Defects and hydrogen in amorphous silicon nitride

The behaviour of defects in amorphous silicon nitride (a-Si3N4) and the hydrogenated amorphous silicon nitride (a-SiNx: H) alloys are described. The main defects in a-Si3N4 are the Si and N dangling bonds (DBs). The Si DB forms a sp3 state near midgap, while the N DB forms a highly localized pπ level just above the valence-band edge. The behaviour of the alloys changes near x ≈ 1.1, the percolation threshold of Si-Si bonds. In the Si-rich alloys (x<1.1), both band edges are Si like, only Si DBs are seen and their density is controlled by equilibration with weak Si-Si bonds. The x>1.1 alloys behave largely like silicon nitride; the valence band changes towards N pπ like, both Si and N DBs can arise and the Si DB can have a high density and prefers to be in its charged diamagnetic configurations. The density of defects in a-SiN x : H, via the equilibration with weak Si-Si bonds, depends on the diffusion coefficient of hydrogen. A theory of H diffusion is developed which shows that the optimum deposition temperature varies linearly with the bandgap of the alloys.