Homoepitaxial growth mechanism of ZnO(0001): Molecular-dynamics simulations

We clarified here an epitaxial growth mechanism of ZnO(0001) surface on an atomic scale, by using molecular-dynamics crystal-growth simulations. It was observed that the crystal growth starts at the step of ZnO(0001), but not at the terrace of ZnO(0001). This phenomenon is clearly justified from the coordination number of adsorbed ZnO molecules on the ZnO(0001) surface. The ZnO molecule can form bonds with the smooth ZnO surface through only single coordination since the topmost surface is constructed by only one atomic species, i.e., either a Zn or O atomic plane. Hence, a ZnO molecule adsorbed on the smooth ZnO surface can be readily evaporated, indicating the rare growth of the ZnO(0001) from the terrace. On the other hand, double coordination can be observed at the step, since both Zn and O atoms are exposed to the surface at the step. Hence, the adsorbed ZnO molecule at the step is stabilized and the crystal growth starts from the step. it indicates that the ZnO molecule adsorbed at the step has a role of nucleation center. The above epitaxial growth mechanism is completely different from that of MgO(001) [M. Kubo et al., J. Chem. Phys. 107, 4416 (1997)]. This difference is clearly interpreted from the different surface structures of the ZnO(0001) and MgO(001).