Photoconducting Urbach edge in amorphous Se

Abstract Photoconductivity spectra in amorphous Se have been analyzed on the basis of the geminate-pair model. It predicts that around the Urbach edge the photoconductivity spectrum exhibits also an exponential variation ~ exp( hν / E U p ) with the steepness parameter given as E U p ≈ k B T , which may be universally held in other materials. In amorphous Se films, however, the exponential variation tends to be masked by below-gap responses, the origin being speculative. We also discuss the photoconductivity spectra in super-gap regions.