Optical gap and photoluminescence properties of amorphous silicon alloys

The effect of alloying of hydrogenated amorphous silicon (a-Si:X:H, with X = O, N or C) on low-temperature photoluminescence, and: the optical properties is described and discussed. It is found that, for all alloys with a wider optical gap than a-Si:H, that is a-Si:O:H, a-Si:N:H and a-Si:C:H, the photoluminescence energy and the linewidth increase with increasing optical gap in the same way. Alloying with germanium leads to a decreasing optical gap. Differences between the wide-bandgap alloys and a-Si:Ge:H are observed when the changes in the optical gap, the photoluminescence energy, the linewidth and the slope of the absorption edge are compared. A surprising result is, however, that the difference in the optical gap and the photoluminescence energy is related to the photoluminescence linewidth and the slope of the band tail in the same way for all alloys. In a limited energy range, hydrogen has a similar effect on the optical gap as oxygen, nitrogen and carbon but the relation between the optical gap and the photoluminescence energy and linewidth is different. The data point to a contribution of two effects: the distribution of carriers in energy and space due to disorder and the electron-phonon interaction. The effect of alloying can be regarded as the replacement of Si-Si bonds by Si-X bonds with bonding and antibonding energies deep in the valence and conduction band respectively. Hydrogen seems to act in a similar way but the density of Si-H bonds is likely limited by the formation of polysilanes.