DEFECT DISTRIBUTIONS IN A-SIXGE1-X-H

Gap states in a-SiGe:H alloys were examined by numerical simulations o f sub-bandgap absorption spectra measured by the constant photocurrent method and photothermal deflection spectroscopy. In contrast to simpl e deconvolution methods our analysis uses occupation statistics and ta kes into account the condition of charge neutrality. The simulations y ield information on the energy distribution and the charge state of th e defects. The results reveal the coexistence of charged and neutral d efects. The defect distributions are similar to those found in amorpho us hydrogenated silicon. In the investigated range of compositions cha rged states dominate the defect density. Taking the position of the de fect states as a reference level both band edges shift towards the def ect states with decreasing band gap. In contrast to electron spin reso nance measurements, no evidence for a distinction between Si-related a nd Ge-related defect states can be found in sub-bandgap absorption spe ctra. (C) 1998 Elsevier Science B.V. All rights reserved.