Study Of Properties And ZnO Films Grown By MOCVD

ZnO is a direct wide-band gap semiconductor with WZ crystal structure. Due the lower growth temperature and high exciton binding energy (60meV) at room temperature, it is an ideal semiconductor material in blue light range and the research in ZnO field is named as "low temperature blue project". At the beginning, ZnO film with high quality couldn't be obtained due to the limit of the semiconductor growth technology, and there was no progress in ZnO emitting light research. With the development of semiconductor growth technology, high quality ZnO can be obtained, and the research in ZnO film attracts much more attention again. Above all, the upsurge in ZnO research is coming due to the realization of the light pumped exciton emitting and discovery of self-formed cavity. In this paper, high quality ZnO thin films on sapphire and Si substrate have been obtained by MOCVD system designed by ourselves. At the same time, methods of pretreatment and growing buffer layer on the substrate were employed to improve the qualities of ZnO films, and good results have been achieved. By measuring X-ray diffraction of samples, we observed effects of surface pretreatments of the substrates on quality of crystal ZnO thin films. After the surface thermal treatments, the ZnO (0002) peak with 2θapproximately 34.50°arising from hexagonal wurtzite structure is predominant for all films, which indicates that the films are all highly c-axis oriented. And X-ray diffraction peak of samples after treatments with O₂ and N₂ lie in 34.48°and 34.56°, respectively. FWHM of the (002) ZnO peak decrease after the sapphire nitrification. In this case, a very thin N-polar AlN layer was formed after the sapphire nitrification and this layer acted as the template of the following ZnO layer growth. This AlN layer was formed with epitaxial relationship of [1010]AlN//[1120]Al2O3 as confirmed by RHEED monitoring. The AlN layer had the same hexagonal structure as ZnO and the following ZnO film was grown with the epitaxial relationship of [1010]ZnO//[1010]AlN, resulting in single domain ZnO film. Although the AlN layer was not fully relaxed as confirmed by the RHEED in-situ d-spacing measurement, it largely decreased the mismatch between the ZnO and the sapphire substrate. The evaporation of oxygen, the surface of Al2O3 is believed to be Al rich after high temperature thermal cleaning (TC) under UHV condition. It largely affected the mismatch between the ZnO and the sapphire substrate. For pretreatments with the oxygen can dispel this influence, the crystallization quality of ZnO was improved. In order to investigate the effect of the low-temperature grown ZnO buffer layer on the quality of the ZnO thin film, epitaxial ZnO thin film were grown on Si with and without the low-temperature grown buffer layer at the optimal substrate temperature of 610°C. For ZnO epitaxial layer employing the buffer layer, one peak from the ZnO (002) orientation is observed in the XRD measurement, and the FWHM of the (002) ZnO peak becomes narrower and has a value of 0.18°. It clearly shows that the crystal quality of the ZnO thin film is improved by employing the low-temperature grown ZnO buffer layer. The existence of the low-temperature buffer layer may reduce the effects of the oxide layer and the lattice mismatch on the quality of the ZnO films. The nucleation of the ZnO buffer layer grown at low-temperature is relatively easy to form on the Si surface,...