Temperature-dependent electroluminescence of red high-In-content MQWs of dual-wavelength micro-LED.

Temperature-dependent electroluminescence (TDEL) measurements have been employed to investigate the carrier transport and recombination processes of InGaN red micro-LED based on dual-wavelength InGaN/GaN MQWs structure. EL peak energy and carrier transport of the red micro-LED both show temperature dependence, due to temperature-induced changes in defect activation. In addition, the current density at which the blue peak of the low-In-content appears in the EL spectrum varies with temperature. As the temperature increases, the blue peak of the low In component tends to appear at higher current densities, which may be attributed to the increase in thermally activated defects hindering the injection of holes into the low-In-content MQWs further away from p-GaN. Furthermore, the IQEs of the high-In-content MQWs are estimated from the TDEL method and then reveal the temperature-dependent efficiency droop. The IQE decreases as temperature increases, particularly above 50 K, where it drops sharply due to temperature-dependent nonradiative recombination. And the two different variation trends in IQE of MQWs with high and low In content reveal a competitive mechanism in carrier distribution, implying that more escaping holes from high-In-content MQWs will further reduce red emission efficiency but enhance carrier injection and blue emission in low-In-content MQWs.

Manipulating Surface Band Bending of III-Nitride Nanowires with Ambipolar Charge-Transfer Characteristics: A Pathway Toward Advanced Photoswitching Logic Gates and Encrypted Optical Communication.

The operational principle of semiconductor devices critically relies on the band structures that ultimately govern their charge-transfer characteristics. Indeed, the precise orchestration of band structure within semiconductor devices, notably at the semiconductor surface and corresponding interface, continues to pose a perennial conundrum. Herein, for the first time, this work reports a novel postepitaxy method: thickness-tunable carbon layer decoration to continuously manipulate the surface band bending of III-nitride semiconductors. Specifically, the surface band bending of p-type aluminum-gallium-nitride (p-AlGaN) nanowires grown on n-Si can be precisely controlled by depositing different carbon layers as guided by theoretical calculations, which eventually regulate the ambipolar charge-transfer behavior between the p-AlGaN/electrolyte and p-AlGaN/n-Si interface in an electrolyte environment. Enabled by the accurate modulation of the thickness of carbon layers, a spectrally distinctive bipolar photoresponse with a controllable polarity-switching-point over a wide spectrum range can be achieved, further demonstrating reprogrammable photoswitching logic gates "XOR", "NAND", "OR", and "NOT" in a single device. Finally, this work constructs a secured image transmission system where the optical signals are encrypted through the "XOR" logic operations. The proposed continuous surface band tuning strategy provides an effective avenue for the development of multifunctional integrated-photonics systems implemented with nanophotonics.

Highly Responsive Switchable Broadband DUV-NIR Photodetector and Tunable Emitter Enabled by Uniform and Vertically Grown III-V Nanowire on Silicon Substrate for Integrated Photonics.

Low-dimensional semiconductor nanostructures, particularly in the form of nanowire configurations with large surface-to-volume-ratio, offer intriguing optoelectronic properties for the advancement of integrated photonic technologies. Here, a bias-controlled, superior dual-functional broadband light detecting/emitting diode enabled by constructing the aluminum-gallium-nitride-based nanowire on the silicon-platform is reported. Strikingly, the diode exhibits a stable and high responsivity (R) of over 200 mAW-1 covering an extremely wide operation band under reverse bias conditions, ranging from deep ultraviolet (DUV: 254 nm) to near-infrared (NIR: 1000 nm) spectrum region. While at zero bias, it still possesses superior DUV light selectivity with a high off-rejection ratio of 106. When it comes to the operation of the light-emitting mode under forward bias, it can achieve large spectral changes from UV to red simply by coating colloid quantum dots on the nanowires. Based on the multifunctional features of the diodes, this study further employs them in various optoelectronic systems, demonstrating outstanding applications in multicolor imaging, filterless color discrimination, and DUV/NIR visualization. Such highly responsive broadband photodetector with a tunable emitter enabled by III-V nanowire on silicon provides a new avenue toward the realization of integrated photonics and holds great promise for future applications in communication, sensing, imaging, and visualization.

266 nm ultraviolet communication under unknown interference using UVC micro-LED.

Ultraviolet C (UVC) micro light-emitting diode (LED) can achieve symbol communication rate up to 100Msps at distance 40 meters without transmitter-side lens to guarantee certain communication mobility. We consider what we believe to be a new scenario where high speed UV communciation is realized under unknown low-rate interference. The signal amplitude properties are characterized, and the interference intensity is categorized into three cases, namely weak, medium and high interference intensity. The achievable transmission rates for the three cases are derived, where the achievable transmission rate for medium interference intensity can approach those in weak interference intensity and strong interference intensity cases. We provide Gaussian approximation and related log-likelihood ratio (LLR) calculation, which are fed into the subsequent message-passing decoder. In the experiment, the data is transmitted with symbol rate 20 Msps under unknown interference with symbol rate 1 Msps, received by one photomultiplier tube (PMT). Experimental results show that the proposed interference symbol estimation approach shows negligibly higher bit error rate (BER) compared with those with perfect knowledge on the interference symbols.

Light-Induced Bipolar Photoresponse with Amplified Photocurrents in an Electrolyte-Assisted Bipolar p-n Junction.

The p-n junction with bipolar characteristics sets the fundamental unit to build electronics while its unique rectification behavior constrains the degree of carrier tunability for expanded functionalities. Herein, a bipolar-junction photoelectrode employed with a gallium nitride (GaN) p-n homojunction nanowire array that operates in electrolyte is reported, demonstrating bipolar photoresponse controlled by different wavelengths of light. Significantly, with rational decoration of a ruthenium oxides (RuOx ) layer on nanowires guided by theoretical modeling, the resulting RuOx /p-n GaN photoelectrode exhibits unambiguously boosted bipolar photoresponse by an enhancement of 775% and 3000% for positive and negative photocurrents, respectively, compared to the pristine nanowires. The loading of the RuOx layer on nanowire surface optimizes surface band bending, which facilitates charge transfer across the GaN/electrolyte interface, meanwhile promoting the efficiency of redox reaction for both hydrogen evolution reaction and oxygen evolution reaction which corresponds to the negative and positive photocurrents, respectively. Finally, a dual-channel optical communication system incorporated with such photoelectrode is constructed with using only one photoelectrode to decode dual-band signals with encrypted property. The proposed bipolar device architecture presents a viable route to manipulate the carrier dynamics for the development of a plethora of multifunctional optoelectronic devices for future sensing, communication, and imaging systems.

Anisotropic photoresponse behavior of a LaAlO3 single-crystal-based vacuum-ultraviolet photodetector.

Nowadays, vacuum-ultraviolet (VUV) photodetectors (PDs) have attracted extensive attention owing to their potential applications in space exploration, radiation monitoring, and the semiconductor industry. Benefiting from its intrinsic ultra-wide band-gap, chemical robustness, and low-cost features, LaAlO3 shows great promise in developing next-generation compact, cheap, and easy-to-fabricate VUV PDs. In this work, we report the unique anisotropic photoresponse behavior of LaAlO3 single crystals for VUV photodetection applications. First of all, with the guidance of density functional theory (DFT) calculations along with the comprehensive material characterization, the anisotropic carrier transport behavior of LaAlO3 single crystals was confirmed. Thereafter, after exploring the metal-semiconductor-metal (MSM) device configuration along different substrate orientations, including (100), (110), and (111)-LaAlO3 single crystals, we found that the (110)-LaAlO3 VUV PD exhibits the best device performance under VUV illumination, with a responsivity of 2.23 mA W-1, a high detectivity of 3.72 × 1011 Jones, and a photo-to-dark-current ratio of 5.48 × 103. This work not only provides a feasible avenue to explore the anisotropic optoelectronic behavior of ultra-wide band-gap semiconductors but also expands the application of the low-cost oxide perovskite family in the field of VUV photodetection.

AlGaN Quantum Disk Nanorods with Efficient UV-B Emission Grown on Si(111) Using Molecular Beam Epitaxy.

AlGaN nanorods have attracted increasing amounts of attention for use in ultraviolet (UV) optoelectronic devices. Here, self-assembled AlGaN nanorods with embedding quantum disks (Qdisks) were grown on Si(111) using plasma-assisted molecular beam epitaxy (PA-MBE). The morphology and quantum construction of the nanorods were investigated and well-oriented and nearly defect-free nanorods were shown to have a high density of about 2 × 1010 cm-2. By controlling the substrate temperature and Al/Ga ratio, the emission wavelengths of the nanorods could be adjusted from 276 nm to 330 nm. By optimizing the structures and growth parameters of the Qdisks, a high internal quantum efficiency (IQE) of the AlGaN Qdisk nanorods of up to 77% was obtained at 305 nm, which also exhibited a shift in the small emission wavelength peak with respect to the increasing temperatures during the PL measurements.

Observation of polarity-switchable photoconductivity in III-nitride/MoSx core-shell nanowires.

III-V semiconductor nanowires are indispensable building blocks for nanoscale electronic and optoelectronic devices. However, solely relying on their intrinsic physical and material properties sometimes limits device functionalities to meet the increasing demands in versatile and complex electronic world. By leveraging the distinctive nature of the one-dimensional geometry and large surface-to-volume ratio of the nanowires, new properties can be attained through monolithic integration of conventional nanowires with other easy-synthesized functional materials. Herein, we combine high-crystal-quality III-nitride nanowires with amorphous molybdenum sulfides (a-MoSx) to construct III-nitride/a-MoSx core-shell nanostructures. Upon light illumination, such nanostructures exhibit striking spectrally distinctive photodetection characteristic in photoelectrochemical environment, demonstrating a negative photoresponsivity of -100.42 mA W-1 under 254 nm illumination, and a positive photoresponsivity of 29.5 mA W-1 under 365 nm illumination. Density functional theory calculations reveal that the successful surface modification of the nanowires via a-MoSx decoration accelerates the reaction process at the electrolyte/nanowire interface, leading to the generation of opposite photocurrent signals under different photon illumination. Most importantly, such polarity-switchable photoconductivity can be further tuned for multiple wavelength bands photodetection by simply adjusting the surrounding environment and/or tailoring the nanowire composition, showing great promise to build light-wavelength controllable sensing devices in the future.

Development of highly efficient ultraviolet LEDs on hybrid patterned sapphire substrates.

A hybrid patterned sapphire substrate (HPSS) aiming to achieve high-quality Al(Ga)N epilayers for the development of GaN-based ultraviolet light-emitting diodes (UV LEDs) has been prepared. The high-resolution X-ray diffraction measurements reveal that the Al(Ga)N epilayers grown on a HPSS and conventional patterned sapphire substrate (CPSS) have similar structural quality. More importantly, benefiting from the larger refractive index contrast between the patterned silica array and sapphire, the photons can escape from the hybrid substrate with an improved transmittance in the UV band. As a result, in comparison with the UV LEDs grown on the CPSS, the LEDs grown on the HPSS exhibit a significantly enhanced light output power by 14.5% and more than 22.9% higher peak external quantum efficiency, owing to the boost of the light extraction efficiency from the adoption of the HPSS which can be used as a promising substrate to realize high-efficiency and high-power UV LEDs of the future.

Enhanced light extraction of the deep-ultraviolet micro-LED via rational design of chip sidewall.

In this Letter, we perform a comprehensive investigation on the optical characterization of micro-sized deep-ultraviolet (DUV) LEDs (micro-LEDs) emitting below 280 nm, highlighting the light extraction behavior in relation to the design of chip sidewall angle. We found that the micro-LEDs with a smaller inclined chip sidewall angle (∼33∘) have improved external quantum efficiency (EQE) performance 19% more than that of the micro-LEDs with a larger angle (∼75∘). Most importantly, the EQE improvement by adopting an inclined sidewall can be more outstanding as the diameter of the LED chip reduces from 40 to 20 µm. The enhanced EQE of the micro-LEDs with smaller inclined chip sidewall angles can be attributed to the stronger reflection of the inclined sidewall, leading to enhanced light extraction efficiency (LEE). In the end, the numerical optical modeling further reveals and verifies the impact of the sidewall angles on the LEE of the micro-LEDs, corroborating our experiment results. This Letter provides a fundamental understanding of the light extraction behavior with optimized chip geometry to design and fabricate highly efficient micro-LEDs in a DUV spectrum of the future.

The C5 protein encoded by Ageratum leaf curl Sichuan virus is a virulence factor and contributes to the virus infection.

Earlier reports have indicated that begomoviruses encode four proteins (AC1/C1, AC2/C2, AC3/C3, and AC4/C4 proteins) using complementary-sense DNA as the template. In recent years, several reports have shown that some begomoviruses also encode an AC5/C5 protein from the complementary DNA strand, and these AC5/C5 proteins play different roles in virus infections. Here, we provide evidence showing that Ageratum leaf curl Sichuan virus (ALCScV), a monopartite begomovirus, also encodes a C5 protein that is important for disease symptom formation and can affect viral replication. Infection of Nicotiana benthamiana plants with a potato virus X (PVX)-based vector carrying the ALCScV C5 gene resulted in more severe disease symptoms and higher virus accumulation levels. ALCScV C5 protein can be found in the cytoplasm and the nucleus. Furthermore, this protein is also a suppressor of posttranscriptional gene silencing. Mutational analysis showed that knockout of C5 gene expression significantly reduced ALCScV-induced disease symptoms and virus accumulation, while expression of the C5 gene using the PVX-based vector enhanced ALCScV accumulation in coinfected N. benthamiana plants.

AlGaN-based deep ultraviolet micro-LED emitting at 275 nm.

The investigation of electrical and optical properties of micro-scale AlGaN deep ultraviolet (DUV) light-emitting diodes (LEDs) emitting at ∼275nm was carried out, with an emphasis on fabricated devices having a diameter of 300, 200, 100, 50, and 20 µm, respectively. It was revealed that the LED chips with smaller mesa areas deliver considerably higher light output power density; meanwhile, they can sustain a higher current density, which is mainly attributed to the enhanced current spreading uniformity in micro-scale chips. Importantly, when the diameter of LED chips decreases from 300 µm to 20 µm, the peak external quantum efficiency (EQE) increases by 20%, and the EQE peak current density can be boosted from 8.85A/cm2 and 99.52A/cm2. Moreover, we observed a longer wavelength emission with enlarged full-width at half-maximum (FWHM) in the LEDs with smaller chip sizes because of the self-heating effect at high current injection. These experimental observations provide insights into the design and fabrication of high-efficiency micro-LEDs emitting in the DUV regime with different device geometries for various future applications.

Compositionally graded III-nitride alloys: building blocks for efficient ultraviolet optoelectronics and power electronics.

Wide bandgap aluminum gallium nitride (AlGaN) semiconductor alloys have established themselves as the key materials for building ultraviolet (UV) optoelectronic and power electronic devices. However, further improvements to device performance are lagging, largely due to the difficulties in precisely controlling carrier behavior, both carrier generation and carrier transport, within AlGaN-based devices. Fortunately, it has been discovered that instead of using AlGaN layers with fixed Al compositions, by grading the Al composition along the growth direction, it is possible to (1) generate high-density electrons and holes via polarization-induced doping; (2) manipulate carrier transport behavior via energy band modulation, also known as 'band engineering'. Consequently, such compositionally graded AlGaN alloys have attracted extensive interest as promising building blocks for efficient AlGaN-based UV light emitters and power electronic devices. In this review, we focus on the unique physical properties of graded AlGaN alloys and highlight the key roles that such graded structures play in device exploration. Firstly, we elaborate on the underlying mechanisms of efficient carrier generation and transport manipulation enabled by graded AlGaN alloys. Thereafter, we comprehensively summarize and discuss the recent progress in UV light emitters and power electronic devices incorporating graded AlGaN structures. Finally, we outline the prospects associated with the implementation of graded AlGaN alloys in the pursuit of high-performance optoelectronic and power electronic devices.

Pt/AlGaN Nanoarchitecture: Toward High Responsivity, Self-Powered Ultraviolet-Sensitive Photodetection.

Energy-saving photodetectors are the key components in future photonic systems. Particularly, self-powered photoelectrochemical-type photodetectors (PEC-PDs), which depart completely from the classical solid-state junction device, have lately intrigued intensive interest to meet next-generation power-independent and environment-sensitive photodetection. Herein, we construct, for the first time, solar-blind PEC PDs based on self-assembled AlGaN nanostructures on silicon. Importantly, with the proper surface platinum (Pt) decoration, a significant boost of photon responsivity by more than an order of magnitude was achieved in the newly built Pt/AlGaN nanoarchitectures, demonstrating strikingly high responsivity of 45 mA/W and record fast response/recovery time of 47/20 ms without external power source. Such high solar-blind photodetection originates from the unparalleled material quality, fast interfacial kinetics, as well as high carrier separation efficiency which suggests that embracement of defect-free wide-bandgap semiconductor nanostructures with appropriate surface decoration offers an unprecedented opportunity for designing future energy-efficient and large-scale optoelectronic systems on a silicon platform.

Designing Mechanical Metamaterials with Kirigami-Inspired, Hierarchical Constructions for Giant Positive and Negative Thermal Expansion.

Advanced mechanical metamaterials with unusual thermal expansion properties represent an area of growing interest, due to their promising potential for use in a broad range of areas. In spite of previous work on metamaterials with large or ultralow coefficient of thermal expansion (CTE), achieving a broad range of CTE values with access to large thermally induced dimensional changes in structures with high filling ratios remains a key challenge. Here, design concepts and fabrication strategies for a kirigami-inspired class of 2D hierarchical metamaterials that can effectively convert the thermal mismatch between two closely packed constituent materials into giant levels of biaxial/uniaxial thermal expansion/shrinkage are presented. At large filling ratios (>50%), these systems offer not only unprecedented negative and positive biaxial CTE (i.e., -5950 and 10 710 ppm K-1 ), but also large biaxial thermal expansion properties (e.g., > 21% for 20 K temperature increase). Theoretical modeling of thermal deformations provides a clear understanding of the microstructure-property relationships and serves as a basis for design choices for desired CTE values. An Ashby plot of the CTE versus density serves as a quantitative comparison of the hierarchical metamaterials presented here to previously reported systems, indicating the capability for substantially enlarging the accessible range of CTE.

Advantages of AlGaN-based deep-ultraviolet light-emitting diodes with an Al-composition graded quantum barrier.

AlGaN-based deep-ultraviolet light-emitting diodes (DUV LEDs) still suffer from poor quantum efficiency and low optical power. In this work, we proposed a DUV LED structure that includes five unique AlxGa1-xN quantum barriers (QBs); Each QB has a linear-increment of Al composition by 0.03 along the growth direction, unlike those commonly used flat QBs in conventional LEDs. As a result, the electron and hole concentration in the active region was considerably increased, attributing to the success of the electron blocking effect and enhanced hole injection efficiency. Importantly, the optical power was remarkably improved by 65.83% at the injection current of 60 mA. After in-depth device optimization, we found that a relatively thinner graded QB layer could further boost the LED performance because of the increased carrier concentrations and enhanced electron and hole wave function overlap in the QW, triggering a much higher radiative recombination efficiency. Hence, the proposed graded QBs, which have a continuous increment of Al composition along the growth direction, provide us with an effective solution to boost light output power in the pursuit of high-performance DUV emitters.