Ion Beam Synthesis by Tungsten Implantation into 6H-Silicon Carbide at Elevated Temperatures
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ABSTRACT We studied high dose implantation of tungsten into 6H-silicon carbide in order to synthesize I ×1017 an electrically conductive layer. Implantation was performed at 200 keV with a dose of 2 either at subsequently annealed The samples were of 90'C and 500'C. W+cm" at temperatures 950'C or 1100IC. The influence of implantation and annealing temperatures on the reaction of W with SiC was investigated. Rutherford backscattering spectrometry (RBS), x-ray diffiraction (XRD) and Auger electron spectroscopy (AES) contributed to study the structure and composition of the implanted layer as well as the chemical state of the elements. The implantation temperature influences the depth distribution of C, Si and W as well as the damage production in SiC. The W depth profile exhibits a bimodal distribution for high temperature implantation and a customary gaussian distribution for room temperature implantation. Formation of tungsten carbide and silicide was observed in each sample already in the as-implanted state. Implantation at 90'C and annealing at 950°C lead to crystallization of W2C; tungsten silicide, however, remains amorphous. After implantation at 500'C and subsequent annealing at 1100'C crystalline W5 Si3 forms, while tungsten carbide is amorphous. INTRODUCTION Silicon carbide is an auspicious material for high-temperature semiconductor devices because of its superior properties such as high thermal conductivity, high electron velocity and wide bandgap [ 1 ,2 ]. High-temperature devices, however, are in need of reliable metallization since their working temperature can be as high as 7001C. A proper ohmic contact 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 metallization on 6H-SiC [3 - 10 ]. Contacts that could meet all of the above-mentioned requirements are either layered structures consisting of a reactive metal to ensure adhesion, a diffusion barrier and a low resistivity metal, or highly temperature stable low resistivity materials. Promising materials of the latter class are tungsten silicide 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, 11] and (ii) sputter deposition, subsequent ion beam mixing and final annealing of W/Si multilayers [ 12 ]. However, the first technique requires annealing temperatures in excess of 1000°C and, moreover, produces an inhomogenous mixture of polycrystalline tungsten silicide, tungsten carbide and tungsten. The latter needs too much expenditure to form a homogeneous layer of tungsten silicide. The reaction of a deposited W layer with crystalline SiC was investigated earlier [ 13 ,14 ] using Auger electron spectroscopy. Already after deposition, little amounts of tungsten carbide and silicide were found, which formed at defects within the W/SiC-interfacial region. Baud et al. showed that annealing at 950'C is neces
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