Effects of nickel and titanium thickness on nickel/titanium ohmic contacts to n-type silicon carbide

Abstract

Titanium/nickel/titanium ohmic contacts to 4H n-SiC nitrogen doped to 3.55×1018 and 1.22×1019cm−3 have been studied. As-deposited Ti∕Ni∕Ti∕SiC structures were rectifying, but became ohmic upon annealing at 1000°C for 2min in a N2 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 20nm bottom Ti layer showed a contact resistance of 1×10−4Ωcm2, while maintaining an interfacial rms roughness of 7.5nm. Contacts with thicker bottom Ti layers (>20nm) 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 90to30nm over a 20nm Ti bottom layer, but the specific contact resistances (ρC) (3.3×10−4±2.5×10−4Ωcm2) did not vary significantly with Ni thickness. Thicker Ni(>30nm) contacts showed a nonuniform carbon distribution with graphite-rich zones at the silicide/SiC interface. Thin Ni contacts (30nm) showed more uniform carbon distribution than in the contacts with thicker Ni, and no significant carbon-rich zone at the silicide/SiC interface.