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Characterization of semi-insulating CdTe crystals grown by horizontal seeded physical vapor transport
Journal of Crystal Growth 191 (1998) 377—385
Characterization of semi-insulating CdTe crystals grown by horizontal seeded physical vapor transport K. Chattopadhyay!, S. Feth!, H. Chen!, A. Burger!,*, Ching-Hua Su" ! Center for Photonic Materials and Devices, Department of Physics, Fisk University, Nashville, TN 37208, USA " Space Science Laboratory, NASA George C. Marshall Space Flight Center, AL 35812, USA Received 17 January 1992
Abstract CdTe crystals were grown by horizontal seeded physical vapor transport technique in uncoated and boron nitride coated fused silica ampoules with the source materials near the congruent sublimation condition. The grown crystals were characterized by current—voltage measurements, low-temperature photoluminescence spectroscopy, near IR transmission optical microscopy, spark source mass spectroscopy and chemical etching. The measured resistivities of the crystals were in the high-108 ) cm range. The photoluminescence spectra of the crystal grown in the boron nitride coated ampoule showed similar features previously observed in the CdTe crystals doped with group III elements. Although the crystal was contaminated with boron, the boron nitride coating of the growth ampoule has yielded a single crystal with no inclusions or precipitates. ( 1998 Published by Elsevier Science B.V. All rights reserved.
1. Introduction High-resistivity CdTe crystals with high mobility-lifetime product are needed for room temperature nuclear radiation detectors [1—7]. The desired high resistivity in CdTe is usually accomplished by the introduction of dopants, such as halogens [8—10]. In Ref. [6], Ti [11,12] or V [11,12], to compensate for the native defect centers, which are generally thought to be related to Cd vacancies although a Te anti-site defect was proposed re-
* Corresponding author. Fax: #1615 329 8634; e-mail: [email protected].
cently as the predominant native defect [13,14]. The requirement of a high mobility-lifetime product means that the crystal needs to show good crystalline quality and have a low concentration of defects, including native or impurity point defects as well as the two- and three-dimensional structural defects such as grain boundaries and Te inclusions or precipitates. In the past, the main method used to grow detector grade CdTe material was the traveling heater method (THM) from Te-rich solvent [2,4,5,15—17]. This low-temperature technique has some advantages over the other high-temperature growth methods, such as the Bridgman technique, in that it results in less thermal strain in the grown crystals
0022-0248/98/$19.00 ( 1998 Published by Elsevier Science B.V. All rights reserved. PII S 0 0 2 2 - 0 2 4 8 ( 9 8 ) 0 0 1 3 4 - 1
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and it extracted impurities [2,15,16] as well as reduced possible contamination from or through the fused silica ampoule [2,18]. The disadvantage of this non-stoichiometric (Te-excess) growth condition is the introduction of Te inclusions and precipitates as well as the native defects such as Cd vacancies [6]. Consequently, in previous investigation doping was necessary to compensate for the native-point defects. The resistivity of the undoped or doped sample grown by the THM is generally in the 105—109 ) cm range [17,19]. Vapor growth techniques offer similar advantages as the THM in that the growth temperature is considerably lower than the melting point. Physical vapor transport (PVT) and its various modifications were used to grow undoped and doped CdTe [9,10,20—22]. The resistivity of the undoped CdTe crystals varied from 1]103 to 3]109 ) cm. This variation was mainly determined by the deviations from stoichiometry of the samples. The Cl- and V-doped CdTe samples grown by PVT showed a resistivity as high as 1.5]1010 ) cm, however, the problem of polarization causing time instability or degradation during the performance of the nuclear detectors has been reported on Cl-doped CdTe [23]. In theory, the highest resistivity that a CdTe crystal can possibly achieve can be estimated from intrinsic carrier concentration as determined from the energy band gap and carrier mobilities. An intrinsic CdTe sample has no electrically active defects, including native-point defects such as vacancies, interstitials, anti-sites, etc., as well as foreign impurities. In principle, with accurate control of the stoichiometry during the PVT growth process one should obtain CdTe crystal with high resistivity. However, it is difficult to reproducibly grow CdTe crystals with the desired electrical properties and free from Te-precipitates when the partial pressures of the predominant vapor species, Cd and Te , vary over orders of magnitude at the 2 growth temperature due to deviations from stoichiometry of the source materials [24]. Additionally, oxygen and other impurities from fused silica crucibles can cause significant contamination to the grown crystals during the growth process even with the employment of carbon coated fused silica ampoules. Therefore, about a decade ago, the high-pressure Bridgman (HPB)
method was adopted to grow high-resistivity crystals of undoped CdTe and CdZnTe [25,26]. With the high inert pressure applied over the growth chamber, graphite could be used as the growth crucible. Using the HPB technique, the undoped CdTe crystals showed a resistivity as high as 3]109 ) cm, although the problem of Te-inclusions and precipitates, due to the retrograde Te solubility of CdTe at the elevated growth temperatures, has not been solved. Recently, a heat-treatment technique on the CdTe starting material for the PVT process aiming at adjusting the vapor-phase stoichiometry toward that of the congruent sublimation condition has been demonstrated [27]. In this paper, we report the results of the growth and characterization of the CdTe crystals grown by a horizontal seeded PVT process using this heat treatment technique. The prevention of contamination from the fused silica ampoules was also investigated by growing the CdTe crystal in a boron nitride (BN) coated (inside) fused silica ampoule. The quality of the grown crystals were assessed by current—voltage measurements, low-temperature photoluminescence (PL) spectroscopy, near IR transmission optical microscopy, spark source mass spectroscopy (SSMS) and chemical etching.
2. Experimental procedure The starting materials were homogenized from pure elements. The homogenization ampoules were made from 24 mm OD, 20 mm ID (24]20) fused silica tubing supplied by Heraeus Amersil, Inc. The starting elements were quadruple-zone refined (QZR) or double zone refined, six—nine grade, Cd rods and QZR, six—nine grade Te bars from Johnson Matthey, Inc. The elements were weighed to the accuracy of 0.1 mg. A total amount of 100—220 g of the pure elements were loaded into each ampoule with the nominal Te content ranging from 0.50001 to 0.50006. The sealed ampoules were heated slowly from room temperature to the homogenization temperature of 990°C and maintained at the temperature for 35—50 h before slow cooling to room temperature. The ampoules were opened and the reacted sponge-like CdTe was
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crushed and ground into particles with diameter of less than 0.5 mm. The seeded ampoule was made by fusing 18]16 and 12]8 mm silica tubings with a 7 mm diameter joint in between as a seed restriction. The BN coating inside the ampoule wall was accomplished by high-temperature treatment of BN powder which resulted in the fusion of BN to the silica tubing. The empty ampoule was cleaned and loaded with high-purity grade BN powder supplied by Union Carbide, Inc. Radially uniform heating was applied using a hydrogen—oxygen torch on the ampoule which was mounted on a glass blowing lathe. After 10—20 min under white flame the ampoule was cooled and the excess BN powder was removed with distilled water. A uniform foggy appearance on the inside wall of the ampoule was visible. After heating the ampoule again by the torch, the hazy appearance disappeared and a green emission could be seen from the ampoule inner wall under the heat of the torch. The attempt to measure the thickness of the BN coating by examining the cross section of the fused silica tubing under scanning electron microscope was unsuccessful although B and N peaks were detected using energy dispersive X-ray spectroscopy. About 20 g of the ground, homogenized CdTe was loaded into each ampoule. The heat treatment used to adjust the stoichiometry of the starting materials towards that of the congruent sublimation condition [27] was performed by baking the loaded ampoule at elevated temperature under dynamic vacuum for 7—10 min. An extra step of hydrogen reduction by baking the sample at 1000°C under 0.57 atm H pressure for 25 min was adopted 2 for the BN coated ampoule prior to the above baking heat treatment. After sealing under vacuum the ampoules were placed inside a high-gradient furnace to transport the material to the source end. Each ampoule was then opened and a CdTe seed (7 mm long and 7 mm diameter supplied by Keystone Corp.) and a fused silica rod were loaded and the ampoules sealed under vacuum. All the seeds had a (1 1 1) orientation except a (2 1 1) seed was used for the BN coated ampoule. The ampoule configuration and the initial ampoule position along the axial thermal profile during the growth run is given schematically in Fig. 1. The growth
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furnace and process have been described in detail elsewhere [28]. In brief, the thermal profile was provided by a three-zone furnace with an adiabatic zone between the central and the end zone. The furnace translated to the right during the growth. The initial ampoule position was carefully adjusted such that the middle section of the seed was at the supersaturation position. Under such condition, part of the seed was thermally etched and sublimed before the grown crystal nucleated and grew on top of the seed. For the crystal, CdTe-10, an uncoated fused silica ampoule and a (1 1 1) orientation seed were used. The source temperature was 825°C with a central maximum of 844°C and the cold zone of 715°C. The thermal gradient at the crystal interface was about 30°C/cm. The furnace translation was 3.32 mm/day and the crystal was cooled at a rate of 19°C/h to room temperature. A BN coated fused silica ampoule and a (2 1 1) seed were used to grow crystal CdTe-22. In this case, the source was at 852°C with the maximum of 863°C and the cold zone was at 800°C. The gradient at the supersaturation position was 15°C/cm. The furnace translation rate was 3.87 mm/day and a cooling rate of 5°C/h was adopted. Spark source mass spectrographic (SSMS) analysis was performed on two samples. The first sample was the homogenized CdTe starting material and the second one was a slice cut from a crystal grown by the seeded PVT technique with no coating applied to the ampoule wall. The samples were polished on a mechanical polisher with a final abrasive of 0.05 lm particle size alumina suspension and then rinsed with methanol. Au contacts were deposited by thermal evaporation immediately after polishing and rinsing to minimize the surface oxidation. After the deposition, Pt leads were attached to the contacts using a colloidal graphite suspension in water (“aquadag” from Achelson Inc.). Finally, the device was covered with protective coating (“Humiseal” from Chase Corp.) to help with surface passivation and also to improve the mechanical stability. For I—» characterization, each crystal was mounted on a Teflon holder and then placed in a closed aluminum box. The measurements were carried out at room temperature using a Keithley
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Fig. 1. Schematics of the furnace thermal profile and the initial position of the growth ampoule.
programmable electrometer (Model 617) in conjunction with a Bertran high—voltage power supply (series 225). The data acquisition was computer controlled via a GPIB interface. For the etch-pit density (EPD) study the crystals were chemico-mechanically polished on both surfaces as mentioned above. Then they were chemically polished with E solution for 1 min prior to being etched. The E solution was composed of 10 ml of HNO , 20 ml of H O and 4 g of K Cr O . 3 2 2 2 7 The EAg-1 etching solution was selected since it produces an etch-pit configuration that varies in shape depending on both the surface orientation of the crystal and the Ag` ion configuration. The samples were etched with fresh EAg-1, which is composed of 10 ml of E solution and 0.5 mg of AgNO , for 1 min. 3 For the low-temperature PL measurements, the samples were cooled to 11 K using an ADP Cryogenic, Inc., system equipped with dual HC4MK1 helium compressors. The samples were illuminated with the 488 nm line of an ILT 5500A air-cooled Ar-ion laser at a power density of
about 2 W/cm2. The PL spectra were detected using a SPEX 1877D Triplemate spectrometer in conjunction with a liquid-nitrogen cooled charge-couple-device detector. A 0.1 mm slit and a 300 grooves/mm grating were employed for the spectrometer which was calibrated with a Hg lamp before each measurement. The signals were recorded by a PC using the SPEX DM3000 software.
3. Results and discussion Table 1 gives the SSMS results on the impurity levels in the homogenized CdTe starting material and in the grown CdTe crystal. The major impurities in both samples are S and In and a low level of Li and S contamination was found from the growth process. Table 2 gives the resistivities and current density of both CdTe-10 and CdTe-22 crystals. Fig. 2 shows the I—» curves of these samples. The intrinsic carrier concentration of CdTe at 300 K is about
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Table 1 Impurity levels by spark source mass spectrographic analysis Element (ppm, atomic)
Homogenized CdTe starting material
Grown CdTe crystal (No BN coating)
Li Na Al Si P S Cl K Ca Cr Cu Se In
(0.001! 0.003 0.03 0.3 (0.02! 2 (0.01! 0.005 0.05 0.03 (5! 0.1 1
0.002 0.003 0.03 0.3 (0.02! 3 (0.01! 0.005 0.05 0.02 (5! 0.1 1
! The impurity level is below the indicated detection limit of the SSMS.
Table 2 Resistivity and current density for the CdTe(s) crystals Sample number
Current density at 500 V/cm (nA/cm2)
Slope of J—E curve () cm)
CdTe-10 CdTe-22
277 1000
9.0]108 7.0]108
1]106 cm~3 [29] and the highest mobilities measured at 300 K are roughly 1100 and 100 cm2/V cm for electrons and holes [8,30], respectively. The calculated resistivity of the intrinsic CdTe crystal at 300 K from the above values is 5]109 ) cm. The measured apparent resistivity of high-108 ) cm range is slightly lower than this value and is considered not high enough to be detector grade. The surface treatment with different chemical etchants [31] and the metal contacting process affects the defect states near the surface region, both extrinsic and intrinsic. These factors determine the surface leakage current which in turn significantly affects the performance of the device. We therefore believe that the high leakage current might be due to the surface defects introduced by mechanical and chemical treatment. Figs. 3 and 4 show the PL spectra of the CdTe samples measured at 11 K. The PL spectrum for
Fig. 2. I—» curves of different samples of CdTe grown by horizontal seeded PVT method.
CdTe-10, shown in Fig. 3, is a typical result for CdTe crystals grown in the uncoated ampoules. The spectrum has dominant bound exciton peaks that consist of a weak emission of the free exciton at 1.598 eV and a shallow donor peak (Do, X) at 1.593 eV. It has been reported [32] that the (Do, X) line mainly originates from Cl . However, the reT% sults of SSMS analysis in Table 1 indicates the Cl content is less than 0.01 ppm in the crystal. Indium is the other possible donor with an appreciated amount listed in Table 1. The origins of other two peaks at 1.557 and 1.549 eV with their LO phonon replicas are not very clear. The peaks in this region have been previously observed in As and Sb-doped CdTe crystals [33], and were assigned as peaks due to electron—acceptor (e—A°) recombination and a shallow donor—acceptor pair (DAP) recombination, respectively. In our case we think that the presence of an appreciable amount of In as an impurity in the CdTe leads to the occurrence of these peaks. No deep level donor—acceptor-pair (DAP) lines around 1.45 eV were observed. The PL spectrum of CdTe-22, which was grown in the BN coated ampoule, is shown in Fig. 3. The (Do, X) line is observed at 1.592 eV. It was a common feature to observe peaks near 1.584 eV in high-resistivity CdTe crystals doped with group III elements, such as 1.584 [34] and 1.585 [35] eV for Ga and similar lines for In and Al-doped CdTe [36]. The line at 1.584 eV in Fig. 3 is therefore assumed to be caused by the unintentional doping
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Fig. 3. Low-temperature (11 K) PL spectra showing a strong bound exciton (D°, X) peak for CdTe-10 sample.
Fig. 4. Low temperature (11 K) PL spectra showing the bound-acceptor peak due to boron doping for CdTe-22 sample.
of boron from the BN coating of the ampoule. There is a strong emission line at 1.545 eV followed by its phonon replica at 1.523 eV. We believe that this peak is due to a DAP recombination. The peak at 1.545 eV is due to an impurity or dopant, and in this case the diffused boron enhances the intensity of the emission. At lower energy the DAP emission at 1.455 eV and its phonon replicas (DAP-1LO,
DAP-2LO and DAP-3LO) were observed. Similar DAP emission bands were also observed in Ga[34], In- [36] and Al-doped [3] CdTe crystals. The intense emission of these lines is expected in the doped samples. Fig. 5 shows pictures of the CdTe-10 crystal using the near IR transmission microscope as a function of depth from the surface. The regions opaque
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in nature. No precipitates were observed in the CdTe-22 crystal. Results of the EPD study in Fig. 6a show that the CdTe-10 crystal consists of several grains with two different kinds of etch pits on both sides of the grain boundary. The pyramidal pits indicate a (1 1 1)A orientation whereas the rectangular pits indicate a (1 0 0) orientation. Fig. 6b shows the etch-pits on the CdTe-22 crystal. Only one kind of etch-pit lined up along the straight parallel lines. Twin boundaries were observed in this crystal grown by PVT on a 2 1 1 seed. This is not consistent with the reported results that single CdTe crystal without twins were successfully grown using homoepitaxial growth by sublimation on the 2 1 1 seed [37]. The crystal grown on the (1 1 1) seed in the uncoated quartz ampoule has a larger number of grain boundaries than the crystal grown on the (2 1 1) seed in the boron nitride coated ampoule. Similar results were reported by Shetty et al. [38], although in their case, the CdTe crystal were grown by the Bridgman method. Since the formation of the dislocation etch pits have been related to stress between the crystal and the ampoule, the lower etch-pit density in CdTe-22 clearly indicates that the boron nitride coating helps in reducing the stress between the crystal and the inner walls of the ampoule.
4. Conclusions
Fig. 5. Near IR transmission pictures of seeded CdTe-10 as a function of depth, with the microscope focused (a) near the surface, (b) just below the surface, and (c) deep inside the bulk.
to near IR are believed to be Te precipitates and the definite geometrical shape of the precipitates indicates that the precipitates may be crystalline
In this article, we have presented the results on the growth and characterization of CdTe crystals grown by horizontal seeded PVT with the source material near the congruent sublimation condition. The prevention of contamination from the fused silica ampoules was also investigated by growing the CdTe crystal in a boron nitride (BN) coated fused silica ampoule. The resistivities of the grown crystals were in the high-108 ) cm range. The PL spectra of the crystal grown in the BN coated ampoule showed similar features observed in the CdTe crystals doped with group III elements. Thus, the crystal was believed to be unintentionally doped with boron. On the other hand, BN coating of the growth ampoules yielded a single crystal with no grain boundaries nor precipitates.
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Fig. 6. The EPD pictures showing the etch pits and the grain boundaries of the two crystals (a) etch pits for CdTe-10 near the grain boundary, the long sides of the rectangular pits are parallel to the grain boundary when the neighboring pits are pyramidal. (b) small etch pits that are roughly triangular for CdTe-22, and there are no pits in the twinned region.
Acknowledgements This work was supported by National Aeronautics and Space Administration through the Fisk University center for Photonic Materials and Devices, Grant No. NAGW-2925 and by Microgravity Research Division, National Aeronautics and Space Administration.
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[10] C. Eiche, W. Joerger, M. Fiederle, D. Ebling, M. Salk, R. Schwarz, K.W. Benz, J. Crystal Growth 166 (1996) 245. [11] C. Eiche, W. Joerger, M. Fiederle, D. Ebling, R. Schwarz, K.W. Benz, Opt. Mater. 4 (1995) 214. [12] R. Schwarz, W. Joerger, C. Eiche, M. Fiederle, K.W. Benz, J. Crystal Growth 146 (1995) 92. [13] M.A. Berding, M. van Schifgaarde, A.T. Paxon, A. Sher, J. Vac. Sci. Technol. A 8 (1990) 1103. [14] R.F. Brebrick, R. Fang, J. Phys. Chem. Solids 57 (1996) 451. [15] R.O. Bell, N. Hemat, F. Wald, Phys. Stat. Sol. (a) 1 (1970) 375. [16] R. Triboulet, Y. Marfaing, A. Cornet, P. Siffert, J. Appl. Phys. 45 (1974) 2759. [17] R. Triboulet, Y. Marfaing, J. Crystal Growth 51 (1981) 89. [18] R. Triboulet, A. Aoudia, A. Lusson, J. Electron. Mater. 24 (1995) 1061. [19] L. Verger, N. Baffert, M. Rosaz, J. Rustique, Nucl. Instr. and Meth. A 380 (1996) 121. [20] M. Bruder, R. Nitsche, J. Crystal Growth 72 (1985) 705. [21] N. Yellin, A. Zemel, R. Tenne, J. Electron. Mater. 14 (1985) 85. [22] M. Laasch, R. Schwarz, W. Joerger, C. Eiche, M. Fiederle, K.W. Benz, K. Grasza, J. Crystal Growth 146 (1995) 125. [23] H.L. Malm, M. Martin, IEEE Trans. Nucl. Sci. NS-21 (1974) 322. [24] R.F. Brebrick, J. Electrochem. Soc. 118 (1971) 2014. [25] F.P. Doty, J.F. Butler, J.F. Schetzina, K.A. Bowers, J. Vac. Sci. Technol. B 10 (1992) 1418.
K. Chattopadhyay et al. / Journal of Crystal Growth 191 (1998) 377–385 [26] J.F. Butler, C.L. Lingren, F.P. Doty, IEEE Trans. Nucl. Sci. NS-39 (1992) 605. [27] C.-H. Su, Y.-G. Sha, S.L. Lehoczky, H.-C. Liu, R. Fang, R.F. Brebrick, J. Crystal Growth, to be published. [28] C.-H. Su, SPIE Proc. 3123 (1997) 7. [29] C.-H. Su, P.-K. Liao, R.F. Brebrick, J. Electron. Mater. 12 (1983) 771. [30] B. Segall, M.R. Lorenz, R.E. Halsted, Phys. Rev. 129 (1963) 2471. [31] H. Chen, S.U. Egarievwe, Z. Hu, J. Tong, D.T. Shi, G.H. Wu, K.-T. Chen, M.A. George, W.E. Collins, A. Burger, R.B. James, C.M. Stahle, L.M. Bartlett, J. Appl. Phys. 80 (5) (1996) 1.
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[32] B. Yang, Y. Ishikawa, Y. Doumae, T. Miki, T. Ohyama, M. Isshiki, J. Crystal Growth 172 (1997) 370. [33] M. Soltani, M. Certier, R. Errard, E. Kartheuser, J. Appl. Phys. 78 (9) (1995) 5626. [34] S. Seto, A. Tanaka, K. Suzuki, M. Kawashima, J. Crystal Growth 101 (1990) 430. [35] J.M. Wrobel, J.J. Dubowski, P. Becla, J. Vac. Sci. Technol. A 7 (1989) 338. [36] N.C. Giles, S. Hwang, J.F. Schetzina, S. McDevitt, C.J. Johnson, J. Appl. Phys. 64 (1988) 2656. [37] Y. Yoshioka, H. Yoda, M. Kasuga, J. Crystal Growth 115 (1991) 705. [38] Rajaram Shetty, William R. Wilcox, Liya L. Regel, J. Crystal Growth 153 (1995) 103.
Characterization of semi-insulating CdTe crystals grown by horizontal seeded physical vapor transport K. Chattopadhyay!, S. Feth!, H. Chen!, A. Burger!,*, Ching-Hua Su" ! Center for Photonic Materials and Devices, Department of Physics, Fisk University, Nashville, TN 37208, USA " Space Science Laboratory, NASA George C. Marshall Space Flight Center, AL 35812, USA Received 17 January 1992
Abstract CdTe crystals were grown by horizontal seeded physical vapor transport technique in uncoated and boron nitride coated fused silica ampoules with the source materials near the congruent sublimation condition. The grown crystals were characterized by current—voltage measurements, low-temperature photoluminescence spectroscopy, near IR transmission optical microscopy, spark source mass spectroscopy and chemical etching. The measured resistivities of the crystals were in the high-108 ) cm range. The photoluminescence spectra of the crystal grown in the boron nitride coated ampoule showed similar features previously observed in the CdTe crystals doped with group III elements. Although the crystal was contaminated with boron, the boron nitride coating of the growth ampoule has yielded a single crystal with no inclusions or precipitates. ( 1998 Published by Elsevier Science B.V. All rights reserved.
1. Introduction High-resistivity CdTe crystals with high mobility-lifetime product are needed for room temperature nuclear radiation detectors [1—7]. The desired high resistivity in CdTe is usually accomplished by the introduction of dopants, such as halogens [8—10]. In Ref. [6], Ti [11,12] or V [11,12], to compensate for the native defect centers, which are generally thought to be related to Cd vacancies although a Te anti-site defect was proposed re-
* Corresponding author. Fax: #1615 329 8634; e-mail: [email protected].
cently as the predominant native defect [13,14]. The requirement of a high mobility-lifetime product means that the crystal needs to show good crystalline quality and have a low concentration of defects, including native or impurity point defects as well as the two- and three-dimensional structural defects such as grain boundaries and Te inclusions or precipitates. In the past, the main method used to grow detector grade CdTe material was the traveling heater method (THM) from Te-rich solvent [2,4,5,15—17]. This low-temperature technique has some advantages over the other high-temperature growth methods, such as the Bridgman technique, in that it results in less thermal strain in the grown crystals
0022-0248/98/$19.00 ( 1998 Published by Elsevier Science B.V. All rights reserved. PII S 0 0 2 2 - 0 2 4 8 ( 9 8 ) 0 0 1 3 4 - 1
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and it extracted impurities [2,15,16] as well as reduced possible contamination from or through the fused silica ampoule [2,18]. The disadvantage of this non-stoichiometric (Te-excess) growth condition is the introduction of Te inclusions and precipitates as well as the native defects such as Cd vacancies [6]. Consequently, in previous investigation doping was necessary to compensate for the native-point defects. The resistivity of the undoped or doped sample grown by the THM is generally in the 105—109 ) cm range [17,19]. Vapor growth techniques offer similar advantages as the THM in that the growth temperature is considerably lower than the melting point. Physical vapor transport (PVT) and its various modifications were used to grow undoped and doped CdTe [9,10,20—22]. The resistivity of the undoped CdTe crystals varied from 1]103 to 3]109 ) cm. This variation was mainly determined by the deviations from stoichiometry of the samples. The Cl- and V-doped CdTe samples grown by PVT showed a resistivity as high as 1.5]1010 ) cm, however, the problem of polarization causing time instability or degradation during the performance of the nuclear detectors has been reported on Cl-doped CdTe [23]. In theory, the highest resistivity that a CdTe crystal can possibly achieve can be estimated from intrinsic carrier concentration as determined from the energy band gap and carrier mobilities. An intrinsic CdTe sample has no electrically active defects, including native-point defects such as vacancies, interstitials, anti-sites, etc., as well as foreign impurities. In principle, with accurate control of the stoichiometry during the PVT growth process one should obtain CdTe crystal with high resistivity. However, it is difficult to reproducibly grow CdTe crystals with the desired electrical properties and free from Te-precipitates when the partial pressures of the predominant vapor species, Cd and Te , vary over orders of magnitude at the 2 growth temperature due to deviations from stoichiometry of the source materials [24]. Additionally, oxygen and other impurities from fused silica crucibles can cause significant contamination to the grown crystals during the growth process even with the employment of carbon coated fused silica ampoules. Therefore, about a decade ago, the high-pressure Bridgman (HPB)
method was adopted to grow high-resistivity crystals of undoped CdTe and CdZnTe [25,26]. With the high inert pressure applied over the growth chamber, graphite could be used as the growth crucible. Using the HPB technique, the undoped CdTe crystals showed a resistivity as high as 3]109 ) cm, although the problem of Te-inclusions and precipitates, due to the retrograde Te solubility of CdTe at the elevated growth temperatures, has not been solved. Recently, a heat-treatment technique on the CdTe starting material for the PVT process aiming at adjusting the vapor-phase stoichiometry toward that of the congruent sublimation condition has been demonstrated [27]. In this paper, we report the results of the growth and characterization of the CdTe crystals grown by a horizontal seeded PVT process using this heat treatment technique. The prevention of contamination from the fused silica ampoules was also investigated by growing the CdTe crystal in a boron nitride (BN) coated (inside) fused silica ampoule. The quality of the grown crystals were assessed by current—voltage measurements, low-temperature photoluminescence (PL) spectroscopy, near IR transmission optical microscopy, spark source mass spectroscopy (SSMS) and chemical etching.
2. Experimental procedure The starting materials were homogenized from pure elements. The homogenization ampoules were made from 24 mm OD, 20 mm ID (24]20) fused silica tubing supplied by Heraeus Amersil, Inc. The starting elements were quadruple-zone refined (QZR) or double zone refined, six—nine grade, Cd rods and QZR, six—nine grade Te bars from Johnson Matthey, Inc. The elements were weighed to the accuracy of 0.1 mg. A total amount of 100—220 g of the pure elements were loaded into each ampoule with the nominal Te content ranging from 0.50001 to 0.50006. The sealed ampoules were heated slowly from room temperature to the homogenization temperature of 990°C and maintained at the temperature for 35—50 h before slow cooling to room temperature. The ampoules were opened and the reacted sponge-like CdTe was
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crushed and ground into particles with diameter of less than 0.5 mm. The seeded ampoule was made by fusing 18]16 and 12]8 mm silica tubings with a 7 mm diameter joint in between as a seed restriction. The BN coating inside the ampoule wall was accomplished by high-temperature treatment of BN powder which resulted in the fusion of BN to the silica tubing. The empty ampoule was cleaned and loaded with high-purity grade BN powder supplied by Union Carbide, Inc. Radially uniform heating was applied using a hydrogen—oxygen torch on the ampoule which was mounted on a glass blowing lathe. After 10—20 min under white flame the ampoule was cooled and the excess BN powder was removed with distilled water. A uniform foggy appearance on the inside wall of the ampoule was visible. After heating the ampoule again by the torch, the hazy appearance disappeared and a green emission could be seen from the ampoule inner wall under the heat of the torch. The attempt to measure the thickness of the BN coating by examining the cross section of the fused silica tubing under scanning electron microscope was unsuccessful although B and N peaks were detected using energy dispersive X-ray spectroscopy. About 20 g of the ground, homogenized CdTe was loaded into each ampoule. The heat treatment used to adjust the stoichiometry of the starting materials towards that of the congruent sublimation condition [27] was performed by baking the loaded ampoule at elevated temperature under dynamic vacuum for 7—10 min. An extra step of hydrogen reduction by baking the sample at 1000°C under 0.57 atm H pressure for 25 min was adopted 2 for the BN coated ampoule prior to the above baking heat treatment. After sealing under vacuum the ampoules were placed inside a high-gradient furnace to transport the material to the source end. Each ampoule was then opened and a CdTe seed (7 mm long and 7 mm diameter supplied by Keystone Corp.) and a fused silica rod were loaded and the ampoules sealed under vacuum. All the seeds had a (1 1 1) orientation except a (2 1 1) seed was used for the BN coated ampoule. The ampoule configuration and the initial ampoule position along the axial thermal profile during the growth run is given schematically in Fig. 1. The growth
379
furnace and process have been described in detail elsewhere [28]. In brief, the thermal profile was provided by a three-zone furnace with an adiabatic zone between the central and the end zone. The furnace translated to the right during the growth. The initial ampoule position was carefully adjusted such that the middle section of the seed was at the supersaturation position. Under such condition, part of the seed was thermally etched and sublimed before the grown crystal nucleated and grew on top of the seed. For the crystal, CdTe-10, an uncoated fused silica ampoule and a (1 1 1) orientation seed were used. The source temperature was 825°C with a central maximum of 844°C and the cold zone of 715°C. The thermal gradient at the crystal interface was about 30°C/cm. The furnace translation was 3.32 mm/day and the crystal was cooled at a rate of 19°C/h to room temperature. A BN coated fused silica ampoule and a (2 1 1) seed were used to grow crystal CdTe-22. In this case, the source was at 852°C with the maximum of 863°C and the cold zone was at 800°C. The gradient at the supersaturation position was 15°C/cm. The furnace translation rate was 3.87 mm/day and a cooling rate of 5°C/h was adopted. Spark source mass spectrographic (SSMS) analysis was performed on two samples. The first sample was the homogenized CdTe starting material and the second one was a slice cut from a crystal grown by the seeded PVT technique with no coating applied to the ampoule wall. The samples were polished on a mechanical polisher with a final abrasive of 0.05 lm particle size alumina suspension and then rinsed with methanol. Au contacts were deposited by thermal evaporation immediately after polishing and rinsing to minimize the surface oxidation. After the deposition, Pt leads were attached to the contacts using a colloidal graphite suspension in water (“aquadag” from Achelson Inc.). Finally, the device was covered with protective coating (“Humiseal” from Chase Corp.) to help with surface passivation and also to improve the mechanical stability. For I—» characterization, each crystal was mounted on a Teflon holder and then placed in a closed aluminum box. The measurements were carried out at room temperature using a Keithley
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Fig. 1. Schematics of the furnace thermal profile and the initial position of the growth ampoule.
programmable electrometer (Model 617) in conjunction with a Bertran high—voltage power supply (series 225). The data acquisition was computer controlled via a GPIB interface. For the etch-pit density (EPD) study the crystals were chemico-mechanically polished on both surfaces as mentioned above. Then they were chemically polished with E solution for 1 min prior to being etched. The E solution was composed of 10 ml of HNO , 20 ml of H O and 4 g of K Cr O . 3 2 2 2 7 The EAg-1 etching solution was selected since it produces an etch-pit configuration that varies in shape depending on both the surface orientation of the crystal and the Ag` ion configuration. The samples were etched with fresh EAg-1, which is composed of 10 ml of E solution and 0.5 mg of AgNO , for 1 min. 3 For the low-temperature PL measurements, the samples were cooled to 11 K using an ADP Cryogenic, Inc., system equipped with dual HC4MK1 helium compressors. The samples were illuminated with the 488 nm line of an ILT 5500A air-cooled Ar-ion laser at a power density of
about 2 W/cm2. The PL spectra were detected using a SPEX 1877D Triplemate spectrometer in conjunction with a liquid-nitrogen cooled charge-couple-device detector. A 0.1 mm slit and a 300 grooves/mm grating were employed for the spectrometer which was calibrated with a Hg lamp before each measurement. The signals were recorded by a PC using the SPEX DM3000 software.
3. Results and discussion Table 1 gives the SSMS results on the impurity levels in the homogenized CdTe starting material and in the grown CdTe crystal. The major impurities in both samples are S and In and a low level of Li and S contamination was found from the growth process. Table 2 gives the resistivities and current density of both CdTe-10 and CdTe-22 crystals. Fig. 2 shows the I—» curves of these samples. The intrinsic carrier concentration of CdTe at 300 K is about
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Table 1 Impurity levels by spark source mass spectrographic analysis Element (ppm, atomic)
Homogenized CdTe starting material
Grown CdTe crystal (No BN coating)
Li Na Al Si P S Cl K Ca Cr Cu Se In
(0.001! 0.003 0.03 0.3 (0.02! 2 (0.01! 0.005 0.05 0.03 (5! 0.1 1
0.002 0.003 0.03 0.3 (0.02! 3 (0.01! 0.005 0.05 0.02 (5! 0.1 1
! The impurity level is below the indicated detection limit of the SSMS.
Table 2 Resistivity and current density for the CdTe(s) crystals Sample number
Current density at 500 V/cm (nA/cm2)
Slope of J—E curve () cm)
CdTe-10 CdTe-22
277 1000
9.0]108 7.0]108
1]106 cm~3 [29] and the highest mobilities measured at 300 K are roughly 1100 and 100 cm2/V cm for electrons and holes [8,30], respectively. The calculated resistivity of the intrinsic CdTe crystal at 300 K from the above values is 5]109 ) cm. The measured apparent resistivity of high-108 ) cm range is slightly lower than this value and is considered not high enough to be detector grade. The surface treatment with different chemical etchants [31] and the metal contacting process affects the defect states near the surface region, both extrinsic and intrinsic. These factors determine the surface leakage current which in turn significantly affects the performance of the device. We therefore believe that the high leakage current might be due to the surface defects introduced by mechanical and chemical treatment. Figs. 3 and 4 show the PL spectra of the CdTe samples measured at 11 K. The PL spectrum for
Fig. 2. I—» curves of different samples of CdTe grown by horizontal seeded PVT method.
CdTe-10, shown in Fig. 3, is a typical result for CdTe crystals grown in the uncoated ampoules. The spectrum has dominant bound exciton peaks that consist of a weak emission of the free exciton at 1.598 eV and a shallow donor peak (Do, X) at 1.593 eV. It has been reported [32] that the (Do, X) line mainly originates from Cl . However, the reT% sults of SSMS analysis in Table 1 indicates the Cl content is less than 0.01 ppm in the crystal. Indium is the other possible donor with an appreciated amount listed in Table 1. The origins of other two peaks at 1.557 and 1.549 eV with their LO phonon replicas are not very clear. The peaks in this region have been previously observed in As and Sb-doped CdTe crystals [33], and were assigned as peaks due to electron—acceptor (e—A°) recombination and a shallow donor—acceptor pair (DAP) recombination, respectively. In our case we think that the presence of an appreciable amount of In as an impurity in the CdTe leads to the occurrence of these peaks. No deep level donor—acceptor-pair (DAP) lines around 1.45 eV were observed. The PL spectrum of CdTe-22, which was grown in the BN coated ampoule, is shown in Fig. 3. The (Do, X) line is observed at 1.592 eV. It was a common feature to observe peaks near 1.584 eV in high-resistivity CdTe crystals doped with group III elements, such as 1.584 [34] and 1.585 [35] eV for Ga and similar lines for In and Al-doped CdTe [36]. The line at 1.584 eV in Fig. 3 is therefore assumed to be caused by the unintentional doping
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Fig. 3. Low-temperature (11 K) PL spectra showing a strong bound exciton (D°, X) peak for CdTe-10 sample.
Fig. 4. Low temperature (11 K) PL spectra showing the bound-acceptor peak due to boron doping for CdTe-22 sample.
of boron from the BN coating of the ampoule. There is a strong emission line at 1.545 eV followed by its phonon replica at 1.523 eV. We believe that this peak is due to a DAP recombination. The peak at 1.545 eV is due to an impurity or dopant, and in this case the diffused boron enhances the intensity of the emission. At lower energy the DAP emission at 1.455 eV and its phonon replicas (DAP-1LO,
DAP-2LO and DAP-3LO) were observed. Similar DAP emission bands were also observed in Ga[34], In- [36] and Al-doped [3] CdTe crystals. The intense emission of these lines is expected in the doped samples. Fig. 5 shows pictures of the CdTe-10 crystal using the near IR transmission microscope as a function of depth from the surface. The regions opaque
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in nature. No precipitates were observed in the CdTe-22 crystal. Results of the EPD study in Fig. 6a show that the CdTe-10 crystal consists of several grains with two different kinds of etch pits on both sides of the grain boundary. The pyramidal pits indicate a (1 1 1)A orientation whereas the rectangular pits indicate a (1 0 0) orientation. Fig. 6b shows the etch-pits on the CdTe-22 crystal. Only one kind of etch-pit lined up along the straight parallel lines. Twin boundaries were observed in this crystal grown by PVT on a 2 1 1 seed. This is not consistent with the reported results that single CdTe crystal without twins were successfully grown using homoepitaxial growth by sublimation on the 2 1 1 seed [37]. The crystal grown on the (1 1 1) seed in the uncoated quartz ampoule has a larger number of grain boundaries than the crystal grown on the (2 1 1) seed in the boron nitride coated ampoule. Similar results were reported by Shetty et al. [38], although in their case, the CdTe crystal were grown by the Bridgman method. Since the formation of the dislocation etch pits have been related to stress between the crystal and the ampoule, the lower etch-pit density in CdTe-22 clearly indicates that the boron nitride coating helps in reducing the stress between the crystal and the inner walls of the ampoule.
4. Conclusions
Fig. 5. Near IR transmission pictures of seeded CdTe-10 as a function of depth, with the microscope focused (a) near the surface, (b) just below the surface, and (c) deep inside the bulk.
to near IR are believed to be Te precipitates and the definite geometrical shape of the precipitates indicates that the precipitates may be crystalline
In this article, we have presented the results on the growth and characterization of CdTe crystals grown by horizontal seeded PVT with the source material near the congruent sublimation condition. The prevention of contamination from the fused silica ampoules was also investigated by growing the CdTe crystal in a boron nitride (BN) coated fused silica ampoule. The resistivities of the grown crystals were in the high-108 ) cm range. The PL spectra of the crystal grown in the BN coated ampoule showed similar features observed in the CdTe crystals doped with group III elements. Thus, the crystal was believed to be unintentionally doped with boron. On the other hand, BN coating of the growth ampoules yielded a single crystal with no grain boundaries nor precipitates.
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Fig. 6. The EPD pictures showing the etch pits and the grain boundaries of the two crystals (a) etch pits for CdTe-10 near the grain boundary, the long sides of the rectangular pits are parallel to the grain boundary when the neighboring pits are pyramidal. (b) small etch pits that are roughly triangular for CdTe-22, and there are no pits in the twinned region.
Acknowledgements This work was supported by National Aeronautics and Space Administration through the Fisk University center for Photonic Materials and Devices, Grant No. NAGW-2925 and by Microgravity Research Division, National Aeronautics and Space Administration.
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