Semi-empirical calculation of the optical spectra of silica clusters in spark-processed silicon

Substantial part of the volume of the luminescent spark-processed silicon (sp-Si) material consists of amorphous SiO2. The nitrogen atoms incorporated in the material substantially influence its optical properties. In order to understand the photoluminescence (PL) behavior of sp-Si, we calculated UV/visible optical absorption spectra of a large variety of silicon-oxynitride clusters and compared them with experimentally measured absorption spectra of sp-Si. All structures in our study were optimized with the AM1 method. The optical absorption spectra were calculated using the quantum-mechanical Intermediate Neglect of Differential Overlap (INDO) method together with configuration interaction, which was parameterized for Si. The experimentally measured absorption spectrum of sp-Si shows electron transitions at 245, 277, 325 and 389 nm. Our theoretical results indicate that the transition at 245 nm can be attributed to certain specific nitrogen-substituted SiO2 structures, in which the double SiO bond plays a crucial role.