Theoretical Studies of Electronic States Produced by Hydrogenation of Amorphous Silicon

First-principles calculations of electronic energies of hydrogenated amorphous silicon have been performed for a series of realistic structural models in which hydrogen appears as SiH, Si${\mathrm{H}}_2$, Si${\mathrm{H}}_3$, ${\left(\mathrm{S}\mathrm{i}{\mathrm{H}}_2\right)}_2$, and SiHHSi (a broken Si-Si with two H atoms). Whereas all these models are consistent with photoemission experiment (less so for Si${\mathrm{H}}_3$), only the last two are found to give band-gap states. The broken-band model is in good agreement with several sets of experiments.