Advances in the Crystal Growth of Semi-insulating … of CdZnTe single crystal growth is performed with a variant of the directional solidification technique. In these techniques,

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Abstract-- The growth of large-volume semi-insulating

CdZnTe single crystals with improved structural perfection hasbeen demonstrated by the electro-dynamic gradient (EDG)technique and active control of the Cd partial pressure in theampoule. The EDG furnace nearly completely eliminates theuncontrolled radiative heat transport commonly encountered intraditional Bridgman systems where the charge and furnacemove relative to each other. Since the new furnace utilizeselectronically controlled high-precision gradient translation itachieves superior thermal stability throughout the growth. Thecontrol of the Cd partial pressure allowed the solidification andcool-down of the ingots close to the stoichiometric composition.As a result, the formation and incorporation of large size (≥≥≥≥ 1 µµµµmdiameter) Te inclusions was avoided during crystallization andingots with high structural perfection were achieved. Adequateelectrical compensation has been achieved in most of the crystalgrowth experiments yielding CdZnTe crystals with bulkelectrical resistivity in the 109 – 1010 ΩΩΩΩcm range and electronmobility-lifetime product as high as µµµµττττe = 1.2××××10-3 cm2/V. Thematerials exhibit good spectral performance in the parallel platedetector configuration.

I. INTRODUCTION

he industrial scale growth of semi-insulating Cd1-xZnxTe(0≤x≤0.2) for room-temperature x-ray and gamma-ray

radiation detector applications has been hampered by lowyields of large-volume single crystals with uniform chargetransport properties. All of the crystal growth techniquesemployed today such as the traveling heater method,conventional Bridgman, high-pressure Bridgman and gradientfreeze techniques are essentially producing large-grainpolycrystalline ingots [1]-[5]. Although, all of thesetechniques have occasionally yielded single crystal CdZnTeingots up to 50 mm diameter they yet to demonstrate thestable and reproducible growth of single crystal ingots on theindustrial scale with diameter 100 mm and more. Much ofthese difficulties are related to the inherent thermo-physical

Manuscript received November 25, 2001. This work was supported inpart by the U.S. Department of Energy Office of Nonproliferation Researchand Engineering.

Csaba Szeles, Scott E. Cameron, Jean-Olivier Ndap and William C.Chalmers are with eV PRODUCTS a division of II-VI Incorporated,Saxonburg, PA 16056 USA (telephone: 724-352-5288, e-mail: [email protected]).

properties of CdZnTe at temperatures close to the meltingpoint and the resulting uncontrolled formation and evolutionof lattice defects during crystallization and the subsequentcool-down of the ingots. Advancement of the crystal growthsystems and much improved control over heat transport bothin the melt and the solid are needed to achieve better controlover the defect formation and evolution during the CdZnTecrystal growth process. Here we report on the development ofa crystal growth system designed to reduce defect formationand improve the crystalline perfection of radiation detectorgrade CdZnTe ingots. The new electro-dynamic gradient(EDG) furnace is based on our research on the gradient freezegrowth technique. The new furnace nearly completelyeliminates the problems resulting from uncontrolled radiativeheat transport commonly encountered in traditional Bridgmansystems where the charge and furnace move relative to eachother. Since the new furnace utilizes electronically controlledgradient transport they achieves superior thermal stabilitythroughout the growth. The new conventional EDG furnaceallows high precision translation of the temperature gradientand precise control of the Cd partial pressure in the ampouleduring the crystal growth.

II. CRYSTAL GROWTH CHALLENGES

The growth of semi-insulating (SI) CdZnTe crystals, withspatially uniform charge transport properties required byradiation detector applications, poses a considerablechallenge for crystal growers. In addition to the difficulties ofgrowing large-volume, uniform single crystals with lowdislocation, precipitate and inclusion density, the stringentrequirements for tuned electrical transport properties addadditional challenges. The need to maintain a very low netfree-carrier density (105 – 106 cm-3), and low density ofcarrier traps and recombination centers requires very goodcontrol over the purity and the stoichiometry of the material.

The fundamental material science challenges of CdZnTecrystal growth can be divided into three categories: A) chargetransport properties, B) single crystal volume, and C) singlecrystal uniformity.

A. Charge Transport Properties

Radiation detector applications require sensor materialswith high electrical resistivity and long lifetime

Advances in the Crystal Growth of Semi-insulating CdZnTe for Radiation Detector

Applications

Csaba Szeles, Scott E. Cameron, Jean-Olivier Ndap and William C. Chalmers

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(recombination and trapping) of the charge carriers. Toachieve the maximum resistivity allowed by the band gap ofthe material, and sufficiently low defect concentrations forreasonably long carrier lifetimes, careful control of theincorporation of electrically active defects is required duringthe crystal growth of semi-insulating CdZnTe.

It is now widely accepted that complete electricalcompensation in SI CdZnTe is achieved by localized defectswith deep electronic levels close to the middle of the bandgap, as discussed by Neumark in her seminal paper [7].Unfortunately, the same deep level defects that assure theelectrical compensation and high resistivity of the CdZnTecrystals also serve as recombination centers and carrier trapsand seriously deteriorate the charge transport in the material.Based on the measured electron, τe ≈ 10-6 s, and holelifetimes, τh ≈ 10-7 s, and the published capture cross sectionsof deep levels [8], one can estimate the density of electron(Ne) and hole (Nh) traps in SI CdZnTe. Today the typicalvalues are about Ne ≈ 1014 cm-3 and Nh ≈ 1015 cm-3,respectively. As a rule of thumb, an order of magnitudeincrease of the carrier lifetimes would require an order ofmagnitude reduction of the density of the correspondingcarrier traps.

B. Single Crystal Volume

Except for a few experimental programs, at present almostall of CdZnTe single crystal growth is performed with avariant of the directional solidification technique. In thesetechniques, (Bridgman, gradient freeze, traveling heatermethod) a temperature gradient is passed through the melt toobtain large single crystals. The probability of obtaining largesingle crystals depends on the physical properties of thematerial in the liquid and solid phase and the thermalproperties of the crystal growth equipment. Single crystalgrowth is therefore a thermodynamic and heat transportproblem. The progress of the solidification is the function ofthe heat flow at the solid-liquid interface.

Φs = ρsHfR + Φl (1)

where Φs and Φl are the heat flux in the solid and the liquid,ρs is the density of the solid, Hf is the heat of fusion and R isthe growth rate. In the approximation that heat is primarilytransported by thermal conduction at the solid-liquidinterface, the heat balance equation simplifies to

ksGs =ρsHfR + klGl (2)

where ks and kl are the thermal conductivity of the solid andthe liquid, and Gs and Gl are the temperature gradients in thesolid and liquid, respectively. The above equation shows thatthe solid has to transport the heat from the melt and the heatreleased during solidification (the solidification of CdZnTebeing an exothermic reaction). The low thermal conductivityof solid CdZnTe poses a significant challenge to maintainingthe heat flow balance at the interface and dictates the use of

slow growth rates and relatively large temperature gradientsto grow large single crystals. A realistic description of theheat transport in real crystal growth systems is a complexproblem that takes into account all the heat transport modes(conductive, convective and radiative) in the melt and solid,growth crucible and furnace. This can only be accomplishedby numerical modeling of the crystallization process. Anexample of thermal modeling for a multi-zone vertical EDGsystem is shown in Fig. 1.

Besides the adequately designed heat flow, the thermalstability of the crystal growth system is the most critical factorfor successful single crystal growth. Temperature fluctuationsand thermal drift of the growth furnace can causeuncontrolled changes of the shape of the growth interface andinduce spurious nucleation. In a classical Bridgman growth,where the charge and the furnace move relative to each other,it is difficult to achieve adequate long-term stability of thegrowth interface, as the radiative heat transport continuouslychanges at the ends of the growth crucible with the progressof the translation (Fig. 2a). Such effects can be eliminated,

and the heat-transport control stabilizedtechnique where the growth cruciblestationary and the translation of the teachieved by programmed lowering opoints in a multi-zone furnace (Fig. 2bthe gradient freeze technique is the contemperature gradient at the melt-sosolidification progresses. As a result brate and the convection in the melt cgrowth and need to be taken into considsingle crystals.

This effect can be eliminated iftemperature gradient zone is achievetechnique is called Elecro-Dynamic Gin the literature. With state of the a

Fig. 1. Temperature distribution and melt flow pattern in a multi-zoneEDG CdZnTe crystal growth system.

, in the gradient freeze

mperature gradient isf the temperature set-). The disadvantage oftinuous change of thelid interface as theoth the crystallizationhanges during crystaleration to obtain lager

the translation of thed electronically. Thisradient (EDG) growthrt temperature control

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techniques versatile crystal growth systems with superiorthermal stability can be designed and constructed today.

We have adopted the EDG technique for both theconventional and high-pressure growth systems. Results fromthe conventional vertical directional solidification of semi-insulating CdZnTe using an advanced EDG furnace with Cdpartial pressure control will be discussed in this article.

C. Single Crystal Uniformity

There are numerous structural defects that hamper theuniformity of CdZnTe single crystals such as dislocations,sub-grain boundaries, Te precipitates and Te inclusions.Many of these defects are associated with charge trapping andrecombination, and were found to adversely affect chargetransport. Recently it was shown that dislocations introducedby deformation in CdZnTe are associated with a localizeddefect level with ionization energy of 0.27 eV [9]. Significant

concentration of dislocations is typically introduced duringthe growth process of the CdZnTe crystals, due to the thermalstress during solidification and cooldown. Dislocationsintersecting a polished surface of a <111> oriented CdZnTecrystal can be revealed by appropriate defect etching [10].Fig. 3 shows dislocation-related etch pits for a CdZnTe singlecrystal with a very high dislocation density. In this case,cellular arrangements of the etch pits are observed. Such astructure is typically observed if the crystal is grown in a hightemperature gradient where the dislocations introducedundergo polygonalization, and form lower energy cellularstructures or sub-grain boundaries. Charge trapping andrecombination along the sub-grain boundaries is expected toproduce non-uniform charge transport and poor performanceof radiation detectors fabricated from such materials.

Almost all of the detector-grade CdZnTe produced today isgrown from Te rich melts and the resulting ingots containlarge concentrations of Te inclusions and precipitates. Here,we use the definitions of Rudolph and Mühlberg for

Tm

Temperature

Z position

Gradientmotion

Temperature

Z position

Tm

Heatertemperature

Classical Bridgman Gradient Freeze

a) b)

Furnacemotion

Tm

Temperature

Z position

Gradientmotion

Temperature

Z position

Tm

Heatertemperature

Classical Bridgman Gradient Freeze

a) b)

Furnacemotion

Fig. 2. Comparison of the classical vertical Bridgman a) and the gradient freeze crystal growth technique b). Tm is the melting point andthe horizontal dashed lines show the progress of the solid-liquid interface. In the Electro-Dynamic Gradient (EDG) system the gradientmotion is achieved electronically (as in a)) without moving the furnace or crucible relative to each other.

50 µµµµm50 µµµµm

Fig. 3. Cellular arrangements of etch pits or sub-grain boundaries(dark spots) in a (111) oriented CdZnTe crystal containing large densityof dislocations.

1 mm1 mm

Fig. 4. IR image of randomly distributed Te inclusions in a CdZnTesingle crystal.

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inclusions and precipitates [11]. Precipitates are formedduring the cooling process and originate from the retrogradeslope of the solidus line [12]. The nucleation and growth ofTe precipitates is controlled by atomic diffusion. The averagediameter of Te precipitates is 10 − 30 nm. In contrast, thetypical diameter of Te-rich inclusions formed in CdZnTe is inthe 1 − 50 µm range. Inclusions originate from morphologicalinstabilities at the growth interface, as Te-rich melt dropletsare captured from the boundary layer ahead of the interface.Fig. 4 shows an infrared (IR) microscopy image of a typicalCdZnTe slice showing Te inclusions as dark spots. Fig. 5shows a transmission electron microscopy (TEM) image of atriangular Te inclusion. Inclusions are typically surrounded bya dense field of dislocations as seen in the x-ray topographyimage in Fig. 6. Isothermal annealing of CdZnTe in Cdvapors can dissolve Te inclusions; however, the dislocationfield surrounding the inclusion stays behind. Te inclusionsformed at the growth interface, and embedded into singlecrystal grains, can migrate during the cool-down of theCdZnTe ingot under the influence of the temperaturegradients existing in the ingot (thermo-migration) [13]. Thisprocess can be understood as the dissolution of the CdZnTeby the Te melt at the high temperature side of the inclusionand the re-crystallization of the CdZnTe matrix at the coldend of the inclusion [14]. Upon arriving at structural defectssuch as grain boundaries, twin boundaries and sub-grainboundaries, the Te inclusions get pinned, hindering theirfurther migration. Such a process can explain the often-observed decoration of large defects in CdZnTe grown fromTe rich melts.

Te inclusions, and the dislocation fields associated withthem, are expected to adversely affect charge transport innuclear radiation detectors fabricated from CdZnTe singlecrystals. In, particular non-uniform distribution of Teinclusions likely causes a severe deterioration of theperformance of the devices. It is well documented thatcorrelated arrangements of Te inclusions such as decoratedgrain boundaries, cause severe deterioration of chargetransport due to charge trapping at these defects and thedistortion of the internal electric field distribution. As a result,detectors fabricated from polycrystalline CdZnTe show poorperformance compared to single crystal devices. The materialnon-uniformity leads to a point-by-point spatial variation ofthe charge transport through the device and results in a severedeterioration of the energy resolution and efficiency of thedevice. Since most of the studies were performed on CdZnTepolycrystals with decorated structural defects, it is unclear atthis point what the effect of un-decorated grain boundaries,twins and sub-grain boundaries is on the charge transportproperties in CdZnTe. Recently, it was shown that non-uniform distribution of Te inclusions also deteriorates thespectroscopic performance of large-volume single-crystalCdZnTe detectors. It was also shown from a geometricargument that the region of degraded charge transport near

the Te inclusions extends beyond the volume of the inclusionitself [15].

Te inclusions can be eliminated from CdZnTe crystals bycontrolling the Cd partial pressure during crystal growth [13],[16]. The suppression of the Te inclusion formation duringcrystal growth by controlling the melt composition offers

several benefits over post-growth annealing of SI CdZnTe.

When the formation of Te inclusions is suppressed, theassociated dislocation fields are eliminated as well. Impuritiesusually trapped at Te inclusions are segregated at the first-to-freeze and last-to-freeze section of the ingots. It is anticipatedthat with this approach CdZnTe single crystals with moreuniform charge transport properties can be grown for nucleardetector applications.

In this paper, we present and discuss experimental resultsfor the growth of CdZnTe single crystals free of Te inclusionsgrown with the EDG and the vertical gradient freezetechnique employing Cd partial pressure control.

3 µµµµm3 µµµµm

Fig. 5. TEM image of a triangular shaped Te inclusion inCdZnTe.

Fig. 6. X-ray topography image of the dislocation fieldsurrounding a Te inclusion in CdZnTe.

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III. EXPERIMENTAL RESULTS

A. Crystal growth experiments

We performed as series of crystal growth experiments intwo different advanced multi-zone EDG furnace using Cdpartial pressure control. The geometry of the furnace/ampouleassembly and a typical gradient freeze temperature program isillustrated in Fig. 7 for one of the furnaces. Thecrucible/ampoule assembly with the Cd reservoir is shown inFig. 8. We used high-purity silica tubes to prepare theampoules and glassy carbon crucibles to grow the Cd1-xZnxTe(x = 0.1) ingots. 6N purity Cd, Zn and Te were used to

synthesize the CdZinto a silica ampoulewas evacuated to ≤growth was perfotranslating the teelectronically. Wegrowth rates to min

with low dislocation density and to avoid constitutional super-cooling effects. The Cd reservoir temperature was variedbetween 800°C and 850°C in these experiments to achievevarious melt stoichiometry during crystallization. Thetemperature of the Cd reservoir was programmed throughoutthe solidification and cool-down in such a way to maintain thedesired stoichiometry of the CdZnTe melt and solid.

Fig. 9 shows a 92 mm diameter 4 kg CdZnTe ingot grownby the vertical EDG technique using Cd partial pressurecontrol. 95% of the ingot is one large single crystal. Anumber of twins are observed throughout the crystal as seenin Fig. 10 that shows a radial slice form the ingot. The one

Fig. 8. The crysta

Fig. 7.

Cd

Am

0

2 0 0

4 0 0

6 0 0

8 0 0

1 0 0 0

1 2 0 0

8 0 0 9 0 0 1 0 0 0 1 1 0 0 1 2 0 0

T e m p e ra tu re (C )

Po

sit

ion

(mm

)

Tm = 1100oC

TCd = 800-850oCCd

reservoir

Crucible

Heaterzones

0

2 0 0

4 0 0

6 0 0

8 0 0

1 0 0 0

1 2 0 0

8 0 0 9 0 0 1 0 0 0 1 1 0 0 1 2 0 0

T e m p e ra tu re (C )

Po

sit

ion

(mm

)

Tm = 1100oC

TCd = 800-850oCCd

reservoir

Crucible

Heaterzones

Vertical Bridgman furnace and gradient freeze temperature program used in some of the CdZnTe crystal growth

nTe compound. The charge was placedwith an extension for a Cd reservoir that

2×10-7 torr and sealed under vacuum. Thermed using the EDG technique i.e.mperature gradient through the meltused low temperature gradients and lowimize thermal stress and produce material

dominant large single crystal grain clearly demonstrates theexcellent thermal stability of the EDG furnace. The presenceof twins suggests that excessive stresses persisted duringsolidification of the ingot. The elimination of the twins willrequire further optimization of the heat-flow pattern of theEDG furnace and refinement of the crystal growth program.

l growth ampoule and Cd reservoir configuration.

source

poule extension

Vacuum-sealed ampoule with crucible

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B. Stoichiometry control

We have performed a set of experiments to suppress Te

inclusiohave varratio ofduring sinfraredslices ofstoichioconditiosolidificstructuragrain bstoichiodiametemicrosc

Fig. 12 compares higher magnification IR images(approximately 10×10 mm2 surface area) of the inclusion-freeingot to a typical detector-grade SI CdZnTe single crystal

Fig. 10EDG t

TwinsTwins

Fig. 9. 92 mm diameter 4kg SI CdZnTe ingot grown by the EDG technique and Cd partial pressure control.

n formation in CdZnTe crystals. For this purpose weied the starting composition of the melt as well as thethe melt temperature and the Cd reservoir temperatureolidification. Fig. 11 illustrates the results in a set of(IR) microscopy images taken on 12 – 30 mm thick50 mm diameter CdZnTe ingots grown under various

metry conditions. Under poor stoichiometry controlns large density of Te inclusions are formed duringations both inside single crystal grains and alongl defects such as grain boundaries, twins and sub-oundaries. For the ingot grown with favorablemetry conditions no Te inclusions are visible withr larger than 1 µm (the resolving power of IRopes).

with the same surface area and thickness. The dark spots inthe latter material show the presence of approximately 50 µmsize Te inclusions. Examination of the CdZnTe slices underhigher magnification (400X) did not reveal the presence of Teinclusions at the ~1 µm spatial resolution of the IRmicroscope. It is clear from Fig. 12 that by controlling thepartial pressure of Cd in the growth ampoule, we maintainedthe composition of the CdZnTe melt and solid close tostoichiometry and suppressed the incorporation of the Teinclusions to the crystal. The presence or absence of Teinclusions smaller in size than ~1 µm cannot be established inthe present investigation. The results were confirmed onseveral CdZnTe ingots grown under nominally identicalconditions.

C. Charge transport properties

To study the charge transport properties of the CdZnTecrystals grown with the vertical EDG or gradient freezetechniques and Cd partial pressure control, 5×5×2 mm3 singlecrystal samples were fabricated from radial slices cut close tothe first-to-freeze (tip) and last-to-freeze (heel) section of theingot. The samples were etched in dilute Br - methanolsolution to remove the surface damage introduced duringcutting. Platinum electrodes were deposited by sputtering the5×5 mm2 area surfaces of the crystals, to form parallel platedetector structures. Current-voltage (I-V) measurements wereperformed to estimate the bulk electrical resistivity of thematerial. Examination of the I-V curves at low bias voltagesshowed the back-to-back Schottky junction characteristicstypical for SI CdZnTe with Pt electrodes. The measured bulkresistivity of the CdZnTe crystals was in the 109 –1010 Ωcmrange.

. Radial slice 92 mm diameter CdZnTe ingot grown by theechnique.

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The mobility lifetime product (µτ) of the charge carrierswas determined from the bias dependence of the chargecollection efficiency. To evaluate the collection efficiency,and estimate the µτ values, the shift of the pulse height of the

photopeak from the 5.5 MeV alpha particles from a 241Amsource was measured as a function of the bias voltage and theresulting data fitted to the Hecht equation [17]. Electronmobility-lifetime product as high as µτe = 1.2×10-3 cm2/V wasobtained in some crystals. The hole mobility-lifetime productof the material was estimated to be less than 5×10-5 cm2/V.

Fig. 13 illustrates the detector performance of the CdZnTecrystals grown with the EDG technique and Cd partialpressure control. 13. Such an excellent spectral performanceis adequate for most of x-ray and gamma spectroscopyapplications where sufficient detector efficiency can beachieved with 5 mm or thinner CdZnTe crystals.

IV. CONCLUSIONS

The growth of large-volume semi-insulating Cd1-xZnxTe (x= 0.1) single crystals with improved structural perfection hasbeen demonstrated by the electro-dynamic gradient (EDG)technique and active control of the Cd partial pressure in theampoule. The crystal growth experiments were performed inEDG furnaces that nearly completely eliminate the problemsresulting from uncontrolled radiative heat transportcommonly encountered in traditional Bridgman systemswhere the charge and furnace move relative to each other.Since these new furnaces utilize electronically controlledgradient translation they achieve superior thermal stabilitythroughout the growth. The control of the Cd partial pressureallowed the solidification and cool-down of the ingots close

to the stoichiometric composition. As a result, the formationand incorporation of large size (≥ 1 µm diameter) Teinclusions was avoided during crystallization and ingots withhigh structural perfection were obtained. Adequate electrical

compensation has been achieved in most of the crystal growth

experiments, yielding bulk electrical resistivity of the CdZnTecrystals in the 109 – 1010 Ωcm range. The materials yieldeddetectors exhibiting good spectral performance in the parallelplate detector configuration and electron mobility-lifetimeproduct as high as µτe = 1.2×10-3 cm2/V.

V. ACKNOWLEDGMENT

The authors are indebted to R. Triboulet (CRNS, Bellevue,France), H.R. Vydyanath (Avyd Devices, Costa Mesa, USA),F.P. Doty (Sandia National Laboratory, Livermore, USA), V.

Te inclusions

Poor control

Successful control

Te inclusions

Poor control

Successful control

Fig. 11. IR transmission microscopy images of 12 – 30 mm thick radial slices from 50 mm diameter CdZnTe ingots grown undervarious stoichiometry conditions.

b)a) b)a)

Fig. 12. CdZnTe single crystals grown a) using Cd partial pressure controland b) without Cd partial pressure control. The dark spots in b) indicatethe presence of Te inclusion of ~ 50 µm diameter.

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Balakrishna (Carnegie-Mellon University, Pittsburgh, USA),K.G. Lynn, T. Rule (Washington State University, Pullman,USA), M. Bliss (PNNL, Richland, USA), K.B. Parnham, andS.A. Soldner (eV PRODUCTS) for many useful discussions.

VI. REFERENCES

[1] E. Raiskin and J.F. Butler, “CdTe low level gamma detectors based ona new crystal growth method,” IEEE Trans. Nucl. Sci. vol. NS-35, pp.81-84, 1988.

[2] F.P. Doty, J.F. Butler, J.F. Schetzina, and K.A. Bowers, “Properties ofCdZnTe crystals grown by a high-pressure Bridgman method,” J. Vac.Sci. Technol. vol. B10, pp. 1418-1422, 1992.

[3] Cs. Szeles and E.E. Eissler, “Current issues of high-pressure Bridgmangrowth of semi-insulating CdZnTe,” MRS Symposium Proceedings,vol. 487, MRS, Warrendale, 1998, pp. 3-12.

[4] Acrorad, Okinawa, Japan and Eurorad, Strasbourg, France.[5] T.E. Schlesinger, M. Greaves, S. Ross; B.A. Brunett, J.M. Van Scyoc,

R.B. James, “Role of uniformity and geometry in IMARAD-typegamma-ray spectrometers” SPIE Proceedings Series, vol. 3768, pp.16-26, 1999.

[6] L. Cirignano, K.S. Shah, P. Bennett, L. Li, F. Lu, J. Buturlia, W. Yao,G. Wright, and R.B. James “Characterization of Multi-element CZTarrays” SPIE Proceedings Series, vol. 4141, pp. 23-28, 2000.

[7] G.F. Neumark, “Effect of deep levels on semiconductor carrierconcentration in the case of “strong” compensation,” Phys. Rev. vol.B26, pp. 2250-2252, Aug. 1981.

[8] A. Castaldini, A. Cavallini, B. Fraboni, P. Fernandez, and J. Piqueras,“Deep energy levels in CdTe and CdZnTe,” J. Appl. Phys. vol. 83, pp.2121-2126, 1998.

[9] N. Krsmanovic, K.G. Lynn, M.H. Weber, R. Tjossem, S.A. Awadalla,Cs. Szeles, J.P. Flint, and H.L. Glass, “Electrical compensation inCdTe and CdZnTe by intrinsic defects,” SPIE Proceedings Series, vol.4141, pp. 219-225, 2000.

[10] K. Nakagawa, K. Maeda and S. Takeguchi, Appl. Phys. Lett. vol. 34,pp. 574-575, 1979.

[11] P. Rudolph and M. Mühlberg, “Basic problems of vertical Bridgmangrowth of CdTe,” Mater. Sci. Eng. vol. B16, pp. 8-16, 1993.

[12] J.H. Greenberg, V.N. Guskov, V.B. Lazarev, and O.V. Shebershneva,“Vapor pressure scanning of nonstoichiometry in CdTe,” J. Solid StateChem. vol. 102, pp. 382-389, 1993.

[13] H.R. Vydyanath, J. Ellsworth, J.J. Kennedy, B. Dean, C.J. Johnson,G.T. Neugebauer, J. Sepich, P.-K. Liao, “Recipe to minimize Teprecipitation in CdTe and (Cd,Zn)Te crystals,” J. Vac. Sci. Technol.vol. B10, pp. 1476-1484 1992.

[14] P. Rudolph, A. Engel, I. Schentke, and A. Grochocki, “Distributionand genesis of inclusions in CdTe and (Cd,Zn)Te single crystals grown

by the Bridgman method and the traveling heater method,” J. CrystalGrowth, vol. 147, pp. 297-304, 1995.

[15] M. Amman, J.S. Lee, and P.N. Luke, private communication.[16] S. Sen, C.S. Liang, D.R. Rhiger, J.E. Stennard, and H.F. Arlinghaus,

“Reduction of CdZnTe substrate defects and relation to epitaxialHgCdTe quality,” J. Electron. Mater., vol. 25, pp. 1188-1195, 1996.

[17] K. Hecht, Zeits. Phys. vol. 77, 235 (1932).

0 200 400 600 800 1000 1200C hannels

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Co

un

ts

2 00 400 600 800 1000 1200 1400C hannels

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1000

1500

2000

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Co

unts

5 7Co5x5x2 m m 3

400 V0.5 µs

137Cs5x5x2 m m 3

400 V4 µs

122 keV

662 keV

a) b)

Fig. 13. Detector response of the CdZnTe crystals grown with Cd partial pressure control to the 122 keV and 662 keV gamma radiationfrom the 57Co and 137Cs sources.