Current status of GaN crystal growth by sublimation sandwich technique
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Internet Journal Nitride Semiconductor Research
Current status of GaN crystal growth by sublimation sandwich technique P. G. Baranov1, E. N. Mokhov1, A. O. Ostroumov1, M. G. Ramm1, M. S. Ramm1, V. V. Ratnikov1, A. D. Roenkov1, Yu. A. Vodakov1, A. A. Wolfson1, G. V. Saparin2, S. Yu. Karpov3, D. V. Zimina3, Yu. N. Makarov4 and Holger Juergensen5 1Ioffe
Physical-Technical Institute, State Lomonosov University, 3Soft-Impact Ltd (St.Petersburg, Russia), 4Lehrstuhl für Strömungsmechanik, University of Erlangen-Nürnberg, 5AIXTRON AG, 2Moscow
(Received Monday, August 31, 1998; accepted Monday, November 2, 1998)
The current status of GaN crystal growth using the Sublimation Sandwich Technique is discussed in the paper. We use modeling to analyze gas dynamics in the reactor and the supply of the main gaseous species into the growth cell under growth conditions used in experiments. Important features of growth process — non-equilibrium cracking of ammonia, partial sticking of ammonia at the growing surface and kinetic limitation of GaN thermal decomposition — are taken into account in the model. Growth is carried out on sapphire and 6H-SiC substrates in ammonia atmosphere using a Ga/GaN mixture as the group-III element source. Single crystals of GaN of size 15×15 mm and up to 0.5 mm thick are normally grown with the optimized growth rates of 0.25-0.35 mm/h. The GaN crystals are characterized by photoluminescence, by the Color Cathodoluminescence Scanning Electron Microscopy technique, by differential double-crystal and triple-crystal X-ray diffractometry, and by electron paramagnetic resonance. Mechanisms of sublimation growth of GaN and physical limitations of the growth process are discussed.
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Introduction
Homoepitaxy on GaN substrates is one of the successful ways of fabricating device-quality nitride-based heterostructures. To get such substrates, bulk GaN crystals of an appropriate size are necessary. Nowadays three techniques can be regarded as promising to obtain GaN bulk crystals − High Pressure Crystal Growth [1], Chloride Hydride Vapor Phase Epitaxy (especially in combination with lateral regrowth) [2], and Sublimation Sandwich Technique [3]. The latter has been initially developed for growth of high-quality SiC crystals and then applied to group-III nitrides, first of all, to GaN [4]. Recently the sublimation technique also succeeded in growth of large size AlN crystals [5]. The sublimation technique is one of the traditional ways to grow bulk AlN crystals [6]. This is possible due to congruent evaporation of this material at high temperatures providing nearly stoichiometric composition of the vapor phase. The pressure of the main species (Al and N2) over a AlN surface reaches ~1 atm only at
2260°C. That is the reason why the temperature range of 1950-2250°C is normally used for sublimation growth of AlN. As a result crystals up to 1 mm thick with the size of 10×10 mm can be obtained by this technique. In contrast to the case of AlN the nitrogen pressure over GaN under practical growth conditions (temperat
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