Growth of high quality ZnO thin films with a homonucleation on sapphire

Abstract

ZnO thin films were epitaxially grown on c-plane sapphire substrates by plasma-assisted molecular beam epitaxy. A low temperature homonucleation ZnO layer was found crucial at the interfacial region to absorb the defects formed by the lattice mismatch between the sapphire and ZnO, resulting in a smooth surface that enables smooth 2D epitaxial growth. High quality ZnO films were achieved after careful optimization of critical growth conditions: the sequence of Zn and O source shutters, growth temperature for both the ZnO nucleation and growth layer, and Zn/O ratio. Oxygen plasma pretreatment was not applied prior to the growth, thus shortening the growth time and reducing oxidation of the metallic sources. Resultant epitaxial ZnO films on sapphire demonstrated a root-mean-square surface roughness of 0.373 nm for 1 μm × 1 μm atomic force microscope images with clear hexagonal structure and terrace steps. The x-ray diffraction full width at half maximum (FWHM) for ω and ω-2θ ZnO (0002) triple-crystal rocking curves were measured to be 13 and 26 arc/s, respectively. This FWHM value is lower than any reported to date in the literature, with ω and ω-2θ values indicating excellent coherence of the epitaxial layer along the interface and the growth direction, accordingly. These x-ray diffraction and surface roughness values are lower than those obtained using common nucleation layers such as MgO, indicating that growth with ZnO nucleation layers on sapphire may lead to higher quality electrical and optical devices.