Photogating Effect Research Articles

High responsivity graphene photodetectors from visible to near-infrared by photogating effect

Graphene photodetectors are highly attractive owing to its ultra-fast and wide-range spectral response from visible to infrared benefit from the superior carrier mobility and the linear dispersion with zero bandgap of graphene. The application of graphene photodetectors however is seriously limited by the low intrinsic responsivity in the order of ∼10 mA/W. Here, we demonstrate photogating field-effect transistors based on pure monolayer graphene with simple device structures. The light absorption in the heavily n-doped silicon/silicon oxide (Si/SiO2) substrate generates an additional photovoltage that effectively modulates the conductance of graphene, leading to room temperature graphene photodetectors with high responsivity of ∼500 A/W for 450 nm light and ∼4 A/W for 1064 nm light, respectively. The generated photocurrent changes with applied gate voltage and shows a strongly nonlinear power dependence. Meanwhile, the photoresponse of graphene exhibits a cut-off wavelength of ∼1100 nm, confirming the dominance photogating effect caused by light absorption in Si/SiO2 substrate. Considering the great compatibility of graphene to Si technology, our result paves a way for high-performance chip-integrated photodetectors.

Photoresponsive Devices: Ultrahigh Photoresponsive Device Based on ReS2/Graphene Heterostructure (Small 45/2018)

In article number 1802593, Changgu Lee and co-workers develop a photodetector based on a ReS2/graphene heterostructure, which exhibits outstanding photoresponsivity, detectivity, and ultrafast responsivity due to the excellent light absorption property of ReS2 and high carrier mobility of graphene. The interface between ReS2 and graphene facilitates a high photocurrent because of the photogating effect under the applied gate voltage.

Synthesis of few-layer 2H-MoSe2 thin films with wafer-level homogeneity for high-performance photodetector

Abstract The unique structural and physical properties of two-dimensional (2D) atomic layer semiconductors render them promising candidates for electronic or optoelectronic devices. However, the lack of efficient and stable approaches to synthesize large-area thin films with excellent uniformity hinders their realistic applications. In this work, we reported a method involving atomic layer deposition and a chemical vapor deposition chamber to produce few-layer 2H-MoSe2 thin films with wafer-level uniformity. The reduction of MoO3 was found indispensable for the successful synthesis of MoSe2 films due to the low vaporization temperature. Moreover, a metal-semiconductor-metal photodetector (PD) was fabricated and investigated systematically. We extracted an ultrahigh photoresponsivity approaching 101 A/W with concomitantly high external quantum efficiency up to 19,668% due to the produced gain arising from the holes trapped at the metal/MoSe2 interface, the band tail state contribution, and the photogating effect. A fast response time of 22 ms was observed and attributed to effective nonequilibrium carrier recombination. Additionally, the ultrahigh photoresponsivity and low dark current that originated from Schottky barrier resulted in a record-high specific detectivity of up to 2×1013 Jones for 2D MoSe2/MoS2 PDs. Our findings revealed a pathway for the development of high-performance PDs based on 2D MoSe2 that are inexpensive, large area, and suitable for mass production and contribute to a deep understanding of the photoconductivity mechanisms in atomically thin MoSe2. We anticipate that these results are generalizable to other layer semiconductors as well.

Photocurrent Engineering of Silicon Nanowire Field‐Effect Transistors by Ultrathin Poly(3‐hexylthiophene)

AbstractPhotoresponse on the silicon nanowire (SiNW) and organic semiconductor interfaces embedding an insulating barrier is understood by a photogating effect associated with charge separations. Still elusive one is when the thickness of organic semiconductor is decreased down to a few molecular layers, where the photoresponse can be strongly altered by the spatial confinement of photoinduced carriers. In this work, the photoresponse modulation of SiNW field‐effect transistors coated with an ultrathin organic semiconductor poly(3‐hexylthiophene) (P3HT) is reported, where the P3HT layer thickness is changed by an order of magnitude. In the absence of laser illumination, the P3HT interface on SiNW slightly decreases the SiNW channel current, irrespective of the P3HT thickness. Upon the laser illumination, the thick case of P3HT layers (≈100 nm) supports the dissociation of photoinduced electrons and holes, hence the photogating effect largely decreases the SiNW channel current. When the P3HT layer thickness is decreased down to the P3HT hole diffusion length (≈8.5 nm), the photoinduced P3HT holes screen out the P3HT interface states, effectively increasing the SiNW channel current up to the intrinsic level. The method of engineering the organic semiconductor thickness suggests a complementary optoelectronic operation, verifying the controllable photoresponse within a compact nanodevice.

P‐Type SnO Thin Film Phototransistor with Perovskite‐Mediated Photogating

AbstractA p‐type phototransistor based on tin monoxide (SnO) thin film and a perovskite‐mediated photogating effect to enhance the device performance are reported. Without the perovskite layer, the SnO thin film phototransistor exhibits a good figure of merit including on/off ratio (Ion/Ioff) over 103, hole mobility of 3.55 cm2 V−1 s−1, photoresponsivity of 1.83 × 103 A W−1, and detectivity of 2.11 × 1013 Jones at 655 nm; these values are among the highest reported for oxide‐based devices. Furthermore, it is shown that when a hybrid perovskite MAPbI3 overlayer is deposited on the p‐type SnO channel, the phototransistor behavior is significantly modified. Specifically, the field‐effect mobility increases to 5.53 cm2 V−1 s−1 and the on/off ratio increases to 2.7 × 103 compared with an on/off ratio of 519 under dark. These effects can be ascribed to a perovskite‐mediated photogating effect with favorable band alignment and interfacial charge transfer. This work not only introduces SnO as a new p‐type phototransistor material but also presents a general approach to enhance the channel transport via coating a photoactive perovskite overlayer.

Room-Temperature Phototransistor with Negative Photoresponsivity of 108 A W-1 Using Fullerene-Sensitized Aligned Carbon Nanotubes.

Detection of low intensity light down to a few photons requires photodetectors with high gain. A new photodetector is reported based on C60 -sensitized aligned carbon nanotube (CNT) transistors with an extremely high responsivity of 108 A W-1 (gain > 108 ) in the ultraviolet and visible range, and 720 A W-1 (gain = 940) in the infrared range. In contrast to most sensitized phototransistors that operate on the photogating effect, the new photodetector operates on the modulation of the electrons scattering in the CNTs, leading to negative photoconductivity. Comparison with similar photodetectors using random CNT networks shows the benefit of using aligned CNTs. At room temperature, the aligned CNT photodetectors are demonstrated to detect a few tens of photons per CNT.

Graphene-Based Light Sensing: Fabrication, Characterisation, Physical Properties and Performance.

Graphene and graphene-based materials exhibit exceptional optical and electrical properties with great promise for novel applications in light detection. However, several challenges prevent the full exploitation of these properties in commercial devices. Such challenges include the limited linear dynamic range (LDR) of graphene-based photodetectors, the lack of efficient generation and extraction of photoexcited charges, the smearing of photoactive junctions due to hot-carriers effects, large-scale fabrication and ultimately the environmental stability of the constituent materials. In order to overcome the aforementioned limits, different approaches to tune the properties of graphene have been explored. A new class of graphene-based devices has emerged where chemical functionalisation, hybridisation with light-sensitising materials and the formation of heterostructures with other 2D materials have led to improved performance, stability or versatility. For example, intercalation of graphene with FeCl is highly stable in ambient conditions and can be used to define photo-active junctions characterized by an unprecedented LDR while graphene oxide (GO) is a very scalable and versatile material which supports the photodetection from UV to THz frequencies. Nanoparticles and quantum dots have been used to enhance the absorption of pristine graphene and to enable high gain thanks to the photogating effect. In the same way, hybrid detectors made from stacked sequences of graphene and layered transition-metal dichalcogenides enabled a class of devices with high gain and responsivity. In this work, we will review the performance and advances in functionalised graphene and hybrid photodetectors, with particular focus on the physical mechanisms governing the photoresponse, the performance and possible future paths of investigation.

Ultrahigh Photoresponsive Device Based on ReS2 /Graphene Heterostructure.

Heterostructures that combine graphene and transition metal dichalcogenides, such as MoS2 , MoTe2 , and WS2 , have attracted attention due to their high performances in optoelectronic devices compared to homogeneous systems. Here, a photodevice based on a hybrid van der Waals heterostructure of rhenium disulfide (ReS2 ) and graphene is fabricated using the stacking method. The device presents a remarkable ultrahigh photoresponsivity of 7 × 105 A W-1 and a detectivity of 1.9 × 1013 Jones, along with a fast response time of less than 30 ms. Tremendous photocurrents are generated in the heterostructure due to the direct bandgap, high quantum efficiency, and strong light absorption by the multilayer ReS2 and the high carrier mobility of graphene. The ReS2 /graphene heterostructured device displays a high photocurrent under the applied gate voltages due to the photogating effect induced by the junction between graphene and ReS2 . The ReS2 /graphene heterostructure may find promising applications in future optoelectronic devices, providing a high sensitivity, flexibility, and transparency.

High responsivity middle-wavelength infrared graphene photodetectors using photo-gating

In this work, high-responsivity graphene photodetectors operating in the middle-wavelength infrared (MWIR) spectral band were fabricated by taking advantage of the photo-gating effect. Graphene-based field effect transistors were fabricated on indium antimonide (InSb) substrates. The InSb generated photo-carriers in response to incident IR light modulated the graphene channel gate voltage and induced a large photocurrent. These graphene-based photodetectors exhibited a clear photoresponse during irradiation with 4.6 μm MWIR laser light and an ultrahigh responsivity of 33.8 A/W was achieved at 50 K due to the photo-gating effect. These devices were found to maintain an MWIR photoresponse up to 150 K. Our graphene-based photodetector design is expected to contribute to the development of high-performance MWIR image sensors.

Double-layer heterostructure of graphene/carbon nanotube films for highly efficient broadband photodetector

Graphene-semiconducting single-wall carbon nanotubes' (graphene-s-SWCNTs) thin-film photodetector based on a double-layer stacked heterostructure was fabricated. The carbon-based heterostructure exhibits excellent long-range van der Waals interactions. The as-fabricated device was demonstrated with an ultra-broadband photodetection characteristic with a high responsivity of 78 A/W at a visible wavelength and a fast response time of 80 μs. Moreover, the high photoconductive gain based on the photogating effect for the graphene-s-SWCNTs device was realized. In addition, the temperature-dependent photoresponse performance was also demonstrated. Benefiting from the high photoconductive gain, ultra-fast response time, and high stable quality of carbon materials, our devices exhibit great potential applications for high sensitivity detection to weak target signals in extreme environments.

Ultrascaled Germanium Nanowires for Highly Sensitive Photodetection at the Quantum Ballistic Limit.

We report an experimental study on quasi-one-dimensional Al-Ge-Al nanowire (NW) heterostructures featuring unmatched photoconductive gains exceeding 107 and responsivities as high as 10 A/μW in the visible wavelength regime. Our observations are attributed to the presence of GeO x related hole-trapping states at the NW surface and can be described by a photogating effect in accordance with previous studies on low-dimensional nanostructures. Utilizing an ultrascaled photodetector device operating in the quantum ballistic transport regime at room temperature we demonstrate for the first time that individual current channels can be addressed directly by laser irradiation. The resulting quantization of the photocurrent represents the ultimate limit of photodetectors, allowing for advanced concepts including highly resolved imaging, light effect transistors and single photon detectors with practically zero off-state current.

Hybrid Field-Effect Transistors and Photodetectors Based on Organic Semiconductor and CsPbI3 Perovskite Nanorods Bilayer Structure

The outstanding performances of nanostructured all-inorganic CsPbX3 (X = I, Br, Cl) perovskites in optoelectronic applications can be attributed to their unique combination of a suitable bandgap, high absorption coefficient, and long carrier lifetime, which are desirable for photodetectors. However, the photosensing performances of the CsPbI3 nanomaterials are limited by their low charge-transport efficiency. In this study, a phototransistor with a bilayer structure of an organic semiconductor layer of 2,7-dioctyl [1] benzothieno[3,2-b] [1] benzothiophene and CsPbI3 nanorod layer was fabricated. The high-quality CsPbI3 nanorod layer obtained using a simple dip-coating method provided decent transistor performance of the hybrid transistor device. The perovskite layer efficiently absorbs light, while the organic semiconductor layer acts as a transport channel for injected photogenerated carriers and provides gate modulation. The hybrid phototransistor exhibits high performance owing to the synergistic function of the photogating effect and field effect in the transistor, with a photoresponsivity as high as 4300 A W−1, ultra-high photosensitivity of 2.2 × 106, and excellent stability over 1 month. This study provides a strategy to combine the advantages of perovskite nanorods and organic semiconductors in fabrication of high-performance photodetectors.

Enhanced photoresponsivity and hole mobility of MoTe2 phototransistors by using an Al2O3 high-κ gate dielectric

Molybdenum ditelluride (MoTe2) has been demonstrated great potential in electronic and optoelectronic applications. However, the reported effective hole mobility remains far below its theoretical value. Herein, taking advantage of high-κ screening effect, we have fabricated back-gated MoTe2 transistors on an Al2O3 high-κ dielectric and systematically investigated the electronic and optoelectronic properties. A high current on/off ratio exceeding 106 is achieved in the Al2O3-based MoTe2 transistors, and the hole mobility is demonstrated to be 150 cm2 V−1 s−1, compared to 0.2–20 cm2 V−1 s−1 ever obtained from back-gated MoTe2 transistors in the literatures. Moreover, a considerable hole concentration of 1.2 × 1013 cm−2 is attained in our Al2O3-based MoTe2 transistors owing to the strong gate control capability, leading to a high on-state hole current of 6.1 μA μm−1. After optimization, our Al2O3-based MoTe2 phototransistor exhibits outstanding photodetective performance, with a high responsivity of 543 A W−1 and a high photogain of 1,662 at 405 nm light illumination, which are boosted around 419 times compared to the referential SiO2-based control devices. The mechanisms of photoconductivity in the Al2O3-based MoTe2 phototransistors have been analyzed in detail, and the photogating effect is considered to play an important role. This work may provide useful insight to improve carrier mobility in two-dimensional layered semiconductors and open opportunities to facilitate the development of high-performance photodetectors in the future.

Distinct Optoelectronic Signatures for Charge Transfer and Energy Transfer in Quantum Dot–MoS2 Hybrid Photodetectors Revealed by Photocurrent Imaging Microscopy

AbstractAtomically thin transition metal dichalcogenides (TMDCs) have intriguing nanoscale properties like high charge mobility, photosensitivity, layer‐thickness‐dependent bandgap, and mechanical flexibility, which are all appealing for the development of next generation optoelectronic, catalytic, and sensory devices. Their atomically thin thickness, however, renders TMDCs poor absorptivity. Here, bilayer MoS2 is combined with core‐only CdSe QDs and core/shell CdSe/ZnS QDs to obtain hybrids with increased light harvesting and exhibiting interfacial charge transfer (CT) and nonradiative energy transfer (NET), respectively. Field‐effect transistors based on these hybrids and their responses to varying laser power and applied gate voltage are investigated with scanning photocurrent microscopy (SPCM) in view of their potential utilization in light harvesting and photodetector applications. CdSe–MoS2 hybrids are found to exhibit encouraging properties for photodetectors, like high responsivity and fast on/off response under low light exposure while CdSe/ZnS–MoS2 hybrids show enhanced charge carrier generation with increased light exposure, thus suitable for photovoltaics. While distinguishing optically between CT and NET in QD–TMDCs is nontrivial, it is found that they can be differentiated by SPCM as these two processes exhibit distinctive light‐intensity dependencies: CT causes a photogating effect, decreasing the photocurrent response with increasing light power while NET increases the photocurrent response with increasing light power, opposite to CT case.

Pronounced photogating effect in atomically thin WSe2 with a self-limiting surface oxide layer

The photogating effect is a photocurrent generation mechanism that leads to marked responsivity in two-dimensional (2D) semiconductor-based devices. A key step to promote the photogating effect in a 2D semiconductor is to integrate it with a high density of charge traps. Here, we show that self-limiting surface oxides on atomically thin WSe2 can serve as effective electron traps to facilitate p-type photogating. By examining the gate-bias-induced threshold voltage shift of a p-type transistor based on single-layer WSe2 with surface oxide, the electron trap density and the trap rate of the oxide are determined to be >1012 cm−2 and >1010 cm−2 s−1, respectively. White-light illumination on an oxide-covered 4-layer WSe2 transistor leads to the generation of photocurrent, the magnitude of which increases with the hole mobility. During illumination, the photocurrent evolves on a timescale of seconds, and a portion of the current persists even after illumination. These observations indicate that the photogenerated electrons are trapped deeply in the surface oxide and effectively gate the underlying WSe2. Owing to the pronounced photogating effect, the responsivity of the oxide-covered WSe2 transistor is observed to exceed 3000 A/W at an incident optical power of 1.1 nW, suggesting the effectiveness of surface oxidation in facilitating the photogating effect in 2D semiconductors.

Graphene/black phosphorus heterostructured photodetector

Graphene photodetectors exhibit a low photoresponsivity due to their weak light absorbance. In this study, we fabricated a graphene/black phosphorus (BP) heterostructure, in which the multilayer BP flake with a ∼0.3 eV direct band gap functions as an enhanced light-absorption material. Further, the photoexcited electrons are trapped in the trap states of the BP, which creates a photogating effect and causes holes to flow into the graphene layer driven by the built-in potential between BP and graphene. The photocarrier lifetime is therefore prolonged by trapping, and as a result of the high carrier mobility of graphene, the holes that transfer into the graphene channel can travel through the circuit before they recombine with trapped electrons. These combined effects result in a high photoresponsivity: 55.75 A/W at λ = 655 nm, 1.82 A/W at λ = 785 nm, and 0.66 A/W at λ = 980 nm.

Highly Enhanced Photoresponsivity of a Monolayer WSe2 Photodetector with Nitrogen-Doped Graphene Quantum Dots

Hybrid structures of two-dimensional (2D) materials and quantum dots (QDs) are particularly interesting in the field of nanoscale optoelectronic devices because QDs are efficient light absorbers and can inject photocarriers into thin layers of 2D transition-metal dichalcogenides, which have high carrier mobility. In this study, we present a heterostructure that consists of a monolayer of tungsten diselenide (ML WSe2) covered by nitrogen-doped graphene QDs (N-GQDs). The improved photoluminescence of ML WSe2 is attributed to the dominant neutral exciton emission caused by the n-doping effect. Owing to strong light absorption and charge transfer from N-GQDs to ML WSe2, N-GQD-covered ML WSe2 showed up to 480% higher photoresponsivity than that of a pristine ML WSe2 photodetector. The hybrid photodetector exhibits good environmental stability, with 46% performance retention after 30 days under ambient conditions. The photogating effect also plays a key role in the improvement of hybrid photodetector performance. On applying the back-gate voltage modulation, the hybrid photodetector shows a responsivity of 2578 A W-1, which is much higher than that of the ML WSe2-based device.

Ultra-high Photoresponsivity in Suspended Metal-Semiconductor-Metal Mesoscopic Multilayer MoS2 Broadband Detector from UV-to-IR with Low Schottky Barrier Contacts

The design, fabrication, and characterization of ultra-high responsivity photodetectors based on mesoscopic multilayer MoS2 is presented, which is a less explored system compared to direct band gap monolayer MoS2 that has received increasing attention in recent years. The device architecture is comprised of a metal-semiconductor-metal (MSM) photodetector, where Mo was used as the contact metal to suspended MoS2 membranes. The photoresponsivity {boldsymbol{R}} was measured to be ~1.4 × 104 A/W, which is > 104 times higher compared to prior reports, while the detectivity D* was computed to be ~2.3 × 1011 Jones at 300 K at an optical power P of ~14.5 pW and wavelength λ of ~700 nm. In addition, the dominant photocurrent mechanism was determined to be the photoconductive effect (PCE), while a contribution from the photogating effect was also noted from trap-states that yielded a wide spectral photoresponse from UV-to-IR (400 nm to 1100 nm) with an external quantum efficiency (EQE) ~104. From time-resolved photocurrent measurements, a decay time τd ~ 2.5 ms at 300 K was measured from the falling edge of the photogenerated waveform after irradiating the device with a stream of incoming ON/OFF white light pulses.

Field effect transistors and phototransistors based upon p-type solution-processed PbS nanowires

We demonstrate the fabrication of solution processed highly crystalline p-type PbS nanowires via the oriented attachment of nanoparticles. The analysis of single nanowire field effect transistor (FET) devices revealed a hole conduction behaviour with average mobilities greater than 30 cm2 V−1 s−1, which is an order of magnitude higher than that reported to date for p-type PbS colloidal nanowires. We have investigated the response of the FETs to near-infrared light excitation and show herein that the nanowires exhibited gate-dependent photo-conductivities, enabling us to tune the device performances. The responsivity was found to be greater than 104 A W−1 together with a detectivity of 1013 Jones, which benefits from a photogating effect occurring at negative gate voltages. These encouraging detection parameters are accompanied by relatively short switching times of 15 ms at positive gate voltages, resulting from a combination of the standard photoconduction and the high crystallinity of the nanowires. Collectively, these results indicate that solution-processed PbS nanowires are promising nanomaterials for infrared photodetectors as well as p-type nanowire FETs.

Broadband MoS2 Field-Effect Phototransistors: Ultrasensitive Visible-Light Photoresponse and Negative Infrared Photoresponse.

Inverse photoresponse is discovered from phototransistors based on molybdenum disulfide (MoS2 ). The devices are capable of detecting photons with energy below the bandgap of MoS2 . Under the illumination of near-infrared (NIR) light at 980 and 1550 nm, negative photoresponses with short response time (50 ms) are observed for the first time. Upon visible-light illumination, the phototransistors exhibit positive photoresponse with ultrahigh responsivity on the order of 104 -105 A W-1 owing to the photogating effect and charge trapping mechanism. Besides, the phototransistors can detect a weak visible-light signal with effective optical power as low as 17 picowatts (pW). A thermally induced photoresponse mechanism, the bolometric effect, is proposed as the cause of the negative photocurrent in the NIR regime. The thermal energy of the NIR radiation is transferred to the MoS2 crystal lattice, inducing lattice heating and resistance increase. This model is experimentally confirmed by low-temperature electrical measurements. The bolometric coefficient calculated from the measured transport current change with temperature is -33 nA K-1 . These findings offer a new approach to develop sub-bandgap photodetectors and other novel optoelectronic devices based on 2D layered materials.