Ultra-smooth and lattice relaxed ZnO thin films

Abstract

The crystal structure and quality of ZnO thin films were enhanced by high temperature vacuum annealing. High quality ZnO thin films have been grown on a-plane sapphire substrate by radio frequency (rf) magnetron sputtering method at a substrate temperature of 600°C. A remarkable improvement in the epilayer quality were established by in situ high temperature annealing. The film quality, smoothness, the in plane stress, and the degree of epitaxy of the films have been evaluated. The crystalline quality was investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM) and Raman spectroscopy analyses. An extremely smooth ZnO films were achieved at higher annealing temperatures with root mean square roughness of 0.3nm. The transverse optical mode A1(TO) observed in all the samples and the longitudinal optical mode A1(LO) appeared only at higher annealing temperatures over 800°C in the micro-Raman scattering measurements. The strain of c-axis were relaxed and the lattice parameter was comparable to that of bulk ZnO at high annealing temperature of 900°C.