We present an extensive study of the electronic properties of amorphous silicon nitride for a wide range of N concentrations. Local densities of states on Si and N for the ‘‘ideal’’ random network structure, defect states introduced into the semiconducting gap by both Si and N dangling bonds, the effect of wrong bonds (N—N bonds below stoichometry and Si—Si bonds above stoichiometry) on the local density of states, and, the energy dependence (from 0 to 10 eV) of the imaginary part of the dielectric function have been obtained as a function of N content. The main purpose has been the development of a theoretical model for amorphous compound that were both internally consistent and in overall agreement with experiment. We stress the following main results: (i) The semiconducting gap opens continuously from pure a-Si to the stoichiometric compound a-${\mathrm{Si}}_{3/7}$${\mathrm{N}}_{4/7}$. For low N content the gap widening is linear in x/(1-x) whereas, just before stoichiometry is reached, the gap opens very sharply to its maximum value. (ii)

The sharp opening of the gap when stoichiometry is achieved is due to the narrowing and final disappearance of a band of localized states produced by the presence of Si—Si bonds in the random network. This band is centered about the energy of an isolated Si—Si defect and both its width and its height decrease as stoichiometry is approached. (iii) A narrow band originated in the N ‘‘lone-pair’’ bonds is always present in the upper part of the valence band. This N ‘‘lone-pair’’ band defines the valence-band maximum of the stoichiometric semiconductor. For N contents above stoichiometry the band broadens and the gap becomes smaller. (iv) Below stoichiometry, the ‘‘optical gap’’ energy closely follows the behavior of the spectral gap. Nevertheless, in spite of the gap reduction that takes place above stoichiometry, the optical gap energy remains large due to the small cross section of optical excitations from lone-pair states to conduction-band states. (v) Both Si and N dangling bonds introduce localized states into the gap. Their energy levels show very small dependence of the N concentration. N states lie 1–2 eV lower in energy than Si states. (vi) Finally, our work shows that further experimental and theoretical investigation is needed to clarify several unknown properties of this semiconducting material.

• Received 31 October 1986

DOI:https://doi.org/10.1103/PhysRevB.35.9683