Photodetectors with zigzag and armchair graphene nanoribbon channels and asymmetric source and drain contacts: Detectors for visible and solar blind applications.

Employing the non-equilibrium Green's function with the third order tight binding Hamiltonian, we show that 12.7 nm long and 1.11 nm wide zigzag and armchair graphene nanoribbons (ZGNR and AGNR) in contact with asymmetric source and drain, consisting of graphene topped with Au and Ti, respectively, can operate as visible and solar blind photodetectors under zero externally applied biases. The carbon atoms at the channels' edges are considered to be hydrogen passivated. Numerical simulations show that the unbiased Au-ZGNR-Ti and Au-AGNR-Ti photodetectors can detect photons of energies 2.72 eV (456 nm, visible reign) and 5.02 eV (247 nm, solar blind reign), respectively, with the corresponding quantum efficiencies as high as 90% and 60% and responsivities of 93.8 and 13.4 A/mW. We also show that in Au-ZGNR-Ti the local photocurrents are distributed more around the ZGNR edges in regions near the source and drain sides of the channel, for the zero gate-source voltage (VGS=0), whereas in Au-AGNR-Ti for the same biasing condition, the local photocurrents are distributed far from the AGNR edges within regions near the source and drain sides of the channel. As the applied gate voltage moves away from the Dirac point, the peaks of the local photocurrents move away from the source and drain sides of the channel in opposite directions, in either asymmetric photodetector. Polarities of the local photocurrents are altered, as the gate-source voltage approaches the Dirac point, where the total photocurrent is maximized for both asymmetric photodetectors. [ABSTRACT FROM AUTHOR]