Abstract
The electronic structure and the optical properties of α-Si3 N4, β-Si3 N4, and Si2 N2O crystals are studied by means of first-principles local-density calculations. Together with the earlier published result on α-SiO2, a comparative study of the electronic bonding and optical excitations in these crystals becomes possible. It is found that the electronic structure of the β and the α phases of Si3 N4 are similar, but with some subtle differences that can be traced to different stacking sequences of the atomic layers and the slightly different local bonding structure. The electronic structure of the Si2 N2O crystal cannot be adequately described as a simple superposition of Si3 N4 and α-SiO2. The presence of both Si-N and Si-O bonds in Si2 N2O crystals affects the local potential and results in a downward shift of the O levels. The calculated density of states (DOS) and the orbital-resolved partial DOS are in good general agreement with the photoemission and x-ray emission measurements. Better resolution in the calculated DOS shows that the upper valence band has five peaks rather than three.

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