Band tails in hydrogenated amorphous silicon and silicon-germanium alloys

The temperature dependence of the conduction- and valence-band tails has been determined by total-photoelectron-yield spectroscopy for doped and undoped a-Si:H and a-SiGe:H alloys. We find that all films possess purely exponential conduction- and valence-band-tail densities of states; however, the characteristic energy of the conduction-band tail increases much more rapidly with temperature than that of the valence-band tail. This indicates that the conduction-band tail is considerably more susceptible to thermal disorder than to structural disorder, whereas the reverse holds for the valence-band tail.