Stucture And Optical Properties Of ZnO Nanowires/nanosheets Grown By MBE Technique

Because its excellent optical and electrical properties, ZnO have a wide range of applications in many fields such as solar cells,light emitting diodes and gas sensors, and can be also applied in the life sciences for biocompatibility. In order to satify demands, researchers have prepared a variety of ZnO nanostructures.Lots of research work for the preparation of high-quality ZnO films has been accomplished by molecular beam epitaxy (MBE). On the one hand, MBE system is in (ultra) high vacuum which can effectively avoid doping of the impurities; On the other hand, one can precisely alter the evaporation rate of the source material and the flow rate of gas in order to control the growth of nano-materials. This paper describes the optical and electrical properties of ZnO nanowires/nanosheet grown by plasma-assisted MBE. The main results are described as follows:1. Several growth factors are discussed for the ZnO growth, including the substrate temperature, the size of Au catalyst, the oxygen flow rate and growth time. The results show that the progressive increase of XRD intensity with the substrate in lower temperature, but the opposite of PL spectra. The size of Au catalyst is about 20~200nm after annealing. ZnO films are prepared with oxygen flowed into the system at a rate of 5.2sccm, but nanowires are grown on the substrate when the oxygen flow rate is 5.8sccm. PL spectra show the deep-level emissions decrease with the increase of growth time, indicating reduction in defects.2. ZnO nanowire arrays were grown on the Si (111) substrates coated with 1nm Au catalyst by plasma assisted molecular beam epitaxy (P-MBE) through Vapor-Liquid-Solid (VLS) growth mechanism at low temperature. Field-emission scanning electron microscopy (FE-SEM) reveals that ZnO nanowire arrays grown densely and vertically to the substrate surface with the average diameter of 20-30nm, the length is ~200nm. The structure properties of ZnO nanowirs are measured by X-ray diffraction (XRD) and high resolution transmission electron microscopy (HRTEM), the results clearly show that the ZnO nanowire exhibits a typical wurtzite structure with c-axis (002) preferred orientation and good crystal quality. The room temperature photoluminescence (PL) spectrum shows that the ZnO nanowire arrays possess good photoluminscent properties with strong near band edge excitonic UV emission around 380nm and weak defect-related emission in the visible region of 475-650nm. The V-L-S growth mechanism of ZnO nanowire arrays is discussed.3. The high-quality ZnO nanosheets are prepared by P-MBE. FE-SEM exhibits that ZnO nanorod grown densely on the Si (111) substrate with the length and diameter of 1-3μm and 0.4-0.8μm, respectively. The ZnO nanosheets attach to the nanorod, the range of the ZnO nanosheet dimension is 100-300nm. XRD and HRTEM results clearly show that ZnO nanosheet exhibits a typical wurtzite structure with c-axis preferred orientation, the spacing of the lattice fringes along the c axis of the ZnO nanowire is 0.26nm. The room temperature PL spectra reveal the strong emission at the wavelength ~380nm, which indicates the good crystallity of ZnO nanostructure with few defects.