| Spark-processed silicon (sp-Si) is an amorphous, luminescent material that exhibits emission spectra, peaked around 385, 525 and 650 nm. In order to understand the photoluminescence (PL) behavior of sp-Si, UV/visible optical absorption spectra of a large variety of silicon-based molecular clusters were calculated and compared with experimentally measured absorption spectra of sp-Si. All structures in this study were optimized with the AM1 or the PM3 method. The optical absorption spectra were calculated using the quantum-mechanical INDO method together with configuration interaction, which was parameterized for Si.; The experimentally measured absorption spectrum of sp-Si exhibits peaks at 245, 277, 325 and 389 nm. In an attempt to reproduce this spectrum computationally, the present work includes a detailed study of the optical properties of silica clusters, Si rings, Si clusters and cages, Si oxides and oxynitrides.; While the spectra of silica, Si rings and OH-terminated Si clusters resemble certain features of the experimental absorption spectrum of sp-Si, remarkable agreement is achieved in the case of a Si cluster, surrounded by an amorphous matrix. The agreement improves when the size of the model Si cluster increases from Si3 to Si14. χ2 statistical analysis of the calculated spectra shows that the presence or absence of N influences the optical properties of such complexes.; Silicon spark-processed in pure O does not photoluminesce when excited by laser light having a wavelength of 325 nm. The absorption spectrum of this material differs from the one obtained on Si spark-processed in air. The statistical analysis comparing various cluster models to the two spectra shows that certain Si cluster sizes are favored in the case of Si processed in air, while this is not true for Si processed in O. |
