Graphene-based Photodetectors Research Articles

Photocurrent enhancement of graphene photodetectors by photon tunneling of light into surface plasmons

We demonstrate that surface plasmon resonances excited by photon tunneling through an adjacent dielectric medium enhance the photocurrent detected by a graphene photodetector. The device is created by overlaying a graphene sheet over an etched gap in a gold film deposited on glass. The detected photocurrents are compared for five different excitation wavelengths, ranging from to Although the device is not optimized, the photocurrent excited with incident p-polarized light (which excites resonant surface plasmons) is significantly amplified in comparison with that for s-polarized light (without surface plasmon resonances). We observe that the photocurrent is greater for shorter wavelengths (for both s- and -polarizations) with increased photothermal current. Position-dependent Raman spectroscopic analysis of the optically-excited graphene photodetector indicates the presence of charge carriers in the graphene near the metallic edge. In addition, we show that the polarity of the photocurrent reverses across the gap as the incident light spot moves across the gap. Graphene-based photodetectors offer a simple architecture which can be fabricated on dielectric waveguides to exploit the plasmonic photocurrent enhancement of the evanescent field. Applications for these devices include photodetection, optical sensing and direct plasmonic detection.

Highly Sensitive Graphene–Semiconducting Polymer Hybrid Photodetectors with Millisecond Response Time

Graphene–semiconducting light absorber hybrid photodetectors have attracted increasing attention because of their ultrahigh photoconductive gain and superior sensitivity. However, most graphene-based hybrid photodetectors reported previously have shown a relatively long response time (on the order of seconds) caused by numerous long-lived traps in these hybrid systems, which greatly restricts device speed. In this work, graphene–thieno[3,4-b]thiophene/benzodithiophene polymer hybrid photodetectors fabricated on self-assembled-monolayer (SAM)-functionalized SiO2 substrates are demonstrated with a maximum responsivity of ∼1.8 × 105 A W–1 and a relatively short photocurrent response time of ∼7.8 ms. The fast and highly sensitive device characteristics provide great potential in low-light imaging applications. The hybrid photodetector on the SAM-coated SiO2 substrate shows better performance in responsivities and response times as compared with those of the device on the bare SiO2 substrate. The improved resp...

Graphene-based optical photodetector exploiting hybrid plasmonic waveguide to enhance photo-thermoelectric current

A graphene-based optical photodetector with a symmetric hybrid plasmonic waveguide that integrates two high-refractive-index dielectric slabs as a finite-width insulator-metal-insulator (IMI) structure is proposed and investigated at . It is shown that by electrically tuning the graphene refractive index, the gap between the silicon slabs and graphene strip as a hybrid plasmonic waveguide results in modified modal behavior over the graphene layer. This highly lossy hybrid surface plasmon polariton strip mode enhances optical absorption in the graphene channel. An analytical approach reveals that this highly confined absorption creates a highly asymmetric hot-carrier spatial temperature profile along the channel. Based on photo-thermoelectric (PTE) effect, this asymmetric temperature profile along with applied source-drain voltage (Vds) creates a PTE current. Coupled to the super plasmonic mode of hybrid plasmonic waveguide (HPW) mechanism, this photodetector exhibits a maximum responsivity of . In addition, for the proposed HPW graphene photodetector (HPW-GPD), the calculated effective mode area, the propagation length and modal propagation loss are , and , respectively.

Investigation on the extended range of absorbing film for a microcavity enhanced graphene photodetector

Microcavity is the preferred graphene-based photodetector structure for its perfect feature of narrow spectral width and absorption enhancement, thus its application sheds light on ultra-fast detection in optic telecommunication and sensing fields. Due to an extremely thin film of graphene, the present study naturally deems it essential to locate the graphene in the exact position of resonant peak intensity. Here an extended graphene position margin in asymmetric planar microcavity with absorption higher than 97.35% was demonstrated. The shift of the centre wavelength caused by graphene was revealed to be non-negligible for telecommunication applications and graphene applied devices. The maximum shift beyond the designed wavelength of 1550 nm has reached 1.07 nm in the cavity, which may have severe impact on the DWDM system. Our theoretical investigation amplifies the absorption features and the parasitical spectrum alterations of a designed photodetector with graphene.

Enhancing light absorption in graphene with plasmonic lattices(a)(a)Contribution to the Focus Issue Self-assemblies of Inorganic and Organic Nanomaterials edited by Marie-Paule Pileni.

We present a novel strategy based on metallic arrays of nanoparticles for enhancing the optical absorption in a monolayer of graphene. The collective resonances sustained by the metallic array and arising from the radiative coupling of localized surface plasmon resonances favour the interaction between the graphene layer and the light distributed in the structure. We measure the dispersion diagram of the allowed hybrid plasmonic-photonic modes and we calculate that one of these modes leads to an enhancement of the optical absorption of graphene by a factor 7. We propose to exploit this result for the rational design of graphene-based photodetectors.

Position-dependent and millimetre-range photodetection in phototransistors with micrometre-scale graphene on SiC.

The extraordinary optical and electronic properties of graphene make it a promising component of high-performance photodetectors. However, in typical graphene-based photodetectors demonstrated to date, the photoresponse only comes from specific locations near graphene over an area much smaller than the device size. For many optoelectronic device applications, it is desirable to obtain the photoresponse and positional sensitivity over a much larger area. Here, we report the spatial dependence of the photoresponse in backgated graphene field-effect transistors (GFET) on silicon carbide (SiC) substrates by scanning a focused laser beam across the GFET. The GFET shows a nonlocal photoresponse even when the SiC substrate is illuminated at distances greater than 500 µm from the graphene. The photoresponsivity and photocurrent can be varied by more than one order of magnitude depending on the illumination position. Our observations are explained with a numerical model based on charge transport of photoexcited carriers in the substrate.

A Broadband Fluorographene Photodetector.

High-performance photodetectors operating over a broad wavelength range from ultraviolet, visible, to infrared are of scientific and technological importance for a wide range of applications. Here, a photodetector based on van der Waals heterostructures of graphene and its fluorine-functionalized derivative is presented. It consistently shows broadband photoresponse from the ultraviolet (255 nm) to the mid-infrared (4.3 µm) wavelengths, with three orders of magnitude enhanced responsivity compared to pristine graphene photodetectors. The broadband photodetection is attributed to the synergistic effects of the spatial nonuniform collective quantum confinement of sp2 domains, and the trapping of photoexcited charge carriers in the localized states in sp3 domains. Tunable photoresponse is achieved by controlling the nature of sp3 sites and the size and fraction of sp3 /sp2 domains. In addition, the photoresponse due to the different photoexcited-charge-carrier trapping times in sp2 and sp3 nanodomains is determined. The proposed scheme paves the way toward implementing high-performance broadband graphene-based photodetectors.

Improved Performance of All Solution-Processed Graphene Photodetectors via Plasmonic Nanoparticles

A cost-effective approach of building graphene-based photodetectors with Ag nanowires (AgNWs) as the contact electrode on the glass substrate via solution process has been developed. The transmittance of the graphene/glass and the AgNWs/graphene/glass at a wavelength of 550 nm is 74% and 67%, respectively. To further improve the performance of the graphene photodetector, Ag nanoparticles are applied on the graphene to obtain the enhanced photocurrent and the improved ON/OFF ratio of 505 by the surface plasmon resonance effect. This semi-transparent graphene photodetector shows the superior optical and electrical performance and it paves the way to the future development of graphene-based devices by costly and high-throughput techniques.

Strong tunable absorption enhancement in graphene using dielectric-metal core-shell resonators

We theoretically investigate light absorption by a graphene monolayer that is coated on the outside of dielectric-metal core-shell resonators (DMCSRs). We demonstrate that light absorption of graphene can be greatly enhanced in such multi-layered core-shell architectures as a result of the excitation of the hybridized bonding plasmon resonance supported by the DMCSRs. We also demonstrate that the absorption enhancement in graphene can be easily tuned over a wide range from the visible to the near-infrared, and particularly the enhancement factor can be optimally maximized at any selective wavelength, by simultaneously varying the dielectric core size and the metal shell thickness. Our results suggest that the graphene-wrapped DMCSRs with strong and highly wavelength-tunable absorption enhancement in graphene could be attractive candidates for applications in graphene-based photodetectors and image sensors.

Silicon-graphene conductive photodetector with ultra-high responsivity

Graphene is attractive for realizing optoelectronic devices, including photodetectors because of the unique advantages. It can easily co-work with other semiconductors to form a Schottky junction, in which the photo-carrier generated by light absorption in the semiconductor might be transported to the graphene layer efficiently by the build-in field. It changes the graphene conduction greatly and provides the possibility of realizing a graphene-based conductive-mode photodetector. Here we design and demonstrate a silicon-graphene conductive photodetector with improved responsivity and response speed. An electrical-circuit model is established and the graphene-sheet pattern is designed optimally for maximizing the responsivity. The fabricated silicon-graphene conductive photodetector shows a responsivity of up to ~105A/W at room temperature (27 °C) and the response time is as short as ~30 μs. The temperature dependence of the silicon-graphene conductive photodetector is studied for the first time. It is shown that the silicon-graphene conductive photodetector has ultra-high responsivity when operating at low temperature, which provides the possibility to detect extremely weak optical power. For example, the device can detect an input optical power as low as 6.2 pW with the responsivity as high as 2.4 × 107 A/W when operating at −25 °C in our experiment.

Synergistic Effects of Plasmonics and Electron Trapping in Graphene Short-Wave Infrared Photodetectors with Ultrahigh Responsivity.

Graphene's unique electronic and optical properties have made it an attractive material for developing ultrafast short-wave infrared (SWIR) photodetectors. However, the performance of graphene SWIR photodetectors has been limited by the low optical absorption of graphene as well as the ultrashort lifetime of photoinduced carriers. Here, we present two mechanisms to overcome these two shortages and demonstrate a graphene-based SWIR photodetector with high responsivity and fast photoresponse. In particular, a vertical built-in field is employed in the graphene channel for trapping the photoinduced electrons and leaving holes in graphene, which results in prolonged photoinduced carrier lifetime. On the other hand, plasmonic effects were employed to realize photon trapping and enhance the light absorption of graphene. Thanks to the above two mechanisms, the responsivity of this proposed SWIR photodetector is up to a record of 83 A/W at a wavelength of 1.55 μm with a fast rising time of less than 600 ns. This device design concept addresses key challenges for high-performance graphene SWIR photodetectors and is promising for the development of mid/far-infrared optoelectronic applications.

Photoconductive multi-layer graphene photodetectors fabricated on etched silicon-on-insulator substratesr**Project supported by the National Key Research and Development Program of China (Grant No. 2016YFB0402404), the High-Tech Research and Development Program of China (Grant Nos. 2013AA031401, 2015AA016902, and 2015AA016904), and the National Natural Science Foundation of China (Grant Nos. 61674136, 61176053, 61274069, and 61435002).

Recently, graphene-based photodetectors have been rapidly developed. However, their photoresponsivities are generally low due to the weak optical absorption strength of graphene. In this paper, we fabricate photoconductive multi-layer graphene (MLG) photodetectors on etched silicon-on-insulator substrates. A photoresponsivity exceeding is obtained, which enables most optoelectronic application. In addition, according to the analyses of the high photoresponsivity and long photoresponse time, we conclude that the working mechanism of the device is photoconductive effect. The process of photons conversion into conducting electrons is also described in detail. Finally, according to the distinct difference between the photoresponses at 1550 nm and 808 nm, we estimate that the position of the trapping energy is somewhere between 0.4 eV and 0.76 eV, higher than the Fermi energy of MLG. Our work paves a new way for fabricating the graphene photoconductive photodetectors.

Numerical Study on Plasmonic Absorption Enhancement by a Rippled Graphene Sheet

We investigate the absorption property of a single layer of periodically rippled graphene adhering to dielectric substrate. By varying the geometric parameters of the ripple and chemical potential of graphene, the absorption can remarkably exceed that of planar graphene and reaches over 50%. It is found that the absorption peak undergoes a redshift as the ripple height increases. The variation of the incident angle will basically not influence the absorption wavelength but may yield additional absorption peaks due to higher order oscillation. The study reveals the optical response of graphene in the intrinsically nonplanar profile and will benefit to the design of graphene-based absorbers and photodetectors.

ZnO quantum dot-doped graphene/h-BN/GaN-heterostructure ultraviolet photodetector with extremely high responsivity

A ZnO quantum dot photo-doped graphene/h-BN/GaN-heterostructure ultraviolet photodetector with extremely high responsivity of more than 1915 A W−1 and detectivity of more than 1.02 × 1013 Jones (Jones = cm Hz1/2 W−1) has been demonstrated. The interfaced h-BN layer increases the barrier height at the graphene/GaN heterojunction, which decreases the dark current and improves the on/off current ratio of the device. The photo-doping effect increases the barrier height and carrier concentration at the graphene/h-BN/GaN heterojunction, thus the responsivity is improved from 1473 A W−1 to 1915 A W−1 and the detectivity is improved from 5.8 × 1012 to 1.0 × 1013 Jones. Moreover, all of the responsivity and detectivity values are the highest values among all the graphene-based ultraviolet photodetectors.

Ultra-sensitive graphene photodetector with plasmonic structure

We report a graphene-based photodetector with ultra-high photoresponsivity and wavelength selectivity, targeting at the mid-infrared (MIR) regime. To enhance the spectral selectivity, a gold-grating structure is designed and implemented under the graphene layer to excite surface plasmon polaritons (SPPs). The electromagnetic field with specific wavelength can be guided to and confined within the designed subwavelength structures. The graphene layer contacted by metal is slightly p-type doped due to gold grating, improving the interband transition rate of electrons. The built-in potential established in the contact region facilitates the separation of non-equilibrium carriers generated on graphene layer, leading to a photovoltage. With optimized structural design the photodetector exhibits excellent photoresponsivity of 1 V/μW at the wavelength of 9 μm.

THz-circuits driven by photo-thermoelectric, gate-tunable graphene-junctions.

For future on-chip communication schemes, it is essential to integrate nanoscale materials with an ultrafast optoelectronic functionality into high-frequency circuits. The atomically thin graphene has been widely demonstrated to be suitable for photovoltaic and optoelectronic devices because of its broadband optical absorption and its high electron mobility. Moreover, the ultrafast relaxation of photogenerated charge carriers has been verified in graphene. Here, we show that dual-gated graphene junctions can be functional parts of THz-circuits. As the underlying optoelectronic process, we exploit ultrafast photo-thermoelectric currents. We describe an immediate photo-thermoelectric current of the unbiased device following a femtosecond laser excitation. For a picosecond time-scale after the optical excitation, an additional photo-thermoelectric contribution shows up, which exhibits the fingerprint of a spatially inverted temperature profile. The latter can be understood by the different time-constants and thermal coupling mechanisms of the electron and phonon baths within graphene to the substrate and the metal contacts. The interplay of the processes gives rise to ultrafast electromagnetic transients in high-frequency circuits, and it is equally important for a fundamental understanding of graphene-based ultrafast photodetectors and switches.

Controlled Generation of a p-n Junction in a Waveguide Integrated Graphene Photodetector.

With its electrically tunable light absorption and ultrafast photoresponse, graphene is a promising candidate for high-speed chip-integrated photonics. The generation mechanisms of photosignals in graphene photodetectors have been studied extensively in the past years. However, the knowledge about efficient light conversion at graphene p-n junctions has not yet been translated into high-performance devices. Here, we present a graphene photodetector integrated on a silicon slot-waveguide, acting as a dual gate to create a p-n junction in the optical absorption region of the device. While at zero bias the photothermoelectric effect is the dominant conversion process, an additional photoconductive contribution is identified in a biased configuration. Extrinsic responsivities of 35 mA/W, or 3.5 V/W, at zero bias and 76 mA/W at 300 mV bias voltage are achieved. The device exhibits a 3 dB bandwidth of 65 GHz, which is the highest value reported for a graphene-based photodetector.

Bolometric effect in a waveguide-integrated graphene photodetector**Project supported by the National Key Research and Development Program of China (Grant No. 2016YFB0402204), the High-Tech Research and Development Program of China (Grant Nos. 2013AA031401, 2015AA016902, and 2015AA016904), and the National Natural Science Foundation of China (Grant Nos. 61674136, 61176053, 61274069, and 61435002).

Graphene is an alternative material for photodetectors owing to its unique properties. These include its uniform absorption of light from ultraviolet to infrared and its ultrahigh mobility for both electrons and holes. Unfortunately, due to the low absorption of light, the photoresponsivity of graphene-based photodetectors is usually low, only a few milliamps per watt. In this letter, we fabricate a waveguide-integrated graphene photodetector. A photoresponsivity exceeding 0.11 A·W−1 is obtained which enables most optoelectronic applications. The dominating mechanism of photoresponse is investigated and is attributed to the photo-induced bolometric effect. Theoretical calculation shows that the bolometric photoresponsivity is 4.6 A·W−1. The absorption coefficient of the device is estimated to be 0.27 dB·μm−1.

High-performance graphene photodetector using interfacial gating

Graphene-based photodetectors have recently received much attention for their potential to detect weak signals and their short response time, both of which are crucial in applications such as optical positioning, remote sensing, and biomedical imaging. However, existing devices for detecting weak signals are limited by the current photogating mechanism, so the price for achieving ultrahigh sensitivity is to sacrifice response time. In this work, we bridge the gap between ultrafast response and ultrahigh sensitivity by employing a graphene/SiO2/lightly doped Si architecture with an interfacial gating mechanism. Our device is capable of detecting a signal of <1 nW (with a responsivity of ∼1000 AW−1), and the spectral response extends from the visible to near-IR. More important, the photoresponse time of our device has been pushed to ∼400 ns. The current device structure does not need a complicated fabrication process and is fully compatible with silicon technology. This work not only will open up a route to graphene-based high-performance optoelectronic devices but also has great potential for ultrafast weak signal detection.

Terahertz-induced photothermoelectric response in graphene-metal contact structures

We report on the photoresponse of a graphene-metal contact device under terahertz (THz) illumination. The device has an extremely simple structure consisting of a large-area monolayer graphene stripe contacted with two gold electrodes. A significant position-dependent photovoltage is observed across the device by THz excitation, exhibiting a linear relationship with the incident beam power. Experimental results show that the graphene channel length and the substrate thermal conductivity have obvious influence on the photovoltage amplitude and response time, which is consistent with the photothermoelectric mechanism. This compact and powerless device is expected to have a promising application in THz detection. Our work provides theoretical and experimental evidence for the development of high-performance graphene-based THz photodetectors.