Layer-by-layer growth of ZnO epilayer on Al2O3(0001) by using a MgO buffer layer

By introducing a thin MgO buffer, layer-by-layer growth of ZnO epilayers on ${\mathrm{Al}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{3}}\mathrm{(0001)}$ substrates is achieved by plasma-assisted molecular beam epitaxy. The MgO buffer is very effective on the improvement of surface morphology during the initial growth stage, which eventually leads to an atomically flat surface. As a result, $\text{(3×3)}$ surface reconstruction of ZnO is observed and reflection high-energy electron diffraction intensity oscillations are recorded. Structural analysis indicates that the twin defect with a 30° in-plane crystal orientation misaligned is completely eliminated, while the total dislocation density is reduced. Free exciton emissions at 3.3774 eV ${\mathrm{(X}}_A)$ and 3.383 eV ${\mathrm{(X}}_B)$ are observed in photoluminescence at 4.2 K further indicating the high quality of the resulting ZnO epilayers.