Optical absorption and defects in amorphous SiN_x and SiO_x

Abstract

Photothermal deflection spectroscopy has been used to measure optical absorption for two series of amorphous silicon-based alloys prepared by the decomposition of silane/ammonia or silane/nitrous oxide mixtures to produce a-SiN_x or a-SiO_x respectively. Measurements have been made in the energy range 1 to 5 e V. Each sample shows a well-defined exponential edge and a low-absorption region. The inverse slope of the exponential edge increases monotonically with nitrogen or oxygen content. The low-absorption region is analysed and discussed in terms of the defect-state density. For the silicon nitride samples the results can be interpreted in terms of silicon dangling-bond defect states. On the incorporation of some nitrogen the density of defect states increases, but the defect remains at a certain energy above the valence band edge. As stoichiometry is approached the number of such states decreases. For the silicon oxide samples, oxygen incorporation results in a decrease in the density of silicon dangling-bond states and the appearance of states associated with an oxygen vacancy.