Band-Tail Characteristics in Amorphous Semiconductors Studied by the Constant-Photocurrent Method

Optical absorption edges in five chalcogenide glasses, Se, As 2 S(Se) 3 , and GeS(Se) 2 , and a-Si:H have been measured using the constant-photocurrent method at 10–400 K. In the chalcogenide glasses the weak-absorption tail detected by the method is much smaller than that evaluated from transmission spectra, while in a-Si:H the magnitudes are similar. Photoconductive Urbach tails in all the chalcogenide glasses blue-shift from the optical Urbach tails, and marked non-photoconducting spectral gaps appear. The width and the residual absorption of the spectral gap depend upon the dielectric constant and the average coordination number of atoms, respectively. These characteristics of the chalcogenide glass are discussed in comparison with those in a-Si:H and crystalline chalcogenide materials.