Preparation of Conductive Tungsten Carbide Layers for SiC High Temperature Applications
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ABSTRACT Thin tungsten carbide films of different compositions were prepared by DC magnetron sputtering of tungsten and carbon and subsequent annealing in different environments. The onset of carbide formation was around 800'C. Annealing in a pure hydrogen ambient generally results in carbon depletion in the layers with the formation of a dominant W 2C phase. Adding propane enhances the carbon content in the layers and stimulates the formation of the WC phase. On silicon nitride substrates, variation of the propane concentration in an annealing environment allows a continuous alteration of the layer structure between polycrystalline single phase WC and a mixed layer with dominant W 2C and with it, the adjustment of different values of the electrical resistance. In contrast, on thin (100)SiC layers a textured W2C phase was grown after annealing in propane/hydrogen at 900'C whereas at higher temperatures the formation of silicides was observed. In addition, the chemical composition and the temperature dependence of the electrical specific resistance were investigated and are also discussed.
INTRODUCTION In recent years, increasing interest has been directed to wide band-gap semiconductors like silicon carbide (SiC) due to its potential for applications in power and high temperature electronics [I]. SiC devices show very good high temperature, chemical and mechanical stability and high breakdown voltages. Recent improvements in crystal growth techniques now provide industry and research with high quality bulk and epilayer material. However, there are still a variety of factors limiting the commercialization of devices in this area [2]. One of the most important factors with respect to high temperature devices is the requirement of a metallization, which should maintain a low contact resistivity, have good adhesion to the substrate material and have high stability at elevated temperatures, particularly at the interface. Several attempts have been made to achieve good Ohmic contacts to SiC. However, at elevated temperatures most metals are not stable on SiC and form rather good rectifying contacts with a barrier height > 1 eV [3]. The formation of good stable Ohmic contacts to p-type SiC remains an important technological problem that is hindering the industrial production of high temperature and power devices. A theoretical study based on an ideal band bending model of a semiconductor-metal interface has shown the impossibility of forming enhancement contacts to p-type SiC [4], because no contact material exists with the required work function. In all cases, to form Ohmic contacts the width of the depletion zone has to be minimized to increase thermionic emission. This can be attained 99 Mat. Res. Soc. Symp. Proc. Vol. 572 © 1999 Materials Research Society
by a high doping of the surface region. Nevertheless, a high value of the contact material work function is a precondition for the formation of Ohmic contacts. Metal carbides are promising contact materials that could fulfill the above mentioned requirements. Tran
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