NONLINEAR-OPTICAL SUSCEPTIBILITY OF THE (111)SURFACE AND (001)SURFACEOF SILICON

The nonlinear optical susceptibility chi((2)) for second-harmonic gene ration at the (111) and (001) surfaces of silicon is studied in the fr amework of the self-consistent tight-binding theory. Equilibrium atomi c structures of the (001) and (111) surfaces of silicon are determined by minimizing the total energy of the system. To achieve a better des cription of the conduction band of bulk silicon, a peripheral s state is added to the minimal sp(3) tight-binding basis. The electron-elect ron and electron-ion Coulomb-screened interactions are included by sol ving the secular equation self-consistently. The dependence of the chi ((2)) spectra on the atomic structure of the surfaces of silicon is in vestigated. Intensive response of chi((2)) is found in the spectral re gions corresponding to optical transitions involving surface states. O ptical structures of chi((2)) which correspond to the direct E(1) and E(2) gaps in silicon are also found. The results obtained are discusse d in comparison with the data available in the literature.