THEORY ON LASER SPUTTERING BY HIGH-DENSITY VALENCE-ELECTRON EXCITATION OF SEMICONDUCTOR SURFACES

A theory on sputtering by sub-bandgap lasers is developed on the presumption by Itoh and Nakayama that rupture of a surface bond by two-hole localization leads finally to ejection of an atom from the surface due to Coulombic repulsion between the two holes and lattice relaxations. The lasers excite electrons in the valence band to unoccupied intra-gap surface states, giving rise to free holes in the valence band and surface negative charges which attract holes and let them move two-dimensionally along the surface. The theory is applied to calculate the ejection efficiency per laser pulse of Ga neutrals from the intrinsic Ga sites and that of Ga cations from defect sites of Ga adatoms on the GaP(110) surface. It is predicted that both of them should increase exponentially with the laser intensity when it exceeds a certain critical value, although the former should increase only quadratically and the latter only linearly when the laser intensity does not exceed this value. The former is much smaller than unity, meaning that the sputtering yield of Ga neutrals does not fade with the repetition of laser pulses, while the latter is not much smaller than unity, meaning that the sputtering yield of Ga cations fades rapidly with repetition, in agreement with experiment. Abrupt superlinear rise of the sputtering yield observed can be reproduced in the region of exponential increase of the efficiencies.