Origin of photoresponse in black phosphorus phototransistors

We study the origin of a photocurrent generated in doped multilayer black phosphorus (BP) phototransistors, and find that it is dominated by thermally driven thermoelectric and bolometric processes. The experimentally observed photocurrent polarities are consistent with photothermal processes. The photothermoelectric current can be generated up to a micrometer away from the contacts, indicating a long thermal decay length. With an applied source-drain bias, a photobolometric current is generated across the whole device, overwhelming the photothermoelectric contribution at a moderate bias. The photoresponsivity in the multilayer BP device is two orders of magnitude larger than that observed in graphene.