Dynamics of laser sputtering at GaN, GaP, and GaAs surfaces

Laser sputtering of GaAs, GaP, and GaN has been studied by measuring angle-resolved time-of-flight (TOF) distributions of emitted neutral particles. The observed mean translational energy and sputtering yield were found to show strong forward peaking depending on the laser power densities, thence on the desorption yield. This fact can be explained in terms of postdesorption collisions among desorbed particles as predicted theoretically [I. NoorBatcha, R. R. Lucchese, and Y. Zeiri, J. Chem. Phys. 86, 5816 (1987); 898, 5251 (1988), and Kelly and R. W. Dreyfus, Surf Sci. 198, 263 (1988); Nucl. Instr. Methods B 32, 341 (1988); J. Chem. Phys. 92, 5048 (1990)]. However, the observed velocity distributions of sputtered particles were found to be in contradiction with the so-called shifted Maxwellian, because the best-fitted center of mass velocities for Ga were always found to be negative. In addition, Kelly's Mach number M, estimated from the energy spectra, was found to be significantly larger than those estimated from the yield spectra. It is suggested that energy transfer from the internal states of diatomic molecules to the translational motion of Ga may play an important role in the final TOF spectra.