Vanadium Metal Doping of Monolayer MoS2 for p-Type Transistors and Fast-Speed Phototransistors.

Modulating the electrical properties of two-dimensional (2D) materials is a fundamental prerequisite for their development to advanced electronic and optoelectronic devices. Substitutional doping has been demonstrated as an effective method for tuning the band structure in monolayer 2D materials. Here, we demonstrate a facile selective-area growth of vanadium-doped molybdenum disulfide (V-doped MoS2) flakes via pre-patterned vanadium-metal-assisted chemical vapor deposition (CVD). Optical microscopy characterization revealed the presence of flake arrays. Transmission electron microscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy were employed to identify the chemical composition and crystalline structure of as-grown flakes. Electrical measurements indicated a light p-type conduction behavior in monolayer V-doped MoS2. Furthermore, the response time of phototransistors based on V-doped MoS2 monolayers exhibited a remarkable capability of 3 ms, representing approximately 3 orders of magnitude faster response than that observed in pure MoS2 phototransistors. This work hereby provides a feasible approach to doping of 2D materials, promising a scalable pathway for the integration of these materials into emerging electronic and optoelectronic devices.

Low-threshold AlGaN-based deep ultraviolet laser enabled by a nanoporous cladding layer.

We demonstrated an AlGaN-based multiple-quantum-well (MQW) deep ultraviolet (DUV) laser at 278 nm using a nanoporous (NP) n-AlGaN as the bottom cladding layer grown on the sapphire substrate. The laser has a very-low-threshold optically pumped power density of 79 kW/cm2 at room temperature and a transverse electric (TE)-polarization-dominant emission. The high optical confinement factor of 9.12% benefiting from the low refractive index of the nanoporous n-AlGaN is the key to enable a low-threshold lasing. The I-V electrical measurement demonstrates that an ohmic contact can be still achieved in the NP n-AlGaN with a larger but acceptable resistance, which indicates it is compatible with electrically driven laser devices. Our work provides insights into the design and fabrication of low-threshold lasers emitting in the DUV regime.

Origin of Water-Stable CsPbX3 Quantum Dots Assisted by Zwitterionic Ligands and Sequential Strategies for Enhanced Luminescence Based on Crystal Evolution.

Water stability is a crucial issue always addressed for commercial practical application of perovskite quantum dots (QDs). Recent advances in ligand engineering for in situ synthesis of water-stable perovskite QDs have attracted growing interest. However, the exact mechanism remains unclear. Here, the function of 4-bromobutyric acid (BBA) and oleylamine (OLA) is systematically studied in water-stable CsPbX3 (X = Br and I) QDs and confirms that the zwitterionic ligands generated in situ by BBA and OLA are anchored on the QDs surface, thus preventing water from penetrating into the QDs. Cs4PbBr6 intermediate crystal found in the crystal structure evolution process of CsPbX3 QD further reveals a complete crystallization process: PbX2 + CsX + Br- → Cs4PbBr6 crystals + X-→ CsPbX3 QDs + Br-. Furthermore, it is found that the solvent coordination of the precursor solution has a significant effect on the crystallinity of Cs4PbBr6 intermediate crystal, while the Rb+ doping can effectively passivate the surface defects of CsPbX3 QDs, thereby jointly achieving photoluminescence quantum yields (PLQY) of 94.6% for CsPbBr3 QDs (88.2% for CsPbI3 QDs). This work provides new insights and guiding ideas for the green synthesis of high-quality and water-stable perovskite QDs.

Synergistic effects of extrinsic photoconduction and photogating in a short-wavelength ZrS3 infrared photodetector.

Two-dimensional (2D) material-based photodetectors, especially those working in the infrared band, have shown great application potential in the thermal imaging, optical communication, and medicine fields. Designing 2D material photodetectors with broadened detection band and enhanced responsivity has become an attractive but challenging research direction. To solve this issue, we report a zirconium trisulfide (ZrS3) infrared photodetector with enhanced and broadened response with the assistance of the synergistic effects of extrinsic photoconduction and photogating effect. The ZrS3 photodetectors can detect infrared light up to 2 µm by extrinsic photoconduction and exhibit a responsivity of 100 mA W-1 under 1550 nm illumination. Furthermore, the ZrS3 infrared photodetectors with an oxide layer show a triple enhanced responsivity due to the photogating effect. Additionally, the infrared imaging capability of the ZrS3 infrared photodetectors is also demonstrated. This work provides a potential way to extend the response range and improve the responsivity for nanomaterial-based photodetectors at the same time.

Fabrication of wafer-scale nanoporous AlGaN-based deep ultraviolet distributed Bragg reflectors via one-step selective wet etching.

In this paper, we reported on wafer-scale nanoporous (NP) AlGaN-based deep ultraviolet (DUV) distributed Bragg reflectors (DBRs) with 95% reflectivity at 280 nm, using epitaxial periodically stacked n-Al0.62Ga0.38N/u-Al0.62Ga0.38N structures grown on AlN/sapphire templates via metal-organic chemical vapor deposition (MOCVD). The DBRs were fabricated by a simple one-step selective wet etching in heated KOH aqueous solution. To study the influence of the temperature of KOH electrolyte on the nanopores formation, the amount of charge consumed during etching process was counted, and the surface and cross-sectional morphology of DBRs were characterized by Scanning electron microscopy (SEM) and atomic force microscopy (AFM). As the electrolyte temperature increased, the nanopores became larger while the amount of charge reduced, which revealed that the etching process was a combination of electrochemical and chemical etching. The triangular nanopores and hexagonal pits further confirmed the chemical etching processes. Our work demonstrated a simple wet etching to fabricate high reflective DBRs, which would be useful for AlGaN based DUV devices with microcavity structures.

Flexible assembly of the PEDOT: PSS/ exfoliated ß-Ga2O3 microwire hybrid heterojunction for high-performance self-powered solar-blind photodetector.

Motivated by the goals of fabricating highly reliable, high performance, and cost-efficient self-powered photodetector (PD) for numerous scientific research and civil fields, an organic-inorganic hybrid solar-blind ultraviolet (UV) PD based on PEDOT: PSS/exfoliated ß-Ga2O3 microwire heterojunction was fabricated by a flexible and cost-effective assembly method. Benefiting from the heterojunction constructed by the highly crystalline ß-Ga2O3 and the excellent hole transport layer PEDOT: PSS, the device presents a high responsivity of 39.8 mA/W at 250 nm and a sharp cut-off edge at 280 nm without any power supply. Additionally, the ultra-high normalized photo-to-dark current ratio (> 104 mW-1cm2) under reverse bias and the superior detectivity of 2.4×1012 Jones at zero bias demonstrate the excellent detection capabilities. Furthermore, the hybrid PD exhibits a rapid rise time (several milliseconds) and high rejection ratio (R250/R365: 5.8 × 103), which further highlights its good spectral selectivity for solar-blind UV. The prominent performance is mainly ascribed to the efficient separation of the photogenerated carriers by the large built-in electric field of the advanced heterojunction. This flexible assembly strategy for solar-blind UV PD combines the advantages of high efficiency, low cost and high performance, providing more potential for PD investigation and application in the future.

Ionic Liquid Passivation Eliminates Low-n Quantum Well Domains in Blue Quasi-2D Perovskite Films.

In quasi-two-dimensional (quasi-2D) perovskite films, carriers transport in the cascade structural systems involving various quantum wells (QWs) n, but their efficiency is limited by the severe nonradiative recombination within plentiful n = 1, 2, 3 domains induced by traditional ammonium bromide passivation. Here, we fabricate the quasi-2D films with the elimination of n = 1, 2, 3 domains by introducing the ionic liquid n-butylamine acetate (BAAc) instead of n-butylamine hydrobromide (BABr), which increases the photoluminescence quantum yield (PLQY) and lowers the surface roughness of films. Due to the anion exchange between BAAc and methylamine hydrobromide (MABr), BAAc exhibits a sole passivation effect on methylamine-based perovskites. As a result, the ionic liquid-derived perovskite light-emitting diodes (PeLEDs) display blue emission at 479 nm and show significantly improved performance on external quantum efficiency (EQE) and luminance. Our finding provides insights into the passivating effect of ionic liquid on quasi-2D perovskites and will benefit fabricating PeLEDs with enhanced performance.

Near-octave-spanning breathing soliton crystal in an AlN microresonator.

The soliton crystal (SC) was recently discovered as an extraordinary Kerr soliton state with regularly distributed soliton pulses and enhanced comb line power spaced by multiples of the cavity free spectral ranges (FSRs), which will significantly extend the application potential of microcombs in optical clock, signal processing, and terahertz wave systems. However, the reported SC spectra are generally narrow. In this Letter, we demonstrate the generation of a breathing SC in an aluminum nitride (AlN) microresonator (FSR ∼374GHz), featuring a near-octave-spanning (1150-2200 nm) spectral range and a terahertz repetition rate of ∼1.87THz. The measured 60 fs short pulses and low intensity-noise characteristics confirm the high coherence of the breathing SC. Broadband microcombs with various repetition rates of ∼0.75, ∼1.12, and ∼1.5THz were also realized in different microresonators of the same size. The proposed scheme shows a reliable design strategy for broadband soliton generation with versatile dynamic control over the comb line spacing.

Full wafer scale electroluminescence properties of AlGaN-based deep ultraviolet LEDs with different well widths.

Deep ultraviolet (DUV) LEDs have great potential in sterilization, water, air purification, and other fields. In this work, DUV LED wafers with different quantum well (QW) widths were grown by metal-organic chemical vapor deposition. It is found that the light output power (LOP) and peak wavelength of all chips are not only related to the QW thickness but also affected by warpage. For the first time, to the best of our knowledge, a positive correlation between the LOP and peak wavelength of DUV LED chips on the same wafer was observed, which is very important for improving the yield of DUV LEDs and reducing costs. Furthermore, the influence of QW thickness on the external quantum efficiency (EQE) of DUV LED has also been investigated. As the thickness of the QW increases, the exciton localization effect decreases and the quantum confinement Stark effect increases. Consequently, DUV LED wafers with a QW thickness of 2 nm have the highest EQE and yield. These findings not only help to improve the efficiency of DUV LEDs but also provide new insights for evaluating the performance of DUV LED wafers.

Blackbody-sensitive room-temperature infrared photodetectors based on low-dimensional tellurium grown by chemical vapor deposition.

Blackbody-sensitive room-temperature infrared detection is a notable development direction for future low-dimensional infrared photodetectors. However, because of the limitations of responsivity and spectral response range for low-dimensional narrow bandgap semiconductors, few low-dimensional infrared photodetectors exhibit blackbody sensitivity. Here, highly crystalline tellurium (Te) nanowires and two-dimensional nanosheets were synthesized by using chemical vapor deposition. The low-dimensional Te shows high hole mobility and broadband detection. The blackbody-sensitive infrared detection of Te devices was demonstrated. A high responsivity of 6650 A W-1 (at 1550-nm laser) and the blackbody responsivity of 5.19 A W-1 were achieved. High-resolution imaging based on Te photodetectors was successfully obtained. All the results suggest that the chemical vapor deposition-grown low-dimensional Te is one of the competitive candidates for sensitive focal-plane-array infrared photodetectors at room temperature.

Octave-spanning Kerr frequency comb generation with stimulated Raman scattering in an AlN microresonator.

Octave-spanning optical frequency combs (OFCs) are essential for various applications, such as precision metrology and astrophysical spectrometer calibration. In this Letter, we demonstrate, for the first time to our knowledge, the generation of octave-spanning Kerr frequency combs ranging from 1150 to 2400 nm in aluminum nitride (AlN) microring resonators, by pumping the TM00 modes at 250 mW on-chip power. By simply adjusting the pump detuning, we observe the transition and coexistence of Kerr OFC and stimulated Raman scattering. For the TE00 mode in the same device, a broadband Raman-assisted frequency comb is demonstrated by adjusting the pump power and tuning. These results indicate a crucial development for the fundamentals of nonlinear dynamics and comb applications in AlN.

Growth of InGaN-based blue-LED on AlN/sapphire sputtered with different oxygen flow rate.

Indium gallium nitride (InGaN) based blue light-emitting diodes (LEDs) suffer from insufficient crystal quality and serious efficiency droop in large forward current. In this paper, the InGaN-based blue LEDs are grown on sputtered aluminum nitride (AlN) films to improve the device light power and weaken the efficiency droop. The effects of oxygen flow rate on the sputtering of AlN films on sapphire and device performance of blue LEDs are studied in detail. The mechanism of external quantum efficiency improvement is related to the change of V-pits density in multiple quantum wells. The external quantum efficiency of 66% and 3-V operating voltage are measured at a 40-mA forward current of with the optimal oxygen flow rate of 4 SCCM.

Photolithography allows high-Q AlN microresonators for near octave-spanning frequency comb and harmonic generation.

Single-crystal aluminum nitride (AlN) possessing both strong Pockels and Kerr nonlinear optical effects as well as a very large band gap is a fascinating optical platform for integrated nonlinear optics. In this work, fully etched AlN-on-sapphire microresonators with a high-Q of 2.1 × 106 for the TE00 mode are firstly demonstrated with the standard photolithography technique. A near octave-spanning Kerr frequency comb ranging from 1100 to 2150 nm is generated at an on-chip power of 406 mW for the TM00 mode. Due to the high confinement, the TE10 mode also excites a Kerr comb from 1270 to 1850nm at 316 mW. In addition, frequency conversion to visible light is observed during the frequency comb generation. Our work will lead to a large-scale, low-cost, integrated nonlinear platform based on AlN.

Highly Integrated In Situ Photoenergy Gas Sensor with Deep Ultraviolet LED.

According to the demands of the Internet of Things (IoTs), a gas sensor is demanded to be small, portable, and easy to integrate with the environment or structure in its application. Herein, an ingenious form of in situ photoenergy gas sensor integrated with a deep ultraviolet light-emitting diode (LED) has been designed to achieve ppb level NO2 gas detection at room temperature. In this gas sensor, the deep ultraviolet LED based on AlGaN materials, which has a wider band gap and higher photoexcitation energy, acts as the substrate with emission at 280 nm. The ZnO nanorods of the gas-sensing material were directly grown on 2 inch AlGaN-based LEDs containing thousands of independent light-emitting chips, leading to photosensitive materials with uniform and controllable, as well as a sensor with low power consumption and mass manufacture with low cost. The result shows that responses of over 500% to 500 ppb of NO2 were observed by in situ irradiation of just 3 mW optical power. Meanwhile, sensitivity without real-time photoenergy is defined as the ratio of the resistance change rate in pollutant to that in air has been presented. Interestingly, the prototype gets in situ photoenergy charge for 5 min and then has responses of over 200% to 500 ppb of NO2 for 5 days in the dark. It may open a new avenue for the integrated microchip design of a gas sensor and give a novel sight into the sensitivity study without real-time assisted energy.

Ripening-resistance of Pd on TiO2(110) from first-principles kinetics.

Suppressing sintering of supported particles is of importance for the study and application of metal-TiO2 system. Theoretical study of Ostwald ripening of TiO2(110)-supported Pd particles would be helpful to extend the understanding of the sintering. In this paper, based on density functional theory (DFT), the surface energy of Pd and the total activation energy (the sum of formation energy and diffusion barrier) of TiO2-supported Pd were calculated. Since the total activation energy is mainly contributed from the formation energy, it is indicated that the ripening of Pd particles would be in the interface control limit. Subsequently, the calculated surface energy and total activation energy were used to simulate Ostwald ripening of TiO2(110)-supported Pd particles. As a result, in comparison with larger particles, smaller particles would worsen the performance of ripening-resistance according to its lower onset temperature and shorter half-life time. The differences on ripening-resistance among different size particles could be mitigated along with the increase of temperature. Moreover, it is verified that the monodispersity can improve ripening resistance especially for the smaller particles. However, the different performances of the ripening originating from difference of the relative standard deviation are more obvious at higher temperature than lower temperature. This temperature effect for the relative standard deviation is the inverse of that for the initial main particle size. It is indicated that the influence of dispersity of TiO2(110)-supported Pd particles on ripening may be more sensitive at higher temperature. In this contribution, we extend the first principle kinetics to elaborate the ripening of Pd on TiO2(110). It is expected that the information from first principle kinetics would be helpful to the study in experiments.

Enhanced Performance of AlGaN-Based Deep Ultraviolet Light-Emitting Diodes with Chirped Superlattice Electron Deceleration Layer.

AlGaN-based deep ultraviolet (DUV) light-emitting diodes (LEDs) suffer from electron overflow and insufficient hole injection. In this paper, novel DUV LED structures with superlattice electron deceleration layer (SEDL) is proposed to decelerate the electrons injected to the active region and improve radiative recombination. The effects of several chirped SEDLs on the performance of DUV LEDs have been studied experimentally and numerically. The DUV LEDs have been grown by metal-organic chemical vapor deposition (MOCVD) and fabricated into 762 × 762 µm2 chips, exhibiting single peak emission at 275 nm. The external quantum efficiency of 3.43% and operating voltage of 6.4 V are measured at a forward current of 40 mA, indicating that the wall-plug efficiency is 2.41% of the DUV LEDs with ascending Al-content chirped SEDL. The mechanism responsible for this improvement is investigated by theoretical simulations. The lifetime of the DUV LED with ascending Al-content chirped SEDL is measured to be over 10,000 h at L50, due to the carrier injection promotion.

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.

Optical polarization characteristics and light extraction behavior of deep-ultraviolet LED flip-chip with full-spatial omnidirectional reflector system.

Optical polarization characteristics and light extraction behavior of deep-ultraviolet (DUV) light-emitting diode (LED) flip-chip with full-spatial omnidirectional reflector (FSODR) have been investigated. FSODR is fabricated to be simultaneously covered on the whole flip-chip, except the sapphire surface. It is found that the FSODR greatly enhance both transverse-electric (TE) and transverse-magnetic (TM) mode light extraction at every space angle, resulting in total enhancement of 73.1% and 79.8%, respectively. Moreover, the four individual ODR structures separated from FSODR, which are covered on the surface of n-AlGaN, the interface of p-GaN/p-AlGaN, the sidewall of mesa and the sidewall of n-AlGaN/AlN, respectively, show considerably different optical polarization characteristics and extraction behaviors between each other. The achievements of FSODR cannot be obtained by any separated ODR, and all of the individual ODRs can contribute to the FSODR. Especially, the synergy effect of TM extraction behavior obviously exists in FSODR. As a result, the light extraction efficiency (LEE) enhancement of FSODR is approximately 60% at a high current density of 140A/cm2. This study is significant for understanding and modulating the extraction behavior of polarized light to realize high efficiency AlGaN-based DUV LEDs.

Strain modulated nanostructure patterned AlGaN-based deep ultraviolet multiple-quantum-wells for polarization control and light extraction efficiency enhancement.

AlGaN-based deep ultraviolet (DUV) multiple-quantum-wells (MQWs) incorporating strain-modulated nanostructures are proposed, demonstrating enhanced degree of polarization (DOP) and improved light extraction efficiency (LEE). The influence of Al composition and bi-axial strains on the optical behaviors of the DUV-MQWs were carefully examined. Compared with planar DUV-MQWs, strain-modulated nanostructure patterned MQWs show three times higher photoluminescence and increased DOP from -0.43 to -0.16. Moreover, nanostructure patterned DUV-MQWs under compressive strains further illustrate higher DOP and LEE values than those under tensile strains due to more efficient diffraction of the guided modes of the transverse electric (TE) polarized light. Our work demonstrates, for the first time, that a combination of compressive in-plane strain and surface nanostructure show unambiguous advantages in facilitating TE mode emission, thus have great promises in the design and optimization of highly efficient polarized DUV optoelectronic devices.

Bio-Inspired Flexible Fluoropolymer Film for All-Mode Light Extraction Enhancement.

Enhancing the light extraction efficiency is a prevalent but vital challenge for most solid-state lighting technologies, especially for deep ultraviolet light-emitting diodes (DUV-LEDs). In this paper, inspired by the microstructure of the butterfly's eye, we propose and fabricate a flexible fluoropolymer film (FFP film) to tackle this issue for all-mode, full-wavelength light extraction enhancement for most solid-state lighting technologies compatibly. The experimental results demonstrate that compared with one mounted with a smooth FFP film, the light output power of DUV-LED is enhanced up to 26.7% by mounting the FFP film with 325 nm radius nanocones at a driving current of 200 mA. Importantly, thanks to the super-flexible feature of the FFP film, it can both cover the top surface and sidewalls of the DUV-LED chip, leading to the improvement of transverse electric and transverse magnetic mode light extraction by 20.5 and 21.8%, respectively. Finite element analysis (FEA) simulations of the electric field distribution of DUV-LEDs with the FFP film reveal the underlying physics. The present strategy is proposed from the view of the packaging level, which is cost-effective, able to be manufactured at a large scale, and compatible with the solid-state lighting technologies.