Crystallinity improvement of ZnO thin film by hierarchical thermal annealing

Abstract

Molecular beam epitaxy (MBE) was adopted to grow zinc oxide (ZnO) film on sapphire substrate and improve the quality of ZnO thin film epitaxy using a magnesium oxide (MgO) buffer layer and a two-segment temperature scheme for ZnO thin film growth. The influence of thermal annealing of different layers on the optical and crystalline features, stress expression, as well as surface morphology of ZnO thin film was examined. SEM images showed smooth surfaces were formed, and these surfaces allowed the low-temperature ZnO buffer layer to have better epitaxial environment at the very beginning. X-ray diffraction (XRD) analysis revealed that a lower thermal annealing temperature more effectively released the stress of materials. The thermally annealed MgO buffer layer had 26% less strain than the purely thermally annealed, high-temperature-grown ZnO (HT-ZnO), and 33% less strain than the unannealed samples. Atomic force microscopy results showed that the root-mean-square of surface roughness of thermally annealed MgO was 0.255 nm, which was 80% lower than that of thermally annealed HT-ZnO (1.241 nm). Photoluminescence measurement showed that the thermally annealed MgO buffer layer had the highest strength for near-band-edge emission because of improved crystalline quality. HRTEM results showed that the stress caused by the mismatch between the sapphire lattice was effectively released because the MgO buffer layer was annealed at a high temperature. The surface of the MgO buffer layer became smooth and the stress mismatching with the ZnO lattice did not obviously extend upwards. When MBE was used to grow ZnO thin film, a lower thermal annealing temperature for the MgO buffer layer more effectively controlled stress accumulation and produced high-quality ZnO thin film.