A XANES study of Cu valency in Cu-doped epitaxial ZnO

Near-edge X-ray absorption (XANES) at the Cu K-edge was employed to study the valency of Cu (a potential p-dopant) in Cu-doped ZnO grown by molecular beam epitaxy. For a similar chemical environment, a shift in the onset of absorption can be interpreted as being due to a change in effective valency. We have studied this shift for both as-grown and argon ambient annealed (1000 °C,30 min) Cu-doped ZnO samples. The valency shift was measured against ab-initio standards calculated using feff8 as well as the standard samples Cu₂O (+1 valence) and CuO (+2 valence). It was found for the flux region explored here that as-incorporated Cu assumes an effective valence of approximately +1 which increases towards +2 upon a 1000 °C argon ambient anneal. X-ray diffraction also shows the presence of both metallic Cu and Cu₂O depending upon growth and annealing conditions.