Oxidation of GaAs(110) withNO2: Infrared spectroscopy

The oxidation of the GaAs(110) surface with ${\mathrm{NO}}_2$ has been studied by Fourier-transform infrared spectroscopy, molecular-beam techniques, and transport measurements. We find that ambient ${\mathrm{NO}}_2$ dissociatively adsorbs with an average low-coverage probability of 0.03 at room temperature and 0.14 at liquid-nitrogen temperature. In contrast, the molecular-sticking probability is greater than 0.5. We have identified the vibrational modes related to oxygen atoms, nitrogen atoms, NO, ${\mathrm{N}}_2$O, and dimerized ${\mathrm{NO}}_2$ adsorbed on the (110) surface. We find significantly different vibrational frequency distributions for oxygen atoms directly deposited on the surface compared to those derived from ${\mathrm{NO}}_2$ dissociative adsorption. Based on a variety of measurements we argue that ${\mathrm{NO}}_2$ dissociation requires steps or defects. Both vibrational spectra and transport properties indicate that a small fraction of the adsorbed oxygen atoms migrates into the bulk of the GaAs crystal, creating an electronic level in the band gap.