Room temperature oxidation of GaAs(110) using high translational kinetic energy molecular beams of O2 visualized by STM

This study examines the reactive surface dynamics of GaAs(110) oxidation with molecular oxygen at room temperature over a range of impinging kinetic energies. Visualization of the surface by scanning tunneling microscopy (STM) after exposures to O-2 with kinetic energies of 0.4-1.2 eV provides morphological and kinetic data that were obtained utilizing a novel instrument that combines a supersonic molecular beam with an in-line, in-situ STM. Oxidation was found to proceed by two morphologically distinct, competing mechanisms: a spatially homogeneous process with randomly distributed chemisorbed oxygen atoms leading to layer-by-layer oxide growth, and a spatially heterogeneous process with oxides nucleating on structural surface defects and growing vertically and laterally with continued exposure. Both oxidation mechanisms exhibit enhanced reactivity with increasing kinetic energy. Only trace oxidation was observed with O-2 kinetic energies below 0.7 eV; a rapid increase in the rate of oxidation from 1.0 to 1.2 eV was found with homogeneous and heterogeneous oxidation proceeding simultaneously until full surface coverage was reached. In addition, the relative rates of the two mechanisms appear to change with O-2 kinetic energy: spatially homogeneous oxidation is expected to dominate at lower kinetic energies (< 0.7 eV) while the heterogenous growth of oxide islands increasingly dominates with higher kinetic energies (>= 1.0 eV). The results obtained in this study conclusively demonstrate that a heterogenous oxidation mechanism is activated on GaAs(110) at high O-2 kinetic energies, and reveal that thin oxide layers can be achieved with higher efficiency at room temperature using molecular beams of oxygen. These results provide vital information about the morphological evolution of the surface in conjunction with the overall kinetics, and identify a controlled method of enhanced oxidation at moderate temperatures that could potentially improve abruptness at oxide interfaces and be used in the fabrication of GaAs semiconductor devices.