Growth of high-quality ZnO thin films on () a-plane sapphire substrates by plasma-assisted molecular beam epitaxy

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/Al 2 O 3 interface. The full width at half maximum values of the 0002 and ZnO ω -rocking curves are 467.8 and 813.5 arc sec for a 600 nm thick ZnO film. A screw dislocation density of 4.35×10 8 cm −2 and an edge dislocation density of 3.38×10 9 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 6.85×10 17 cm −3 , and a mobility of 76.5 cm 2 V −1 s −1 at room temperature. Low temperature photoluminescence measurements show good optical properties comparable to ZnO single crystals.