Ultraviolet photodetectors (UV PDs) with “5S” (high sensitivity, high signal-tonoise ratio, excellent spectrum selectivity, fast speed, and great stability) have been proposed as promising optoelectronics in recent years. To realize highperformance UV PDs, heterojunctions are created to form a built-in electrical field for suppressing recombination of photogenerated carriers and promoting collection efficiency. In this progress report, the fundamental components of heterojunctions including UV response semiconductors and other materials functionalized with unique effects are discussed. Then, strategies of building PDs with lattice-matched heterojunctions, van der Waals heterostructures, and other heterojunctions are summarized. Finally, several applications based on heterojunction/heterostructure UV PDs are discussed, compromising flexible photodetectors, logic gates, and image sensors. This work draws an outline of diverse materials as well as basic assembly methods applied in heterojunction/heterostructure UV PDs, which will help to bring about new possibilities and call for more efforts to unleash the potential of heterojunctions.

Recent Progress of Heterojunction Ultraviolet Photodetectors: Materials, Integrations, and Applications

2D materials are considered to be the most promising materials for photodetectors due to their unique optical and electrical properties. Since the discovery of graphene, many photodetectors based on 2D materials have been reported. However, the low quantum efficiency, large noise, and slow response caused by the thinness of 2D materials limit their application in photodetectors. Here, recent progress on 2D material photodetectors is reviewed, covering the spectrum from ultraviolet to terahertz waves. First the interaction of 2D materials with light is analyzed in terms of optical physics. Then the present methods to improve the performance of 2D material photodetectors are summarized, such as defect engineering, p-n junctions and hybrid detectors, and the issue of serious overestimation of the performance in reported photodetectors based on 2D materials is discussed. Next, a comparison of 2D material photodetectors with traditional commercially available detectors shows that it is difficult to balance the current 2D material photodetectors with regard to having simultaneously both high sensitivity and fast response. Finally, a possible novel EIW mechanism is suggested to advance the performance of 2D material photodetectors in the future.

Photodetectors of 2D Materials from Ultraviolet to Terahertz Waves

Two-dimensional crystals have emerged as a class of materials that may impact future electronic technologies. Experimentally identifying and characterizing new functional two-dimensional materials is challenging, but also potentially rewarding. Here, we fabricate field-effect transistors based on few-layer black phosphorus crystals with thickness down to a few nanometres. Reliable transistor performance is achieved at room temperature in samples thinner than 7.5 nm, with drain current modulation on the order of 10(5) and well-developed current saturation in the I-V characteristics. The charge-carrier mobility is found to be thickness-dependent, with the highest values up to ∼ 1,000 cm(2) V(-1) s(-1) obtained for a thickness of ∼ 10 nm. Our results demonstrate the potential of black phosphorus thin crystals as a new two-dimensional material for applications in nanoelectronic devices.

http://absimage.aps.org/image/MAR14/MWS_MAR14-2013-005031.pdf

Black Phosphorus Field-effect Transistors

Recently, much progress has been made in polarization-sensitive photodetectors based on the newly-emerging metal-halide perovskites. However, compared to extensive work on visible/infrared-light photodetection, polarization-sensitive perovskite photodetectors targeted at the ultraviolet (UV) region have not been reported. In addition, most of the previously reported photodetectors concentrated on lead-halide perovskites, but the lead-toxicity and poor stability severely restricted their practical applications. In this study, for the first time, one-dimensional CsCu2I3 nanowires (NWs) were used as the light absorber to fabricate a polarization-sensitive UV photodetector because of their electric and optical anisotropy of the asymmetric structure and external morphology anisotropy, and a high photocurrent anisotropy ratio of ∼3.16 was achieved. Owing to the high crystallinity of the CsCu2I3 NWs, the device exhibits remarkable photodetection ability with a photoresponsivity of ∼32.3 A W−1, a specific detectivity of 1.89 × 1012 Jones, and a fast response speed of 6.94/214 μs. Moreover, such a device fabricated on a flexible substrate shows almost no photodetection degradation after extreme bending for 1000 cycles, demonstrating good flexibility and bending endurance. The results obtained highlight the great potential of such copper halides as a stable and environmentally-friendly candidate for polarization-sensitive UV photodetectors, rendering them potentially useful for assembly of optoelectronic systems in the future.

Solution-processed one-dimensional CsCu2I3 nanowires for polarization-sensitive and flexible ultraviolet photodetectors

Polarization-sensitive ultraviolet (UV) photodetection is of great technological importance for both civilian and military applications. Two-dimensional (2D) group-10 transition-metal dichalcogenides (TMDs), especially palladium diselenide (PdSe2), are promising candidates for polarized photodetection due to their low-symmetric crystal structure. However, the lack of an efficient heterostructure severely restricts their applications in UV-polarized photodetection. Here, we develop a PdSe2/GaN Schottky junction by in situ van der Waals growth for highly polarization-sensitive UV photodetection. Owing to the high-quality junction, the device exhibits an appealing UV detection performance in terms of a large responsivity of 249.9 mA/W, a high specific detectivity, and a fast response speed. More importantly, thanks to the puckered structure of the PdSe2 layer, the device is highly sensitive to polarized UV light with a large dichroic ratio up to 4.5, which is among the highest for 2D TMD material-based UV polarization-sensitive photodetectors. These findings further enable the demonstration of the outstanding polarized UV imaging capability of the Schottky junction, as well as its utility as an optical receiver for secure UV optical communication. Our work offers a strategy to fabricate the PdSe2-based heterostructure for high-performance polarization-sensitive UV photodetection.

Shuai Zhang

Papers

Direct Wide Bandgap 2D GeSe2 Monolayer toward Anisotropic UV Photodetection