Density functional calculations were used to study the structure and formation energy of the surface oxygen vacancy (VO) in a free-standing, very-thin film of ZnO and substitutionally doped ZnO very-thin film. It was found that the structure of the ZnO(00•¯1) very-thin film was strongly dependent upon the film thickness. Decreasing the thickness of the ZnO(00•¯1) film increased the distances between the atomic planes along the (00•¯1) direction. At a film thickness of five layers, the energy of the very-thin film with a wurtzite structure was the same as the energy of very-thin film with an hexagonal structure. For thickness <5 layers, the hexagonal very-thin film was more stable than the wurtzite film. The formation of VO was exothermic and led to a decrease in the interplanar distance along the (00•¯1) direction in the entire film, making the structure close to that of wurtzite. The exothermic nature of VO formation suggested that the ZnO(00•¯1) very-thin film with vacancies was thermodynamically stable at low oxygen pressures in the gas phase. The structure of the ZnO(10•0) very-thin film did not depend strongly upon the film thickness. The formation of a VO at the surface of the ZnO(10•0) very-thin film was endothermic and affected the film structure only in the vicinity of the VO. Replacement of some of the Zn atoms at the surface of the stoichiometric ZnO(00•¯1) very-thin film by a dopant (Au, Ag, Cu, Ga or Al) decreased the interplanar distance along the (00•¯1) direction over the entire film, making its structure close to that of wurtzite. The formation of VO at the surface of a doped very-thin film was endothermic and therefore the presence of the dopant stabilized the stoichiometric wurtzite structure. Doping the surface of the ZnO(10•0) very-thin film with Au, Ag or Cu lowered the energy of VO formation at the surface and doping with Ga or Al increased it. It was also found that Au, Ag and Cu, which facilitated VO formation, segregated to the surface of the ZnO (10•0) very-thin film and Ga and Al, which hindered VO formation, segregated to the bulk.

The Structure and Energy of Oxygen Vacancy Formation in Clean and Doped, Very Thin Films of ZnO. Pala, R.G.S., Metiu, H.: Journal of Physical Chemistry C, 2007, 111[34], 12715-22