Output Power Of Light-emitting Diodes Research Articles

High thermal stability and high-performance efficiency capability of light sources–based rate equation models in optical fiber transmission systems

Abstract This paper clarified the high thermal stability and high-performance efficiency capability of silicon light sources–based rate equation models in the optical fiber transmission systems. The laser diode and light emitting diode output power variations are measured with input injection current under various ambient temperature effects. The light source differential quantum efficiency is demonstrated for both LED and laser diode with different injection input current and various temperature variations. The LED/laser diode light source external efficiency is clarified with different injection input current and various temperature variations. The LED/laser diode light source total efficiency is measured with different injection input current and various temperature variations. As well as the laser diode and light emitting diode, threshold current variations are demonstrated clearly with input injection current under room temperature and various spectral thermal effects. The laser diode and light emitting diode power dissipation variations are measured with input injection current under room temperature and various spectral thermal effects.

60Co γ-irradiation of AlGaN UVC light-emitting diodes

270 nm AlGaN UV Light-Emitting Diodes (LEDs) were exposed to 1–5 Mrad fluences of Co-60 γ-rays. The effect of the exposure to radiation was a ∼40% reduction in optical output after the highest fluence. No significant midgap emission was induced in the electro-luminescence spectra of the irradiated LEDs. We ascribe the decrease in optical output to creation of non-radiative states within the active regions. There were small (5–10%) increases in forward and reverse current as a result of irradiation with an effective carrier removal rate of <1 cm−1. The irradiation did not produce any increase in degradation rate of the LEDs output power under high drive current (95 mA) compared to unirradiated devices, which is consistent with the lack of midgap emission. The relatively small changes in electrical and optical properties, along with the resistance of the AlxGa1-xN/AlN to displacement damage effects indicate these devices may be well-suited to harsh terrestrial and space radiation applications.

Research on Light Extraction of GaN-based LED with Surface/Interface Coarsening

Light-emitting diodes (LEDs) are a highly versatile light source that have been widely employed in various fields. In order to maximize its quantum efficiency, it is essential to boost the efficiency of light extraction. Internal photon reflection can have a distinct negative impact on the luminous output power of GaN-based LEDs, which can be addressed by surface and interface texturing of different LED layers. This article discusses the mechanisms of light extraction losses in GaN-based LEDs and summarizes various surface and interface coarsening techniques proposed for improving the light extraction rate, including a range of physical and chemical etching methods.

A Review on the Progress of AlGaN Tunnel Homojunction Deep-Ultraviolet Light-Emitting Diodes

Conventional deep-ultraviolet (UV) light-emitting diodes (LEDs) based on AlGaN crystals have extremely low light-emission efficiencies due to the absorption in p-type GaN anode contacts. UV-light-transparent anode structures are considered as one of the solutions to increase a light output power. To this end, the present study focuses on developing a transparent AlGaN homoepitaxial tunnel junction (TJ) as the anode of a deep-UV LED. Deep-UV LEDs composed of n+/p+-type AlGaN TJs were fabricated under the growth condition that reduced the carrier compensation in the n+-type AlGaN layers. The developed deep-UV LED achieved an operating voltage of 10.8 V under a direct current (DC) operation of 63 A cm−2, which is one of the lowest values among devices composed of AlGaN tunnel homojunctions. In addition, magnesium zinc oxide (MgZnO)/Al reflective electrodes were fabricated to enhance the output power of the AlGaN homoepitaxial TJ LED. The output power was increased to 57.3 mW under a 63 A cm−2 DC operation, which was 1.7 times higher than that achieved using the conventional Ti/Al electrodes. The combination of the AlGaN-based TJ and MgZnO/Al reflective contact allows further improvement of the light output power. This study confirms that the AlGaN TJ is a promising UV-transmittance structure that can achieve a high light-extraction efficiency.

Smart Dimmable LED Lighting Systems.

This paper proposes energy-efficient solutions for the smart light-emitting diode (LED) lighting system, which provides minimal energy consumption while simultaneously satisfying illuminance requirements of the users in a typical office space. In addition to artificial light from dimmable LED lamps, natural daylight coming from external sources, such as windows, is considered as a source of illumination in an indoor environment. In order to reduce total energy consumption, the smart LED system has the possibility to dim LED lamps, resulting in reduced LED output power. Additionally, various LED lamps' functionality, such as semi-angle of the half illuminance and LED tilting, are introduced as an additional parameter to be optimized to achieve greater energy saving of the designed system. In order to properly exploit external lighting, the idea to reduce overall daylight intensity at a users' location is realized by the option to dim the windows with a shading factor. Based on the users' requirements for a minimal and desired level of illumination, the proposed optimization problems can be solved by implementing different optimization algorithms. The obtained solutions are able to give instructions to a smart LED system to manage and control system parameters (LEDs dimming levels, semi-angles of the half illuminance, orientation of LEDs, the shading factor) in order to design total illumination, which ensures minimal energy consumption and users' satisfaction related to illuminance requirements.

Defect Dynamics and Internal Quantum Efficiency Decay Resultant Ultraviolet C‐Band Light‐Emitting Diode Lifetime Performance

The internal quantum efficiency (IQE) decay resultant lifetime performance of ultraviolet C‐band (UVC) light‐emitting diodes (LEDs) is investigated, and an LED lifetime model is deduced assuming that defect generation is the sole cause for IQE decay and the defect generation process is to maximize the entropy of the defects. The proposed model reveals that the lifetime of an LED is proportional to its radiative recombination coefficient and inversely proportional to the product of its initial nonradiative recombination coefficient and defect growth interest rate, thus providing explicit dependence of the lifetime on the LED junction temperature and current density. The model fits excellently well to full sets of lifetime measurement data, which were collected from UVC LEDs stressed at different currents and different ambient temperatures for 2500 up to ≈10 000 h. The sensitivity of the lifetime to the junction temperature and current density is revealed for some UVC LEDs. Using the model, a lifetime of ≈96 000 h is projected to maintain 70% of the initial optical output power for a UVC LED based on a stress test for ≈10 000 h.

High Luminous Efficacy Phosphor-Converted Mass-Produced White LEDs Achieved by AlN Prebuffer and Transitional-Refraction-Index Patterned Sapphire Substrate.

Constant advance in improving the luminous efficacy (ηL) of nitride-based light-emitting diodes (LEDs) plays a critical role for saving measurable amounts of energy. Further development is motivated to approach the efficiency limit for this material system while reducing the costs. In this work, strategies of using thin AlN prebuffer and transitional-refraction-index patterned sapphire substrate (TPSS) were proposed, which pushed up the efficiency of white LEDs (WLEDs). The AlN prebuffer was obtained through physical vapor deposition (PVD) method and TPSS was fabricated by dry-etched periodic silica arrays covered on sapphire. Devices in mass production confirmed that PVD AlN prebuffer was able to improve the light output power (φe) of blue LEDs (BLEDs) by 2.53% while increasing the productivity by ~8% through shortening the growth time. Additionally, BLEDs on TPSS exhibited an enhanced top ηext of 5.65% in contrast to BLEDs on the conventional PSS through Monte Carlo ray-tracing simulation. Consequently, φe of BLEDs was experimentally enhanced by 10% at an injected current density (Jin) of 40 A/cm2. A peak ηL of 295.2 lm/W at a Jin of 0.9 A/cm2 and the representative ηL of 282.4 lm/W at a Jin of 5.6 A/cm2 for phosphor-converted WLEDs were achieved at a correlated color temperature of 4592 K.

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%.

Review of Low-Frequency Noise Properties of High-Power White LEDs during Long-Term Aging.

Low-frequency noise investigation is a highly sensitive and very informative method for characterization of white nitride-based light-emitting diodes (LEDs) as well as for the evaluation of their degradation. We present a review of quality and reliability investigations of high-power (1 W and 3 W) white light-emitting diodes during long-term aging at the maximum permissible forward current at room temperature. The research was centered on the investigation of blue InGaN and AlInGaN quantum wells (QWs) LEDs covered by a YAG:Ce3+ phosphor layer for white light emission. The current-voltage, light output power, and low-frequency noise characteristics were measured. A broadband silicon photodetector and two-color (blue and red) selective silicon photodetectors were used for the LED output power detection, which makes it possible to separate physical processes related to the initial blue light radiation and the phosphor luminescence. Particular attention was paid to the measurement and interpretation of the simultaneous cross-correlation coefficient between electrical and optical fluctuations. The presented method enables to determine which part of fluctuations originates in the quantum well layer of the LED. The technique using the two-color selective photodetector enables investigation of changes in the noise properties of the main blue light source and the phosphor layer during the long-term aging.

Performance improvement of InGaN LEDs by using strain compensated last quantum barrier and electron blocking layer

The influence of u-AlGaN/InGaN superlattices (SLs) last quantum barrier (LQB) and p-AlGaN/InGaN SLs electron blocking layer (EBL) on the performance of InGaN light-emitting diodes (LEDs) are analyzed experimentally and theoretically. Research results indicated that LED C with u-AlGaN/InGaN SLs LQB and p-AlGaN/InGaN SLs EBL exhibit the highest output power among the three groups of LED samples. At 200 mA, the output power of LED C is enhanced by 27.5% as compared with that of reference LED A with u-GaN LQB and p-AlGaN/GaN SLs EBL. The efficiency droop of LED C has also been suppressed effectively. At 200 mA, the efficiency droop of LED C was 28.5%, which was smaller than that of LED A 34%. By analyzing the distribution of electron/hole concentration in multiple quantum wells, it is indicated that the leakage of electrons from the active layer can be effectively reduced and the injection efficiency of holes into the active layer can be dramatically enhanced by using strain compensated AlGaN/InGaN SLs LQB and EBL. The reason is that the potential energy barriers for electrons and holes could be effectively regulated by using the strain compensated LQB and EBL, which is consistent with simulation results of energy band diagram.

UV Light-Emitting Diode With Buried Polarization- Induced n-AlGaN/InGaN/p-AlGaN Tunneling Junction

The polarization-induced electric field in the III-nitride UV light-emitting diode (LED) allows for significant flexibility in device design to address the electron overflow and hole injection issues. The conventional AlGaN-based UV LED with the PIN structure suffers from insufficient carriers especially hole concentration due to the large valence band barrier for hole injection and p-type doping challenge. Our systematic study reveals that the inverse design of the n-type and p-type layer shall build an opposite polarization-induced field to suppress electron overflow as well as simultaneously enhance hole injection. To design this p-side down UV LED and improve the hole injection, we adopt the n-AlGaN/i-InGaN/p-AlGaN buried tunneling junction (BTJ) instead of the bottom p-layer. The tunneling probability and output power of the LED are further investigated by optimizing the composition and thickness of the InGaN layer. Simulation results show that the optimized 3 nm In <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.3</sub> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.7</sub> N tunneling layer could lead to several orders of magnitude enhancement for LED output power. This study is significant for the pursuit of highly efficient UV LEDs.

Comparison of Ultraviolet B Light‐Emitting Diodes with Single or Triple Quantum Wells

Light‐emitting diodes (LEDs) with an emission wavelength of 310 nm containing either a single or a triple quantum well are compared regarding their efficiency and long‐term stability. In addition, the influence of the thickness of the lower quantum well barrier and the quantum well thickness in single quantum well (SQW) LEDs is investigated. Electroluminescence measurements show a 28% higher initial output power for the SQW LEDs compared with the triple quantum well (TQW) LEDs because of larger spatial overlap of the carriers in the SQW as revealed by electro‐optical simulations of the LED heterostructures. However, TQW LEDs show a higher output power than SQW LEDs after 1 h operation under harsh conditions. For SQW LEDs, it is found that for a thicker lower quantum well barrier (65 nm instead of 25 nm) the initial output power decreases by ≈15%. A thicker SQW (3 nm instead of 1.6 nm) reduces the initial output power by even 45% but increases the lifetime by a factor of 6 which is attributed to reduced Auger recombination from an enhanced spatial separation of electrons and holes in the quantum wells due to the quantum‐confined Stark effect.

Active Thermal Management of High-Power LED Through Chip on Thermoelectric Cooler

In this work, we proposed a chip on thermoelectric cooler (chip-on-TEC) for the active thermal management of high-power light-emitting diode (LED). The chip-on-TEC was prepared by electroplating a circuit layer on the cold-side substrate of thermoelectric cooling (TEC) and directly attaching LED chips on the circuit layer. Due to the Peltier effect of TE materials and the low thermal resistance of chip-on-TEC structure, the heat generated from the chips can be dissipated to the surrounding environment effectively. The performances of chip-on-TEC were studied by using the thermal simulation and packaging experiment. Compared with the traditional packaging structure, the working temperature of chip-on-TEC is greatly reduced under various chip currents. At the chip current of 1.0 A, the chip-on-TEC can reduce the working temperature from 232 °C to 114 °C (the reduction of 51%). Moreover, the light output power of LED is increased by 35.3%. The chip-on-TEC also can improve the light intensity and the light saturation point of LED. The results demonstrate that the chip-on-TEC is a promising thermal management for high-power LED packaging.

Cascaded deep ultraviolet light-emitting diode via tunnel junction

The AlGaN-based deep ultraviolet (DUV) light-emitting diode (LED) is an alternative DUV light source to replace traditional mercury-based lamps. However, the state-of-the-art DUV LEDs currently exhibit poor wall-plug efficiency and low light output power, which seriously hinder their commercialization. In this work, we design and report a tunnel-junction-cascaded (TJC) DUV LED, which enables multiple radiative recombinations within the active regions. Therefore, the light output power of the TJC-DUV LEDs is more than doubled compared to the conventional DUV LED. Correspondingly, the wall-plug efficiency of the TJC-DUV LED is also significantly boosted by 25% at 60 mA.

Strategically constructed patterned sapphire with silica array to boost substrate performance in GaN-based flip-chip visible light-emitting diodes

A strategically constructed substrate, patterned sapphire with silica array (PSSA), was developed to boost the efficiency of patterned sapphire substrate (PSS) in GaN-based light-emitting diodes (LEDs) application. The light output power of a flip-chip LED on PSSA improved by 16.5% at 120 mA than that of device grown on PSS. The XRD and STEM measurements revealed that the GaN epilayer grown on PSSA had better crystalline quality compared to the epilayer grown on PSS, which was the result of decreased misfit at coalescence boundary in the PSSA case. Moreover, the light extraction efficiency of the flip-chip LED on PSSA was significantly enhanced, benefiting from the small refractive-index contrast between the patterned silica array and air. This small refractive-index contrast also contributed to a more convergent emission pattern for the flip-chip LED on PSSA, as demonstrated by the far-field radiation pattern measurements. The discovery that PSSA could excel at defect suppression and light extraction revealed a new substrate platform for III-nitride optoelectronic devices.

Improved Light Extraction of Nitride-Based Light-Emitting Diodes by Al2O3-Doped UV-Curing Encapsulant

Silicone-based UV curing has recently been developed as a novel kind of encapsulation technology, which is time-saving and cost-effective for the package of light-emitting diodes (LEDs). In this work, to improve the light output power (LOP) and device reliability of LEDs packaged by UV curing, Al2O3 nanoparticles (NPs) are doped into silicone. Different compositions of Al2O3 NPs are used and the optimal composition is found to be 0.3%. Compared with LEDs packaged by UV curing without Al2O3 doping (256.3 mW) and by traditional thermal-curing (264.1 mW) encapsulants, the LOP is increased to 272.9 mW for LED with Al2O3-NP-doped encapsulant. The enhancement of LOP is attributed to both the increased refractive index and light scattering effect of the Al2O3 NPs, which improves the light extraction efficiency (LEE) of the encapsulant. Furthermore, the improvement of LEE also suppresses the heat generation and thus improving the long-term reliability of the devices. These results indicate that using high refractive index materials as doping, UV curing could be an excellent choice as a novel package technology for the LED industry.

Enhancement of light extraction efficiency of AlGaInP-based light emitting diodes by silicon oxide hemisphere array

Surface texturing has been demonstrated to be very effective in improving the extraction efficiency of light emitting diodes (LEDs). In this paper, high-brightness AlGaInP-based vertical LEDs (VLEDs) with nanoscale silicon oxide (SiO2) hemisphere array were successfully fabricated by photoresist thermal reflux technique and laser interference exposure. It is the first time to combine interferential lithography and thermal reflux technique to make a regular nanoscale hemisphere array to enhance LEE for LEDs Compared to LEDs with microscale SiO2 hemisphere array, with planar SiO2 and without SiO2, electroluminescent measurements show that four LEDs have a similar threshold voltage. Simultaneously, the light output power (LOP) of LEDs with nanoscale and microscale SiO2 hemisphere array has been improved up to 40% and 20% at 200 mA, respectively, in comparison to LEDs without SiO2. It fits the simulated results well, which were obtained by the finite-difference time-domain (FDTD) method. Furthermore, it is demonstrated that more light emits out in the direction perpendicular to the epitaxial wafer by angle-resolved EL spectra for LEDs with nanoscale SiO2 hemisphere array, which accords to the simulated optical field distribution images. This kind of high-brightness LEDs with nanoscale SiO2 hemisphere array will have promising applications in meeting different requirements in special situations, such as headlights, flashlights. The new method is capable of fabricating large-scale nanostructured materials with good controllability and low cost, which will be widely used in improving properties of all kinds of LEDs, especially Mini LEDs and Micro LEDs.

Development and Analysis of a Modular LED Array Light Source

Light emitting diodes (LEDs) have experienced rapid technological development in the past decade, making them a winning alternative to conventional light sources in many applications. LED arrays allow precise control of the desired irradiance profile in a target area by adjusting the position and output power of individual LEDs. However, despite increased efficiency, many LEDs still transform a large proportion of the input electrical power into heat, requiring an efficient cooling system. This paper presents a modular LED array light source mounted on a water-cooled aluminum plate. Novel electronic LED driver modules, connected via a serial communication bus in a daisy-chain topology, were developed with the ability to set the operating current of individual LEDs. A modular layout of cooling and mounting system and LED driver modules, as well as a specialized design for the LED soldering footprint, was able to house a variety of common commercial LEDs, enabling easy adjustment of the lighting system to the required application and size of the irradiated area. In a prototype of one plate containing 10 LEDs, individual LED radiance was optimized for a better irradiance homogeneity in the target area. Array characterization showed a low standard deviation of the irradiance of 1.8% and a good fit between measured and calculated irradiance. A test of the array at elevated temperatures showed moderate LED radiance degradation and a wavelength shift of the measured spectra after extended use.

Enhancing reliability of InGaN/GaN light-emitting diodes by controlling the etching profile of the current blocking layer

SiO2 was used as the current blocking layer (CBL) during fabricating the InGaN/GaN-based light-emitting diodes (LEDs). The SiO2 film was prepared by plasma enhanced chemical vapor deposition (PECVD) at a lower temperature (LT) of 180 °C and a higher temperature (HT) of 280 °C for characterizing the reliability of LEDs. The degradation of output power in LT-CBL LED is as high as 6.8% during 1000 h in the high-temperature and humidity (85 °C/85 RH) condition. Experimental results demonstrate the low temperature grown CBL forms a larger side-wall angle via wet etching. The thinner side-wall ITO film cracks and the current spreading effect is suppressed, causing drastic power degradation. On the contrary, the HT-CBL SiO2 demonstrates optimal step coverage of ITO film for current spreading and then the HT-CBL LEDs slightly degrade as low as 5% in the accelerated reliability test. A dense quality of HT-CBL SiO2 as well as a good CBL decreased parasitic optical absorption in the p-pad electrode and p-finger. Besides, the HT-CBL SiO2 showed a small side-wall angle of 40˚ which increased the step coverage and current spreading of ITO. An approach is conducted to confirm the side-wall profile of CBL for each process.

Improved Leakage and Output Characteristics of Pixelated LED Array for Headlight application

We investigated the effects of etching conditions on the performance of light-emitting diodes (LEDs) of various sizes aimed at vehicle headlamp applications. Photoluminescence (PL) images showed that after wet etching, the percentage of bad LED arrays significantly increased from 75% to 94%, and the leakage current at −5 V significantly increased from 1.14 × 10−9 A to 5.02 × 10−6 A. It was shown that plasma etching turned an Ag layer into Ag particles, the size and density of which depended on the treatment time and Ag layer thickness. These Ag particles served as micro-masks during dry etching. Plasma etching produced relatively uniform hillocks of diameters 0.9–1.43 μm and heights 0.85–2.5 μm. Moreover, the PL images showed that dry etching did not degrade the LED arrays. Furthermore, the light output power of the dry-etched LEDs was higher than that of the wet-etched LEDs. For example, the output power levels of the dry-etched LEDs (chip sizes: 240 × 290 μm2, 240 × 490 μm2, and 490 × 1190 μm2) at 100 mA were 20.4%, 15.0%, and 11.2% higher than those of the corresponding wet-etched LEDs, respectively. Moreover, we demonstrated a vehicle headlamp unit consisting of Ag-particle-based plasma-etched LEDs.