DFT Study of Water Adsorption and Decomposition on a Ga-Rich GaP(001)(2x4) Surface

Abstract

We investigate the adsorption and decomposition states of a water molecule on a Ga-rich GaP(001)(2x4) surface using the PBE flavor of density functional theory (DFT). We selected the GaP(001)(2x4) mixed dimer surface reconstruction model to represent the Ga-rich GaP(001)(2X4) surface. Because our focus is on reactions between a single water molecule and the surface, the surface water coverage is kept at 0.125 ML, which corresponds to one water molecule in the (2X4) unit cell. We report here the geometries and energies for an exhaustive set of adsorption and decomposition states induced by a water molecule on the (2x4) unit cell. Our results support a mechanism in which (1) the first step is the molecular adsorption, with the water molecule forming a Lewis acid-Lewis base bond to the sp(2) Ga atom of either the first-layer Ga-P mixed dimer or the second layer Ga-Ga dimers using an addition reaction, (2) which is followed by dissociation of the adsorbed H2O to form the HO/H decomposition state in which the hydroxyl moiety bonds with surface sp(2) Ga atoms, while the hydrogen moiety binds with the first-layer P atom, (3) which is followed by the O/2H decomposition state, in which the oxygen moiety forms bridged Ga-O-Ga structures with surface Ga dimers while one H bonds with the first-layer P atom and the other to surface sp(2) Ga atoms. (4) We find that driving off the hydrogen as H-2 leads to the surface oxide state, bridged Ga-O-Ga structures. This surface oxide formation reaction is exothermic relative to the energy of H2O plus the reconstructed surface. These results provide guidelines for experiments and theory to validate the key steps and to obtain kinetics data for modeling the growth processes.