Growth of GaN crystals under ammonothermal conditions
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Growth of GaN crystals under ammonothermal conditions Michael J. Callahan1, Buguo Wang2*, Lionel O. Bouthillette1, Sheng-Qi Wang3*, Joseph W. Kolis2, David F. Bliss1 1) Sensors Directorate, Air Force Research Laboratory, Hanscom AFB, MA 01731, USA 2) Department of Chemistry, Clemson University, SC 29634, USA 3) Solid State Scientific Corporation, 27-2 Wright Road, Hollis, NH 03049, USA
ABSTRACT Growth of GaN bulk crystals under ammonothermal conditions has been developed. The experiments were performed in ammono-basic solutions in high nickel content autoclaves for up to 3 weeks. Nutrients were crystalline GaN made from vapor phase transport growth. Single crystal clusters of GaN on the order of 500 µm – 1 mm long were obtained. These crystals were spontaneously nucleated on the walls of the autoclave. Transport growth on polycrystalline GaN seeds and single crystal HVPE seeds was also achieved. GaN has a high solubility in ammono-basic solutions, on the order of several weight percent. The ammonothermal crystals were characterized by photoluminescence (PL), X-ray diffraction and SEM. INTRODUCTION The nitride device market has grown to $1.35 billion annually and is expected to grow to $4.5B in 20071. Despite the advancement of nitride based devices such as LEDs, high power FETs, and laser diodes many these devices could have even greater reliability, lower cost, and higher efficiency if an inexpensive commercial GaN substrate were available. Currently, thin films for device structures of gallium nitride and its corresponding alloys of aluminum and indium are grown on heterogeneous substrates such as sapphire and silicon carbide which generate a large number of defects (up to 1010 dislocations/cm2). Various growth schemes and thick film techniques have reduced the dislocations to less than 108 /cm2 but these techniques increase complexity and drive up cost2-4. Thus, there is a great demand for low cost, low dislocation (< 104 dislocations/cm2) GaN substrates. Traditional molten techniques used to synthesize Si, GaAs, and InP bulk crystals cannot be employed in GaN crystal growth because of the extreme nitrogen pressure that is needed to melt GaN (estimated to be in excess of 2200° C and 60 kbars5). Other techniques, such as sublimation, extreme high-pressure gallium solution growth, and flux growth, 6-8 have been suggested to grow large crystals at high growth rates but none have yet proven to be cost effective. Intensive research has been done over the years on the growth of large single crystals (> 1cm3) of many different compounds (ZnS, KTP, ZnO, AlPO4, etc.) by the hydrothermal method. None of these have had the success of α-quartz, which is produced by the millions of kilograms each year by the hydrothermal method. Ammonothermal growth is the analogue of hydrothermal growth using super critical ammonia as the solvent instead of water. This is because water and ammonia are both polar solvents with similar properties. Very high purity *
Research performed at Air Force Research Laboratory, Sensors Direct
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