Epitaxial growth of non-polar m-plane AlN film on bare and ZnO buffered m-sapphire

• Highly m-axis oriented non-polar AlN films were epitaxial growth on bare and ZnO buffered sapphire substrate by laser molecular beam epitaxy.
• ZnO layer improves the crystal quality, bond areal density and reduce the out-of-plane residual strain, anisotropic characteristic of AlN film.
• ZnO layer transforms the surface morphology of AlN film from grains to transverse stripes elongated.
• Epitaxial growth of non-polar m-plane AlN film is of great significance to realize a higher internal quantum efficiency photodevice than polar AlN-based.

Non-polar m-plane AlN films were grown epitaxially on bare and ZnO buffered m-plane sapphire by laser molecular beam epitaxy. Based on X-ray θ−2θ and Φ scanning results, the in-plane epitaxial relationships of AlN/sapphire and AlN/ZnO/sapphire are [0001]AlN∥[112¯0]sapphire and [0001]AlN∥[0001]ZnO∥[112¯0]sapphire, respectively. A direct epitaxy without in-plane rotation is observed between AlN and ZnO, due to a small lattice mismatch, which results in a higher bond areal density of AlN/ZnO/sapphire than that of AlN/sapphire. ZnO buffer layer transforms the in-plane residual strain from compressive strain to tensile strain along the direction of [0001]AlN, owing to the change in the in-plane lattice mismatch. X-ray rocking curve and atomic force microscope reveal that ZnO buffer layer can change the mosaicity spread along the direction of [0001]AlN and [112¯0]AlN, leading to the opposite striated anisotropic morphology of AlN/sapphire and AlN/ZnO/sapphire. The decrease in anisotropic characteristic of AlN/ZnO/sapphire is consistent with the result of in-plane lattice mismatch.