We measure the channel potential of a graphene transistor using a scanning photocurrent imaging technique. We show that at a certain gate bias, the impact of the metal on the channel potential profile extends into the channel for more than 1/3 of the total channel length from both source and drain sides, hence most of the channel is affected by the metal. The potential barrier between the metal controlled graphene and bulk graphene channel is also measured at various gate biases. As the gate bias exceeds the Dirac point voltage, VDirac, the original p-type graphene channel turns into a p-n-p channel. When light is focused on the p-n junctions, an impressive external responsivity of 0.001 A/W is achieved, given that only a single layer of atoms are involved in photon detection.
@article{arxiv.0912.4788,
title = {Photocurrent imaging and efficient photon detection in a graphene transistor},
author = {Fengnian Xia and Thomas Mueller and Roksana Golizadeh-mojarad and Marcus Freitag and Yu-ming Lin and James Tsang and Vasili Perebeinos and Phaedon Avouris},
journal= {arXiv preprint arXiv:0912.4788},
year = {2009}
}