Metal contacts in carbon nanotube field-effect transistors: Beyond the Schottky barrier paradigm

The observed performances of carbon nanotube field-effect transistors are examined using first-principles quantum transport calculations. We focus on the nature and role of the electrical contact of Au and Pd electrodes to open-ended semiconducting nanotubes, allowing the chemical contact at the surface to fully develop through large-scale relaxation of the contacting atomic configuration. As expected from their respective work functions, the Schottky barrier heights for Au and Pd turn out to be fairly similar for realistic contact models. We present, however, direct numerical evidence of Pd contacts exhibiting perfect transparency for hole injection as opposed to that of Au contacts. These findings support experimental data reported to date.

Figure 1

(Color online) Transmission for an (8,0) CNT composed of six unit cells contacted to (a) Au and (b) Pd, as shown in the inset. Panels (c) and (d) show the same but for a nine unit-cell CNT.

Figure 2

Density of states projected on succesive carbon rings starting from the interface for the (c) (upper panel) and (d) (lower panel) cases shown in Fig. 1. Similar features are obtained for (a) and (b).

Figure 3

(Color online) Transmission for six [(a) and (b)] and nine [(c) and (d)] unit-cell (8,0) CNTs contacted to Au [(a) and (c)] and Pd [(b) and (d)] electrodes as shown in the inset. The nanotube ends are embedded in the metal one atom layer deep.

Figure 4

(Color online) Atomic cross section of the metal-CNT interface for (a) Au and (b) Pd electrodes. Contour density plot at the interface for (c) Au and (d) Pd.