Oxygen adsorption on $\mathrm{Cu}∕\mathrm{Zn}\mathrm{O}\left(0001\right)\text{−}\mathrm{Zn}$

We hereby describe the modifications induced in a Cu thin film induced by different oxidation processes. By using plasma assisted deposition, we have adsorbed oxygen onto the $\mathrm{Cu}∕\mathrm{Zn}\mathrm{O}\left(0001\right)\text{−}\mathrm{Zn}$ surface. Cu was deposited on the sputtered-annealed ZnO substrate at room temperature, which was later exposed to oxygen. Using x-ray photoelectron spectroscopy, we verified the effect of the surface treatment on the electronic structure. Our findings are consistent with a partially oxidized Cu layer, with the CuO located at the interface between ZnO and the adsorbed Cu islands. Further Cu deposition induces the formation of ${\mathrm{Cu}}_2\mathrm{O}$ as judged by the evolution of the spectra. Annealing the sample up to $750°\mathrm{C}$ in UHV induces further reduction of the oxide, metallic Cu is recovered on the top layer, with evidence of Cu desorption into the vacuum or incorporation into the substrate.

Figure 1

ARXPS of $\mathrm{Cu}2p_{3∕2}$ after depositing (i) 0.3 ML, (ii) 1 ML, and (iii) 5 ML at room temperature on the clean ZnO(0001)-Zn surface. Starting at 1 ML coverage, the XPS intensity shows diffraction features consistent with a crystalline ordering of the Cu clusters. The inset shows the angles for the strongest forward focusing peaks expected for a Cu(111) crystal which is oriented azimuthally in the plane along the $⟨\overline{1}100⟩$ substrate direction.

Figure 2

XPS spectra of (a) $\mathrm{Zn}2p$, (b) $\mathrm{Cu}2p$, and (c) $\mathrm{O}1s$ for (i) ZnO(0001)-Zn clean surface, (ii) after depositing 3 ML Cu, at room temperature, (iii) the same sample after oxidation in oxygen plasma at ${10}^{-6}\mathrm{mbar}$, (iv) after an additional deposition of 3 ML Cu on the previously oxidized sample, and (v) the sample annealed to $750°\mathrm{C}$. Clear differences are evident between CuO and ${\mathrm{Cu}}_2\mathrm{O}$ features in the spectra.

Figure 3

Cu deposition on ZnO(0001) induces a reduction in the binding energy of the $\mathrm{Zn}2p_{3∕2}$ peak. Consistently the band bending increases as a function of Cu overlayer thickness.

Figure 4

$\mathrm{Cu}+\mathrm{Zn}$ LMM spectra for (i) ZnO(0001)-Zn clean surface, (ii) after depositing 3 ML Cu at room temperature, (iii) the same sample after oxidation in oxygen plasma at ${10}^{-6}\mathrm{mbar}$, (iv) after an additional deposition of 3 ML Cu on the previously oxidized sample, and (v) the sample annealed to $750°\mathrm{C}$.

Figure 5

Series of spectra of the $\mathrm{Cu}2p$ states in the oxidized sample (3 ML $\mathrm{Cu}+\text{plasma}$ oxidation) as a function of emission angle. The XPS intensity of the satellite at $942\mathrm{eV}$ BE has its maximum at 0° and decreases gradually as the emission angle is increased. This result is consistent with Cu having a higher oxidation state closer to the interface.

Figure 6

(Color online) Schematic illustration of the different changes induced in the $\mathrm{Cu}∕\mathrm{Zn}\mathrm{O}$ system by controlled temperature treatments under different atmospheres. The main compounds of the clusters are highlighted. The shape and apparent size of the clusters in the diagrams are arbitrary and not related to experimental results: (i) the $\mathrm{Cu}∕\mathrm{Zn}\mathrm{O}$ system after an oxidation treatment in oxygen plasma at ${10}^{-6}\mathrm{mbar}$ and (ii) the oxidized sample after an additional deposition of Cu.