Synthesis And Properties Of GaN Nanostructures And GaN Thin Films By Ammoniating Ga₂O₃/Co Films Deposited On Si(111) Substrates

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. In contrast to the intensive theoretical and experimental efforts made on optical and electrical properties of nitrides, kinetic process in growth of nitrides films remains less explored.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. However, there are several diffieulties to grow single crystalline GaN Films directly on the Si substrate l) the high lattices mismatch (17%) and the differences in the thermal expansion coefficient (100%) between GaN (in wurtzite strueture) and Si; 2) the Poor wetting of GaN on Si; 3) the nitridation of the Si surface during growth of nitrides. To overcome these impediment, two-step method are used extensively: a buffer layer is firstly grown between the Si substrate and the GaN film, then GaN epilayer is grown on it. Results indicate that the quality of the film increased remarkably by using buffer layer.In this paper, GaN nanostructures have been synthesized on Si(111) substrates by ammoniating Ga₂O₃/Co films. First, Co 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 nanostructuresCo buffer layers are deposited on Si substrates by radio frequency magnetron sputtering system. A thick Ga2O3 films (about 500nm) are sputtering on Co buffer layer by JCK-500A magnetron sputtering system. Then Ga₂O₃/Co 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 mooth 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 10min). As the annealing time increasing, the quality and surface morphology of GaN nanostructure increased and then decreased. When the annealing time rose to 25 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 Co was deposited on the Si substrates as the buffer layer with the thickness of 10nm and 30nm. When the thickness of the buffer layer was 10nm, the GaN nanostructures with higher quality and better morphology were acquired.5.Exploration of the growth mechanism of GaN nanostructureDuring the course of the growth of GaN nanostructure, the buffer layer (Co layer) is very important. At the ammoniating temperature, Co layer with the thickness only tens of nanometer melted into small liquid drop. The small liquid drop distribute on the surface of the substrate, so the energy of the surface will be changed, and which can produce some perfect place to grow GaN nanostructures. We first present the GaN growth theory of defect energy gathering, and explain the growth process of nanostructures.6.Preparation of GaN films by ammoniatingThe Ga2O3/Co films were deposited on the Si substrates by sputtering Co target for 10 min and Ga2O3 target for 90 min, separately. Then, the Ga₂O₃/Co films were ammoniated in an ammonia atmosphere. The temperatures of the ammoniating were 900℃, 950℃, 1000℃and 1050℃, respectively. We studied the structure and morphology of as-synthesized GaN films.