Photoconductivity studies of discrete defect levels in amorphous chalcogenides

The disordered nature of the amorphous semiconductors results in the presence of a significant density of localised electronic states in the energy gap between their valence and conduction bands. In the chalcogenide materials this leads to band edges being widened into tails of localised states with densities well above 10¹⁶ cm^-3eV^-1 as far as 0.5 to 0.7 eV into the gap. Discrete defect levels, caused by specific co-ordination defeds, often have a lower density and remain, consequently, frequently undetected by optical spectroscopies. Through the use of photoconductivity techniques, where transport and recombination processes follow the initial optical absorption, it becomes possible to locate some of those discrete defect levels.