Degradation Mechanisms of the Detector Properties of CdTe and CdZnTe Under the Influence of Gamma Irradiation

Abstract

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 τ_n and holes τ_p, in CdTe:Cl and Cd_0.9Zn_0.1Te 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 τ_n and τ_p, 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 Cd_0.9Zn_0.1Te 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 V_Cd and acceptor complexes V_Cd – 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 Te_Zn. 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 E_F in these semiconductors under gamma irradiation. E_F 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.