The Simulation Study Of ZnO Films Initial Growth On α-Al₂O₃(0001) Surface

The high-quality ZnO thin films growth on the sapphire(0001) substrate, using pulse laser deposition (PLD) or molecule beam epitaxy (MBE) technology, has a wide application prospect in short wavelength optical devices including light emitting diodes or laser diodes. But there is no theoretical calculation on the initial growing mechanism of the ZnO thin films deposited on the α-Al2O3(0001) surface. The first principle is considered for ZnO films heterogeneous growth simulation through an analysis of the difficulties of complex oxide films growth and a summary of the modeling and computer simulation methods of films growth. A series of models of ZnO films growth on the α-Al2O3(0001) surface and their theoretical calculation are carried out by using the CASTEP code based on the density-functional theory(DFT). Electron wave functions are expanded in terms of a plane wave basis set in the periodic k-space and the ultrasoft pseudo-potentials (USPs) are employed. The electron-electron interaction is treated within the local density approximation (LDA) and the generalized gradient approximation(GGA). In this paper, the author attempts to have a detail discussion of the following points such as the geometric and electronic structure of the α-Al2O3(0001) surface, the adsorption geometries sites on the substrate surface, the ZnO films growing process in different temperature conditions, the surface and interface structure of the Al2O3/ZnO, and the defect in films growth. The mechanism of ZnO adsorption on the α-Al2O3(0001) surface, the surface and interface structure and the growing orientation, the temperature dependent influence of the atoms behavior character of the surface/interface, and the point defect of the surface AI and the O effect on ZnO adsorption growth etc. are calculated the first time by the author. The results are well in accordance with experimental reports.The theoretical calculation gives a further evidence that the single Al-terminated structure of the a-Al2O3(0001) surface is much more stable, showing that the surface relaxation mainly takes place in the top Al -O layer with an inward distance of 0.079~0.082nm. The relaxation has caused the re-distribution of large quantity of surface O2- charge which is mainly from the O-2p states, thus the locations which the electronic density increase on the surface areas can enhance the adsorption of cations and the particles with positive charge.ZnO has experienced strong chemisorbed on the α-Al2O3(0001) surface where the preferential adsorption sites has a 30° away from the axis of hexagonal symmetry of oxygen cells on the surface. The chemical bonding of the (Zn)O-Al(substrate) is shown as having the character of ionic bonding, while chemical bonding of Zn-O(substrate) has a clearly covalent feature, manly from the hybridization between Zn4s and O2p, as well as part of the hybridization between Zn3d and O2p. In view of the adsorption energy and the adsorptive position, it is favorable for forming the tetrahedral coordination by sp3 hybridization between Zn 4s and the 2p orbits of the surface O atoms, which is favorable for forming the tetrahedral coordination in ZnO wurtzite structure. These adsorption sites are the most preferred growth sites of ZnO films.The ab initio dynamic simulation of the ZnO adsorption on the α-Al2O3(0001) surface and the calculation on the surface and interface energy indicate that the growth structure at c-axis is of substrate-O-layer-Al layer-Zn layer-O-layer structure, and that the films with (0001) -O surface is the most stabilized structure. The temperature has an evident impact on the reaction diffusivity of the O atoms so as to significantly act on the regular films growth, which has a decisive effect on the ZnO films' growth model. ZnO films with a spiral-in plane growth at 400℃, the temperature around 400 ℃ is favorable for forming the Zn-terminated surface, while an aligned in-plane growth at 600℃ is observed. There are a surface phase transition during ZnO films growth at 600 ℃, one with the surface structure characteristic of the Zn-terminated surface and the ZnO films [1010] // substrate[1010], and the other with the O-terminated surface and the films [1010]// substrate [1120]. The barrier energy in the two changes of the surface structure is about 1.6 eV.The simulation results reveal that there is more disordered layer structure of ZnO films at 400℃. With the temperature up to 800℃, the dissociation of the Al-O atoms on the Al2O3 surface results in forming intermixed amorphous interface. The temperature of about 600 ℃ is the ideal temperature condition of ZnO heterogeneous growth on sapphire (0001) surface. The vacancies of the Zn on the interface where close to the α-Al2O3(0001) surface are more than that of the O. Therefore, we suggest that a proper increase of the deposition rate and a treatment of the α-Al2O3(0001) surface with quantitative Zn atoms as a surfactant should serve as the nucleation sites for the further formation of high quality films.