Determination of surface polarity of c-axis oriented ZnO films by coaxial impact-collision ion scattering spectroscopy

We have identified the surface polar structure of wurtzite-type ZnO films by coaxial impact-collision ion scattering spectroscopy. High-quality ZnO epitaxial films were prepared on sapphire $\mathrm{(\alpha -}{\mathrm{Al}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{3}})$ (0001) substrates by laser molecular beam epitaxy using a ZnO ceramic target. The (0001̄) crystallographic plane (the O face) was found to terminate the top surface of the ZnO film by comparing spectra of the films with those of well-defined (0001) and (0001̄) surfaces of bulk single crystals. The preferential [0001̄] growth direction of ZnO films is discussed from the viewpoints of the chemical interaction at the interface and surface stability against sublimation.