Density functional theory study of the structural and electronic properties of amorphous silicon nitrides: Si<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msub><mml:mrow /><mml:mn>3</mml:mn></mml:msub></mml:math>N<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msub><mml:mrow /><mml:mrow><mml:mn>4</mml:mn><mml:mo>−</mml:mo><mml:mi>x</mml:mi></mml:mrow></mml:msub></mml:math>:H

We present ab initio density functional theory studies for stoichiometric as well as nonstoichiometric amorphous silicon nitride, varying the stoichiometry between Si${}_{3}$N${}_{4.5}$ and Si${}_{3}$N${}_{3}$. Stoichiometric amorphous Si${}_{3}$N${}_{4}$ possesses the same local structure as crystalline Si${}_{3}$N${}_{4}$, with Si being fourfold coordinated and N being threefold coordinated. Only few Si-Si and N-N bonds and other defects are found in stoichiometric silicon nitride, and the electronic properties are very similar to the crystalline bulk. In over-stoichiometric Si${}_{3}$N${}_{4+x}$, the additional N results in N-N bonds, whereas in under-stoichiometric Si${}_{3}$N${}_{4\ensuremath{-}x}$ the number of homopolar Si-Si bonds increases with decreasing N content. Analysis of the structure factor and the local coordination of the Si atoms indicates a slight tendency towards Si clustering, although at the investigated stoichiometries, phase separation is not observed. In the electronic properties, the conduction-band minimum is dominated by Si states, whereas the valence-band maximum is made up by lone pair N states. Towards Si rich samples, the character of the valence-band maximum becomes dominated by Si states corresponding to Si-Si bonding linear combinations. Adding small amounts of hydrogen, as typically used in passivating layers of photovoltaic devices, has essentially no impact on the overall structural and electronic properties.