Next-Generation Photodetectors beyond Van Der Waals Junctions

Next-Generation Image Sensors Based on Low-Dimensional Semiconductor Materials

With the rapid advancement of technology of big data and artificial intelligence (AI), the exponential increase in visual information leads to heightened demands for the quality and analysis of imaging results, rendering traditional silicon-based image sensors inadequate. This review serves as a comprehensive overview of next-generation image sensors based on low-dimensional semiconductor materials encompassing 0D, 1D, 2D materials, and their hybrids. It offers an in-depth introduction to the distinctive properties exhibited by these materials and delves into the device structures tailored specifically for image sensor applications. The classification of novel image sensors based on low-dimensional materials, in particular for transition metal dichalcogenides (TMDs), covering the preparation methods and corresponding imaging characteristics, is explored. Furthermore, this review highlights the diverse applications of low-dimensional materials in next-generation image sensors, encompassing advanced imaging sensors, biomimetic vision sensors, and non-von Neumann imaging systems. Lastly, the challenges and opportunities encountered in the development of next-generation image sensors utilizing low-dimensional semiconductor materials, paving the way for further advancements in this rapidly evolving field, are proposed.

High-Performance Broadband Mixed-Dimensional Phototransistors Based on the Boron Nitride Quantum Dots/MoSe2 Heterostructure with Enhanced UV Sensitivity

Two-dimensional (2D) semiconductors have been of great interest in phototransistors in recent years due to their unique optoelectronic and electronic properties. However, their discernible spectral range and the efficiency of light absorption are usually restricted. Here, we present phototransistors based on mixed-dimensional heterostructures formed by zero-dimensional (0D) boron nitride quantum dots (BNQDs) and molybdenum diselenide (MoSe2), which have high responsivity (R), specific detectivity (D*), and external quantum efficiency (EQE), especially in the ultraviolet (UV) spectral range. The heterostructure phototransistors showed a 440% increase in R at 375 nm (from 5.6 to 24.7 A/W) and a 260% increase in D* (from 3.3 to 8.7 × 1011 Jones) compared to bare MoSe2 at the wavelength of 375 nm and a bias of 1 V. A series of characterization and comparison experiments show that charge transfer on BNQDs/MoSe2 results in the photogating effect and optical gain. Meanwhile, the high-performance BNQDs/MoSe2 heterostructure phototransistors exhibit broadband imaging capabilities and thus hold great promise for ultrasensitive light detection, neuromorphic visual sensing, and in-sensor computing applications.

Lensless Polarimetric Imaging and Encryption Enabled by Te/ReSe2 van der Waals Heterostructure Polarization-Sensitive Photodetector

Polarimetric imaging and encryption improve target recognition precision and information security, enhancing image sensors' perceptual acuity and interference resilience. However, the miniaturization of sensing systems faces challenges due to the complex integration of dispersive optical components such as polarizers. To address this, we propose a polarization-sensitive photodetector using a Te/ReSe2 van der Waals heterostructure. This design leverages type-II band alignment for efficient photocarrier segregation. The anisotropic crystal orientations of ReSe2 and Te layers integrate photon absorption with photocarrier extraction, boosting the functionality. The Te/ReSe2 device offers a broad spectral photoresponse (300-965 nm), a high polarization ratio of 8.9, and a fast response time of 55.4/55.7 μs at 635 nm. These properties enable high-resolution polarimetric imaging and precise image processing. This study provides a blueprint for developing miniaturized polarization-sensitive photodetectors and advancing lensless polarimetric optoelectronics.

High-sensitivity, high-speed, broadband mid-infrared photodetector enabled by a van der Waals heterostructure with a vertical transport channel

The realization of room-temperature-operated, high-performance, miniaturized, low-power-consumption and Complementary Metal-Oxide-Semiconductor (CMOS)-compatible mid-infrared photodetectors is highly desirable for next-generation optoelectronic applications, but has thus far remained an outstanding challenge using conventional materials. Two-dimensional (2D) heterostructures provide an alternative path toward this goal, yet despite continued efforts, their performance has not matched that of low-temperature HgCdTe photodetectors. Here, we push the detectivity and response speed of a 2D heterostructure-based mid-infrared photodetector to be comparable to, and even superior to, commercial cooled HgCdTe photodetectors by utilizing a vertical transport channel (graphene/black phosphorus/molybdenum disulfide/graphene). The minimized carrier transit path of tens of nanometers facilitates efficient and fast carrier transport, leading to significantly improved performance, with a mid-infrared detectivity reaching 2.38 × 1011 cmHz1/2W-1 (approaching the theoretical limit), a fast response time of 10.4 ns at 1550 nm, and an ultrabroadband detection range spanning from the ultraviolet to mid-infrared wavelengths. Our study provides design guidelines for next-generation high-performance room-temperature-operated mid-infrared photodetectors.

Graphene Readout Silicon-Based Microtube Photodetectors for Encrypted Visible Light Communication

The implementation of an advanced light receiver is imperative for the widespread application of visible light communication. However, the integration of multifunctional and high-performance visible light receivers is still limited by device structure and system complexity. Herein, a graphene-readout silicon-based microtube photodetector is proposed as the receiver for omnidirectional Mbps-level visible light communication. The integration of graphene-semiconductor material systems simultaneously ensures the effective absorption of incident light and rapid readout of photogenerated carriers, and the device exhibits an ultrafast response speed of 75 ns and high responsivity of 6803 A W-1. In addition, the microtube photodetector realizes the omnidirectional light-trapping and enhanced polarization photodetection. As the receiving end of the visible light communication system, the microtube photodetector achieves a data rate of up to 778 Mbps, a field of view of 140°, and the encrypted visible light communication of polarized light, providing a new possibility for the future development of the internet of things and information security.

Reconfigurable Homojunction Phototransistor for Near-Zero Power Consumption Artificial Biomimetic Retina Function

Semiconductor photodetectors integrating preliminary signal-processing functions play a vital role in artificial biomimetic retina systems. Herein, we propose a tungsten diselenide (WSe2) phototransistor with a dual-layer gate dielectric and an asymmetric graphene insert structure. This phototransistor exhibits a bidirectional self-powered photocurrent by controlling the gate voltage via the formation of reconfigurable p+-p and n-p homojunctions in the channel from the asymmetric graphene insert. At the same time, the nonvolatile electron and hole stored in the dual-layer gate dielectric are generated using a temporary gate voltage, which can replace the gate voltage to regulate the channel charge. Moreover, the photocurrent shows a linear relation with the temporary programming gate voltage. The phototransistor exhibits a rectification ratio of >4 orders of magnitude without the gate voltage, indicating its significant capability to operate in a fully self-powered mode with near-zero power consumption. Based on the device characteristics, we successfully simulate the biological functions of the photoreceptor layer and bipolar cell layer in the retinal receptive field. The identification of the object motion direction in the receptive field can be realized by integrating three programmable devices on the chip. Furthermore, edge enhancement of the image is achieved by independently modulating the light response of each pixel in the sensor by varying the programming gate voltage. This study will promote the developing progress of future artificial biomimetic retina systems.

An Ultrasensitive Bi2O2Se/In2S3 Photodetector with Low Detection Limit and Fast Response toward High-Precision Unmanned Driving

The machine vision utilized in unmanned driving systems must possess the ability to accurately perceive scenes under low-light illumination conditions. To achieve this, photodetectors with low detection limits and a fast response are essential. Current systems rely on avalanche diodes or lidars, which come with the drawbacks of increased energy consumption and complexity. Here, we present an ultrasensitive photodetector based on a two-dimensional (2D) Bi2O2Se/In2S3 heterostructure, incorporating a homotype unilateral depletion band design. This innovative architecture effectively modulates the transport of both free and photoexcited carriers, suppressing the dark current and facilitating the rapid and efficient separation of photocarriers. Owing to these features, this device exhibits a responsivity of 144 A/W, a specific detectivity of 1.2 × 1014 Jones, and a light on/off ratio of 1.1 × 105. These metrics rank among the top values reported for state-of-the-art 2D devices. Moreover, this device also demonstrates a fast response time of 170/296 μs and a low noise equivalent power of 0.57 fW/Hz1/2, attributes that endow it with ultraweak light imaging capabilities. Furthermore, we have successfully integrated this device into an unmanned driving system, providing a perspective on the design and fabrication of future optoelectronic devices.

Dual-Electrically Configurable MoTe2/In2S3 Phototransistor toward Multifunctional Applications

Photodetectors, essential for a wide range of optoelectronic applications in both military and civilian sectors, face challenges in balancing responsivity, detectivity, and response time due to their inherent unidirectional carrier transport mechanism. Multifunctional photodetectors that address these trade-offs are highly sought after for their potential to reduce costs, simplify system design, and surpass Moore's Law limitations. Herein, we present a multimodal phototransistor based on a 2D MoTe2/In2S3 heterostructure. Through dual electrical modulation employing bias voltage and gate voltage, we engineer the energy band to achieve switchable photoresponse mechanisms between photoconductive and photovoltaic modes. In photoconductive mode, the device exhibits a responsivity of 320 A/W and a specific detectivity of 1.2 × 1013 Jones. Meanwhile, in photovoltaic mode, it exhibits a light on/off ratio of 2 × 105 and response speed of 0.68/0.60 ms. These capabilities enable multifunctional applications such as high-resolution imaging across various wavelengths, a conceptual optoelectronic logic gate, and dual-channel optical communication. This work makes an advancement in the development of future multifunctional optoelectronic devices.

Spin-Enhanced Self-Powered Light Helicity Detecting Based on Vertical WSe2-CrI3 p-n Heterojunction

Two-dimensional (2D) magnetic semiconductors offer an intriguing platform for investigating magneto-optoelectronic properties and hold immense potential in developing prospective devices when they are combined with valley electronic materials like 2D transition-metal dichalcogenides. Herein, we report various magneto-optoelectronic response features of the vertical hBN-FLG-CrI3-WSe2-FLG-hBN van der Waals heterostructure. Through a sensible layout and exquisite manipulation, an hBN-FLG-CrI3-FLG-hBN heterostructure was also fabricated on identical CrI3 and FLGs for better comparison. Our results show that the WSe2-CrI3 heterostructure, acting as a p-n heterojunction, has advantageous capability in light detection, especially in self-powered light helicity detecting. In the WSe2-CrI3 heterojunction, the absolute value of photocurrent IPH exhibits obvious asymmetry with respect to the bias V, with the IPH of reversely biased WSe2-CrI3 p-n heterojunction being larger. When the CrI3 is fully spin-polarized under a 3 T magnetic field, the reversely biased WSe2-CrI3 heterojunction exhibits advantageous capability in light helicity detecting. Both the short-circuit currents ISC and IPH show one-cycle fluctuation behaviors when the quarter-wave plate rotates 180°, and the corresponding photoresponsivity helicities can be as high as 18.0% and 20.1%, respectively. We attribute the spin-enhanced photovoltaic effect in the WSe2-CrI3 heterojunction and its contribution to circularly polarized light detection to the coordination function of the spin-filter CrI3, the valley electronic monolayer WSe2, and the spin-dependent charge transfer between them. Our work helps us understand the interplay between the magnetic and optoelectronic properties of WSe2-CrI3 heterojunctions and promotes the developing progress of prospective 2D spin optoelectronic devices.

High-Spike Barrier Photodiodes Based on 2D Te/WS2 Heterostructures

Two-dimensional (2D) van der Waals (vdWs) heterojunctions have been actively investigated in low-power-consumption and fast-response photodiodes owing to their atomically smooth interfaces and ultrafast interfacial charge transfer. However, achieving ultralow dark current and ultrafast photoresponse in the reported photovoltaic devices remains a challenge as the large built-in electric field in a heterojunction can not only speed up photocarrier transport but also increase the minority-carrier dark current. Here, we propose a high-spike barrier photodiode that can achieve both an ultralow dark current and an ultrafast response. The device is fabricated by the Te/WS2 heterojunction, while the band alignment can transition from type-II to type-I with a high electron barrier and a large hole built-in electronic field. The high electron barrier can greatly reduce the drift current of minority carriers and the generation current of the thermal carriers, while the large built-in electronic field can still speed up the photocarrier transport. The designed Te/WS2 vdWs photodiode yields an ultralow dark current of 8 × 10-14 A and an ultrafast photoresponse of 10/13 μs. Furthermore, a high-performance visible-light imager with a pixel resolution of 100 × 40 is demonstrated using the Te/WS2 vdWs photodiode. This work provides a comprehensive understanding of designing 2D-material-based photovoltaics with excellent overall performance.

Schottky infrared detectors with optically tunable barriers beyond the internal photoemission limit

Internal photoemission is a prominent branch of the photoelectric effect and has emerged as a viable method for detecting photons with energies below the semiconductor bandgap. This breakthrough has played a significant role in accelerating the development of infrared imaging in one chip with state-of-the-art silicon techniques. However, the performance of these Schottky infrared detectors is currently hindered by the limit of internal photoemission; specifically, a low Schottky barrier height is inevitable for the detection of low-energy infrared photons. Herein, a distinct paradigm of Schottky infrared detectors is proposed to overcome the internal photoemission limit by introducing an optically tunable barrier. This device uses an infrared absorbing material-sensitized Schottky diode, assisted by the highly adjustable Fermi level of graphene, which subtly decouples the photon energy from the Schottky barrier height. Correspondingly, a broadband photoresponse spanning from ultraviolet to mid-wave infrared is achieved, with a high specific detectivity of 9.83 × 1010 cm Hz1/2 W-1 at 2,700 nm and an excellent specific detectivity of 7.2 × 109 cm Hz1/2 W-1 at room temperature under blackbody radiation. These results address a key challenge in internal photoemission and hold great promise for the development of the Schottky infrared detector with high sensitivity and room temperature operation.

Anisotropic charge transfer and gate tuning for p-SnS/n-MoS2 vertical van der Waals diodes

2D-material-based van der Waals heterostructures (vdWhs) have shown great potential in next-generation multi-functional microelectronic devices. Thanks to their sharp interface and ultrathin thickness, 2D p-n junctions with high rectification properties have been established by combining p-type monochalcogenides with n-type transition metal dichalcogenides. However, the anisotropic rectification together with the charge transfer and gate effect has not been clarified. Herein, the electrical anisotropy of p-SnS/n-MoS2 diodes was studied. Optimum ideality factors within 1.08-1.18 have been achieved for the diode with 6.6 nm thick SnS on monolayer MoS2, and a high rectification ratio of 3.1 × 104 with strong in-plane anisotropy is observed along the zigzag direction of SnS. A strong gate effect on the anisotropic series resistance has been verified and an effective tuning over the transport length of the SnS channel can be established through adjustment of the current orientation and gate voltage. A thickness-dependent minority carrier transport mechanism has also been demonstrated for the reverse drain current, and Fowler-Nordheim tunneling and direct tunneling are proposed for the increase of the reverse current of the thicker and thinner diodes, respectively. This work will provide another strategy for high-performance diodes based on vdWhs via the control of the current orientation and the gate effect.

High-Performance Planar Field-Emission Photodetector of Monolayer Tungsten Disulfide with Microtips

Monolayer tungsten disulfide (ML WS2 ) is believed as an ideal photosensitive material due to its small direct bandgap, large exciton/trion binding energy, high carrier mobility, and considerable quantum conversion efficiency. Compared with other photosensitive devices, planar field emission (FE)-type photodetectors with a full-plane structure should simultaneously have rapider switching speed and lower power consumption. In this work, ML WS2 microtips are fabricated by electron beam lithography (EBL) way and used to construct a planar FE-type photodetector. By optimization design, ML WS2 with three microtips can exhibit the maximum current density as high as  52 A cm-2 (@300 V µm-1 ), and the largest photoresponsivity is up to 6.8 × 105 A W-1 under green light irradiation, superior to that of many other ML transition metal dichalcogenide (TMDC) detectors. More interestingly, ML WS2 devices with microtips can effectively solve the contradictory problem between large photoresponsivity and rapid switching speed. The excellent photoresponse performances of ML WS2 with microtips should be attributed to their high carrier mobility, sharp emission edge, ultrahigh quantum yield, and unique planar FE device structure. Our research may shed new light on exploring the fabrication technology and photosensitive mechanism of two dimensional (2D) material-based planar FE photodetectors.