Photoenhanced metastable deep trapping in amorphous chalcogenides near room temperature

Deep trapping of injected holes or electrons is temporarily enhanced at room temperature in chalcogenide films which have undergone prior near-band-gap photoexcitation at room temperature. Persistence of this effect can be measured by xerographic techniques which are very sensitive to relatively small changes in trapped-space-charge density. Xerographic techniques can also be used to monitor and compare the release rates of carriers trapped in photosensitized and dark-rested samples. Measurements have been made on glasses in the As: Se: Te system but the present results are restricted to $a$-Se, which is ambipolar. There are four principal results: (1) Distinct decay times for the persistence of photoenhanced metastable hole and electron deep trapping are observed. Metastable electron traps decay away more slowly than their hole counterparts. (2) Photosensitization of deep trapping in $a$-Se is a bulk effect which exhibits a characteristic wavelength and exposure dependence. (3) In both dark-rested and photoexcited samples, average release time of trapped electrons is larger than average release times of trapped holes. (4) ESR is not observed in either the photoexcited or dark-rested samples at room temperature. Results are discussed phenomenologically and in terms of a specific low-energy point-defect model proposed for chalcogenides.