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

The desire for developing ultraviolet optoelectronic devices has prompted extensive studies toward wide-bandgap semiconductor ZnO and its related alloys. Bandgap engineering as well as p-type doping is the key toward practical applications of ZnO. As yet, stable and reproducible p-type doping of ZnO remains a formidable challenge. To circumvent p-type conductivity, ZnO-based optoelectronic devices have been developed with hetero-structures of ZnO alloys. In past decades, substantial efforts have been made to engineer the band structure of ZnO via isovalent cation- or anion-substitution for obtaining desired material properties, and considerable progresses have been achieved. The purpose of this review is to summarize recent advances in the experimental and theoretical studies on bandgap engineering of ZnO by formation of multi-component alloys, and the development of related hetero-structures and optoelectronic devices. First, we briefly introduce the general properties, epitaxial growth techniques, and bandgap engineering of ZnO. Then, we focus on presenting the current status of researches on ZnO ternary and quaternary alloys for bandgap engineering. The issues about substituent solubility limit and phase separation, as well as variations of lattice parameters and bandgap with the substituent content in the alloys are discussed in detail. Further, ZnO alloys based hetero-structures including hetero-junctions, quantum wells, and superlattices are reviewed, and recent achievements in the area of optoelectronic devices based on ZnO multi-component alloys are summarized. The review closes with outlooking the likely developing trend of multi-component alloys for the bandgap engineering of ZnO and related hetero-structures, and the potential and pathway of multi-component alloys in settling the p-type doping of ZnO. 

Multi-component ZnO alloys: Bandgap engineering, hetero-structures, and optoelectronic devices

High-mobility induced high-performance self-powered ultraviolet photodetector based on single ZnO microwire/PEDOT:PSS heterojunction via slight ga-doping

The photodetectors based on the wide bandgap semiconductor (WBS)/Si heterojunction have attracted more and more attention in recent years due to their excellent photoelectric characteristics and easy integration capabilities. In this work, we have demonstrated a self-powered solar-blind ultraviolet (UV) photodetector based on the ZnGa2O4/Si heterojunction. A typical rectification characteristic with a rectification ratio exceeding 103 within ±5 V can be obtained. At 0 V bias, the −3 dB cutoff wavelength of ∼255 nm and the UV-visible rejection ratio of ∼3 × 102 show that the device has excellent self-powered solar-blind UV detection performance. In addition, the responsivity and the response speed of ZnGa2O4/Si heterojunction can be efficiently enhanced by a transient spike current at 0 V bias when turning on and off the 254 nm UV light. The interface pyroelectric effect of the ZnGa2O4 film should be responsible for this transient spike photocurrent phenomenon. Our findings in this work pave a feasible way to realize high-performance WBS/Si heterojunction self-powered solar-blind photodetectors.

Performance enhancement of a self-powered solar-blind UV photodetector based on ZnGa2O4/Si heterojunction via interface pyroelectric effect

The heterojunction UV photodetector based on p-CuZnS and n-GaN is prepared by a simple chemical bath deposition. The CuZnS/GaN film device shows a significantly enhanced photocurrent at 3 V and 350 nm, a good rectifying behavior (ratio of 19 000 at ±3 V) and a large open circuit voltage (0.55 V). Furthermore, the CuZnS/GaN photodetector exhibits excellent self-powered characteristics under 350 nm light illumination, including a high photocurrent (19 μA), a fast response speed (0.14/40 ms) and an ultrahigh on/off ratio (3 × 108). More importantly, the CuZnS/GaN photodetector presents a large detectivity of 8 × 1013 Jones and an ultrahigh linear dynamic range of 137 dB at 320 nm light illumination at zero bias. These results suggest that the CuZnS/GaN film devices have great potential as high-performance self-powered UV photodetectors for practical applications.

High performance self-powered CuZnS/GaN UV photodetectors with ultrahigh on/off ratio (3 × 108)

A novel self-powered solar-blind ZnGa2O4 ultraviolet (UV) photodetector has been constructed by fabricating asymmetric pairs of Au Schottky electrodes on the ZnGa2O4 film. The current-voltage curve of the device exhibits an obvious rectifying characteristic with a rectification ratio of 11.3 at ±8 V. At 0 V bias, the peak responsivity of our device at 246 nm is about 22.2 mA/W with an UV–vis rejection ratio of 1.3× 10 4 , indicating the excellent self-powered solar-blind UV photodetector performance. More interestingly, the rise time and decay time of the device are only 10 ns and 30 ns, respectively, which are much faster than that of any other previously reported self-powered solar-blind UV photodetectors to the best of our knowledge. The ultra-fast response speed of the device should be attributed to the asymmetric Schottky junctions and the high crystalline quality of our ZnGa2O4 film with low trap density. Our work provides a new method to constructing high-performance self-powered solar-blind UV photodetectors.

Self-powered solar-blind ZnGa2O4 UV photodetector with ultra-fast response speed

A high performance p-GaN/n-ZnMgO heterojunction photodiode was demonstrated and investigated. A high quality p-GaN film was grown on GaN/sapphire templates by plasma-assisted molecular beam epitaxy and subsequently an n-ZnMgO layer was deposited on p-GaN by a metal organic chemical vapor deposition technique. The p-GaN/n-ZnMgO heterojunction photodetector shows a clear rectifying I–V characteristic with a turn-on voltage of 2.5 V. At zero-bias voltage, the device shows a high peak responsivity of 196 mA W−1 at 362 nm. The 10–90% rise time and 90–10% decay time of the device can be as short as 1.7 ms and 3.3 ms, respectively. The excellent crystal quality and electrical properties of p-GaN play an important role in the high performance of the photodiode. This work provides a feasible way for the development of high-performance heterojunction self-powered UV detectors.

A high performance self-powered ultraviolet photodetector based on a p-GaN/n-ZnMgO heterojunction

Fabrication and Characteristics of MgZnO Ultraviolet Detector Based on Ag Microporous Array Structure Electrode

The invention provides a ZnMgO ultraviolet detector and a preparation method thereof, and the method comprises the steps: S1, taking an organic zinc compound as a zinc source, taking an organic magnesium compound as a magnesium source, taking high-purity oxygen as an oxygen source, and growing a ZnMgO film on the surface of a substrate by adopting a metal organic compound chemical vapor depositionmethod; S2, forming an interdigital electrode mask on the ZnMgO film by using negative photoresist photoetching, and removing the interdigital electrode mask after sputtering metal on the interdigital electrode mask to form an interdigital electrode; and S3, pressing In particles on the interdigital electrode to obtain the ZnMgO ultraviolet detector with the MSM structure. Compared with the priorart, the oxygen flow is increased, the oxygen partial pressure is increased, and oxygen defects are reduced; the prepared ZnMgO film has the characteristics of high crystallization quality, no phasesplitting, steep absorption cut-off edge and the like, and the ZnMgO film 2 with the mixed-phase structure can simultaneously meet high responsivity and low dark current, so that the ZnMgO ultravioletphotoelectric detector has lower dark current and higher photoresponse speed.

ZnMgO ultraviolet detector and preparation method thereof

The invention provides a preparation method of a high-performance mixed-phase ZnMgO film. A substrate is put into a growth cavity, an organic zinc compound is used as a zinc source, an organic magnesium compound is used as a magnesium source, high-purity oxygen is used as an oxygen source, the ZnMgO thin film grows under the high-temperature condition, growth of the high-quality ZnMgO thin film isachieved through accurate control over the growth temperature, the zinc source, the magnesium source and the oxygen flow, and a good material foundation is laid for preparing a corresponding high-performance ultraviolet photoelectric device.

Hou Qichao

Papers

Self-powered solar-blind ZnGa2O4 UV photodetector with ultra-fast response speed

A high performance self-powered ultraviolet photodetector based on a p-GaN/n-ZnMgO heterojunction

Fabrication and Characteristics of MgZnO Ultraviolet Detector Based on Ag Microporous Array Structure Electrode

ZnMgO ultraviolet detector and preparation method thereof

Preparation method of high-performance mixed-phase ZnMgO film and ZnMgO film