Interfacial reactions in nickel/titanium ohmic contacts to N-type silicon carbide

Titanium/nickel/titanium ohmic contacts to n-SiC have been studied. The Ti/Ni/Ti/SiC structures were rectifying as the as-deposited state but became ohmic upon annealing at 1000°C for 2 minutes in a N₂ ambient. A nickel silicide phase (Ni₂Si) was formed, resulting in a specific contact resistance (ρC) as low as 1 × 10−4 Ω cm2. Smooth interface profiles were obtained due to the Ti layer between the Ni and SiC. Auger depth profile data and transmission electron microscopy analysis showed that Ni moved through the Ti layer to decompose the SiC and form Ni₂Si, leaving excess carbon as a reaction product. Some of the carbon reacted with the two Ti layers to form titanium carbide (TiC). These phases are observed because titanium carbide has lower free energy of formation than titanium silicide, and nickel silicide has a lower free energy of formation than titanium silicide. The carbon that was not bonded as TiC was distributed in the silicide layer and at the silicide/SiC interface as a graphite phase. A schematic model of the layers in the Ti/Ni/Ti/SiC contacts was deduced. The dependence of ohmic contact resistance on Ni and Ti layer thickness was also studied. Contacts with a 20 nm bottom Ti and Ni/Ti ratio of 3.5 showed a contact resistance of 1 × 10−4 Ω cm2, while maintaining an interfacial roughness of 7.5 nm. Thicker bottom Ti (>20 nm) contacts were rectifying with a non-linear I–V behavior, and there was still a Ti layer between the Ni and SiC after annealing. The lack of an ohmic contact was attributed to the Ti layer acting as a diffusion barrier preventing the formation of Ni₂Si. The Ni thickness was varied from 90 to 30 nm over 20 nm Ti bottom layers, but the specific contact resistances (ρ_c) (3.3 × 10−4 ± 2.5 × 10−4Ω cm2 did not vary systematically with Ni thickness. Thicker Ni (>30 nm) contacts showed a non-uniform carbon distribution with carbon peaks at the silicide/SiC interface as graphite. Thin Ni contacts (30 nm) showed more uniform carbon distribution than in the contacts with thicker Ni. No significant carbon peak layer was found at the silicide/SiC interface.