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The remarkable progress in nonpolar and semipolar devices based on gallium nitride (GaN) in recent years has been driven by not only advancements in the epitaxial growth technique but also improvements in the quality of bulk nonpolar and semipolar GaN substrates. At present, high-quality nonpolar/semipolar substrates are only made by slicing thick bulk GaN crystals grown by hydride vapor-phase epitaxy (HVPE). Although HVPE is currently the most successful method for obtaining high-quality bulk GaN crystals, it is still difficult to obtain uniform crystals with large diameters and thicknesses. The size of the nonpolar/semipolar substrates has been limited by the growth thickness along the c-axis of bulk GaN crystals. Here we review the growth of bulk GaN crystals by HVPE to achieve high-quality and large-sized nonpolar and semipolar substrates.
Ammonothermal growth of GaN was studied to determine its eventual utility for mass production of GaN bulk crystals. Dissolution of GaN in supercritical ammonia with 1 M NaNH2 was investigated through a weight loss method. The time dependence of the weight loss was examined at 500°C and 525°C. Since the weight loss did not reach saturation as a function of time, the solubility limit was not realized. However, experiments demonstrate that GaN has a negative temperature dependence of solubility in supercritical ammonobasic solutions. Based on this result, GaN was grown via fluid transport from metallic Ga to a free-standing GaN single crystal seed by placing the seed crystal in a higher temperature zone and the nutrient in a lower temperature zone. GaN films with thickness of 5 μm (Ga face) and 4 μm (N face) were simultaneously grown on the seed in three days. The surface morphology, optical property, and defect density were found to be different for films on Ga face and N face.
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