Crucible Selection in AIN Bulk Crystal Growth
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Crucible Selection in AlN Bulk Crystal Growth Rafael Dalmau1, Balaji Raghothamachar2, Michael Dudley2, Raoul Schlesser1, and Zlatko Sitar1 Dept. of Materials Science and Engineering, North Carolina State University, Raleigh, NC 27695-7919, U.S.A. 2 Dept. of Materials Science and Engineering, State University of New York at Stony Brook, Stony Brook, NY 11794-2275, U.S.A. 1
ABSTRACT Growth of AlN bulk single crystals by sublimation of AlN powder was carried out in a resistively heated reactor at temperatures up to 2300°C. A variety of crucible materials, such as BN, W, Ta, Re, ZrO2, TaN, and TaC, were evaluated. Our studies have shown that the morphology of crystals grown by spontaneous nucleation strongly depends on the growth temperature and contamination in the reactor. Crucible selection had a profound effect on contamination in the crystal growth environment, thus influencing nucleation, coalescence, and crystal morphology. Spontaneously grown single crystals up to 15 mm in size were characterized by x-ray diffraction (XRD), x-ray topography (XRT), glow discharge mass spectrometry (GDMS), and secondary ion mass spectrometry (SIMS). Average dislocation densities were on the order of 103 cm-3, with extended areas virtually dislocation-free, while high-resolution XRD showed rocking curves as narrow as 7 arcsec. INTRODUCTION The lack of closely lattice-matched substrates for GaN based epitaxy has limited potential applications for devices based on III-nitride semiconductors. The lattice mismatch between currently used substrates, such as silicon carbide and sapphire, and the overgrown layers leads to a high density of defects in the heterostructures, deteriorating device performance and lifetime. A significant reduction in the defect density of the device layers may be achieved by growing them on single crystalline AlN or GaN substrates. The high thermal conductivity, large optical bandgap, similar thermal expansion coefficient, and small lattice mismatch of AlN make it an excellent substrate choice for GaN epitaxy. AlN bulk crystal growth has been demonstrated by a number of groups [1-5]. Based on these results, it appears that the most promising method for producing large, high-quality AlN single crystals is by sublimation of AlN source material. Due to the very high process temperatures (typically >2100°C) required to achieve commercially-viable growth rates, furnace design and materials selection is critical for achieving durability of the growth hardware and keeping crystal impurity levels low [4, 6, 7]. In particular, crucible materials must withstand repeated temperature cycling and highly reactive Al vapor. In the present study a survey of the influence of reaction crucibles on crystal growth and morphology will be presented. EXPERIMENT Crystal growth experiments were carried out in a resistively heated reactor equipped with a cylindrical graphite heater. Details on the growth equipment have been published elsewhere [3]. Typical growth parameters included 400–600 Torr reactor pressure, 100 sccm nitro
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