TIGHT-BINDING MODEL-CALCULATIONS OF THE NONLINEAR-OPTICAL RESPONSE OFFREE AND HYDROGENATED SI(111) SURFACES

A tight-binding model involving a third-nearest neighbour parameteriza tion of the electronic Hamiltonian is applied in a calculation of the second-order nonlinear susceptibility of a slab representing an ideal unreconstructed Si(111) surface. The dispersion of the linear optical properties is reasonably well reproduced by this approach. The nonline ar susceptibility shows qualitatively similar features already known e xperimentally for reconstructed surfaces. For adsorption of a monolaye r of hydrogen atoms \chi((2))\ decreases typically by an order of magn itude from the free-surface value. The different elements of the chi(( 2)) tenser are found to be of comparable magnitude with existing data for a reconstructed surface if the effect of the microscopic fields is accounted for.