Growth and Characterization of Bulk GaN Crystals at High Pressure and High Temperature
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Growth and Characterization of Bulk GaN Crystals at High Pressure and High Temperature M. P. D’Evelyna, K. J. Naranga, D.-S. Parka, H. C. Honga, M. Barbera, S. A. Tysoea, J. Lemana, J. Balcha, V. L. Loua, S. F. LeBoeufa, Y. Gaoa, J. A. Teetsova, P. J. Codellaa, P. R. Tavernierb, D. R. Clarkeb, and R. J. Molnarc a GE Global Research Center, Niskayuna, NY 12309 USA b Materials Department, College of Engineering, University of California at Santa Barbara, Santa
Barbara, CA 93106-5050 USA c Massachusetts Institute of Technology, Lincoln Laboratory, Lexington, MA 02173-9108 USA
ABSTRACT We report the growth and characterization of bulk GaN single crystals by temperaturegradient recrystallization at high pressure and high temperature (HPHT), using apparatus adapted from that used to synthesize gem-grade diamond crystals. The bulk crystals are grown on seeds that were synthesized by hydride vapor phase epitaxy (HVPE) and subsequently removed from their sapphire substrate. Our largest crystals to date are 15×18 mm in diameter; however, the process is scalable to 50 mm and above. The crystals are transparent and well faceted, and dislocation densities below 100 cm-2 have been achieved. Additional characterization of the GaN crystals is also presented. INTRODUCTION Gallium nitride based optoelectronic and electronic device applications have grown spectacularly during the past decade, led by light-emitting diodes (LEDs) and laser diodes. However, the performance and reliability of these devices are compromised by the lack of high quality, cost-effective native substrates. Threading dislocations, with a typical concentration of 109-1010 cm-2 in heteroepitaxial GaN layers on sapphire or SiC, decrease the efficiency and maximum intensity of light emission in ultraviolet LEDs [1] and the lifetime of laser diodes [2]. Many laboratories and companies are working on growth of bulk and pseudo-bulk GaN substrates but no fully satisfactory solution is yet at hand. By far the most mature technology is the growth of thick quasi-bulk GaN substrates, typically on sapphire, GaAs, or LiAlO2, by hydride/halide vapor phase epitaxy, or HVPE [3,4]. HVPE is currently capable of routinely producing free-standing GaN wafers with a dislocation density of about 107 cm-2, although lower values have been reported over at least restricted regions (e.g., within a grain) [4]. However, as a one- or few-at-a-time growth technique, wafers produced by this method are likely to remain rather expensive. In addition, for a number of device applications, lower dislocation densities may be required. Growth of GaN in liquid Ga under a high N2 pressure is capable of growing high-quality platelets, approximately 10 mm × 10 mm × 100 µm in dimension with dislocation densities in the range of 10-105 cm-2 [5]. However, the growth conditions are severe (10-15 kbar pressure), growth rates are rather small, and no one has been able to demonstrate routine growth of larger crystals by this method, despite extensive efforts by several groups [5,6]. Interesting results
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