Ion Beam Synthesis by Tungsten-Implantation Into 6H-SIC
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ABSTRACT We studied high-dose implantation of tungsten into 6H-silicon carbide in order to synthesize a highly conductive surface layer. Implantation was performed at 200 keV at room temperature. Subsequently, the samples were annealed in two steps at 500'C and 700'C or 950'C, respectively. The influence of dose and annealing temperature on the reaction of W with SiC was investigated. Rutherford Backscattering Spectrometry (RBS), X-Ray Diffraction (XRD) and Auger Electron Spectroscopy (AES) contributed to study structure and composition of the layer as well as chemical states of the elements. During implantation sputtering became significant at a dose exceeding l.OX1017 W+cm-2. Formation of tungsten carbide and silicide was observed already in the as-implanted state. An annealing temperature of 950'C was necessary to crystallize tungsten carbide. However, tungsten silicide remained amorphous at this temperature. Therefore, a mixture of polycrystalline tungsten carbide and amorphous tungsten silicide evolved under these conditions. The resistivity of such a layer implanted with 1.O 1017 W'cm"2 and annealed at 950'C is 565 p_(cm.
INTRODUCTION Silicon carbide is presently one of the most promising materials for high-temperature semiconductor devices because of its superior properties such as high thermal conductivity, high electron mobility [ I ] and wide bandgap [ 2 ]. High temperature devices, however, are in need of reliable metallizations for their working temperature of up to 700'C. A proper contact material should have a low resistivity, good adhesion to the underlying SiC, as well as high chemical stability at elevated temperatures for more than 1000 hours. Several attempts have been made to develop such metallizations on 6H-SiC [ 3 - 6 ]. However, to our knowledge, no universally applicable ohmic contact with long term stability at high temperatures has been developed up to now. The best result yet was obtained by Crofton et al. [6] who proved that a TiW layer can have a considerable long term stability on 6H-SiC. However, this contact was ohmic only on highly doped substrates. Promising contact materials that could meet all of the above mentioned requirements are tungsten disilicide and tungsten carbide. Fabrication of W based contact layers was achieved up to now by (i) deposition of pure tungsten on SiC and subsequent annealing [5, 7 1and (ii) sputter deposition, subsequent ion beam mixing and final annealing of W/Si multilayers [8 ]. However, the first technique requires annealing temperatures of more than 1000°C and, moreover, produces an inhomogeneous mixture of polycrystalline tungsten silicides, tungsten carbides and tungsten. The latter needs three preparation steps to form ohmic contacts. The reaction of a deposited W layer with the underlying SiC was investigated by Geib et al. [9 ] using Auger electron spectroscopy. They observed no formation of new bonds up to annealing temperatures of 900'C. However, they found already after deposition little amounts of tungsten carbide and silicide, which forme
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