Article

# Asymmetrically Encapsulated Vertical ITO/MoS 2 /Cu 2 O Photodetector with Ultrahigh Sensitivity

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## 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.

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... 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]. ...
Preprint
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. ...
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... 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]. ...
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... 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. ...
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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.
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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.
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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.
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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.
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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.
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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.
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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).
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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.
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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
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
Article
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.
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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.
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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