Graphene photodetectors’ intrinsically low responsivity (sensitivity) has been a long-standing issue that overshadows graphene’s other excellent optical properties as a photodetection material. The key to improving the graphene photodetector responsivity lies in enhancing the photothermoelectric (PTE) effect, which has already been demonstrated to be the dominant photocarrier generation mechanism. To maximize the PTE current, one would need a strong optically-induced temperature gradient to overlap with a graphene p-n junction spatially. Here, the temperature gradient drives the charge carrier movement, while the graphene p-n junction separates the different charge carrier types (electrons and holes) and makes them drift in opposite directions. In this work, we show that these two conditions can be met simultaneously in a meticulously designed device, combining a gap plasmon structure and a pair of split-gates. The gap plasmon structure absorbs 71% of incident light creating localized heating (thereby large temperature gradient), and the split-gates create a p-n junction at the center of the localized thermal gradient. We fabricated a graphene photodetector with the proposed configuration, and experimentally verified the dominance of PTE effect in photocurrent generation in good agreement with theoretical calculations. More importantly, we obtained a responsivity 70 times higher than the previously reported value from a similar device without plasmon-enhancement. Moreover, originating from the combination of gap plasmon-enhanced optical absorption and optimized p-n junction, our responsivity is 5~7 times higher than reported values for other graphene photodetectors with different types of plasmon-enhancement and no junction control.
Due to its high charge carrier mobility, broadband light absorption, and ultrafast carrier dynamics, graphene is a promising material for the development of high-performance photodetectors. Graphene-based photodetectors have been demonstrated to date using monolayer graphene operating in conjunction with either metals or semiconductors. Most graphene devices are fabricated on doped Si substrates with SiO2 dielectric used for back gating. Here, we demonstrate photodetection in graphene field effect phototransistors fabricated on undoped semiconductor (SiC) substrates. The photodetection mechanism relies on the high sensitivity of the graphene conductivity to the local change in the electric field that can result from the photo-excited charge carriers produced in the back-gated semiconductor substrate. We also modeled the device and simulated its operation using the finite element method to validate the existence of the field-induced photoresponse mechanism and study its properties. Our graphene phototransistor possesses a room-temperature photoresponsivity as high as ~7.4 A/W, which is higher than the required photoresponsivity (1 A/W) in most practical applications. The light power-dependent photocurrent and photoresponsivity can be tuned by the source-drain bias voltage and back-gate voltage. Graphene phototransistors based on this simple and generic architecture can be fabricated by depositing graphene on a variety of undoped substrates, and are attractive for many applications in which photodetection or radiation detection is sought.
Graphene has been demonstrated to be a promising photodetection material because of its atomic-thin nature, broadband and uniform optical absorption, etc. Photovoltaic and photothermoelectric, which are considered to be the main contributors to photo current/voltage generation in graphene, enable photodetection through driving electrons via built-in electric field and thermoelectric power, respectively. Graphene photovoltaic/photothermoelectric detectors are ideal for ultrafast photodetection applications due to the high carrier mobilities in graphene and ultrashort time the electrons need to give away heat. Despite all the advantages for graphene photovoltaic/photothermoelectric detectors, the sensitivity in such detectors is relatively low, owing to the low optical absorption in the single atomic layer. In the past, our research group has used delicately designed snowflake-like fractal metasurface to realize broadband photovoltage enhancement in the visible spectral range, on SiO2 thin film backed by Si substrates. We have also demonstrated that the enhancement from the proposed fractal metasurface is insensitive to the polarization of the incident light. In this current work, we have carried out experiments of the same fractal metasurface on transparent SiO2 substrates, and obtained higher enhancement factor on the fractal metasurface than that achieved on SiO2/Si substrates. Moreover, the device allows more than 70% of the incident light to transmit during the detection, enabling photodetection in the optical path without any significant distortion. Another possibility to make use of the large portion of transmitted light is to stack multiple such devices along the optical path to linearly scale up the sensitivity.
Graphene has been demonstrated to be a promising photo-detection material because of its ultra-broadband absorption, compatibility with CMOS technology, and dynamic tunability. There are multiple known photo-detection mechanisms in graphene, among which the photovoltaic effect has the fastest response time thus is the prioritized candidate for ultrafast photodetector. There have been numerous efforts to enhance the intrinsically low sensitivity in graphene photovoltaic detectors using metallic plasmonic structures, but such plasmonic enhancements are mostly narrowband and polarization dependent. In this work, we propose a gold Cayley-tree fractal metasurface design that has a multi-band resonance, to realize broadband and polarization-insensitive plasmonic enhancement in graphene photovoltaic detectors. When illuminated with visible light, the fractal metasurface exhibits multiple hotspots at the metal-graphene interface, where the electric field of the incident electromagnetic wave is enhanced and contributes to generating excess electron-hole pairs in graphene. The large metal-graphene interface length in the fractal metasurface also helps to harvest at a higher efficiency the electron-hole pairs by built-in electric field due to metal-graphene potential gradient. To demonstrate the concept, we carried out experiment using Ar-Kr CW laser, an optical chopper, and lock-in amplifier. We obtained experimentally an almost constant ten-fold enhancement of photocurrent generated on the fractal metasurface compared to that generated on the plain gold-graphene edge, at all tested wavelengths (488 nm, 514 nm, 568 nm, and 647 nm). We also observed an unchanged photoresponse with respect to incident light polarization angles, which is a result of the highly symmetric geometry of the fractal metasurface.