In-depth insights into polarization-dependent light extraction mechanisms of AlGaN-based deep ultraviolet light-emitting diodes.

The AlGaN-based deep ultraviolet light-emitting diodes (DUV LEDs) dominated by transverse-magnetic (TM) polarized emission suffer from extremely poor light extraction efficiency (LEE) from their top surface, which severely limits the device performance. In this study, the underlying physics of polarization-dependent light extraction mechanisms of AlGaN-based DUV LEDs has been explored in depth via simple Monte Carlo ray-tracing simulations with Snell's law. It is especially worth noting that the structures of the p-type electron blocking layer (p-EBL) and multi-quantum wells (MQWs) have a significant impact on light extraction behavior, especially for TM-polarized emission. Thus, an artificial vertical escape channel (named GLRV) has been constructed to efficiently extract the TM-polarized light through the top surface, by adjusting the structures of the p-EBL, MQWs, sidewalls, and using the adverse total internal reflection in a positive manner. The results show that the enhancement times of the top-surface LEE is up to 18 for TM-polarized emission in the 300 × 300 µm2 chip comprising a single GLRV structure, and further increases to 25 by dividing this single GLRV structure into a 4 × 4 micro-GLRV array structure. This study provides a new perspective for understanding and modulating the extraction mechanisms of polarized light to overcome the inherently poor LEE for the TM-polarized light.

Upconversion under Photon Trapping in ZnO/BN Nanoarray: An Ultrahigh Responsivity Solar-Blind Photodetecting Paper.

Solar-blind photodetectors (PDs) are widely applicable in special, military, medical, environmental, and commercial fields. However, high performance and flexible PD for deep ultraviolet (UV) range is still a challenge. Here, it is demonstrated that an upconversion of photon absorption beyond the energy bandgap is achieved in the ZnO nanoarray/h-BN heterostructure, which enables the ultrahigh responsivity of a solar-blind photodetecting paper. The direct growth of ultralong ZnO nanoarray on polycrystalline copper paper induced by h-BN 2D interlayer is obtained. Meanwhile, strong photon trapping takes place within the ZnO nanoarray forest through the cyclic state transition of surface oxygen ions, resulting in an extremely high absorption efficiency (> 99.5%). A flexible photodetecting paper is fabricated for switchable detections between near UV and deep UV signals by critical external bias. The device shows robust reliability, ultrahigh responsivity up to 700 A W-1 @ 265-276 nm, and high photoconductive gain of ≈2 × 103 . A negative differential resistance effect is revealed for driving the rapid transfer of up-converted electrons between adjacent energy valleys (Γ to A) above the critical bias (3.9 V). The discovered rationale and device structure are expected to bring high-efficiency deep UV detecting and future wearable applications.

Regulating the valence level arrangement of high-Al-content AlGaN quantum wells using additional potentials with Mg doping.

Quantum states and arrangement of valence levels determine most of the electronic and optical properties of semiconductors. Since the crystal field split-off hole (CH) band is the top valence band in high-Al-content AlGaN, TM-polarized optical anisotropy has become the limiting factor for efficient deep-ultraviolet (DUV) light emission. Additional potentials, including on-site Coulomb interaction and orbital state coupling induced by magnesium (Mg) doping, are proposed in this work to regulate the valence level arrangement of AlN/Al0.75Ga0.25N quantum wells (QWs). Diverse responses of valence quantum states |pi〉 (i = x, y, or z) of AlGaN to additional potentials due to different configurations and interactions of orbitals revealed by first-principles simulations are understood in terms of the linear combination of atomic orbital states. A positive charge and large Mg dopant in QWs introduce an additional Coulomb potential and modulate the orbital coupling distance. For the CH band (pz orbital), the Mg-induced Coulomb potential compensates the orbital coupling energy. Meanwhile, the heavy/light hole (HH/LH) bands (px and py orbitals) are elevated by the Mg-induced Coulomb potential. Consequently, HH/LH energy levels are relatively shifted upward and replace the CH level to be the top of the valence band. The inversion of optical anisotropy and enhancement of TE-polarized emission are further confirmed experimentally via spectroscopic ellipsometry.

Extraordinary N atom tunneling in formation of InN shell layer on GaN nanorod m-plane sidewall.

We report the extraordinary tunneling process that finds the lower cohesive energy route for stablizing InN shell layer on m-plane sidewall of GaN nanorod. The [0001] orientated GaN nanorod array is grown on sapphire substrate patterned with Ga nanoparticle by metal-organic vapor deposition method, based on which the simulation structures of c-plane top surface and m-plane sidewall surface is constructed for the first-principles calculations. The results show that the introduction of In wetting monolayer could effectively lower the cohesive energy of adalayers on non-polar GaN surfaces. Most importantly, it is revealed that there exists an extraordinary tunneling process in which the N atoms will drag out the In wetting atoms and tunnel through to form stable InN shell layer on the nanorod sidewall.

Rhodium-embedded UV photodetectors based on localized surface plasmon resonance on AlN/GaN.

We report a remarkably enhanced responsivity of metal-semiconductor-metal photodetectors embedded with a large-scale periodicity and highly uniform rhodium nanoparticle array based on localized surface plasmon resonance. In this study, we used theoretical simulations of the absorption, scattering, and extinction behaviors, as well as the near electromagnetic field distributions to predict the plasmon resonance wavelength of quasi-triangular-shaped rhodium nanoparticles. More specifically, we successfully implemented a hexagonal close-packed structure with the individual quasi-triangular-shaped rhodium nanoparticle on the AlN/GaN structure by self-assembly nanosphere technology. The characterization results showed that the device embedded with rhodium nanoparticles had a reduced dark current of 7 × 10-14 A, and the maximum responsivity was shifted to a longer wavelength of approximately 310 nm compared to the device without rhodium nanoparticles. Moreover, at a wavelength of 324 nm, the enhancement ratio of the responsivity was as high as 56. Our study makes a significant contribution to the literature with a highly uniform, large-scale distributed rhodium nanoparticle array for enhancing the performance of AlGaN-based photodetectors in the UV region.

Enhancing deep-UV emission at 234 nm by introducing a truncated pyramid AlN/GaN nanostructure with fine-tuned multiple facets.

The external quantum efficiency of a high-Al content (>0.6) AlGaN deep-ultraviolet (DUV) light-emitting diode is typically below 1% in the sub-250 nm wavelength range. One of the main reasons for this low efficiency is the fundamental properties of high-Al content AlGaN comprising the transverse-magnetic (TM)-dominant emission and low light extraction due to the total internal reflection (TIR). This work demonstrates a truncated pyramid nanostructure with fine-tuned multiple facets in an (AlN)8/(GaN)2 digital alloy to achieve highly efficient DUV emission at 234 nm. By applying nanoimprint lithography, dry and wet etching, a hexagonal truncated pyramid nanohole structure is fabricated featuring multiple crystal facets of (0001), (10-13), and (20-21) planes. These fine-tuned multiple facets act as reflecting mirrors that can effectively modulate the light propagation and extraction patterns to overcome the TIR via multiple reflections and enhanced scattering. Consequently, significant light extraction enhancements of 5.6 times and 1.1 times for TM and transverse-electric emissions are achieved in the truncated pyramid nanohole structure, respectively. The total luminous intensity of this unique nanostructure is greatly increased by 191% compared to that of a conventional planar structure. The truncated pyramid AlN/GaN nanostructure with fine-tuned multiple facets used in this work provides a promising approach for realizing highly efficient sub-250 nm DUV light-emitting devices.

Towards n-type conductivity in hexagonal boron nitride.

Asymmetric transport characteristic in n- and p-type conductivity has long been a fundamental difficulty in wide bandgap semiconductors. Hexagonal boron nitride (h-BN) can achieve p-type conduction, however, the n-type conductivity still remains unavailable. Here, we demonstrate a concept of orbital split induced level engineering through sacrificial impurity coupling and the realization of efficient n-type transport in 2D h-BN monolayer. We find that the O 2pz orbital has both symmetry and energy matching to the Ge 4pz orbital, which promises a strong coupling. The introduction of side-by-side O to Ge donor can effectively push up the donor level by the formation of another sacrificial deep level. We discover that a Ge-O2 trimer brings the extremely shallow donor level and very low ionization energy. By low-pressure chemical vapor deposition method, we obtain the in-situ Ge-O doping in h-BN monolayer and successfully achieve both through-plane (~100 nA) and in-plane (~20 nA) n-type conduction. We fabricate a vertically-stacked n-hBN/p-GaN heterojunction and show distinct rectification characteristics. The sacrificial impurity coupling method provides a highly viable route to overcome the n-type limitation of h-BN and paves the way for the future 2D optoelectronic devices.

Role of Strain-Induced Microscale Compositional Pulling on Optical Properties of High Al Content AlGaN Quantum Wells for Deep-Ultraviolet LED.

A systematic study was carried out for strain-induced microscale compositional pulling effect on the structural and optical properties of high Al content AlGaN multiple quantum wells (MQWs). Investigations reveal that a large tensile strain is introduced during the epitaxial growth of AlGaN MQWs, due to the grain boundary formation, coalescence and growth. The presence of this tensile strain results in the microscale inhomogeneous compositional pulling and Ga segregation, which is further confirmed by the lower formation enthalpy of Ga atom than Al atom on AlGaN slab using first principle simulations. The strain-induced microscale compositional pulling leads to an asymmetrical feature of emission spectra and local variation in emission energy of AlGaN MQWs. Because of a stronger three-dimensional carrier localization, the area of Ga segregation shows a higher emission efficiency compared with the intrinsic area of MQWs, which is benefit for fabricating efficient AlGaN-based deep-ultraviolet light-emitting diode.

Nonvolatile Electrical Valley Manipulation in WS2 by Ferroelectric Gating.

Valleytronics in transition metal dichalcogenides has been intensively investigated for potential applications in next-generation information storage, data processing, and signal transmission devices. Here a ferroelectric gating approach is engaged in achieving nonvolatile electrical tuning of the valley-excitonic properties of monolayer and bilayer WS2. The gating effects include carrier doping and ferroelectric coupling, which are further distinguished by comparing two geometries where the gate electrodes are in direct contact with or insulated from the WS2 crystal. The results show that the carrier doping from gate electrodes acts on WS2 through carrier screening, which only moderately alters the valley polarization. In contrast, the ferroelectric gating promotes electron-phonon interaction, introduces a strong surface polarization field, and controls the interfacial charge trapping/detrapping, causing a Stark shift in exciton energy and strongly enhancing room-temperature valley polarization. In bilayer WS2, the intralayer-interlayer exciton transition is further induced, contributing to even higher valley polarization. The ferroelectric coupling effect can still be maintained after the removal of gate voltage, showing its nonvolatile nature. The role of ferroelectricity is further verified by the anomalous temperature dependence in valley polarization. This work has revealed effective electrical control over valley excitons in semiconductors through interaction with ferroelectric materials. The reported high room-temperature valley polarization in WS2 will boost the development of valleytronics devices.

High-Efficient Spin Injection in GaN at Room Temperature Through A Van der Waals Tunnelling Barrier.

Achieving high-efficient spin injection in semiconductors is critical for developing spintronic devices. Although a tunnel spin injector is typically used, the construction of a high-quality tunnel barrier remains a significant challenge due to the large lattice mismatch between oxides and semiconductors. In this work, van der Waals h-BN films with the atomically flat interface were engaged as the tunnel barrier to achieve high spin polarization in GaN, and the spin injection and transport in GaN were investigated systematically. Based on the Hanle precession and magnetic resistance measurements, CoFeB was determined as an optimal spin polarizer, bilayer h-BN tunnelling barrier was proven to yield a much higher spin polarization than the case of monolayer, and appropriate carrier concentration as well as higher crystal equality of n-GaN could effectively reduce the defect-induced spin scattering to improve the spin transport. The systematic understanding and the high efficiency of spin injection in this work may pave the way to the development of physical connotations and the applications of semiconductor spintronics.

Colorful Conductive Threads for Wearable Electronics: Transparent Cu-Ag Nanonets.

Electronic textiles have been regarded as the basic building blocks for constructing a new generation of wearable electronics. However, the electronization of textiles often changes their original properties such as color, softness, glossiness, or flexibility. Here a rapid room-temperature fabrication method toward conductive colorful threads and fabrics with Ag-coated Cu (Cu-Ag) nanonets is demonstrated. Cu-Ag core-shell nanowires are produced through a one-pot synthesis followed by electroless deposition. According to the balance of draining and entraining forces, a fast dip-withdraw process in a volatile solution is developed to tightly wrap Cu-Ag nanonets onto the fibers of thread. The modified threads are not only conductive, but they also retain their original features with enhanced mechanical stability and dry-wash durability. Furthermore, various e-textile devices are fabricated such as a fabric heater, touch screen gloves, a wearable real-time temperature sensor, and warm fabrics against infrared thermal dissipation. These high quality and colorful conductive textiles will provide powerful materials for promoting next-generation applications in wearable electronics.

Unidirectional Elimination of Hydrogen by a Giant Local Field Saves First- and Last-Mile Performances of Semiconductor Devices.

Hydrogen, the smallest element, easily forms bonds to host/dopant atoms in semiconductors, which strongly passivates the original electronic characteristics and deteriorates the final reliability. Here, we demonstrate a concept of unidirectional elimination of hydrogen from semiconductor wafers as well as electronic chips through a giant local electric field induced by compact chloridions. We reveal an interactive behavior of chloridions, which can rapidly approach and take hydrogen atoms away from the device surface. A universal and simple technique based on a solution-mediated three-electrode system achieves efficient hydrogen elimination from various semiconductor wafers (p-GaN, p-AlGaN, SiC, and AlInP) and also complete light emitting diodes (LEDs). The p-type conductivity and light output efficiency of H-eliminated UVC LEDs have been significantly enhanced, and the lifetime is almost doubled. Moreover, we confirm that under a one-second irradiation of UVC LEDs, bacteria and COVID-19 coronavirus can be completely killed (>99.93%). This technology will accelerate the further development of the semiconductor-based electronic industry.

Multiple fields manipulation on nitride material structures in ultraviolet light-emitting diodes.

As demonstrated during the COVID-19 pandemic, advanced deep ultraviolet (DUV) light sources (200-280 nm), such as AlGaN-based light-emitting diodes (LEDs) show excellence in preventing virus transmission, which further reveals their wide applications from biological, environmental, industrial to medical. However, the relatively low external quantum efficiencies (mostly lower than 10%) strongly restrict their wider or even potential applications, which have been known related to the intrinsic properties of high Al-content AlGaN semiconductor materials and especially their quantum structures. Here, we review recent progress in the development of novel concepts and techniques in AlGaN-based LEDs and summarize the multiple physical fields as a toolkit for effectively controlling and tailoring the crucial properties of nitride quantum structures. In addition, we describe the key challenges for further increasing the efficiency of DUV LEDs and provide an outlook for future developments.

Designs of InGaN Micro-LED Structure for Improving Quantum Efficiency at Low Current Density.

Here we report a comprehensive numerical study for the operating behavior and physical mechanism of nitride micro-light-emitting-diode (micro-LED) at low current density. Analysis for the polarization effect shows that micro-LED suffers a severer quantum-confined Stark effect at low current density, which poses challenges for improving efficiency and realizing stable full-color emission. Carrier transport and matching are analyzed to determine the best operating conditions and optimize the structure design of micro-LED at low current density. It is shown that less quantum well number in the active region enhances carrier matching and radiative recombination rate, leading to higher quantum efficiency and output power. Effectiveness of the electron blocking layer (EBL) for micro-LED is discussed. By removing the EBL, the electron confinement and hole injection are found to be improved simultaneously, hence the emission of micro-LED is enhanced significantly at low current density. The recombination processes regarding Auger and Shockley-Read-Hall are investigated, and the sensitivity to defect is highlighted for micro-LED at low current density.Synopsis: The polarization-induced QCSE, the carrier transport and matching, and recombination processes of InGaN micro-LEDs operating at low current density are numerically investigated. Based on the understanding of these device behaviors and mechanisms, specifically designed epitaxial structures including two QWs, highly doped or without EBL and p-GaN with high hole concentration for the efficient micro-LED emissive display are proposed. The sensitivity to defect density is also highlighted for micro-LED.

Non-contact electrical detection of intrinsic local charge and internal electric field at nanointerfaces.

A nanoscale non-contact electrical measurement has been developed based on Auger electron spectroscopy. This approach used the specialty of an Auger electron, which is self-generated and free from external influences, to overcome the technical limitations of conventional measurements. The detection of the intrinsic local charge and internal electric field for nanostructured materials was achieved with a resolution below 10 nm. As an example, the electrical properties at the GaN/AlGaN/GaN nanointerfaces were characterized. The concentration of the intrinsic polarization sheet charges embedded in GaN/AlGaN nanointerfacial layers were accurately detected to be -4.4 e nm(-2). The mapping of the internal electric field across the nanointerface revealed the actual energy-band configuration at the early stage of the formation of a two-dimensional electron gas.

Programmed Ultrafast Scan Welding of Cu Nanowire Networks with a Pulsed Ultraviolet Laser Beam for Transparent Conductive Electrodes and Flexible Circuits.

Metal nanowires (NWs) have shown superior advances for the next-generation transparent conducting (TC) materials. Most concerns were focused on uniform conductive films; however, fabrication of a programmed circuit is still lacking. Here, we demonstrate a programmable ultrafast welding method by pulsed laser beam scanning under ambient conditions to achieve a Cu NW pattern-free TC circuit as well as various size films. High-aspect ratio Cu NWs (> 3000) are synthesized through an oleylamine-mediated solution system. Pulsed ultraviolet laser irradiation together with a programmed moving station is set up for the welding of Cu NW networks. Finite element simulations reveal that the transient heating by efficient absorption of UV light (∼ 250 nm) could remove the organic residues on the surface and realize local welding of interlaced NW junctions. With only 10 ms pulsed irradiation, high optoelectronic performance (33 ohm/sq. at 87% transmittance at 550 nm) and excellent stability of the Cu NW TC film have been achieved. The line-by-line and selected route scanning modes could rapidly make large area TC films and directly write flexible circuits. Moreover, completely transparent micron-size UV and blue LED chips are fabricated and successfully lit with bright emission. This method opens up a future way of circuit and device fabrication by direct one-step laser writing.

Cu Nanowires Passivated with Hexagonal Boron Nitride: An Ultrastable, Selectively Transparent Conductor.

The copper nanowire (Cu NW) network is considered a promising alternative to indium tin oxide as transparent conductors for advanced optoelectronic devices. However, the fast degradation of copper in ambient conditions largely overshadows its practical applications. Here we demonstrate a facile method for epitaxial growth of hexagonal boron nitride (h-BN) of a few atomic layers on interlaced Cu NWs by low-pressure chemical vapor deposition, which exhibit excellent thermal and chemical stability under high temperature (900 °C in vacuum), high humidity (95% RH), and strong base/oxidizer solution (NaOH/H2O2). Meanwhile, their optical and electrical performances remain similar to those of the original Cu NWs (e.g., high optical transmittance (∼93%) and high conductivity (60.9 Ω/□)). A smart privacy glass is successfully fabricated based on a Cu@h-BN NW network and liquid crytal, which could rapidly control the visibility from transparent to opaque (0.26 s) and, at the same time, strongly block the mid-infrared light for energy saving by screening radiative heat. This precise engineering of epitaxial Cu@h-BN core-shell nanostructure offers broad applications in high-performance electronic and optoelectronic devices.

Enhanced Emission of Deep Ultraviolet Light-Emitting Diodes through Using Work Function Tunable Cu Nanowires as the Top Transparent Electrode.

Deep ultraviolet light-emitting diodes (DUV LEDs) (<280 nm) have been important light sources for broad applications in, e.g., sterilization, purification, and high-density storage. However, the lack of excellent transparent electrodes in the DUV region remains a challenging issue. Here, we demonstrate an architectural engineering scheme to flexibly tune the work function of Cu@shell nanowires (NWs) as top transparent electrodes in DUV LEDs. By fast encapsulation of shell metals on Cu NWs and a shift of electron binding energy, the electronic work function could be widely tailored down to 4.37 eV and up to 5.73 eV. It is revealed that the high work function of Cu@Ni and Cu@Pt NWs could overcome the interfacial barrier to p-AlGaN and achieve direct ohmic contact with high transparency (91%) in 200-400 nm. Completely transparent DUV LED chips are fabricated and successfully lighted with sharp top emission (wall-plug efficiency reaches 3%) under a turn-on voltage of 6.4 V. This architectural strategy is of importance in providing highly transparent ohmic electrodes for optoelectronic devices in broad wavelength regions.

Highly transparent light emitting diodes on graphene encapsulated Cu nanowires network.

The internal quantum efficiency of blue LEDs is almost close to the limit, therefore, advanced transparent electrode has been long explored for gaining high external quantum efficiency. However, work function mismatch at electrode-semiconductor interface remains the fundamental difficulty in obtaining low resistance ohmic contact. Here, we demonstrate the gas phase encapsulation of graphene layer on superfine Cu nanowires network by chemical vapor deposition for highly transparent LEDs. The fast encapsulation of graphene shell layer on Cu nanowires achieves high optoelectronic performance (33 Ω/sq @ 95% T), broad transparency range (200~3000 nm) and strong antioxidant stability. A novel phenomenon of scattered-point contact is revealed at the Cu nanowires/GaN interface. Point discharge effect is found to produce locally high injection current through contact points, which can effectively overcome Schottky barrier and form ohmic contact. The transparent LED on Cu@graphene nanowire network is successfully lighted with bright blue emission.

p-Type conductivity of hexagonal boron nitride as a dielectrically tunable monolayer: modulation doping with magnesium.

Hexagonal boron nitride (h-BN) is the widest band gap 2D material (>6 eV), which has attracted extensive attention. For exploring potential applications in optoelectronic devices, electrical conductivity modulation (n or p type) is of extreme importance. Here, we report the achievement of a large-scale and high quality h-BN monolayer with p-type conductivity by modulation doping of Mg using a low pressure chemical vapor deposition method. A large-scale monolayer h-BN (>10 inches) was grown by using a wound Cu foil roll on a multi-prong quartz fork. Magnesium nitride is used as a dopant precursor in a separate line due to its appropriate melting point and decomposition temperature. Density functional theory calculations revealed that the acceptor level introduced by Mg is almost pinned into the valence band and the activated holes are highly delocalized into the surrounding h-BN lattices. The h-BN:Mg monolayer showed a p-type conductivity with a considerable surface current of over 12 µA and a hole density of 1.7 × 1014 cm-2. The dielectrically tunable h-BN monolayer makes the fabrication of advanced 2D optoelectronic devices in short wavelength possible.