Radiation-induced defects and their annealing behavior in cadmium telluride

Abstract

Radiation-induced defects and their annealing behavior in p- and n-type CdTe irradiated with 2-MeV electrons, neutrons, and 60Co gamma rays have been investigated by means of photoluminescence, Hall, time-of-flight measurements, and the performance of CdTe gamma-ray detectors. The 1.1-eV emission band produced after electron irradiation at 77 K anneals at a stage centered around about 120 K and originates from a defect including Te vacancy. From the isothermal annealing experiment, an activation energy of about 0.2 eV is obtained for the annealing stage. The electron and gamma irradiation at 77 K with room-temperature anneal, and neutron irradiation at 300 K enhance 1.53- and 1.55-eV edge emission bands. It has been suggested that the 1.53-eV band is attributed to a complex involving Cd vacancies. On the other hand, the 1.55-eV band is attributed to a complex involving Te vacancies. An annealing stage centered at about 370 K is observed in electrical properties in relatively pure p-type crystals irradiated with electrons and gamma rays. It is found from the isothermal annealing that the annealing behavior of the stage obeys first-order kinetics. The activation energy for the migration of the defects is estimated to be about 0.8 eV and the pre-exponential factor is found to be about 108 sec−1, suggesting long-range migration of Cd vacancies. Electron-trap levels lying at 0.058 and 0.5 eV below the conduction band are observed in crystals irradiated with gamma rays. The 0.058-eV electron trap anneals at about 600 K and the origin is tentatively assigned to a complex involving Te vacancies as inferred from photoluminescence results. Electron-drift mobility at room temperature is remarkably reduced after gamma irradiation, possibly due to the introduction of an electron-trap level at about 0.5 eV below the conduction band. The 0.5-eV electron-trap level and the electron-drift mobility anneal in the vicinity of about 600 K. The effects due to successive detector irradiations with 60Co gamma rays markedly degraded the output pulse-height spectra until ultimately the 59.5-keV photopeak of the 241Am gamma ray can not be discerned. Following annealing at 600 K, the pulse-height spectrum recovers almost to the preirradiation value. This annealing behavior coincides with the recovery of the electron-drift mobility of bulk crystals.