Electron-spectroscopic studies of LiNbO3and LiTaO3surfaces

An analysis is presented of the Auger, uv photoemission, and electron-energy-loss spectra of LiNb${\mathrm{O}}_3$ and LiTa${\mathrm{O}}_3$ single-crystal surfaces. Results are reported for the clean surface before and after ${\mathrm{Ar}}^{+}$-ion bombardment and for bombarded surfaces after exposure to ${\mathrm{O}}_2$. Both ${\mathrm{Ar}}^{+}$-ion and electron bombardment are found to remove surface oxygen, leading to characteristic changes in the low-kinetic-energy (∼ 100-250 eV) Auger spectra. Both materials readily form surface carbides when carbon contamination is present. In uv photoemission, formation of oxygen-ion vacancies by ion bombardment produces a peak above the valence-band edge in both LiNb${\mathrm{O}}_3$ and LiTa${\mathrm{O}}_3$. Subsequent exposure to ${\mathrm{O}}_2$ strongly attenuates the bombardment-induced photoemission. The ultraviolet photoemission spectroscopy (UPS) data are interpreted in terms of $M^{4+}$-(oxygen vacancy) complexes ($M=\mathrm{Nb}\mathrm{or}\mathrm{Ta}$). Electron-energy-loss spectroscopy (ELS) data in the 2-90 eV range of loss energies show transitions from the valence band and upper core levels to states in the conduction band. The effects of variation of the primary-beam energy and of ${\mathrm{O}}_2$ on the intrinsic ELS are examined. ${\mathrm{Ar}}^{+}$-ion bombardment results in a low-energy extrinsic ELS peak at ∼2.5-3.0 eV in LiNb${\mathrm{O}}_3$ and ∼3.1-3.5 eV in LiTa${\mathrm{O}}_3$. ${\mathrm{O}}_2$ adsorption has little effect on the loss intensity in LiNb${\mathrm{O}}_3$ but causes a noticeable decrease in LiTa${\mathrm{O}}_3$. Annealing at 650°C, to diffuse ${\mathrm{Li}}_2$O out of the bulk, diminishes the defect loss peak in LiNb${\mathrm{O}}_3$ and almost eliminates it in LiTa${\mathrm{O}}_3$. Defects removed by annealing but not by ${\mathrm{O}}_2$ adsorption are believed to involve ${\mathrm{Li}}^{+}$-ion vacancies. The same low-energy-defect ELS peaks induced by ion bombardment are also observed, with less intensity, in UHV-fractured samples prior to bombardment, and are indicative of a small deficiency of oxygen and lithium. In LiTa${\mathrm{O}}_3$, a transition at ∼22 eV, thought to be from the O $2s$ level to the bottom of the conduction band, is observed and found to be dependent on surface oxidation and reduction.