Molecular electronic structure theory in the study of localised defects

Abstract

Deep-level defect centres occupy a unique position intermediate between isolated molecules and the solid state. Their electronic wavefunctions are often strongly localised about the defect site and resemble small-molecule orbitals; but they are also embedded in an extended host, and they interact with the host matrix. The authors discuss simple concepts and models from molecular electronic structure theory which are equally useful in the study of localised defect centres. First they outline the construction and use of Walsh diagrams, which predict conformations and conformational change as a function of electron number, electronic excitation and atomic orbital energy (electronegativity). Second, they discuss in detail the approximations of the widely used local-hybrid analysis of ESR and ENDOR spectra, point to the situations where those approximations are likely to fail and give an explicit comparison of theoretical calculations at several levels of sophistication for a particular molecular analogue, the silyl radical SiH3. In both cases they concentrate on broken-bond systems of the form AB3, which represent the most important intrinsic defects in silicon and silica. The importance and utility of molecular analogies in the study of defect centres are emphasised throughout.