分压控制下的高阻Cd_（0.9）Zn_（0.1）Te晶体生长及其热处理研究
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摘要
Cd_(1-x)Zn_xTe核辐射探测器由于可在室温下工作,并且对X、γ射线有较高的探测效率和较好的能量分辨率,因此可广泛应用在核安全、环境监测、医学诊断、天体物理研究等领域。而高质量的Cd_(1-x)Zn_xTe晶体材料是研制高性能Cd_(1-x)Zn_xTe x-、γ-射线探测器的关键。目前国际上商用的Cd_(1-x)Zn_xTe晶体是采用高压垂直布里奇曼法(HPVB)制备的,其晶体电阻率已达10~(11)Ωcm。但由于其设备复杂,成品率低(4%左右),价格特别高昂。为此,当前国际上的研究热点是采用低压垂直布里奇曼法(LPVB)制备出高阻、高质量、低成本的探测器级Cd_(1-x)Zn_xTe晶体。但是目前对于探测器级Cd_(1-x)Zn_xTe晶体的LPVB生长,还存在一些基本的理论与工艺问题尚未解决。因此开展这方面的研究具有重要的学术意义和实际应用价值。为此,本文采用改进的垂直布里奇曼法(MVB)系统地研究了探测器级Cd_(0.9)Zn_(0.1)Te晶体的制备理论与工艺。本文的主要研究内容与结果如下:
1、采用有限元法对Cd_(0.9)Zn_(0.1)Te的晶体生长过程进行了模拟。研究了垂直布里奇曼法(VB)晶体生长中不同的因素对晶体生长过程中固液界面形状的影响。在模拟计算中,考虑了热传导、对流、辐射等热交换过程和相变过程,利用傅立叶变换等形式给出了变化炉温加载的函数表达式。同时根据模拟结果,进行了相应的晶体生长实验,并将实验结果和模拟结果进行了比较分析。模拟结果表明,当坩埚下降速度Vp≈0.9～1mm/h时,可获得接近水平的固-液界面,这将有利于获取径向组成分布均匀、高质量的Cd_(0.9)Zn_(0.1)Te晶体。这一模拟结果在实验中也得到了验证。
2、在前期研究工作的基础上,利用热力学关系,估算了Cd_(1-x)Zn_xTe和Cd_(1-x)Zn_x熔体的平衡蒸汽分压,并建立了两者各组元平衡蒸汽压之间的关系。首次获得了Cd_(0.9)Zn_(0.1)Te晶体在其熔点1393K附近的Cd/Zn分压,它们分别为P_(Cd)=4.04×10~5 Pa和P_(Zn)=4.99×10~4 Pa,同时估算出了与之相平衡的Cd_(1-x)Zn_x合金控制源的组成(x=0.19)与温度(1195K)。
3、为克服传统ZnTe合成法存在合成不充分和合成温度高且反应剧烈致使石英合成管易爆炸等困难,本工作研究设计并采用了ZnTe合成新工艺,即EPS(Evaporating Pressure Synthesis)法,成功地解决了ZnTe合成的技术关键。此项工艺国内外还未见有报道。
4、采用改进的垂直布里奇曼法(MVB),即采用Cd_(0.81)Zn_(0.19)合金源作为晶体生长时的气相分压控制,进行了Cd_(0.9)Zn_(0.1)Te的晶体生长实验。获得了分压控制与晶体电阻率的关系,成功生长出了高阻、高质量的Cd_(0.9)Zn_(0.1)Te晶体。晶体电阻率最高可达1.1×10~(10)Ω·cm,x射线摇摆曲线的半峰宽约为53”,平均位错密度为7×10~4cm~(-2),红外透过率在5000～450cm~(-1)波数范围内高达60%,径
Cd_(1-x)Zn_xTe radiation detector not only can work at room temperature, but also has high detection efficiency and good energy resolution for Y - and x- ray. So it has been used in many fields such as nuclear security, environment inspecting, medicine diagnoses and sphere physics research. The high-quality Cd_(1-x)Zn_xTe crystal material is the key issue for developing high-performance Cd_(1-x)Zn_xTe nuclear-detectors. Presently commercial Cd_(1-x)Zn_x crystals are mostly prepared by HPVB method in the world and the resistivity of the as-grown crystal reached up to 10~(11) Ωcm. However, the method needed complex equipments and the useable part of the as-grown crystal was lower than 4%, the cost of crystal was very high. At present, the research on crystal growth of Cd_(1-x)Zn_xTe in the world is focus on how to grow crystal with high resisitivity, high quality and low cost by the LPVB method, but there are still some problems on basic theory and technology unresolved, so it is of important academic significance and applied worth to perform the research on these problems. In this paper, we systematically studied the preparation theory and technology of Cd_(0.9)Zn_(0.1)Te crystal. The main contents and conclusions of the paper are summarized as follows:Finite element method (FEM) was used to simulate the growth process of Cd_(0.9)Zn_(0.1)Te crystal and the effects of different crucible moving rates on crystal growth rate and solid-liquid interface configuration were studied as well. Thermal conduction, convection, radiation and phase-change process were considered in our calculation model. The intrinsic relationship between the interface shape and the crucible descending rate was discussed in details. In addition,Cd_(0.9)Zn_(0.1)Te crystal growth experiments were carried out, experimental results and the simulation results were compared as well. Simulation results show that when crucible descends at the rate of about lmm/h, nearly flat solid/liquid interface can be attained which is very helpful to gain Cd_(0.9)Zn_(0.1)Te crystal with uniform composition distribution in the radial direction and higher quality. And the simulation result is well consistent with that of experiments.Equilibrium partial pressures over Cd_(1-x)Zn_xTe and Cd_(1-x)Zn_x melt with different composition in different temperatures were estimated by thermodynamic relationship based on previous work, and the Cd, Zn partial pressures of P_(Cd)=4.037×10~5 Pa and
pZn=4.995xl04 Pa over Cdo.9Zno.1Te melt at the melting temperature of 1393K were firstly obtained, which are consistent with those over the Cdj_xZnx melt at the temperature of 1195K.A new synthesis method called EPS (Evaporating Pressure Synthesis) was performed to synthesize ZnTe, which overcame the weaknesses of conventional synthesis method with high synthesis temperature, insufficient synthesis, severe reaction and easy to explode, and homogeneous ZnTe was obtained.MVB method was used to grow the Cdo.9Zno.1Te crystals under Cd/Zn partial pressures provided by Cdo.siZno 19 alloy reservoir in our lab, we succeed to obtain the high quality CdogZno.iTe crystals. The best result for the resistivity, which has reached up to about l.l><10l0Q-cm, has been obtained under the equilibrium partial pressures. FWHM of the X-ray rocking curve was about 53'", which indicates excellent structural integrity of the crystal. The etch pit density of the crystal was 7X 104cm"2 on average. Infrared transmissivity of the crystal was up to 60% in the range of 5000~450cm'\ and the radial variation of Zn concentration was less than 0.2al%. In addition, the relationship between the resistivities of Cdo.9Zno.1Te crystals and the partial pressures controlled during the crystal growth was also obtained.The upper limit value of annealing temperature was determined by exploring the high-temperature phase of CdTe. Based on the estimation of equilibrium partial pressures over Cdi_xZnx melt, the as-grown high resistivity Cdo.9Zno.1Te slices have been annealed under controlled Cd/Zn equilibrium pressures provided by Cdi_xZnx alloy source. The relationship between the properties of Cdo.9Zno.1Te slices and the annealing parameters such as annealing temperature, holding time. Cd/Zn partial pressure have been discussed. The results show that the integrality of Cdo.9Zno.1Te slices with high resistivity were improved after annealing, Infrared transmissivity of the slices can be raised by 3% up to 65% in the range of 5000~450cm'' and their resistivities were further raised. In addition, the Cdo.9Zno.1Te with relative low resistivity of 7.0 X 104Q-cm slices can be raised by 4 orders, up to 6.97 X 108fi-cm, the value of IR of the slices an be raised by over 10%. up to 64% in the range of 5000~450crrf', and after they were annealed under optimizing conditions.A novel process of diffusion into as-grown Cdo.9Zno.1Te slices by In as gas-doping
1、采用有限元法对Cd_(0.9)Zn_(0.1)Te的晶体生长过程进行了模拟。研究了垂直布里奇曼法(VB)晶体生长中不同的因素对晶体生长过程中固液界面形状的影响。在模拟计算中,考虑了热传导、对流、辐射等热交换过程和相变过程,利用傅立叶变换等形式给出了变化炉温加载的函数表达式。同时根据模拟结果,进行了相应的晶体生长实验,并将实验结果和模拟结果进行了比较分析。模拟结果表明,当坩埚下降速度Vp≈0.9～1mm/h时,可获得接近水平的固-液界面,这将有利于获取径向组成分布均匀、高质量的Cd_(0.9)Zn_(0.1)Te晶体。这一模拟结果在实验中也得到了验证。
2、在前期研究工作的基础上,利用热力学关系,估算了Cd_(1-x)Zn_xTe和Cd_(1-x)Zn_x熔体的平衡蒸汽分压,并建立了两者各组元平衡蒸汽压之间的关系。首次获得了Cd_(0.9)Zn_(0.1)Te晶体在其熔点1393K附近的Cd/Zn分压,它们分别为P_(Cd)=4.04×10~5 Pa和P_(Zn)=4.99×10~4 Pa,同时估算出了与之相平衡的Cd_(1-x)Zn_x合金控制源的组成(x=0.19)与温度(1195K)。
3、为克服传统ZnTe合成法存在合成不充分和合成温度高且反应剧烈致使石英合成管易爆炸等困难,本工作研究设计并采用了ZnTe合成新工艺,即EPS(Evaporating Pressure Synthesis)法,成功地解决了ZnTe合成的技术关键。此项工艺国内外还未见有报道。
4、采用改进的垂直布里奇曼法(MVB),即采用Cd_(0.81)Zn_(0.19)合金源作为晶体生长时的气相分压控制,进行了Cd_(0.9)Zn_(0.1)Te的晶体生长实验。获得了分压控制与晶体电阻率的关系,成功生长出了高阻、高质量的Cd_(0.9)Zn_(0.1)Te晶体。晶体电阻率最高可达1.1×10~(10)Ω·cm,x射线摇摆曲线的半峰宽约为53”,平均位错密度为7×10~4cm~(-2),红外透过率在5000～450cm~(-1)波数范围内高达60%,径
Cd_(1-x)Zn_xTe radiation detector not only can work at room temperature, but also has high detection efficiency and good energy resolution for Y - and x- ray. So it has been used in many fields such as nuclear security, environment inspecting, medicine diagnoses and sphere physics research. The high-quality Cd_(1-x)Zn_xTe crystal material is the key issue for developing high-performance Cd_(1-x)Zn_xTe nuclear-detectors. Presently commercial Cd_(1-x)Zn_x crystals are mostly prepared by HPVB method in the world and the resistivity of the as-grown crystal reached up to 10~(11) Ωcm. However, the method needed complex equipments and the useable part of the as-grown crystal was lower than 4%, the cost of crystal was very high. At present, the research on crystal growth of Cd_(1-x)Zn_xTe in the world is focus on how to grow crystal with high resisitivity, high quality and low cost by the LPVB method, but there are still some problems on basic theory and technology unresolved, so it is of important academic significance and applied worth to perform the research on these problems. In this paper, we systematically studied the preparation theory and technology of Cd_(0.9)Zn_(0.1)Te crystal. The main contents and conclusions of the paper are summarized as follows:Finite element method (FEM) was used to simulate the growth process of Cd_(0.9)Zn_(0.1)Te crystal and the effects of different crucible moving rates on crystal growth rate and solid-liquid interface configuration were studied as well. Thermal conduction, convection, radiation and phase-change process were considered in our calculation model. The intrinsic relationship between the interface shape and the crucible descending rate was discussed in details. In addition,Cd_(0.9)Zn_(0.1)Te crystal growth experiments were carried out, experimental results and the simulation results were compared as well. Simulation results show that when crucible descends at the rate of about lmm/h, nearly flat solid/liquid interface can be attained which is very helpful to gain Cd_(0.9)Zn_(0.1)Te crystal with uniform composition distribution in the radial direction and higher quality. And the simulation result is well consistent with that of experiments.Equilibrium partial pressures over Cd_(1-x)Zn_xTe and Cd_(1-x)Zn_x melt with different composition in different temperatures were estimated by thermodynamic relationship based on previous work, and the Cd, Zn partial pressures of P_(Cd)=4.037×10~5 Pa and
pZn=4.995xl04 Pa over Cdo.9Zno.1Te melt at the melting temperature of 1393K were firstly obtained, which are consistent with those over the Cdj_xZnx melt at the temperature of 1195K.A new synthesis method called EPS (Evaporating Pressure Synthesis) was performed to synthesize ZnTe, which overcame the weaknesses of conventional synthesis method with high synthesis temperature, insufficient synthesis, severe reaction and easy to explode, and homogeneous ZnTe was obtained.MVB method was used to grow the Cdo.9Zno.1Te crystals under Cd/Zn partial pressures provided by Cdo.siZno 19 alloy reservoir in our lab, we succeed to obtain the high quality CdogZno.iTe crystals. The best result for the resistivity, which has reached up to about l.l><10l0Q-cm, has been obtained under the equilibrium partial pressures. FWHM of the X-ray rocking curve was about 53'", which indicates excellent structural integrity of the crystal. The etch pit density of the crystal was 7X 104cm"2 on average. Infrared transmissivity of the crystal was up to 60% in the range of 5000~450cm'\ and the radial variation of Zn concentration was less than 0.2al%. In addition, the relationship between the resistivities of Cdo.9Zno.1Te crystals and the partial pressures controlled during the crystal growth was also obtained.The upper limit value of annealing temperature was determined by exploring the high-temperature phase of CdTe. Based on the estimation of equilibrium partial pressures over Cdi_xZnx melt, the as-grown high resistivity Cdo.9Zno.1Te slices have been annealed under controlled Cd/Zn equilibrium pressures provided by Cdi_xZnx alloy source. The relationship between the properties of Cdo.9Zno.1Te slices and the annealing parameters such as annealing temperature, holding time. Cd/Zn partial pressure have been discussed. The results show that the integrality of Cdo.9Zno.1Te slices with high resistivity were improved after annealing, Infrared transmissivity of the slices can be raised by 3% up to 65% in the range of 5000~450cm'' and their resistivities were further raised. In addition, the Cdo.9Zno.1Te with relative low resistivity of 7.0 X 104Q-cm slices can be raised by 4 orders, up to 6.97 X 108fi-cm, the value of IR of the slices an be raised by over 10%. up to 64% in the range of 5000~450crrf', and after they were annealed under optimizing conditions.A novel process of diffusion into as-grown Cdo.9Zno.1Te slices by In as gas-doping
引文
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[3] T.umay O. T.umera, Shi Yina, Victoria Cajipea et al., High-resolution pixel detectors for second generation digital mammography Nuclear Instruments and Methods in Physics Research A 497 (2003) 21-29
[4] E. Bertolucci, M, Maiorino, G. Mettivier, M.C. Montesi, P. Russo, Preliminary test of an imaging probe for nuclear medicine using hybrid pixel detectors, Nuclear Instruments and Methods in Physics Research A 487 (2002) 193-201
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[6] K.H. Czock, R. Arlt, Use of CdZnTe detectors to analyze gamma emission of safeguards samples in the field Nuclear Instruments and Methods in Physics Research A 458 (2001) 175-182
[7]K. Abbas, J. Morel, M. Etcheverry, G. Nicolaou, Use of miniature CdZnTe X/ Y detector in nuclear safeguards: characterization of spent nuclear fuel and uranium enrichment determination, Nuci. Instrum .and Methods A 405, (1998) 153-158
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[2] L.Verger et al., New developments in CdTe and CdZnTe detectors for X and g-ray applications, J. Electron. Mater., Vol.26, (1997) 738
[3] T.umay O. T.umera, Shi Yina, Victoria Cajipea et al., High-resolution pixel detectors for second generation digital mammography Nuclear Instruments and Methods in Physics Research A 497 (2003) 21-29
[4] E. Bertolucci, M, Maiorino, G. Mettivier, M.C. Montesi, P. Russo, Preliminary test of an imaging probe for nuclear medicine using hybrid pixel detectors, Nuclear Instruments and Methods in Physics Research A 487 (2002) 193-201
[5] M. Bavdaz, A. Peacock, A. Owens, Future space applications of compound semiconductor X-ray detectors, Nuclear Instruments and Methods in Physics Research A 458 (2001) 123-131
[6] K.H. Czock, R. Arlt, Use of CdZnTe detectors to analyze gamma emission of safeguards samples in the field Nuclear Instruments and Methods in Physics Research A 458 (2001) 175-182
[7]K. Abbas, J. Morel, M. Etcheverry, G. Nicolaou, Use of miniature CdZnTe X/ Y detector in nuclear safeguards: characterization of spent nuclear fuel and uranium enrichment determination, Nuci. Instrum .and Methods A 405, (1998) 153-158
[8]T.Asahi, et al., Growth and characterization of 100mm diameter CdZnTe single crystals by the vertical gradient freezing method, J. Crystal Growth 161 (1996) 20.
[9] A.Niemala, et al., Improving CdZnTe X-ray detector performance by cooling and rise time discrimination, Nucl. Instrument. Methods A 377 (1996) 484.
[10]Y.Eisen, et al., A gamma camera based on CdTe detecors, Nucl. Instrument Methods A 380 (1996)474.
[11] T.-C. Yu,et al., The Hg-Cd-Zn-Te Phase Diagram, J.Phase Equilibria 13(1992)476
[12] J.M.Francou.et al., Shallow donors in CdTe, Phys.Rev. B. 41(1990) 12035
[13] S.Sen,et al., Crystal growth of large-area single-crystal CdTe and CdZnTe by the computer-controlled vertical modified-bridgman process, J. Cryst. Growth 86 (1987) 111-117
[14]K. Lynn et al., Improved CdZnTe detectors grown by vertical Bridgman process, Mater. Res. Soc. Symp. 487(1998)229
[15] K. Chattopadhyay et al., Gamma ray spectrometers fabricated from modified Bridgman/annealed CZT crystal material, Mater. Res. Soc. Proc. 487(1998) 123
[16] P.Cheuvart, U.E1-Hanani,et al., CdTe and CdZnTe crystal growth by horizontal bridgman technique, J.Crystal Growth 101 (1990) 270
[17] E,Raiskin,et al., CdTe low level gamma detectors based on a new crystal growth method, IEEE Trans, on Nucl.Science,Vol.NS-35,No.l (1988)82
[18] M.AzouIay,et al., Crystalline perfection of melt-grown CdTe , J.Crystal Growth 101 (1990) 256
[19] Hwa-Yuh Shin,et al., Temperature-gradient-solution grown CdTe crystals for y-ray detectors, J.Crystal Growth 186 (1998) 67
[20] Kh.Allachen,et al., Photoconductivity studies in vanadium-doped CdTe and Cdl-xZnxTe J.Crystal Growth 184/185(1998) 1142
[21] R.Triboulet,et al., Undoped high-resistivity cadmium telluride for nuclear radiation detectors, J.Appl.Phys. 45 (1974)2759-2765
[22] K. Grasza et al., Novel method of crystal growth by physical vapour transport and its application to CdTe, J.Crystal Growth 123 (1992) 519
[23] J. Boone et al., Electrical and crystallographic characterization of CdTe grown by the vapor transport method, J.Crystal Growth 139 (1994) 27
[24] A. Melnikov, A. Sigov, K. Vorotiov, A. Davydov, L. Topalova, Growth of CdZnTe single crystals for radiation detectors , J. Cryst. Growth 197 (1999) 666
[25] W. Palosz, S. Lehoczky, F. Szofran, Thermochemical model of physical vapor transport of cadmium-zinc telluride, J.Crystal Growth 148 (1995) 49
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