First principle calculation of the photocurrent in a short carbon nanotube: the effect of an external bias.

We study the transport properties of a short carbon nanotube between two different metal electrodes. Sepcificly, the photocurrents under a series of bias voltages are investigated. The calculations are completed within non-equilibrium Green's function method where the photon-electron interaction is taken as a perturbation. The rule-of-thumb that a forward bias decreases while a reverse bias increases the photocurrent under the same illumination is verified. The first principle results demonstrate the characteristic of Franz-Keldysh effect where the photocurrent response edge shows a clear red-shift trend in electric fields of both axial directions. An obvious Stark splitting is observed when some reverse bias is applied on the system due to the very large field strength. In this shortchannel situation, intrinsic nanotube states are strongly hybridized with metal electrodes states, which results in dark current leakage and some specific features such as long tail and fluctuation in the photocurrent response.