Photothermoelectric and Photoelectric Contributions to Light Detection in Metal–Graphene–Metal Photodetectors

Abstract

Graphene's high mobility and Fermi velocity, combined with its constant light absorption in the visible to far-infrared range, make it an ideal material to fabricate high-speed and ultrabroadband photodetectors. However, the precise mechanism of photodetection is still debated. Here, we report wavelength and polarization-dependent measurements of metal−graphene−metal photodetectors. This allows us to quantify and control the relative contributions of both photothermo-and photoelectric effects, both adding to the overall photoresponse. This paves the way for a more efficient photodetector design for ultrafast operating speeds.KEYWORDS: Graphene, photodetectors, Raman spectroscopy, photoresponse, optoelectronics T he unique optical and electronic properties of graphene make it ideal for photonics and optoelectronics. 1 A variety of prototype devices have already been demonstrated, such as transparent electrodes in displays 2 and photovoltaic modules, 3 optical modulators, 4 plasmonic devices, 4−9 microcavities, 10,11 and ultrafast lasers. 12 Among these, a significant effort is being devoted to photodetectors (PDs). 6,10,11,13−25 Various photodetection schemes and architectures have been proposed to date. The simplest configuration is the metal− graphene−metal (MGM) PD, in which graphene is contacted with metal electrodes as the source and drain. 13−18 These PDs can be combined with metal nanostructures enabling local surface plasmons and increased absorption, realizing an enhancement in responsivity (i.e., the ratio of the lightgenerated electrical current to the incident light power). 6,26 Microcavity based PDs were also used, with increased light absorption at the cavity resonance frequency, again achieving an increase in responsivity. 10,11 Another scheme is the integration of graphene with a waveguide to increase the effective interaction length with light. 25,27 Hybrid approaches employ semiconducting nanodots as light-absorbing media. 22 In this case, light excites electron−hole (e−h) pairs in the nanodots; the electrons are trapped in the nanodot, while the holes are transferred to graphene, thus effectively gating it. 22 Under applied drain−source bias, this results in a shift in the Dirac point, thus a modulation of the drain−source current. 22 Due to the long trapping time of the electrons within the dot, the transferred holes can cycle many times through the phototransistor before relaxation and e−h recombination. This gives a photoconductive gain; i.e., one absorbed photon effectively results in an electrical current of several electrons. Responsivities >10 7 A/W were reported, 22 but with a millisecond time scale, not suitable for, e.g., high-speed optical communications. Devices were also fabricated for detection of THz light. 28,29 In this low energy range, Pauli blocking forbids the direct excitation of e−h pairs due to finite doping. Instead, an antenna coupled to source and gate of the device excites plasma waves within the channel. These are rectified, leading to a detectable dc out...

Competing Channels for Hot-Electron Cooling in Graphene

“…We emphasize that this nonmonotonic temperature dependence due to the hot-electron cooling is unique to the PTE response and is not expected from the conventional photovoltaic (PV) effect, in which the separation of excited carriers by the built-in electric field leads to a net current [32][33][34][35]. Therefore, this serves as a strong indication that the PTE effect dominates the response to laser illumination of the G-M interface, consistent with recent reports where the photovoltaic contribution at 800 nm wavelength is relatively small [36].…”

Hot-carrier photocurrent effects at graphene–metal interfaces

“…This vector map clearly reveals that the photocurrent is enhanced when light polarization is perpendicular to the contact edge. A similar effect was observed in [14], whereas [13] reports the opposite effect, i.e. a maximum photocurrent for polarization parallel to the metal contact edge.…”

Surface Plasmon Polariton Graphene Photodetectors

“…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].…”

“…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]. At the metal-graphene junction, a work-function difference causes charge transfer and a shift of the graphene Fermi level underneath the contact [4,5,7,23], compared to that of graphene away from the contact [4,5,7,23], resulting into a build-up of an internal electric field (photovoltaic mechanism) [5,7,24,25] and into a difference of Seebeck coefficients (photo-thermoelectric mechanism) [11,21,26].…”