Defect distributions in a-SixGe1-x:H

Gap states in a-SiGe:H alloys were examined by numerical simulations of sub-bandgap absorption spectra measured by the constant photocurrent method and photothermal deflection spectroscopy. In contrast to simple deconvolution methods our analysis uses occupation statistics and takes into account the condition of charge neutrality. The simulations yield information on the energy distribution and the charge state of the defects. The results reveal the coexistence of charged and neutral defects. The defect distributions are similar to those found in amorphous hydrogenated silicon. In the investigated range of compositions charged states dominate the defect density. Taking the position of the defect states as a reference level both band edges shift towards the defect states with decreasing band gap. In contrast to electron spin resonance measurements, no evidence for a distinction between Si-related and Ge-related defect states can be found in sub-bandgap absorption spectra. (C) 1998 Elsevier Science B.V. All rights reserved.