Titanium/nickel/titanium ohmic contacts to 4H n-SiC nitrogen doped to 3.55 X 10(18) and 1.22. X 10(19) cm(-3) have been studied. As-deposited Ti/Ni/Ti/SiC structures were rectifying, but became ohmic upon annealing at 1000 degrees C for 2 min in a N-2 ambient. A nickel silicide phase (Ni2Si) Was formed during annealing, and smooth interfaces were observed when a Ti layer was present between the Ni and SiC. The dependence of ohmic contact resistance on Ni and Ti layer thickness was studied. Contacts with a 20 nm bottom Ti layer showed a contact resistance of 1 X 10(-4) Omega cm(2), while maintaining an interfacial rms roughness of 7.5 nm. Contacts with thicker bottom Ti layers (> 20 nm) were rectifying with a nonlinear current-voltage behavior even after annealing, and there was still a distinct Ti layer between the Ni and SiC. The lack of an ohmic contact was attributed to the Ti layer acting as a diffusion barrier restricting the formation of Ni2Si. The Ni thickness was varied from 90 to 30 nm over a 20 nm Ti bottom layer, but the specific contact resistances (pc) (3.3 X 10(-4) +/- 2.5 X 10(-4) Omega cm(2)) did not vary significantly with Ni thickness. Thicker Ni (> 30 nm) contacts showed a nonuniform carbon distribution with graphite-rich zones at the silicide/SiC interface. Thin Ni contacts (30 nm) showed more uniform carbon distribution than in the contacts with thicker Ni, and no significant carbon-rich zone at the silicide/SiC interface. (c) 2005 American Vacuum Society.