Showing papers on "Photodetector published in 2015"

Highly narrowband perovskite single-crystal photodetectors enabled by surface-charge recombination

Abstract: Organolead trihalide perovskite is an emerging low-cost, solution-processable material with a tunable bandgap from the violet to near-infrared, which has attracted a great deal of interest for applications in high-performance optoelectronic devices. Here, we present hybrid perovskite single-crystal photodetectors that have a very narrow spectral response with a full-width at half-maximum of <20 nm. The response spectra are continuously tuned from blue to red by changing the halide composition and thus the bandgap of the single crystals synthesized by solution processes. The narrowband photodetection can be explained by the strong surface-charge recombination of the excess carriers close to the crystal surfaces generated by short-wavelength light. The excess carriers generated by below-bandgap excitation locate away from the surfaces and can be much more efficiently collected by the electrodes, assisted by the applied electric field. This provides a new design paradigm for a narrowband photodetector with broad applications where background noise emission needs to be suppressed. Perovskite-based devices typically exhibit broadband spectral responses. Here narrowband (< 20 nm FWHM) response is achieved for a photodetector application.

Polarization-sensitive broadband photodetector using a black phosphorus vertical p–n junction

Abstract: The ability to detect light over a broad spectral range is central to practical optoelectronic applications and has been successfully demonstrated with photodetectors of two-dimensional layered crystals such as graphene and MoS2. However, polarization sensitivity within such a photodetector remains elusive. Here, we demonstrate a broadband photodetector using a layered black phosphorus transistor that is polarization-sensitive over a bandwidth from ∼400 nm to 3,750 nm. The polarization sensitivity is due to the strong intrinsic linear dichroism, which arises from the in-plane optical anisotropy of this material. In this transistor geometry, a perpendicular built-in electric field induced by gating can spatially separate the photogenerated electrons and holes in the channel, effectively reducing their recombination rate and thus enhancing the performance for linear dichroism photodetection. The use of anisotropic layered black phosphorus in polarization-sensitive photodetection might provide new functionalities in novel optical and optoelectronic device applications. The anisotropic optical properties of black phosphorus can be exploited to fabricate photodetectors with linear dichroism operating over a broad spectral range.

Ultrahigh-Gain Photodetectors Based on Atomically Thin Graphene-MoS2 Heterostructures

Abstract: Due to its high carrier mobility, broadband absorption, and fast response time, the semi-metallic graphene is attractive for optoelectronics. Another two-dimensional semiconducting material molybdenum disulfide (MoS2) is also known as light- sensitive. Here we show that a large-area and continuous MoS2 monolayer is achievable using a CVD method and graphene is transferable onto MoS2. We demonstrate that a photodetector based on the graphene/MoS2 heterostructure is able to provide a high photogain greater than 10(8). Our experiments show that the electron-hole pairs are produced in the MoS2 layer after light absorption and subsequently separated across the layers. Contradictory to the expectation based on the conventional built-in electric field model for metal-semiconductor contacts, photoelectrons are injected into the graphene layer rather than trapped in MoS2 due to the presence of a perpendicular effective electric field caused by the combination of the built-in electric field, the applied electrostatic field, and charged impurities or adsorbates, resulting in a tuneable photoresponsivity.

Waveguide-integrated black phosphorus photodetector with high responsivity and low dark current

Abstract: A gated multilayer black phosphorus photodetector integrated on a silicon photonic waveguide operating in the telecom band is demonstrated with intrinsic responsivity up to 135 mA W−1 and 657 mA W−1 in 11.5-nm- and 100-nm-thick devices, respectively.

High-Performance Perovskite–Graphene Hybrid Photodetector

Abstract: A high-performance novel photodetector is demonstrated, which consists of graphene and CH3 NH3 PbI3 perovskite layers. The resulting hybrid photodetector exhibits a dramatically enhanced photo responsivity (180 A/W) and effective quantum efficiency (5× 10(4) %) over a broad bandwidth within the UV and visible ranges.

Ultrasensitive and Broadband MoS2 Photodetector Driven by Ferroelectrics

Abstract: A few-layer MoS2 photodetector driven by poly(vinylidene fluoride-trifluoroethylene) ferroelectrics is achieved. The detectivity and responsitivity are up to 2.2 × 10(12) Jones and 2570 A W(-1), respectively, at 635 nm with ZERO gate bias. E(g) of MoS2 is tuned by the ultrahigh electrostatic field from the ferroelectric polarization. The photoresponse wavelengths of the photodetector are extended into the near-infrared (0.85-1.55 μm).

Hybrid 2D–0D MoS2–PbS Quantum Dot Photodetectors

Abstract: A hybrid phototransistor consisting of colloidal PbS quantum dots and few layers of MoS2 (≥2 layers) is demonstrated. The hybrid benefits from tailored light absorption in the quantum dots throughout the visible/near infrared region, efficient charge-carrier separation at the p-n interface, and fast carrier transport through the MoS2 channel. It shows responsivity of up to 10(6) A W(-1) and backgate-dependent sensitivity.

MoS2/Si Heterojunction with Vertically Standing Layered Structure for Ultrafast, High‐Detectivity, Self‐Driven Visible–Near Infrared Photodetectors

Abstract: As an interesting layered material, molybdenum disulfide (MoS2) has been extensively studied in recent years due to its exciting properties. However, the applications of MoS2 in optoelectronic devices are impeded by the lack of high-quality p–n junction, low light absorption for mono-/multilayers, and the difficulty for large-scale monolayer growth. Here, it is demonstrated that MoS2 films with vertically standing layered structure can be deposited on silicon substrate with a scalable sputtering method, forming the heterojunction-type photodetectors. Molecular layers of the MoS2 films are perpendicular to the substrate, offering high-speed paths for the separation and transportation of photo-generated carriers. Owing to the strong light absorption of the relatively thick MoS2 film and the unique vertically standing layered structure, MoS2/Si heterojunction photodetectors with unprecedented performance are actualized. The self-driven MoS2/Si heterojunction photodetector is sensitive to a broadband wavelength from visible light to near-infrared light, showing an extremely high detectivity up to ≈1013 Jones (Jones = cm Hz1/2 W−1), and an ultrafast response speed of ≈3 μs. The performance is significantly better than the photodetectors based on mono-/multilayer MoS2 nanosheets. Additionally, the MoS2/Si photodetectors exhibit excellent stability in air for a month. This work unveils the great potential of MoS2/Si heterojunction for optoelectronic applications.

Highly Sensitive, Encapsulated MoS2 Photodetector with Gate Controllable Gain and Speed.

Abstract: Semiconducting, two-dimensional molybdenum disulfide (MoS2) is considered a promising new material for highly sensitive photodetection, because of its atomically thin profile and favorable bandgap. However, reported photodetectors to date show strong variation in performance due to the detrimental and uncontrollable effects of environmental adsorbates on devices due to large surface to volume ratio. Here, we report on highly stable and high-performance monolayer and bilayer MoS2 photodetectors encapsulated with atomic layer deposited hafnium oxide. The protected devices show enhanced electronic properties by isolating them from the ambience as strong n-type doping, vanishing hysteresis, and reduced device resistance. By controlling the gate voltage the responsivity and temporal response can be tuned by several orders of magnitude with R ∼ 10-10(4) A/W and t ∼ 10 ms to 10 s. At strong negative gate voltage, the detector is operated at higher speed and simultaneously exhibits a low-bound, record sensitivity of D* ≥ 7.7 × 10(11) Jones. Our results lead the way for future application of ultrathin, flexible, and high-performance MoS2 detectors and prompt for further investigation in encapsulated transition metal dichalcogenide optoelectronics.

Circularly polarized light detection with hot electrons in chiral plasmonic metamaterials.

Abstract: Circularly polarized light is utilized in various optical techniques and devices. However, using conventional optical systems to generate, analyse and detect circularly polarized light involves multiple optical elements, making it challenging to realize miniature and integrated devices. While a number of ultracompact optical elements for manipulating circularly polarized light have recently been demonstrated, the development of an efficient and highly selective circularly polarized light photodetector remains challenging. Here we report on an ultracompact circularly polarized light detector that combines large engineered chirality, realized using chiral plasmonic metamaterials, with hot electron injection. We demonstrate the detector's ability to distinguish between left and right hand circularly polarized light without the use of additional optical elements. Implementation of this photodetector could lead to enhanced security in fibre and free-space communication, as well as emission, imaging and sensing applications for circularly polarized light using a highly integrated photonic platform.

Ultrathin SnSe2 Flakes Grown by Chemical Vapor Deposition for High‐Performance Photodetectors

Abstract: High-quality ultrathin single-crystalline SnSe2 flakes are synthesized under atmospheric-pressure chemical vapor deposition for the first time. A high-performance photodetector based on the individual SnSe2 flake demonstrates a high photoresponsivity of 1.1 × 10(3) A W(-1), a high EQE of 2.61 × 10(5)%, and superb detectivity of 1.01 × 10(10) Jones, combined with fast rise and decay times of 14.5 and 8.1 ms, respectively.

Resolving Weak Light of Sub-picowatt per Square Centimeter by Hybrid Perovskite Photodetectors Enabled by Noise Reduction.

Abstract: A highly sensitive hybrid perovskite photodetector is demonstrated to be able to directly resolve light irradiance down to sub-picowatts per square centimeter, in good accordance with the calculated noise equivalent power, which is enabled by electron and hole transport layer engineering, especially the trap passivation effect of the double fullerene layer.

Filterless narrowband visible photodetectors

Abstract: Wavelength-selective light detection is crucial for many applications, including imaging and machine vision. Narrowband spectral responses are required for colour discrimination, and current systems use broadband photodiodes combined with optical filters. This approach increases the architectural complexity and limits the quality of colour sensing. Here we report a method for tuning the spectral response to give filterless, narrowband red, green and blue photodiodes. The devices have simple planar junction architectures with the photoactive layer being a solution-processed mixture of either an organohalide perovskite or lead halide semiconductor and an organic (macro)molecule. The organic (macro)molecules modify the optical and electrical properties of the photodiode and facilitate charge collection narrowing of the device's external quantum efficiency. These red, green and blue photodiodes all possess full-width at half-maxima of <100 nm and performance metrics suitable for many imaging applications. Photodiodes with an intrinsic narrow spectral response make it possible to discriminate between red, green and blue light without the need for any optical filters.

Narrowband light detection via internal quantum efficiency manipulation of organic photodiodes

Abstract: Spectrally selective light detection is vital for full-colour and near-infrared (NIR) imaging and machine vision. This is not possible with traditional broadband-absorbing inorganic semiconductors without input filtering, and is yet to be achieved for narrowband absorbing organic semiconductors. We demonstrate the first sub-100 nm full-width-at-half-maximum visible-blind red and NIR photodetectors with state-of-the-art performance across critical response metrics. These devices are based on organic photodiodes with optically thick junctions. Paradoxically, we use broadband-absorbing organic semiconductors and utilize the electro-optical properties of the junction to create the narrowest NIR-band photoresponses yet demonstrated. In this context, these photodiodes outperform the encumbent technology (input filtered inorganic semiconductor diodes) and emerging technologies such as narrow absorber organic semiconductors or quantum nanocrystals. The design concept allows for response tuning and is generic for other spectral windows. Furthermore, it is material-agnostic and applicable to other disordered and polycrystalline semiconductors.

Solar-Blind Avalanche Photodetector Based On Single ZnO–Ga2O3 Core–Shell Microwire

Abstract: High-performance solar-blind (200–280 nm) avalanche photodetectors (APDs) were fabricated based on highly crystallized ZnO–Ga2O3 core–shell microwires. The responsivity can reach up to 1.3 × 103 A/W under −6 V bias. Moreover, the corresponding detectivity was as high as 9.91 × 1014 cm·Hz1/2/W. The device also showed a fast response, with a rise time shorter than 20 μs and a decay time of 42 μs. The quality of the detectors in solar-blind waveband is comparable to or even higher than that of commercial Si APD (APD120A2 from Thorlabs Inc.), with a responsivity ∼8 A/W, detectivity ∼1012 cm·Hz1/2/W, and response time ∼20 ns. The high performance of this APD make it highly suitable for practical applications as solar-blind photodetectors, and this core–shell microstructure heterojunction design method would provide a new approach for realizing an APD device.

Sensitive, Fast, and Stable Perovskite Photodetectors Exploiting Interface Engineering

Abstract: Organometallic halide perovskites are a class of solution-processed semiconductors exhibiting remarkable optoelectronic properties. They have seen rapid strides toward enabling efficient third-generation solar cell technologies. Here, we report the first material-tailoring of TiO2/perovskite/spiro-OMeTAD junction-based photodiodes toward applications in photodetection, a field in need of fast, sensitive, low-cost, spectrally tunable materials that offer facile integration across a broad range of substrates. We report photodetection that exhibits 1 μs temporal response, and we showcase stable operation in the detection of over 7 billion transient light pulses through a continuous pulsed-illumination period. The perovskite diode photodetector has a peak responsivity approaching 0.4 A W–1 at 600 nm wavelength, which is superior to red light detection in crystalline silicon photodiodes used in commercial image sensors. Only by developing a composite Al2O3/PCBM front contact interface layer were we able to sta...

Air-Stable and Solution-Processable Perovskite Photodetectors for Solar-Blind UV and Visible Light.

Abstract: Stable perovskite CH3NH3PbI3-xClx for a photodetector was prepared through spin-coating of a fluorous polymer as a light protection layer. The best responsivity of photodetector was 14.5 A/W to white light and 7.85 A/W for solar-blind UV light (λ = 254 nm). The response time was in the submicrosecond range. The fluorous polymer coating increases the lifetime of the devices to almost 100 days.

High-Performance Planar-Type Photodetector on (100) Facet of MAPbI3 Single Crystal.

Abstract: Recently, the discovery of organometallic halide perovskites provides promising routes for fabricating optoelectronic devices with low cost and high performance. Previous experimental studies of MAPbI3 optoelectronic devices, such as photodetectors and solar cells, are normally based on polycrystalline films. In this work, a high-performance planar-type photodetector fabricated on the (100) facet of a MAPbI3 single crystal is proposed. We demonstrate that MAPbI3 photodetector based on single crystal can perform much better than that on polycrystalline-film counterpart. The low trap density of MAPbI3 single crystal accounts for the higher carrier mobility and longer carrier diffusion length, resulted in a significant performance increasement of MAPbI3 photodetector. Compared with similar planar-type photodetectors based on MAPbI3 polycrystalline film, our MAPbI3 single crystal photodetector showed excellent performance with good stability and durability, broader response spectrum to near-infrared region, about 10(2) times higher responsivity and EQE, and approximately 10(3) times faster response speed. These results may pave the way for exploiting high-performance perovskites photodetectors based on single crystal.

Flexible and Semitransparent Organolead Triiodide Perovskite Network Photodetector Arrays with High Stability.

Abstract: Organolead triiodide perovskite (CH3NH3PbI3) as a light-sensitive material has attracted extensive attention in optoelectronics. The reported perovskite photodetectors (PDs) mainly focus on the individual, which limits their spatial imaging applications. Uniform perovskite networks combining transparency and device performance were synthesized on poly(ethylene terephthalate) (PET) by controlling perovskite crystallization. Photodetector arrays based on above network were fabricated to demonstrate the potential for image mapping. The trade-off between the PD performance and transparency was systematically investigated and the optimal device was obtained from 30 wt % precursor concentration. The switching ratio, normalized detectivity, and equivalent dark current derived shot noise as the critical parameters of PD arrays reached 300, 1.02 × 1012 Jones, and 4.73 × 10–15A Hz–1/2, respectively. Furthermore, the PD arrays could clearly detect spatial light intensity distribution, thus demonstrating its prelimin...

X-ray imaging with scintillator-sensitized hybrid organic photodetectors

Abstract: Medical X-ray imaging requires cost-effective and high-resolution flat-panel detectors for the energy range between 20 and 120 keV. Solution-processed photodetectors provide the opportunity to fabricate detectors with a large active area at low cost. Here, we present a disruptive approach that improves the resolution of such detectors by incorporating terbium-doped gadolinium oxysulfide scintillator particles into an organic photodetector matrix. The X-ray induced light emission from the scintillators is absorbed within hundreds of nanometres, which is negligible compared with the pixel size. Hence, optical crosstalk, a limiting factor in the resolution of scintillator-based X-ray detectors, is minimized. The concept is validated with a 256 × 256 pixel detector with a resolution of 4.75 lp mm−1 at a MTF = 0.2, significantly better than previous stacked scintillator-based flat-panel detectors. We achieved a resolution that proves the feasibility of solution-based detectors in medical applications. Time-resolved electrical characterization showed enhanced charge carrier mobility with increased scintillator filling, which is explained by morphological changes.

Light-induced pyroelectric effect as an effective approach for ultrafast ultraviolet nanosensing.

Abstract: Zinc oxide is potentially a useful material for ultraviolet detectors; however, a relatively long response time hinders practical implementation. Here by designing and fabricating a self-powered ZnO/perovskite-heterostructured ultraviolet photodetector, the pyroelectric effect, induced in wurtzite ZnO nanowires on ultraviolet illumination, has been utilized as an effective approach for high-performance photon sensing. The response time is improved from 5.4 s to 53 μs at the rising edge, and 8.9 s to 63 μs at the falling edge, with an enhancement of five orders in magnitudes. The specific detectivity and the responsivity are both enhanced by 322%. This work provides a novel design to achieve ultrafast ultraviolet sensing at room temperature via light-self-induced pyroelectric effect. The newly designed ultrafast self-powered ultraviolet nanosensors may find promising applications in ultrafast optics, nonlinear optics, optothermal detections, computational memories and biocompatible optoelectronic probes.

An Atomically Layered InSe Avalanche Photodetector

Abstract: Atomically thin photodetectors based on 2D materials have attracted great interest due to their potential as highly energy-efficient integrated devices. However, photoinduced carrier generation in these media is relatively poor due to low optical absorption, limiting device performance. Current methods for overcoming this problem, such as reducing contact resistances or back gating, tend to increase dark current and suffer slow response times. Here, we realize the avalanche effect in a 2D material-based photodetector and show that avalanche multiplication can greatly enhance the device response of an ultrathin InSe-based photodetector. This is achieved by exploiting the large Schottky barrier formed between InSe and Al electrodes, enabling the application of a large bias voltage. Plasmonic enhancement of the photosensitivity, achieved by patterning arrays of Al nanodisks onto the InSe layer, further improves device efficiency. With an external quantum efficiency approaching 866%, a dark current in the pic...

Hybrid Graphene–Perovskite Phototransistors with Ultrahigh Responsivity and Gain

Abstract: Graphene is an attractive optoelectronic material for light detection because of its broadband light absorption and fast response time. However, the relatively low absorption cross-section, fast recombination rate, and the absence of gain mechanism have limited the responsivity of pure graphene-based phototransistor to ≈10−2 A W−1. In this work, a photoconductive gain of ≈109 electrons per photon and a responsivity of ≈6.0 × 105 A W−1 are demonstrated in a hybrid photodetector that consists of monolayer graphene covered with a thin layer of dispersive organolead halide perovskite (CH3NH3PbBr2I) islands. The unprecedented performance is attributed to the effective charge transfer and photogating effect, which were evidenced by photoluminescence quenching, time-resolved photoluminescence decay, scanning near-field optical microscopy, and photocurrent mapping. Unlike previous report which used perovskite bulk thin film, the perovskite islands have low bulk recombination rate of photogenerated carriers. The device also shows broad photodetection spectral range from ultraviolet to visible (250–700 nm), affording new opportunities for scalable UV detectors and imaging sensors.

Solution-Processed Graphene Quantum Dot Deep-UV Photodetectors

Abstract: Fast-response and high-sensitivity deep-ultraviolet (DUV) photodetectors with detection wavelength shorter than 320 nm are in high demand due to their potential applications in diverse fields. However, the fabrication processes of DUV detectors based on traditional semiconductor thin films are complicated and costly. Here we report a high-performance DUV photodetector based on graphene quantum dots (GQDs) fabricated via a facile solution process. The devices are capable of detecting DUV light with wavelength as short as 254 nm. With the aid of an asymmetric electrode structure, the device performance could be significantly improved. An on/off ratio of ∼6000 under 254 nm illumination at a relatively weak light intensity of 42 μW cm–2 is achieved. The devices also exhibit excellent stability and reproducibility with a fast response speed. Given the solution-processing capability of the devices and extraordinary properties of GQDs, the use of GQDs will open up unique opportunities for future high-performance...

Piezo-phototronic Effect Enhanced UV/Visible Photodetector Based on Fully Wide Band Gap Type-II ZnO/ZnS Core/Shell Nanowire Array.

Abstract: A high-performance broad band UV/visible photodetector has been successfully fabricated on a fully wide bandgap ZnO/ZnS type-II heterojunction core/shell nanowire array. The device can detect photons with energies significantly smaller (2.2 eV) than the band gap of ZnO (3.2 eV) and ZnS (3.7 eV), which is mainly attributed to spatially indirect type-II transition facilitated by the abrupt interface between the ZnO core and ZnS shell. The performance of the device was further enhanced through the piezo-phototronic effect induced lowering of the barrier height to allow charge carrier transport across the ZnO/ZnS interface, resulting in three orders of relative responsivity change measured at three different excitation wavelengths (385, 465, and 520 nm). This work demonstrates a prototype UV/visible photodetector based on the truly wide band gap semiconducting 3D core/shell nanowire array with enhanced performance through the piezo-phototronic effect.

Ultrahigh photo-responsivity and detectivity in multilayer InSe nanosheets phototransistors with broadband response†

Abstract: We demonstrate the strategies and principles for the performance improvement of layered semiconductor based photodetectors using multilayer indium selenide (InSe) as the model material. It is discovered that multiple reflection interference at the interfaces in the phototransistor device leads to a thickness-dependent photo-response, which provides a guideline to improve the performance of layered semiconductor based phototransistors. The responsivity and detectivity of InSe nanosheet phototransistor can be adjustable using applied gate voltage. Our InSe nanosheet phototransistor exhibits ultrahigh responsivity and detectivity. An ultrahigh external photo-responsivity of ∼104 A W−1 can be achieved from broad spectra ranging from UV to near infrared wavelength using our InSe nanosheet photodetectors. The detectivity of multilayer InSe devices is ∼1012 to 1013 Jones, which surpasses that of the currently exploited InGaAs photodetectors (1011 to 1012 Jones). This research shows that multilayer InSe nanosheets are promising materials for high performance photodetectors.

Low-Dimensional Structure Vacuum-Ultraviolet-Sensitive (λ < 200 nm) Photodetector with Fast-Response Speed Based on High-Quality AlN Micro/Nanowire.

Abstract: A low-dimensional-structure vacuum-ultraviolet-sensitive photodetector based on high-quality aluminum nitride (AlN) micro-/nanowires is reported. This work, for the first time, demonstrates that a semiconductor nanostructure can be applied in vacuum-ultraviolet (VUV) photon detection and opens a way for developing diminutive, power-saving, and low-cost VUV materials and sensors that can be potentially applied in geospace sciences and solar-terrestrial physics.

Ultra-broadband and high response of the Bi2Te3–Si heterojunction and its application as a photodetector at room temperature in harsh working environments

Abstract: Broadband photodetection is central to various technological applications including imaging, sensing and optical communications. On account of their Dirac-like surface state, Topological insulators (TIs) are theoretically predicted to be promising candidate materials for broadband photodetection from the infrared to the terahertz. Here, we report a vertically-constructed ultra-broadband photodetector based on a TI Bi2Te3–Si heterostructure. The device demonstrated room-temperature photodetection from the ultraviolet (370.6 nm) to terahertz (118 μm) with good reproducibility. Under bias conditions, the visible responsivity reaches ca. 1 A W−1 and the response time is better than 100 ms. As a self-powered photodetector, it exhibits extremely high photosensitivity approaching 7.5 × 105 cm2 W−1, and decent detectivity as high as 2.5 × 1011 cm Hz1/2 W−1. In addition, such a prototype device without any encapsulation suffers no obvious degradation after long-time exposure to air, high-energy UV illumination and acidic treatment. In summary, we demonstrate that TI-based heterostructures hold great promise for addressing the long lasting predicament of stable room-temperature high-performance broadband photodetectors.

High-Responsivity Graphene-Boron Nitride Photodetector and Autocorrelator in a Silicon Photonic Integrated Circuit.

Abstract: Graphene and other two-dimensional (2D) materials have emerged as promising materials for broadband and ultrafast photodetection and optical modulation These optoelectronic capabilities can augment complementary metal–oxide–semiconductor (CMOS) devices for high-speed and low-power optical interconnects Here, we demonstrate an on-chip ultrafast photodetector based on a two-dimensional heterostructure consisting of high-quality graphene encapsulated in hexagonal boron nitride Coupled to the optical mode of a silicon waveguide, this 2D heterostructure-based photodetector exhibits a maximum responsivity of 036 A/W and high-speed operation with a 3 dB cutoff at 42 GHz From photocurrent measurements as a function of the top-gate and source-drain voltages, we conclude that the photoresponse is consistent with hot electron mediated effects At moderate peak powers above 50 mW, we observe a saturating photocurrent consistent with the mechanisms of electron–phonon supercollision cooling This nonlinear photore