Investigation Of NiCl₂ Catalytic Synthesis Of GaN Nanostructures

GaN based III-V nitrides have attracted great attention recently, as driven by the commercial success in their application in optoelectronic devices, such as blue light-emitting-diode (LED), ultraviolet (UV) photo-detector and short wavelength laser diode. 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 letter, GaN nanowires were fabricated on Si substrates coated with NiCl₂ thin films using chemical vapor deposition (CVD) method by evaporating Ga₂O₃ powder at the temperature of 1050℃, 1100℃and 1150℃, which are higher than those that have been reported for GaN nanowires growth. But this growth method allows a continuous synthesis and produces single-crystal GaN nanowires at relatively high purity and low cost. 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. All the results are as follows:1. Synthesis and characteristics of one-dimensional GaN nanostructures through CVD methodFirstly, the NiCl₂ thin films were formed by dipping method. GaN nanostructures were synthesized with Ga2O3 powder as Ga source and NH3 as N source through CVD method. We research on the synthesis of GaN nanostructure by changing the reaction time, reaction temperature, source of volume, ammonia flow, the distance between source and substrate, as well as the concentration of NiCl₂.The results reveal that different condition have a great influence on the synthesis of GaN nanostructures. The synthesized nanostructures are of hexagonal wurtzite single-crystal GaN, showing their morphologies as nanowires, nanorods, and radial-shaped nanostructures.2. Optical properties of GaN nanostructuresA strong emission peak at 367nm (ultraviolet band) was found from the GaN nanostructures, which were excited by 325nm light at room temperature. Because the as-prepared GaN nanostructures is too large for quantum confinement, and in fact the Bohr exciton radius of GaN is 11nm. The emission peak at 367nm has no blue shift of the band-gap emission compared with the bulk GaN.3. Exploration of the growth mechanism for GaN nanostructuresThe NiCl₂ film breaks down at the reaction temperature and Ni liquid droplets could subsequently supply favorable nucleation sites for GaN nanowires. At the same time, NH₃ decomposes step by step to NH₂, NH, H₂ and N when the 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. The NiCl₂ thin film plays an important role in the nanowires growth because we did not find any of this kind of nanostructures on the unpolished Si substrates without NiCl₂ under the same conditions. Consequently, we believe that Ni 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.