The electronic levels of vanadium in semi-insulating 4H- and 6H-SiC have been reinvestigated using temperature dependent Hall effect and resistivity measurements at temperatures up to 1000K in conjunction with electron paramagnetic resonance (EPR) and optical absorption measurements which were used to identify the charge state of vanadium in the material. Two distinct thermal activation energies were found for each polytype. The shallower of the two levels correlated with the presence of both V3+ and V4+ in the EPR and absorption experiments, demonstrating that this level is the vanadium acceptor level while the deeper level is the donor level for which the V4+ charge state was observed. The results for the V4+∕5+ donor level, EC−1.57±0.09eV for 4H-SiC and EC−1.54±0.06eV for 6H-SiC, are in agreement with the generally accepted values. However, the results for the V3+∕4+ acceptor level, EC−0.85±0.03eV in 6H-SiC and EC−1.11±0.08eV in 4H-SiC, are significantly higher than previously assumed. Variations in crystal quality and purity may explain the differences in the previously reported values for the donor and acceptor levels.

Electrical contact properties and graphitic structures of metal/carbon/4H–SiC structures are investigated. Metals studied include Ni, Co, Cr, NiCr, Ti, W, Mo, Al, and Au. Ohmic contacts are formed on Ni/C, Co/C, Cr/C, and NiCr/C films on 4H–SiC with n-type, C-face, and a doping concentration of 1.8×1019 cm−3 . Only Ni/C and Co/C films exhibit Ohmic contact behavior on SiC with n-type, Si-face, and a doping concentration of 1.6×1018 cm−3. Ni and Co are well known as excellent graphitization catalysts. Raman spectra show that the formation of graphitic carbon is related to the formation of Ohmic contacts in the annealed metal/carbon/SiC structures. Generally accepted catalytic graphitization mechanisms are applied to explain the scanning electron microscopy images, which demonstrate a relationship between the catalytically reacted morphology and Ohmic contact behavior. This study provides evidence that the metals with better catalytic graphitization activities form better Ohmic contacts on metal/carbon/SiC structures.

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We demonstrate ohmic contacts to SiC using carbon films after thermal annealing. Carbon films are deposited on 4H-SiC and 6H-SiC substrates using a radio frequency sputtering method. The carbon/SiC samples convert from Schottky behavior to ohmic behavior after annealing in the temperature range from 1150 to 1350°C. Nanosize graphitic flakes are identified after annealing by Raman spectroscopy and are associated with the ohmic behavior of the contacts. The contact behavior of carbon films on 4H-SiC and 6H-SiC after annealing are compared and the polytype of the SiC has no effects on the structural evolution of the carbon films during annealing. This study reveals that the structural evolution of carbon is associated with formation of ohmic contacts on SiC and that nanosize graphitic flakes play a determinative role in the formation of ohmic contacts.

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