The influence of temperature on the optical absorption edge shift induced by band-gap illumination in thin amorphous GeSe2 films
Faculty of Applied Economics
MECHANICS ELECTRONIC OPTICAL AND MAGNETIC PROPERTIES
Philosophical magazine: B: physics of condensed matter: electronic, optical and magnetic properties. - London, 1978 - 2002
, p. 213-221
University of Antwerp
The photoinduced shift in the optical gap (E(g)) was studied in amorphous GeSe2 thin films at temperatures T = 77 K, 300 K 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 him, a small increase of the gap was observed in comparison with the preceding annealed state. The light-saturated state at T = 400 K 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.