Investigation Of Synthesis Of GaN Nanostructures Through Ammoniating Ga₂O₃/Tb Films Magnetron Sputtered

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.In this paper, one-dimensional nanostructures are synthesized by ammoniating the Ga2O3/Tb films. The structure, composition, morphology, and optical properties of the as-prepared GaN nanostructures are studied thoroughly. The growth mechanism of GaN nanostructures is explored. The properties of the intermediate layer and its effect on GaN nanostructures are also studied. All the results are as follows:1. Synthesis and characteristics of one-dimensional GaN nanostructures through magnetron sputtering and ammonification methodRare earth metal Tb is used as the intermediate layer; Tb films are deposited on Si substrates by radio frequency magnetron sputtering system. Then thick Ga2O3 films (about 500nm) are sputtered on Tb layer by JCK-500A magnetron sputtering system. The results reveal that different anneal temperature and different ammoniating time of Ga2O3/Tb films and different thickness of Tb layer 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 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 two peaks located at 369nm and 387nm, respectively. With changes of experimental parameters,the locations of the two peaks move hardly, and only the intensity of them change. Band-edge emission is observed in these nanostructured samples located at 369nm. As GaN films are formed with Tb intermediate layers, so the emission peak at 387nm may be ascribed to the electron transition from the deep acceptor level, resulting from C impurity taking place of N atoms, to the conductor band, and or the existence of defects or surface states, which is attributed to the rearrangement of GaN during the ammonization process.3. Exploration of the growth mechanism for GaN nanostructuresAlthough the melting point (1360℃) of bulk metal Tb is higher than the reaction temperature we used, the melting point of nanoparticles is lower than the corresponding bulk materials. Tb film breaks up at the reaction temperature and melted Tb 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 Tb 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.