Electronic transitions associated with small crystalline silicon inclusions within an amorphous silicon host

Abstract

Amorphous silicon films were prepared by dc reactive magnetron sputtering under a range of hydrogen or deuterium partial pressures approaching the phase transition to full microcrystallinity. Tunneling electron microscopy imaging indicated that these films consisted of 5{endash}50-nm-sized Si crystallites embedded in an amorphous silicon matrix. The sub-band-gap optical spectra of these films were recorded using photocapacitance and transient photocurrent spectroscopy. These spectra appear to consist of a superposition of a sub-band-gap spectrum typical of amorphous silicon together with a unique optical transition, with a very large optical cross section, corresponding to valence-band electrons being optically inserted into empty levels lying within the amorphous silicon mobility gap. We believe these empty levels are associated with defect states at the crystalline-amorphous boundary. {copyright} {ital 1999} {ital The American Physical Society}