Coaxial impact-collision ion scattering spectroscopy analysis of ZnO thin films and single crystals

Coaxial impact-collision ion scattering spectroscopy (CAICISS) was employed to analyze the surface structure of ZnO single crystals and thin films with various thickness. Epitaxial ZnO films grown on sapphire (α-Al2O3) (0001) by laser molecular beam epitaxy was found to choose the [0001̄] growth direction (the (0001̄) O face termination) among asymmetric c-axis directions of wurtzite structure by comparing the spectra and their incident angle dependences of the films with those of well-defined (0001) Zn face and (0001̄) O face surfaces of bulk single crystals. The incident angle dependences of Zn signal intensity for single crystals and thick (>10 nm) films can be well fitted with the simulated curves without taking reconstruction of the surface into account. However, the CAICISS results for the very thin film (2 nm) showed considerable deviation from the data of single crystals, suggesting the existence of the distorted region during the initial growth stage induced by the large lattice mismatch between ZnO and sapphire (18%). The growth mechanism and preferential [0001̄] growth direction of ZnO are discussed based on preferential tetrahedral bonding between Zn and O atoms on the sapphire surface.