X-ray-induced low-temperature oxidation:N2O/GaAs(110)

We have studied the interaction of ${\mathrm{N}}_2$O with GaAs(110) at 25 K as a function of photon beam exposure, using photoemission to detect and characterize reactions. The results show that x rays (hν=1486 and 1253 eV, photon flux 6.8×${10}^9$ and 2.3×${10}^9$ photons ${\mathrm{cm}}^{\mathrm{-}2}$ ${\sec }^{\mathrm{-}1}$) induce ${\mathrm{N}}_2$O dissociation and surface oxidation while ultraviolet photons do not within our detectability limit (hν=21.2 and 40.8 eV). The primary dissociation process for physisorbed ${\mathrm{N}}_2$O involves the attachment of low-energy secondary electrons created by photoillumination. Following electron capture, ${\mathrm{N}}_2$O dissociation produces ${\mathrm{O}}^{\mathrm{-}}$ ions that react with GaAs to yield surface Ga and As oxides. Dissociation also produces ${\mathrm{N}}_2$ molecules that desorb without reacting but can be kinetically trapped at low temperature. The As oxides exhibit ${\mathrm{As}}_2$ ${\mathrm{O}}_5$-like, ${\mathrm{As}}_2$ ${\mathrm{O}}_3$-like, and intermediate ${\mathrm{AsO}}_{\mathit{x}}$-like bonding configurations in relative amounts determined by kinetic constraints and oxygen availability. The photon-enhanced formation of a thick oxide at low temperature is limited by diffusion through the oxide layer, and the formation of ${\mathrm{O}}_2$ molecules is observed. Warming to 300 K enhances ${\mathrm{Ga}}_2$ ${\mathrm{O}}_3$ growth at the expense of As-O configurations.