Carrier excitation by atomic collision on GaAs(110)

We have measured the excitation yield for carriers produced at the outermost atomic layer of a GaAs(110) surface by impinging hyperthermal Xe and Kr neutral atoms. The excitation yield roughly scales as exponential in the inverse of the mean energy deposited in the collisions. This is consistent with previous results for InP, which indicated a rapid electronic equilibration with the local lattice kinetic energy dissipated in the vicinity of the atomic impact. However, for GaAs(110) the maximum in the excitation yield, as a function of incident angle, is not normal to the surface. Instead, the maximum yield is found for polar angles nearly perpendicular to the tilted GaAs surface atom bonds. In addition the yield is two orders of magnitude greater than that estimated from equilibrium arguments. These results are in apparent disagreement with the simplest ideas of local equilibration, and may provide the first insights into the collisional excitation mechanism of electrons by neutral atoms.