the temperature profile for crystal growth of different materials. Silicon carbide heating elements are known to have a temperature varying resistance, and therefore require a method of limiting the current to the elements in order to provide a stable controlled temperature environment for crystal growth. In addition, the elements’ resistance increases with age (use), and this must be compensated for by increasing the voltage. The system will automatically maintain the same power to the furnace throughout the life of the elements, without overdriving, which could cause control deviations and premature heating element failure compromising the quality of the crystal being grown. There are several temperature readout type R thermocouple sensors that are located in the center of each hot zone. A Mellen ADAPT integrated furnace temperature control and data acquisition system has been set up to query each thermocouple, control the temperature at a precise set point temperature and collect run data for each hot zone. The controlling capability of this system for each zone is ± 0.1oC.

2.1 Crystal Growth2. EXPERIMENTAL PROCEDURES


(a) (b) (c)

Figure 1. High temperature, computer controlled vertical Bridgman furnaces capable of growing crystals of diameters up to (a) 1.5”, (b) 2” and (c) 4”.

Before growth, precursor materials are loaded into quartz ampoules and heated under vacuum (~10-6 Torr) overnight. The ampoules are then sealed and the material is first synthesized in a horizontal furnace before being loaded into the growth furnaces. After the growth furnaces have been heated to the appropriate profile (shown in Figure 2), the ampoules are then lowered through the zones at rates of 0.5-4.8 cm/day and are rotated at 12-30 rph. Once the ampoules are at 700oC near the end of the growth run, they are cooled at a rate of 20oC/hr to room temperature.

(a) (b)

Figure 2. Simplified furnace temperature profiles for (a) GaSe crystal growth and (b) GaTe crystal growth.

2.2 Characterization The photoluminescence (PL) spectrum of a freshly cleaved GaTe and GaSe crystals was measured at both Fisk University and at EIC. The tests at Fisk were at 10 K using an Ar+ laser (488 nm) as an excitation source. The luminescence was analyzed with a SPEX 1877D Triplemate spectrometer and was detected by a CCD (charge-coupled device) camera. At EIC, the samples were cooled down to about 9 K using an APD Cryogenic, Inc. dual HC-4MK I helium compressor. The crystals were illuminated with the 488-nm line of an ILT 5500A air-cooled argon-ion laser with density of 2.0 W/cm2. The PL spectra were detected using a SPEX 1877D Triplemate spectrometer in conjunction with a liquid nitrogen cooled charge coupled device (CCD) detector. A 0.1-mm slit and a grating with 300 grooves/mm were employed for the spectrometer.

X-ray photoelectron spectroscopy (XPS) was used to study the surface chemistry of polished, etched and oxidized samples and was performed on a PHI Quantum 2000 system using a focused monochromatic Al Kα x-ray (1486.7 eV) source for excitation and a spherical section analyzer. The instrument has a 16-element multichannel detection system. A 100 µm diameter x-ray beam was used for analysis. The x-ray beam is incident normal to the sample and the x-ray detector is at 45° away from the normal. The pass energy was 23.5 eV giving an energy resolution of 0.3 eV that when


WI

LL±

IZL'

L±

i

i!1

— -IllIP II H huH ii u,i

combined with the 0.85 eV full width at half maximum (FWHM). Al Kα line width gives a resolvable XPS peak width of 1.2 eV FWHM. Deconvolution of non-resolved peaks was accomplished using Multipak 6.1A (PHI) curve fitting routines. The collected data were referenced to an energy scale with binding energies for Cu 2p3/2 at 932.72± 0.05 eV and Au 4f7/2 at 84.01± 0.05 eV. Binding energies were also referenced to the C 1s photoelectron line arising from adventitious carbon at 284.8 eV. Low energy electrons and argon ions were used for specimen neutralization.

To measure current-voltage (I-V) characteristics and to determine resistivity, I-V curves were recorded using a Keithley 237 high voltage source measurement unit and Metrics ICS data collection software. Samples measured were freshly cleaved to provide an oxide free surface and had either silver paint, sputtered gold or e-beam evaporated indium contacts applied in a symmetrical configuration.

To measure bulk carrier concentration and charge carrier mobility, we have performed Hall measurements. The contacts consisted of indium soldered in a van der Pauw configuration to the laminar surface of freshly cleaved samples. The measurement set-up was an Ecopia HMS-3000 Hall Measurement System with a 0.55 Tesla magnetic flux density.

3. RESULTS 3.1 Crystal Growth The GaSe and GaTe crystals were grown using a modified vertical Bridgman method. Undoped GaSe single crystal, shown in Figure 3, was grown by one of our Mellen high temperature, computer controlled, 3-zone furnace. The crystal boule was cut using a diamond impregnated wire saw and then the surfaces were lapped and polished.

(a) (b)

Figure 3. Photograph of undoped GaSe single crystals after lapping and mild polishing.

GaTe crystals were grown using a Mellen furnace as described above with a slower growth rate than GaSe to achieve high quality single crystals. We have used a modified pre-fabricated capillary ampoule to induce and enhance single-crystal growth as shown in Figure 4.

(a) (b)

Figure 4. Photographs of a grown GaTe crystal ingot (a) in a capillary growth ampoule and (b) removed from the growth ampoule.


L

iiIliT11 :IlHII1I1I1tc_Z 3 4 5 6 7 8 10

I ABOR\ I ( IRIES. INC. 6 8

iflf ii111111 If 111i1111111111111

200 210

Figure 5 shows an as-grown and unprocessed 2” diameter Ge-doped GaTe crystal. For large diameter crystals the growth rate was further reduced to about 0.5cm/day.

Figure 5. Photograph of a 2” diameter Ge-doped GaTe crystal after removal from the growth ampoule and before processing.

GaTe crystals are cut, lapped, polished and cleaved for characterization. Figure 6 shows a lapped cross section of a Sn-doped GaTe crystal that clearly demonstrates the single crystal quality. The lapped surfaces are easily cleavable parallel to the growth axis and we are able to obtain large single crystal samples for detector fabrication.

Figure 6. Lapped cross section of a Sn-doped (50 ppm) GaTe crystal.

3.2 Characterization The hardness of the laminar surface of GaTe crystals has been measured. In this method, the applied load is divided by the area of the deformation. A schematic of the indenter is pictured in Figure 7(a). An example of the depression created in a GaTe surface is shown in Figure 7(b).


U— 22-

(a) (b)

Figure 7. (a) A schematic of the indenter and (b) an example of the indentation in the crystal sample.

The data in Table 1 shows the load, dimensions of the deformation, force divided by the area, and the deduced hardness in GPa. The hardness of GaTe is seen to reduce by half when doped with germanium.

Table 1. Summary of hardness data.

Sample Load (gf) D1 (µm) D2 (µm) HV (gf/mm2) Hardness (GPa)

GaTe 25 19.5 21.6 110.2 1.08

GaTe (Ge-doped) 5 12.8 13.5 54.2 0.53

Optical infrared microscopy studies have been conducted to search for Te inclusions (or precipitates) in a well-polished sample of GaTe. The crystal was prepared by first setting the material in an epoxy and then polishing the structure by conventional means. The optical surface was the non-laminar plane. In Figure 8, four successive vertically-displaced micrographs at intervals of 100 µm are shown for a 1 mm thick GaTe sample. No inclusions or precipitates were identifiable under conditions in which it is straightforward to observe them in CZT crystals.

Figure 8. Infrared micrographs of GaTe revealing the absence of Te-inclusions or precipitates.


To investigate the nature of the oxide layer that can form on the GaTe surface, polished samples were etched in a mixture of H3PO4:H2O2:H2O and allowed to oxidize in air at ambient temperature. X-ray photoelectron spectroscopy (XPS) was used to study the surface chemistry of the polished, etched and oxidized samples in an effort to understand the effect of surface composition on room temperature radiation detector performance.

After encapsulation in Struers Epofix for metallographic sample preparation, sequential polishing of the GaTe with progressively finer diamond paste followed by colloidal silica resulted in a surface with a mirror finish and rms surface roughness of 20 nm as determined with Zygo optical interferometry. Wet etching of the polished GaTe surface was performed using a mixture of H3PO4, H2O2 and H2O with the ratio of 10:10:100 by volume respectively.

Photoemission measurements on the Ga 2p, Te 3d and O 1s core lines were used to evaluate the structural chemistry of the chemically treated surfaces. The polished surface has a Ga 2p3/2 peak binding energy of 1118.4 eV representative of Ga2O3. The Te 3d spectrum shown in Figure 9 for the as received GaTe surface shows two sets of Te 3d5/2,3/2 spin-orbit pairs. The higher binding energy of the Te 3d5/2 peak at 576.3 eV represents Te4+ in TeO2 and the lower binding energy peak at 573 eV represents lattice bound Te in GaTe. In addition, the O 1s peak has two components with 530.8 and 532.8 eV binding energies attributed to Ga2O3 and TeO2, respectively.19 The binding energies for the photoelectron peaks are summarized in Table 2.

Table 2. Summary of XPS binding energies (eV) for the processed GaTe crystals.

GaTe Ga 2p3/2 Te 3d5/2, 3/2 O 1s

As received 1118.4 576.3, 573.0 530.8, 532.8

1 min etch 1118.4 576.4 531.8, 533.1

3 min etch 1118.9 576.6, 573.0

5 min etch 1119.4 572.9 –

5 min air exposure 573.1 –

10 min air exposure – 576.9, 573.2 –

20 min air exposure 1119.3 576.8, 573.1 531.0, 532.3

40 min air exposure 1119.4 576.5, 573.1 531.0, 532.3

80 min air exposure 1119.1 576.4, 573.0 530.6, 532.1

After 1 minute in the H3PO4:H2O2:H2O solution, the GaTe surface becomes fully oxidized as evidenced by the Te 3d5/2 peak at 576.4 eV indicative of Te4+ in TeO2 and the absence of the lower binding energy component. The lower binding energy Te 3d5/2 peak reappears following 3 minutes in the H3PO4:H2O2:H2O solution. Finally, after 5 minutes in the H3PO4:H2O2:H2O solution, the GaTe surface is oxide free as evidenced by the sole Te 3d3/2 peak at 572.9 eV. Based on this data we conclude that the H3PO4:H2O2:H2O solution first oxidizes the GaTe surface and then completely removes the oxide leaving a pristine surface ideal for metal contact application.

The etched GaTe surface was then allowed to oxidize in air at ambient temperature for timed intervals and the surface was characterized after each timed exposure. Monitoring the growth of the oxide was accomplished using the components of the Te 3d5/2 peak and their respective binding energies.

From Figure 9, we note that minimal oxide growth has occurred on the etched GaTe surface after 5 minutes in ambient air. However, after 10 min. in air, a small Te 3d5/2 component indicative of TeO2 begins to appear at 576.9 eV. This oxide peak continues to grow after each air exposure but never attains its original intensity. This data suggests that the H3PO4:H2O2:H2O solution treatment passivates the GaTe surface.


4000

3000

2000

1000

0590 585 580 575 570 565

Binding Energy (eV)

GaTe EIC 7Te 3d 3/2 Te 3d 5/2

Te4+

Te2-

as rec

1 min H 3 PO 4 /H 2 O2 /H 2 O

3 min etch

5 min etch

5 min air

10 min air

20 min air

40 min air

80 min air

Te0

Figure 9. High resolution XPS spectra of Te 3d in GaTe (etched and oxidized surfaces).

To investigate defect trap levels, we have carried out low temperature photoluminescence (PL) studies. Figure 10 shows the PL spectrum of undoped GaTe at 10 K with three clear peaks (A, B, C). A strong free excitonic peak was observed at 1.779 eV (A) with a full width at half maxima (FWHM) of 2 meV. The exciton binding energy was found to be 22 meV as the bandgap of GaTe at 10 K is 1.801 eV.20 The peak at 1.748 eV was assigned to an acceptor bound exciton (A0 X) with an activation energy of 33 meV (B). This acceptor state is attributed to Ga vacancies as it is well known that undoped GaTe is p-type due to Ga vacancies.21 The peak at 1.575 eV (C) is assigned to a donor-acceptor pair (DAP). The shoulder peak at 1.700 eV has not been previously reported and may be assigned as a longitudinal-optical (LO) phonon replica of peak B. The temperature dependence of the excitonic peak intensity is shown in Figure 10(b). The excitonic peak intensity is almost reduced to zero above 40 K. Wan et al21 have reported that electron and optical-phonon interaction is the dominant scattering process above 30 K and effectively dissociates exciton as the energy of the optical phonon mode is 14 meV close to excitonic energy.


1.4 1.5 1.6 1.7 1.8 1.9 2.0

C

B

(4 meV)1.575 eV

1.700 eV

1.748 eV

1.779 eV

Inte

nsity

(arb

. Uni

t)

Energy (eV)

A

1.76 1.78 1.80

10 K18 K30 K40 K50 K

Inte

nsity

(arb

. uni

t)

Energy (eV) (a) (b)

Figure 10. (a) PL spectrum of undoped GaTe crystal at 10 K and (b) temperature dependent intensity of free exciton.

The 10 K PL spectra of GaSe is shown in Figure 11, exhibiting three peaks. The most intense GaSe peak at around 2.093 eV is an acceptor bound exciton, and the highest energy peak is related to a free exciton emission around 2.114 eV.22 The free excitonic peak position for GaSe was reported by Taylor and Ryan23 at 2.106 eV. The peak at 2.065 eV is related to impurities and is yet to be assigned.

1.9 2.0 2.1 2.2

2.065 eV

2.093 eV

Inte

nsity

(arb

. uni

t)

Energy (eV)

2.114 eV

Figure 11. PL spectrum of GaSe crystals at 10 K.

Contact studies have been performed to study the formation of ohmic contacts to laminar GaTe surfaces. Indium contacts were sputtered onto cleaved Ge-doped GaTe surfaces. Photolithography was then performed to create a transfer length measurement pattern which consists of 100 µm squares separated by 5, 10, 25, 40, 50 and 75 µm spacings. The unwanted indium was then etched away using dilute aqua regia and the photoresist was removed with acetone. A sequence of anneals at 100°C, 150°C, 180°C, 200°C, 225°C and 250°C for 2 minutes in a N2 ambient were carried out. In between each anneal cycle electrical characterization was performed and these results are shown in Figure 12. The contacts were ohmic as deposited, although with a high specific contact resistivity of 2.8 x103 Ω cm2. The specific contact resistivity decreases with increasing anneal temperature up to 200 °C, reaching the minimum at 1.2 Ω cm2, after which it begins to increase.


1.OE+04

4

1.OE+03

+4

1.OE+02

+I flP+flI

4

I.OE+OOI UU IbU ZUU ZbU UU

th&I Tpft (C)

Figure 12. Specific contact resistivity of indium on a laminar (cleaved) GaTe surface as a function of the anneal temperature.

Rectifying behavior was observed for GaSe devices with Au contacts, however there is a significant amount of reverse bias bulk leakage current. This response is shown in Figure 13 along with I-V data for In-doped GaSe. The calculated resistivity for the undoped GaSe ranges from 1x106 to 1.5x107 Ω cm. The In-doped GaSe showed a more symmetric response with a resistivity of ~ 6x106 Ω cm.

-3 -2 -1 0 1 2 3

1E-9

1E-8

1E-7

1E-6

In doped GaSe

Cur

rent

(am

p)

Voltage (V)

GaSeArea=0.03 cm2

th=0.02 cm

Figure 13. I-V characteristics of undoped GaSe and In-doped GaSe crystals.

Doping in GaTe, particularly with Ge, has a greater effect on electrical conductivity than doping in GaSe. We have tested GaTe crystals doped with either Sn or Ge in concentrations between 50 and 8000 ppm. Currently, the best resistivity has been observed in 4000 ppm Ge-doped GaTe. These crystals have demonstrated resistivities ≥1010 Ω cm. The I-V curve and resistivity data for one of these samples are shown in Figure 14.


Resistivity for 080102 GaTe:Ge (4000 ppm)1012.

-io-ibo -so ó ióoioApplied Voltage [V]

(a) (b)

Figure 14. (a) I-V data and (b) resistivity vs. voltage data for GaTe: Ge (4000 ppm).

Hall measurements have been performed on a series of lightly doped GaTe samples. Crystals doped with 50, 200 and 600 ppm Sn and 50, 200 and 600 ppm Ge were compared along with an undoped GaTe crystal. The bulk carrier concentration and carrier mobility data have been extracted. The crystals have been characterized and demonstrated p-type conductivity. The bulk carrier concentration data shows an overall trend of decreasing with increasing dopant concentration, with a drop of up to five orders of magnitude from the undoped crystal to the 600 ppm doped crystals of both dopants (1015-1010 cm-3). The mobility data for both dopants shows an initial drop of up to 32% from 145 cm2/V.s through a concentration of 200 ppm. However, the mobility recovers and even improves slightly when doped at the 600 ppm level (121% when compared to the undoped crystal).

4. CONCLUSION We have grown and characterized high quality undoped and doped GaSe and GaTe single crystals. Large diameter, up to 2”, ingots have been grown using a modified vertical Bridgman method. Infrared micrographs have shown the GaTe to be precipitate free. Photoluminescence studies have provided further evidence of the crystalline quality of cleaved single crystal samples. Our Ge-doped GaTe crystals have demonstrated high resistivities of ≥1010 Ω cm. Hall data has confirmed p-type conductivity for GaTe and has shown that doping GaTe with Sn or Ge lowers bulk carrier concentrations. It has also been shown that indium contacts on GaTe yield a minimum contact resistance when annealed at 200°C for 2 min in a N2 atmosphere.

ACKNOWLEDGEMENTS The authors acknowledge partial financial support provided by the DHS/DNDO under contract number HSHQTC-07-C-00034.

REFERENCES 1. R.B. James, T.E. Schlesinger, J. Lund, and M. Schieber, Semiconductors for Room Temperature Nuclear Detector

Applications, New York, Academic, 1995, vol. 43, pp. 335-381. 2. K.C. Mandal, S.H. Kang, M. Choi, A. Kargar, M.J. Harrison, D.S. McGregor, A.E. Bolotnikov, G.A. Carini, G.C.

Camarda, and R.B. James, “Characterization of Low-Defect Cd0.9Zn0.1Te and CdTe Crystals for High-Performance Frisch Collar Detectors”, IEEE Trans. Nucl. Sci., 54, pp. 802-806, 2007.

3. S.U. Egarievwe, K.-T. Chen, A. Burger, R.B. James, and M. Lisse, “Detection and electrical properties of Cd1-

xZnxTe detectors at elevated temperatures”, J. X-ray Sci. Tech., 6, pp. 309-315, 1996. 4. L.S. Darken, and C.E. Cox, Semiconductors for Room Temperature Nuclear Detector Applications, New York,

Academic, 1995, vol. 43, pp. 23-83. 5. M. Amman and P. N. Luke, “Position-sensitive germanium detectors for gamma-ray imaging and spectroscopy”,

Proc. SPIE, vol. 4141, pp. 144-156, 2000.


6. K.C. Mandal, S.H. Kang, M. Choi, J. Bello, L. Zheng, H. Zhang, M. Groza, U.N. Roy, A. Burger, G.E. Jellison, D.E. Holcomb, G.W. Wright, and J.A. Williams, “Simulation, Modeling, and Crystal Growth of Cd0.9Zn0.1Te for Nuclear Spectrometers”, J. Electron. Mater., 35, pp. 1251-1256, 2006.

7. K.C. Mandal, C. Noblitt, M. Choi, R. David Rauh, U.N. Roy, M. Groza, A. Burger, D.E. Holcomb, and G.E. Jellison, “Crystal Growth, Characterization and Testing of Cd0.9Zn0.1Te Single Crystals for Radiation Detectors”, Proc. SPIE, vol. 5540, pp. 186-195, 2004.

8. K.C. Mandal, S.H. Kang, M. Choi, J. Wei, L. Zheng, H. Zhang, G.E. Jellison, M. Groza, and A. Burger, “Component Overpressure Growth and Characterization of High-Resistivity CdTe Crystals for Radiation Detectors”, J. Electron. Mater., 36, pp. 1013-1020, 2007.

9. G. Koley, J. Liu, and K.C. Mandal, “Investigation of CdZnTe crystal defects using scanning probe microscopy”, Appl. Phys. Lett., 90, pp. 102121-1-3, 2007.

10. C. Manfredotti, R. Murri and L. Vasanelli, “GaSe as nuclear particle detector”, Nucl. Instr. and Meth., 115, pp. 349-353, 1974.

11. K. Liu, J. Xu and X.-C. Zhang, “GaSe crystals for broadband terahertz wave detection”, Appl. Phys. Lett., 85, pp. 863-865, 2004.

12. K.C. Mandal, S.H. Kang, M. Choi, “Layered compound semiconductor GaSe and GaTe crystals for THz applications”, Proc. MRS, 969, pp. 111-116, 2007.

13. B. L. Yu, F. Zeng, V. Kartazayev, R. R. Alfano and K. C. Mandal, “Terahertz studies of the dielectric response and second-order phonons in a GaSe crystal”, Appl. Phys. Lett., 87, pp. 182104-1-3, 2005.

14. K.C. Mandal, C. Noblitt, M. Choi, A. Smirnov and R. David Rauh, “Crystal growth, characterization and anisotropic electrical properties of GaSe single crystals for THz source and radiation detector applications”, AIP Proc., CP772, pp. 159-160, 2005.

15. K.C. Mandal, M. Choi, S.H. Kang, R.D. Rauh, J. Wei, H. Zhang, L. Zheng, Y. Cui, M. Groza, A. Burger, “GaSe and GaTe anisotropic layered semiconductors for radiation detectors”, Proc. SPIE, 6706, pp. 67060E-1-10, 2007.

16. C. Paorici, N. Romeo, and L. Tarricone, “Negative resistance and memory effects in GaTe single crystals”, J. Phys. D: Appl. Phys., 9, pp. 245-251, 1976.

17. Y. Cui, R. Dupere, A. Burger, D. Johnstone, K.C. Mandal, and S.A. Payne, “Acceptor levels in GaSe:In crystals investigated by deep-level transient spectroscopy and photoluminescence”, J. Appl. Phys., 103, pp. 013710-1-4, 2008.

18. Z. Rak, S.D. Mahanti, K.C. Mandal and N.C. Fernelius, “Electronic structure of Cd, In, Sn, substitutional defects in GaSe”, Proc. MRS, 994, pp. 10-15, 2007.

19. O.A. Balitskii and W. Jaegermann, “XPS study of InTe and GaTe single crystals oxidation”, Mater. Chem. Phys., 97, pp. 98–101, 2006.

20. H.S.Guder, B. Abay, H. Efeoglu, and Y. Yogurtcu, “Photoluminescence characterization of GaTe single crystals” J. Luminescence, 93, pp. 243-248, 2001.

21. J.Z. Wan, J.L. Brebner, L. Leonelli, G. Zhao and J.T. Graham, “Temperature dependence of free-exciton photoluminescence in crystalline GaTe”, Phys. Rev. B, 48, pp. 5197-5201, 1993.

22. V. Kapozi and K. Maschke, “Thermalization of photoexcited localized excitons in GaSe samples with stacking disorder ”, Phys. Rev. B, 34, pp. 3924-3931, 1986.

23. R. A. Taylor and J. F. Ryan, “Time-resolved exciton photoluminescence in GaSe and GaTe”, J. Phys. C: Solid State Phys., 20, pp. 6175-6187, 1987.


spie proceedings [spie optical engineering + applications - san diego, ca (sunday 10 august 2008)]...

Documents