Experimental and Theoretical Analysis of Heat and Mass Transport in the System for AlN Bulk Crystal Growth
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Experimental and Theoretical Analysis of Heat and Mass Transport in the System for AlN Bulk Crystal Growth M.V. Bogdanov1, S.Yu. Karpov1, A.V. Kulik1, M.S. Ramm1, Yu.N. Makarov1, R. Schlesser2, R.F. Dalmau2, Z. Sitar2 1 Semiconductor Technology Research, Inc., P.O.Box 70604, Richmond, VA 23255-0604, USA 2 Dept. Mat. Sci. Eng., North Caroline State University, 1001 Capability Dr., Raleigh, NC 27695 -7919, USA ABSTRACT Bulk AlN crystal growth by Physical Vapor Transport (PVT) is studied both experimentally and numerically. This paper presents the analysis of heat and mass transport mechanisms in closed and partially open crucible geometries. The heat transfer in the growth system used at North Carolina State University (NCSU) is simulated. The computed temperature profiles are used in the analysis of mass transport in the growth cell to gain understanding of the effect of species exchange between the crucible and environment on the AlN growth rate. The model predictions are in reasonable agreement with observations. INTRODUCTION Bulk aluminum nitride, a promising substrate material for nitride-based high-power electronics and ultra-violet optoelectronics, is normally grown by the sublimation (PVT) technique suggested by Slack and co-workers in the mid ‘70s [1,2]. A specific feature of the PVT growth is that temperature distribution in a furnace controls the crystal growth rate, the shape of the crystallization front, the stability of AlN powder source, the thermoelastic stress, and, eventually, the quality of the grown material. In the temperature range of interest (1800-2300°C), it is problematic to monitor experimentally the thermal field inside the growth system. Measurements of the temperature distribution, obtained with the crucible removed from the hot zone, do not provide sufficiently accurate information because the crucible considerably disturbs the thermal field. Another important issue is the control of the V/III ratio in the growth system. In a tightly closed crucible, this ratio is nearly equal to unity due to congruent AlN powder sublimation. The V/III ratio can change due to species exchange between the crucible and its environment, occurring through narrow openings. This, however, results in additional material losses and it is unclear how critical the effect of species exchange on the AlN growth is. In this paper, we simulate the heat transfer in the growth system used at North Carolina State University (NCSU) [3,4]. The data on axial temperature distribution obtained experimentally in the system with the crucible removed are used for adjustment of the unknown model parameters. Then, the model is applied to predict the temperature perturbations due to crucible insertion into the growth chamber. The computed temperature profiles are used as a basis for studying the mass transport in the growth cavity with the focus on the effect of species exchange between the crucible and ambient
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on the material losses and the AlN growth rate. The theoretical predictions are compared with observati
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