Substrate-Induced Photofield Effect in Graphene Phototransistors

A single atomic layer of graphene, integrated onto an undoped bulk substrate in a back-gated transistor configuration, demonstrates surprising strong photoconduction, and yet, the physical origin of the photoresponse is not fully understood. Here, we use a detailed computational model to demonstrate that the photoconductivity arises from the electrostatic doping of graphene, induced by the surface accumulation of photogenerated carriers at the graphene/substrate interface. The accumulated charge density depends strongly on the rate of charge transfer between the substrate and the graphene; the suppression of the transfer rate below that of carrier's thermal velocity is an essential prerequisite for a substantial photoinduced doping in the graphene channel under this mechanism. The contact-to-graphene coupling (defined by the ratio of graphene-metal contact capacitance to graphene's quantum capacitance) determines the magnitude of photoinduced doping in graphene at the source/drain contacts. High-performance graphene phototransistors would, therefore, require careful engineering of the graphene-substrate interface and optimization of graphene-metal contacts.