Dielectric functions and electronic band states of a-Si and a-Si:H

We have measured dielectric spectra of unhydrogenated (a-Si) and hydrogenated (a-Si:H) amorphous silicon films grown by dc magnetron sputtering deposition, using in situ spectroscopic ellipsometry. We have determined the bulk dielectric functions, using information on the film surface and bulk morphology obtained from in situ real-time ellipsometry. Compared with the a-Si film, the a-Si:H films display a decrease in the peak height of the imaginary part of the dielectric spectrum and a shift in the peak position toward higher photon energy. We derive the density of states in both the valence band (SiSi states) and conduction band (SiSi and SiH states) by fitting the bulk dielectric spectra. Hydrogenation reduces the density of states near the valence-band edge, and increases those deeper in the valence band. In addition, SiH antibonding states are observed about 1.5 eV above the conduction-band edge. The distribution of these states broadens as the hydrogen content increases.