| Gallium nitride (GaN) is one of the most important materials for semiconductor devices since silicon due to its very large direct band gap (3.39eV), high saturation velocity (3×107cm/s) and some other excellent properties. Currently, GaN has been extensive applied in photoelectric devices (LEDs and LDs) and microelectronic devices (high-temperature, high-power, and high-frequency transistors). Although GaN-based devices exhibit a fast and astonishing development in the past few years, there are still many difficulties remain challenging and unresolved.As is known, the performance of GaN-based devices is highly dependent on the substrate materials. Since the high low solubility of N in metal Ga and high steam pressure of N in GaN, the growth of large area single crystal GaN substrate is extremely difficulty. At present, GaN are deposited mainly by high temperature metal organic chemical vapor deposition (MOCVD), mostly on a single-crystal sapphire or silicon carbon substrate, to ensure high crystal quality. However, the performance of GaN devices is often restricted by the poor electric conductivity and the low thermal conductivity of the sapphire substrate as well as a remaining lattice mismatch between GaN and the substrates. Moreover, SiC substrates are expensive and of small-area compared with sapphire substrates, which is not suitable for practical application. On the contrary, metal can be chosen as substrates for its perfect thermal conductivity, electrical conductivity, as well as low cost and large area to achieve vertical transmission of current in subsequent GaN-based devices. Ni is considered to be one of the most promising substrates for the large area low cost GaN-based photonic devices due to its excellent properties such as its excellent electrical and thermal conductivity, relatively low lattice mismatch with GaN, high temperature resistance, good corrosion resistance, high light reflection, as well as low cost and large area. These features are absolutely imperative for the subsequent GaN-based semiconductor devices.In this article, highly c-axis oriented GaN films were grown on Ni metal substrate by electron cyclotron resonance plasma enhanced metal organic chemical vapor deposition (ECR-PEMOCVD) technique with TMGa and N2employed as Ga and N source, respectively. During the conventional deposition process, the high temperature (>1050℃) may cause serious interfacial reactions as well as tremendous thermal stress between the GaN films and the metal substrates, thus severely degrade the device performance. In order to suppress the adverse effects of high deposition temperatures, ECR-PEMOCVD technique was highly desirable to deposit crystalline GaN films on Ni metal substrate at low temperature.In this article, by means of reflection high energy electron diffraction (RHEED), X-ray diffraction (XRD), atomic force microscopy (AFM), scanning electron microscope (SEM), room temperature photoluminescence (PL) and Current-voltage (Ⅰ-Ⅴ) measurement, crystal structure, surface morphology, optical performance and electrical properties of GaN films were investigated systematically. The influences mechanism of nitridition time, deposition temperature, TMGa flux on the properties of GaN films on Ni metal substrates were investigated intensively, as well as the optimum parameters of nitridition time, deposition temperature and TMGa flux were achieved. The results suggest that the crystalline quality of GaN films is dependent on the nitridation time. When the nitridation time is0min, GaN films with high the crystalline quality and strong ultraviolet (UV) emission peak were successfully obtained. Meanwhile, the crystalline quality of GaN films is also dependent on deposition temperature and TMGa flux. Deposition temperature of480℃and TMGa flux of1.2sccm is the optimized parameter to obtain GaN films with high the crystalline quality and strong ultraviolet (UV) emission peak. Under the optimized parameter, the contacts characteristics between GaN films and Ni substrates are excellent ohmic contacts. |
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