Gate-Activated Photoresponse in a Graphene p–n Junction

Abstract

We study photodetection in graphene near a local electrostatic gate, which enables active control of the potential landscape and carrier polarity. We find that a strong photoresponse only appears when and where a p-n junction is formed, allowing on-off control of photodetection.Photocurrents generated near p-n junctions do not require biasing and can be realized using submicron gates. Locally modulated photoresponse enables a new range of applications for graphene-based photodetectors including, for example, pixilated infrared imaging with control of response on subwavelength dimensions. MANUSCRIPT TEXTGraphene is a promising photonic material 1 whose gapless band structure allows electron-hole pairs to be generated over a broad range of wavelengths, from UV, visible 2 , and telecommunication bands, to IR and THz frequencies 3 . Previous studies of photocurrents in graphene have demonstrated photoresponse near metallic contacts [4][5][6][7] , at the interface between single-layer and bilayer regions 8 , or at the edge of chemically doped regions 10 . Photocurrents generated near metal contacts were attributed to electric fields in the graphene that arise from band bending near the contacts 5-7 , and could be modulated by sweeping a global back-gate voltage with the potential of the contacts fixed. In these studies, photocurrent away from contacts and interfaces was typically very weak. In contrast, the present study concerns devices with top gates, separated from otherwise homogeneous graphene by an insulator, Al2O3, deposited by atomic layer deposition (ALD). When the top gate inverts the carrier type under the gate, a p-n junction is formed at the gate edges, and a highly localized photocurrent is observed using a 1 * These authors contributed equally to this work. focussed scanning laser. A density difference induced by the top gate that does not create a p-n junction does not create local photosensitivity.Comparing experimental results to theory suggests that the photocurrent generated at the p-n interface results from a combination of direct photogeneration of electron-hole pairs in a potential gradient, and a photothermoelectric effect in which electric fields result from optically induced temperature gradients 8,11 . Both effects are strongly enhanced at p-n interfaces: The enhancement of direct photocurrent results from its scaling inversely with local conductivity, while the thermoelectric contribution is enhanced by the strong spatial dependence of the Seebeck coefficient near the p-n interface. As neither mechanism is wavelength selective, the overall effect should provide broadband photosensitivity. We further anticipate that the ability to activate local photosensitive regions using gate voltages will provide pixel-controlled bolometers for imaging or spectroscopy with broadband sensitivity and subwavelength spatial resolution.A typical device layout and micrograph are shown in Fig. 1. Graphene was deposited onto ~300 nm of silicon dioxide on degenerately doped silicon by mechanical exfoliat...

Surface Plasmon Polariton Graphene Photodetectors

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“…The simplest graphene-based photodetection scheme relies on the metal-graphene-metal (MGM) architecture [5,7,8,11,[20][21][22], where the photoresponse is due to a combination of photo-thermoelectric and photovoltaic effects [5,7,8,11,[20][21][22]. For both mechanisms, the presence of a junction is required to spatially separate excited electronhole (e-h) pairs [5,7,8,11,[20][21][22].…”

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