Kinetics of the photostructural changes in a-Se films

The kinetics of the photodarkening effect has been studied experimentally for amorphous selenium (a-Se) layers at room temperature and at an elevated temperature (35 °C) close to the glass transition. By switching an intense pumping light on and off with a period of 100 s, we have studied the kinetics of both the buildup of photodarkening and its relaxation (recovery). It was found that at 35 °C, only a reversible component of photodarkening has been observed. This result has been interpreted within the framework of a phenomenological model assuming that photodarkening is caused by light-induced transitions of structural units from their ground states into metastable states. Our estimate for the energy barrier EB between these states obtained for the photodarkening process (EB ∼0.8 eV) coincides with that obtained from the analysis of the relaxation process. At room temperature, an irreversible component of photodarkening has been observed along with the reversible one. The energy barrier responsible for the relaxation of the reversible component at room temperature appears the same as at 35 °C. This suggests that the energy barrier identified represents a fundamental feature of the photoinduced structural metastability in amorphous selenium.