Dissociative Adsorption of Water on an H2O/GaAs(100) Interface: In Situ Near-Ambient Pressure XPS Studies

Dissociative water adsorption on a GaAs(100) surface is demonstrated using X-ray photoelectron spectroscopy (XPS) carried out in situ under near-ambient conditions. These in situ XPS studies enable us to monitor the evolution of water dissociation on GaAs under elevated pressures and temperatures for the first time. In pressure-dependent XPS studies, the GaAs surface was exposed to room-temperature water vapor at pressures ranging from UHV to 5 mbar. With an increase in H2O pressure, enhancement of oxygenation and hydroxylation of surface Ga atoms is observed. In addition, strong XPS signals are also detected that originate from water molecules that are molecularly adsorbed onto the GaAs surface and hydrogen-bonded to surface OH species. Peak broadening and shifts in the As 2p XPS spectra are correlated with the signal increase resulting from surface HO-Ga species, suggesting the formation of surface H-As bonds. In temperature-dependent XPS studies, the GaAs surface was annealed from room temperature up to 773 K at a water vapor pressure of 0.1 mbar. The temperature increase leads to desorption of weakly adsorbed water molecules or their dissociation, resulting in surface Ga oxidation. However, no changes in chemical state of the surface As atoms are observed at higher temperatures.