The lattice relaxation of ZnO single crystal (0001) surface

Wurtzite-type ZnO is a polar crystal whose (0001) surface is terminated by Zn ions. The ZnO single crystals were etched for the surfaces to have regular step/terrace structures and analyzed by reflection high-energy electron diffraction (RHEED) and coaxial impact-collision ion scattering spectroscopy (CAICISS). RHEED patterns showed that the surface had a 1×1 structure. From the polar angle dependence of CAICISS time-of-flight spectra for Zn ions, a peak due to a focusing effect for fifth-layer Zn ions by the first-layer Zn ions was observed at 49.5°, which was 2° higher than the corresponding angle simulated for a non-relaxed surface. These results indicated that the uppermost Zn ions were surface-normally relaxed toward the outside of the surface by about 7% of the c-axis lattice constant.