Optical second harmonic spectra of silicon-adatom surfaces: theory and experiment

A recently-developed ab initio theory of second harmonic generation (SHG) s pectroscopy of semiconductor surfaces is used to develop a new 'modified' s emi-empirical tight binding model (SETBM) of SHG spectroscopy. Some key qua ntitative features of experimental SHG spectra for H- and Ge-covered Si(001 ) are not well explained by a simple two-parameter SETBM, despite its succe ss in explaining linear semiconductor surface spectra. The ab initio theory identifies the source of this short-coming as the stronger contribution of optical transitions involving surface atom/adatom states to the SHG respon se, and is then used to quantify additional SETBM parameters describing the se transitions, resulting in a complete, accurate, and convenient descripti on of surface orbital hybridization and experimental results. (C) 2000 Publ ished by Elsevier Science S.A.