Solid-state ionizing radiation detectors based on high-resistance semiconductors can be used to monitor the safety of nuclear reactors. High-resistance CdTe and CdZnTe have very good electrophysical and detector properties. The objective of this study was to use computer simulation to determine how impurities and structural defects, as well as their clusters, affect electrophysical and detector properties of Cd1-xZnxTe (0≤x≤0.3). The calculations were based on well-tested models, the reliability of which was confirmed when comparing simulation results with well-known experimental data. It has been established that deep donors with energy levels near the middle of the band gap considerably extend the area of the high-resistance state of CdTe and CdZnTe, which is suitable for the creation of radiation detectors. The capture and recombination of non-equilibrium charge carriers occurs at the deep levels of cadmium vacancies owing to the influence of Ti, V, Ge, Ni, and Sn impurities. For this reason, such impurities are considered to be harmful, noticeably reducing the efficiency of charge collection η in CdTe and CdZnTe detectors. The decrease of electron mobility in CdTe and CdZnTe can be caused by the distribution heterogeneity of impurities (impurity clusters).When concentration of harmful impurities Ti, V, Ni, Sn, Ge does not exceed the content of the "background", provided that the impurities are distributed over the crystal uniformly, it is possible to obtain high-resistance CdZnTe of an acceptable detector quality. The obtained results could help determining conditions for producing CdTe and CdZnTe materials of high detector quality.
A promising material for semiconductor detectors of ionizing radiation is CdTe:Cl which allows obtaining detectors with high resistivity ρ and electron mobility μn. During operation, the detector materials may be exposed to neutron irradiation, which causes radiation defects to form in crystal lattice and deep levels to appear in the band gap, acting as centers of capture and recombination of nonequilibrium charge carriers, thus reducing the detection capability. The aim of this study was to use computer simulation to investigate the mechanisms of the influence of such radiation defects on the electrophysical properties (ρ, μn) of CdTe:Cl and the charge collection efficiency η of radiation detectors based on this material. The simulations were based on the models tested for reliability. It was found that the increase of the CdTe:Cl resistivity ρ during low-energy neutrons bombardment and at the initial stages of high-energy neutrons bombardment is caused by an increase in the concentration of radiation donor defect Z (with an energy level EC – 0.47 eV), presumably interstitial tellurium, which shifts the Fermi level into the middle of the band gap. The sharp rise of ρ observed at high-energy neutron bombardment is probably caused by the restructuring of the crystalline structure of the detector material with a change in the lattice constant and with an increase of the band gap, accompanied by a change in the conductivity properties. The degradation of the detector properties of CdTe:Cl during neutron irradiation is due to the capture and recombination of nonequilibrium electrons at radiation defects: Te interstitial, Te substitutional at the cadmium site, on tellurium vacancies and cadmium vacancies. The degradation of electron mobility μn can be caused by the scattering of electrons at microscopic areas of radiation defect clusters. The increase in concentration of the defects over the volume of the crystal at their uniform distribution of up to 1016 cm–3 does not significantly affect the electron mobility at room temperature.
This work is devoted to the study of the mechanisms of the influence of radiation defects, arising under the influence of gamma irradiation, on the change in resistivity ρ, lifetime of nonequilibrium electrons τn and holes τp, in CdTe:Cl and Cd0.9Zn0.1Te as well as the collection efficiency η of uncooled radiation detectors based on these materials, by computer simulation method. Radiation defects, that are corresponded by deep energy levels in the band gap, act as trapping centers of nonequilibrium charge carriers, noticeably affect the degree of compensation by changing ρ of the detector material, the recombination processes by decreasing τn and τp, what ultimately can cause degradation of the charges collection efficiency η. The specific reasons for the deterioration of the detector properties of CdTe:Cl and Cd0.9Zn0.1Te under the influence of gamma irradiation were identified, and the main factors leading to complete degradation of the recording ability of detectors based on these semiconductors during their bombardment by 60Co gamma quanta were determined. The gradual degradation of the spectroscopic performance of CdTe:Cl-based detectors during gamma irradiation occurs due to the continuous formation of cadmium vacancies VCd and acceptor complexes VCd – Cl, which continuously shift Fermi level towards valence band and decrease ρ. The complete performance degradation of CdTe detectors takes place mainly due to the capture of nonequilibrium electrons at energy level of interstitial tellurium Te(I). The invariable spectroscopic properties of CdZnTe-based sensors under gamma irradiation up to 25 kGy occur due to the mechanism of radiative self-compensation by formation of substitutional defect TeZn. At the final stage of irradiation, a sharp deterioration in the detector properties of CdZnTe occurs, mainly due to the capture and recombination of nonequilibrium charge carriers at the level of the Te(I) defect. The different radiation resistances of CdZnTe and CdTe:Cl can be explained by different behavior of Fermi level EF in these semiconductors under gamma irradiation. EF in CdZnTe under radiation exposure remains near the middle of band gap, and in CdTe it drifts to the valence band. The rate of capture and recombination through Te(I) donor level in CdTe:Cl is lower than in CdZnTe due to the larger difference between the Fermi level and the radiation defect Te(I) level in cadmium telluride. Thereby, the complete degradation of the CdTe:Cl detector occurs at a higher concentration of radiation defect Te(I), and hence after a higher irradiation dose of 50 kGy compared with a dose of 30 kGy required for degradation of CdZnTe detector properties.
The work is dedicated to studying by computer modeling the mechanisms of the influence of radiation defects, originating under high energy proton irradiation, on the resistivity ρ, lifetime of nonequilibrium electrons n and holes p in CdTe:Cl and Cd0.9Zn0.1Te, and charge collection efficiency η of room temperature ionizing radiation detectors based on these materials. The effect of recombination at deep levels of radiation defects on the degradation of n, p, and of detectors based on CdTe:Cl and Cd0.9Zn0.1Te was studied. Energy levels of radiation defects also substantially effect on compensation degree of semiconductor decreasing ρ. The main factors affecting the abrupt or gradual decrease in the resistivity and charge collection efficiency of these detectors during their bombardment by high-energy protons, leading to complete degradation of their recording ability, were found. The important role of purity and deep donor concentration in initial state of the detector material was indicated.
Clarification of the influence of defects on detecting properties of CdZnTe detectors and understanding of the behavior of defects under the influence of aggressive radiation environment are very important to improve detector performance. The objective was to study the charges collection efficiency and the resistivity of Cd0,9Zn0,1Te:Al detectors operating under the influence of low dose γ-radiation. The study was carried out by computer simulation, where initial data were provided by the experiment results of other researchers. The possible reason for the change of measured signatures of defect levels in high resistance Cd0.9Zn0.1Te:Al during gamma irradiation and 1 month later is the change in compensation degree of the material. The changes in the properties of Cd0.9Zn0.1Te:Al detector have been researched depending on the concentration and energy level of the deep donor for different concentrations of deep acceptors, as well as on the degree of alloying with aluminum. The negative factor for registering properties of Cd0.9Zn0.1Te:Al detector is increased concentration of zinc vacancies, which may arise at manufacturing stage and under influence of -irradiation during operation. The degradation of properties of irradiated detector may occur due to the offset dependence of the resistivity on the aluminum dopant concentration N(Al) towards to higher concentrations of Al when the value of doping is not enough large. Only resistivity will be reduced and charge collection efficiency may increase. The increase in resistivity of Cd0.9Zn0.1Te and charges collection efficiency of the detector occur when there is a sufficiently high level of doping the material with aluminum.
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