| GaN is an excellentⅢ-Ⅴwide-band gap semiconductor material, and can endure high temperature, radiation, and acid and alkali, with high luminescent efficiency, high thermal conductance efficiency, high intensity and high rigidity. The Eg = 3.4eV for GaN at room temperature. Light emission from infrared light to ultraviolet light and the full color panel display of red, yellow, and blue light can be actualized; therefore, GaN has important applications foreground in 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 techniques 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 hexagonal GaN materials on Si substrates 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. According to the requirement to decrease the size of the device, nano-sized materials is of great significance nanodevices because of their excellent natures. GaN nanowires are the most promising material.In this paper, GaN nanostructures have been synthesized on Si(111) substrates by ammoniating Ga2O3/ Cr films. First, Cr films were deposited on Si(111) substrates by magnetron sputtering system. The structure, elemental composition and morphology of the GaN nanostructures were determined by X-ray diffraction (XRD), Fourier transformed infrared spectroscopy (FTIR), X-ray photoelectron energy spectroscopy (XPS), scanning electronic microscope (SEM) and high-resolution transmission electronic microscope (HRTEM). According to the analysis and discussion about the results, we have a primary discussion about the growth progress of GaN nanostructures. We have synthesized GaN crystal films with novel and simple two steps growth pattern and studied the influence of annealing temperature on the properties of GaN films.1. Synthesis of one-dimensional GaN nanostructuresCr buffer layers are deposited on Si substrates by radio frequency magnetron sputtering system. A thick Ga2O3 films (about 500nm) are sputtering on Cr buffer layer by JCK-500A magnetron sputtering system. Then Ga2O3/Cr films are annealed in NH3 ambient. After reaction, a deposit of light-yellow layer was found on the substrate surface. The results showed that the as-synthesized light-yellow layer was composited with large-scale one-dimensional GaN nanostructures.2. Influence of ammoniated temperature on the properties of GaN nanostructureThe results reveal that different annealing temperature have a great influence on the crystal quality and the surface morphology of the GaN nanostructures. And we find the optimally ammoniated temperature is 950℃. The synthesized nanostructures are of hexagonal wurtzite single-crystal GaN and have smooth surface.3 . Effect of annealing time on the properties of GaN nanostructuresThe morphologies and quality of GaN nanostructures were affected greatly by annealing time. Choosing suitable annealing time can get ideal one-dimensional GaN nanostructures. There were no synthesized one-dimensional GaN nanostructures if the annealing time was too low (lower than 5min). As the annealing time increasing, the quality and surface morphology of GaN nanostructure increased and then decreased. When the annealing time rose to 20 min, we could not found the one-dimensional GaN nanostructures anymore.4. Constract of the GaN nanostructure synthesized with different thickness of the buffer layerMetal Cr was deposited on the Si substrates as the buffer layer with the thickness of 5nm,20nm and 100nm.When the thickness of the buffer layer was 5nm, the GaN nanostructures with the highest quality and the best morphology were acquired.5. Constract of the GaN nanostructure synthesized with Polished substrates and un-polished substrates.Polished substrates and un-polished substrates were used to fabricated GaN nanostructures. When the substrates were un-polished, the GaN nanostructures with higher quality and better morphology were acquired. 6. Exploration of the growth mechanism for GaN nanostructuresAlthough the melting point (1857℃) of bulk metal Cr is higher than the reaction temperature we used, the melting point of nanoparticles is lower than the corresponding bulk materials. Cr film breaks up at the reaction temperature and melted Cr liquid droplets could supply subsequently favorable nucleation sites for GaN nanostructures. At the same time, NH3 decomposes step by step to NH2, NH, H2 and N when ammoniating temperature reaches 850℃. Ga2O3 particles are reduced to gaseous Ga2O by H2 and then GaN molecules are synthesized through the reaction of Ga2O and ammonia. Ga2O and NH3 gases evaporate and travel to the substrate, where the catalytic reaction takes place. The formed GaN molecules diffuse and agglomerate into GaN crystalline nuclei, and then the very small GaN crystalline nuclei grow up gradually with the progress of the ammonification. Nanodroplets are found at the tips of many nanostructures; the growth process follows probably VLS mechanism. We also deposited the Ga2O3 film directly onto Si substrates with the same condition, but no nanostructures were formed. Consequently, we believe that Cr probably acts as the nucleation sites for GaN embryos and plays the role of catalyst in the reaction, as nanostructures tend not to grow without it.
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