Deep Ultraviolet Light-emitting Diodes Research Articles

Action Spectra of Bacteria and Purification of Pollutant Water at Faucets Using a Water Waveguide Method

Ultraviolet (UV) radiation treatment is an effective method for purifying pollutant water contaminated with bacteria and/or chemicals. As an emerging technology, purification by deep ultraviolet light-emitting diodes (DUV-LEDs) is promising. Few studies have used the point-source characteristics of LEDs and have instead replaced mercury vapor lamps with LEDs. Here, we show our recent progress in the instantaneous purification of contaminated water by combining the point-source characteristics of DUV-LEDs with a water waveguide (WW). Before the demonstration, we determined the efficacy of disinfection as a function of irradiation wavelength (action spectra) by constructing a wavelength tunable DUV light source. We found that, as a function of irradiation wavelength, there is a strong correlation between the dose-based inactivation rate constants and deoxyribonucleic acid (DNA) absorbance. Based on this correlation, the emission wavelength of 265 nm was determined as the most effective wavelength for disinfecting water contaminated with bacteria. Instantaneous 2-log disinfection levels of water contaminated with Escherichia coli O1 or Pseudomonas aeruginosa were demonstrated by using the DUV-LED WW method. We also discuss how far-UVC radiation shorter than 230 nm, which has recently been attracting attention and is known as a safe and effective disinfection wavelength for the human body, cannot give a higher-dose-based inactivation rate constant compared to that of 265 nm irradiation due to the larger absorption coefficient of water with a wavelength shorter than 230 nm.

AlGaN nanowire deep ultraviolet light emitting diodes with graphene electrode

Despite graphene being an attractive transparent conductive electrode for semiconductor deep ultraviolet (UV) light emitting diodes (LEDs), there have been no experimental demonstrations of any kind of semiconductor deep UV LEDs using a graphene electrode. Moreover, although aluminum gallium nitride (AlGaN) alloys in the format of nanowires are an appealing platform for surface-emitting vertical semiconductor deep UV LEDs, in particular, at short wavelengths, there are few demonstrations of AlGaN nanowire UV LEDs with a graphene electrode. In this work, we show that transferred graphene can serve as the top electrode for AlGaN nanowire deep UV LEDs, and devices emitting down to around 240 nm are demonstrated. Compared to using metal, graphene improves both the light output power and external quantum efficiency. Nonetheless, devices with a graphene electrode show a more severe efficiency droop compared to devices with metal. Here, we attribute the heating effect associated with the large contact resistance to be the major reason for the severe efficiency droop in the devices with a graphene electrode. Detailed scanning electron microscopy and Raman scattering experiments suggest that the nanowire height nonuniformity is the main cause for the large contact resistance; this issue could be potentially alleviated by using nanowires grown by selective area epitaxy that is able to produce nanowires with uniform height. This work, therefore, not only demonstrates the shortest wavelength LEDs using a graphene electrode but also provides a viable path for surface-emitting vertical semiconductor deep UV LEDs at short wavelengths.

AlGaN-Based Deep Ultraviolet Light-Emitting Diodes with Thermally Oxidized Al x Ga2-x O3 Sidewalls.

AlGaN and GaN sidewalls were turned into AlxGa2–xO3 and Ga2O3, respectively, by thermal oxidation to improve the optoelectrical characteristics of deep ultraviolet (DUV) light-emitting diodes (LEDs). The thermally oxidized Ga2O3 is a single crystal with nanosized voids homogenously distributed inside the layer. Two oxidized AlxGa2–xO3 layers were observed on the sidewall of the AlGaN layer in transmission electron microscopy images. The first oxidized AlxGa2–xO3 layer is a single crystal, while the second oxidized AlxGa2–xO3 layer is a single crystal with numerous nanosized voids inside. The composition of Al in the first oxidized AlxGa2–xO3 layer is higher than that in the second one. The thermal oxidation at high temperature degrades the quality of the p-GaN layer and increases the forward voltage from 8.18 to 11.36 V. The thermally oxidized AlxGa2–xO3 sidewall greatly enhances the light extraction efficiency of the lateral light of the DUV LEDs by combined mechanisms of holey structure, graded refractive index, high transparency, and tensile stress. Consequently, the light output power of the DUV LEDs increases from 0.69 to 0.88 mW by introducing a 420 nm thick AlxGa2–xO3 sidewall oxidized at 900 °C, in which the enhancement of light output power can reach 27.5%.

Graphene-driving strain engineering to enable strain-free epitaxy of AlN film for deep ultraviolet light-emitting diode

The energy-efficient deep ultraviolet (DUV) optoelectronic devices suffer from critical issues associated with the poor quality and large strain of nitride material system caused by the inherent mismatch of heteroepitaxy. In this work, we have prepared the strain-free AlN film with low dislocation density (DD) by graphene (Gr)-driving strain-pre-store engineering and a unique mechanism of strain-relaxation in quasi-van der Waals (QvdW) epitaxy is presented. The DD in AlN epilayer with Gr exhibits an anomalous sawtooth-like evolution during the whole epitaxy process. Gr can help to enable the annihilation of the dislocations originated from the interface between AlN and Gr/sapphire by impelling a lateral two-dimensional growth mode. Remarkably, it can induce AlN epilayer to pre-store sufficient tensile strain during the early growth stage and thus compensate the compressive strain caused by hetero-mismatch. Therefore, the low-strain state of the DUV light-emitting diode (DUV-LED) epitaxial structure is realized on the strain-free AlN template with Gr. Furthermore, the DUV-LED with Gr demonstrate 2.1 times enhancement of light output power and a better stability of luminous wavelength compared to that on bare sapphire. An in-depth understanding of this work reveals diverse beneficial impacts of Gr on nitride growth and provides a novel strategy of relaxing the vital requirements of hetero-mismatch in conventional heteroepitaxy.

Enhancement of the light extraction characteristics and wide-angle emissive behavior of deep-ultraviolet flip-chip light-emitting diodes by using optimized optical films.

We propose the use of optical films to enhance the light extraction efficiency (LEE) and wide-angle emission of traditional packaged deep-ultraviolet light-emitting diodes (DUV-LEDs). Total internal reflection occurs easily in DUV-LEDs because they contain sapphire, which has a high refractive index. DUV-LEDs also contain an aluminum nitride (AlN) ceramic substrate, which has high light absorption in the ultraviolet band. Photons are absorbed by the sapphire and AlN ceramic substrate, which reduces the LEE of DUV-LEDs. By adding a brightness enhancement film (BEF) on the sapphire surface and a high-reflection film (HRF) on the surface of the AlN ceramic substrate, the LEE of DUV-LEDs can be increased. Moreover, we designed a single-layer metal reflective film (SMRF) on the upper surface of the quartz glass in order to achieve wide-angle emission. Experimental results indicated that compared with traditional packaged DUV-LEDs, the light output power and external quantum efficiency of DUV-LEDs with a plated BEF, HRF, and SMRF increased by 18.3% and 18.2%, respectively. Moreover, an emission angle of 160° was achieved. In a reliability test, DUV-LEDs maintained more than 95% of the initial forward voltage and light output power after 1000 h of operation at 25°C, which indicated that the addition of an optical film can improve the light efficiency and long-term reliability of DUV-LEDs.

Structural design optimization of 279 nm wavelength AlGaN homojunction tunnel junction deep-UV light-emitting diode

We demonstrated the structural optimization of AlGaN tunnel junction (TJ) deep-ultraviolet light-emitting diodes by changing the thickness and impurity concentrations of p+-type and n+-type AlGaN constituting the TJ. By decreasing the total thickness of the TJ to 23 nm, the operating voltage reached a minimum of 8.8 V at 63 A cm−2. Further decrease in TJ thickness markedly increases the operating voltage. This finding implies that the depletion layer width becomes greater than the TJ thickness if it is smaller than 12 nm. Therefore, we conclude that the TJ thickness must be greater than the depletion layer width.

Fabrication of vertical AlGaN-based deep-ultraviolet light-emitting diodes operating at high current density (∼43 kA cm−2) using a laser liftoff method

We have successfully fabricated vertical LEDs by separating a 1 × 1 cm2 wafer composed of deep-ultraviolet LED on a sapphire substrate from the substrate using a laser liftoff (LLO) method. Reproducible substrate separation was achieved by the LLO method using an Al0.68Ga0.32N underlayer film on an AlN template with periodic pillars. The fabricated vertical LED successfully demonstrated notable luminescence (peak wavelength: 298 nm) characteristics up to a current density of ∼43 kA cm−2 at room temperature and in pulsed drive, which is expected to be used in high-power LEDs and laser diodes.

Sub-nanometer ultrathin epitaxy of AlGaN and its application in efficient doping

Solving the doping asymmetry issue in wide-gap semiconductors is a key difficulty and long-standing challenge for device applications. Here, a desorption-tailoring strategy is proposed to juggle the carrier concentration and transport. Specific to the p-doping issue in Al-rich AlGaN, self-assembled p-AlGaN superlattices with an average Al composition of over 50% are prepared by adopting this approach. The hole concentration as high as 8.1 × 1018 cm−3 is thus realized at room temperature, which is attributed to the significant reduction of effective Mg activation energy to 17.5 meV through modulating the activating path, as well as the highlighted Mg surface-incorporation by an intentional interruption for desorption. More importantly, benefiting from the constant ultrathin barrier thickness of only three monolayers via this approach, vertical miniband transport of holes is verified in the p-AlGaN superlattices, greatly satisfying the demand of hole injection in device application. 280 nm deep-ultraviolet light-emitting diodes are then fabricated as a demo with the desorption-tailored Al-rich p-AlGaN superlattices, which exhibit a great improvement of the carrier injection efficiency and light extraction efficiency, thus leading to a 55.7% increase of the light output power. This study provides a solution for p-type doping of Al-rich AlGaN, and also sheds light on solving the doping asymmetry issue in general for wide-gap semiconductors.

Rapid inactivation of Dabie bandavirus (SFTSV) by irradiation with deep-ultraviolet light-emitting diode.

Severe fever with thrombocytopenia syndrome (SFTS) caused by Dabie bandavirus (SFTSV) is a serious public health concern in endemic areas, particularly in Asian and Southeast Asian countries. SFTSV is transmitted by direct contact with body fluids from infected humans and animals. Therefore, environmental hygiene in hospitals and veterinary clinics in SFTSV‐endemic areas is highly important. This study assessed the effects of continuous and intermittent irradiation with deep‐ultraviolet light‐emitting diode (DUV‐LED) on SFTSV. Evaluation was performed by conducting plaque assay in which SFTSV irradiated with deep‐ultraviolet (DUV; 280 ± 5 nm) was inoculated onto Vero cells. The results showed that continuous and intermittent irradiation for 5 s, resulting in 18.75 mJ/cm2 of cumulative UV exposure, led to a >2.7 and >2.9 log reduction, respectively, corresponding to a >99.8% reduction in infectivity. These results demonstrate that DUV can be utilized for inactivation of SFTSV to maintain environmental hygiene in hospitals and veterinary clinics in endemic countries.

Polarization-doped quantum wells with graded Al-composition for highly efficient deep ultraviolet light-emitting diodes

The efficiency performance of AlGaN-based deep ultraviolet light-emitting diodes (DUV LEDs) suffers from the strong polarization effect in multiple quantum wells (MQWs). Here, we propose V-shaped QWs to improve the quantum efficiency of DUV LEDs. We numerically investigated the effect of QWs with graded Al-composition on the device performance of DUV LEDs. Simulation results show that the QWs with graded Al-composition can mitigate the detrimental factors of traditional QWs with flat Al-composition, such as tilted energy band and separation of carrier wave function. Furthermore, compared to the QWs with linearly decreased or increased Al-composition along the growth direction, the V-shaped QWs can simultaneously alleviate the slope of valance and conduction band and obtain the better carrier confinement ability. Therefore, the DUV LED with V-shaped QWs exhibits a better optical performance.

Enhanced hole concentration in strain-compensated BAlN/AlGaN superlattice for deep ultraviolet light-emitting diodes

The hole concentration in the strain-compensated B0.14Al0.86N/Al0.5Ga0.5N superlattice (SCSL) is investigated. Compared with the Al0.6Ga0.4N/Al0.5Ga0.5N SL, the effective hole concentration in the SCSL could be improved from 1.1 ​× ​1017 ​cm−3 to 8.7 ​× ​1018 ​cm−3 due to the remarkably enlarged valence band bending from 53 ​meV to 533 ​meV. We then propose the ultraviolet light-emitting diode (UV LED) structure emitting at 284 ​nm with the SCSL p-region. Compared with the bulk p-region structure, the UV LED with the SCSL p-region shows improved output characteristics due to the reduced electron leakage and improved hole injection ability. The internal quantum efficiency (IQE) and droop ratio of the SCSL LED structure increases from 49.5% to 54.1% and decreases from 36.8% to 13.6%, respectively. Moreover, the light output powers of the Al0.6Ga0.4N/Al0.5Ga0.5N SL structure and SCSL structure have been improved by 16.3% and 49.1% over the bulk p-region counterpart. Our results indicate that the SCSL structure can be a promising candidate for the high-performance UV LED.

Enhancement of optical phonons limited electron mobility in an AlGaN step-shaped quantum well

Achieving high conductivity in n-type AlGaN of deep ultraviolet light-emitting diodes is still a challenge nowadays. In the Lei–Ting equilibrium equation method framework, a four-layer quantum well with n-Al0.7Ga0.3N barriers and an Al0.55Ga0.45N/AlxGa1−xN bi-component well could obtain a much higher electron mobility μT than that in a single-component quantum well over a relatively broad Al content range 0.56 < x < 0.68. While the built-in electric field induces strong confinement, pushing the electron to the interface, the optical phonon mode at this interface could be component-modulated to disappear due to the ternary mixed crystal effect. A significant enhancement of electron mobility can be achieved by avoiding the scattering from optical phonons at the interface where the electron gathers. The optical phonon limited μT could reach 7966 cm2/V s at x = 0.58, which is almost five times of 1518 cm2/V s in the case of x = 0.55 and ten times of 822 cm2/V s for x = 0.70. Such a step-shaped quantum well with high electron mobility could be an alternate or insert layer to the high Al content n-layer to alleviate the current crowding.

Visualization of depletion layer in AlGaN homojunction p–n junction

We analyzed the p–n junction of an aluminum gallium nitride (AlGaN) homojunction tunnel junction (TJ) deep-ultraviolet light-emitting diode by phase-shifting electron holography. We clearly obtained a phase image reflecting the band alinement of the p–n homojunction and derived a depletion layer width of approximately 10 nm. In addition, the observed depletion layer width for the AlGaN TJ was in good agreement with the simulated one reflecting the diffusion profile of Mg and Si, thus enabling a discussion on the electrical conduction mechanism for an AlGaN p–n junction.

Enhanced Light Extraction Efficiency and Modulation Bandwidth of Deep-Ultraviolet Light-Emitting Diodes with Al Nanospheres

Planar, nanopillar and Al nanosphere structure AlGaN-based deep-ultraviolet light-emitting diodes (DUV-LEDs) were numerically investigated via a three-dimensional finite difference time domain (3D FDTD) method. The three types of DUV-LEDs were compared and analyzed in terms of light extraction efficiency (LEE), Purcell factor (FP) and modulation bandwidth. The results showed that nanopillar structure DUV-LEDs with optimal nanopillar height, width and spacing can enhance transverse electric (TE)-polarized LEE to 39.7% and transverse magnetic (TM)-polarized LEE to 4.4%. The remarkable improvement was mainly due to the increased scattering effect, decreased absorption of the p-GaN layer and total internal reflection (TIR) effect. After adopting the Al nanospheres, the TE-polarized modulation bandwidth was increased by 71 MHz and the TM-polarized LEE was enhanced approximately 4.3-fold as compared to the nanopillar LED structure, while the Al nanosphere diameter was 120 nm. The reasons for promotion are mainly attributed to the coupling behavior of diploe and localized surface plasmon induced by Al nanospheres. The designed structures provide a meaningful solution for realization of high-efficiency DUV-LEDs.

On the Near-Pole Hole Insertion Layer and the Far-Pole Hole Insertion Layer for Multi-Quantum-Well Deep Ultraviolet Light-Emitting Diodes.

A novel Multi-Quantum-Well Deep Ultra Violet Light Emitting Diode (DUV-LED) device with a near-pole hole insertion layer and far-pole hole insertion layer was proposed and carefully studied. It was found that remarkable enhancements both in the light output power (LOP) and the internal quantum efficiency (IQE) could be realized by using the far-electrode hole insertion layer and near-electrode hole insertion layer compared to the conventional DUV-LED device. Inserting the near-polar hole insertion layer can increase the electric field in the hole injection layer, which will promote the ionization of the acceptor, increase the hole concentration, and enhance the light-emitting performance of the device. In addition, inserting the far-pole hole insertion layer can suppress electron leakage and promote the hole injection. At the same time, the updated electron barrier height of P-AlGaN/GaN will indirectly weaken the electrostatic field in the hole injection layer, which remains inconducive to the ionization of the acceptor, implying that the electrostatic field between the P-AGaN/GaN layer can optimize the efficiency droop of the device.

Enhanced performance of an AlGaN-based deep ultraviolet light-emitting diode using a p+-GaN/SiO2/ITO tunnel junction.

In this work, a 280-nm-wavelength deep-ultraviolet light-emitting diode (DUV LED) with a p+-GaN/SiO2/ITO tunnel junction is fabricated and investigated. Due to the decreased tunnel region width and enhanced electric field intensity in the 1-nm-thick SiO2 layer, the interband tunneling efficiency and the corresponding hole injection efficiency are promoted. Therefore, the external quantum efficiency (EQE) for the proposed device is increased when compared with a traditional DUV LED. In addition, an improved current spreading effect is observed for our proposed device. As a result, improved wall-plug efficiency (WPE) is obtained owing to the increased optical power and decreased forward operating voltage. Meanwhile, the enhanced electric field intensity in the SiO2 layer reduces the voltage drop in the p-n junction region for the proposed device, and thus the leakage current is reduced.

Optimized carrier confinement in deep-ultraviolet light-emitting diodes with AlInGaN/AlInN superlattice electron blocking layer

The optoelectronic characteristics of AlGaN-based deep ultraviolet light-emitting diodes (DUV LEDs) with AlInGaN/AlInN superlattices electron blocking layer (EBL) are numerically investigated. The results show that DUV LED with AlInGaN/AlInN EBL has superior carrier confinement capability over the DUV LED with conventional AlGaN EBL. Hence, the internal quantum efficiency (IQE) and radiative recombination rate of the proposed LED is increased by 49% and 219%, respectively as compared to conventional LED. This is assigned to the optimal recombination of electron-hole pairs in the active zone of the proposed LED. Interestingly, the proposed LED also exhibits near-droop-free efficiency.

Solid–Liquid Hybrid‐State Organic Lens for Highly Efficient Deep Ultraviolet Light‐Emitting Diodes

Deep ultraviolet light‐emitting diodes (DUV‐LEDs) are considered to be the most promising ultraviolet light sources for effective and fast sterilization under the worldwide spread of the coronavirus disease (COVID‐19). However, it reminds challenging to promote the light extraction of DUV‐LEDs owing to the strong DUV light absorption of conventional encapsulation materials. Herein, solid–liquid hybrid‐state gels (SLHGs) to fabricate organic lenses for DUV‐LEDs, which largely enhance the light extraction efficiency without sacrificing the working stability, are proposed. Solid‐state cross‐linked networks of polydimethylsiloxane are used in the SLHG structure to maintain the liquid components of silicone oil and demonstrate excellent formability, DUV transparency (over 90% transmittance in the UVC range), and thermal stability. The radiant power of DUV‐LEDs (275 nm) with the optimized SLHG organic lens is enhanced by 52.5% compared to a commercial quartz lens, achieving a record packaging optical effiency of 1.31 among solid‐state packaging DUV‐LEDs. A sterilization evaluation for SARS‐CoV‐2 demonstrates that the inactivation time can be reduced by ≈25% using the SLHG organic lens. Consequently, herein, a feasible and effective method to enhance the optical performance of DUV‐LEDs and advance their promising potential for disinfection applications is provided.

Transcriptome analysis of molecular response to UVC irradiation in zebrafish embryos

Ultraviolet (UV) rays can be both harmful and beneficial to humans. This study aimed to investigate the toxicity and safety of ultraviolet C (UVC) exposure in living organisms and the corresponding biodefense molecular mechanisms. Zebrafish embryos, at an early developmental stage (5–6 h post-fertilization), were irradiated with increasing UVC dosages using high-efficiency deep-ultraviolet light-emitting diodes (278 nm). Morphological phenotypes including survival rate, hatching rate, heart rate, and malformation rate were evaluated. Compared to un-irradiated controls, all zebrafish embryos exposed to 4.5 mJ/cm2 UVC survived and showed no significant difference in hatching and heart rate. However, 7.5 mJ/cm2 of UVC irradiation caused a significantly decreased survival rate (37.5%) and an increased malformation rate (81.8%). Therefore, 4.5 mJ/cm2 was chosen as the limit dosage that the internal biodefense system of zebrafish embryos can protect against UVC radiation. Transcriptome analysis (RNA sequencing) performed on 3 min and 3 days post-irradiation embryos (4.5 mJ/cm2) revealed the molecular mechanisms underlying the response of zebrafish embryos to irradiation. The embryos quickly responded to UVC-induced stress by activating the p53 signaling pathway. In addition, after 3 days of recuperation, the embryos showed activation of signal transducer and activator of transcription (STAT) signaling pathway. To our knowledge, this is the first study to evaluate the toxicological effects and the molecular mechanism of biodefense in zebrafish embryos upon 278 nm UVC irradiation.

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.