Surface segregation of W doped in ZnO thin films

We observed surface segregation of W (0.05–4 mol%) doped in ZnO films by annealing above 900 K. The segregation coefficient was related to the crystal quality of the film, where slower segregation occurs in higher-quality crystalline films. Using low-energy He+ ion scattering spectroscopy for structure analysis, we found that the W–ZnO surface terminates with an O-layer, and W is located in a substitutional site of Zn at the second surface layer as a consequence of segregation. On the other hand, we observed no indication that W occupies certain sites in the ZnO lattice at the subsurface. Ultraviolet photoelectron spectroscopy (He I) on the W-segregated ZnO surface indicates that W is hexavalent at the Zn site. The segregation of the W atom is likely accompanied by two Zn vacancies. Ion beam mixing followed by annealing of the ZnO surface deposited with W provides a surface electronic structure similar to that of W-segregated ZnO.