Several physical mechanismscan give rise to a photoresponse, but only two can play a significant role ingraphene-based photodetectors. These are the photovoltaic and the photo-thermoelectriceffects. In the case of photovoltaic effect, incident photonsgenerate a density of carriers that are separated, and induce current at the electrodeswhen an electric field is applied. In our experiments the photocurrent inprocessed structures has linear dependence on bias voltage without any photovoltaiceffect. The photo-thermoelectric effect, on the other hand,originates from a difference in Seebeck coefficients from areas with differentdensity of states.
The different local temperatures at the junction result inthe diffusion of carriers after irradiation. Hot-carrier dynamics are generallyrecognized to dominate photocurrent generation in supported graphene devicesbecause of inefficient cooling of electrons with the lattice. Bolometric effect is negligible due to electrodespassivation with photostructurable polyimide.The gate-modulated currentgives clear evidence of Dirac point shift under continuous laser irradiation. Theshift can reach up to 30 V in backgate measurements converting thegraphene channel from heavy p-doped to almost intrinsic.
Such decrease of thehole concentration and increase of the off-current can be related to the raiseof electron concentration through reducing the energy barrier between attachedfunctional groups and graphene and charge traps occupation at the graphene-SiO2interface. In pristine graphene we did not observe prominentshift under laser irradiation.By tuning the gatevoltage V, the majority carriers in the processed graphene channelchange from holes to electrons.
The photocurrent is minimal near the Diracpoint. It rises with increasing of the V-V difference for both types of carriers. When we consider the energy levels farfrom Fermi level, all carriers are involved in charge transport and thequantity of charges generated upon laser irradiation is minimal, hence thephotocurrent tends to zero. For these observations the photo-thermoelectriceffect dominates as was shown previously by Xu et al.
The physical pictureof photocurrent generation due to the photo-thermoelectric effect can bedescribed as following: after the electrons are excited from the valence bandto the conduction band, they relax back to the Fermi level on the time scale offemtoseconds by phonon emission and form a hot Fermion distribution.The hot free carrierstend to diffuse from the pristine part of the channel into the functionalizedone due to the temperature gradient across the channel, which leads to apositive current for hole doped graphene. In this case the photocurrent can beformulated