Thermodynamic Analysis of Impurities in the Sublimation Growth of AlN Single Crystals
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Thermodynamic Analysis of Impurities in the Sublimation Growth of AlN Single Crystals Li Du and James H Edgar Kansas State University, Manhattan, KS, 66502 ABSTRACT The vapor phase species responsible for the transport of impurities in the sublimation growth of bulk AlN crystals was predicted by thermodynamic analysis. AlN powder containing oxygen was investigated in Al-O-N system for an inert reactor. Dialuminum monoxide (Al2O) is strongly favored over all other possible oxygen containing species including NO and NO2. For AlN crystal growth in a graphite furnace, the Al-O-C-N system was studied. CO is the main species containing carbon and oxygen, and has a partial pressure more than one hundred times higher than all other carbon or oxygen containing species. Its partial pressure even exceeds that of Al vapor. Pure AlN growth on SiC seed was represented in the Al-N-Si-C system. SiC is not stable at high temperatures, the presence of nitrogen accelerates the decomposition of the SiC, and the most probable volatile species originating from the SiC seed are Si, CN and C2N2. INTRODUCTION Owing to its excellent properties, such as a wide band-gap, a low thermal expansion coefficient, good mechanical strength, resistance to chemical attack and excellent thermal conductivity, aluminum nitride has attracted extensive attention for many applications [1]. In addition, AlN single crystals are also good substrates for AlGaN epitaxy. However, the properties of AlN crystals are deeply affected by the type and concentration of the impurities it contains. For example, its thermal conductivity degrades sharply with increasing oxygen (O) concentration. Oxygen substitutes on nitrogen sites and because of charge compensation to maintain overall neutrality, this leads to a high concentration of Al vacancies, which in turn reduces the lattice constants of AlN. Oxygen and other impurities can also cause high concentrations of point and extended defects. The majority of studies of AlN bulk crystal growth have been experimental in nature, few of which focused on the incorporation of impurities. Kakanakpva et al. studied C and O impurity incorporation through vapor species such as NO, NO2, CN and Al2C by applying cohesive energy per atom [2]. Fukuyama et al. [3] reported single crystal AlN films were fabricated by nitriding Al2O3 in carbon-saturation condition, demonstrating that the present of C could reduce oxide to nitride. Other studies have investigated electronic structure, atomic configurations, formation energies and other properties of impurity doping (C, Mg, Si, Zn, and Ge) in AlN [4-7]. In this paper, a simplified thermodynamic model was applied to determine the major species responsible for impurity incorporation in the AlN crystal growth process. The products from the sublimation of an AlN source containing oxygen were predicted for a nonreacting furnace (the Al-O-N system) and for a graphite furnace (the Al-O-C-N system). The products produced during AlN seeded growth on SiC were predicted for the Al-N-Si-C system
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