OXYGEN 2S SPECTROSCOPY OF TIN OXIDES WITH SYNCHROTRON-RADIATION-INDUCED PHOTOEMISSION

Oxygen 2s spectroscopy can be especially useful in studies using synch rotron radiation (SR) where the deeper O 1s level frequently can not b e excited. For tin oxides (SnO and SnO2), the weak emission from the O 2s levels has been previously found degenerate with the intense photo emission peak from Sn 4d levels around a binding energy (BE) of 26 eV. By working at the Cooper minimum of the interfering tin signal, a dis tinct peak near BE approximate to 22.5 eV could be unambiguously attri buted to emission from O 2s levels in tin oxides considering photoioni zation cross-section arguments, The O 2s intensity could now be used f or the quantitative evaluation of the near-surface oxygen-species conc entration. We have used the O 2s in combination with the Sn 4d areas t o study the variations of the [Sn]/[O] ratio as a function of the prep aration of a SnO2 single crystal. Although the dominant formal valence state for tin determined from a Sn 4d lineshape analysis remains esse ntially Sn4+, we show that this [Sn]/[O] ratio can vary from simple to double. This can be explained by oxygen interstitials buried in a sub -surface region during ion bombardment, which segregate at the surface upon annealing.