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Unipolar barrier structures are used to suppress dark current in photodetectors by blocking majority carriers. Designing unipolar barriers with conventional materials is challenging due to the strict requirements of lattice and band matching. Two-dimensional materials have self-passivated surfaces and tunable band structures, and can thus be used to design unipolar barriers in which lattice mismatch and interface defects are avoided. Here, we show that band-engineered van der Waals heterostructures can be used to build visible and mid-wavelength infrared unipolar barrier photodetectors. Our nBn unipolar barrier photodetectors, which are based on a tungsten disulfide/hexagonal boron nitride/palladium diselenide heterostructure, exhibit a low dark current of 15 pA, a photocurrent of 20 μA and a detectivity of 2.7 × 1012 cm Hz1/2 W−1. Our pBp unipolar barrier photodetectors, which are based on a black phosphorus/molybdenum disulfide/graphene heterostructure, exhibit a room-temperature detectivity of 2.3 × 1010 cm Hz1/2 W−1 in the mid-wavelength infrared region under blackbody radiation. The pBp devices also show a dichroic ratio of 4.9 under blackbody radiation, and a response time of 23 μs under 2 μm laser illumination. Band-engineered van der Waals heterostructures that block dark current without suppressing photocurrent can be used to build detectors with high room-temperature detectivity for visible light and blackbody infrared light.

Uncooled infrared photodetectors have evoked widespread interest in basic research and military manufacturing because of their low-cost, compact detection systems. However, existing uncooled infrared photodetectors utilize the photothermoelectric effect of infrared radiation operating at 8-12 μm, with a slow response time in the millisecond range. Hence, the exploration of new uncooled mid-wavelength infrared (MWIR) heterostructures is conducive to the development of ultrafast and high-performance nano-optoelectronics. This study explores a van der Waals heterojunction on epitaxial HgCdTe (vdWs-on-MCT) as an uncooled MWIR photodetector, which achieves fast response as well as high detectivity for spectral blackbody detection. Specifically, the vdWs-on-MCT photodetector has a fast response time of 13 ns (77 MHz), which is approximately an order of magnitude faster than commercial uncooled MCT photovoltaic photodetectors. Importantly, the device exhibits a photoresponsivity of 2.5 A/W, quantum efficiency as high as 85%, peak detectivity of 2×1010 cm Hz1/2 W-1 under blackbody radiation at room temperature, and peak detectivity of up to 1011 cm Hz1/2 W-1 at 77 K. Thereby, our work facilitates the effective design of high-speed and high-performance heterojunction uncooled MWIR photodetectors. This article is protected by copyright. All rights reserved.

Single-photon detectors (SPDs) that can sense individual photons are the most sensitive instruments for photodetection. Established SPDs such as conventional silicon or III-V compound semiconductor avalanche diodes and photomultiplier tubes have been used in a wide range of time-correlated photon-counting applications, including quantum information technologies, in vivo biomedical imaging, time-of-flight 3D scanners, and deep-space optical communications. However, further development of these fields requires more sophisticated detectors with high detection efficiency, fast response, and photon-number-resolving ability, etc. Thereby, significant efforts have been made to improve the performance of conventional SPDs and to develop new photon-counting technologies. In this review, the working mechanisms and key performance metrics of conventional SPDs are first summarized. Then emerging photon-counting detectors (in the visible to infrared range) based on 0D quantum dots, 1D quantum nanowires, and 2D layered materials are discussed. These low-dimensional materials exhibit many exotic properties due to the quantum confinement effect. And photodetectors built from these nD-materials (n = 0, 1, 2) can potentially be used for ultra-weak light detection. By reviewing the status and discussing the challenges faced by SPDs, this review aims to provide future perspectives on the research directions of emerging photon-counting technologies.

Low-dimensional Te-based photodetectors exhibit blackbody response and achieve record-set performance metrics., Blackbody-sensitive room-temperature infrared detection is a notable development direction for future low-dimensional infrared photodetectors. However, because of the limitations of responsivity and spectral response range for low-dimensional narrow bandgap semiconductors, few low-dimensional infrared photodetectors exhibit blackbody sensitivity. Here, highly crystalline tellurium (Te) nanowires and two-dimensional nanosheets were synthesized by using chemical vapor deposition. The low-dimensional Te shows high hole mobility and broadband detection. The blackbody-sensitive infrared detection of Te devices was demonstrated. A high responsivity of 6650 A W−1 (at 1550-nm laser) and the blackbody responsivity of 5.19 A W−1 were achieved. High-resolution imaging based on Te photodetectors was successfully obtained. All the results suggest that the chemical vapor deposition–grown low-dimensional Te is one of the competitive candidates for sensitive focal-plane-array infrared photodetectors at room temperature.

The avalanche is the foundation of the understanding and vast applications of the breakdown of semiconductors and insulators. Present numerical theories analyzing the avalanche photodetectors are mainly split into two categories: the macroscopic empirical model with fitting parameters and the microscopic process simulation with statistical estimations. Here, we present a parameter-free analytic theory of the avalanche for a narrow-band material, HgCdTe, originated from quantum mechanics, avoiding any fitting parameter or any statistical estimation while taking advantage of both categories. Distinct from classical theory, we propose a full spatial description of an avalanche with basic concepts such as transition rate and equation of motion modified. As a stochastic process, the probability density function (PDF) of impact ionization is utilized in a generalized history-dependent theory. On account of different carrier generation character of light and leakage current, we suggest that carrier generated at different positions should be considered separately, which is done by generalized history-dependent theory in our work. Further, in our calculation, the reason for the abnormal rise of excess noise factor (ENF) observed in the experiment in single-carrier avalanche photodetectors is clarified.

2D layered photodetectors have been widely researched for intriguing optoelectronic properties but their application fields are limited by the bandgap. Extending the detection waveband can significantly enrich functionalities and applications of photodetectors. For example, after breaking through bandgap limitation, extrinsic Si photodetectors are used for short-wavelength infrared or even long-wavelength infrared detection. Utilizing extrinsic photoconduction to extend the detection waveband of 2D layered photodetectors is attractive and desirable. However, extrinsic photoconduction has yet not been observed in 2D layered materials. Here, extrinsic photoconduction-induced short-wavelength infrared photodetectors based on Ge-based chalcogenides are reported for the first time and the effectiveness of intrinsic point defects are demonstrated. The detection waveband of room-temperature extrinsic GeSe photodetectors with the assistance of Ge vacancies is broadened to 1.6 µm. Extrinsic GeSe photodetectors have an excellent external quantum efficiency (0.5%) at the communication band of 1.31 µm and polarization-resolved capability to subwaveband radiation. Moreover, room-temperature extrinsic GeS photodetectors with a detection waveband to the communication band of 1.55 µm further verify the versatility of intrinsic point defects. This approach provides design strategies to enrich the functionalities of 2D layered photodetectors.

Two-dimensional (2D) materials with excellent optoelectronic properties have attracted tremendous research interest in recent years. The promising performances of photodetectors based on 2D materials, such as ultrafast photoresponse and ultrahigh photoresponsivity, have been demonstrated in the visible to short-wavelength infrared spectrum range (0.8–2 μm). However, high performance, room temperature operation long-wavelength infrared (LWIR) photodetection is challenging. The detectors based on graphene usually exhibit low photoresponsivity due to the low optical absorption and short carrier lifetime. In addition, the relatively large bandgap of transition metal dichalcogenides limited the photoresponse bandwidth. Here, we report a way to fabricate a scalable device array of room-temperature operation LWIR PtSe2-G heterostructure detectors. The photoresponsivity at 10.6 μm up to ∼300 mA/W is obtained. The long-wave infrared light in the pico-watt range could be detected at room temperature by the PtSe2-G heterostructure detector. This result indicates that the PtSe2-G heterostructure device could be a highly competitive candidate for an uncooled LWIR detector. It also opens a way for a scalable array infrared focus plane device for the LWIR image.

Zinc oxide (ZnO) nanosheets have demonstrated outstanding electrical and optical properties, which are well suited for ultraviolet (UV) photodetectors. However, they have a high density of intrinsically unfilled traps, and it is difficult to achieve p-type doping, leading to the poor performance for low light level switching ratio and a high dark current that limit practical applications in UV photodetection. Here, UV photodetectors based on ZnO nanosheets are demonstrated, whose performance is significantly improved by using a ferroelectric localized field. Specifically, the photodetectors have achieved a responsivity of up to 3.8 × 105 A W-1 , a detectivity of 4.4 × 1015 Jones, and a photocurrent gain up to 1.24 × 106 . These device figures of merit are far beyond those of traditional ZnO ultraviolet photodetectors. In addition, the devices' initial dark current can be easily restored after continuous photocurrent measurement by using a positive gate voltage pulse. This study establishes a new approach to produce high-sensitivity and low-dark-current ultraviolet photodetectors and presents a crucial step for further practical applications.

Nanostructured zinc oxide (ZnO) semiconductors have emerged as promising materials for high-performance photodetectors due to their natural direct bandgap and extraordinary physicochemical properties. However, the oxygen vacancy defects of nano-ZnO can easily trap oxygen molecules in air and generate charge transfer at the interface, which induced continuous photoconductance that limited the development and application of ZnO in photodetection. Here, we demonstrate a homojunction ultrathin ZnO nanosheet photodetector with high performance and propose a better dominant photoresponse mechanism of the ZnO nanosheet driven by the charge transfer induced local field. The strong localized electric field significantly accelerates the separation of photo-generated carriers and effectively suppresses the dark current. Thus, the photodetector of the charge transfer induced homojunction exhibits ultralow dark current (10−12 A), ultra-high specific detectivity (up to ∼1014 Jones), and fast rising (300 ms) and decaying times (310 ms), taking advantages of high-performance and fast response speed and subverting the limitation of traditional ZnO photodetectors in the field of application. An easy-fabrication, fast response, and high-performance photodetector proposed here provides a good paradigm for the next-generation photodetectors based on two-dimensional nanostructures.