Chemisorption of H2O on GaN(0001)

The chemisorption of H2O on the clean Ga-polar GaN(0001) surface near room temperature has been studied using mainly synchrotron ultraviolet photoemission spectroscopy. H2O adsorbs dissociatively, with a total sticking probability of ≥0.45, up to a saturation coverage of ca. 0.46 monolayers. In contrast O2 exposure gives about the same saturation coverage but with a total sticking probability of ∼2.7×10−3. Surface states at the valence band maximum are removed with, however, only a small apparent reduction in upward band bending (which may be influenced by surface photovoltage). An additional density of states, extending into the gap, is not removed by exposure to H2O or O2 and may be derived from bulk defects. Changes in the valence band and in the electron affinity suggest that annealing at ∼200°C decomposes adsorbed OH to form O and, presumably, H. The resulting surface appears to be essentially the same as that formed by O2 exposure at room temperature. Further annealing (to ∼500°C) of the surface with chemisorbed O, from either OH decomposition or O2 chemisorption, causes additional changes in the spectra which may represent the conversion of chemisorbed O to an oxide-like phase. Evidence is found for a small (∼0.1 eV) surface photovoltage effect on band bending for the clean surface which increases to ∼0.33 eV for the H2O-exposed surface after annealing.