Model for growth of a-Si:H and its alloys

Alloys of a-Si:H with Ge, C, N, O, etc., have higher defect densities and poorer electronic properties than a-Si:H. Recent ideas of hydrogen equilibration during growth are used to explain the structural trends in the alloys responsible for the different electronic properties. The analysis finds two characteristically different types of behavior, distinguished by the hydrogen-bond strengths of the alloy elements. Type-1 alloys have predominantly silicon dangling bonds and a large disorder broadening of the valence-band tail, while type-2 alloys have dangling defects on the nonsilicon element and low disorder. Examples are a-Si:C:H and a-Si:Ge:H, respectively. The same approach to understanding the electronic structure is applied to alloys with halogens such as fluorine, to the different elemental hydrogenated amorphous semiconductors, and to microcrystalline thin films.