| The GaN material system is gathering great interest because of its potential application to photonic and electronic devices, as well as high-voltage, high-power devices, due to its wide band gap (3.39eV) and other excellent properties such as high saturated drift velocity, high breakdown voltage. GaN films are manufactured in factory for application as blue and green light emitting diodes (LED), blue laser diodes (LD), high power diodes and so on.GaN-visible LED have rapid advancements to beceome a potential viable alternatives to traditional lighting fixture because the achievement of the white-light sources based on InGaN/GaN multiple-quantum-well blue LED chip with phosphor coating or a combination of red-, green-and blue-emitting LED chips.As is well known that the performance of GaN-based devices is highly dependent on the substrate materials. GaN are fabricated on a single crystal sapphire substrate, SiC substrate or Si substrate, generally. But for these substrates, there are some defects in lattice-match, conductivity and so on. Large mismatch in lattice constants between GaN film and the sapphire substrate is a cumbersome problem. In addition, the poor conductivity in electronic and heat of sapphire substrate is a big problem. For SiC, high cost limits its commercialization. Mismatch in thermal expansion coefficients between GaN film and Si substrate always cause GaN film chapped.Recently, there are researchers want to prepare GaN on metal substrates directly for metals'good properties such as good conductivity, low cost and reflection of light. These properties may improve the heat dispersion and the light out-coupling efficiency of the device. Importantly, metals can be used as electrode down to make vertical structure GaN devices which easier than laser lift-off.In this article, highly c-axis oriented GaN films are deposited on metal Ti and Mo by electron cyclotron resonance plasma enhanced metal organic chemical vapor deposition (ECR-PEMOCVD) technology, with TMGa and high purity N2 as Ga and N sources, respectively. During the traditional MOCVD process, high temperature, always above 1050?, will cause serious interface reaction and huge thermal stress between GaN and the substrate, which seriously affect the properties of the subsequent GaN-based device. We introduce ECR plasma activation technology to produce non-equilibrium plasma under low pressure, providing the high activation of N ion, which will greatly reduce the deposition temperature.In this article, we study the crystal structure, surface morphology, optical properties and electrical properties of the GaN films by the reflective high energy electron diffraction (RHEED), X-ray diffraction (XRD), scanning electron microscope (SEM), room temperature photoluminescence spectrum (PL spectrum) and current-voltage (I-V) test. We have mainly studied the influnces of deposited temperatures and TMGa flows on properties of GaN films. Results show that the best deposition temperature for GaN films on Ti substrates is 480 ?, and the best TMGa flow rate is 1.4sccm. Good crystal quality, uniform, density and weak photoluminescence GaN film has be preparated under optimal conditions, and the conduct characteristics betwwen GaN films and the Ti substrates are schottky contact. The best deposition temperature for GaN films on Mo substrate is 570 ?, and the best TMGa flow rate is 1.6 seem. Under the optimal conditions, highly c-axis oriented GaN film has be deposited, and they have better crystal quality and stronger photoluminescence. The conduct characteristic betwwen GaN films and the Mo substrates are schottky contact. |
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