Separating the effects of hydrogen and bond-angle variation on the amorphous-silicon band gap

The effects of 1-MeV Si⁺ implantation on the optical band gap Eg of pure (a-Si) and hydrogenated amorphous silicon (a-Si:H) have been determined. An identical decrease in Eg of 0.46±{}0.02 eV upon ion implantation is observed for a-Si and a-Si:H. Eg decreases because the valence and conduction bands broaden upon ion irradiation. Raman scattering shows that the broadening of the energy bands is accompanied by an increase in the average bond-angle variation. Hydrogenation results in a separate increase of Eg by 0.22±{}0.02 eV for 11 at. % hydrogen.