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 degrees C for 1 h, niobium carbide (Nb(2)C) is formed at the Nb/SWCNT interface. The Nb(2)C/SWCNT contacts demonstrate a very small Schottky barrier height of approximately 18 meV (decreased by > 80% relative to that of pristine Nb/SWCNT contact of approximately 98 meV) to p-type transport. This is attributed to the higher work function of Nb(2)C (approximately 5.2 eV) than Nb (approximately 4.3 eV) and better bonding between Nb(2)C and SWCNTs. The performance of Nb(2)C-contacted SWCNT FETs is as follows: the p-channel ON current is as high as 0.5 microA at V(DS) = 0.1 V, the I(ON)/I(OFF) ratio is up to approximately 10(5) and the subthreshold slope is approximately 550 mV/dec, which is as good as that of titanium carbide (TiC-) and Pd-contacted SWCNT FETs. Compared with TiC, Nb(2)C contacts yield more unipolar p-type SWCNT FETs, as a result of the Nb(2)Cs higher work function. More importantly, Nb(2)C contacts can form near-ohmic contacts to both large-(>or=1.6 nm) and small-diameter (approximately 1 nm) SWCNTs, while Pd can only form near-ohmic contacts for large-diameter SWCNTs. Moreover, the Nb(2)C contacts demonstrate good stability in air.