Formation of O− in ZnO from the dissociation of adsorbed N2O

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Photoinduced O− ions or V-type centers have been detected in ZnO following ultraviolet irradiation at −196°C. The EPR spectrum of the ions is characterized by g⊥=2.021 and g∥ = 2.0026. The concentration of O− was greatly enhanced when N2O had been adsorbed on the surface prior to irradiation. No oxygen-17 hyperfine structure was detected when N2O enriched with 17O was adsorbed. The mechanism, therefore, for the formation of the observed O− ions is not the same on a semiconductor as on an insulator such as MgO, which was previously studied. The paramagnetic center on ZnO was formed only on samples that were degassed at temperatures near 450°C. It is proposed that both anion and cation vacancies exist on the surface and in the bulk following this pretreatment. Ultraviolet irradiation produces electron-hole pairs, and the electrons react with N2O forming O− ions and N2. The O− is then trapped at an anion vacancy where it gives up its hole (traps another electron) and becomes an oxide ion. Holes formed in this manner, as well as those produced directly by the ultraviolet light, migrate through the lattice and are trapped at zinc ion vacancies. These centers are stable at −196°C, but they rapidly decay at 25°C. Reactions of O− with CO and O2 suggest that the ions may be near but not on the surface of ZnO.