MICROSCOPIC THEORY OF 2ND-HARMONIC GENERATION AT SI(100) SURFACES

We apply a microscopic formulation to calculate the second harmonic sp ectra of clean and hydrogenated Si(100) surfaces. We find a trend of t he surface-allowed Ei resonance as a function of the hydrogen coverage , which is consistent with experiment. Another resonance at lower ener gy, recently observed at the clean surface, is also reproduced and exp lained in terms of transitions across surface states. Its quenching is clearly demonstrated as a function of hydrogen coverage. Our results for the energetically more favorable c(4 X 2) structure are in better agreement with low temperature experimental data than those calculated for the 2 X 1 reconstruction. We find that the spectra are dominated by the chi(III perpendicular to)(S) component of the second-order nonl inear surface susceptibility. [S0031-9007(98)07375-X].