The influence of temperature on the optical absorption edge shift induced by band‐gap illumination in thin amorphous GeSe₂ films

Abstract

The photoinduced shift in the optical gap (E _g) was studied in amorphous GeSe₂ thin films at temperatures T = 77 K, 300 ? and 400 K. It was found that at T = 77 K, both virgin and well‐annealed films are darkened by band‐gap illumination. By annealing the film up to room temperature, the darkening is reduced and the film slowly relaxes to a state close to the initial one. Room‐temperature illumination results in a blue shift of the gap. The process can be described by first‐order kinetics. Annealed films, when illuminated at room temperature, are ‘quasireversibly’ darkened, meaning that after annealing of the previously darkened film, a small increase of the gap was observed in comparison with the preceding annealed state. The light‐saturated state at ? = 400 ? has been found to be more ordered, and a blue shift of the gap was observed, in contrast with the well‐annealed state. It is speculated that the photoinduced changes have their origin in self‐trapped exciton‐like defects and random‐pair formation. An equilibrium between these defects is most probably determined by both the temperature and light intensity. At higher annealing temperatures (T _A), these defects (induced by illumination) probably exist as short life‐time intermediate states during the transition of the initial state of the film to a more ordered state, which otherwise can probably be reached by annealing at higher T _A, i.e. at T _A close to the glass‐transition temperature. Thus, as proposed by Fritzsche (1993, Phil. Mag. B, 68, 561), recombination bond‐induced rearrangements could be responsible for the observed light‐induced changes of the gap.