Improvement of the crystalline quality of the ZnO epitaxial layer on a low-temperature grown ZnO buffer layer

Abstract

In this paper, a high-quality ZnO thin film was first grown on a Si(1 1 1) substrate by plasma-assisted molecular beam epitaxy. A ZnO thin film grown at 300°C was used as an intermediate layer for the growth. The dependence of the ZnO thin-film quality on the substrate temperature was studied by using X-ray diffraction and photoluminescence (PL). Only poly crystal ZnO could be obtained from direct growth on the Si substrate, because of the large differences in lattice constant between the Si substrate and ZnO thin film and the existence of an oxide layer on the Si surface. The best ZnO was obtained at a substrate temperature of 550°C. A low-temperature ZnO buffer layer is induced between the substrate and ZnO film to reduce the effects of the oxide layer and the mismatch. Next, a high-temperature ZnO layer is deposited on this buffer layer to obtain a high-quality thin film. The effects of the buffer layer on the structure and PL of ZnO thin films are studied. For the ZnO films employing the buffer layer, only one peak from the ZnO (0 0 2) orientation is obtained in the X-ray diffraction measurement, and the full-width at half-maximum (FWHM) of the (0 0 2) ZnO peak is less than 0.180°. The ZnO thin film exhibits a smooth surface as measured by atomic force microscopy, and the PL spectra show a strong ultraviolet band emissions with little or no deep-level emission related to defects. The FWHMs of the PL spectra are narrower compared to the ZnO film without a buffer layer. These measurements indicate that high-quality ZnO films can be obtained by employing a low-temperature grown ZnO buffer layer.