Comparison of Ni/Ti and Ni ohmic contacts on n-type 6H–SiC

Ni (50 nm)/Ti (10 nm) and Ni (50 nm) contact structures were deposited by vacuum evaporation on n-type 6H–SiC with various doping level. Prior to deposition, part of the substrates had been subjected to plasma cleaning. To achieve ohmic character, the samples were annealed in vacuum. Electrical parameters of the contacts were determined by measuring specific contact resistances. The results were similar for both contact structures; we have not found any influence of plasma cleaning. The lowest value was 1.4 × 10−4 Ω cm2 for substrate with doping level of 1.9 × 1019 cm−3. Using XPS depth profiling it was found, that the titanium layer was shifted upon annealing of the Ni/Ti structures from the interface towards contact surface and that this layer consists of TiC. Between the TiC layer and the substrate was a layer of nickel silicides and carbon. In the plasma-cleaned Ni/Ti/SiC samples, increased content of nickel at the expense of carbon was detected just below the TiC layer. We suggest the snowplow effect of dopants in the SiC substrate upon annealing of the structures as a main factor in ohmic contact formation.