Ultrathin Photodetectors Research Articles

High performance CH3NH3PbI3 perovskite photodetector functionalized by T-Ag plasmonic nanostructure

Photodetectors with ultrathin thickness and high efficiency are increasingly needed in the developing of wearable devices. However, the very limited active layer thickness has seriously restricted the photoelectric conversion efficiency. Here, the optical nanostructure fabricated with the assistance of self-assembly polystyrene mask layer has been integrated to increase the light absorption capacity by simultaneously utilizing the local surface plasma resonance, gap plasma, and the microcavity resonance effect. With the incorporation of triangular silver (T-Ag) nanostructure, the photogenerated current and response speed have significantly enhanced 481.25% and 26.30%, respectively. This strategy provides a viable way to design high-efficiency ultra-thin photodetectors.

High-speed long-wave infrared ultra-thin photodetectors

The primary challenge for long-wavelength infrared (λ = 8–13 µm) detection has long been the mitigation of dark current while achieving a high conversion efficiency of optical to electrical signals. Often overlooked is the bandwidth of detector response, despite several existing and expected future long-wave infrared high bandwidth applications. Here, we demonstrate ultra-fast response times in long-wave infrared detectors leveraging ultra-thin absorbers. The time response of the detectors is characterized using mid-infrared femtosecond pulses generated by an optical parametric amplifier, as a function of the device temperature and operating bias, as well as excitation wavelength. An equivalent circuit model for the detectors is presented and compared to our experimental results with excellent agreement. We demonstrate detector impulse response times of <100ps and 3 dB bandwidths in the GHz frequency range (f3dB > 3.5 GHz). Spectral response measurements confirm that the detectors have a resonant cavity mode enhanced response in the long-wave infrared, peaking at 10.2 µm. The presented detectors offer a potential solution for a range of high-frequency applications in the long-wave infrared.

100 Gbps PAM4 ultra-thin photodetectors integrated on SOI platform by micro transfer printing.

The integration of compact high-bandwidth III-V active devices in a scalable manner is highly significant for Silicon-on-insulator (SOI) photonic integrated circuits. To address this, we demonstrate the integration of pre-fabricated 21 × 57 µm2 InGaAs photodetector (PD) coupons with a thickness of 675 nm to a 500 nm SOI platform using a direct bonding micro-transfer printing process. The common devices are coupled to the Si waveguides via butt, grating and evanescent coupling schemes with responsivities of 0.13, 0.3 and 0.6 A/W respectively, in line with simulations. The thin device facilitates simplified high-speed connections without the need for an interlayer dielectric. A back-to-back data communication rate of 50 Gb/s is achieved with on-off keying and with post processing of four-level pulse-amplitude modulation (PAM4) 100 Gb/s is realized. Potentially, around 1 million devices per 75 mm InP wafer can be attained. The integration of compact PDs exhibited in this work can be extended to modulators and lasers in the future.

Ultrathin Self-Powered Heavy-Metal-Free Cu-In-Se Quantum Dot Photodetectors for Wearable Health Monitoring.

Mechanically deformable photodetectors (PDs) are key device components for wearable health monitoring systems based on photoplethysmography (PPG). Achieving high detectivity, fast response time, and an ultrathin form factor in the PD is highly needed for next-generation wearable PPG systems. Self-powered operation without a bulky power-supply unit is also beneficial for point-of-care application. Here, we propose ultrathin self-powered PDs using heavy-metal-free Cu-In-Se quantum dots (QDs), which enable high-performance wearable PPG systems. Although the light-absorbing QD layer is extremely thin (∼40 nm), the developed PD exhibits excellent performance (specific detectivity: 2.10 × 1012 Jones, linear dynamic range: 102 dB, and spectral range: 250-1050 nm at zero bias), which is comparable to that of conventional rigid QD-PDs employing thick Pb-chalcogenide QD layers. This is attributed to material and device strategies─materials that include Cu-In-Se QDs, a MoS2-nanosheet-blended poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) hole transport layer, a ZnO nanoparticle electron transport layer, Ag and ITO electrodes, and an ultrathin form factor (∼120 nm except the electrodes) that enable excellent mechanical deformability. These allow the successful application of QD-PDs to a wearable system for real-time PPG monitoring, expanding their potential in the field of mobile bioelectronics.

A distinctive architecture design of lateral p–n type GaN ultraviolet photodetectors via a numerical simulation

The conventionally vertical p–n type gallium nitride (GaN)-based ultraviolet (UV) photodetectors (PDs) suffer from the drawbacks of insufficient light absorption in the depletion region, thus resulting in poor carrier separation efficiency. The architecture of lateral p–n type GaN-based UV PDs has attracted much attention with applications in numerous fields due to their unique photoelectric properties. However, the potential of this type of devices has not yet been fully unlocked because of lacking a complete understanding of the design principles and working mechanisms. Here, a detailed photoelectronic model was developed to address the processes of carrier generation, transport, recombination and extraction to clarify the underlying mechanisms of the lateral p–n type UV PDs. By comparing with the vertical UV PDs, the lateral PDs show similar optical response, but higher photo-to-dark current ratio and responsivity when GaN is heavily doped. Moreover, it is also confirmed that the performance of the lateral p–n devices is almost independent of GaN thickness, providing significant inspiration for the development of ultra-thin UV PDs.

Wafer‐Scale Two‐Dimensional Molybdenum Diselenide Phototransistor Array via Liquid‐Precursor‐Assisted Chemical Vapor Deposition

AbstractMonolayer transition metal dichalcogenides (TMDs) have received considerable interest as a candidate material for ultrathin photodetectors, but their practical applications are hindered by difficulties in synthesizing high‐quality films over centimeter‐scale areas. Although chemical vapor deposition (CVD) based on liquid‐phase precursors has shown promise for large‐area synthesis of TMDs, the resulting films typically exhibit much lower optoelectronic performance due to the lack of wafer‐scale uniformity with monolayer thickness and inferior electrical properties induced by defects. In this article, the authors present a wafer‐scale, gate‐tunable photodetector array from high‐quality monolayer molybdenum diselenide (MoSe2) films synthesized with the liquid‐phase CVD process assisted by a growth promoter. Continuous monolayer MoSe2 can form by introducing potassium iodide as a growth promoter for selenizing the metal‐precursor film. Side‐by‐side comparison between MoSe2 formed with and without the potassium iodide reveal that the promoter‐assisted growth strategy significantly improves the crystallinity, which results in enhanced optoelectronic properties. The resulting photodetector array is highly photosensitive over the visible wavelengths with photoresponsivities higher than those of the previously reported devices based on CVD‐synthesized monolayer TMDs and even comparable to the devices fabricated from mechanical exfoliation approach.

Transfer-free, scalable photodetectors based on MOCVD-grown 2D-heterostructures

2D semiconductors based on transition metal dichalcogenides are highly promising for ultrathin photodetectors due to their thickness in the nanometer range and their exceptional light absorption properties. To enable efficient separation of optically generated electron–hole pairs heterostructures have to be implemented, which are usually prepared by poorly controlled mechanical steps such as exfoliation, transfer and stacking processes that prevent industrial upscaling. Here, semitransparent photodetectors in the mm2 range based on MoS2/WS2 heterostructures are presented that are realized without any transfer step by a scalable metal-organic chemical vapor deposition process on a sapphire substrate in a continuous growth run. The heterostructure device exhibits a responsivity, which is enhanced by about 5–6 orders of magnitude with respect to reference devices based on either MoS2 or WS2 monolayers only. The large gain enhancement is attributed to efficient charge carrier separation at the MoS2/WS2 heterointerface combined with hole trapping, leading to an improved electron transport in the heterostructure under illumination.

An ultrathin MoSe2 photodetector with near-perfect absorption

An ultrathin near-perfect MoSe2 absorber working in the visible regime is demonstrated theoretically and experimentally, and it consists of a MoSe2/Au bi-layer film. The polymer-assisted deposition method is used to synthesize MoSe2 films, which can reduce the roughness and thus improve the film absorption. Simulation results show that the absorption of the absorber with 22 nm MoSe2 reaches to larger than 90% between 628.5 nm and 718 nm with a peak value up to 99.5% at 686 nm. Moreover, the measured absorption also shows near-perfect absorption of this simple absorber. Finally, an ultrathin photodetector is fabricated based on this perfect absorber and shows on/off reproducibility and remarkable photocurrent, which is three orders of magnitude higher than the dark current.

Theoretical study on Schottky regulation of WSe<sub>2</sub>/graphene heterostructure doped with nonmetallic elements

采用平面波超软赝势方法研究了非金属元素掺杂对二硒化钨/石墨烯肖特基电子特性的影响. 研究表明二硒化钨与石墨烯层间以范德瓦耳斯力结合形成稳定的结构. 能带结果表明二硒化钨与石墨烯在稳定层间距下形成n型肖特基势垒. 三维电子密度差分图表明石墨烯中的电子向二硒化钨移动, 使二硒化钨表面带负电, 石墨烯表面带正电, 界面形成内建电场. 分析表明, 将非金属原子掺杂二硒化钨可以有效地调控二硒化钨/石墨烯肖特基势垒的类型和高度. C, O原子掺杂二硒化钨时, 肖特基类型由p型转化为n型, 并有效降低了肖特基势垒的高度; N, B原子掺杂二硒化钨时, 掺杂二硒化钨体系表现出金属性, 与石墨烯接触表现为欧姆接触. 本文结果可为二维场效应晶体管的设计与制作提供相关指导.

High-Responsivity MSM Solar-Blind UV Photodetector Based on Annealed ITO/Ag/ITO Structure Using RF Sputtering

In this paper, a novel ITO/Ag/ITO (IAI) multilayer UV photodetector was successfully prepared via RF magnetron sputtering technique to achieve the dual-benefit of improved photoresponse and outstanding visible blindness properties. The influence of the annealing process on the performance of the elaborated IAI UV photodetector was carried out. Moreover, the structural and photoelectrical properties of the sputtered tri-layered structures with and without annealing effects were also analyzed. It was found that the elaborated UV -photodetector provides near perfect UV absorbance of 96%. This achievement is attributed to the improved light trapping capability and tunable plasmonic resonance effects induced by the efficient optical coupling between free-space UV photons and Ag plasmons. It was also revealed that the heat treatment paves a new path toward preparing efficient visible-blind UV photodetectors, where the annealed sample demonstrated a superior rejection ratio of 225 as compared to that of the unannealed counterpart. Besides, electrical characterization underlined the ability of the annealed IAI UV photodetector at 500°C for offering exciting opportunities, where it exhibits a high responsivity of 3.8mA/W at self-powered condition. Therefore, by well optimizing both IAI multilayer geometry and annealing conditions, we were able to elaborate an ultrathin photodetector suitable for high-performance and flexible UV sensing applications.

Spectrally Selective Ultrathin Photodetectors Using Strong Interference in Nanocavity Design

Thinning the active layer’s thickness of the semiconductor down to a level comparable with the carriers’ diffusion length while keeping its absorption high is an ultimate goal to boost the performance of optoelectronic devices. Strong interference in multilayer structures is one of the promising and practical solutions owing to their simple and large-scale compatible fabrication route. These nanocavity designs not only provide near unity absorption, but they can also be designed in a way that a spectrally selective absorption response can be achieved. In this letter, we will demonstrate the functionality of a metal–insulator–semiconductor (MIS) cavity to obtain spectrally selective ultrathin photodetectors. To prove our theoretical and numerical findings, a 4-nm-thick amorphous silicon (a-Si)-based MIS cavity is designed, fabricated, and characterized. The experimental results show that the optimized cavity design can act as an efficient visible blind ultraviolet (UV) photodetector. The proposed design shows the responsivity values of 120 and 2.5 mA/W in the UV ( $\lambda =\textsf {350}$ nm) and visible ( $\lambda =\textsf {500}$ nm) regions, respectively.

Rapid-throughput solution-based production of wafer-scale 2D MoS2

Two-dimensional semiconductors, such as MoS2, are leading candidates for the production of next-generation optoelectronic devices such as ultrathin photodetectors and photovoltaics. However, the commercial application of 2D semiconductors is hindered by growth techniques requiring hours of heating and cooling cycles to produce large-area 2D materials. We present here a growth technique that leverages high-intensity optical irradiation of a solution-processed (NH4)2MoS4 precursor to synthesize MoS2 in one-tenth the time of typical furnace-based CVD. From start to finish, the technique produces uniform 2D MoS2 across 4-in. wafers within 15 min. Raman spectroscopy, in-plane XRD, and XPS show a 2H MoS2 crystal structure with a stoichiometry of 1.8:1 S:Mo. AFM scans show that the films are 2.0 nm thick MoS2 with a roughness of 0.68 nm. Photoluminescence spectroscopy reveals the characteristic 1.85 eV bandgap. The as-grown films were used to make field-effect transistors with a mobility of 0.022 cm2 V−1 s−1 and photodetectors with a responsivity of 300 mA/W and an external quantum efficiency of 0.016%, demonstrating their potential for optoelectronic device development. This rapid thermal processing growth technique reduces MoS2 synthesis time by an order of magnitude relative to comparable techniques and enables greater accessibility to 2D semiconductors for researchers and developers.

Interfacial interaction and Schottky contact of two-dimensional WS<sub>2</sub>/graphene heterostructure

Two-dimensional (2D) materials exhibit massive potential in research and development in the scientific world due to their unique electrical, optical, thermal and mechanical properties. Graphene is an earliest found two-dimensional material, which has many excellent properties, such as high carrier mobility and large surface area. However, single layer graphene has a zero band gap, which limits its response in electronic devices. Unlike graphene, the transition metal sulfides (TMDs) have various band structures and chemical compositions, which greatly compensate for the defect of zero gap in graphene. The WS<sub>2</sub> is one of the 2D TMDs exhibiting a series of unique properties, such as strong spin-orbit coupling, band splitting and high nonlinear susceptibility, which make it possess many applications in semiconducting optoelectronics and micro/nano-electronics. The 2D semiconductors along with semimetallic graphene are seen as basic building blocks for a new generation of nanoelectronic devices. In this way, the artificially designed TMD heterostructure is a promising option for ultrathin photodetectors. There are few reports on the physical mechanism of carrier mobility and charge distribution at the interface of WS<sub>2</sub>/graphene heterostructure, by varying the interfacial distance of WS<sub>2</sub>/graphene heterostructure to investigate the effect on the electronic properties. Here in this work, the corresponding effects of interface cohesive interaction and electronic properties of WS<sub>2</sub>/graphene heterostructure are studied by first-principles method. The calculation results indicate that the lattice mismatch between monolayer WS<sub>2</sub> and graphene is low, the equilibrium layer distance <i>d</i> of about 3.42 Å for the WS<sub>2</sub>/graphene heterostructure and a weak van der Waals interaction forms in interface. Further, by analyzing the energy band structures and the three-dimensional charge density difference of WS<sub>2</sub>/graphene, we can identify that at the interface of the WS<sub>2</sub> layer there appears an obvious electron accumulation: positive charges are accumulated near to the graphene layer, showing that WS<sub>2</sub> is an n-type semiconductor due to the combination with graphene. Furthermore, the total density of states and corresponding partial density of states of WS<sub>2</sub>/graphene heterostructure are investigated, and the results show that the valence band is composed of hybrid orbitals of W 5d and C 2p, whereas the conduction band is comprised of W 5d and S 3p orbitals, the orbital hybridization between W 5d and S 3p will cause photogenerated electrons to transfer easily from the internal W atoms to the external S atoms, thereby forming a build-in internal electric field from graphene to WS<sub>2</sub>. Finally, by varying the interfacial distance for analyzing the Schottky barrier transition, as the interfacial distance is changed greatly from 2.4 Å to 4.2 Å, the shape of the band changes slightly, however, the Fermi level descends relatively gradually, which can achieve the transition from a p-type Schottky contact to an n-type Schottky contact in the WS<sub>2</sub>/graphene. The plane-averaged charge density difference proves that the interfacial charge transfer and the Fermi level shift are the reasons for determining the Schottky barrier transition in the WS<sub>2</sub>/graphene heterostructure. Our studies may prove to be instrumental in the future design and fabrication of van der Waals based field effect transistors.

Spatially-Resolved Photoluminescence of Monolayer MoS2 under Controlled Environment for Ambient Optoelectronic Applications

Monolayer (ML) MoS2 has become a very promising two-dimensional material for photorelated applications, potentially serving as the basis for an ultrathin photodetector, switching device, or transistors because of its strong interaction with light in ambient conditions. Establishing the impact of individual ambient gas components on the optical properties of MoS2 is a necessary step toward application development. By using in situ Raman microspectroscopy with an environment-controlled reaction cell, the photoluminescence (PL) intensity of chemical vapor deposition (CVD)-grown MoS2 MLs is monitored at different intralayer locations under ambient and controlled gas environments, such as N2, O2, and H2O. This new approach enables us to monitor the optical properties of MoS2 at different locations on the flakes and separate the role of photoreaction of various gases during laser irradiation. Upon mild photoirradiation in ambient conditions, the PL intensity in the interior of the ML MoS2 flakes remains unchang...

Ultraflexible Near-Infrared Organic Photodetectors for Conformal Photoplethysmogram Sensors.

Flexible organic optoelectronic devices simultaneously targeting mechanical conformability and fast responsivity in the near-infrared (IR) region are a prerequisite to expand the capabilities of practical optical science and engineering for on-skin optoelectronic applications. Here, an ultraflexible near-IR responsive skin-conformal photoplethysmogram sensor based on a bulk heterojunction photovoltaic active layer containing regioregular polyindacenodithiophene-pyridyl[2,1,3]thiadiazole-cyclopentadithiophene (PIPCP) is reported. The ultrathin (3 µm thick) photodetector exhibits unprecedented operational stability under severe mechanical deformation at a bending radius of less than 3 µm, even after more than 103 bending cycles. Deliberate optimization of the physical dimensions of the active layer used in the device enables precise on/off switching and high device yield simultaneously. The response frequency over 1 kHz under mechanically deformed conditions facilitates conformal electronic sensors at the machine/human interface. Finally, a mechanically stretchable, flexible, and skin-conformal photoplethysmogram (PPG) device with higher sensitivity than those of rigid devices is demonstrated, through conformal adherence to the flexuous surface of a fingerprint.

Lateral Graphene-Contacted Vertically Stacked WS2 /MoS2 Hybrid Photodetectors with Large Gain.

A demonstration is presented of how significant improvements in all-2D photodetectors can be achieved by exploiting the type-II band alignment of vertically stacked WS2 /MoS2 semiconducting heterobilayers and finite density of states of graphene electrodes. The photoresponsivity of WS2 /MoS2 heterobilayer devices is increased by more than an order of magnitude compared to homobilayer devices and two orders of magnitude compared to monolayer devices of WS2 and MoS2 , reaching 103 A W-1 under an illumination power density of 1.7 × 102 mW cm-2 . The massive improvement in performance is due to the strong Coulomb interaction between WS2 and MoS2 layers. The efficient charge transfer at the WS2 /MoS2 heterointerface and long trapping time of photogenerated charges contribute to the observed large photoconductive gain of ≈3 × 104 . Laterally spaced graphene electrodes with vertically stacked 2D van der Waals heterostructures are employed for making high-performing ultrathin photodetectors.

Ultrathin 2D Photodetectors Utilizing Chemical Vapor Deposition Grown WS2 With Graphene Electrodes

In this report, graphene (Gr) is used as a 2D electrode and monolayer WS2 as the active semiconductor in ultrathin photodetector devices. All of the 2D materials are grown by chemical vapor deposition (CVD) and thus pose as a viable route to scalability. The monolayer thickness of both electrode and semiconductor gives these photodetectors ∼2 nm thickness. We show that graphene is different to conventional metal (Au) electrodes due to the finite density of states from the Dirac cones of the valence and conduction bands, which enables the photoresponsivity to be modulated by electrostatic gating and light input control. We demonstrate lateral Gr-WS2-Gr photodetectors with photoresponsivities reaching 3.5 A/W under illumination power densities of 2.5 × 10(7) mW/cm(2). The performance of monolayer WS2 is compared to bilayer WS2 in photodetectors and we show that increased photoresponsivity is achieved in the thicker bilayer WS2 crystals due to increased optical absorption. This approach of incorporating graphene electrodes in lateral TMD based devices provides insights on the contact engineering in 2D optoelectronics, which is crucial for the development of high performing ultrathin photodetector arrays for versatile applications.

Tunable functionalization of graphene nanosheets for graphene-organic hybrid photodetectors

Graphene-organic hybrid thin films are promising candidates for use as advanced transparent electrodes and high-performance photodetectors. In this work, we fabricated hybrid thin film structures consisting of graphene and either tetraphenyl-porphyrin (H2TPP) or metalloporphyrins such as aluminum (III) tetraphenyl-porphyrin (Al(III)TPP) and zinc tetraphenyl-porphyrin (ZnTPP). The optical and electrical characteristics of ultrathin photodetectors based on the graphene-organic hybrid layers were subsequently evaluated. A hybrid deposition system capable of both thermal evaporation and vapor phase metalation was employed to synthesize the tunable metalloporphyrin-based thin films. As a proof of concept, we successfully fabricated various graphene-based photodetectors via the simple and efficient vapor-phase metalation of porphyrin. This work may facilitate the development of new architectures for flexible graphene-organic devices.

Nanometre optical coatings based on strong interference effects in highly absorbing media

Optical coatings, which consist of one or more films of dielectric or metallic materials, are widely used in applications ranging from mirrors to eyeglasses and photography lenses. Many conventional dielectric coatings rely on Fabry-Perot-type interference, involving multiple optical passes through transparent layers with thicknesses of the order of the wavelength to achieve functionalities such as anti-reflection, high-reflection and dichroism. Highly absorbing dielectrics are typically not used because it is generally accepted that light propagation through such media destroys interference effects. We show that under appropriate conditions interference can instead persist in ultrathin, highly absorbing films of a few to tens of nanometres in thickness, and demonstrate a new type of optical coating comprising such a film on a metallic substrate, which selectively absorbs various frequency ranges of the incident light. These coatings have a low sensitivity to the angle of incidence and require minimal amounts of absorbing material that can be as thin as 5-20 nm for visible light. This technology has the potential for a variety of applications from ultrathin photodetectors and solar cells to optical filters, to labelling, and even the visual arts and jewellery.

Novel III‐nitride based transparent photodetectors for standing wave interferometry

AbstractThe concept of a standing wave (SW) interferometer has been extensively investigated for the last decade. A key problem within these efforts is the development of a transparent ultra‐thin photodetector for sampling the intensity profile of the generated SW. We report on the simulation and fabrication as well as on the structural and optoelectronical characterization of highly transparent AlGaN/GaN double‐heterostructure and InGaN metal–semiconductor–metal photodetectors based on intraband and band‐to‐band transitions, respectively. Both detectors have a 20 nm thick optically active layer designed for the light absorption at the wavelength of ∼633 nm. Besides good structural properties of both Al0.35Ga0.65N/GaN and In0.56Ga0.44N structures, the absorbance of the InGaN‐based detector was found ∼20 times higher compared to the AlGaN/GaN photodetectors. The temporal characteristics of the detectors are limited by both the RC‐constant and by the persistent photocurrent effect. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)