Ab initio total energy study of ZnO adsorption on a sapphire (0001) surface

The sapphire (0001) surface and the adsorption of ZnO are theoretically calculated by using a plane wave ultrasoft pseudo-potential method based on density functional theory. By comparing the PDOS of the Al and the O before and after the surface relaxation, we demonstrate different sapphire (0001) surface states, and changes in surface chemical bonding resulting from Al and O are analyzed. The bonding processing of a ZnO molecule on the surface of Al ₂O₃, adsorption energy and bonding orientation, and change in surface structure as well as the characters of surface chemical bonding are further investigated. We conclude that the surface inward relaxation in the first layer Al-O is weakened, even eliminated after the surface adsorption of the ZnO; chemical bonding energy achieved 4.4±0.4 eV. There is a deflected angle of 30° between the chemical bonding of the ZnO (0.185±0.01 nm) and the adjacent surface Al-O bonding; the stable chemical adsorption site of Zn is just about 30° deflected from the O-hexagonal symmetry of the α-Al₂O₃(0001) surface. Through analysis of the atomic populations, density of state, and bonding electronic density before and after the adsorption, it is revealed that the chemical bonding formed by the O^2- of the ZnO and the surface Al ³⁺ is characterized by strong ionic bonding, while the bond of the Zn²⁺ and the surface O^2- has a covalent character, which mainly comes from the hybridization of the Zn 4s and the O 2p and partially from the hybridization of the Zn 3d and the O 2p, so as to facilitate the forming of tetrahedral coordination in the initial stage of the ZnO films growth. Hence it is favorable for the formation of the wurtzite structure.