In recent years, there has been an increasing interest in the development of SiC, III-nitrides and their solid solutions as replacements for conventional III-V optoelectronic devices fabricated using Asand P-based semiconductors. Of these compounds, solid solutions of the (SiC)1−x (AlN)x system are promising hightemperature wide-gap semiconductor materials with an energy gap that ranges from 3.30 eV (for 2H-SiC) to 6.0 eV (for AlN) [1, 2]. The synthesis of AlN-SiC solid solutions has been successfully accomplished through a variety of methods, such as hot pressing of AlN and α or β-SiC [3], metalorganic chemical vapor deposition (MOCVD) [4], the carbothermal reduction of silica and aluminum in a nitrogen atmosphere [5], plasma-assisted gas source molecular beam epitaxy (MBE) [6], and selfpropagating combustion synthesis [7, 8], However, the refractory nature of the these two ceramics and their relatively low solid state diffusivities, results in the formation of their solid solutions requiring high temperature processing for extended periods of time. One way to overcome these difficulties is the use of in situ reactive synthesis techniques. The results of such an investigation for the synthesis of AlN-SiC solid solutions are repated here. Silicon nitride (98% pure, average particle size of 0.5 μm), aluminum powder (99.8% pure, sieve classification of −200 mesh), and carbon powder (99.9% pure, average particle size of 0.7 μm) were used as reactant materials. Powders of Si3N4, Al, and C, were dry-mixed according to the stoichiometric ratios of the equation (Si3N4 + 4Al + 3C = 4AlN + 3SiC), and put in a porous graphite crucible. Sintering was performed in a graphite furnace in a nitrogen atmosphere (99.99% pure) at temperatures in the range 1600–1900 ◦C for 1 h to 2 h. Phase analysis of the products was performed using X-ray diffraction (XRD) with CuKa radiation. For curve fitting of the (110) peak, the region near 2θ = 60◦ was examined in a step-scan mode with 2θ = 0.02◦ and a collection time of 125 s. The morphology of the products was examined using scanning electron microscopy (SEM) with an energy dispersive X-ray spectrometer (EDX). The XRD patterns of the synthesized products sintered at temperatures in the range 1600–1900 ◦C for 1 h are shown in Fig. 1. For the products obtained at 1600 ◦C, the major crystalline phase was a AlN-SiC solid solution, along with a trace of residual Si3N4 (Fig. 1a). As the temperature increased, the amount of residual Si3N4 decreased (Fig. 1b). When the temperature was increased to 1900 ◦C, the synthesized product contained peaks corresponding to the AlN-SiC only. The strong lines corresponding to the hexagonal AlN-SiC solid solution, with the absence of any splitting, indicates that this product is a single-phase, homogeneous AlN-SiC solid solution (Fig. 1c). The formation of a solid solution will be confirmed in more detail by the SEM-EDX analysis of synthesized products as described later. Although the diffraction peaks of AlN and 2H-SiC are closely matched with differences of 2θ values of less than 1◦ for the major peaks [8], the (110) diffraction line of commercial 2H-AlN and 2H-SiC can be resolved very well into two peaks since it has the largest difference in 2θ . In order to determine the exact nature of the AlN-SiC solid solution careful examination of the (110) peak near 2θ = 60◦ for the products obtained at 1900 ◦C for different soaking times was conducted (see Fig. 2). On soaking for 1 h, there was a single line of the (110) peak with the absence of any splitting (Fig. 2a), which suggests that the formation of a homogenous AlN-SiC solid solution had been performed completely.
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Aug 23, 2004
Ling Wu + 4
Open Access