Oxidation of Gallium Nitride Epilayers in Dry Oxygen
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ABSTRACT The oxidation of GaN epilayers in dry oxygen has been studied. The 1-µm-thick GaN epilayers grown on (0001) sapphire substrates by Rapid-Thermal-Processing/Low Pressure Metalorganic Chemical Vapor Deposition were used in this work. The oxidation of GaN in dry oxygen was performed at various temperatures for different time. The oxide was identified as the monoclinic β-Ga2O3 by a θ-2θ scan X-ray diffraction (XRD). The scanning electron microscope observation shows a rough oxide surface and an expansion of the volume. XRD data also showed that the oxidation of GaN began to occur at 800oC. The GaN diffraction peaks disappeared at 1050oC for 4 h or at 1100oC for 1 h, which indicates that the GaN epilayers has been completely oxidized. From these results, it was found that the oxidation of GaN in dry oxygen was not layer-by-layer and limited by the interfacial reaction and diffusion mechanism at different temperatures.
I. INTRODUCTION With the great progress of the applications of GaN on optoelectronic devices and high temperature / high power electronics, GaN is more attractive than ever[1,2]. The metal-oxide-semiconductor field effect transistor (MOSFET) based on β-Ga2O3/GaN has been reported[3], and it shows a better performance at 400oC than that at room temperature. In the many polymorphs of Ga2O3, β-Ga2O3 is believed to be the equilibrium phase. It is very important to understand the oxidation mechanism of GaN at high temperature for the realization of these electronic devices. The kinetics of the oxidation of GaN powder in dry air has been reported by Wolter et al.[4]. An oxide forms at 900oC for 1 h, and is identified as the monoclinic β-Ga2O3. The initial stage of the oxidation is found to be limited by the rate of an interfacial reaction. The same mechanism was also found in the oxidation of GaN epilayers at 900oC in dry air in their report. However, there is little known about the oxidation of GaN epilayers in dry oxygen. In this letter, we report a scanning electron microscope (SEM) observation and the oxidation behavior of GaN epilayers at temperatures from 800oC to 1100oC in dry oxygen. The results reveal the different oxidation mechanism at different temperatures.
II. EXPERIMENT The GaN epilayers (about 1-µm thick) used in the experiments are grown on a cplane of sapphire substrates by rapid thermal processing low-pressure metalorganic chemical vapor deposition (RTP/LP-MOCVD). The detailed growth process has been published previously[5]. The full width at half maximum (FWHM) of 9.8 min of X-ray
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rocking curve (XRC) indicates the high quality of the GaN film. A 1.5-inch diameter sample is divided into several pieces for oxidizing under various conditions. After cleaned in the solvents and DI water, the samples are etched in HCl:DI water (1:1) to remove surface contamination and the native oxide. Then the samples are placed in a quartz boat, which will be transferred into a horizontal quartz tube, and the tube is placed in a electric furnace to oxidize the samples at temperatures (from
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