Degradation Under Influence of Radiation Defects of Detector Properties of CdTe and Cd0.9Zn0.1Te Irradiated by Neutrons

Abstract

This work is devoted to the study by computer simulation of the mechanisms of the influence of radiation defects, arising under the influence of neutron irradiation, on the changes in electrical properties: resistivity ρ, electron mobility μ_n, lifetime of nonequilibrium electrons τ_n and holes τ_p in Cd_0.9Zn_0.1Te and charge collection efficiency η of uncooled ionizing radiation detectors based on this material. Radiation defects, which 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, decreasing τ_n and τ_p, and also the scattering of conduction electrons, decreasing μ_n, that ultimately can cause degradation of the charges collection efficiency η. The specific reasons for the deterioration of the electrophysical and detector properties of this semiconductor under the influence of neutron irradiation were identified, and the main factors affecting the increase in the resistivity of Cd_0.9Zn_0.1Te during its bombardment by low-energy and high-energy neutrons, leading to complete degradation of the recording ability of detectors based on  this materials, were found. The recombination of nonequilibrium charge carriers is noticeably stronger than the decrease in μ_n affects the degradation of detector properties, therefore, the effect of recombination processes at deep levels of radiation defects on the degradation of τ_n, τ_p, and η of detectors based on Cd_0.9Zn_0.1Te was studied.  A comparative analysis of the properties of Cd_0.9Zn_0.1Te with the previously studied CdTe:Cl was made. An attempt was made to explain the higher radiation resistance of Cd_0.9Zn_0.1Te compared to CdTe:Cl under neutron irradiation by the influence of the radiation self-compensation mechanism with participation of deep donor energy levels: interstitial tellurium and tellurium at the site of cadmium. In addition, the rate of recombination at defect levels in Cd_0.9Zn_0.1Te is, ceteris paribus, lower than in CdTe:Cl due to the smaller difference between the Fermi level and the levels of radiation defects in cadmium telluride. The relationship between the band gaps of Cd_0.9Zn_0.1Te and CdTe:Cl, the concentration of radiation defects, the Fermi level drift during irradiation, and the radiation resistance of the detectors were also noted. The important role of purity and dopant shallow donor concentration in initial state of the detector material is indicated.