AB INITIOTEMPERATURE-DEPENDENT SIMULATION OFZnOADSORBED ON THEα-Al2O3(0001)SURFACE

The adsorption and the growth ofZnOonα-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 ofZnOat 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 ofZnO/α-Al2O3(0001). It is found that theZn-hexagonal symmetry deflexion does not appear during the adsorption growth ofZnOat 400°C, and that the${\rm ZnO}[10\overline{1}0]$is parallel with the$[10\overline{1}0]$of theα-Al2O3(0001), which is favorable for forming theZnOfilm with theZn-terminated surface. It can be observed from our simulation that there are two surface structures in the adsorption ofZnOat 600°C: one is theZnOsurface that has theZn-terminated structure, and whose$[10\overline{1}0]$parallels with the substrate surface$[10\overline{1}0]$, and the other is theZnO$[10\overline{1}0]$// sapphire$[11\overline{2}0]$with theO-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 ofZnOon sapphire is about 600°C, and it facilitates the formation of wurtzite structure containingZn-O–Zn-O–Zn-Odouble layers as a growth unit cell. At 600°C, the average bond length ofZnOis 0.190±0.01 nm, and the electron localization function value indicates that the bond of (substrate)–O–Zn–Ohas 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 ofAlandOatoms on the surface of theα-Al2O3(0001)leads to a disordered surface and interface structure. Thus, theZn-hexagonal symmetry structure of theZnOfilm is not observed under the condition.