Radio Frequency Plasma Assisted Molecular Beam Epitaxy (rf-mbe) Preparation Of High-quality Zno Films

Growth of high quality epitaxial ZnO thin films on highly mismatched sapphire(0001) substrates have been performed using radio frequency plasma-assisted molecular beam epitaxy. It has been found that the quality of ZnO thin films on sapphire(0001) substrates can be significantly improved by using the following three methods.1.The surface modification by a thin Ga wetting layer after oxygen plasma pretreatment of the sapphire(0001) substrate. Experiments reveal that the Ga wetting layer suppresses completely the formation of rotation domains and inversion domains which are the main causes of the quality degradation of ZnO thin films. In situ reflection high energy electron diffraction (RHEED) observation and ex situ high resolution X-ray diffraction (HRXRD), transmission electron microscopy (TEM) and convergent beam electron diffraction (CBED) measurements demonstrate that the as-grown ZnO thin films are of high quality and have uniform Zn unipolarity. The effects of the Ga wetting layer on the selection of ZnO polarity and the reduction of defect density in epitaxial thin films have been discussed in detail. Meanwhile, a Ga bilayer model has been proposed to explain the mechanism of Zn unipolarity growth with this method.2.Deposition of a MgO buffer layer after oxygen plasma pretreatment of sapphire(0001) substrate. In situ RHEED observation and HRXRD measurement demonstrated that no rotation domains exist and the as-grown ZnO thin films are of O unipolarity. The effects of the MgO buffer layer on quality improvement and O polarity selection of ZnO thin films have been discussed.3.The substrate nitridation followed by deposition of a A1N layer. RHEED patterns clearly show that the A1N thin film was formed after nitridation. This A1N thin film plays an important role in the ZnO epitaxy on sapphire. The lattice mismatch is reduced from 18.4% (between ZnO(0001) and sapphire(0001))to 4.5%(between ZnO(0001) and AlN(0001)). Rotation domains were not observedin AIN thin film, neither in ZnO film. Furthermore, the polarity can be controlled by the substrate nitridation. The effects of the AIN thin layer on high quality ZnO thin film epitaxy have been discussed.By our systematic studies, we found that the three methods developed are very effective in reducing the rotation and inversion domains and thus improving the quality of ZnO epitaxial thin films. The information is very useful for development of ZnO based optoelectric devices.