Oxidation of GaN{0001}-11 surfaces at room temperature

The interaction of unexcited oxygen molecules with clean GaN{0001}-11 surfaces was investigated using X-ray photoemission spectroscopy (XPS), Auger electron spectroscopy (AES), and low-energy electron diffraction (LEED). Clean surfaces were prepared by a HF dip followed either by desorption of Ga films deposited at room temperature or by nitrogen-ion bombardment and annealing. During exposures in the range from 0.3 up to 10¹⁵ L-O₂ any excitations of the oxygen were avoided. Oxygen coverages determined from the XPS and the AES data differ by a factor of two. The larger XPS-derived coverages are considered to be more reliable since the AES signals decayed during data recording. The oxygen uptake takes place in two consecutive stages. The first one is identified as dissociative chemisorption and the second one is tentatively attributed to field-assisted diffusion by the Mott-Cabrera mechanism. The dissociative chemisorption is characterized by an initial sticking coefficient of and a saturation coverage of monolayers that is reached after exposures of 10³ L-O₂. The second mechanism sets in at exposures to 10⁸ L-O₂ but reaches no saturation even with the largest doses applied. Received 4 September 1998 and Received in final form 6 November 1998