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Growth and interface study of 2 in diameter CdZnTe by THM technique
Journal of Crystal Growth 312 (2010) 2840–2845
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Growth and interface study of 2 in diameter CdZnTe by THM technique U.N. Roy n, S. Weiler, J. Stein ICx Radiation Inc., 100 Midland Road, Oak Ridge, TN 37830, USA
a r t i c l e in f o
a b s t r a c t
Article history: Received 1 March 2010 Accepted 16 May 2010 Communicated by T.F. Kuech Available online 19 July 2010
Growth interface of large diameter CdZnTe ingots grown from Te solution by travelling heater method have been studied. Both macroscopic and microscopic investigations were carried out. The results indicated that the shape of the interface strongly governs the grain growth on the ingot, while the microscopic morphology of the growth interface is responsible for Te inclusions in the grown crystal. & 2010 Elsevier B.V. All rights reserved.
Keywords: A1. Growth interface A2. THM B2. CdZnTe
1. Introduction For many years CdZnTe (CZT) has been the material of interest for various applications. CZT has long been known as a good substrate material for HgCdTe due to its lattice matching for the IR detector and night vision applications [1–3]. However, CZT has attracted most of the attention from researchers and industries as a room temperature nuclear detector material [4–8]. Its excellent optoelectronic properties and appropriate band gap with high average atomic number ( 50) makes CZT the ideal semiconductor for nuclear detector applications operable at room temperature. All these applications demand good quality CZT crystals with good uniformity of composition, both along the diameter and length of the ingots, at lower cost. Extensive research efforts have been undertaken for more than last two decades on the growth of bulk CZT crystals, especially from melt, such as high pressure Bridgman [9,10] and low pressure Bridgman [11,12], in order to achieve large single crystals with good uniformity at lower cost. However, several detrimental intrinsic properties such as high vapor pressure of Cd at and above the melting point, low thermal conductivity and the segregation of Zn mostly entail a low yield of detector grade material, reduced reproducibility and eventually an increase of the overall production cost. Thus, growth of large grain good quality uniform CZT with reasonable yield, from melt growth technique still remains a challenge for commercial production. In the recent years travelling heater method (THM) technique has proven to be the most viable technique to grow large diameter CdTe (100 mm diameter) and CZT(75 mm diameter),
n
Corresponding author. E-mail address: [email protected] (U.N. Roy).
0022-0248/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2010.05.046
with good composition uniformity and better yield [13–15]. Due to the lower temperature growth process, much below than the melting point, THM has many advantages over melt growth technique. At lower growth temperature, problems related to vapor pressure are excluded. There is less contamination from the crucible, reduced risk of ampoule explosion and less defect density. As the technique involves dissolving the fresh feed material and depositing at the growth interface, the problem of Zn segregation is overruled. The THM technique thus ensures the macroscale composition uniformity of the grown ingot, which results in production at significantly lower cost. In addition to the above said advantages, the THM technique is also known to be a self purification technique as reported by Mokri et al. [16]. A recent finding of impurity gettering effect of Te inclusions [17] also confirms that the THM is indeed a self purification technique. The first 2 in diameter THM growth of CZT was reported by Mokri et al. [16], with fairly good longitudinal and radial uniformity of composition. Schoenholz et al. [18] reported lower etch pit density for THM grown CdTe compared to Bridgman grown CdTe. They also demonstrated that the defect density reduces drastically in the grown crystal than the seed. Thus the crystals with less thermal stress, less defects and higher purity with composition uniformity can be grown by THM technique. However, unlike melt growth techniques, the growth interface in THM is between CZT and Te-rich CZT alloy. A clean interface during growth is essential to achieve good quality CZT. The macroscopic shape of the interface significantly governs the crystal quality of the grown ingot, while the microscopic irregularities introduce Te inclusions due to trapping of Te-rich CZT alloy. It is vital to optimize the growth conditions in order to obtain better interface. The optimization of the growth parameter is essentially the optimization of the height of Te-rich CZT solvent zone, which in turn governs the shape of the resulting interface. The height of the solvent zone however depends on various parameters such as
U.N. Roy et al. / Journal of Crystal Growth 312 (2010) 2840–2845
amount of tellurium, growth temperature, growth rate, heat flow, temperature gradient near the interface, etc. In spite of the commercial success of the THM technique for CZT growth, not many studies are available in the literature on the macroscopic and microscopic investigation on the interface. In the recent past, Wang et al. [19,20] have studied microscopic interface of CZT (11 mm diameter ingot) by THM technique, and demonstrated that influence of external magnetic field drastically improves the growth interface. However, for commercial application on large diameter growth, applying an appropriate external magnetic field is technically rather challenging and expensive. In the present study, the growth interface between CZT and Terich CZT solvent was investigated for THM grown ingots. Both macroscopic and microscopic study was performed to understand the crystal quality on the shape of the interface and the mechanism of Te formation due to the microscopic irregularities of the interface of 2 in diameter ingots grown by THM technique.
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Fig. 1. THM grown CZT ingot of diameter 52 mm.
2. Experiments All the CZT crystals were grown from pre-synthesized 6 N purity Cd0.9Zn0.1Te as starting material from 5N Plus Inc. without any further purification. Excess Te used was 6 N purity from Alfa Aeser and 6 N purity In was used as dopant. All the growth experiments were un-seeded and were carried out in conically tipped quartz ampoules. The quartz ampoules were freshly cleaned with trichloroethylene, acetone and methanol, followed by mixture of nitric acid and HF, and finally rinsed repeatedly with 18 M O de-ionized water. Prior to loading the charge, the ampoules were coated with graphite layer by cracking high purity acetone. After graphite coating, the ampoules were baked at 950 1C for degassing. The required amount of Te, In and CZT were loaded and subsequently sealed under dynamic vacuum of 3 10 7 Torr. Crystals were grown by THM technique, where the Te-rich CZT solvent zone moves from the bottom of the ampoule towards the top during the process of the growth. Both 40 and 52 mm diameter ingots were grown by THM technique, and were weighing about 650 g and more than 1 kg, respectively. The crystals were grown in a three zone vertical furnace. The growth rate varied from 3 to 5 mm/day and the temperature gradient near the interface was between 10 and 15 1C/cm. Growth runs were actuated by lowering the ampoule through the furnace using multigeared DC motor. To investigate the interface shape, the ingots were slowly cooled after completion of the growth process. Some of the ingots were cooled rapidly, by switching the furnace off to cool down naturally to room temperature. For infra-red (IR) imaging, 52 mm diameter slabs with a thickness of 5–10 mm were cut perpendicular to the growth direction. To study the interface, wafers were cut along the growth direction near the interface. All the samples were lapped and polished to mirror finish using 0.05 mm alumina suspension for final polishing. The polished slabs were investigated by IR imaging of the full wafers, and with high magnification optical microscope (Zeiss Axio Imager M1m). For all the transmission mode imaging, an IR filter was used to filter out the visible light.
3. Results and discussion After complete cooling to room temperature, the grown ingots were taken out from the ampoule. The ingots could be easily removed from the ampoules without any sticking problem due to the protective graphite layer between the ingot and the inner wall of the ampoules. A thin Te layer was found on the surface of the ingots. Fig. 1 shows a typical CZT ingot grown by THM technique.
Fig. 2. Schematic diagram of growth interface: (a) convex, (b) concave and (c) flat.
The shiny silvery layer, covering the ingot, is the Te layer. A strip of the ingot was sand blasted as shown in Fig. 1, the Te-rich CZT layer with sharp interface is prominent near the top of the ingot. The THM grown ingots were of uniform compositional homogeneity with moderately good detector quality [21]. As is well known, optimization of the growth parameter is essentially optimization of the height of the solvent zone (Te+ CZT layer), and the height of the solvent zone is governed by many parameters such as growth temperature, temperature gradient near the solvent zone, growth rate, heat flow and of course the initial amount of Te. The height of the solvent zone ultimately governs the shape of the growth interface, and the crystalline quality (grain size) strongly depends on the shape of the growth interface. Fig. 2 shows the schematic diagram of different growth interface shapes. The shape of the interface is strongly governed by the heat flow through the solid portion of the ingot and the ampoule in addition to the convective flow of the liquid zone. As shown in Fig. 2a, the interface shape is convex which favors single crystalline growth. While, the concave interface (Fig. 2b) results in growth of polycrystalline ingot. However, the ideal growth interface is flat for the growth of single crystalline ingot as shown in Fig. 2c. It is thus evident that the control of growth interface shape plays an important role in the growth of large grain or single crystal. As is well known, the natural convection in the molten zone is strongly induced by the temperature and concentration gradients, and affects the shape of the growth interface. Reduced convection is more favorable for convex growth interface shape, hence the growth of large grain/single crystalline ingots. In THM technique, large temperature gradient is generally used in practice, which produces strong convection and adversely affects the crystalline quality [22]. In order to reduce the natural convection, in the present study the temperature gradient near the molten zone was kept moderately low between 10 and 15 1C/cm.
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In order to study the interface characteristics, a thin slice near the Te-rich part (top of the ingot) was cut along the growth direction. Fig. 3 shows a typical cross-sectional view of the interface and the corresponding wafer cut perpendicular to the growth direction just below the interface of a 52 mm diameter THM grown CZT ingot grown before optimization of the growth parameters. The white line on the wafer indicates the cut position of the cross-sectional interface. It is evident from Fig. 3 that the overall shape of the interface is convex, while near the arrow the shape is slightly concave. As discussed earlier that the concave interface is not favorable for single crystalline growth, formation of grain boundary is observed near the concave portion of the interface. Similar effect was also reported for THM grown CdTe (100 mm diameter ingot) by Shiraki et al. [15]. Although flat interface is the most favorable for single crystalline growth, in practice the interfaces are rarely flat because of low thermal conductivity of CZT. So, the growth conditions were optimized to obtain convex interface in order to achieve large grain or single crystalline CZT by THM technique. Fig. 4 shows the convex interface resulting in single crystalline growth of 40 mm diameter CZT by THM technique. The interface after optimization of the growth parameters for 52 mm diameter ingot is shown in Fig. 5. The corresponding slice near the interface cut perpendicular to the growth direction is also shown in the bottom part of Fig. 5. The interface, as shown in Fig. 5, is reasonably flat with slightly concave near the left side of the interface. It is thus evident that the yield of single grain increased dramatically compared to the ingot shown in Fig. 3. Some scattered cracks are visible on the
Fig. 4. Cross-section of the growth interface and the corresponding CZT wafer of diameter 40 mm, grown by THM technique.
Fig. 3. Cross-section of the growth interface and the corresponding CZT wafer of diameter 52 mm, grown by THM technique.
wafer; this is due to rapid cooling of the ingot. The crystal shown in Fig. 5 was cooled rapidly by switching off the furnace and allowed it to cool to room temperature naturally. It is well known that the Te inclusions/precipitations have detrimental effects on the device performance [23,24], microscopic investigations of the interface between Te-rich CZT (molten zone) and CZT were carried out to understand the formation of Te inclusions due to trapping of Te-rich CZT resulting from microscopic irregularities of the interface. Unlike melt grown CZT, mechanisms of formation of Te inclusions/precipitations are two folds. Firstly, due to retrograde solubility of Te and secondly, trapping of Te-rich CZT solution at the interface. Formation of Te inclusions/precipitations due to retrograde solubility is common for both melt growth and THM technique. Te-inclusion due to trapping of the Te-CZT solution is the added cause for THM technique. Since THM technique is a low temperature growth compared to melt growth technique, the concentration and size of Te inclusions/precipitations formed due to retrograde solubility of Te is supposed to be less than melt growth technique. Thus, by controlling the growth interface by optimization of the growth parameters, it is possible to reduce the concentration of Te inclusions substantially due to trapping of Te-rich CZT solution. Hence, the total concentration of Te inclusions/precipitations (formed due to retrograde solubility and trapping of Te-rich CZT solution) in THM grown ingots could be lower than melt grown CZT ingots.
U.N. Roy et al. / Journal of Crystal Growth 312 (2010) 2840–2845
Fig. 5. Cross-section of the growth interface and the corresponding CZT wafer of diameter 52 mm, grown by THM technique.
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irregular or non-uniform interface is clearly evident from Fig. 6b, as shown by the arrows. It is to be noted that the shape and size of the Te inclusions formed because of trapping of Te-CZT solution are random as shown in Fig. 6b. Another example of trapping of Te-rich CZT solution at the irregular interface is shown in Fig. 7. The non-uniform growth interface is obviously the result of convection of the molten solution. Wang et al. [20] showed similar irregular interface for THM grown CZT ingots after quenching during growth. It was also demonstrated that smooth wavelike interface could be achieved by applying magnetic field during growth [19,20]. The applied magnetic field during growth suppresses the convection in the molten zone resulting in a smooth interface [20]. It is worth mentioning here that, because the molten zone is saturated with CZT dissolved in Te during the growth and at the end of the ingot is slightly under saturated. Quenching of the ingots during the growth or even at the end of the growth, during cooling, leads to random deposition of CZT from the molten zone at the interface and in the zone, until the solidification of Te. The shape of the interface of the quenched ingot by THM might not be altered macroscopically, but undeniably the interface will be rough/non-uniform because of the random deposition of the CZT from the molten zone during quenching. Figs. 8 and 9 are the examples of random deposition of CZT near the interface and in the bulk of the Te-rich CZT zone of a quenched THM grown ingot. The random deposition of CZT in the Te-rich zone is also visible in the cross-section of the quenched interface as shown in Fig. 5. In our growth experiments, for microscopic interface study, the ampoules were lowered down at the same rate till the solidification temperature of Te, in order to avoid any random
Fig. 7. Morphology of the growth interface.
Fig. 6. (a) Polished cross-section of 52 mm diameter growth interface. (b) Magnified version of the highlighted portion of (a).
Fig. 6a shows the polished interface, and a macroscopic nonuniformity of the interface is visible in the region inside the marked rectangular box. Fig. 6b shows a magnified version of the corresponding region of the interface. The bright portion is the Terich CZT portion, while the black region is the grown CZT. Trapping of Te-rich CZT solution near the interface and hence formation mechanism of Te inclusions in the grown CZT due to
Fig. 8. Morphology of the growth interface of quenched ingot shows random deposition of CZT.
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Fig. 12. Smooth growth interface at high magnification.
Fig. 9. Morphology of the growth interface of quenched ingot.
Fig. 13. IR transmission image of 52 mm diameter, 10.5 mm thick CZT as-grown slab, grown by THM technique.
Fig. 10. Smooth growth interface after optimization of growth parameters.
after optimization of the parameters, it is possible to obtain a smooth growth interface without any applied external magnetic field during growth. This smooth interface also suggest that even at moderately low temperature gradient near the growth zone, the effect of constitutional supercooling is negligible. Fig. 13 shows a typical IR image of the full wafer of as-grown CZT by THM, of diameter 52 mm and thickness of about 10.5 mm. The Te inclusions due to trapping of the solution at the interface are considerably low as shown in Fig. 13.
4. Summary
Fig. 11. Smooth steplike growth interface.
deposition of CZT at the interface. Fig. 10 shows a smooth interface of the THM grown ingot after optimization of the growth parameters. A steplike growth interface is shown in Fig. 11. High magnification of the interface is shown in Fig. 12. It is evident that
In the present study, both macroscopic and microscopic morphology of the growth interface were investigated for large diameter CZT ingots grown by THM technique using Te as the solvent. It has been demonstrated that after optimization of the growth parameters, reasonably flat/convex interface could be obtained. In addition to that, microscopically smooth interface could be achieved by optimizing the parameters, which eventually reduces the Te inclusions in the grown ingots. It was observed that quenching is not a suitable technique to study the microscopic growth interface as random deposition of CZT in the Te-rich CZT solution and the interface results in a rough or nonuniform interface. Good quality, large grain CZT with low Te inclusions originating from the interface could be grown by THM technique.
U.N. Roy et al. / Journal of Crystal Growth 312 (2010) 2840–2845
References [1] M. Yoshikawa, K. Maruyama, T. Saito, T. Maekawa, H. Takigawa, J. Vac Sci. Technol. A 5 (1987) 3052. [2] J.M. Arias, J.G. Pasko, M. Zandian, J. Bajaj, L.J. Kozlowski, R.E. Dewames, W.E. Tennant, Proc. SPIE 2021 (1993) 114. [3] T. Aoki, Y. Chang, G. Badano, J. Zhao, C. Grein, S. Sivananthan, David J. Smith, J. Cryst. Growth 265 (2004) 224. [4] R.B. James, T.E. Schlesinger, J. Lund, M. Schieber, Semicond. Semimet. 43 (1995) 335. [5] A. Burger, M. Groza, Y. Cui, U.N. Roy, D. Hillman, M. Guo, L. Li, G.W. Right, R.B. James, Phys. Status Solidi C 2 (2005) 1586. [6] C. Szeles, S.A. Soldner, S. Vydrin, J Graves, D.S. Bale, IEEE Trans. Nucl. Sci. 55 (2008) 572. [7] J. Hong, B. Allen, J. Grindlay, N. Chammas, S. Barthelemy, R. Baker, N. Gehrels, K.E. Nelson, S. Labov, J. Collins, W.R. Cook, R. McLean, F. Harrison, Nucl. Instrum. Methods Phys. Res. A 605 (2009) 364. [8] Nam-Ho Lee Se-Hwan Park, S.M. Lee, Jang Ho Ha, Han Soo Jae-Hyung Lee, Kim, Yong Kyun Kim, Appl. Radiat. Isot. 67 (2009) 1471. [9] P. Fougeres, M. Hage-Ali, J.M. Koebel, P. Siffert, S. Hassan, A. Lusson, R. Triboulet, G. Marrakchi, A. Zerrai, K. Cherkaoui, R. Adhiri, G. Bremond, O. Kaitasov, M.O. Ruault, J. Crestou, J. Cryst. Growth 184 (1998) 1313. [10] C. Szeles, S.E. Cameron, S.A. Soldner, J.O. Ndap, M.D. Reed, J. Electron. Mater. 33 (2004) 742. [11] L. Li, F. Lu, C. Lee, H.W. Yao, A. Burger, M. Groza, G.W. Wright, R.B. James, R.W. Olsen, P.N. Luke, K.S. Shah, L.J. Cirignano, M.R. Squillante, D.R. Ouimette, Proc. SPIE 5198 (2004) 41.
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Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Growth and interface study of 2 in diameter CdZnTe by THM technique U.N. Roy n, S. Weiler, J. Stein ICx Radiation Inc., 100 Midland Road, Oak Ridge, TN 37830, USA
a r t i c l e in f o
a b s t r a c t
Article history: Received 1 March 2010 Accepted 16 May 2010 Communicated by T.F. Kuech Available online 19 July 2010
Growth interface of large diameter CdZnTe ingots grown from Te solution by travelling heater method have been studied. Both macroscopic and microscopic investigations were carried out. The results indicated that the shape of the interface strongly governs the grain growth on the ingot, while the microscopic morphology of the growth interface is responsible for Te inclusions in the grown crystal. & 2010 Elsevier B.V. All rights reserved.
Keywords: A1. Growth interface A2. THM B2. CdZnTe
1. Introduction For many years CdZnTe (CZT) has been the material of interest for various applications. CZT has long been known as a good substrate material for HgCdTe due to its lattice matching for the IR detector and night vision applications [1–3]. However, CZT has attracted most of the attention from researchers and industries as a room temperature nuclear detector material [4–8]. Its excellent optoelectronic properties and appropriate band gap with high average atomic number ( 50) makes CZT the ideal semiconductor for nuclear detector applications operable at room temperature. All these applications demand good quality CZT crystals with good uniformity of composition, both along the diameter and length of the ingots, at lower cost. Extensive research efforts have been undertaken for more than last two decades on the growth of bulk CZT crystals, especially from melt, such as high pressure Bridgman [9,10] and low pressure Bridgman [11,12], in order to achieve large single crystals with good uniformity at lower cost. However, several detrimental intrinsic properties such as high vapor pressure of Cd at and above the melting point, low thermal conductivity and the segregation of Zn mostly entail a low yield of detector grade material, reduced reproducibility and eventually an increase of the overall production cost. Thus, growth of large grain good quality uniform CZT with reasonable yield, from melt growth technique still remains a challenge for commercial production. In the recent years travelling heater method (THM) technique has proven to be the most viable technique to grow large diameter CdTe (100 mm diameter) and CZT(75 mm diameter),
n
Corresponding author. E-mail address: [email protected] (U.N. Roy).
0022-0248/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2010.05.046
with good composition uniformity and better yield [13–15]. Due to the lower temperature growth process, much below than the melting point, THM has many advantages over melt growth technique. At lower growth temperature, problems related to vapor pressure are excluded. There is less contamination from the crucible, reduced risk of ampoule explosion and less defect density. As the technique involves dissolving the fresh feed material and depositing at the growth interface, the problem of Zn segregation is overruled. The THM technique thus ensures the macroscale composition uniformity of the grown ingot, which results in production at significantly lower cost. In addition to the above said advantages, the THM technique is also known to be a self purification technique as reported by Mokri et al. [16]. A recent finding of impurity gettering effect of Te inclusions [17] also confirms that the THM is indeed a self purification technique. The first 2 in diameter THM growth of CZT was reported by Mokri et al. [16], with fairly good longitudinal and radial uniformity of composition. Schoenholz et al. [18] reported lower etch pit density for THM grown CdTe compared to Bridgman grown CdTe. They also demonstrated that the defect density reduces drastically in the grown crystal than the seed. Thus the crystals with less thermal stress, less defects and higher purity with composition uniformity can be grown by THM technique. However, unlike melt growth techniques, the growth interface in THM is between CZT and Te-rich CZT alloy. A clean interface during growth is essential to achieve good quality CZT. The macroscopic shape of the interface significantly governs the crystal quality of the grown ingot, while the microscopic irregularities introduce Te inclusions due to trapping of Te-rich CZT alloy. It is vital to optimize the growth conditions in order to obtain better interface. The optimization of the growth parameter is essentially the optimization of the height of Te-rich CZT solvent zone, which in turn governs the shape of the resulting interface. The height of the solvent zone however depends on various parameters such as
U.N. Roy et al. / Journal of Crystal Growth 312 (2010) 2840–2845
amount of tellurium, growth temperature, growth rate, heat flow, temperature gradient near the interface, etc. In spite of the commercial success of the THM technique for CZT growth, not many studies are available in the literature on the macroscopic and microscopic investigation on the interface. In the recent past, Wang et al. [19,20] have studied microscopic interface of CZT (11 mm diameter ingot) by THM technique, and demonstrated that influence of external magnetic field drastically improves the growth interface. However, for commercial application on large diameter growth, applying an appropriate external magnetic field is technically rather challenging and expensive. In the present study, the growth interface between CZT and Terich CZT solvent was investigated for THM grown ingots. Both macroscopic and microscopic study was performed to understand the crystal quality on the shape of the interface and the mechanism of Te formation due to the microscopic irregularities of the interface of 2 in diameter ingots grown by THM technique.
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Fig. 1. THM grown CZT ingot of diameter 52 mm.
2. Experiments All the CZT crystals were grown from pre-synthesized 6 N purity Cd0.9Zn0.1Te as starting material from 5N Plus Inc. without any further purification. Excess Te used was 6 N purity from Alfa Aeser and 6 N purity In was used as dopant. All the growth experiments were un-seeded and were carried out in conically tipped quartz ampoules. The quartz ampoules were freshly cleaned with trichloroethylene, acetone and methanol, followed by mixture of nitric acid and HF, and finally rinsed repeatedly with 18 M O de-ionized water. Prior to loading the charge, the ampoules were coated with graphite layer by cracking high purity acetone. After graphite coating, the ampoules were baked at 950 1C for degassing. The required amount of Te, In and CZT were loaded and subsequently sealed under dynamic vacuum of 3 10 7 Torr. Crystals were grown by THM technique, where the Te-rich CZT solvent zone moves from the bottom of the ampoule towards the top during the process of the growth. Both 40 and 52 mm diameter ingots were grown by THM technique, and were weighing about 650 g and more than 1 kg, respectively. The crystals were grown in a three zone vertical furnace. The growth rate varied from 3 to 5 mm/day and the temperature gradient near the interface was between 10 and 15 1C/cm. Growth runs were actuated by lowering the ampoule through the furnace using multigeared DC motor. To investigate the interface shape, the ingots were slowly cooled after completion of the growth process. Some of the ingots were cooled rapidly, by switching the furnace off to cool down naturally to room temperature. For infra-red (IR) imaging, 52 mm diameter slabs with a thickness of 5–10 mm were cut perpendicular to the growth direction. To study the interface, wafers were cut along the growth direction near the interface. All the samples were lapped and polished to mirror finish using 0.05 mm alumina suspension for final polishing. The polished slabs were investigated by IR imaging of the full wafers, and with high magnification optical microscope (Zeiss Axio Imager M1m). For all the transmission mode imaging, an IR filter was used to filter out the visible light.
3. Results and discussion After complete cooling to room temperature, the grown ingots were taken out from the ampoule. The ingots could be easily removed from the ampoules without any sticking problem due to the protective graphite layer between the ingot and the inner wall of the ampoules. A thin Te layer was found on the surface of the ingots. Fig. 1 shows a typical CZT ingot grown by THM technique.
Fig. 2. Schematic diagram of growth interface: (a) convex, (b) concave and (c) flat.
The shiny silvery layer, covering the ingot, is the Te layer. A strip of the ingot was sand blasted as shown in Fig. 1, the Te-rich CZT layer with sharp interface is prominent near the top of the ingot. The THM grown ingots were of uniform compositional homogeneity with moderately good detector quality [21]. As is well known, optimization of the growth parameter is essentially optimization of the height of the solvent zone (Te+ CZT layer), and the height of the solvent zone is governed by many parameters such as growth temperature, temperature gradient near the solvent zone, growth rate, heat flow and of course the initial amount of Te. The height of the solvent zone ultimately governs the shape of the growth interface, and the crystalline quality (grain size) strongly depends on the shape of the growth interface. Fig. 2 shows the schematic diagram of different growth interface shapes. The shape of the interface is strongly governed by the heat flow through the solid portion of the ingot and the ampoule in addition to the convective flow of the liquid zone. As shown in Fig. 2a, the interface shape is convex which favors single crystalline growth. While, the concave interface (Fig. 2b) results in growth of polycrystalline ingot. However, the ideal growth interface is flat for the growth of single crystalline ingot as shown in Fig. 2c. It is thus evident that the control of growth interface shape plays an important role in the growth of large grain or single crystal. As is well known, the natural convection in the molten zone is strongly induced by the temperature and concentration gradients, and affects the shape of the growth interface. Reduced convection is more favorable for convex growth interface shape, hence the growth of large grain/single crystalline ingots. In THM technique, large temperature gradient is generally used in practice, which produces strong convection and adversely affects the crystalline quality [22]. In order to reduce the natural convection, in the present study the temperature gradient near the molten zone was kept moderately low between 10 and 15 1C/cm.
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In order to study the interface characteristics, a thin slice near the Te-rich part (top of the ingot) was cut along the growth direction. Fig. 3 shows a typical cross-sectional view of the interface and the corresponding wafer cut perpendicular to the growth direction just below the interface of a 52 mm diameter THM grown CZT ingot grown before optimization of the growth parameters. The white line on the wafer indicates the cut position of the cross-sectional interface. It is evident from Fig. 3 that the overall shape of the interface is convex, while near the arrow the shape is slightly concave. As discussed earlier that the concave interface is not favorable for single crystalline growth, formation of grain boundary is observed near the concave portion of the interface. Similar effect was also reported for THM grown CdTe (100 mm diameter ingot) by Shiraki et al. [15]. Although flat interface is the most favorable for single crystalline growth, in practice the interfaces are rarely flat because of low thermal conductivity of CZT. So, the growth conditions were optimized to obtain convex interface in order to achieve large grain or single crystalline CZT by THM technique. Fig. 4 shows the convex interface resulting in single crystalline growth of 40 mm diameter CZT by THM technique. The interface after optimization of the growth parameters for 52 mm diameter ingot is shown in Fig. 5. The corresponding slice near the interface cut perpendicular to the growth direction is also shown in the bottom part of Fig. 5. The interface, as shown in Fig. 5, is reasonably flat with slightly concave near the left side of the interface. It is thus evident that the yield of single grain increased dramatically compared to the ingot shown in Fig. 3. Some scattered cracks are visible on the
Fig. 4. Cross-section of the growth interface and the corresponding CZT wafer of diameter 40 mm, grown by THM technique.
Fig. 3. Cross-section of the growth interface and the corresponding CZT wafer of diameter 52 mm, grown by THM technique.
wafer; this is due to rapid cooling of the ingot. The crystal shown in Fig. 5 was cooled rapidly by switching off the furnace and allowed it to cool to room temperature naturally. It is well known that the Te inclusions/precipitations have detrimental effects on the device performance [23,24], microscopic investigations of the interface between Te-rich CZT (molten zone) and CZT were carried out to understand the formation of Te inclusions due to trapping of Te-rich CZT resulting from microscopic irregularities of the interface. Unlike melt grown CZT, mechanisms of formation of Te inclusions/precipitations are two folds. Firstly, due to retrograde solubility of Te and secondly, trapping of Te-rich CZT solution at the interface. Formation of Te inclusions/precipitations due to retrograde solubility is common for both melt growth and THM technique. Te-inclusion due to trapping of the Te-CZT solution is the added cause for THM technique. Since THM technique is a low temperature growth compared to melt growth technique, the concentration and size of Te inclusions/precipitations formed due to retrograde solubility of Te is supposed to be less than melt growth technique. Thus, by controlling the growth interface by optimization of the growth parameters, it is possible to reduce the concentration of Te inclusions substantially due to trapping of Te-rich CZT solution. Hence, the total concentration of Te inclusions/precipitations (formed due to retrograde solubility and trapping of Te-rich CZT solution) in THM grown ingots could be lower than melt grown CZT ingots.
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Fig. 5. Cross-section of the growth interface and the corresponding CZT wafer of diameter 52 mm, grown by THM technique.
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irregular or non-uniform interface is clearly evident from Fig. 6b, as shown by the arrows. It is to be noted that the shape and size of the Te inclusions formed because of trapping of Te-CZT solution are random as shown in Fig. 6b. Another example of trapping of Te-rich CZT solution at the irregular interface is shown in Fig. 7. The non-uniform growth interface is obviously the result of convection of the molten solution. Wang et al. [20] showed similar irregular interface for THM grown CZT ingots after quenching during growth. It was also demonstrated that smooth wavelike interface could be achieved by applying magnetic field during growth [19,20]. The applied magnetic field during growth suppresses the convection in the molten zone resulting in a smooth interface [20]. It is worth mentioning here that, because the molten zone is saturated with CZT dissolved in Te during the growth and at the end of the ingot is slightly under saturated. Quenching of the ingots during the growth or even at the end of the growth, during cooling, leads to random deposition of CZT from the molten zone at the interface and in the zone, until the solidification of Te. The shape of the interface of the quenched ingot by THM might not be altered macroscopically, but undeniably the interface will be rough/non-uniform because of the random deposition of the CZT from the molten zone during quenching. Figs. 8 and 9 are the examples of random deposition of CZT near the interface and in the bulk of the Te-rich CZT zone of a quenched THM grown ingot. The random deposition of CZT in the Te-rich zone is also visible in the cross-section of the quenched interface as shown in Fig. 5. In our growth experiments, for microscopic interface study, the ampoules were lowered down at the same rate till the solidification temperature of Te, in order to avoid any random
Fig. 7. Morphology of the growth interface.
Fig. 6. (a) Polished cross-section of 52 mm diameter growth interface. (b) Magnified version of the highlighted portion of (a).
Fig. 6a shows the polished interface, and a macroscopic nonuniformity of the interface is visible in the region inside the marked rectangular box. Fig. 6b shows a magnified version of the corresponding region of the interface. The bright portion is the Terich CZT portion, while the black region is the grown CZT. Trapping of Te-rich CZT solution near the interface and hence formation mechanism of Te inclusions in the grown CZT due to
Fig. 8. Morphology of the growth interface of quenched ingot shows random deposition of CZT.
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Fig. 12. Smooth growth interface at high magnification.
Fig. 9. Morphology of the growth interface of quenched ingot.
Fig. 13. IR transmission image of 52 mm diameter, 10.5 mm thick CZT as-grown slab, grown by THM technique.
Fig. 10. Smooth growth interface after optimization of growth parameters.
after optimization of the parameters, it is possible to obtain a smooth growth interface without any applied external magnetic field during growth. This smooth interface also suggest that even at moderately low temperature gradient near the growth zone, the effect of constitutional supercooling is negligible. Fig. 13 shows a typical IR image of the full wafer of as-grown CZT by THM, of diameter 52 mm and thickness of about 10.5 mm. The Te inclusions due to trapping of the solution at the interface are considerably low as shown in Fig. 13.
4. Summary
Fig. 11. Smooth steplike growth interface.
deposition of CZT at the interface. Fig. 10 shows a smooth interface of the THM grown ingot after optimization of the growth parameters. A steplike growth interface is shown in Fig. 11. High magnification of the interface is shown in Fig. 12. It is evident that
In the present study, both macroscopic and microscopic morphology of the growth interface were investigated for large diameter CZT ingots grown by THM technique using Te as the solvent. It has been demonstrated that after optimization of the growth parameters, reasonably flat/convex interface could be obtained. In addition to that, microscopically smooth interface could be achieved by optimizing the parameters, which eventually reduces the Te inclusions in the grown ingots. It was observed that quenching is not a suitable technique to study the microscopic growth interface as random deposition of CZT in the Te-rich CZT solution and the interface results in a rough or nonuniform interface. Good quality, large grain CZT with low Te inclusions originating from the interface could be grown by THM technique.
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