Controls Of Microscopic Defects In CdZnTe Crystal And Their Electrical Transport Process

Cadmium zinc telluride?CdZnTe or CZT?crystal,with excellent optical and electrical properties,is recognized as an attractive II-VI compound semiconductor material for fabricating room-temperature nuclear radiation detectors.These radiation detectors have great energy and space resolution for X/gamma ray spectroscopy at room temperature,fast response and high sensitivity.However,in the CZT crystal growth processes of the melt technique,a variety of microscopic defects inevitably exist in CZT single crystal,having serious impacts on the electrical proformance and the carrier transport behavior and thus limiting the application of CZT radiation detector.Therefore,this paper deeply researches In doped Cd_0.9Zn_0.1Te crystal grown by the modified vertical Bridgman?MVB?method aiming at this problem.All kinds of point defects in crystal and their effects on electrical property and the carrier transport behavior are mainly investigated in this paper,getting semi-quantitative descriptions of various point defects and revealing their microscopic origins.On these bases,the high-resistivity mechanism has been discussed,laying the certain theoretical foundation for obtaining high-quality CZT crystal using for detector.A piece of In doped Cd_0.9Zn_0.1Te ingot with a diameter of 60 mm was grown by the MVB method.A lot of 5×5×2 mm³ CZT wafers were obtained after axial cutting,mechanical polishing and chemical etching.Then the two types of wafers with obvious bulk resistivity difference were selected from above wafers by current-voltage?I-V?at room temperature.The wafer of relatively high bulk resistivity was named CZT1(about 1.28×10¹⁰?·cm)and the wafer of relatively low bulk resistivity was named CZT2?about 1.46×10⁹?·cm?,respectively.Firstly,the electrical performance and the carrier transport behavior of the two wafers were investigated.The result of Hall measurement shows the both conductive type is n type.The results of ²⁴¹Am?59.5 keV?gamma-ray spectroscopy response tests with variable bias voltages manifest that the electron mobility lifetime products of CZT1(about 1.15×10^-33 cm²·V^-1)is higher than that of CZT2(about 5.41×10^-44 cm²·V^-1).The results of Time of Flight?TOF?spectroscopy indicate the ranges of impulse voltage increase with bias voltage increasing,while the drift time(t_dr),trapping time??^*?and de-trapping time??_D?of carriers decrease,and the decreasing degree of?_D is larger than?^*.The response time and collection efficiency of carriers can be enhanced under high bias voltage.The electron mobility of CZT1 and CZT2 are 936 cm²/Vs and 547 cm²/Vs by calculated respectively.The carriers transport performances of CZT1 are superior to CZT2.Meanwhile,the analytical results of ultraviolet-visiable-near infrared?UV-Vis-NIR?transmission spectra show the both band gap is about 1.493 eV.Secondly,the distribution of all kinds of micro-defects in crystal and their impacts on electrical transport process were investigated.The various recombination center defects were studied by photoluminescence?PL?spectra.The results show that the carrier mobility decreases owing to the existing deep-level recombination center that has an impact on carrier transport performance.The defect levels introduced by various traps were researched by thermally stimulated current spectroscopy?TSC?and theroelectronic effect spectroscopy?TEES?.The simultaneous multiple peak analysis?SIMPA?method was adopted to determine trap-related parameters,such as thermal activation energy,carrier concentration,capture cross section and trap type.The microscopic origins of these traps were identified.The sum of trap levels concentration of CZT2 is greater than CZT1,and thus the ionized impurity scattering possibility of CZT2 is larger than that of CZT1.This may be the reason that CZT2 has relatively low electron mobility.The reducing of deep-level defects amounts in crystal can improve the carrier transport performance under the premise of high resistivity.Lastly,the Fermi-level E_F of CZT1 and CZT2 are 0.676 eV and 0.609 eV by I-V curves at variable temperature combined with the relationship between E_F and resistivity respectively.The deep donor level E_DD leads to E_F pinned close to the mid-gap.The more pinned degree,the higher resistivity.The high-resistivity formation mechanism of n type CZT crystal can be explained by combining shallow donor level(In⁺_Cd)and deep donor level(Te_Cd²⁺)compensating acceptor level V_Cd^2-derived from Cd vacancy model.The sum of V_Cd^2-concentration and[V_Cd^2--In⁺_Cd]^-concentration is greater than In⁺_Cd concentration,and Te_Cd²⁺concentration is greater than the value of V_Cd^2-concentration plus[V_Cd^2--In⁺_Cd]^-concentration minus In⁺_Cd concentration.