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 photoemis sion spectroscopy. H2O adsorbs dissociatively, with a total sticking probab ility of greater than or equal to 0.45, up to a saturation coverage of ca. 0.46 monolayers. In contrast O-2 exposure gives about the same saturation c overage but with a total sticking probability of similar to 2.7 x 10(-3). S urface states at the valence band maximum are removed with, however, only a small apparent reduction in upward band bending (which may be influenced b y surface photovoltage). An additional density of states, extending into th e gap, is not removed by exposure to H2O or O-2 and may be derived from bul k defects. Changes in the valence band and in the electron affinity suggest that annealing at similar to 200 degrees C decomposes adsorbed OH to form O and, presumably, H. The resulting surface appears to be essentially the s ame as that formed by O-2 exposure at room temperature. Further annealing ( to similar to 500 degrees C) of the surface with chemisorbed O, from either OH decomposition or O-2 chemisorption, causes additional changes in the sp ectra which may represent the conversion of chemisorbed O to an oxide-like phase. Evidence is found for a small (similar to 0.1 eV) surface photovolta ge effect on band bending for the clean surface which increases to similar to 0.33 eV for the H2O-exposed surface after annealing. (C) 2000 Elsevier S cience B.V. All rights reserved.