Defect Distributions in Doped and Undoped A-SiGe:H Alloys

Abstract

ABSTRACTIn this work, gap states in doped and undoped a-SiGe:H alloys are examined by numerical simulations of sub-bandgap absorption spectra measured by the constant photocurrent method and photothermal deflection spectroscopy. Deconvolution methods, neglecting the condition of charge neutrality, can be used for a rough estimate of the defect density value but not for ob- taining detailed information on the distribution of gap states in undoped samples. Our numerical analysis uses adapted occupation statistics and takes into account the condition of charge neutrality. Good agreement between measured and simulated PDS and CPM spectra is obtained. For a certain composition, i.e. a certain bandgap, the investigation of doped films yields infor- mation on the density and the position of charged defect states in the bandgap. In addition, the density of neutral defect states can be derived from a comparison of CPM and PDS spectra. The results reveal the coexistence of charged and neutral defects. In doped as well as in undoped films, charged defect states dominate the defect density. In the investigated range of compo- sitions the defect distribution of a-SiGe:H is similar to those found in a-Si:H. The width of the defect distributions does not decrease with decreasing bandgap. No evidence for a different be- havior of Si- and Ge-related defect states can be found in sub-bandgap absorption spectra.