| GaN based onⅢ-Ⅴsemiconductors with wide-band gap, thermal stability, chemical stability, wave band from visible light to ultraviolet light, have attracted intensive interest due to their promising applications in the domain of optoelectronics and microelectronics. Until recently, the majority of the GaN based devices has been fabricated on sapphire substrates. However, because sapphire itself is very expensive, insulated and hard to incise, low thermal conductivity as well as difficult technics and high cost for devices, it is disadvantageous to fabricate high power electronics devices. But the Si substrates can make up sapphire's shortcoming. Therefore, the investigation of GaN epitaxy on silicon is of extreme practical importance. Although the direct epitaxial growth of GaN on Si is very difficult owing to the large lattice mismatch and the thermal expansion coefficient, Si is very attractive because of its considerable advantages: high quality, relatively low cost, doping capability, availability of large and high-quality wafers, thermal and electrical conductivity, and potential integration on Si technology. It has become a strong competitor for sapphire.In this paper, one-dimensional nanostructures are synthesized by ammoniating the Ga2O3/ZnO films. The structure, composition, morphology and optical properties of the as-prepared GaN nanostructures are studied thoroughly. The optical mechanism and growth mechanism of GaN nanostructure are firstly explored. Buffer layer's properties and effect on the GaN nanostructure are also studied. GaN epitaxial has been carried out on ZnO as a buffer layer by radio frequency magnetron sputtering system. All the results are as follows:1. Effect of annealing temperature on the properties of ZnO buffer layerIn ammonia ambient, the results show that the increase of annealing temperature makes for the improvement in the crystal quality and surface morphology below the temperature of 650℃. However, when the annealing temperature is above 650℃, several microholes are found on the surface, demonstrating the ZnO films will volatilize. When ZnO films are annealed in NH3 ambient at 700℃, the crystal grains become smaller and the films become rough with many microholes on the surface, revealing that ZnO films have been partly volatile during the annealing process. Especially at 750℃, there is nothing on the surface and only Si substrates are observed, demonstrating that ZnO films have volatilized completely.In O2 ambient, the optimally annealing temperature is 900℃. High quality, a uniform and smooth morphology ZnO films are obtained with a preferred (002) orientation. A further increase in the temperature up to 1000°C leads to the decrease of crystal quality because ZnO is thermodynamically unstable with in an oxygen-free ambient leading to its evaporative dissociation into Zn and O2. The results of air are similar to the results of oxygen, showing that the optimally annealing temperature is 600℃.2. Synthesis of one-dimensional GaN nanostructuresZnO buffer layers are fabricated on Si substrates by two method, pulsed laser deposition and radio frequency magnetron sputtering system, respectively. A thick Ga2O3 films (about 500nm) are sputtering on ZnO buffer layer by JCK-500A magnetron sputtering system. The results reveal that different annealing temperature of ZnO buffer layer and different ammoniating temperature of Ga2O3 films have a great influence on the synthesis of GaN nanostructure. The synthesized nanostructures are of hexagonal wurtzite single-crystal GaN, showing their morphologies as nanowires, nanorods, nanograins, and cluster-shaped nanostructures.3. Optical properties of GaN nanostructuresFor the optical property, the measurement of PL spectrum was performed with a Xe lamp as the excitation source (wavelength was 298 nm) at room temperature. There are three peaks located at 373nm, 436nm, 474nm, respectively. With changes of experimental parameters,the locations of the three peaks do not move, and only the intensity of them change. Band-edge emission is observed in these nanostructured samples located at 373nm. As GaN films are formed with ZnO buffer layers, Zn may diffused in GaN in the process of the ammoniation. So the emission peak at 436nm may correspond to the transition of Zn impurity energy lever. It may be also ascribed to the electron transition from the deep acceptor level, resulting from C impurity taking place of N atoms, to the conductor band. Another peak at 474nm may be ascribed to existence of defects or surface states, which is attributed to the rearrangement of GaN and ammoniation process.4. Exploration of GaN growth mechanism Firstly, ZnO films are annealed in oxygen ambient. A better crystal structure and more relaxed ZnO films are achieved, which conduces to the subsequent growth of Ga2O3 films. It could improve nucleation by lowering the surface energy and providing an improved lattice match in the growth of Ga2O3 films. Secondly, during the high-temperature ammoniating process, ZnO buffer layer will volatilize. The volatilization of ZnO buffer layer plays an important role in the formation process of GaN nanostructures. It is unavoidable to form some defects and dislocations, which can subsequently be used as a mask to fabricate the nanostructures or act as potential nucleation sites for the GaN nanostructures. Simultaneously, it is well known that when the ammoniating temperature is above 850℃, NH3 decomposes stepwise to NH2, NH, H2 and N. The Ga2O3 particles should be reduced to gaseous Ga2O by H2 and then the reaction of Ga2O with ammonia result in formation of GaN crystal nuclei at the above-mentioned sites. Then the very small GaN granules grow up with the progress of the ammoniating process and accordingly lay the foundations for the growth of nanostructured GaN. These GaN molecules continuously come out and agglomerate into micrograins. When the growth direction of the micrograins orientate in the same direction, the single GaN nanostructures are formed. The growth mechanism can be decribed as vapor-solid (VS) mechanism.5. Preparation of high quality GaN filmsGaN films are fabricated on ZnO layer as a buffer layer on Si substrates by magnetron sputtering system. The results reveal that The thickness of the buffer layer has great effect on the properties of the GaN film: if the buffer layer is too thin, it can not relax the strain which is created during the growth due to the large differences between the lattice constants and the thermal coefficients, resulting in many cracks on the surface of the films and bad crystal quality; if the buffer is too thick, the orientation relationship between the GaN and the Si substrates could not be established, at the same time a thick buffer layer could not serve as a smooth and planar template for the GaN epitaxy, resulting in a rough surface and bad crystal quality of the GaN film. The best thick of the buffer layer in this method is 15min. |
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