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Purification and optical properties of TlBr crystals
ARTICLE IN PRESS Nuclear Instruments and Methods in Physics Research A 602 (2009) 484–488
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Purification and optical properties of TlBr crystals Shijin Yu, Dongxiang Zhou , Shuping Gong, Zhiping Zheng, Yunxiang Hu, Chuan Wang, Lin Quan Department of Electronic Science and Technology, Engineering Research Center for Functional Ceramics of the Ministry of Education, Huazhong University of Science and Technology, Wuhan 430074, China
a r t i c l e in f o
a b s t r a c t
Article history: Received 17 October 2008 Received in revised form 27 December 2008 Accepted 5 January 2009 Available online 21 January 2009
TlBr was synthesized by chemical precipitation, purified by the hydrothermal re-crystallization method, and grown by the vertical Bridgman method. Trace element analysis at ppb/ppm level was made using inductively coupled plasma mass spectroscopy. A significant reduction of the impurity concentration was observed as a function of the cooling rate. The concentrations of impurities of Ca, Fe, Mg, K, Zn, Cu, Na, and Si decreased obviously after hydrothermal re-crystallization. In addition, the optical properties of TlBr wafers were compared using spectra in the wavelength ranges of 400–600 and 2500–20 000 nm. Compared to the wafer grown from the re-crystallized powder, the wafer grown from the non-purified powder exhibits a large absorption coefficient and small transmission in the range of 450–600 nm, a shift of the absorption edge to a longer wavelength region, and a decrease of the average IR transmittance value. & 2009 Elsevier B.V. All rights reserved.
Keywords: TlBr Crystals Purification Absorption Transmittance Trace element analysis
1. Introduction Over the past six decades, considerable efforts have been expended in developing TlBr crystals as X- and gamma-ray detectors [1–5]. This is due to several interesting properties of TlBr material, such as high photon stopping power and room temperature operation. In spite of the excellent promise shown by TlBr crystals, the material quality has not been optimized. In particular, purity of TlBr crystal is a crucial factor in its properties [6,7]. Therefore, impurity concentration in TlBr crystals must be decreased. The first study of TlBr as a radiation detector material was preformed by Rahman and Hofstadter [8], but it was not very successful due to impurity contamination of starting materials. Recently, several studies have been carried out on the preparation of TlBr semiconductor detectors, and progress has been achieved by improvements in techniques of purification and characterization of the crystals [9,10]. The problem of TlBr crystal purity is related to raw material purity, purification, and crystal growth techniques, and further treatments. Every stage affects crystal purity, but the former two factors are crucial. Synthesis from elements [11] and chemical reaction of precipitation [12] are the primary methods for preparing TlBr powder. The process of direct synthesis from elements is complicated because of the strong oxidation by Br2 resulting in TlBr3 formation. In addition, toxicity of Tl materials
makes their technological process difficult. Therefore, commercial TlBr is synthesized by the wet chemical method. TlBr melts congruently at 460 1C and does not exhibit destructive phase transition between the solidification point and room temperature. Hence purification of TlBr can be performed from the melt. Vacuum distillation (VD) and zone refining (ZR) have been carried out for TlBr purification, and high-purity TlBr powder is obtained [12–14]. Kozlov et al. [15] demonstrated that the hydrothermal recrystallization method was effective for removal of relevant element impurities from TlBr powder. In this work, TlBr was synthesized by chemical precipitation from pre-purified TlNO3 and HBr and purified by the hydrothermal re-crystallization method. The number of runs of hydrothermal re-crystallization and cooling rate of solution were expected to be important factors for the purification efficiency. The impurity concentration was analyzed by trace element analysis using inductively coupled plasma mass spectrometry (ICP-MS). TlBr crystals were grown by the vertical Bridgman method (VBM). Measurements of absorption and transmission spectra were performed to clarify the effectiveness of hydrothermal re-crystallization purification.
2. Experimental 2.1. Sample preparation
Corresponding author. Tel.: +86 27 87547447.
E-mail addresses: [email protected] (S. Yu), [email protected] (D. Zhou). 0168-9002/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2009.01.022
TlBr powder was synthesized by chemical precipitation using TlNO3 and hydrobromic acid. Raw TlNO3 (commercial grade, Linda
ARTICLE IN PRESS S. Yu et al. / Nuclear Instruments and Methods in Physics Research A 602 (2009) 484–488
Chemistry Co. Ltd., China) powder was dissolved in de-ionized water at 80 1C, and TlNO3 powder was precipitated when the solution was cooled down from 80 1C to room temperature. These operations of dissolution and precipitation were repeated three times. Hydrobromic acid (analytical grade, Signophrm Chemical Reagent Co. Ltd., China) was purified by the decompression distillation method. Synthesis of TlBr was carried out by the following reaction: TlNO3+HBr ¼ TlBr+HNO3. Reactants TlNO3 and HBr were mixed at the stoichiometric ratio of 1:1. The yellow TlBr precipitate was isolated and rinsed by de-ionized water repeatedly until the pH value became neutral. The hydrothermal re-crystallization purification of TlBr powder was carried out as follows: 100 g of TlBr and 1200 ml of deionized water were loaded in an autoclave with a polytetrafluoroethylene (PTFE) cup. The autoclave was put into a two-zone vertical furnace so that it could hold a steady temperature gradient (see Fig. 1). Then the furnace was heated to 170 1C and held for 15 h to prepare a saturated TlBr solution. Then the top of the furnace was cooled at different cooling rates between 0.75 and 6 1C/h, and the bottom of the furnace was cooled at different cooling rates between 0.5 and 4 1C/h. Due to the temperature gradient inside the PTFE cup, mass transfer of TlBr took place through the solution and TlBr dendrites were crystallized at the top of the cup. The parameters of the re-crystallization processes are shown in Table 1. TlBr ingots grown by VBM were cut into wafers along the radial direction using a wire saw. The wafers grown from powder T000
485
Table 1 Particular parameters in the hydrothermal re-crystallization process. Symbol
T000 T100 T111 T123 T146 T223 T523
Number of re-crystallization runs
0 1 1 1 1 2 5
Cooling rate (1C/h) Bottom
Top
0 0.5 1 2 4 2 2
0 0.75 1.5 3 6 3 3
T000 is as-synthesized and non-purified by hydrothermal re-crystallization.
and grown from powder T123 were designated T000 wafers and T123 wafers, respectively. The surface of wafers was treated mechanically and chemically in order to remove surface damage layers. For mechanical polishing, alumina abrasives were used. Chemical etching was then performed using 5% bromine– methanol solution. The etched wafers were rinsed by methanol. Final wafers were transparent and had good surface quality. 2.2. Characterization Phase identification was performed by X-ray diffraction (XRD; X’Pert PRO, PANalytical B.V., Netherlands). The purification efficiency was evaluated by measuring concentration of impurities using ICP-MS (Elan DRC-e, Perkin-Elmer, USA). In order to prevent interference from Tl ion in the measurement of impurity concentration, isopropanol was used as an extractant of TlBr. Optical properties were studied by Bruker Vertex70 FT-IR (wavelength range 2500–20 000 nm) and Perkin-Elmer Lambda35 ultraviolet spectrophotometer (wavelength range 190–900 nm) at room temperature.
3. Results and discussion 3.1. Synthesis and purification
Fig. 1. Schematic drawing of hydrothermal re-crystallization system: 1—autoclave, 2—PTFE cup, 3—furnaces, 4—thermocouples, 5—purified TlBr powder, and 6—raw TlBr powder.
The powder synthesized by chemical precipitation was characterized by XRD (Fig. 2). There were no other crystalline phases in the synthetic powder, and all detected peaks corresponded to the TlBr phase (JCPDS No. 78-0631). In the synthetic process, the pH value of the solution plays an important role. Weak acidic conditions were the best for TlBr synthesis. Under these conditions, TlBr hydrolysis and formation of Tl, Tl2O, Tl2O3, and TlBrO3 were not observed. Table 2 shows the concentration of impurities in powder T000. The sum of impurity concentrations in powder T000 was lower than that in commercial powder [9]. The hydrothermal re-crystallization purification of TlBr is based on the positive temperature coefficient of TlBr solubility [16]. Some impurities can be directly separated, if they cannot be dissolved, and other impurities can be rejected from TlBr re-crystallized material during re-crystallization process. The elements Ca, Mg, Fe, K, Zn, Cu, Na, Si, Al, and Ti in Table 2 were selected for evaluation of the re-crystallization process. Fig. 3 shows the impurity concentration of TlBr powder before and after re-crystallization. There was a strong reduction in the concentration of impurities and the sum of impurity concentrations was about 10 ppm in powder T123. The purification effect was evaluated by the ratio of impurity concentration in powder T000 to that in powder T123. The ratio values of Ca, Fe, Mg, K, Zn, Cu, Na, and Si were 67.26, 28.05, 26.76, 18.78, 11.08, 8.69, 5.86, and
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5.40, respectively. In contrast with the result of Kozlov et al. [15], high purification efficiencies of Ca, Na, and Si were achieved, which may be attributed to the small temperature gradient inside the cup and slow cooling rate of the furnace. The cooling rate is an important parameter for the recrystallization process. As indicated by powder T146 and by powder T123 in Fig. 4a, the impurity concentration evidently decreased with decreasing cooling rate. When the cooling rate is slow enough, the re-crystallization process is near an equilibrium. Hence the force driving out the impurities from the TlBr host lattice is stronger and the decrease in impurity concentration is more obvious. However, further improvement of the purification was not observed when the cooling rate was slower than the rate of the T123 process (Fig. 4b). The number of hydrothermal re-crystallization runs is expected to be an important figure of merit for achieving the maximum reduction in impurity concentration. Hence, the recrystallization purification process was repeated five times. In Fig. 5 the decease of the impurity concentration as a function of the run number of T123 re-crystallization is shown. There was a strong reduction in impurity concentrations after the first recrystallization process. After further runs, impurity concentrations decreased only slightly. 3.2. Optical characteristics The optical characteristics in the wavelength range of 400–600 nm are shown in Fig. 6. The fundamental absorption edge of T123 wafer at 438 nm corresponds to the data presented in Refs. [11,17]. The T123 wafer has smaller absorption (Fig. 6a) and larger transmission (Fig. 6b) in the range of 450–600 nm than the T000 wafer. Considering the same crystal growth and further treatment techniques, the effect of impurities is the
possible reason why T000 wafer has large absorption and small transmission. Impurities are donors or acceptors in TlBr crystals, and form impurity energy levels. For example, an electron at the donor level gains energy by absorbing a photon and is excited to a higher energy level within the conduction band. Therefore, additional absorption for T000 wafer at wavelengths larger than 450 nm takes place. Similar absorption was observed by Shorohov et al. [18]. Also in Fig. 6, the absorption edge at 438 nm for the T123 wafer is shifted to 445 nm for the T000 wafer. IR transmission spectrum is a powerful tool to evaluate the quality of TlBr crystals. In Fig. 7, the IR transmittance of T000 wafer is below that of T123 wafer. For T000 wafer, the average IR transmittance value was 43%, and 58% for T123 wafer. In the wavelength range of 2500–20 000 nm, the IR transmission is determined by free carrier absorption in crystals [19,20]. Differences in valence states and ionic radii of impurity ions and Tl1+ lead to distortions of the crystal lattices and electronic system of TlBr [17]. Impurity ions entering the TlBr host lattices result in the formation of lone electrons and holes. Absorbing optical energy, the lone electrons and holes can easily escape the bondages of impurity atoms and become free carriers. Therefore, the concentration of free carriers increased with increasing impurity concentration. The absorption coefficient is proportional to the square of free carrier concentration [20]. As a result, the large impurity concentration resulted in the rise of absorption and the fall of IR transmittance in T000 wafer.
4. Conclusions The trace element analyses showed that the purification efficiency of hydrothermal re-crystallization was dominated by
Fig. 3. Impurity concentration of TlBr powder before and after T123 re-crystallization.
Fig. 2. XRD pattern of powder T000.
Table 2 Concentration of impurities (ppm) in powder T000. Element Concentration
Ca 66.721
Na 16.401
Mg 15.363
Fe 14.585
Cu 2.102
Zn 2.549
K 2.404
Si 19.409
Al 0.927
Ti 0.376
Element Concentration
Pd 0.173
Ba 0.156
Li 0.029
Mn 0.116
V 0.202
Ni 0.145
Cd 0.022
Pt 0
Pb 0
As 0
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Fig. 4. (a) Concentration of impurities in powders T123 and T146. (b) Impurity concentration as a function of cooling rate in the re-crystallization process. Impurities: ’—Ca, m—Fe, n—Cu, J—Na, and b—Si. Fig. 6. Spectra of the TlBr wafers in the range 400–600 nm: (a) absorption spectra and (b) transmission spectra; m—T123 wafer, n—T000 wafer.
Fig. 5. Impurity concentration as a function of the run number of re-crystallization. Impurities: ’—Ca, m—Fe, n—Cu, J—Na, and b—Si.
Fig. 7. IR transmission spectra of TlBr wafers. The solid line is from T123 wafer, and the dashed line is from T000 wafer.
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the cooling rate of solution. The impurity concentration was decreased evidently with reduced cooling rate, as was demonstrated by the processes T146 and T123. Hydrothermal re-crystallization was effective for reducing the concentration of impurities in the TlBr material. After T123 hydrothermal re-crystallization, impurities of Ca, Fe, Mg, K, Zn, Cu, Na, and Si were obviously decreased and the sum of impurity concentrations was about 10 ppm. But the decrease in impurity concentration became weaker in the following runs after the first hydrothermal re-crystallization. Optical characteristics were strongly dependent on the purity of TlBr crystals. Compared to the wafer grown from powder T123, the wafer grown from powder T000 exhibited a large absorption coefficient and small transmission in the range 450–600 nm, a shift of the absorption edge from 438 to 445 nm, and a decrease of the average IR transmittance value from 58% to 43%.
Acknowledgement The financial supports of the National Natural Science Foundation of China under Grant nos. 60676050 and 10875046 are acknowledged. References [1] R. Hofstadter, Phys. Rev 72 (1947) 1120. [2] F. Olschner, K.S. Shah, J.C. Lund, J. Zhang, K. Daley, S. Medrick, M.R. Squillante, Nucl. Instr. and Meth. A 322 (1992) 504.
[3] A. Owens, M. Bavdaz, G. Brammertz, V. Gostilo, H. Graafsma, A. Kozorezov, et al., Nucl. Instr. and Meth. A 497 (2003) 370. [4] V. Kozlov, M. Leskela, M. Kemell, H. Sipila, Nucl. Instr. and Meth. A 563 (2006) 58. [5] K. Hitomi, T. Onodera, T. Shoji, Y. Hiratate, Z. He, IEEE Trans. Nucl. Sci. NS-55 (2008) 1781. [6] C.A. Klein, J. Appl. Phys. 39 (1968) 2029. [7] I.B. Oliverira, J.F.D. Chubaci, M.J.A. Armelin, M.M. Hamada, Cryst. Res. Technol. 39 (2004) 849. [8] I.U. Rahman, R. Hofstadter, Phys. Rev. B 29 (1984) 3500. [9] I.B. Oliveira, F.E. Costa, J.F.D. Chubaci, M.M. Hamada, IEEE Trans. Nucl. Sci. NS-51 (2004) 1224. [10] K. Hitomi, T. Onodera, T. Shoji, Nucl. Instr. and Meth. A 579 (2007) 153. [11] I.S. Lisitsky, M.S. Kuznetsov, Y.A. Sultanova, Nucl. Instr. and Meth. A 591 (2008) 213. [12] M.S. Kouznetsov, I.S. Lisitsky, S.I. Zatoloka, V.V. Gostilo, Nucl. Instr. and Meth. A 531 (2004) 174. [13] K. Hitomi, M. Matsumoto, O. Muroi, T. Shoji, Y. Hiratate, IEEE Trans. Nucl. Sci. NS-49 (2002) 2526. [14] K.S. Shah, J.C. Lund, F. Olschner, L. Moy, M.R. Squillante, IEEE Trans. Nucl. Sci. NS-36 (1989) 199. [15] V. Kozlov, M. Leskela, T. Prohasks, G. Schultheis, G. Stingeder, H. Sipila, Nucl. Instr. and Meth. A 531 (2004) 165. [16] A. Benrath, F. Gjedebo, B. Schiffers, H. Wunderlich, Z. Anorg. Allg. Chem. 231 (1937) 285. [17] Y. Dmitriev, P.R. Bennet, L.J. Cirignano, T.K. Gupta, W.M. Higgins, K.S. Shah, P. Wong, Nucl. Instr. and Meth. A 578 (2007) 510. [18] M. Shorohov, L. Grigorjeva, D. Millers, Nucl. Instr. and Meth. A 563 (2006) 78. [19] U. Becker, P. Rudolph, R. Boyn, M. Wienecke, I. Utke, Phys. Stat. Sol. (a) 120 (1990) 653. [20] X. Zong, Y. Wong, The Foundations of Material Physics, in: Fudan University Press, Shanghai, 2001, p. 706 (in Chinese).
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Purification and optical properties of TlBr crystals Shijin Yu, Dongxiang Zhou , Shuping Gong, Zhiping Zheng, Yunxiang Hu, Chuan Wang, Lin Quan Department of Electronic Science and Technology, Engineering Research Center for Functional Ceramics of the Ministry of Education, Huazhong University of Science and Technology, Wuhan 430074, China
a r t i c l e in f o
a b s t r a c t
Article history: Received 17 October 2008 Received in revised form 27 December 2008 Accepted 5 January 2009 Available online 21 January 2009
TlBr was synthesized by chemical precipitation, purified by the hydrothermal re-crystallization method, and grown by the vertical Bridgman method. Trace element analysis at ppb/ppm level was made using inductively coupled plasma mass spectroscopy. A significant reduction of the impurity concentration was observed as a function of the cooling rate. The concentrations of impurities of Ca, Fe, Mg, K, Zn, Cu, Na, and Si decreased obviously after hydrothermal re-crystallization. In addition, the optical properties of TlBr wafers were compared using spectra in the wavelength ranges of 400–600 and 2500–20 000 nm. Compared to the wafer grown from the re-crystallized powder, the wafer grown from the non-purified powder exhibits a large absorption coefficient and small transmission in the range of 450–600 nm, a shift of the absorption edge to a longer wavelength region, and a decrease of the average IR transmittance value. & 2009 Elsevier B.V. All rights reserved.
Keywords: TlBr Crystals Purification Absorption Transmittance Trace element analysis
1. Introduction Over the past six decades, considerable efforts have been expended in developing TlBr crystals as X- and gamma-ray detectors [1–5]. This is due to several interesting properties of TlBr material, such as high photon stopping power and room temperature operation. In spite of the excellent promise shown by TlBr crystals, the material quality has not been optimized. In particular, purity of TlBr crystal is a crucial factor in its properties [6,7]. Therefore, impurity concentration in TlBr crystals must be decreased. The first study of TlBr as a radiation detector material was preformed by Rahman and Hofstadter [8], but it was not very successful due to impurity contamination of starting materials. Recently, several studies have been carried out on the preparation of TlBr semiconductor detectors, and progress has been achieved by improvements in techniques of purification and characterization of the crystals [9,10]. The problem of TlBr crystal purity is related to raw material purity, purification, and crystal growth techniques, and further treatments. Every stage affects crystal purity, but the former two factors are crucial. Synthesis from elements [11] and chemical reaction of precipitation [12] are the primary methods for preparing TlBr powder. The process of direct synthesis from elements is complicated because of the strong oxidation by Br2 resulting in TlBr3 formation. In addition, toxicity of Tl materials
makes their technological process difficult. Therefore, commercial TlBr is synthesized by the wet chemical method. TlBr melts congruently at 460 1C and does not exhibit destructive phase transition between the solidification point and room temperature. Hence purification of TlBr can be performed from the melt. Vacuum distillation (VD) and zone refining (ZR) have been carried out for TlBr purification, and high-purity TlBr powder is obtained [12–14]. Kozlov et al. [15] demonstrated that the hydrothermal recrystallization method was effective for removal of relevant element impurities from TlBr powder. In this work, TlBr was synthesized by chemical precipitation from pre-purified TlNO3 and HBr and purified by the hydrothermal re-crystallization method. The number of runs of hydrothermal re-crystallization and cooling rate of solution were expected to be important factors for the purification efficiency. The impurity concentration was analyzed by trace element analysis using inductively coupled plasma mass spectrometry (ICP-MS). TlBr crystals were grown by the vertical Bridgman method (VBM). Measurements of absorption and transmission spectra were performed to clarify the effectiveness of hydrothermal re-crystallization purification.
2. Experimental 2.1. Sample preparation
Corresponding author. Tel.: +86 27 87547447.
E-mail addresses: [email protected] (S. Yu), [email protected] (D. Zhou). 0168-9002/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2009.01.022
TlBr powder was synthesized by chemical precipitation using TlNO3 and hydrobromic acid. Raw TlNO3 (commercial grade, Linda
ARTICLE IN PRESS S. Yu et al. / Nuclear Instruments and Methods in Physics Research A 602 (2009) 484–488
Chemistry Co. Ltd., China) powder was dissolved in de-ionized water at 80 1C, and TlNO3 powder was precipitated when the solution was cooled down from 80 1C to room temperature. These operations of dissolution and precipitation were repeated three times. Hydrobromic acid (analytical grade, Signophrm Chemical Reagent Co. Ltd., China) was purified by the decompression distillation method. Synthesis of TlBr was carried out by the following reaction: TlNO3+HBr ¼ TlBr+HNO3. Reactants TlNO3 and HBr were mixed at the stoichiometric ratio of 1:1. The yellow TlBr precipitate was isolated and rinsed by de-ionized water repeatedly until the pH value became neutral. The hydrothermal re-crystallization purification of TlBr powder was carried out as follows: 100 g of TlBr and 1200 ml of deionized water were loaded in an autoclave with a polytetrafluoroethylene (PTFE) cup. The autoclave was put into a two-zone vertical furnace so that it could hold a steady temperature gradient (see Fig. 1). Then the furnace was heated to 170 1C and held for 15 h to prepare a saturated TlBr solution. Then the top of the furnace was cooled at different cooling rates between 0.75 and 6 1C/h, and the bottom of the furnace was cooled at different cooling rates between 0.5 and 4 1C/h. Due to the temperature gradient inside the PTFE cup, mass transfer of TlBr took place through the solution and TlBr dendrites were crystallized at the top of the cup. The parameters of the re-crystallization processes are shown in Table 1. TlBr ingots grown by VBM were cut into wafers along the radial direction using a wire saw. The wafers grown from powder T000
485
Table 1 Particular parameters in the hydrothermal re-crystallization process. Symbol
T000 T100 T111 T123 T146 T223 T523
Number of re-crystallization runs
0 1 1 1 1 2 5
Cooling rate (1C/h) Bottom
Top
0 0.5 1 2 4 2 2
0 0.75 1.5 3 6 3 3
T000 is as-synthesized and non-purified by hydrothermal re-crystallization.
and grown from powder T123 were designated T000 wafers and T123 wafers, respectively. The surface of wafers was treated mechanically and chemically in order to remove surface damage layers. For mechanical polishing, alumina abrasives were used. Chemical etching was then performed using 5% bromine– methanol solution. The etched wafers were rinsed by methanol. Final wafers were transparent and had good surface quality. 2.2. Characterization Phase identification was performed by X-ray diffraction (XRD; X’Pert PRO, PANalytical B.V., Netherlands). The purification efficiency was evaluated by measuring concentration of impurities using ICP-MS (Elan DRC-e, Perkin-Elmer, USA). In order to prevent interference from Tl ion in the measurement of impurity concentration, isopropanol was used as an extractant of TlBr. Optical properties were studied by Bruker Vertex70 FT-IR (wavelength range 2500–20 000 nm) and Perkin-Elmer Lambda35 ultraviolet spectrophotometer (wavelength range 190–900 nm) at room temperature.
3. Results and discussion 3.1. Synthesis and purification
Fig. 1. Schematic drawing of hydrothermal re-crystallization system: 1—autoclave, 2—PTFE cup, 3—furnaces, 4—thermocouples, 5—purified TlBr powder, and 6—raw TlBr powder.
The powder synthesized by chemical precipitation was characterized by XRD (Fig. 2). There were no other crystalline phases in the synthetic powder, and all detected peaks corresponded to the TlBr phase (JCPDS No. 78-0631). In the synthetic process, the pH value of the solution plays an important role. Weak acidic conditions were the best for TlBr synthesis. Under these conditions, TlBr hydrolysis and formation of Tl, Tl2O, Tl2O3, and TlBrO3 were not observed. Table 2 shows the concentration of impurities in powder T000. The sum of impurity concentrations in powder T000 was lower than that in commercial powder [9]. The hydrothermal re-crystallization purification of TlBr is based on the positive temperature coefficient of TlBr solubility [16]. Some impurities can be directly separated, if they cannot be dissolved, and other impurities can be rejected from TlBr re-crystallized material during re-crystallization process. The elements Ca, Mg, Fe, K, Zn, Cu, Na, Si, Al, and Ti in Table 2 were selected for evaluation of the re-crystallization process. Fig. 3 shows the impurity concentration of TlBr powder before and after re-crystallization. There was a strong reduction in the concentration of impurities and the sum of impurity concentrations was about 10 ppm in powder T123. The purification effect was evaluated by the ratio of impurity concentration in powder T000 to that in powder T123. The ratio values of Ca, Fe, Mg, K, Zn, Cu, Na, and Si were 67.26, 28.05, 26.76, 18.78, 11.08, 8.69, 5.86, and
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5.40, respectively. In contrast with the result of Kozlov et al. [15], high purification efficiencies of Ca, Na, and Si were achieved, which may be attributed to the small temperature gradient inside the cup and slow cooling rate of the furnace. The cooling rate is an important parameter for the recrystallization process. As indicated by powder T146 and by powder T123 in Fig. 4a, the impurity concentration evidently decreased with decreasing cooling rate. When the cooling rate is slow enough, the re-crystallization process is near an equilibrium. Hence the force driving out the impurities from the TlBr host lattice is stronger and the decrease in impurity concentration is more obvious. However, further improvement of the purification was not observed when the cooling rate was slower than the rate of the T123 process (Fig. 4b). The number of hydrothermal re-crystallization runs is expected to be an important figure of merit for achieving the maximum reduction in impurity concentration. Hence, the recrystallization purification process was repeated five times. In Fig. 5 the decease of the impurity concentration as a function of the run number of T123 re-crystallization is shown. There was a strong reduction in impurity concentrations after the first recrystallization process. After further runs, impurity concentrations decreased only slightly. 3.2. Optical characteristics The optical characteristics in the wavelength range of 400–600 nm are shown in Fig. 6. The fundamental absorption edge of T123 wafer at 438 nm corresponds to the data presented in Refs. [11,17]. The T123 wafer has smaller absorption (Fig. 6a) and larger transmission (Fig. 6b) in the range of 450–600 nm than the T000 wafer. Considering the same crystal growth and further treatment techniques, the effect of impurities is the
possible reason why T000 wafer has large absorption and small transmission. Impurities are donors or acceptors in TlBr crystals, and form impurity energy levels. For example, an electron at the donor level gains energy by absorbing a photon and is excited to a higher energy level within the conduction band. Therefore, additional absorption for T000 wafer at wavelengths larger than 450 nm takes place. Similar absorption was observed by Shorohov et al. [18]. Also in Fig. 6, the absorption edge at 438 nm for the T123 wafer is shifted to 445 nm for the T000 wafer. IR transmission spectrum is a powerful tool to evaluate the quality of TlBr crystals. In Fig. 7, the IR transmittance of T000 wafer is below that of T123 wafer. For T000 wafer, the average IR transmittance value was 43%, and 58% for T123 wafer. In the wavelength range of 2500–20 000 nm, the IR transmission is determined by free carrier absorption in crystals [19,20]. Differences in valence states and ionic radii of impurity ions and Tl1+ lead to distortions of the crystal lattices and electronic system of TlBr [17]. Impurity ions entering the TlBr host lattices result in the formation of lone electrons and holes. Absorbing optical energy, the lone electrons and holes can easily escape the bondages of impurity atoms and become free carriers. Therefore, the concentration of free carriers increased with increasing impurity concentration. The absorption coefficient is proportional to the square of free carrier concentration [20]. As a result, the large impurity concentration resulted in the rise of absorption and the fall of IR transmittance in T000 wafer.
4. Conclusions The trace element analyses showed that the purification efficiency of hydrothermal re-crystallization was dominated by
Fig. 3. Impurity concentration of TlBr powder before and after T123 re-crystallization.
Fig. 2. XRD pattern of powder T000.
Table 2 Concentration of impurities (ppm) in powder T000. Element Concentration
Ca 66.721
Na 16.401
Mg 15.363
Fe 14.585
Cu 2.102
Zn 2.549
K 2.404
Si 19.409
Al 0.927
Ti 0.376
Element Concentration
Pd 0.173
Ba 0.156
Li 0.029
Mn 0.116
V 0.202
Ni 0.145
Cd 0.022
Pt 0
Pb 0
As 0
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Fig. 4. (a) Concentration of impurities in powders T123 and T146. (b) Impurity concentration as a function of cooling rate in the re-crystallization process. Impurities: ’—Ca, m—Fe, n—Cu, J—Na, and b—Si. Fig. 6. Spectra of the TlBr wafers in the range 400–600 nm: (a) absorption spectra and (b) transmission spectra; m—T123 wafer, n—T000 wafer.
Fig. 5. Impurity concentration as a function of the run number of re-crystallization. Impurities: ’—Ca, m—Fe, n—Cu, J—Na, and b—Si.
Fig. 7. IR transmission spectra of TlBr wafers. The solid line is from T123 wafer, and the dashed line is from T000 wafer.
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the cooling rate of solution. The impurity concentration was decreased evidently with reduced cooling rate, as was demonstrated by the processes T146 and T123. Hydrothermal re-crystallization was effective for reducing the concentration of impurities in the TlBr material. After T123 hydrothermal re-crystallization, impurities of Ca, Fe, Mg, K, Zn, Cu, Na, and Si were obviously decreased and the sum of impurity concentrations was about 10 ppm. But the decrease in impurity concentration became weaker in the following runs after the first hydrothermal re-crystallization. Optical characteristics were strongly dependent on the purity of TlBr crystals. Compared to the wafer grown from powder T123, the wafer grown from powder T000 exhibited a large absorption coefficient and small transmission in the range 450–600 nm, a shift of the absorption edge from 438 to 445 nm, and a decrease of the average IR transmittance value from 58% to 43%.
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