Sputter Epitaxy of ZnO Films on Sapphire Substrates Using 3D MgO Buffer Layers

Ultrathin MgO buffers composed of three-dimensional (3D) islands enable sputter epitaxy of atomically flat ZnO films on sapphire, despite an 18% lattice mismatch. The resulting 500 nm-thick ZnO films exhibit an exceptionally narrow full width at half-maximum (fwhm) of 21 arcsec in the 0002 X-ray rocking curve (XRC) and a root-mean-square roughness of only 0.11 nm. These improvements are attributed to the 3D MgO islands, which relax the strain at the MgO/sapphire interface (∼17% mismatch), together with structural features more consistent with the metastable wurtzite phase than with the rocksalt one, thereby providing a favorable template with only 1-2% mismatch with ZnO. In contrast, when the MgO buffers convert to the thermodynamically stable rocksalt phase, the ZnO crystalline quality degrades drastically, with the XRC-fwhm exceeding 1000 arcsec due to the larger mismatch (7%) between the rocksalt MgO and wurtzite ZnO. Furthermore, the clear donor-bound exciton peaks observed in the low-temperature cathodoluminescence confirm the satisfactory optical and structural qualities of the ZnO films. These results demonstrate that 3D wurtzite MgO buffers provide an effective strategy for high-quality epitaxy of ZnO and can be extended to other wurtzite materials on lattice-mismatched substrates.