AB INITIO TEMPERATURE-DEPENDENT SIMULATION OF ZnO ADSORBED ON THE α-Al2O3(0001) SURFACE

The adsorption and the growth of ZnO on α-Al2O3(0001) surface at various temperatures are theoretically calculated by using a plane wave pseudopotentials (USP) method based on density functional theory. The average adsorption energy of ZnO at 400, 600, and 800°C is 4.16±0.08, 4.25±0.11, and 4.05±0.23 eV, respectively. Temperature has a remarkable effect on the structure of the surface and the interface of ZnO/α-Al2O3(0001). It is found that the Zn-hexagonal symmetry deflexion does not appear during the adsorption growth of ZnO at 400°C, and that the ${\rm ZnO}[10\overline{1}0]$ is parallel with the $[10\overline{1}0]$ of the α-Al2O3(0001), which is favorable for forming the ZnO film with the Zn-terminated surface. It can be observed from our simulation that there are two surface structures in the adsorption of ZnO at 600°C: one is the ZnO surface that has the Zn-terminated structure, and whose $[10\overline{1}0]$ parallels with the substrate surface $[10\overline{1}0]$, and the other is the ZnO $[10\overline{1}0]$// sapphire $[11\overline{2}0]$ with the O-terminated surface. The energy barrier of the phase transition between the two different surface structures is about 1.6 eV, and the latter is more stable. Therefore, the ideal temperature for the thin film growth of ZnO on sapphire is about 600°C, and it facilitates the formation of wurtzite structure containing Zn-O–Zn-O–Zn-O double layers as a growth unit cell. At 600°C, the average bond length of ZnO is 0.190±0.01 nm, and the electron localization function value indicates that the bond of (substrate) –O–Zn–O has a distinct covalent character, whereas the (Zn)O–Al (substrate) shows a clear character of ionic bond. However, at the temperature of 800°C, the dissociation of Al and O atoms on the surface of the α-Al2O3(0001) leads to a disordered surface and interface structure. Thus, the Zn-hexagonal symmetry structure of the ZnO film is not observed under the condition.