Atomic-structure characterization of a H:GaAs(110) surface by time-of-flight ion-scattering spectrometry

We have used ion-scattering spectrometry with time-of-flight analysis (TOF) to study the atomic structure of a GaAs(110) surface exposed to atomic hydrogen. The TOF spectra of ions plus neutrals acquired for 6 keV Nei backscattering from both As and Ga top-layer atoms show a strong dependence on the projectile incidence direction and on the hydrogen exposure. The variations in the quasisingle backscattered intensity derived from the TOF spectra were analyzed with a code that calculates shadowing and focusing regions in a two-atom model. Measurements as a function of the hydrogen exposure indicate that the surface derelaxes approaching the ideal bulk termination. At the beginning of the adsorption the fraction of the surface that undergoes derelaxation increases very fast with exposure. At higher exposures, the rate of derelaxation becomes slower and the TOF spectra tend to a steady state. From Ne quasisingle backscattered intensities, measured as a function of the incident angle, we have determined that the spacing between the top As and Ga layers is reduced from (Delta Z = 0.66 +/- 0.08) Angstrom for the clean surface to (Delta Z = 0.0 +/- 0.08) Angstrom for intermediate exposures (2000 L H-2). At this exposure we have found a spacing between the first and second layers of (1.8 +/- 0.1) Angstrom. At higher exposures the results indicate a small counter-relaxation.