ELECTRONIC-STRUCTURE OF SILICON-NITRIDE

This paper reviews the bonding and electronic structure of silicon nitride and amorphous silicon-nitrogen alloys (a-SiN(x)). The conduction band edge of Si3N4 is largely Si s like and is relatively unaffected by disorder, while the valence-band edge is formed of N p-pi states which can tail into the gap because of N-N p-pi interactions. The gap of the amorphous SiN(x) alloys is calculated to open up roughly symmetrically about midgap as x --> 4/3, rather than asymmetrically as suggested by photoemission experiments. Criteria are developed for judging the quality and stoichiometry of silicon nitride from the optical spectra. Residual Si-Si bonds are shown to broaden the absorption edge of even nitrogen-rich plasma-deposited silicon nitride. Both silicon and nitrogen dangling bond defects have recently been observed in silicon nitride. The energy levels of these and of Si-Si, Si-H and N-H bonds are calculated. The correlation energy of the silicon dangling bonds is discussed. The increase in defect density with increasing nitrogen content in plasma-deposited alloys is attributed to the breaking of Si-Si bonds which make up an increasingly wide valence-band tail.