Growth and Characterization of Gan Bulk Crystals Via Vapor Phase Transport
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GROWTH AND CHARACTERIZATION OF GAN BULK CRYSTALS VIA VAPOR PHASE TRANSPORT H. Shin, D. B. Thomson, P. Q. Miraglia, S. D. Wolter, R. Schlesser, Z. Sitar, and R. F. Davis Department of Materials Science and Engineering, North Carolina State University, Raleigh, NC, 27695-7907 ABSTRACT Free-standing single crystals of bulk GaN were grown via unseeded vapor phase transport at 1130oC on hexagonal BN surfaces via direct reaction of Ga with ammonia. The number of nucleation events was reduced and the crystal size increased by introducing the ammonia at high temperatures. The resulting crystals were either needles or platelets depending on the process variables employed. Low V/III ratios achieved via ammonia flow rates ≤ 75sccm and/or ammonia total pressures ≤ 430Torr favored lateral growth. The average lateral growth rate for the platelets was ~50µm/hr; the average vertical growth rate for the needles was ~500µm/hr. Growth rates in all other directions for each of these two morphologies were very low. Seeded growth of both needle and platelet crystals was also achieved; however, the growth rate decreased at longer times and higher pressures due to reaction with H2 from the increased decomposition of ammonia. Nitrogen dilution suppressed this decomposition. A 2mm x 1.5mm GaN crystal was grown with minimal decomposition in a 66.7%NH3 and 33.3%N2 gas mixture. INTRODUCTION Bulk GaN crystals have been produced by high pressure-high temperature solution growth[1], hydride vapor phase epitaxy[2], sublimation growth[3], flux growth[4], and vapor phase transport(VPT)[5]. The characteristics of the VPT technique include high growth rate(>500µm/hr), good aspect ratio control, a continuous supply of Ga and low production costs. However, there are difficulties inherent in this technique that must be surmounted to achieve the growth of crystals and boules of GaN of reasonable size and perfection. Numerous nucleation events and the resulting formation of many small crystals have been major problems in growth via direct reaction of Ga vapor with ammonia[6]. To obtain macroscopic crystals, nucleation during the temperature ramp-up must be avoided. Aspect ratio control is another critical issue in bulk growth. It has been suggested[5,6] that a higher Ga flux promotes lateral growth; however, no systematic investigation has been conducted to date. Prolonged growth times are necessary to obtain large crystals. However, preliminary experiments in the present research revealed that growth ceased after a short period, and the crystals began to decompose. This paper concerns our research regarding the growth of bulk GaN via vapor phase transport. We developed a high temperature nucleation technique to grow seed crystals and investigated processing windows for vertical and lateral growth to control the aspect ratio. We employed seeded growth to avoid multiple nucleation. We also studied the decomposition of GaN as a function of ammonia total pressure for continuous growth. Nitrogen dilution of ammonia has been investigated as a new and effective p
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