Investigations of niobium carbide contact for carbon-nanotube-based devices

Single-walled carbon nanotube (SWCNT) field effect transistors (FETs) with Nb contacts have been fabricated and upon annealing in vacuum at 700 °C for 1h, niobium carbide (Nb₂C) is formed at the Nb/SWCNT interface. The Nb₂C/SWCNT contacts demonstrate a very small Schottky barrier height of ∼ 18meV (decreased by > 80% relative to that of pristine Nb/SWCNT contact of ∼ 98meV) to p-type transport. This is attributed to the higher work function of Nb₂C (∼5.2eV) than Nb (∼4.3eV) and better bonding between Nb₂C and SWCNTs. The performance of Nb ₂C-contacted SWCNT FETs is as follows: the p-channel ON current is as high as 0.5νA at V_DS = 0.1V, the I_ON/I_OFF ratio is up to ∼ 10⁵ and the subthreshold slope is ∼ 550mV/dec, which is as good as that of titanium carbide (TiC-) and Pd-contacted SWCNT FETs. Compared with TiC, Nb₂C contacts yield more unipolar p-type SWCNT FETs, as a result of the Nb₂Cs higher work function. More importantly, Nb₂C contacts can form near-ohmic contacts to both large-(≥1.6nm) and small-diameter (∼1nm) SWCNTs, while Pd can only form near-ohmic contacts for large-diameter SWCNTs. Moreover, the Nb₂C contacts demonstrate good stability in air.