Above-band-gap emission in amorphous semiconductors: Localized states versus surface contamination

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Abstract
The nature of the above-band-gap emission which has been reported by Shah and co-workers in a number of amorphous semiconductors is investigated. The remarkable features of this emission have been attributed to unique properties of the electronic states in these materials. Similar features have been observed in other materials, however, where they were ascribed to contamination effects. Using the Raman phonon spectrum for calibration, this study finds the intensity of the emission from amorphous silicon to be sensitive to surface cleaning procedures. This suggests that the above-band-gap emission may be due to surface contamination, and unrelated to bulk states in the material. A simple model describing the optical properties of a mixture of organic contaminants is proposed which is capable of reproducing the essential features.