GaN Film Growth by a Supersonic Arcjet Plasma
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ABSTRACT This paper reports on preliminary studies of GaN growth obtained using a supersonic nitrogen arcjet plasma expanding into low pressure. A hydrogen-free and carbon-free growth environment was achieved by use of a Ga vapor source positioned downstream of the expanding plume. GaN growth rates of around 8 gin/hour were obtained in these preliminary studies. We believe these growth rates are the highest reported for a non-chemical decomposition process. The growth rates are attributed to the very high nitrogen atom fluxes estimated to be on the order of 10'9 atoms/cm 2/sec. The initial growth rates are believed to be well below those possible with a fully optimized system. The GaN films are characterized by Raman spectroscopy, visible-IR transmission, and x-ray diffraction. The deposited films appear to be single crystal and epitaxial on basal plane sapphire substrates. INTRODUCTION Gallium nitride (GaN) is difficult to grow in thick, high-quality, single-crystal layers. The available epitaxial growth approaches, which can be considered for growing such layers, can be divided into two broad categories. The first growth category encompasses all MBE type approaches which invariably use low pressure electron-cyclotron resonance (ECR) nitrogen plasmas to produce a flux of excited or ionized nitrogen molecules (N 2'), and neutral nitrogen atoms (N) [1-3]. GaN deposition using these approaches generally exhibit growth rates not exceeding I tm/hour (more typically 0.1 - 0.2 nin/hour [2,3]) which is quite inadequate for thick layer growth. The second broad GaN growth category includes all processes based on some form of chemical decomposition to produce Ga and N -containing radicals either at the substrate surface or in the vapor above the substrate. Processes such as vapor phase epitaxy (VPE) or metalorganic
vapor phase epitaxy (MOVPE) are included in this category. The growth rate in these cases is diffusion-limited and rates of up to 100 gin/hour have been reported [4,5]. However, GaN grown by these techniques may contain high concentrations of process by-products such as carbon (i.e., from MOVPE growth). Also, MOVPE and VPE methods seem to lead to GaN films with relatively high background n-type carrier concentrations [2].
We have performed preliminary studies into the possibility of an alternative technique, based on an arcjet plasma supply of a very high flux (10 '9/cm 2/sec) of nitrogen atoms. The arcjet in our experiments was a direct-current (DC) arc plasma discharge expanded to low densities (1016 molecules/cm 3) and high supersonic speeds (Mach numbers of 2-3). This low density, supersonic plasma jet is achieved by appropriately contouring the discharge nozzle and maintaining approximately 0.3 - 1 torr of background pressure in the vacuum chamber into which the jet discharges. Plasma jets of this type, operating on hydrogen, have been previously employed by our group to deposit high quality diamond films [6,7]. Within the context of GaN synthesis, this approach falls into neither of the two general categor
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