Spontaneous ripple formation due to ultraviolet surface oxidation of semiconductors

Many laser microchemical processes are influenced by interactions between surface electromagnetic waves, which are excited by laser radiation, and wavelength-scale surface morphology. This effect has not been extensively documented in laser processing of semiconductors in which low laser intensities have been used. The authors report the formation of subwavelength ripples on semiconductor surfaces during low-power cw UV laser illumination. Specifically, the authors observed these effects in the laser aqueous etching of Si, GaAs, and InP crystals and in the laser-induced oxidation of GaAs. As described previously, the etching and oxidation reaction are induced by UV photogeneration of electron-hole pairs near the semiconductor surface. A low-power cw UV laser source (257 nm), obtained by frequency doubling of the output of a 514-nm Ar/sup +/ laser, was focused (3-5-μm diameter) to moderate intensity levels, i.e. 0.1-10 W/cm/sup 2/, on the semiconductor surface. The substrate was immersed into an etching solution or, in the case of the oxidation, exposed to the atmospheric ambient. Laser heating of the room-temperature substrates is <10C