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Asymmetrically Encapsulated Vertical ITO/MoS 2 /Cu 2 O Photodetector with Ultrahigh Sensitivity
Abstract
Strong light absorption, coupled with moderate carrier transport properties, makes 2D layered transition metal dichalcogenide semiconductors promising candidates for low intensity photodetection applications. However, the performance of these devices is severely bottlenecked by slow response with persistent photocurrent due to long lived charge trapping, and nonreliable characteristics due to undesirable ambience and substrate effects. Here ultrahigh specific detectivity (D*) of 3.2 × 10¹⁴ Jones and responsivity (R) of 5.77 × 10⁴ A W⁻¹ are demonstrated at an optical power density (Pop) of 0.26 W m⁻² and external bias (Vext) of −0.5 V in an indium tin oxide/MoS2/copper oxide/Au vertical multi-heterojunction photodetector exhibiting small carrier transit time. The active MoS2 layer being encapsulated by carrier collection layers allows us to achieve repeatable characteristics over large number of cycles with negligible trap assisted persistent photocurrent. A large D* > 10¹⁴ Jones at zero external bias is also achieved due to the built-in field of the asymmetric photodetector. Benchmarking the performance against existing reports in literature shows a viable pathway for achieving reliable and highly sensitive photodetectors for ultralow intensity photodetection applications.
... Furthermore, unlike bulk semiconductors, different layered materials can be seamlessly integrated in a vertical heterojunction stack without having to worry about the lattice mismatch between two materials [2]- [4]. Consequently, TMDCs (such as MoS 2 , MoSe 2 , WS 2 , and WSe 2 ) have been extensively explored in the recent past as promising candidates to achieve low-cost, sensitive photodetectors [5]- [14]. ...
... However, in the current vertical structure, such current crowding induced resistance is suppressed as the bending of current happens at the highly conducting SnSe 2 layer. This allows carrier collection over the entire photodetector area [14]. ...
... Further, unlike bulk semiconductors, different layered materials can be seamlessly integrated in a vertical heterojunction stack without having to worry about the lattice mismatch between two materials [2]- [4]. Consequently, TMDCs (such as MoS 2 , MoSe 2 , WS 2 and WSe 2 ) have been extensively explored in the recent past as promising candidates to achieve low cost, sensitive photodetectors [5]- [14]. ...
... However, in the current vertical structure, such current crowding induced resistance is suppressed as the bending of current happens at the highly conducting SnSe 2 layer. This allows carrier collection over the entire photodetector area [14]. ...
Two dimensional transition metal di-chalcogenides (TMDCs) are promising candidates for ultra-low intensity photodetection. However, the performance of these photodetectors is usually limited by ambience induced rapid performance degradation and long lived charge trapping induced slow response with a large persistent photocurrent when the light source is switched off. Here we demonstrate an indium tin oxide (ITO)/WSe$_2$/SnSe$_2$ based vertical double heterojunction photoconductive device where the photo-excited hole is confined in the double barrier quantum well, whereas the photo-excited electron can be transferred to either the ITO or the SnSe$_2$ layer in a controlled manner. The intrinsically short transit time of the photoelectrons in the vertical double heterojunction helps us to achieve high responsivity in excess of $1100$ A/W and fast transient response time on the order of $10$ $\mu$s. A large built-in field in the WSe$_2$ sandwich layer results in photodetection at zero external bias allowing a self-powered operation mode. The encapsulation from top and bottom protects the photo-active WSe$_2$ layer from ambience induced detrimental effects and substrate induced trapping effects helping us to achieve repeatable characteristics over many cycles.
... interface expecting similar carrier injection as in MoS 2 sandwiched between layers of ITO and Cu 2 O. [26] All p-and n-type Schottky barrier height values remain close to each other, suggesting a dependence on MoS 2 phase. ...
We report a combination of experimental results with density functional theory (DFT) calculations to understand electronic structure of indium tin oxide and molybdenum disulfide (ITO–MoS 2 ) interface. Our results indicate ITO and MoS 2 conform an n-type Schottky barrier of c.a. − 1.0 eV due to orbital interactions; formation of an ohmic contact is caused by semiconducting and metal behavior of ITO as a function of crystal plane orientation. ITO introduces energy levels around the Fermi level in all interface models in the Γ-Μ-Κ-Γ path. The resulted Van der Waals interface and the values of Schottky barrier height enhance electron carrier injection.
Graphical abstract
... However, the stringent preparation process requirements of Ge nanowires photodetectors result in a high cost. Besides, using a highly conductive and transparentindium-doped tin oxide (ITO) electrode to replace the metal electrode can reduce light reflection by metal, which would be beneficial to the improvement of responsivity [14][15][16][17][18][19]. ...
We propose a simple germanium Schottky photodiode, in which an ultrathin Al2O3 or HfO2 interlayer is inserted between the indium-doped tin oxide (ITO) and n-Ge contact, showing low dark current and ultra-high responsivity (gain). The introduction of 2 nm thick Al2O3 interlayer in a ITO/n-Ge Schottky photodetector results in 138 × reduction of dark current, and 16 × improvement of responsivity, exhibiting a low dark current density of 32 mA/cm2 and high responsivity of 12.5 A/W (external quantum efficiency: 1183%) for 1310 nm and 7.3 A/W (external quantum efficiency: 584%) for 1550 nm at − 4 V reverse bias, respectively. The large responsivity up to 18.5 A/W is obtained for 1310 nm at − 9 V for ITO/Al2O3(2 nm)/n-Ge diode, which corresponds to a large photoconductive gain of 24. Such a high photoconductive gain may be attributed to the capture of holes by the traps at the interface. These results are of significance for the fabrication of Ge photodetector with high performances.
... Two-dimensional layered materials have recently attracted a lot of attention as active material in photodetector applications [39][40][41] . Vertical heterojunction photodetectors based on ultra-thin twodimensional layered materials provide a number of advantages 5,42 compared to lateral structures, namely (1) ultra-short carrier transit time due to nanometer separated electrodes, which is difficult to achieve otherwise in a lithography limited planar structure; (2) large built-in vertical field; (3) suppression of series resistance due to improved carrier collection efficiency that is not limited by transfer length; and (4) repeatable characteristics where encapsulated photoactive layer does not degrade from ambience effects and screened from substrate induced traps. ...
Conventional metals, in general, do not exhibit strong photoluminescence. 2H-TaSe2 is a layered transition metal dichalcogenide that possesses metallic property with charge density wave characteristics. Here we show that 2H-TaSe2 exhibits a surprisingly strong optical absorption and photoluminescence resulting from inter-band transitions. We use this perfect combination of electrical and optical properties in several optoelectronic applications. We show a sevenfold enhancement in the photoluminescence intensity of otherwise weakly luminescent multi-layer MoS2 through non-radiative resonant energy transfer from TaSe2 transition dipoles. Using a combination of scanning photocurrent and time-resolved photoluminescence measurements, we also show that the hot electrons generated by light absorption in TaSe2 have a rather long lifetime unlike conventional metals, making TaSe2 an excellent hot electron injector. Finally, we show a vertical TaSe2/MoS2/graphene photodetector demonstrating a responsivity of >10 AW⁻¹ at 0.1 MHz—one of the fastest reported photodetectors using MoS2.
Transition-metal dichalcogenides (TMDs) are intensively studied for high-performance phototransistors. However, the device performance is limited by the single photoexcitation. Here, we show a unique strategy in which phototransistor performance can be boosted by fabricating the device on top of a distributed Bragg reflector (DBR). Monolayer molybdenum disulfide (MoS2) and tungsten disulfide (WS2) phototransistors were fabricated on DBR and SiO2 substrates for comparison. Furthermore, phototransistor performances including photocurrent, responsivity, photoinduced mobility, and subthreshold swing highlight 582 times enhancement in photoresponsivity ratio and 350 times enhancement in photocurrent ratio in the DBR sample using transparent graphene electrode and hBN encapsulation.
Graphene photodetectors based on photogating effect are promising due to the high photogain, but with the drawback of large dark current due to external bias. In this paper, we realized a graphene photodetector working at zero bias with high responsivity. By establishing asymmetric structure using CdS film on different part of graphene channel, the influence of asymmetric ratio on device performance were found to be opposite for external and zero bias. Without external bias, the responsivity can reach 0.26 A/W under visible light, resulting in a quantum efficiency of 51.5%, which is much larger than other zero-bias graphene photodetectors based on photothermoelectric effect. Furthermore, based on the asymmetric structure, infrared photoresponse beyond the CdS cut-off wavelength was observed at 980 nm wavelength at low temperature. This work provides a feasible scheme for the low energy consumption work of photodetectors with low dimensional materials.
Biological visual system can efficiently handle optical information within the retina and visual cortex of the brain, which suggests an alternative approach for the upgrading of the current low-intelligence, large energy consumption and complex circuitry of the artificial vision system for high-performance edge computing applications. In recent years, retinomorphic machine vision based on the integration of optoelectronic image sensors and processors has been regarded as a promising candidate to improve this phenomenon. This novel intelligent machine vision technology can perform information pre-processing near or even within the sensor in the front end, thereby reducing the transmission of redundant raw data and improving the efficiency of the back-end processor for high-level computing tasks. In this contribution, we try to present a comprehensive review on the recent progress achieved in this emergent field.
Two-dimensional layered materials have emerged prominently in the past decade, largely being investigated fundamentally and practically. Their unique layered structure and atomic-scale thickness make them attractive with exclusive electrical and optical properties compared to their bulk counterparts. Molybdenum disulfide (MoS 2 ) is the most widely studied material in the family of transition metal dichalcogenides. The direct and variable bandgap, high carrier mobility, thermal and chemical stability makes it an attractive choice for next-generation photodetector applications. MoS 2 heterojunction-based photodetectors offer ultrafast charge transfer and broadband photoresponse, adding more functionality beyond their individual counterparts. Enormous efforts have been devoted to adopting a new strategy that can improve photodetector performance in terms of responsivity and response time. This review briefly discusses the photo-induced current mechanism and performance parameters along with some important aspects to realize better device performance. Here, we critically review the current status and progress made towards MoS 2 -based photodetectors, followed by a discussion on open challenges and opportunities in their future application.
Photodetection is one of the important technologies discussed in current scientific and technological circles because of its undeniable and widespread applications in industry and research. The application of conventional bulk semiconductors (GaN, Si, CdS, and InGaAs) to photodetection in industry is being gradually minimized due to poor mechanical firmness (rigidity) and flexibility, the need for expensive substrates, and low charge-carrier mobility. However, 2D materials other than graphene, including transition-metal chalcogenides, nitrides, and carbides, are paving the way towards obtaining more advanced outcomes and overcoming the bottleneck conditions imposed by conventional semiconductors due to their extraordinary electronic and mechanical properties, including flexibility, tunable bandgaps, high mobilities, and abundant potential for heterojunction construction. Due to the recent boom in 2D photodetection research, the area has become so broad and versatile that an organized account of relevant scientific information has become necessary. A categorized review is essentially required to cover topics including 2D material synthesis schemes, assembly techniques, device integration, spectral characteristics, the scope for heterojunctions, and future research directions. This review focuses on the application of 2D materials to photodetection, including comprehensive coverage of synthesis/assembly techniques, heterojunctions, and performance in different spectral regions, including solar-blind, visible, near-infrared, and broadband detectors. We further extend our discussion to hybridized van der Waals heterojunctions (vdWHs) with conventional semiconductors and the application of these materials as photodetectors. Finally, we give a comparative illustration of photodetector performance parameters, providing a clear picture of the acceptability of these materials for use in the development of next-generation photodetectors.
Two-dimensional transition metal dichalcogenides have attracted much attention in the field of optoelectronics due to their tunable bandgaps, strong interaction with light and tremendous capability for developing diverse van der Waals heterostructures with other nanomaterials.
Over the last few years, there has been a rapid growth towards demonstrating highly sensitive, fast photodetectors using photoactive nano-materials. As with any other developing and highly inter-disciplinary field, the existing reports exhibit a large spread in the data due to less optimized materials and non-standardized characterization protocols. This calls for a streamlined performance benchmarking requirement to accelerate technological adoption of the promising candidates. The goal of this paper is four-fold: (i) to address the key challenges to perform such benchmarking exercise; (ii) to elucidate the right figures of merit to look for, and in particular, to demonstrate that noise-equivalent-power ($N\!E\!P$) is a more reliable sensitivity metric than other commonly used ones, such as responsivity ($R$) and specific detectivity ($D^*$); (iii) to propose $N\!E\!P$ versus frequency of operation ($f$) as a single, unified benchmarking chart that could be used for apple-to-apple comparison among heterogeneous detector technologies; and (iv) to propose a simple, yet streamlined characterization protocol that can be followed while reporting photodetector performance.
Photodetectors based on p-type metal oxides are still a challenge for optoelectronic device applications. Many effects have been paid to improve their performance and expand their detection range. Here, high-quality Cu1-xNi
x
O (x = 0, 0.2, and 0.4) film photodetectors were prepared by a solution process. The crystal quality, morphology, and grain size of Cu1-xNi
x
O films can be modulated by Ni doping. Among the photodetectors, the Cu0.8Ni0.2O photodetector shows the maximum photocurrent value (6 × 10-7 A) under a 635 nm laser illumination. High responsivity (26.46 A/W) and external quantum efficiency (5176%) are also achieved for the Cu0.8Ni0.2O photodetector. This is because the Cu0.8Ni0.2O photosensitive layer exhibits high photoconductivity, low surface states, and high crystallization after 20% Ni doping. Compared to the other photodetectors, the Cu0.8Ni0.2O photodetector exhibits the optimal response in the near-infrared region, owing to the high absorption coefficient. These findings provide a route to fabricate high-performance and wide-detection range p-type metal oxide photodetectors.
We study and fabricate graphene nanowalls/silicon hybrid heterojunction photoconductive detector to provide process technology of the device and theoretical foundation for the purpose of preparation of high-performance photodetectors. The graphene nanowalls (GNWs) film is patterned by double-layered photoresist-based photolithography and reactive ion etching (RIE) process to achieve high quality GNWs channel and fabricate three different GNWs/Si heterojunction photoconductive detectors with n-doped, intrinsic and p-doped silicon substrates (n-Si, i-Si, p-Si), respectively. The GNWs film not only acts as a photoconductive channel for carrier transport, but also constructs a Schottky heterojunction with the silicon to participate in the separation and transport of photogenerated carriers. Since the injection of holes needs to pass through the Schottky junction region, the height of the Schottky barrier determines the injection ability of photogenerated carriers, which directly affects the photoconductive gain of the GNWs and silicon. In addition, under low bias VDS, the GNWs/n-Si photoresponse current is maximum and the GNWs/p-Si photoresponse current is minimum. The photoresponse is attributed to the barrier heights of the GNWs/n-Si, GNWs/i-Si, and GNWs/p-Si with values of 0.73 eV, 0.69 eV, and 0.63 eV, respectively. The higher the barrier, the more the number of photogenerated carriers injected into the GNWs will be, and the photoresponse current is large as well.
Polycrystalline cuprous oxide (Cu2O) thin films were sputtered, annealed (900 °C rapid thermal annealing) and subsequently implanted with various hydrogen ion (H+) doses from 5E13 to 2E15 cm−2 with a low acceleration energy of 36 keV at room temperature to tailor the functional properties of the thin films for solar cell application. The annealed and H+ implanted Cu2O thin films were post annealed at low temperatures from 100 °C to 600 °C in an inert atmosphere to promote hydrogen passivation of prevalent intrinsic acceptors and tune the carrier concentration for optimum performance as an absorption layer in a heterojunction solar cell. The H+ incorporation and post annealing tuned the structural, optical and electrical properties of annealed polycrystalline Cu2O thin films. The results show an enhancement of the excitonic feature around ∼2.0 eV with H+ dose. The normalized photoluminescence (PL) area around ∼1.7 eV was drastically enhanced with increasing H+ doses compared to excitonic and copper vacancy related area. The normalized total PL quantum efficiency shows an enhancement in yield with elevated H+ doses by two orders of magnitude. The hole concentration decreases down to ∼1013 cm−3, while hole mobility and resistivity increase to ∼27 cm2/V and ∼2.4 kΩcm, respectively, as the H+ implantation goes from lower to higher doses. In addition, the post annealing and H+ incorporation lead to a change in the energy level of the major acceptor from 0.21 eV to 0.27 eV above the valence band maximum. By following the qualitative (PL analysis) and quantitative (Hall data) outcomes, we can conclude that H+ implantation and post annealing likely indicates the passivation of both acceptor defects and compensating donor defects.
We report on the device demonstration based on vertical transport in multi‐layer α‐In2Se3. Photodetectors realized using a metal/α‐In2Se3/ITO vertical junction exhibited clear signature of band‐edge in spectral responsivity at 680 nm corresponding to an ultra‐high responsivity of 1000 A/W and a detectivity of >1013cm Hz0.5 W−1 at a bias of 0.5 V The variation of responsivity and detectivity with optical power density was studied, and a transient response of 20 ms was obtained for the devices (instrument limitation). Additionally, an asymmetric barrier height arising out of ITO and Au contacts to a vertical α‐In2Se3 junction resulted in photovoltaic effect with VOC ∽ 0.1 V and ISC ∽ 0.4 μA under an illumination of 520 nm. This article is protected by copyright. All rights reserved.
In this work, a sensitive and reliable photoelectrochemical (PEC) biosensor was proposed based on hexagonal carbon nitride tubes (HCNT) as photoactive material for detection of circulating tumor cells (CTCs). Magnetic Fe3O4 nanospheres (MNs) and Cu2O nanoparticles (Cu2O NPs) were utilized for highly efficient magnetic capture of CTCs and for signal amplification, respectively. First, anti-epithelial cell adhesion molecule (EpCAM) antibody was linked onto MNs for capture and enrichment of CTCs. With the captured MCF-7 coated onto the electrode, the photocurrent intensity of HCNT was decreased due to the steric hindrance derived from MCF-7. Then, when the Cu2O-aptamer probe was bound onto the CTC surface, the photocurrent intensity was further decreased because Cu2O NPs competed with HCNT for absorption of exciting light and the aptamer molecules increased the steric hindrance, which leads to significantly decreased photocurrent response, thus realizing dual signal amplification. Using the breast cancer cell MCF-7 as a model, the proposed PEC biosensor displays good performances with a linear range from 3 to 3000 cell mL-1 and limit of detection down to 1 cell mL-1. The HCNT-based PEC biosensor shows good performance for detection of CTCs, which may have potential applications in cancer diagnostics and therapeutics.
Sensitive detection of trace nitrogen dioxide (NO2) gas at room temperature is of urgent necessity in the fields of healthcare and environment monitoring. To achieve this goal, we report on a porous composite film featuring reduced graphene oxide (rGO) nanosheets as the template platform of nanostructured cuprous oxide (Cu2O) nanowires and nanoparticles via a hydrothermal method. The sensor performance was investigated in terms of sensing response, optimal operation temperature, repeatability, long-term stability, selectivity and humidity effect on NO2 sensing. The sensor response achieved 0.66 towards 50 ppb NO2 gas with a full recovery at room temperature (25±2 oC), which was among the best cases of Cu2O-related NO2 detection concerning sensor response and operation temperature. Moreover, a modest repeatability, stability, selectivity as well as a negligible humidity effect on NO2 sensing were exhibited. A mass of interspaces existing within nanostructured composites as well as the synergistic effect between rGO and Cu2O materials endowed the sensing layer with favorable gas accessibility and sufficient gas-solid interaction. Simultaneously, highly conductive rGO nanosheets facilitated an effective electron transfer and collection. In brief, the as-prepared rGO/Cu2O sensors showed a competitive room-temperature detection capability for ppb-level NO2 gas, providing a vast potential in the future applications such as the real-time monitoring of ultralow emission.
Atomically thin 2D materials are promising candidates for miniaturized high‐performance optoelectronic devices. This study reports on multilayer MoTe2 photodetectors contacted with asymmetric electrodes based on n‐ and p‐type graphene layers. The asymmetry in the graphene contacts creates a large (Ebi ∼ 100 kV cm⁻¹) built‐in electric field across the short (l = 15 nm) MoTe2 channel, causing a high and broad (λ = 400–1400 nm) photoresponse even without any externally applied voltage. Spatially resolved photovoltage maps reveal an enhanced photoresponse and larger built‐in electric field in regions of the MoTe2 layer between the two graphene contacts. Furthermore, a fast (∼10 µs) photoresponse is achieved in both the photovoltaic and photoconductive operation modes of the junction. The findings can be extended to other 2D materials and offer prospects for the implementation of asymmetric graphene contacts in future low‐power optoelectronic applications.
A facile, inexpensive solution approach was utilized to prepare novel 3D porous Cu2O-MoS2 microspheres, which were employed for NH3 gas detection. Morphological characterizations indicated porous MoS2 microspheres were decorated with numerous Cu2O nanoparticles. The fabricated gas sensor based on the Cu2O-MoS2 microspheres possessed high gas response of 43 with fast response and recovery speed towards NH3, good reproducibility as well as satisfactory stability, which were mainly ascribed to the distinctive 3D porous microsphere architecture and the formation of p-n heterojunction at the interface between the n-type MoS2 and p-type Cu2O.
In photodetection, the response time is mainly controlled by the device architecture and electron/hole mobility, while the absorption coefficient and the effective separation of the electrons/holes are the key parameters for high responsivity. Here, we report an approach towards the fast and highly responsive infra-red photodetection using n-type SnSe2 thin film on p-Si (100) substrate keeping the overall performance of the device. The I-V characteristics of the device show a rectification ratio of 147 at ± 5 V and enhanced optoelectronic properties under 1064 nm radiation. The responsivity is 0.12 A/W at 5 V and the response/recovery time constants were estimated as ~ 57±25/34±15 μs. Overall, the response times are shown to be controlled by the mobility of the constituent semiconductors of a photodiode. Further, our findings suggest that n-SnSe2 can be intergrated with well established Si technology with enhanced opto-electronic properties and also pave the way in the design of fast response photodetectors for other wavelengths as well.
The Ag modified transparent Cu2O/ZnO p–n junction films were prepared by a simple magnetron sputtering process. The transparence and photoelectric conversion of these devices were investigated, which exhibited an obvious photoelectric conversion enhancement (PCE of 0.48%, ten times) than those of the unmodified heterojunction. Through analysis, the enhancement of the photoelectric conversion could be attributed to the remarkable Cu2O/ZnO p–n junction and Ag⁰ nanoparticles with the performances of photoelectron donor and surface enhanced plasmonic absorption.
Two-dimensional layered materials (2DLMs) have attracted a tremendous amount of attention as photodetectors due to their fascinating features, including high potentials in new-generation electronic devices, wide coverage of bandgaps, ability to construct van der Waals heterostructures, extraordinary light–mass interaction, strong mechanical flexibility, and the capability of enabling synthesis of 2D nonlayered materials. Until now, most attention has been focused on the well-known graphene and transition metal dichalcogenides (TMDs). However, a growing number of functional materials (more than 5619) with novel optoelectronic and electronic properties are being re-discovered, thereby widening the horizon of 2D libraries. In addition to showing common features of 2DLMs, these new 2D members may bring new opportunities to their well-known analogues, like wider bandgap coverage, direct bandgaps independence with thickness, higher mechanical flexibility, and new photoresponse phenomena. The impressive results communicated so far testify that they have shown high potentials with photodetections covering THz, IR, visible, and UV ranges with comparable or even higher performances than well-known TMDs. Here, we give a comprehensive review on the state-of-the-art photodetections of two-dimensional materials beyond graphene and TMDs. The review is organized as follows: fundamentals of photoresponse first are discussed, followed by detailed photodetections of new 2D members including both layered and non-layered ones. After that, photodiodes and hybrid structures based on these new 2D materials are summarized. Then, the integration of these 2D materials with flexible substrates is reviewed. Finally, we conclude with the current research status of this area and offer our perspectives on future developments. We hope that, through reading this manuscript, readers will quickly have a comprehensive view on this research area.
The combination of graphene with semiconductor materials in heterostructure photodetectors enables amplified detection of femtowatt light signals using micrometer-scale electronic devices. Presently, long-lived charge traps limit the speed of such detectors, and impractical strategies, e.g., the use of large gate-voltage pulses, have been employed to achieve bandwidths suitable for applications such as video-frame-rate imaging. Here, atomically thin graphene–WS2 heterostructure photodetectors encapsulated in an ionic polymer are reported, which are uniquely able to operate at bandwidths up to 1.5 kHz whilst maintaining internal gain as large as 106. Highly mobile ions and the nanometer-scale Debye length of the ionic polymer are used to screen charge traps and tune the Fermi level of the graphene over an unprecedented range at the interface with WS2. Responsivity R = 106 A W−1 and detectivity D* = 3.8 × 1011 Jones are observed, approaching that of single-photon counters. The combination of both high responsivity and fast response times makes these photodetectors suitable for video-frame-rate imaging applications.
Tungsten disulfide (WS2), as a typical metal dichalcogenides (TMDs), has aroused keen research interests in photodetection. Here field effect phototransistors (FEpTs) based on heterojunction between monolayer WS2 and PbS colloidal quantum dots are demonstrated to show high photoresponsivity (up to ~ 14 A/W), wide electric bandwidth (~ 396 Hz) and excellent stability. Meanwhile, the devices exhibit fast photoresponse times of ~ 153 μs (rise time) and ~ 226 μs (fall time) due to the assistance of heterojunction on the transfer of photoexcitons. Therefore, excellent device performances strongly underscore monolayer WS2-PbS quantum dot as a promising material for future photoelectronic applications.
The realization of low-cost photodetectors with high sensitivity, high quantum efficiency, high gain and fast photoresponse in the visible and short-wave infrared remains one of the challenges in optoelectronics. Two classes of photodetectors that have been developed are photodiodes and phototransistors, each of them with specific drawbacks. Here we merge both types into a hybrid photodetector device by integrating a colloidal quantum dot photodiode atop a graphene phototransistor. Our hybrid detector overcomes the limitations of a phototransistor in terms of speed, quantum efficiency and linear dynamic range. We report quantum efficiencies in excess of 70%, gain of 105 and linear dynamic range of 110 dB and 3 dB bandwidth of 1.5 kHz. This constitutes a demonstration of an optoelectronically active device integrated directly atop graphene and paves the way towards a generation of flexible highly performing hybrid two-dimensional (2D)/0D optoelectronics.
After a decade of intensive research on two-dimensional (2D) materials inspired by the discovery of graphene, the field of 2D electronics has reached a stage with booming materials and device architectures. However, the efficient integration of 2D functional layers with three-dimensional (3D) systems remains a significant challenge, limiting device performance and circuit design. In this review, we investigate the experimental efforts in interfacing 2D layers with 3D materials and analyze the properties of the heterojunctions formed between them. The contact resistivity of metal on graphene and related 2D materials deserves special attention, while the Schottky junctions formed between metal/2D semiconductor or graphene/3D semiconductor call for careful reconsideration of the physical models describing the junction behavior. The combination of 2D and 3D semiconductors presents a form of p-n junctions that have just marked their debut. For each type of the heterojunctions, the potential applications are reviewed briefly.
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.
Synthesis of large-area, atomically thin transition metal dichalcogenides (TMDs) on diverse substrates is of central importance for the large-scale fabrication of flexible devices and heterojunction-based devices. In this work, we successfully synthesized a large area of highly-crystalline MoSe2 atomic layers on SiO2/Si, mica and Si substrates using a simple chemical vapour deposition (CVD) method at atmospheric pressure. Atomic force microscopy (AFM) and Raman spectroscopy reveal that the as-grown ultrathin MoSe2 layers change from a single layer to a few layers. Photoluminescence (PL) spectroscopy demonstrates that while the multi-layer MoSe2 shows weak emission peaks, the monolayer has a much stronger emission peak at ∼1.56 eV, indicating the transition from an indirect to a direct bandgap. Transmission electron microscopy (TEM) analysis confirms the single-crystallinity of MoSe2 layers with a hexagonal structure. In addition, the photoresponse performance of photodetectors based on MoSe2 monolayer was studied for the first time. The devices exhibit a rapid response of ∼60 ms and a good photoresponsivity of ∼13 mA/W (using a 532 nm laser at an intensity of 1 mW mm(-2) and a bias of 10 V), suggesting that MoSe2 monolayer is a promising material for photodetection applications.
Combining the electronic properties of graphene and molybdenum disulphide (MoS2) in hybrid heterostructures offers the possibility to create devices with various functionalities. Electronic logic and memory devices have already been constructed from graphene-MoS2 hybrids, but they do not make use of the photosensitivity of MoS2, which arises from its optical-range bandgap. Here, we demonstrate that graphene-on-MoS2 binary heterostructures display remarkable dual optoelectronic functionality, including highly sensitive photodetection and gate-tunable persistent photoconductivity. The responsivity of the hybrids was found to be nearly 1 × 10(10) A W(-1) at 130 K and 5 × 10(8) A W(-1) at room temperature, making them the most sensitive graphene-based photodetectors. When subjected to time-dependent photoillumination, the hybrids could also function as a rewritable optoelectronic switch or memory, where the persistent state shows almost no relaxation or decay within experimental timescales, indicating near-perfect charge retention. These effects can be quantitatively explained by gate-tunable charge exchange between the graphene and MoS2 layers, and may lead to new graphene-based optoelectronic devices that are naturally scalable for large-area applications at room temperature.
Two-dimensional materials are an emerging class of new materials with a wide range of electrical properties and potential practical applications. Although graphene is the most well-studied two-dimensional material, single layers of other materials, such as insulating BN (ref. 2) and semiconducting MoS2 (refs 3, 4) or WSe2 (refs 5, 6), are gaining increasing attention as promising gate insulators and channel materials for field-effect transistors. Because monolayer MoS2 is a direct-bandgap semiconductor due to quantum-mechanical confinement, it could be suitable for applications in optoelectronic devices where the direct bandgap would allow a high absorption coefficient and efficient electron-hole pair generation under photoexcitation. Here, we demonstrate ultrasensitive monolayer MoS2 phototransistors with improved device mobility and ON current. Our devices show a maximum external photoresponsivity of 880 A W(-1) at a wavelength of 561 nm and a photoresponse in the 400-680 nm range. With recent developments in large-scale production techniques such as liquid-scale exfoliation and chemical vapour deposition-like growth, MoS2 shows important potential for applications in MoS2-based integrated optoelectronic circuits, light sensing, biomedical imaging, video recording and spectroscopy.
The first GaS nanosheet-based photodetectors are demonstrated on both mechanically rigid and flexible substrates. Highly crystalline, exfoliated GaS nanosheets are promising for optoelectronics due to strong absorption in the UV-visible wavelength region. Photocurrent measurements of GaS nanosheet photodetectors made on SiO2/Si substrates and flexible polyethylene terephthalate (PET) substrates exhibit a photoresponsivity at 254 nm up to 4.2 AW-1 and 19.2 AW-1, respectively, which exceeds that of graphene, MoS2, or other 2D material-based devices. Additionally, the linear dynamic range of the devices on SiO2/Si and PET substrates are 97.7 dB and 78.73 dB, respectively. Both surpass that of currently exploited InGaAs photodetectors (66 dB). Theoretical modeling of the electronic structures indicates that the reduction of the effective mass at the valence band maximum (VBM) with decreasing sheet thickness enhances the carrier mobility of the GaS nanosheets, contributing to the high photocurrents. Double-peak VBMs are theoretically predicted for ultrathin GaS nanosheets (thickness less than five monolayers), which is found to promote photon absorption. These theoretical and experimental results show that GaS nanosheets are promising materials for high-performance photodetectors on both conventional silicon and flexible substrates.
Most electronic devices exploit the electric charge of electrons, but it is also possible to build devices that rely on other properties of electrons. Spintronic devices, for example, make use of the spin of electrons. Valleytronics is a more recent development that relies on the fact that the conduction bands of some materials have two or more minima at equal energies but at different positions in momentum space. To make a valleytronic device it is necessary to control the number of electrons in these valleys, thereby producing a valley polarization. Single-layer MoS(2) is a promising material for valleytronics because both the conduction and valence band edges have two energy-degenerate valleys at the corners of the first Brillouin zone. Here, we demonstrate that optical pumping with circularly polarized light can achieve a valley polarization of 30% in pristine monolayer MoS(2). Our results, and similar results by Mak et al., demonstrate the viability of optical valley control and valley-based electronic and optoelectronic applications in MoS(2) monolayers.
Monolayer Molybdenum disulfide (MoS2), a two-dimensional crystal with a
direct bandgap, is a promising candidate for 2D nanoelectronic devices
complementing graphene. There have been recent attempts to produce MoS2 layers
via chemical and mechanical exfoliation of bulk material. Here we demonstrate
the large area growth of MoS2 atomic layers on SiO2 substrates by a scalable
chemical vapor deposition (CVD) method. The as-prepared samples can either be
readily utilized for further device fabrication or be easily released from SiO2
and transferred to arbitrary substrates. High resolution transmission electron
microscopy and Raman spectroscopy on the as grown films of MoS2 indicate that
the number of layers range from single layer to a few layers. Our results on
the direct growth of MoS2 layers on dielectric leading to facile device
fabrication possibilities show the expanding set of useful 2D atomic layers, on
the heels of graphene, which can be controllably synthesized and manipulated
for many applications.
A new phototransistor based on the mechanically exfoliated single-layer MoS(2) nanosheet is fabricated, and its light-induced electric properties are investigated in detail. Photocurrent generated from the phototransistor is solely determined by the illuminated optical power at a constant drain or gate voltage. The switching behavior of photocurrent generation and annihilation can be completely finished within ca. 50 ms, and it shows good stability. Especially, the single-layer MoS(2) phototransistor exhibits a better photoresponsivity as compared with the graphene-based device. The unique characteristics of incident-light control, prompt photoswitching, and good photoresponsivity from the MoS(2) phototransistor pave an avenue to develop the single-layer semiconducting materials for multifunctional optoelectronic device applications in the future.
Existing models of electrical contacts are often inapplicable at the nanoscale because there are significant differences between nanostructures and bulk materials arising from unique geometries and electrostatics. In this Review, we discuss the physics and materials science of electrical contacts to carbon nanotubes, semiconductor nanowires and graphene, and outline the main research and development challenges in the field. We also include a case study of gold contacts to germanium nanowires to illustrate these concepts.
Germanium-based photodetector is a key component in silicon based photonics because of its unique properties of response at telecommunication band and compatibility with CMOS techniques. However, the limitations of low quantum efficiency and high surface recombination in ultrathin germanium film, especially in the near-infrared range, put huge obstructions on the road towards applications. Nowadays, practical applications require more nano-scale devices with lower power consumption as well as higher responsivity and response speed. In this work, we firstly demonstrate a germanium-graphene hybrid structure photodetector that consists of an ultrathin 20 nm germanium layer and a monolayer graphene. The photodetector can achieve a broadband detection from ultraviolet to near-infrared range. A conductive gain of 155 and a responsivity of 66.2 A W-1 are achieved, which is about three orders of magnitude higher than pure graphene photodetectors and about 4 times larger than pure germanium photodetectors. Such enhancement owes to effective generation, separation and transfer of photo-generated carriers at material interface. The photodetector based on germanium-graphene hybrid structure presents a new paradigm for the realization of small but high performance device in the process of integration in silicon-based optical chips. And it offers new opportunities for imaging, sensing and other optoelectronic field applications.
Screening due to surrounding dielectric medium reshapes the electron-hole interaction potential and plays a pivotal role in deciding the binding energies of strongly bound exciton complexes in quantum confined monolayers of transition metal dichalcogenides (TMDs). However, owing to strong quasi-particle bandgap renormalization in such systems, a direct quantification of estimated shifts in binding energy in different dielectric media remains elusive using optical studies. In this work, by changing the dielectric environment, we show a conspicuous photoluminescence (PL) peak shift at low temperature for higher energy excitons (2s, 3s, 4s, 5s) in monolayer MoSe2, while the 1s exciton peak position remains unaltered-a direct evidence of varying compensation between screening induced exciton binding energy modulation and quasi-particle bandgap renormalization. The estimated modulation of binding energy for the 1s exciton is found to be í µí¿í µí¿. í µí¿% (í µí¿í µí¿. í µí¿% for 2s, í µí¿í µí¿. í µí¿% for 3s, í µí¿í µí¿% for 4s) by coating an Al2O3 layer on top, while the corresponding reduction in quasi-particle bandgap is estimated to be 248 meV. Such a direct evidence of large tunability of the binding energy of exciton complexes as well as the bandgap in monolayer TMDs holds promise of novel device applications.
Van der Waals heterostructures from atomically thin 2D materials have opened up new realms in modern semiconductor industry. Recently, 2D layered semiconductors such as MoS2 and SnSe2 have already demonstrated excellent electronic and optoelectronic properties due to their high electron mobility and unique band structures. Such combination of SnSe2 with MoS2 may provide a novel platform for the applications in electronics and optoelectronics. Thus, we constructed SnSe2/MoS2 based van der Waals heterostructures using MoS2 as templates, which may enrich the family of 2D van der Waals heterostructures. We demonstrate that the vdW heterostructures with high symmetry crystallographic directions show efficient interlayer charge transfer due to the strong coupling. This strong coupling is confirmed by theory calculations, low-temperature photoluminescence (PL) spectra, and electrical transport properties. High performance photodetector based on the vdW heterostructure has been demonstrated with a high responsivity of up to 9.1 × 10³ A W⁻¹ which is higher by two orders of magnitude than those MoS2-only devices. The improved performance can be attributed to the efficient charge transfer from MoS2 to SnSe2 at the interface.
Mercury telluride (HgTe) colloidal quantum dots (CQDs) have been developed as promising materials for the short and mid-wave infrared photodetection applications because of their low cost, solution processing, and size tunable absorption in the short wave and mid-infrared spectrum. However, the low mobility and poor photogain have limited the responsivity of HgTe CQD-based photodetectors to only tens of mA W−1. Here, HgTe CQDs are integrated on a TiO2 encapsulated MoS2 transistor channel to form hybrid phototransistors with high responsivity of ≈106 A W−1, the highest reported to date for HgTe QDs. By operating the phototransistor in the depletion regime enabled by the gate modulated current of MoS2, the noise current is significantly suppressed, leading to an experimentally measured specific detectivity D* of ≈1012 Jones at a wavelength of 2 µm. This work demonstrates for the first time the potential of the hybrid 2D/QD detector technology in reaching out to wavelengths beyond 2 µm with compelling sensitivity.
MoS2 monolayers exhibit excellent light absorption and large thermoelectric power, which are, however, accompanied with very strong exciton binding energy - resulting in complex photoresponse characteristics. We study the electrical response to scanning photo-excitation on MoS2 monolayer (1L) and bilayer (2L) devices, and also on monolayer/bilayer (1L/2L) planar heterojunction and monolayer/few-layer/multi-layer (1L/FL/ML) planar double heterojunction devices to unveil the intrinsic mechanisms responsible for photocurrent generation in these materials and junctions. Strong photoresponse modulation is obtained by scanning the position of the laser spot, as a consequence of controlling the relative dominance of a number of layer dependent properties, including (i) photoelectric effect (PE), (ii) photothermoelectric effect (PTE), (iii) excitonic effect, (iv) hot photo-electron injection from metal, and (v) carrier recombination. The monolayer and bilayer devices show peak photoresponse when the laser is focused at the source junction, while the peak position shifts to the monolayer/multi-layer junction in the heterostructure devices. The photoresponse is found to be dependent on the incoming light polarization when the source junction is illuminated, although the polarization sensitivity drastically reduces at the monolayer/multi-layer heterojunction. Finally, we investigate laser position dependent transient response of photocurrent to reveal trapping of carriers in SiO2 at the source junction is the critical factor to determine the transient response in 2D photodetectors, and also show that, by systematic device design, such trapping can be avoided in the heterojunction devices, resulting in fast transient response. The insights obtained will play an important role in designing fast 2D TMDs based photodetector and related optoelectronic and thermoelectric devices.
The scaling of transistors to sub-10 nm dimensions is strongly limited by their contact resistance (Rc). Here we present a systematic study of scaling MoS2 devices and contacts with varying electrode metals and controlled deposition conditions, over a wide range of temperatures (80 to 500 K), carrier densities (10^12 to 10^13 1/cm^2), and contact dimensions (20 to 500 nm). We uncover that Au deposited in ultra-high vacuum (~10^-9 Torr) yields three times lower Rc than under normal conditions, reaching 740 {\Omega}.{\mu}m and specific contact resistivity 3x10^-7 {\Omega}.cm2, stable for over four months. Modeling reveals separate Rc contributions from the Schottky barrier and the series access resistance, providing key insights on how to further improve scaling of MoS2 contacts and transistor dimensions. The contact transfer length is ~35 nm at 300 K, which is verified experimentally using devices with 20 nm contacts and 70 nm contact pitch (CP), equivalent to the "14 nm" technology node.
We show room temperature valley coherence with in MoS2, MoSe2, WS2 and WSe2 monolayers using linear polarization resolved hot photoluminescence (PL), at energies close to the excitation – demonstrating preservation of valley coherence before sufficient scattering events. The features of the co-polarized hot luminescence allow us to extract the lower bound of the binding energy of the A exciton in monolayer MoS2 as 0.42 (±0.02) eV. The broadening of the PL peak is found to be dominated by Boltzmann-type hot luminescence tail, and using the slope of the exponential decay, the carrier temperature is extracted in-situ at different stages of energy relaxation. The temperature of the emitted optical phonons during the relaxation process are probed by exploiting the corresponding broadening of the Raman peaks due to temperature induced anharmonic effects. The findings provide a physical picture of photo-generation of valley coherent hot carriers, and their subsequent energy relaxation path ways.
As an exotic state of quantum matter, topological insulators have promising applications in new-generation electronic and optoelectronic devices. The realization of these applications relies critically on the preparation and properties understanding of high-quality topological insulators, which however are mainly fabricated by high-cost methods like molecular beam epitaxy. We here report the successful preparation of high-quality topological insulator Bi2Se3/Si heterostructure having an atomically abrupt interface by van der Waals epitaxy growth of single-crystalline Bi2Se3 films on Si wafer. A simple, low-cost physical vapor deposition (PVD) method was employed to achieve the growth of single-crystalline Bi2Se3 films. The Bi2Se3/Si heterostructure exhibited excellent diode characteristics with a pronounced photoresponse under light illumination. The built-in potential at the Bi2Se3/Si interface greatly facilitated the separation and transport of photo-generated carriers, enabling the photodetector to have a high light responsivity of 24.28 A W-1, a high detectivity of 4.39×1012 Jones (Jones=cm Hz1/2 W-1), and a fast response speed of ~µs. These device parameters represent the highest values for topological insulator-based photodetectors. Additionally, the photodetector possessed wide broadband detection ranging from ultraviolet to optical telecommunication wavelengths. Given the simple device architecture and compatibility with silicon technology, the topological insulator Bi2Se3/Si heterostructure holds great promise for high-performance electronic and optoelectronic applications.
Recent advances in the development of atomically thin layers of van der Waals bonded solids have opened up new possibilities for the exploration of 2D physics as well as for materials for applications. Among them, semiconductor transition metal dichalcogenides, MX 2 (M = Mo, W; X = S, Se), have bandgaps in the near-infrared to the visible region, in contrast to the zero bandgap of graphene. In the monolayer limit, these materials have been shown to possess direct bandgaps, a property well suited for photonics and optoelectronics applications. Here, we review the electronic and optical properties and the recent progress in applications of 2D semiconductor transition metal dichalcogenides with emphasis on strong excitonic effects, and spin- and valley-dependent properties.
A graphene/n-type silicon (n-Si) heterojunction has been demonstrated to exhibit strong rectifying behavior and high photoresponsivity, which can be utilized for the development of high-performance photodetectors. However, graphene/n-Si heterojunction photodetectors reported previously suffer from relatively low specific detectivity due to large dark current. Here, by introducing a thin interfacial oxide layer, the dark current of graphene/n-Si heterojunction has been reduced by two orders of magnitude at zero bias. At room temperature, the graphene/n-Si photodetector with interfacial oxide exhibits a specific detectivity up to 5.77 × 10(13) cm Hz(1/2) W(-1) at the peak wavelength of 890 nm in vacuum, which is highest reported detectivity at room temperature for planar graphene/Si heterojunction photodetectors. In addition, the improved graphene/n-Si heterojunction photodetectors possess high responsivity of 0.73 A W(-1) and high photo-to-dark current ratio of ≈10(7) . The current noise spectral density of the graphene/n-Si photodetector has been characterized under ambient and vacuum conditions, which shows that the dark current can be further suppressed in vacuum. These results demonstrate that graphene/Si heterojunction with interfacial oxide is promising for the development of high detectivity photodetectors.
The performance of electronic and optoelectronic devices based on two-dimensional layered crystals, including graphene, semiconductors of the transition metal dichalcogenide family such as molybdenum disulphide (MoS2) and tungsten diselenide (WSe2), as well as other emerging two-dimensional semiconductors such as atomically thin black phosphorus, is significantly affected by the electrical contacts that connect these materials with external circuitry. Here, we present a comprehensive treatment of the physics of such interfaces at the contact region and discuss recent progress towards realizing optimal contacts for two-dimensional materials. We also discuss the requirements that must be fulfilled to realize efficient spin injection in transition metal dichalcogenides.
Semiconducting, two-dimensional Molybdenum Disulfide (MoS2) is considered a promising new material for highly sensitive photodetection, because of its atomically thin profile and favourable 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 (ALD) Hafnium oxide (HfO2). The protected devices show enhanced electronic properties by isolating it 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 -104A/W and t ~ 10ms - 10s. At strong negative gate voltage the detector is operated at higher speed and simultaneously exhibits a low-bound, record sensitivity of D* ≥ 7.7x1011 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 (TMDC) optoelectronics.
A few-layer MoS2 photodetector driven by poly(vinylidene fluoride-trifluoroethylene) ferroelectrics has been 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. Eg of MoS2 is tuned by the ultrahigh electrostatic field from ferroelectric polarization. Photoresponse wavelengths of the photodetector are extended into near-infrared (0.85-1.55 μm).
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.
Layered two-dimensional (2D) semiconductors, such as MoS2 and SnS2, have been receiving intensive attention due to their technological importance for the next-generation electronic/photonic applications. We report a novel approach to the controlled synthesis of thin crystal arrays of SnS2 at predefined locations on chip by chemical vapor deposition with seed engineering and have demonstrated their application as fast photodetectors with photocurrent response time ∼5 μs. This opens a pathway for the large-scale production of layered 2D semiconductor devices, important for applications in integrated nanoelectronic/photonic systems.Keywords: Layered semiconductor; metal dichalcogenide; chemical vapor deposition; low-dimensional materials; single-crystal arrays
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.
Graphene and other two-dimensional materials, such as transition metal dichalcogenides, have rapidly established themselves as intriguing building blocks for optoelectronic applications, with a strong focus on various photodetection platforms. The versatility of these material systems enables their application in areas including ultrafast and ultrasensitive detection of light in the ultraviolet, visible, infrared and terahertz frequency ranges. These detectors can be integrated with other photonic components based on the same material, as well as with silicon photonic and electronic technologies. Here, we provide an overview and evaluation of state-of-the-art photodetectors based on graphene, other two-dimensional materials, and hybrid systems based on the combination of different two-dimensional crystals or of two-dimensional crystals and other (nano)materials, such as plasmonic nanoparticles, semiconductors, quantum dots, or their integration with (silicon) waveguides.
The compelling demand for higher performance and lower power consumption in electronic systems is the main driving force of the electronics industry's quest for devices and/or architectures based on new materials. Here, we provide a review of electronic devices based on two-dimensional materials, outlining their potential as a technological option beyond scaled complementary metal-oxide-semiconductor switches. We focus on the performance limits and advantages of these materials and associated technologies, when exploited for both digital and analog applications, focusing on the main figures of merit needed to meet industry requirements. We also discuss the use of two-dimensional materials as an enabling factor for flexible electronics and provide our perspectives on future developments.
Two-dimensional crystals with a wealth of exotic dimensional-dependent properties are promising candidates for next-generation ultrathin and flexible optoelectronic devices. For the first time, we demonstrate that few-layered InSe photodetectors, fabricated on both a rigid SiO2/Si substrate and a flexible polyethylene terephthalate (PET) film, are capable of conducting broadband photodetection from the visible to near infrared region (450-785 nm) with high photoresponsivities of up to 12.3 AW-1 at 450 nm (on SiO2/Si) and 3.9 AW-1 at 633 nm (on PET). These photoresponsivities are superior to those of other recently reported 2D crystals (graphene, MoS2, GaS, and GaSe)-based photodetectors. The InSe devices fabricated on rigid SiO2/Si substrates possess a response time of ~50 ms and exhibit long-term stability in photo-switching. These InSe devices can also operate on a flexible substrate with or without bending, and reveal comparable performance to those devices on SiO2/Si. With these excellent optoelectronic merits, we envision that the nanoscale InSe layers will not only find applications in flexible optoelectronics, but also act as an active component to configure versatile 2D heterostructure devices.
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We propose a very large scale integration compatible, modified transfer length method (TLM) structure, called sidewall TLM, to minimize the effect of spreading resistance and thus improving the resolution of the TLM method. This is achieved by allowing uniform current collection perpendicularly through the sidewall of the contact. We demonstrate statistically significant specific contact resistivity (ρc) extraction of 2×10-8Ω cm2 and 5×10-9Ω cm2 for n-type and p-type NiSi contacts, respectively, on a 300-mm wafer, which are about 50% less than those extracted using the conventional TLM structure. The proposed structure also shows a tighter distribution in the extracted ρc values. The results show the importance of such test structures to accurately extract ultralow ρc values relevant to sub-14-nm technology nodes.
Optoelectronic devices based on layered materials like graphene have resulted in significant interest due to their unique properties and potential technological applications. The electric and optoelectronic properties of nano GaTe flakes as layered materials are described in this article. The transistor fabricated from multilayer GaTe shows a p-type action with a hole mobility of about 0.2 cm2V-1s-1. The gate transistor exhibits a high photoresponsivity of 104 A/W, which is greatly better than that of graphene, MoS2 and other layered compounds. Meanwhile, the response speed of 6 ms is also very fast. Both the high photoresponsivity and the fast response time described in the present study strongly suggest that multilayer GaTe is a promising candidate for future optoelectronic and photosensitive device applications.
We demonstrate extraordinary photoconductive behavior in two-dimensional (2D) crystalline indium selenide (In2Se3) nanosheets. Photocurrent measurements reveal that semiconducting In2Se3 nanosheets have an extremely high response to visible light, exhibiting a photo-responsivity of 3.95·102 A·W(-1) at 300 nm with an external quantum efficiency greater than 1.63·10(5) % at 5 V bias. The key figures-of-merit exceed that of graphene and other 2D material-based photodetectors reported to-date. In addition, the photodetector has a fast response time of 1.8·10(-2) s and a specific detectivity of 2.26·10(12) Jones. The photoconductive response of α-In2Se3 nanosheets extends into ultraviolet, visible, and near-infrared spectral regions. The high photocurrent response is attributed to the direct bandgap (EG = 1.3 eV) of In2Se3 combined with a large surface-area-to-volume ratio and a self-terminated/native-oxide-free surface which help to reduce carrier recombination while keeping fast response, allowing for real-time detection under very low-light conditions.
Very thin crystals of molybdenum disulphide, less than 100 Å thick, have been prepared by cleavage. The optical absorption spectra in the thickness range several micrometres to less than 100 Å are similar. Absorption coefficients have been measured to values close to 10^6 cm-1. The absorption bands observed with thin crystals are associated with bulk rather than surface properties. Deviations from an exponential relation between transmission and thickness are observed as the crystal thickness is decreased. The transmission begins to oscillate and thick crystals may transmit more light than thinner ones. Three absorption edges have been observed in the absorption spectrum at about 7000, 5000 and 3000 Å, and structure is observed on the first edge at 4 ^circK. It is suggested that the levels at the top of the valence band are split by spin-orbit interaction. The edges at 5000 and 3000 Å may be due to transitions at other values of the wave vector or from deeper valence bands. The absorption peaks correspond to exciton lines close to the conduction band. Photoconductivity has been observed at 77 ^circK. A response at both absorption edges is detected, and photoconduction peaks are observed at the wavelengths of the exciton bands. Values for the refractive index have been obtained in the region of the first absorption edge, and out to 2 mum. Comparison of the optical and low-frequency dielectric constants indicate that the bonding is covalent. The absorption edges and the exciton bands of single crystals and polycrystalline films are compared. The impurity content of the natural crystals has no effect on the properties observed.
A phototransistor based on a chemical vapor deposited (CVD) MoS2 monolayer exhibits a high photoresponsivity (2200 A W(-1) ) and an excellent photogain (5000). The presence of shallow traps contributes to the persistent photoconductivity. Ambient adsorbates act as p-dopants to MoS2 , decreasing the carrier mobility, photoresponsivity, and photogain.
Phototransistors based on multilayer MoS(2) crystals are demonstrated with a wider spectral response and higher photoresponsivity than single-layer MoS(2) phototransistors. Multilayer MoS(2) phototransistors further exhibit high room temperature mobilities (>70 cm(2) V(-1) s(-1) ), near-ideal subthreshold swings (∼70 mV decade(-1) ), low operating gate biases (<5 V), and negligible shifts in the threshold voltages during illumination.
Two-dimensional (2D) semiconductor nanomaterials hold great promises for future electronics and optics. In this paper, a 2D nanosheets of ultrathin GaSe has been prepared by using mechanical cleavage and solvent exfoliation method. Single- and few-layer GaSe nanosheets are exfoliated on an SiO(2)/Si substrate and characterized by atomic force microscopy and Raman spectroscopy. Ultrathin GaSe-based photodetector shows a fast response of 0.02 s, high responsivity of 2.8 AW(-1) and high external quantum efficiency of 1367% at 254 nm, indicating that the two-dimensional nanostructure of GaSe is a new promising material for high performance photodetectors.
Novel physical phenomena can emerge in low-dimensional nanomaterials. Bulk MoS(2), a prototypical metal dichalcogenide, is an indirect bandgap semiconductor with negligible photoluminescence. When the MoS(2) crystal is thinned to monolayer, however, a strong photoluminescence emerges, indicating an indirect to direct bandgap transition in this d-electron system. This observation shows that quantum confinement in layered d-electron materials like MoS(2) provides new opportunities for engineering the electronic structure of matter at the nanoscale.
Polymer Photodetectors
Optical sensing is used in a wide range of applications, such as low-light detection systems in cars and cameras. Most photodetectors have a limited spectral range and can only detect a narrow range of wavelengths. Gong et al. (p. 1665 , published online 13 August) developed polymer photodetectors with extremely broad spectral response and exceptionally high sensitivity that can exceed the response of an inorganic semiconductor detector at liquid helium temperature. A key aspect in the device design is the inclusion of blocking layers to reduce significantly the dark current or noise in the devices.
In this work we present a low cost and scalable technique, via ambient pressure chemical vapor deposition (CVD) on polycrystalline Ni films, to fabricate large area ( approximately cm2) films of single- to few-layer graphene and to transfer the films to nonspecific substrates. These films consist of regions of 1 to approximately 12 graphene layers. Single- or bilayer regions can be up to 20 mum in lateral size. The films are continuous over the entire area and can be patterned lithographically or by prepatterning the underlying Ni film. The transparency, conductivity, and ambipolar transfer characteristics of the films suggest their potential as another materials candidate for electronics and opto-electronic applications.
- X. Wang
- P. Wang
- J. Wang
- W. Hu
- X. Zhou
- N. Guo
- H. Huang
- S. Sun
- H. Shen
- T. Lin
- M. Tang
- L. Liao
- A. Jiang
- J. Sun
- X. Meng
- X. Chen
- W. Lu
- J. Chu