Solid solutions of AlN and SiC grown by plasma-assisted, gas-source molecular beam epitaxy
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Solid solutions of aluminum nitride (AIN) and silicon carbide (SiC), the only intermediate phases in their respective binary systems, have been grown at 1050 °C on a(6H)-SiC(0001) substrates cut 3-4° off-axis toward [1120] using plasma-assisted, gas-source molecular beam epitaxy. A film having the approximate composition of (AlN)o.3(SiC)o.7, as determined by Auger spectrometry, was selected for additional study and is the focus of this note. High resolution transmission electron microscopy (HRTEM) revealed that the film was monocrystalline with the wurtzite (2H) crystal structure.
Silicon carbide is a wide band gap material that exhibits polytypism, a one-dimensional polymorphism arising from the various possible stacking sequences of, e.g., the silicon and carbon layers along the directions of closest packing. The single cubic poly type, /3-SiC, crystallizes in the zinc blende structure, has a room temperature band gap of 2.3 eV, and is commonly referred to as 3C-SiC. In the Ramsdell notation, the three (3) refers to the number of Si and C bilayers necessary to produce a unit cell and the C indicates its cubic symmetry. There are approximately 250 other rhombohedral and hexagonal polytypes1 that are classed under the heading of a-SiC. The most common of these latter polytypes is 6H-SiC, with a room temperature band gap of «3.0 eV. Aluminum nitride normally occurs in the wurtzite (2H) structure; however, the cubic, zinc blende phase has been produced.2-3 The 2H polytype possesses a direct band gap4 and a thermal conductivity5 of 6.28 eV and 3.2 W/cm • K, respectively. As such, this material is of particular interest for high power and optoelectronic devices, the latter of which would emit and absorb ultraviolet radiation. Solid solutions of AIN and SiC have been achieved by two primary routes: reactive sintering of mixtures of powders of a variety of sources and thin film deposition from the vapor phase. Matignon6 first reported the synthesis of an (AlN)x(SiC)!_x material in 1924 formed by heating A12O3, S1O2, and coke in the presence of flowing N2 at an unspecified temperature. Related hot pressing and annealing research coupled with x-ray diffraction and optical and electron microscopy by Rafaniello et a/.7'8 reportedly resulted in single phase,
^Present address: Naval Research Laboratory, Code 4555 Overlook Avenue SW, Washington, DC 20375-5000.
6861,
J. Mater. Res., Vol. 8, No. 7, Jul 1993
http://journals.cambridge.org
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2H material at all compositions hot pressed at 2300 °C, but only within the ranges of 0-15 and 75-100 wt. % AIN for samples prepared at 2100 °C and below. This latter result indicated a miscibility gap, the existence of which was subsequently confirmed by Ruh and Zangvil,9 Zangvil and Ruh,10"12 Kuo and Vikar,13 and Czekaj et al.u using a variety of heat-treatment schedules. The tentative phase diagram proposed by Zangvil and Ruh11 shows a flat miscibility gap at 1900 °C between —20 and 80 wt. % AIN. Above this temperature a 2H solid solution was reported from «20-100
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