Output Power Of LEDs Research Articles

(Invited) Ultrahigh Efficiency Excitonic Micro-LEDs

To date, it has remained challenging to achieve high efficiency micro-LEDs, which are desired for next generation displays and virtual/augmented reality. The performance degradation with reducing dimensions for conventional LEDs has been largely attributed to the plasma damage induced on the mesa sidewalls. Here, we show such a fundamental challenge can be overcome by developing nanowire excitonic LEDs. In this study, InGaN/GaN nanowire LED heterostructures were grown on N-polar GaN-on-sapphire substrates utilizing selective area epitaxy. Studies have shown that the exciton binding energy in strain-relaxed InGaN nanostructures can be dramatically enhanced. Due to the strong Coulombic interaction between electrons and holes, the impact of nonradiative SRH recombination on the performance of an excitonic LED is significantly reduced. By exploiting the large exciton oscillator strength of quantum-confined nanostructures, we have achieved an external quantum efficiency (EQE) exceeding 25% for a green-emitting device with lateral dimensions less than 1 micrometer, which is the highest value reported for any LEDs of this size to the best of our knowledge. It is further observed that the EQE and output power of the excitonic LED show a significantly faster rising trend with current injection, compared to conventional devices. We have identified several important factors for achieving high efficiency excitonic LEDs, including the epitaxy of nanostructures to achieve strain relaxation and reduced dislocation densities and the formation of three-dimensional quantum-confinement to enhance electron-hole wavefunction overlap. We have further investigated the effect of Mg incorporation in p-GaN layer on the device performance. We found out that an optimized Mg dopant incorporation in nanowire structures can contribute significantly to the efficiency of micro-LEDs. We have measured significantly enhanced performance by optimizing Mg dopant incorporation, which leads to reduced leakage current and improved EQE. For example, for a red-emitting micro-LED, a peak external quantum efficiency of >8% was measured for a device with a lateral dimension less than one micrometer by optimizing Mg-dopant incorporation. Through these studies, we show that by harnessing the exciton oscillator strength of nearly dislocation-free nanostructures, together with a careful optimization of p-doping in the contact layer, the efficiency bottleneck of micro-LEDs can be addressed.Conflict of interest: Some IP related to this work has been licensed to NS Nanotech, Inc., which was co-founded by Z. Mi. The University of Michigan and Mi have a financial interest in the company.

Enhancement of Light Efficiency of Deep-Ultraviolet Light-Emitting Diodes by Encapsulation with a 3D Photonic Crystal Reflecting Layer.

Recently, UVC LEDs, which emit deep ultraviolet light, have found extensive applications across various fields. This study demonstrates the design and implementation of thin films of three-dimensional photonic crystals (3D PhCs) as reflectors to enhance the light output power (LOP) of UVC LEDs. The 3D PhC reflectors were prepared using the self-assembly of silica nanospheres on a UVC LED lead frame substrate via the evaporation-induced method (side) and the gravitational sedimentation method (bottom), respectively. These PhCs with the (111) crystallographic plane were deposited on the side wall and bottom of the UVC LED lead frame, acting as functional materials to reflect UVC light. The LOP of UVC LEDs with 3D PhC reflectors at a driving current of 100 mA reached 19.6 mW. This represented a 30% enhancement compared to commercial UVC LEDs with Au-plated reflectors, due to the UVC light reflection by the photonic band gaps of 3D PhCs in the (111) crystallographic plane. Furthermore, after aging tests at 60 °C and 60% relative humidity for 1000 h, the relative LOP of UVC LEDs with 3D PhC reflectors decreased by 7%, which is better than that of commercial UVC LEDs. Thus, this study offers potential methods for enhancing the light output efficiency of commercial UVC light-emitting devices.

High Efficiency Deep Ultraviolet Light-Emitting Diodes With Polarity Inversion of Hole Injection Layer

In this work, we investigate the performance improvement of N-polar AlGaN-based deep ultraviolet light-emitting diodes (DUV LEDs) by the inversion of the hole injection layer from N-polar to Ga-polar. The influence of different inversion points on the performance and the energy band of DUV LED is systematically studied, and the principle of performance improvement of DUV LED is explained in detail through the analysis of the energy band. Furthermore, according to the simulation results and practical application, an appropriate inversion point is selected. Under the current of 120 mA, with polarity inversion of the hole injection layer, the light output power of the DUV LEDs increases from 21.34 mW to 29.71 mW, and the applied voltage reduces from 16.06 V to 9.63 V. The DUV LED with polarity inversion has a 132% increase in wall-plug efficiency (WPE) compared with the DUV LED which is totally along [000-1] direction. Therefore, polarity inversion is an effective way to achieve high-performance UV-LEDs.

Effect of window layer with different growth modes on the photoelectric properties of AlGaInP LED

The AlGaInP window layers with different growth modes of Al in (AlxGa1−x)0.5In0.5P were prepared to investigate the effect of different growth modes of Al on the photoelectric properties of red LEDs. The experimental results show that the forward voltage of (AlxGa1−x)0.5In0.5P LEDs n-window layer with the ridge gradient Al content was 30 mV (x = 0.15) and 190 mV (x = 0.45), respectively, which are lower than that of the n-window layer without ridge gradient Al content samples. Meanwhile, the light output power of the ridge gradient Al content LED is 12.3% (x = 0.15) and 3.6% (x = 0.45) higher than that of the other two samples, respectively. Compared with the Al composition ridge gradient sample, the photoelectric efficiency is 2.45% (x = 0.15) and 5.68% (x = 0.45), respectively, due to the reduction in the voltage and the increase in the light output power.

Enhancement of the light output efficiency and thermal stability of AlGaN-based deep-ultraviolet light-emitting diodes with Ag-nanodot-based p-contacts and an 8-nm p-GaN cap layer.

The efficiency of AlGaN-based deep-ultraviolet light-emitting diodes (DUV LEDs) is limited by the high absorption issue of the p-GaN contact layer or poor contact properties of the transparent p-AlGaN contact layer. Enhancement of the light output efficiency and thermal stability of DUV LEDs with an emission wavelength of 272 nm was investigated in this work. Ag nanodots on an 8-nm p-GaN cap layer were used to form ohmic contact, and Al and Mg reflective mirrors were employed to enhance the light output power (LOP) of DUV LEDs. However, serious deterioration of LOP occurred after the high-temperature process for the LEDs with Al and Mg reflective mirrors, which can be attributed to the damage to the ohmic contact properties. A Ti barrier layer was inserted between the Ag/p-GaN and Al layers to prevent the degeneration of ohmic contact. The wall-plug efficiency (WPE) of DUV LEDs fabricated by the Ag-nanodot/Ti/Al electrode is 1.38 times that of LEDs fabricated by adopting a thick Ag layer/Ti/Al at 10 mA after a high-temperature process. The Ag-nanodot/Ti/Al electrode on thin p-GaN is a reliable technology to improve the WPE of DUV LEDs. The experimental and simulated results show that the ohmic contact is important for the hole-injection efficiency of the DUV LEDs when p-GaN is thin, and a slight increase in the contact barrier height will decrease the WPE drastically. The results highlighted the importance of thermally stable ohmic contacts to achieve high-efficiency DUV LEDs and demonstrated a feasible route for improving the LOP of DUV LEDs with a thin p-GaN cap layer and stable reflective electrodes.

Unexpectedly Simultaneous Increase in Wavelength and Output Power of Yellow LEDs Based on Staggered Quantum Wells by TMIn Flux Modulation.

Pursuing efficient long-wavelength InGaN LED has been a troublesome issue to be solved, which forms interesting subjects for fundamental research, but finds also motivation in extensive applications. Here, we investigate the effect of TMIn (trimethylindium) flux variation for growing bandgap-engineered staggered quantum wells (QWs) on corresponding LED properties and demonstrate the unexpectedly simultaneous increase in light output power (LOP) and emission wavelength. At 20 mA, LEDs based on staggered QWs grown under low flux show an increase of 28% in LOP and longer wavelength compared to that under high flux. The experimental results reveal that TMIn flux affects crystalline quality and indium composition of epilayers. Under high TMIn flux, high in-plane strain exists between adjacent layers, accompanied by the composition pulling effect, which reduces indium incorporation for the following staggered QW growth and hinders realization of yellow light emission. According to simulation results, low-flux-grown staggered QWs contribute to increased carrier wavefunction overlap as well as enhanced electric field. Notably, the former enables high LOP, while the latter results in emissions towards long wavelength, promising to solve an ever-present concern that LED performance deteriorates with increasing emission wavelength. Therefore, this work shows great significance in thoroughly understanding growth conditions for bandgap-engineered staggered QW structures, which offers a facile solution to achieve efficient long-wavelength optoelectronics devices.

Improved performance of deep ultraviolet AlGaN-based light-emitting diode by reducing contact resistance of Al-based reflector

In this study, chlorine (Cl) treatment was carried out on p-AlGaN to enhance the performance of ultraviolet-C light emitting diodes (UVC LEDs) by modifying ITO work function and hence reducing the contact resistance of ITO/Al reflector. The Cl-treated UVC LEDs exhibit the forward voltage of 6.88 V at 20 mA, whereas the reference samples show 7.50 V. The light output power and relative wall plug efficiency (WPE) of the Cl-treated UVC LEDs are enhanced by 17.1% at 500 mW and 19.5% at 100 mA, respectively, as compared to the reference. Additionally, the Cl-treated LEDs also display reduction in both the leakage current and ideality factor. Further, the photoluminescence (PL) intensity of AlGaN micro-disks is also enhanced by the Cl-treatment. X-ray photoemission spectroscopy (XPS) results indicate the formation of Cl-ITO at the ITO/p-AlGaN interface and the passivation of the surface states of AlGaN by Cl radicals. Based on the XPS results, a possible mechanism for the improved performance of Cl-treated UVC AlGaN-based LEDs is described and discussed.

Performance enhancement of ultraviolet light-emitting diodes by manipulating Al composition of InGaN/AlGaN superlattice strain release layer

InGaN/AlGaN ultraviolet light-emitting diodes (UV LEDs) suffer from residual in-plane compressive stress and poor carrier injection efficiency. Here, we simultaneously reduce the stress and improve the carrier concentration in In0.018Ga0.982N/Al0.05Ga0.95N multiple quantum wells by optimizing the Al composition of the underlying InGaN/AlGaN superlattice strain release layer (SRL). On the one hand, higher Al composition of SRL can reduce the residual in-plane compressive stress of epilayers, which is beneficial for increasing radiative recombination rate. On the other hand, higher Al composition can enhance the effective barrier height in SRL, which can decelerate electrons and prevent holes from transporting into the n-region. As a result, the light output power of UV LEDs with In0.01Ga0.99N/Al0.06Ga0.94N SRL is 19.3% higher than that of UV LEDs with In0.01Ga0.99N/Al0.02Ga0.98N SRL at 100 mA.

Characteristics of Micro-Size Light-Emitting Diode With Pentagon-Type Structure

We systemically investigated the effect of chip structures on the optical characteristics of micro-size LEDs. The total internal reflection (TIR) can be improved by selecting a suitable chip structure, thereby increasing the light efficiency. The saturation light output power (LOP) of the micro-size LED using pentagon-type structure is 44.78 mW, an increase of 6.08% compared with that of micro-size LED using circular-type structure. Simulation and experimental results indicate that chip structure of pentagon-type can effectively enhance the light extraction from the top or side of the micro-size LED.

Temperature-dependent study on AlGaN-based deep ultraviolet light-emitting diode for the origin of high ideality factor

Deep ultraviolet light-emitting diodes (DUV LEDs) are promising light sources for disinfection, especially during the pandemic of novel coronavirus (COVID-19). Despite much effort in the development of DUV LEDs, the device temperature and ideality factor are key parameters of devices, which are often neglected. Here, we developed a simple and convenient method to study the behavior of a 280 nm AlGaN-based DUV LED, obtaining the electrical, optical, and thermal properties within one measurement. From the experimental results, we find that the light output power and wall-plug efficiency of the AlGaN-based DUV LED are strongly affected by device temperature, ideality factor (β), and series resistance (Rs). β decreases from 9.3 to 8.1 at 40 mA when the temperature increases from 302 to 317 K. We compared these results with simulations and found that the high potential barriers inside the device and the carrier concentration in n-type or p-type layers, especially the hole concentration in p-type layers, are the two key factors for the high value of the ideality factor from the LED structure. As the device temperature increases, carriers with higher energy would overcome some potential barriers and Mg acceptor activation would be more efficient, which are beneficial for carrier transportation. However, these also lead to the carrier overflow and weaken the radiative recombination rate. The trade-off role of device temperature in carriers between transportation and overflow is needed to be considered in the future development of DUV LEDs with higher efficiency and higher brightness.

Morphological effect of patterned sapphire substrate on efficiency of white-light phosphor LED package

Six-facets patterned sapphire substrates (A-PSS) are manufactured by wet-etching regular semispherical patterned sapphire substrates (r-PSS). The possible crystallographic planes of the etched planes on the semi-sphere pattern is the family of (1102) plane. Metalorganic Chemical Vapor Deposition (MOCVD) GaN epitaxial LED structure was grown on the c-plane sapphire wafer, r-PSS, and A-PSS, which were further processed as horizontal LED chips. Compared to LEDs on the flat c-plane sapphire, the light output power of LEDs on r-PSS and A-PSS is enhanced by 71% and 67%, respectively. The output power of the bare LEDs decreases after the white-light LED package. Remarkably, the output power of the white-light packaged LED on the A-PSS is larger than the white-light packaged LED on the r-PSS. The Monte Carlo ray tracing simulation shows that, before and after silicone-lens encapsulating, the LEDs on r-PSS and A-PSS emit more on the top surface. However, the sidewall emission power ratio of the LED on r-PSS decreases by 3.44% after silicone encapsulating. On the other hand, the sidewall emission power ratio of the LED on A-PSS increases by 1.06% after silicone encapsulating. Note that the emission light of LEDs out of the top surface travels a longer path in the silicone lens than the emission light out of the sidewalls. Hence, more re-extracted light out of the sidewall of LED on A-PSS would be the reason for the LED on A-PSS having a less package loss and the white-light package efficiency than the LED on r-PSS. In conclusion, the light radiation distribution pattern defined by the pattern morphology of A-PSS is the key for the better white-light package efficiency of the LED on A-PSS than that of the LED on r-PSS. • The patterned sapphire substrate morphology can alter the propagation direction of the light emission. • Monte Carlo ray tracing simulation shows the emission power of top surface and sidewall vary before and after lens package. • The emission light of LEDs out of the top surface travels a longer path in the phosphor layer than the sidewalls. • The light radiation distribution defined by the pattern morphology is the key for white-light package efficiency of LED.

Improvement in the Output Power of Near-Ultraviolet LEDs of p-GaN Nanorods through SiO2 Nanosphere Mask Lithography with the Dip-Coating Method.

In this paper, the conditions of the dip-coating method of SiO2 nanospheres are optimized, and a neatly arranged single-layer SiO2 array is obtained. On this basis, a “top-down” inductively coupled plasma (ICP) technique is used to etch the p-GaN layer to prepare a periodic triangular nanopore array. After the etching is completed, the compressive stress in the epitaxial wafer sample is released to a certain extent. Then, die processing is performed on the etched LED epitaxial wafer samples. The LED chip with an etching depth of 150 nm has the highest overall luminous efficiency. Under a 100 mA injection current, the light output power (LOP) of the etched 150 nm sample is 23.61% higher than that of the original unetched sample.

Facile thiol-epoxy click chemistry for transparent and aging-resistant silicone/epoxy composite as LED encapsulant

The thiol-epoxy click chemistry is promising for microelectronic packaging due to its outstanding properties, while there is little knowledge on silicone/epoxy composite fabricated via this approach. Herein, a thiol-functionalized silicone resin (MDT-SH) is synthesized and compounded with epoxy resin to form silicone/epoxy composite. The thermal-curing behaviors, mechanical properties and LED encapsulation of the developed composite are studied detailedly. The thermal-curing kinetics indicate that the thiol-epoxy polymerization in silicone/epoxy composite tends more to the first-order reaction and its active energy is about 60.3 kJ/mol. The mechanical properties of the composite are varied with the molecular structure of MDT-SH and thiol/epoxy ratio. Moreover, the composite has excellent transparency in visible region, and its transparency is little affected by thermal and UV ageing. More importantly, the transparency is only slightly decreased after the composite is serviced at normal temperature and pressure for 2 years. The light output power of LEDs encapsulated by this composite is comparable to the commercial counterparts like Dow Corning OE-6550, but its thermal-curing time and temperature are much lower than the latter, contributing to cost reduction. Therefore, this silicone/epoxy composite based on thiol–epoxy click chemistry would greatly improve the microelectronic packaging.

Investigation of quantum structure in N-polar deep-ultraviolet light-emitting diodes

This study systematically investigates the optical performance of N-polar deep-ultraviolet light-emitting diodes (DUV LEDs) in consideration of different quantum structures in the active region, with a highlight on various thicknesses of quantum barrier (QB), quantum well (QW), and the electron-blocking layer (EBL). The results show that the internal quantum efficiency (IQE), as well as light output power (LOP) of N-polar DUV LED, is not sensitive to QB thickness. On the contrary, the LOP and IQE performance can be significantly enhanced by increasing the QW thickness from 2 to 4 nm. Moreover, a saturated LOP in the N-polar DUV LEDs can be observed after QW thickness increased to a certain level as there is a trade-off between boosted carrier concentration and decreased wave function overlap in the active region. Lastly, the impact of the EBL on the optical performance of the N-polar DUV LED is also investigated. Specifically, a thicker EBL or a higher Al composition in the EBL leads to an increase in the turn-on voltage and series resistance while the LOP value remains unchanged. These findings lay the foundation for the development of high-performance N-polar DUV LEDs of the future.

AlGaN-based deep ultraviolet light emitting diodes with magnesium delta-doped AlGaN last barrier

A magnesium delta-doped AlGaN last barrier (MDDLB) was introduced in the structure of deep ultraviolet light emitting diodes (DUV LEDs) to improve their light output power. The MDDLB effectively improved hole injection efficiency and increased the hole concentration at the last AlGaN well of DUV LEDs. It also raised the potential barrier for electron transport from multiple quantum wells to the p-side. Therefore, it reduced overflow of electrons into the p-side of DUV LEDs. These phenomena improved light emitting efficiency of DUV LEDs with the MDDLB. In addition, the current crowding effect was suppressed by the MDDLB in DUV LEDs. Therefore, the 350 mA-light output power of DUV LEDs with the MDDLB was approximately 30% larger than that of DUV LEDs without the MDDLB. Furthermore, the largest light output power of DUV LEDs with the MDDLB was 55 mW, which was approximately 46% larger than that of DUV LEDs without the MDDLB. The suppressed current crowding effect by the MDDLB also reduced efficiency droops of DUV LEDs with the MDDLB. Therefore, efficiency droops of DUV LEDs decreased from 64% to 55% when the MDDLB was introduced.

Improvement of p-electrode structures for 280 nm AlGaN LED applications

An improvement of Ni/Au/p+-GaN p-electrode for AlGaN deep-ultraviolet light-emitting diodes (DUV LEDs) with the emission wavelength of 280 nm is proposed for both p-side-up and flip-chip structures. An interdigitated multi-finger Ni/Au was employed in p-side-up DUV LED, where the p-GaN contact layer was partially removed to improve the light extraction efficiency without a serious current-crowding effect. The 9- and 12-finger LEDs were determined to have higher thermal dissipation and lower surface temperatures and correlated well with the theoretical simulation. For the comparison of p-side-up emission LEDs, the output power of 9-finger LED is 172% higher than that of conventional LED at the current injection of 350 mA. The optimum p-electrode pattern was further applied to the flip-chip LED structure. It is determined that the output power of 9-finger flip-chip LED at 350 mA is still 14.6% higher than that of a conventional flip-chip LED. The higher output power of 9-finger flip-chip LED with a wall-plug efficiency of 1.05% is attributed to the combination of the improved current-spreading path and the higher reflection through the moderate removal of partial p+-GaN absorbing layer.

Performance enhancement of AlGaN-based deep ultraviolet light-emitting diodes by using stepped and super-lattice n-type confinement layer

Abstract In this paper, a new n-type confinement layer that uses the stepped and super-lattice structure to replace conventional n-type AlGaN layer of deep ultraviolet light-emitting diode (DUV LED) is investigated. The simulation results indicate that the new structure significantly enhances the light output power (LOP) and internal quantum efficiency (IQE) of DUV LED. This is because the capability of carrier confinement in the quantum wells (QWs) is enhanced; the carrier concentrations and the radiative recombination rate in the active region of DUV LED are improved.

High performance electron blocking layer-free InGaN/GaN nanowire white-light-emitting diodes.

We investigated the effect of coupled quantum wells to reduce electron overflow in InGaN/GaN dot-in-a-wire phosphor-free white color light-emitting diodes (white LEDs) and to improve the device performance. The light output power and external quantum efficiency (EQE) of the white LEDs with coupled quantum wells were increased and indicated that the efficiency droop was reduced. The improved output power and EQE of LEDs with the coupled quantum wells were attributed to the significant reduction of electron overflow primarily responsible for efficiency degradation through the near-surface GaN region. Compared to the commonly used AlGaN electron blocking layer between the device active region and p-GaN, the incorporation of a suitable InGaN quantum well between the n-GaN and the active region does not adversely affect the hole injection process. Moreover, the electron transport to the device active region can be further controlled by optimizing the thickness and bandgap energy of this InGaN quantum well. In addition, a blue-emitting InGaN quantum well is incorporated between the quantum dot active region and the p-GaN, wherein electrons escaping from the device active region can recombine with holes and contribute to white-light emission. The resulting device exhibits high internal quantum efficiency of 58.5% with highly stable emission characteristics and virtually no efficiency droop.

Performance Improvement of AlGaN-Based Deep Ultraviolet Light-Emitting Diodes With Step-Like Quantum Barriers

AlGaN-based deep ultraviolet light-emitting diodes (DUV LEDs) still confront many challenges, which is partially limited by the poor carrier injection in the active region. Although incorporating a high Al-composition quantum barrier (QB) may boost carrier confinement capability, it will aggravate the quantum confined Stark effect (QCSE) and thus deteriorate the optical performance. In this article, a DUV LED structure with step-like QBs has been proposed and carefully investigated. This unique QB structure suppresses the electron overflow into the p-side of the device and benefits the hole injection efficiency simultaneously, thereby promoting the radiative recombination rate in the active region. As a result, the internal quantum efficiency (IQE) and light output power (LOP) of the DUV LED with step-like QBs are significantly improved with an enhancement factor of 40% under 60 mA current injection. Therefore, our step-like QB design provides a feasible approach to the enhancement of the optical performance of DUV LEDs.

Compact UV LED Lamp with Low Heat Emissions for Biological Research Applications

Much biomedical research focuses on the effects of UV light on human cells. UV light sources are a prerequisite for such research. This paper presents the design and achieved performance of a UVA (Ultraviolet A: 320–400 nm) and a UVB (Ultraviolet B: 290–320 nm) LED-based lamp suitable for use in bioassays, as well as inside an incubator. Numerical simulations were used to optimise the number, layout and output power of LEDs to achieve good irradiance homogeneity while maintaining low costs. Design was optimised for the efficient transfer of generated heat away from the irradiated samples through the heatsink at the back of the lamps. The average irradiance of the target surface by the UVA lamp was 70.1 W/m2 with a maximum deviation of 4.9%, and the average irradiance by the UVB lamp was 3.1 W/m2 with a maximum deviation of 4.8%. With the UVA and UVB lamps, the temperature of samples undergoing irradiation in the incubator rises from 37 to 42 °C within 40 and 67 min, respectively. This by far exceeds the required UV irradiation time in most cases. Tests on Jurkat and HEK-293 cell cultures confirmed the suitability of our lamps for biomedical research.