Growth of high-quality ZnO thin films on ($11\bar{2}0$) a-plane sapphire substrates by plasma-assisted molecular beam epitaxy

Abstract

High-quality ZnO thin films were grown on a-plane sapphire substrates by plasma-assisted molecular beam epitaxy. X-ray diffraction and transmission electron microscopy reveal that the ZnO films have high structural quality and an atomically sharp ZnO/Al₂O₃ interface. The full width at half maximum values of the 0002 and $30\bar{3}2$ ZnO ω-rocking curves are 467.8 and 813.5 arc sec for a 600 nm thick ZnO film. A screw dislocation density of 4.35×10⁸ cm⁻² and an edge dislocation density of 3.38×10⁹ cm⁻² are estimated by X-ray diffraction. The surface of the ZnO epilayers contains hexagonal pits, which can be observed in the Zn-polar ZnO. The films have a resistivity of 0.119 Ω cm, an electron concentration of 6.85×10¹⁷ cm⁻³, and a mobility of 76.5 cm² V⁻¹ s⁻¹ at room temperature. Low temperature photoluminescence measurements show good optical properties comparable to ZnO single crystals.