New Technique for Sublimation Growth of AlN Single Crystals
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Internet Journal Nitride Semiconductor Research
New Technique for Sublimation Growth of AlN Single Crystals Y. Shi1, B. Liu1, Lianghong Liu1, J.H. Edgar1, E.A. Payzant2, J. M. Hayes3 and Martin Kuball 3 1Kansas
State University, Department of Chemical Engineering, Ridge National Laboratory, High Temperature Materials Laboratory, 3University of Bristol, H. H. Wills Physics Laboratory, 2Oak
(Received Thursday, January 11, 2001; accepted Sunday, March 4, 2001)
Single crystalline platelets of aluminum nitride (AlN) were successfully grown by a new technique. It consists of (1) depositing an AlN buffer layer on a SiC substrate by metal organic chemical vapor deposition (MOCVD) below 1100°C, (2) forming an (AlN)x(SiC)1-x alloy film on the AlN film by condensing vapors sublimated at a temperature of 1800°C from a source mixture of AlN-SiC powders, followed by (3) condensing vapors sublimated from a pure AlN source (at 1800°C). The necessity of the first two steps for the successful AlN sublimation growth on SiC substrate was illustrated by the initial nucleation studies of alloys on SiC substrates with and without MOCVD AlN buffer layers: an AlN MOCVD buffer layer leads to continuous, single grain growth mode; The (AlN)x(SiC)1-x alloy film reduces the crack density because its thermal expansion coefficient is intermediate between SiC and AlN. X-ray diffraction (XRD) and Raman spectroscopy studies indicated the high quality of the AlN single crystal.
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Introduction
Despite the rapid progress made in group III nitride based semiconductor film growth [1], better substrates are still needed for high quality epitaxial growth. Currently, there is worldwide interest in producing highquality single crystals suitable for nitride based electronic and optoelectronic devices. The ideal substrate should be lattice matched, isomorphic, and thermal expansion coefficient matched to the film as much as possible. Single crystal AlN is attractive as a substrate for group III-nitride epitaxial growth due to its relatively small lattice constant mismatch along the a-axis (-2.5%) with GaN, good thermal stability (melting point >2500 °C), high resistivity and similar coefficient of thermal expansion [2]. To date, bulk AlN single crystals are limited in sizes to diameters below 15mm [3]. Bulk AlN crystals are most commonly produced by vapor transport (sublimation) from an AlN source. This technique was most successfully developed by Slack and McNelly [4], [5] in the mid-1970’s. Large AlN single crystals, up to 1 cm long and 0.4 cm in diameter, were grown by self-seeding from the sharp tip of sealed tungsten crucibles heated in an radio frequency induction furnace. Recently, Schowalter et al [3] demonstrated good quality AlGaN epitaxy on bulk AlN crystals produced by self-seeding.
Seeding bulk AlN growth on silicon carbide substrates instead of self-seeding offers several potential advantages including the availability of large diameter seed crystals (up to 100 mm) for rapid process scale-up, and better control of the crystal polarity, th
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