A new approach for fabrications of SiC based photodetectors

Detection of Ultraviolet Light by Graphene Oxide Derived from Epitaxial Graphene on SiC and Graphite

Because of their tunable band gap, flexibility, and high surface-to-volume ratio, two-dimensional materials have appeared as the most promising materials for ultraviolet (UV) light sensors. Here, we report the detection of UV light by oxidized epitaxial graphene (EG) formed on the Si-face of the SiC substrate and graphene oxide (GO) produced by Hummer oxidation of graphite. Both epitaxial graphene oxide (EGO) and GO were characterized by Raman and X-ray photoelectron spectroscopy, and the devices were made simply by placing two parallel copper electrodes onto the graphene oxide layers. Irradiation of UV light onto the graphene oxides was realized by the real-time current measurements between two electrodes at a fixed bias of 1 V. The sudden upward jump of the current (Ids) upon UV light irradiation was observed in both EGO- and GO-based devices, which were returned to the original value, while the UV source was turned OFF. The photocurrent (I ph), the magnitude of the current jump by the UV irradiation, for EGO, was estimated at 8 mA with a channel distance of 2 mm and UV power of 80 mW/cm2. The I phlinearly increases with UV power. In the case of GO, I phwas estimated at 0.2 nA with a similar setup. The photoresponse time and responsivity for EGO are ∼11 s and 5.6 A/W, respectively, which are higher than those of GO. The quantum efficiencies (η) for EGO and GO are calculated as 1907 and 2.3 × 10-6 %, respectively, with an incident power of UV light at 9 mW/cm-2. Because of the advantages of the EG on SiC concerning the stability and wafer scale growth, the present study is expected to lead the development of lab-on-chip-based ultrasensitive UV sensors.

Highly Emissive Lanthanide-Based 0D Metal Halide Nanocrystals for Efficient Ultraviolet Photodetector

Recently, lanthanide-based 0D metal halides have attracted considerable attention for their applications in X-ray imaging, light-emitting diodes (LEDs), sensors, and photodetectors. Herein, lead-free 0D gadolinium-alloyed cesium cerium chloride (Gd3+-alloyed Cs3CeCl6) nanocrystals (NCs) are introduced as promising materials for optoelectronic application owing to their unique optical properties. The incorporation of Gd3+ in Cs3CeCl6 (CCC) NCs is proposed to increase the photoluminescence quantum yield (PLQY) from 57% to 96%, along with significantly enhanced phase and chemical stability. The structural analysis is performed by density functional theory (DFT) to confirm the effect of Gd3+ in Cs3Ce1- xGdxCl6 (CCGC) alloy system. Moreover, the CCGC NCs are applied as the active layer in UVPDs with different Gd3+ concentration. The excellent device performance is shown at 20% of Gd3+ in CCGC NCs with high detectivity (7.938 × 1011 Jones) and responsivity (0.195 A W-1) at -0.1 V at 310 nm. This study paves the way for the development of lanthanide-based metal halide NCs for next-generation UVPDs and other optoelectronic applications.

Fully Reversible and Super-Fast Photo-Induced Morphological Transformation of Nanofilms for High-Performance UV Detection and Light-Driven Actuators

Flexible and highly ultraviolet (UV) sensitive materials garner considerable attention in wearable devices, adaptive sensors, and light-driven actuators. Herein, a type of nanofilms with unprecedented fully reversible UV responsiveness are successfully constructed. Building upon this discovery, a new system for ultra-fast, sensitive, and reliable UV detection is developed. The system operates by monitoring the displacement of photoinduced macroscopic motions of the nanofilms based composite membranes. The system exhibits exceptional responsiveness to UV light at 375 nm, achieving remarkable response and recovery times of < 0.3 s. Furthermore, it boasts a wide detection range from 2.85 µW cm-2 to 8.30 mW cm-2, along with robust durability. Qualitative UV sensing is accomplished by observing the shape changes of the composite membranes. Moreover, the composite membrane can serve as sunlight-responsive actuators for artificial flowers and smart switches in practical scenarios. The photo-induced motion is ascribed to the cis-trans isomerization of the acylhydrazone bonds, and the rapid and fully reversible shape transformation is supposed to be a synergistic result of the instability of the cis-isomers acylhydrazone bonds and the rebounding property of the networked nanofilms. These findings present a novel strategy for both quantitative and qualitative UV detection.

Characteristics of the Discoloration Switching Phenomenon of 4H-SiC Single Crystals Grown by PVT Method Using ToF-SIMS and Micro-Raman Analysis

The discoloration switching appearing in the initial and final growth stages of 4H-silicon carbide (4H-SiC) single crystals grown using the physical vapor transport (PVT) technique was investigated. This phenomenon was studied, investigating the correlation with linear-type micro-pipe defects on the surface of 4H-SiC single crystals. Based on the experimental results obtained using time-of-flight secondary ion mass spectrometry (ToF-SIMS) and micro-Raman analysis, it was deduced that the orientation of the 4H-SiC c-axis causes an axial change that correlates with low levels of carbon. In addition, it was confirmed that the incorporation of additional elements and the concentrations of these doped impurity elements were the main causes of discoloration and changes in growth orientation. Overall, this work provides guidelines for evaluating the discoloration switching in 4H-SiC single crystals and contributes to a greater understanding of this phenomenon.

Epitaxy of (11-22) AlN Films on a Sputtered Buffer Layer with Different Annealing Temperatures via Hydride Vapour Phase Epitaxy

AlN epilayers were grown on magnetron-sputtered (MS) (11-22) AlN buffers on m-plane sapphire substrates at 1450 °C via hydride vapour phase epitaxy (HVPE). The MS buffers were annealed at high temperatures of 1400-1600 °C. All the samples were characterised using X-ray diffraction, atomic force microscopy, scanning electron microscope and Raman spectrometry. The crystal quality of epilayers regrown by HVPE was improved significantly compared to that of the MS counterpart. With an increasing annealing temperature, the crystal quality of both MS buffers and AlN epilayers measured along [11-23] and [1-100] improved first and then decreased, maybe due to the decomposition of MS buffers, while the corresponding anisotropy along the two directions decreased first and then increased. The optimum quality of the AlN epilayer was obtained at the annealing temperature of around 1500 °C. In addition, it was found that the anisotropy for the epilayers decreased significantly compared to that of annealed MS buffers when the annealing temperature was below 1500 °C.

Deep-Ultraviolet Transparent Electrode Design for High-Performance and Self-Powered Perovskite Photodetector

In this study, a highly crystalline and transparent indium-tin-oxide (ITO) thin film was prepared on a quartz substrate via RF sputtering to fabricate an efficient bottom-to-top illuminated electrode for an ultraviolet C (UVC) photodetector. Accordingly, the 26.6 nm thick ITO thin film, which was deposited using the sputtering method followed by post-annealing treatment, exhibited good transparency to deep-UV spectra (67% at a wavelength of 254 nm), along with high electrical conductivity (11.3 S/cm). Under 254 nm UVC illumination, the lead-halide-perovskite-based photodetector developed on the prepared ITO electrode in a vertical structure exhibited an excellent on/off ratio of 1.05 × 104, a superb responsivity of 250.98 mA/W, and a high specific detectivity of 4.71 × 1012 Jones without external energy consumption. This study indicates that post-annealed ITO ultrathin films can be used as electrodes that satisfy both the electrical conductivity and deep-UV transparency requirements for high-performance bottom-illuminated optoelectronic devices, particularly for use in UVC photodetectors.

Flexible All-Inorganic Perovskite Photodetector with a Combined Soft-Hard Layer Produced by Ligand Cross-Linking

Although perovskite nanocrystals have attracted considerable interests as emerging semiconductors in optoelectronic devices, design and fabrication of a deformable structure with high stability and flexibility while meeting the charge transport requirements remain a huge challenge. Herein, a combined soft-hard strategy is demonstrated to fabricate intrinsically flexible all-inorganic perovskite layers for photodetection via ligand cross-linking. Perfluorodecyltrichlorosilane (FDTS) is employed as the capping ligand and passivating agent bound to the CsPbBr₃ surface via Pb-F and Br-F interactions. The SiCl head groups of FDTS are hydrolyzed to produce SiOH groups which subsequently condense to form the SiOSi network. The CsPbBr₃ @FDTS nanocrystals (NCs) are monodispersed cubes with an average particle size of 13.03 nm and exhibit excellent optical stability. Furthermore, the residual hydroxyl groups on the surface of the CsPbBr₃ @FDTS render the NCs tightly packed and cross-linked to each other to form a dense and elastic CsPbBr₃ @FDTS film with soft and hard components. The photodetector based on the flexible CsPbBr₃ @FDTS film exhibits outstanding mechanical flexibility and robust stability after 5000 bending cycles.

High-Performance Ultraviolet Photodetector Based on Single-Crystal Integrated Self-Supporting 4H-SiC Nanohole Arrays

Currently, the photodetectors (PDs) assembled by vertically aligned nanostructured arrays have attracted intensive interest owing to their unique virtues of low light reflectivity and rapid charge transport. However, in terms of the inherent limitations caused by numerous interfaces often existed within the assembled arrays, the photogenerated carriers cannot be effectively separated, thus weakening the performance of target PDs. Aiming at resolving this critical point, a high-performance ultraviolet (UV) PD with a single-crystal integrated self-supporting 4H-SiC nanohole arrays is constructed, which are prepared via the anode oxidation approach. As a result, the PD delivers an excellent performance with a high switching ratio (∼250), remarkable detectivity (6 × 10¹⁰ Jones), fast response (0.5 s/0.88 s), and excellent stability under 375 nm light illumination with a bias voltage of 5 V. Moreover, it has a high responsivity (824 mA/W), superior to those of most reported ones based on 4H-SiC. The overall high performance of the PDs could be mainly attributed to the synergistic effect of the SiC nanohole arrays' geometry, a whole single-crystal integrated self-supporting film without interfaces, established reliable Schottky contact, and incorporated N dopants.

Ultraviolet Photodetectors: From Photocathodes to Low-Dimensional Solids

The paper presents the long-term evolution and recent development of ultraviolet photodetectors. First, the general theory of ultraviolet (UV) photodetectors is briefly described. Then the different types of detectors are presented, starting with the older photoemission detectors through photomultipliers and image intensifiers. More attention is paid to silicon and different types of wide band gap semiconductor photodetectors such as AlGaN, SiC-based, and diamond detectors. Additionally, Ga2O3 is considered a promising material for solar-blind photodetectors due to its excellent electrical properties and a large bandgap energy. The last part of the paper deals with new UV photodetector concepts inspired by new device architectures based on low-dimensional solid materials. It is shown that the evolution of the architecture has shifted device performance toward higher sensitivity, higher frequency response, lower noise, and higher gain-bandwidth products.

Hexagonal Boron Nitride for Photonic Device Applications: A Review

Hexagonal boron nitride (hBN) has emerged as a key two-dimensional material. Its importance is linked to that of graphene because it provides an ideal substrate for graphene with minimal lattice mismatch and maintains its high carrier mobility. Moreover, hBN has unique properties in the deep ultraviolet (DUV) and infrared (IR) wavelength bands owing to its indirect bandgap structure and hyperbolic phonon polaritons (HPPs). This review examines the physical properties and applications of hBN-based photonic devices that operate in these bands. A brief background on BN is provided, and the theoretical background of the intrinsic nature of the indirect bandgap structure and HPPs is discussed. Subsequently, the development of DUV-based light-emitting diodes and photodetectors based on hBN's bandgap in the DUV wavelength band is reviewed. Thereafter, IR absorbers/emitters, hyperlenses, and surface-enhanced IR absorption microscopy applications using HPPs in the IR wavelength band are examined. Finally, future challenges related to hBN fabrication using chemical vapor deposition and techniques for transferring hBN to a substrate are discussed. Emerging techniques to control HPPs are also examined. This review is intended to assist researchers in both industry and academia in the design and development of unique hBN-based photonic devices operating in the DUV and IR wavelength regions.

Achieving High Responsivity and Detectivity in a Quantum-Dot-in-Perovskite Photodetector

This work reports on quantum dots (QDs) in perovskite photodetectors showing high optoelectronic performance via quantum-dot-assisted charge transmission. The self-powered broad-band photodetector constructed with SnS QDs in FAPb_0.5Sn_0.5I₃ perovskite can capture incoming optical signals directly at zero bias. The QDs-in-perovskite photodetector exhibits a high sensitivity in the wavelength range from 300 to 1000 nm. Its responsivity at 850 nm reaches 521.7 mA W^-1, and a high specific detectivity of 2.57 × 10¹² jones can be achieved, which is well beyond the level of previous self-powered broad-band photodetectors. The capability of quantum-dot-in-perovskite photodetectors as data receivers has been further demonstrated in a visible-light communication application.

Self-powered UV photodetectors and imaging arrays based on NiO/IGZO heterojunctions fabricated at room temperature

Self-powered UV photodetectors and imaging arrays based on p-type NiO/n-type InGaZnO (IGZO) heterojunctions are fabricated at room temperature by using ratio-frequency magnetron sputtering. The p-n heterojunction exhibits typical rectifying characteristics with a rectification ratio of 7.4×10⁴ at a ±4 V applied bias. A high photo-responsivity of 28.8 mA/W is observed under zero bias at a wavelength of 365 nm. The photodetector possesses a fast response time of 15 ms which is among the best in reported oxide-based p-n junction-based UV photodetectors. Finally, recognition of an "H" pattern is demonstrated by a 10×10 photodetector array at zero bias. The results indicate that the NiO/IGZO based photodetectors may have a great potential in constructing large-scale self-powered UV imaging systems.

Large-Scale Synthesis h-BN Films on Copper-Nickel Alloy by Atmospheric Pressure Chemical Vapor Deposition

Due to its high thermal and chemical stability, excellent dielectric properties, unique optical properties, corrosion resistance, and oxidation resistance, the two-dimensional hexagonal boron nitride (h-BN) is often used in a thermal conductor protective layer in deep ultraviolet light-emitting detector fields. However, due to the complicated growth conditions of h-BN, it is often necessary to prepare h-BN by the CVD method in a high vacuum environment, which is limited to a certain extent in terms of film size and production cost. In order to solve this problem, we proposed a method to prepare h-BN thin films by atmospheric CVD (APCVD). This method does not need a vacuum environment, which reduces energy consumption and cost, and makes the operation simpler and the experimental environment safer. The preparation of high-quality h-BN film was carried out using a Cu-Ni alloy as the growth substrate. The growth process of h-BN film was studied, and the influence of growth parameters on the structure of the h-BN film was explored. The morphological features and elemental composition pairs of the samples were characterized and analyzed, which confirmed that the high-quality h-BN film could be successfully grown on the Cu-Ni alloy substrate by APCVD. The UV detection device prepared by using the prepared h-BN film as the photoresponse material had good photoresponse characteristics and performance stability. It provides a new idea for the low-cost preparation of large-scale h-BN.

Recent progress of heterostructures based on two dimensional materials and wide bandgap semiconductors

Recent progress in the synthesis and assembly of two-dimensional (2D) materials has laid the foundation for various applications of atomically thin layer films. These 2D materials possess rich and diverse properties such as layer-dependent band gaps, interesting spin degrees of freedom, and variable crystal structures. They exhibit broad application prospects in micro-nano devices. In the meantime, the wide bandgap semiconductors (WBS) with an elevated breakdown voltage, high mobility, and high thermal conductivity have shown important applications in high-frequency microwave devices, high-temperature and high-power electronic devices. Beyond the study on single 2D materials or WBS materials, the multi-functional 2D/WBS heterostructures can promote the carrier transport at the interface, potentially providing novel physical phenomena and applications, and improving the performance of electronic and optoelectronic devices. In this review, we overview the advantages of the heterostructures of 2D materials and WBS materials, and introduce the construction methods of 2D/WBS heterostructures. Then, we present the diversity and recent progress in the applications of 2D/WBS heterostructures, including photodetectors, photocatalysis, sensors, and energy related devices. Finally, we put forward the current challenges of 2D/WBS heterostructures and propose the promising research directions in the future.

High-Performance Flexible Ultraviolet Photodetectors Based on Facilely Synthesized Ecofriendly ZnAl:LDH Nanosheets

Recent studies have focused on the development of efficient, flexible, and highly sensitive ultraviolet photodetectors (UV PDs) with various wide band-gap materials. In the present study, the application of environmentally friendly zinc-aluminum layered double hydroxide (ZnAl-CO₃:LDH) is demonstrated for a high-performance, flexible UV PD. The vertically oriented ZnAl:LDH nanosheets (ZnAl:LDH Ns) are facilely synthesized by dipping the sputtered 10 wt % aluminum-doped zinc oxide thin films in deionized water at room temperature. Without passivation, the UV PDs exhibit an exceptional light-to-dark current ratio of 10⁴ and a responsivity of ∼34.7 mA/W at a bias of 1 V. Moreover, the spectral responsivity and detectivity are enhanced to ∼148.3 mA/W and 2.5 × 10¹² Jones, respectively, by passivating the ZnAl:LDH Ns with polydimethylsiloxane (PDMS), thus making the device suitable for application in UV detectors. In addition, the ambient atmosphere effect on PD performance, which elucidates the clear understanding of the PD working mechanism, is also investigated. The passivation of the Ns by PDMS also helps to enhance the mechanical robustness and long-term stability of the PD. The methodology demonstrated herein highlights the potential of the ZnAl:LDH material in realizing the next generation of flexible UV PDs.

Effects of Seed-Layer N2O Plasma Treatment on ZnO Nanorod Based Ultraviolet Photodetectors: Experimental Investigation with Two Different Device Structures

The crystalline quality of ZnO NR (nanorod) as a sensing material for visible blind ultraviolet PDs (photodetectors) critically depends on the SL (seed layer) material of properties, which is a key to high-quality nanocrystallite growth, more so than the synthesis method. In this study, we fabricated two different device structures of a gateless AlGaN/GaN HEMT (high electron mobility transistor) and a photoconductive PD structure with an IDE (interdigitated electrode) pattern implemented on a PET (polyethylene terephthalate) flexible substrate, and investigated the impact on device performance through the SL N₂O plasma treatment. In case of HEMT-based PD, the highest current on-off ratio (~7) and spectral responsivity R (~1.5 × 10⁵ A/W) were obtained from the treatment for 6 min, whereas the IDE pattern-based PD showed the best performance (on-off ratio = ~44, R = ~69 A/W) from the treatment for 3 min and above, during which a significant etch damage on PET substrates was produced. This improvement in device performance was due to the enhancement in NR crystalline quality as revealed by our X-ray diffraction, photoluminescence, and microanalysis.

Photocurrent enhancement of AlxGa1-xN nanowire arrays photodetector based on coupling effects of pn junction and gradient component

Ultraviolet photodetector has a variety of applications in medical diagnosis, civilian testing and military security. The enhancement of photo response has far been a hot topic regrading to the performance improvement of the devices. In this study, we proposed a self-powered photodetector based on Al_xGa_1-xN nanowire arrays (NWAs) utilizing axial pn junction integrating with gradient Al component. The merit of the coupling structure is demonstrated by theoretical model and simulations. The photoelectric conversion model is built based on a continuity equation derived by its corresponding boundary conditions. The photocurrent for a single nanowire and NWAs are respectively obtained. According to the simulation results of a single nanowire, the optimal nanowire height is obtained with a photocurrent enhancement up to 330%. For NWAs, the aspect ratio of NWAs and incident angle of light synergistically determine the output photocurrent. The optimal aspect ratio for NWAs is 1:1 with an optimal incident angle of 57°. This study provides a reliable method for the design of photodetectors with micro-nano structures.

Investigating the growth of AlGaN/AlN heterostructure by modulating the substrate temperature of AlN buffer layer

We have investigated the impact of AlN buffer layer growth parameters for developing highly single crystalline AlGaN films. The low mobility of Al adatoms and high temperature for compound formation are amongst the major causes that affects the growth quality of AlGaN films. Thus, proper optimization need to be carried out for achieving high quality AlGaN due to an augmented tendency of defect generation compared to GaN films. Thus, growth conditions need to be amended to maximize the incorporation ability of adatoms and minimize defect density. So, this study elaborates the growth optimization of AlGaN/AlN/Si (111) heterostructure with varied AlN buffer growth temperature (760 to 800 °C). It was observed that the remnant Al in low temperature growth of AlN buffer layer resist the growth quality of AlGaN epitaxial films. A highly single crystalline AlGaN film with comparatively lowest rocking curve FWHM value (~ 0.61°) and smooth surface morphology with least surface defect states was witnessed when AlN buffer was grown at 780 °C. From the Vegard’s law, the photoluminescence analysis unveils Aluminium composition of 31.5% with significantly reduced defect band/NBE band ratio to 0.3. The study demonstrates good crystalline quality AlGaN film growth with Aluminium content variation between ~ 30–39% in AlGaN/AlN heterostructure on Si(111) substrate leading to a bandgap range which is suitable for next-generation solar-blind photodetection applications.

Bandgap-Tuned 2D Boron Nitride/Tungsten Nitride Nanocomposites for Development of High-Performance Deep Ultraviolet Selective Photodetectors

This study presents a fast and effective method to synthesize 2D boron nitride/tungsten nitride (BN–WN) nanocomposites for tunable bandgap structures and devices. A few minutes of synthesis yielded a large quantity of high-quality 2D nanocomposites, with which a simple, low-cost deep UV photo-detector (DUV-PD) was fabricated and tested. The new device was demonstrated to have very good performance. High responsivity up to 1.17 A/W, fast response-time of lower than two milliseconds and highly stable repeatability were obtained. Furthermore, the influences of operating temperature and applied bias voltage on the properties of DUV-PD as well as its band structure shift were investigated.

Fabrication of Ultraviolet Photodetectors Based on Fe-Doped ZnO Nanorod Structures

In this paper, 100 nm-thick zinc oxide (ZnO) films were deposited as a seed layer on Corning glass substrates via a radio frequency (RF) magnetron sputtering technique, and vertical well-aligned Fe-doped ZnO (FZO) nanorod (NR) arrays were then grown on the seed layer-coated substrates via a low-temperature solution method. FZO NR arrays were annealed at 600 °C and characterized by using field emission scanning microscopy (FE-SEM) and X-ray diffraction spectrum (XRD) analysis. FZO NRs grew along the preferred (002) orientation with good crystal quality and hexagonal wurtzite structure. The main ultraviolet (UV) peak of 378 nm exhibited a red-shifted phenomenon with Fe-doping by photoluminescence (PL) emission. Furthermore, FZO photodetectors (PDs) based on metal-semiconductor-metal (MSM) structure were successfully manufactured through a photolithography procedure for UV detection. Results revealed that compared with pure ZnO NRs, FZO NRs exhibited a remarkable photosensitivity for UV PD applications and a fast rise/decay time. The sensitivities of prepared pure ZnO and FZO PDs were 43.1, and 471.1 for a 3 V applied bias and 380 nm UV illumination, respectively.

Controlling Defects in Continuous 2D GaS Films for High-Performance Wavelength-Tunable UV-Discriminating Photodetectors

A chemical vapor deposition method is developed for thickness-controlled (one to four layers), uniform, and continuous films of both defective gallium(II) sulfide (GaS): GaS_0.87 and stoichiometric GaS. The unique degradation mechanism of GaS_0.87 with X-ray photoelectron spectroscopy and annular dark-field scanning transmission electron microscopy is studied, and it is found that the poor stability and weak optical signal from GaS are strongly related to photo-induced oxidation at defects. An enhanced stability of the stoichiometric GaS is demonstrated under laser and strong UV light, and by controlling defects in GaS, the photoresponse range can be changed from vis-to-UV to UV-discriminating. The stoichiometric GaS is suitable for large-scale, UV-sensitive, high-performance photodetector arrays for information encoding under large vis-light noise, with short response time (<66 ms), excellent UV photoresponsivity (4.7 A W^-1 for trilayer GaS), and 26-times increase of signal-to-noise ratio compared with small-bandgap 2D semiconductors. By comprehensive characterizations from atomic-scale structures to large-scale device performances in 2D semiconductors, the study provides insights into the role of defects, the importance of neglected material-quality control, and how to enhance device performance, and both layer-controlled defective GaS_0.87 and stoichiometric GaS prove to be promising platforms for study of novel phenomena and new applications.

Nanoplasmonic 1D Diamond UV Photodetectors with High Performance

Diamond nanowires have recently drawn substantial attention because of their unique physical and chemical properties for electrochemical sensors, optoelectronics, and nanophotonics applications. However, diamond nanowire-based ultraviolet photodetectors have not been reported because of the challenges involved in synthesizing crystalline diamond nanowires with controllable morphologies and, more fundamentally, the material's high carrier concentration with low mobilities that limits the obtainable photoresponsivity. The synergetic integration of ultrananocrystalline diamond (UNCD) nanowires with nanoplasmonic enhancement by noble metal nanoparticles is a very promising approach to overcome these shortcomings. Here we report the fabrication of boron-doped ultrananocrystalline diamond nanowires functionalized with the platinum nanoparticles to form self-powered ultraviolet photodetectors that exhibit an ultrahigh photoresponsivity of 388 Amp/Watt at 300 nm wavelength, a fast response time around 20 ms, and a good UV/visible rejection ratio of about 5 orders of magnitude under zero-bias condition.

Zinc oxide ultraviolet photodetectors: rapid progress from conventional to self-powered photodetectors

Currently, the development of ultraviolet (UV) photodetectors (PDs) has attracted the attention of the research community because of the vast range of applications of photodetectors in modern society. A variety of wide-band gap nanomaterials have been utilized for UV detection to achieve higher photosensitivity. Specifically, zinc oxide (ZnO) nanomaterials have attracted significant attention primarily due to their additional properties such as piezo-phototronic and pyro-phototronic effects, which allow the fabrication of high-performance and low power consumption-based UV PDs. This article primarily focuses on the recent development of ZnO nanostructure-based UV PDs ranging from nanomaterials to architectural device design. A brief overview of the photoresponse characteristics of UV PDs and potential ZnO nanostructures is presented. Moreover, the recent development in self-powered PDs and implementation of the piezo-phototronic effect, plasmonic effect and pyro-phototronic effect for performance enhancement is highlighted. Finally, the research perspectives and future research direction related to ZnO nanostructures for next-generation UV PDs are summarized.

Super Stable Pollution Gas Sensor Based on Functionalized 2D Boron Nitride Nanosheet Materials for High Humidity Environments

We report on studies of new gas sensing devices to be used in high humidity environments. Highly thermal-stable, super hydrophobic 2-dimensional (2D) boron nitride nanosheets (BNNSs) functionalized with Pt nanoparticles were prepared and used as an active layer for the prototype. The morphologic surface, crystallographic structures and chemical compositions of the synthesized 2D materials were characterized by using optical microscope, scanning electron microscope (SEM), transmission electron microscope (TEM) and atomic force microscope (AFM) and Raman scattering, respectively. The experimental data reveals that high-quality BNNSs were prepared. A pair of Au electrodes were combined with a basic electrical circuit and the 2D sensing material to form high-performance gas sensors for the detection of pollution gases. The present structure is simple and the fabrication is easy and fast, which ensures the creation of a low-cost prototype with harsh (high humidity, high temperature) environment resistance and potential for miniaturization. The responses of the prototype to different target gases with different concentrations were characterized. The influences of the operating temperature and bias voltage effect on sensing performances were also investigated. The fabricated sensors appear to have high selectivity, high sensitivity and fast response to target gases. The sensing mechanism in the present case is attributed to the electron donation from the target gas molecules to the active layer, leading to the change of electrical properties on the surface of BNNS layer.

Low temperature-processed ZnO thin films for p-n junction-based visible-blind ultraviolet photodetectors

Ultraviolet (UV) photodetectors have drawn extensive attention due to their numerous applications in both civilian and military areas including flame detection, UV sterilization, aerospace UV monitoring, missile early warning, and ultraviolet imaging. Zinc oxide (ZnO)-based UV detectors exhibit remarkable performance; however, many of them are not visible-blind, and the fabrication techniques involve a high-temperature annealing step. Here, we fabricated a p-n junction photodiode based on annealing-free ZnO thin films prepared from ZnO nanoparticles and N,N'-di(1-naphthyl)-N,N'-diphenyl-(1,1'-biphenyl)-4,4'-diamine (NPB). NPB was chosen due to its transparent nature in the visible region and high hole mobility. The ZnO nanoparticles and thin films were characterized by UV-visible absorption spectroscopy, atomic force microscopy (AFM), scanning electron microscopy (SEM), dynamic light scattering (DLS) particle size analysis, Fourier-transform infrared (FTIR) spectroscopy, photoluminescence spectroscopy, XRD and profilometry. The device exhibited responsivity of 0.037 A/W and an external quantum efficiency (EQE) of 12.86% at 5 V bias under 360 nm illumination. In addition, with no biasing, the device exhibited an on-off ratio of more than 103 and a linear dynamic range (LDR) of 63 dB. A high built-in potential at the ZnO/NPB interface could be the reason for this performance at zero bias. The rise and fall times were 156 ms and 319 ms, respectively. The results suggest that a visible-blind UV photodetector with acceptable performance can be fabricated using annealing-free ZnO films, which may lead to the realization of flexible detectors due to the low-temperature processes involved.

Photoelectric Detectors Based on Inorganic p-Type Semiconductor Materials

Photoelectric detectors are the central part of modern photodetection systems with numerous commercial and scientific applications. p-Type semiconductor materials play important roles in optoelectronic devices. Photodetectors based on p-type semiconductor materials have attracted a great deal of attention in recent years because of their unique properties. Here, a comprehensive summary of the recent progress mainly on photodetectors based on inorganic p-type semiconductor materials is presented. Various structures, including photoconductors, phototransistors, homojunctions, heterojunctions, p-i-n junctions, and metal-semiconductor junctions of photodetectors based on inorganic p-type semiconductor materials, are discussed and summarized. Perspectives and an outlook, highlighting the promising future directions of this research field, are also given.

Ultrarapid Multimode Microwave Synthesis of Nano/Submicron β-SiC

This paper presents the design, development and realization of a fast and novel process for the synthesis of 3C silicon carbide (β-SiC) nanorods and submicron powder. Using SiO₂ (or Si) and activated carbon (AC), this process allows β-SiC to be synthesized with almost 100% purity in timeframes of seconds or minutes using multimode microwave rotary tube reactors under open-air conditions. The synthesis temperature used was 1460 ± 50 °C for Si + AC and 1660 ± 50 °C for SiO₂ + AC. The shortest β-SiC synthesis time achieved was about 20 s for Si + AC and 100 s for SiO₂ + AC. This novel synthesis method allows for scaled-up flow processes in the rapid industrial-scale production of β-SiC, having advantages of time/energy saving and carbon dioxide emission reduction over comparable modern processes.

A compact design of a characterization station for far UV photodetectors

A newly fabricated characterization station is presented. It is a compact, cost-effective, and easily adjustable apparatus. Each part including 4-pin probe, manipulators, operating temperature, and applied bias can be independently controlled. The station can provide highly reliable, reproducible, and economical methods to quickly conduct and complete the characterizations of a large amount of sensing materials within a short period of time. It is particularly suitable for studies of various nanostructured materials and their related thermal effect, polarization effect, sensitivity, and electrical and electronic properties.

High-Performance and Self-Powered Deep UV Photodetectors Based on High Quality 2D Boron Nitride Nanosheets

High-quality two-dimensional (2D) crystalline boron nitride nanosheets (BNNSs) were grown on silicon wafers by using pulsed plasma beam deposition techniques. Self-powered deep ultraviolet (DUV) photodetectors (PDs) based on BNNSs with Schottky contact structures are designed and fabricated. By connecting the fabricated DUV photodetector to an ammeter, the response strength, response time and recovery time to different DUV wavelengths at different intensities have been characterized using the output short circuit photocurrent without a power supply. Furthermore, effects of temperature and plasma treatment on the induced photocurrent response of detectors have also been investigated. The experimental data clearly indicate that plasma treatment would significantly improve both induced photocurrent and response time. The BNNS-based DUV photodetector is demonstrated to possess excellent performance at a temperature up to 400 °C, including high sensitivity, high signal-to-noise ratio, high spectral selectivity, high speed, and high stability, which is better than almost all reported semiconducting nanomaterial-based self-powered photodetectors.

Deep Ultraviolet Copper(I) Thiocyanate (CuSCN) Photodetectors Based on Coplanar Nanogap Electrodes Fabricated via Adhesion Lithography

Adhesion lithography (a-Lith) is a versatile fabrication technique used to produce asymmetric coplanar electrodes separated by a <15 nm nanogap. Here, we use a-Lith to fabricate deep ultraviolet (DUV) photodetectors by combining coplanar asymmetric nanogap electrode architectures (Au/Al) with solution-processable wide-band-gap (3.5-3.9 eV) p-type semiconductor copper(I) thiocyanate (CuSCN). Because of the device's unique architecture, the detectors exhibit high responsivity (≈79 A W^-1) and photosensitivity (≈720) when illuminated with a DUV-range (λ_peak = 280 nm) light-emitting diode at 220 μW cm^-2. Interestingly, the photosensitivity of the photodetectors remains fairly high (≈7) even at illuminating intensities down to 0.2 μW cm^-2. The scalability of the a-Lith process combined with the unique properties of CuSCN paves the way to new forms of inexpensive, yet high-performance, photodetectors that can be manufactured on arbitrary substrate materials including plastic.

Ultrahigh Detectivity and Wide Dynamic Range Ultraviolet Photodetectors Based on BixSn1-xO2 Intermediate Band Semiconductor

The ultraviolet (UV) photodetectors have significant applications different fields. High detectivity, high responsivity and wide active area are required to probe a weak UV light in actual ambient. Unfortunately, most practical UV photoconductors based on wide bandgap semiconductor films can hardly have both a high responsivity and a low dark current density. In this study, the intermediate band engineering in semiconductor has been proposed try to solve this problem. The intermediate band UV photodetectors based on Bi_xSn_1-xO₂ (0.017 < x < 0.041) films show a detectivity of 6.1 × 10¹⁵ Jones at 280 nm and a quantum efficiency of 2.9 × 10⁴ %. The dynamic range is 195 dB, which is much higher than other UV photodetector. The recovery time is about 1 s after exposing device into ethanol steam. Our results demonstrate that the intermediate band semiconductor Bi_xSn_1-xO₂ films can serve as a high performance UV photodetector.

High Operating Temperature and Low Power Consumption Boron Nitride Nanosheets Based Broadband UV Photodetector

We extend our work on the use of digitally controlled pulsed laser plasma deposition (PLPD) technique to synthesize high quality, 2-dimensional single crystalline boron nitride nanosheets (BNNSs) at a low substrate temperature for applications in high-performance deep UV photodetectors. The obtained sample consists of a large amount of BNNSs partially overlapping one another with random orientations. Each sheet is composed of a few (from 2 to 10) stacked atomic layers exhibiting high transparency due to its highly ordered hBN crystallinity. Deep UV detectors based on the obtained BNNSs were designed, fabricated, and tested. The bias and temperature effects on the photocurrent strength and the signal-to-noise ratio have been carefully characterized and discussed. A significant shift in the cut off wavelength of the BNNSs based photodetectors was observed suggesting a band gap reduction as a result of the BNNSs’ collective structure. The newly designed photodetector presented exceptional properties: a high sensitivity to weak intensities of radiation in both UVC and UVB range while remaining visible-blind, and a high signal-to-noise ratio operation even at temperatures as high as 400 °C. In addition, the BNNSs based photodetector exhibited potential for self-powered operation.

Low-Voltage High-Performance UV Photodetectors: An Interplay between Grain Boundaries and Debye Length

Accurate detection of UV light by wearable low-power devices has many important applications including environmental monitoring, space to space communication, and defense. Here, we report the structural engineering of ultraporous ZnO nanoparticle networks for fabrication of very low-voltage high-performance UV photodetectors. A record high photo- to dark-current ratio of 3.3 × 10⁵ and detectivity of 3.2 × 10¹² Jones at an ultralow operation bias of 2 mV and low UV-light intensity of 86 μW·cm^-2 are achieved by controlling the interplay between grain boundaries and surface depletion depth of ZnO nanoscale semiconductors. An optimal window of structural properties is determined by varying the particle size of ultraporous nanoparticle networks from 10 to 42 nm. We find that small electron-depleted nanoparticles (≤40 nm) are necessary to minimize the dark-current; however, the rise in photocurrent is tampered with decreasing particle size due to the increasing density of grain boundaries. These findings reveal that nanoparticles with a size close to twice their Debye length are required for high photo- to dark-current ratio and detectivity, while further decreasing their size decreases the photodetector performance.