Deep Ultraviolet Light-emitting Diodes Research Articles

On the integrated p-type region free of electron blocking layer for AlGaN-based deep-ultraviolet light emitting diodes

AlGaN-based deep-ultraviolet light emitting diodes (DUV-LEDs) with thin p-GaN capping layer have been one of the most promising configurations, thanks to their excellent light extraction potential, which are, however, generally accompanied by insufficient hole supply. In this work, multi-graded p-AlGaN layers are adopted as an integrated p-type region, in which the electron blocking layer (EBL) is taken off to promote the carrier transport. The experimental results show that both the operation voltage and light output power of the DUV-LEDs are improved compared to the traditional ones, leading to a remarkable increase (by 114%) in their wall-plug efficiency. Further analysis confirms that the integrated p-type region serving as the hole supply layer helps holes to avoid the barrier between the p-GaN and integrated p-type region during their transport, and the absence of the EBL further eliminates part of the obstacles for hole drifting, giving rise to an enhanced hole concentration in quantum wells. Meanwhile, thanks to the modulation of the negative polarization induced bulk charges for conduction band, which is introduced within the integrated p-type region, the electron leakage is effectively suppressed even without the EBL, thus improving the device performance dramatically.

A compact portable photoacoustic sensor for sub-ppm level SO2 detection with a DUV-LED and a non-resonant photoacoustic cell

A photoacoustic module based on a novel 273 nm deep-ultraviolet light-emitting diode (DUV-LED) was developed for sulfur dioxide (SO2) detection. The DUV-LED with an emission angle of 60° emitted a power of 10 mW. A high-sensitivity non-resonant photoacoustic cell with a gas chamber volume of only 0.39 mL was introduced to reduce the thermal noise generated by the contact of scattered DUV light with the photoacoustic cell. Assembly of the DUV-LED with the non-resonant photoacoustic cell resulted in a robust and portable photoacoustic module without any moving parts. The minimum detection limit of 725 ppb was achieved with 1s integration time at room temperature and atmospheric pressure. Furthermore, the photoacoustic module performance was evaluated in terms of DUV modulation frequency, linearity and the long-term stability.

Temperature Distribution Research on Liquid Packaging Structure of Deep UV LEDs

By showing a packaged device model with 2 × 2 chips, the effects of packaging material, device height, chip spacing, thermal conductivity, and viscosity of silicone oil on temperature distribution of deep ultraviolet (UV) light-emitting diodes (LEDs) were investigated by finite element simulation. The results showed that similar temperature distributions in the horizontal and vertical directions were obtained using different packaging materials including gas, solid, and liquid. The lowest maximum temperature (131.7°C) was obtained with liquid packaging compared to the gas packaging (140.8°C) and solid packing (132.5°C). Accompanied by increasing the device height, the maximum temperature of the liquid packaging structure revealed a more significant drop compared to solid packaging. However, that of gas packaging exhibited a rise and saturation. Larger chip spacing and higher thermal conductivity of silicone oil will dramatically reduce the maximum temperature of the liquid packaging device, and a lower maximum temperature and more uniform temperature distribution were obtained by using a lower viscosity packaging material. Therefore, considering the feasibility of the device process, appropriate liquid packaging structures can be optimized, and the maximum temperature of the liquid packaging structure of 102.8°C has been achieved. Liquid packaging may have a certain impact on the reliability of device sealing due to the current immature technology. For high-power light sources, there may also be a certain impact on their lifespan.

Influence of the thickness of p-GaN ohmic contact layer on the performance of AlGaN-based deep-ultraviolet light-emitting diodes

276 nm AlGaN-based deep-ultraviolet light-emitting diodes (DUV-LEDs) with varying p-GaN thicknesses were investigated. The simulation results demonstrate that DUV-LEDs with an ultra-thin 4 nm p-GaN layer exhibit a 96.9% increase in full-angle total radiant intensity compared to those with a 350 nm p-GaN layer. Meanwhile, the total radiant intensity of the DUV-LEDs with 4 nm p-GaN were all elevated during the degree of polarization (DOP) transition from 1 to −1. The experimental results demonstrate that the maximum external quantum efficiency (EQE) and wall-plug efficiency (WPE) values of 5.3% and 4.0%, respectively, at 4 A/cm2 for the DUV-LEDs with a 4 nm p-GaN layer. These figures represent remarkable improvements of 60.6% and 42.8% when compared to DUV-LEDs featuring a 350 nm p-GaN layer. Nevertheless, it's worth noting that the adoption of an ultra-thin p-GaN ohmic contact layer introduced some surface irregularities, potentially affecting the ohmic contact of the p-type layer and resulting in a slight voltage increase, thus tempering the WPE improvement. The light extraction efficiency (LEE) values of 10.0%, 7.4%, 6.7% and 6.3% were extracted for the four samples after fitting the experimental EQE curves by the ABC model.

Polarization-matching and carrier confinement in III-nitride deep-ultraviolet light-emitting diodes

The polarization-induced quantum confined Stark effect has been recognized as a significant factor contributing to the Internal Quantum Efficiency (IQE) droop in light-emitting diodes (LEDs). This study focuses on the design of LEDs by investigating the InAlN/AlGaN interface. By incorporating InAlN quantum wells, a polarization-matched (PM) multi-quantum well (MQW) LED architecture was developed. While the flat conduction and valence bands on PM MQWs indicate an improved recombination rate, it is crucial to examine the impact on IQE, considering carrier confinement and injection efficiency influenced by the band offsets. This paper presents a numerical analysis comparing two LEDs emitting at 245 and 275 nm, respectively. The results demonstrate that the PM LED operating at 275 nm exhibits enhanced performance, benefiting from high probability density overlap. Conversely, the PM LED emitting at 245 nm demonstrates poor confinement, resulting in an overall low performance, regardless of polarization matching.

Investigation of highly reflective p-electrodes for AlGaN-based deep-ultraviolet light-emitting diodes.

We proposed a "Ni sacrifice" method to fabricate Al-based highly reflective p-electrode in the ultraviolet spectral region for AlGaN-based deep-ultraviolet light-emitting diodes (DUV-LEDs). The "Ni sacrifice" p-electrode could have a high optical reflectivity of around 90% at the DUV spectral region below 300 nm. Compared to Ni/Au, indium tin oxide (ITO), and Pd p-contacts, the "Ni sacrifice" led to a higher resistivity of p-contacts and a slightly higher operated voltage of the DUV-LEDs (within 0.6 V at 20 mA). Although the electrical performance was degraded slightly, the light output power and external quantum efficiency of the DUV-LEDs could be improved by utilizing the "Ni sacrifice" p-electrode. Besides, we introduced a grid of vias in the device mesa and reduced the diameter of the vias to achieve an enhanced peak external quantum efficiency (EQE) up to 1.73%. And the wall-plug efficiency (WPE) of DUV-LEDs with a "Ni sacrifice" p-electrode was higher than that of Ni/Au p-electrode DUV-LEDs at low currents. These results highlight the great potential of the proposed "Ni sacrifice" reflective p-electrode for use in DUV-LEDs.

AlGaN-Based Deep-Ultraviolet Light-Emitting Diodes With Varied Thickness of Sidewall Passivation via Atomic Layer Deposition

AlGaN-Based Deep-Ultraviolet Light-Emitting Diodes With Varied Thickness of Sidewall Passivation via Atomic Layer Deposition

Far-field pattern control and light-extraction enhancement of deep-ultraviolet light-emitting diodes with large-area Fresnel zone plate nano-structures

Conventional methods using high-purity quartz lenses to control deep-ultraviolet light-emitting diode (DUV-LED) far-field patterns have limitations, including small effective apertures and high cost. We apply phase-type Fresnel zone plates to control the beam angle and enhance light extraction efficiency (LEE) for DUV-LEDs on sapphire and AlN substrates. We demonstrate highly-collimated optics-free DUV-LED emissions with full width at half maximum far-field divergence angles of 40° and 10° on sapphire and AlN substrates at a peak emission wavelength of 279 nm and 273 nm, respectively. LEE enhancements of 1.4 and 1.5 times for DUV-LEDs on sapphire and AlN substrates, respectively, are also achieved.

Performance Enhancement of AlGaN-based Deep Ultraviolet Light-emitting Diodes with AlxGa1-xN Linear Descending Layers

Performance Enhancement of AlGaN-based Deep Ultraviolet Light-emitting Diodes with AlxGa1-xN Linear Descending Layers

193 nm laser annealing on p-GaN with enhanced hole concentration and wall-plug-efficiency in deep ultraviolet LED

The high acceptor ionization energy and, thus, low carrier concentration in p-type III-nitride have widely been recognized as the bottleneck preventing the development of high-efficiency light-emitting-diodes (LEDs). In this contribution, the influences of 193 nm pulsed laser annealing on the structure, electrical, and optical properties of p-type GaN were systematically analyzed. The hole concentration of p-GaN first increases and then decreases with increasing laser fluence, regardless of post-growth thermal annealing. The effective dopant activation due to laser annealing can be attributed to the dissociation of Mg–H complexes during the treatment. Laser-annealed p-GaN was utilized in a 280 nm deep ultraviolet LED. A maximum of 1.47 times higher wall-plug-efficiency enhancement factor was obtained compared to that without laser annealing, demonstrating that 193 nm laser annealing plays a decisive role in the boosting of quantum efficiency of optoelectronic devices.

Enhanced hole injection in Ga-polar 290 nm AlGaN-based DUV LEDs with a p-n junction hole accelerator

The limited kinetic energy of holes in AlGaN-based deep ultraviolet light-emitting diodes (DUV LEDs) poses a challenge in their transportation into the active region across the Al-rich electron blocking layer (EBL) and significantly restricts the electrical and optical performance of DUV LEDs. In this work, we propose a hole accelerator structure composing a p-AlxGa1-xN/n-AlxGa1-xN junction to improve the hole injection efficiency and explore the mechanism behind the enhanced performance with the Advanced Physical Models of Semiconductor Devices software (APSYS). The built-in electric field of the p-n junction distributes along the [000-1] direction, which can enhance the hole drift velocity and improve the hole injection into the active region. Moreover, with an optimum Al composition of 50%, [000-1] oriented polarization-induced electric field can be generated at the vicinity of both the p-EBL/accelerator and accelerator/hole supplier interfaces, which further boosts the holes into the active region. Besides, the original steep barrier for holes at the EBL/hole supplier interface can be splited into a two-step barrier which is more favorable for hole transportation. As a result, an enhanced optical power by 49.4% and alleviated efficiency droop by 76.3% can be achieved with the proposed p-n junction-based hole accelerator. The results can pave the way for AlGaN-based DUV LEDs towards high-power and high-efficiency applications.

Achieving atomically ordered GaN/AlN quantum heterostructures: The role of surface polarity.

Interface engineering in heterostructures at the atomic scale has been a central research focus of nanoscale and quantum material science. Despite its paramount importance, the achievement of atomically ordered heterointerfaces has been severely limited by the strong diffusive feature of interfacial atoms in heterostructures. In this work, we first report a strong dependence of interfacial diffusion on the surface polarity. Near-perfect quantum interfaces can be readily synthesized on the semipolar plane instead of the conventional c-plane of GaN/AlN heterostructures. The chemical bonding configurations on the semipolar plane can significantly suppress the cation substitution process as evidenced by first-principles calculations, which leads to an atomically sharp interface. Moreover, the surface polarity of GaN/AlN can be readily controlled by varying the strain relaxation process in core-shell nanostructures. The obtained extremely confined, interdiffusion-free ultrathin GaN quantum wells exhibit a high internal quantum efficiency of ~75%. Deep ultraviolet light-emitting diodes are fabricated utilizing a scalable and robust method and the electroluminescence emission is nearly free of the quantum-confined Stark effect, which is significant for ultrastable device operation. The presented work shows a vital path for achieving atomically ordered quantum heterostructures for III-nitrides as well as other polar materials such as III-arsenides, perovskites, etc.

Deep-ultraviolet light communication in sunlight using 275-nm LEDs

Emerging deep-ultraviolet (DUV) light communication with attractive features fulfills the increasing demand for novel wireless communication without electromagnetic interference or solar noise influence. Here, we propose, manufacture, and characterize a solar-blind full-duplex light communication system using 275-nm DUV light-emitting diodes (LEDs). Four DUV LEDs connected in series form a unit, and the transmitter consists of an array of three independent units, which can be synchronously controlled using a main processing unit to modulate the emitted light. A solar-blind filter is inserted in front of the receiver due to the lack of an ultraviolet photodetector. Both the transmitter and receiver are encapsulated together to manufacture a full-duplex DUV light communication system, which can independently transmit and receive data with a maximal communication distance of 7 m at the same time. All users within a 46 m2 area of the system have access to the DUV communication network through an integrated wireless module. Real-time video is demonstrated and different types of files could be shared. Based on Transmission Control Protocol/Internet Protocol (TCP/IP), the system is finally performed in sunlight with a transmission rate of 10 Mbps and a packet loss rate of 1.28%.

Polymethoxyflavone (PMF) Accumulation and PMF Biosynthesis-Related Gene Expression in Tachibana (Citrus tachibana Tanaka) Fruit Treated Using a Combination of Gibberellic Acid and Prohydrojasmon and Post-harvest Deep-Ultraviolet Light-Emitting Diode Irradiation

Polymethoxyflavone (PMF) Accumulation and PMF Biosynthesis-Related Gene Expression in Tachibana (Citrus tachibana Tanaka) Fruit Treated Using a Combination of Gibberellic Acid and Prohydrojasmon and Post-harvest Deep-Ultraviolet Light-Emitting Diode Irradiation

Multi-wavelength deep-ultraviolet absorbance detector based upon program-controlled pulsing light-emitting diodes

A novel approach for multi-wavelength ultraviolet (UV) absorbance detection has been introduced employing a single board computer (SBC) with a field programmable gate array (FPGA), Red Pitaya SBC, to generate separated micro pulses for three deep-ultraviolet light-emitting diodes (DUV-LEDs), λmax = 235, 250, and 280 nm, along with data acquisition and processing via a custom-made program. The pulse set generation and data acquisition were synchronized using the SBC. The outputs of the three pulsing DUV-LEDs were combined and transmitted to the flow cell via a solarisation resistant trifurcated optical fiber (OF). An ultra-fast responding photodiode was connected to the optical-fiber-compatible flow cell to record the intensity of the DUV pulses. Upper limit of detector linearity (A95 %) was found to be 1917 mAU, 2189 mAU, and 1768 mAU at 235 nm, 250 nm, and 280 nm, respectively, with stray light ≤0.9 %. In addition, the effective path length (Leff) was estimated to be ≥98.0 % of the length of the used flow cell (50 mm). The new pulsed multi-LEDs absorbance detector (PMLAD) has been successfully coupled with a standard liquid chromatograph and utilized for the analysis of pharmaceuticals. Paracetamol, caffeine, and aspirin were simultaneously determined at 250, 280, and 235 nm, respectively, using the PMLAD. The absorbance ratios between the different wavelengths were applied to further confirm the identity of the studied compounds. Excellent linearity was achieved over a range of 0.1–3.2 µg/mL for paracetamol, 0.4–6.4 µg/mL for caffeine, and 0.8–12.8 µg/mL for aspirin with a regression correlation coefficient (r2) ≥ 0.99996. The quantitation limits (LOQs) were 0.10 µg/mL, 0.38 µg/mL, and 0.66 µg/mL for paracetamol, caffeine, and aspirin, respectively.

High‐Power AlGaN‐Based Ultrathin Tunneling Junction Deep Ultraviolet Light‐Emitting Diodes

AbstractTunnel junctions (TJs) offer a unique approach to utilizing nonequilibrium tunneling injection of holes and have demonstrated potential applications in ultraviolet (UV) emitters. However, high operating voltage caused by the wide bandgap of the III‐nitrides has impeded the further promotion of TJ. Here, 275 nm n+‐Al0.45Ga0.55N/p+‐Al0.5Ga0.5N ultrathin tunnel junction (UTJ) deep‐UV light‐emitting diodes (LEDs) are developed for minimizing the electrical losses and achieve, to the current knowledge, the lowest operating voltage (5.7 V) at 300 mA reported to date. This study discovers that Mg‐Si co‐doping occurs in the n+‐Al0.45Ga0.55N layer during the growth of n+‐Al0.45Ga0.55N/p+‐Al0.5Ga0.5N UTJ due to the memory and diffusion effect of Mg, which leads to the formation of Ohmic contact between high‐work‐function Ni/Au and Mg‐Si co‐doped n+‐ Al0.45Ga0.55N layer in UTJ. The Zener diode, fluorine resin, and optimally designed glass lens are incorporated into the chip encapsulation to enhance the reliability and optical performance of the device. Furthermore, a high‐efficiency sterilization deep‐UV light source is developed integrated with 120 UTJ deep‐UV LED chips. Complete surface inactivation at a long irradiation distance (20 cm) is achieved within only seconds using the high‐power sterilization deep‐UV light source. These results indicate that UTJ is promising in developing deep UV LEDs and their integrated light sources.

Synthesis of a waterproof geopolymer adhesive applied in DUV LEDs packaging

This paper presents the synthesis of a waterproof geopolymer based on the modification of silicon phosphate (Si3(PO4)4) that adheres to alumina and quartz glass substrate, which can replace traditional organic polymers for packaging deep-ultraviolet light-emitting diodes. The waterproof property of geopolymer strongly depends on the Si3(PO4)4 loading amount due to its hydrolysate H+ substitutes Na+, which is vulnerable to water. FESEM and microhardness results show that aging by 85°C/85% relative humidity for 100 h, the optimal waterproof geopolymer adhesive is successfully synthesized when the mass ratio of metakaolin, water glass, and Si3(PO4)4 is equal to 0.48: 1: 0.08 respectively, and the corresponding adhesion strength of 6.41 MPa to alumina substrate is achieved after the reliability testing. The rheological properties show that the optimal waterproof geopolymer adhesive, with shear thinning behavior and good thixotropy, is suitable for processing while holding the original shape.

Improving the hole injection efficiency in AlGaN DUV LEDs by minimizing the band offset at the p-EBL/hole supplier interface

In AlGaN-based deep ultraviolet light-emitting diodes (DUV LEDs), the large valence band offset between the Al-rich electron blocking layer (EBL) and p-AlGaN hole supplier weakens the chance of holes being injected into the active region. Only holes with kinetic energy larger than the barrier height at the EBL/p-AlGaN interface are allowed to climb over the EBL before entering the active region, limiting the hole injection efficiency and thus reducing the external quantum efficiency (EQE). In this work, we incorporate a thin AlGaN insertion layer between EBL and the p-AlGaN hole supplier to enhance the hole injection efficiency of DUV LEDs via regulating the energy band at the p-EBL/p-AlGaN interface. By systematically investigating and analyzing the effects of aluminum components in the insertion layers on the hole injection and the electron confinement, we found that the insertion layer with an Al composition of 45% can effectively enhance the EQE of DUV LEDs by 40.5% and suppress efficiency droop by 65.5%. The design strategy provides an effective approach to boost the hole injection efficiency for AlGaN-based DUV LEDs.

Enhanced light output power from AlGaN-based deep ultraviolet LEDs achieved by a four-in-one mesa structure

This study aimed to investigate the impact of mesa geometry on the light output characteristics of AlGaN-based 275 nm deep ultraviolet light-emitting diodes (DUV-LEDs). By dividing the original single-junction mesa into four parts and connecting them serially (four-in-one), high-voltage (HV) DUV-LEDs with rectangular, hexagonal, circular, triangular, and square submesas were realized, achieving significant enhancement of the light output power (LOP) and wall-plug efficiency (WPE). The LOP of HV DUV-LEDs with hexagonal submesas has been promoted substantially compared to that of the original DUV-LEDs. Among the investigated five different types of submesas, hexagonal-type HV DUV-LEDs can achieve the highest LOP and WPE due to the higher sidewall light extraction. Furthermore, it is also demonstrated that pulse current driving can reduce the self-heating effect of HV DUV-LEDs.

Simultaneously improved hole injection and current uniformity in 293 nm AlGaN-based deep ultraviolet light-emitting diodes

Insufficient hole injection and current nonuniformity caused by the nature of the p-AlGaN hole injection layer (HIL) are two issues impeding the advances in the optical and electrical properties of AlGaN-based deep ultraviolet light-emitting diodes (DUV LEDs). To simultaneously improve the hole injection efficiency and current uniformity, an Al-linearly-decreasing AlGaN PN junction (ALD AlGaN PN junction) is proposed to replace the conventional p-AlGaN HIL with a constant Al component. The barrier height for holes at the interface between the p-type electron blocking layer (p-EBL) and HIL as well as the hole concentration and resistance in the p-AlGaN HIL can be modulated at the same time by the proposed ALD AlGaN PN junction. As a result, the hole injection efficiency is enhanced due to the reduced barrier height at the p-EBL/HIL interface and the polarization-induced three-dimensional hole gas (3DHG) induced in the proposed ALD AlGaN PN junction HIL. In addition, the lateral current uniformity is improved by the properly regulated resistance in the p-AlGaN HIL, which can be ascribed to the additional barrier for holes in the valence band because of the incorporated ALD AlGaN PN junction. The proposed DUV LED with ALD AlGaN PN junction exhibits enhanced EQE by a factor of 39.2% at 100 A/cm2 and simultaneously improved current uniformity in the active region. This designed ALD AlGaN PN junction structure provides a promising strategy for achieving high-performance DUV LEDs.