Optically induced metastable defects in a-As2S3

The properties of the metastable PL (PL2) centers, which are generated by band-gap irradiation at low temperature, are studied. The peak energy of the PL2 emission spectrum and the intensity of PL2 are only weakly dependent on the excitation energy from 1.6 to 2.2eV although the absorption coefficient has exponential energy-dependence in the range. This suggests that PL2 occurs by the excitation from extended continuous states to localized defect states or vice versa . From the position of the shoulder of the excitation spectrum, the energy level of the PL2 centers is estimated to be 1.42eV. The processes of the defect creation are also studied. The fatigued PL centers after long-time band-gap irradiation cannot be bleached out by annealing at 220K nor by irradiation with below-gap light while those after short-time irradiation can be bleached out. The same effects have been observed for the PL2 centers. We found out that the component that can not be easily bleached is proportional to the square of the total number of the induced centers. This suggests that the defect sites attacked by photons twice or more is difficult to be bleached out.