Ammonothermal growth of GaN utilizing negative temperature dependence of solubility in basic ammonia
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Ammonothermal growth of GaN utilizing negative temperature dependence of solubility in basic ammonia Tadao Hashimoto, Kenji Fujito, Feng Wu, Benjamin A. Haskell, Paul T. Fini, James S. Speck, and Shuji Nakamura ERATO/JST UCSB group, Santa Barbara, CA 93106-5050, USA ABSTRACT 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 500oC and 525oC. 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. INTRODUCTION Nowadays GaN and its related alloys are used in a variety of light-emitting devices and electron devices. Although heteroepitaxially grown devices are already in the market, basic problems arising from heteroepitaxy (e.g., on sapphire) still need to be solved to realize highefficiency, highly reliable, and low-cost devices. Using GaN wafers sliced from bulk GaN crystals would be a direct and ultimate solution for these problems. Several growth techniques based on a Ga melt have been investigated to grow bulk GaN crystals [1-4]. However, since nitrogen has a very limited solubility in molten Ga, these attempts usually result in thin platelets of GaN. In addition, considering the rapid decrease in the market prices of blue and white LEDs grown on sapphire or SiC, a bulk growth method must be expandable to an industrial production scale to compete with existing substrates. Ammonothermal growth, which is a solvothermal growth method that uses ammonia as a fluid, is a promising candidate due to its high solubility of source materials, fast transport of dissolved species, and excellent scalability. Our research on ammonothermal growth of GaN is strongly motivated by successful mass production of synthetic a-quartz by hydrothermal growth. However, there are only a limited number of groups reporting the ammonothermal growth of GaN so far [5-8], and the chemistry of the GaN-supercritical ammonia system is poorly understood. Although some analogies with quartz growth could be applied to ammonothermal growth of GaN, growth conditions such as temperature, pressure, type of mineralizers, autoclave configuration, etc. are far from optimum. To achieve GaN bulk growth in supercritical ammonia with an appreciable growth rate, the
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following three conditions mus
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