ZnO epitaxial layers grown on c-sapphire substrate with MgO buffer by plasma-assisted molecular beam epitaxy (P-MBE)

Abstract

ZnO films on c-sapphire with and without an MgO buffer were grown by plasma-assisted molecular beam epitaxy. ZnO with an MgO buffer was two dimensionally grown, while ZnO without an MgO buffer was grown three dimensionally, which was confirmed by in situ RHEED (reflection high energy electron diffraction) and AFM (atomic force microscopy) observations. Morphology evolution and growth mechanism of an MgO buffer were studied by in situ RHEED observations. Mosaicity (tilt and twist angle), type and density of dislocation were studied by both TEM (transmission electron microscopy) and HRXRD (high resolution x-ray diffraction). Based on in situ RHEED observations, MgO buffer growth involves three important steps including two-dimensional (2D) growth (wetting layer), 2D–3D growth transition and 3D growth. The mechanism of MgO buffer growth can be attributed to three inter-related effects. They are lowering surface energy through a wetting process, creating nucleation sites through a 2D–3D growth transition, and reducing the defect density by introducing dislocation interactions. It was found that the surface morphology and structural properties of the ZnO layers were improved by employing a thin MgO buffer layer grown at around 500 °C followed by high temperature annealing at 800 °C. By introducing an MgO buffer, the formation of 30° rotational domains in ZnO layers was suppressed, and screw and edge dislocation density of ZnO layers was reduced from 6.1 × 10 8 cm−2 to 8.1 × 105 cm−2 and from 1.3 × 1010 cm−2 to 1.1 × 1010 cm−2, respectively.