Output Power Of Light-emitting Diodes Research Articles

Investigation of lattice-modulated AlInGaN as a barrier layer in near-ultraviolet light-emitting diodes by numerical analysis and fabrication

The effects of using lattice-modulated AlInGaN as barriers in the active region were investigated in near-ultraviolet light-emitting diodes (LEDs). Both a stronger localization effect with wider barriers and a higher energy band gap existed in AlInGaN/InGaN LEDs, compared with GaN/InGaN LEDs. An increase in the carrier concentration in the active layer, a reduction in lattice mismatch that induced polarization mismatch in the active layer, and suppression of electron overflow can be found by numerical simulation. By 100 mA current injection, the AlInGaN/InGaN LED output power can be increased by 33.1%, compared with that of GaN/InGaN LED.

Nitride-Based Blue Light-Emitting Diodes Grown With InN/GaN Matrix Quantum Wells

Nitride-based light-emitting diodes (LEDs) grown with a structure of InN/GaN matrix quantum-wells (QWs) using metal-organic chemical vapor deposition are fabricated and characterized. The InN/GaN matrix QWs can significantly enhance the formation of phase-separated In-rich regions. Under an injection current of 500 A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , the LED output power can be enhanced by 26%, and the efficiency droop can be improved as compared with a conventional LED. These improvements could be attributed to the strong localization effect to remove carriers from the confining potential and capture these carriers at nonradiative centers in the active layer.

Nanopatterned yttrium aluminum garnet phosphor incorporated film for high-brightness GaN-based white light emitting diodes

In this study, we fabricated high-brightness white light emitting diodes (LEDs) by developing a nanopatterned yttrium aluminum garnet (YAG) phosphor-incorporated film. White light can be obtained by mixing blue light from a GaN-based LED and yellow light of the YAG phosphor-incorporated film. If white light sources can be fabricated by exciting proper yellow phosphor using blue light, then these sources can be used instead of the conventional fluorescent lamps with a UV source, for backlighting of displays. In this work, a moth-eye structure was formed on the YAG phosphor-incorporated film by direct spin-on glass (SOG) printing. The moth-eye structures have been investigated to improve light transmittance in various optoelectronic devices, including photovoltaic solar cells, light emitting diodes, and displays, because of their anti-reflection property. Direct SOG printing, which is a simple, easy, and relatively inexpensive process, can be used to fabricate nanoscale structures. After direct SOG printing, the moth-eye structure with a diameter of 220nm was formed uniformly on the YAG phosphor-incorporated film. As a result of moth-eye patterning on the YAG phosphor-incorporated film, the light output power of a white LED with a patterned YAG phosphor-incorporated film increased to up to 13% higher than that of a white LED with a non-patterned film.

Enhanced Internal Quantum Efficiency and Light Extraction Efficiency of Light-emitting Diodes with Air-gap Photonic Crystal Structure Formed by Tungsten Nano-mask

We demonstrate the blue InGaN/GaN multiple quantum wells light-emitting diodes (LEDs) with an embedded air-gap photonic crystal (PC) which was fabricated by the lateral epitaxial overgrowth of GaN layer on the tungsten (W) nano-masks. The periodic air-gap PC was formed by the chemical reaction of hydrogen with GaN on the W nano-mask. The optical output power of LEDs with an air-gap PC was increased by 26% compared to LEDs without an air-gap PC. The enhanced optical output power was attributed to the improvement in internal quantum efficiency and light extraction efficiency by the air-gap PC embedded in GaN layer.

Optoelectrical characteristics of green light-emitting diodes containing thick InGaN wells with digitally grown InN/GaN.

Compared with conventionally grown thin InGaN wells, thick InGaN wells with digitally grown InN/GaN exhibit superior optical properties. The activation energy (48 meV) of thick InGaN wells (generated by digital InN/GaN growth from temperature-dependent integrated photoluminescence intensity) is larger than the activation energy (25 meV) of conventionally grown thin InGaN wells. Moreover, thick InGaN wells with digitally grown InN/GaN exhibit a smaller σ value (the degree of localization effects) of 19 meV than that of conventionally grown thin InGaN wells (23 meV). Compared with green light-emitting diodes (LEDs) with conventional thin InGaN wells, the improvement in 20-A/cm² output power for LEDs containing thick InGaN wells with digitally grown InN/GaN is approximately 23%.

Tailoring of polarization in electron blocking layer for electron confinement and hole injection in ultraviolet light-emitting diodes

The influence of the AlGaN electron blocking layer (EBL) with graded aluminum composition on electron confinement and hole injection in AlGaN-based ultraviolet light-emitting diodes (LEDs) are investigated. The light output power of LED with graded AlGaN EBL was markedly improved, comparing to LED with conventional EBL. In experimental results, a high increment of 86.7% can be obtained in light output power. Simulation analysis shows that via proper modification of the barrier profile from the last barrier of the active region to EBL, not only the elimination of electron overflow to p-type layer can be achieved but also the hole injection into the active region can be enhanced, compared to a conventional LED structure. The dominant factor to the performance improvement is shown to be the modulation of polarization field by the graded Al composition in EBL.

Improved light output power of LEDs with embedded air voids structure and SiO2 current blocking layer

Abstract GaN-based light-emitting diodes (LEDs) with an embedded air voids structure and a SiO2 current blocking layer (CBL) was fabricated and investigated. The air voids structure was formed between cone-shaped patterned sapphire substrate and GaN epitaxial layer by combining laser scribing with H3PO4-based hot chemical etching. The air voids embedded high power LED showed 8.9% higher light output power due to a strong light reflection and redirection at the interface between GaN and air voids, which could increase the top light extraction of the high power LED. Compared to the air voids embedded high power LED, the light output power of the high power LED by integrating air voids structure with SiO2 CBL was 9.1% higher than that of the air voids embedded LED without SiO2 CBL. It was also found that the simulation results agree well with the experimental results.

Diffusion injected multi-quantum well light-emitting diode structure

The attention towards light-emitting diode (LED) structures based on nanowires, surface plasmon coupled LEDs, and large-area high-power LEDs has been increasing for their potential in increasing the optical output power and efficiency of LEDs. In this work we demonstrate an alternative way to inject charge carriers into the active region of an LED, which is based on completely different current transport mechanism compared to conventional current injection approaches. The demonstrated structure is expected to help overcoming some of the challenges related to current injection with conventional structures. A functioning III-nitride diffusion injected light-emitting diode structure, in which the light-emitting active region is located outside the pn-junction, is realized and characterized. In this device design, the charge carriers are injected into the active region by bipolar diffusion, which could also be utilized to excite otherwise challenging to realize light-emitting structures.

GaN-Based Light-Emitting Diodes With Staircase Electron Injector Structure

The authors experimentally studied GaN-based light-emitting diodes (LEDs) with both an staircase electron injector (SEI) structure and a conventional electron blocking layer (EBL). With the EBL, it was found that we could enhance LED output power, reduce forward voltage, and mitigate efficiency droop by inserting the SEI structure. These improvements could all be attributed to the effective cooling of the injected hot electrons. However, it was also found that some of the injected electrons could still leak into the p-GaN layer in the LED with SEI structure but without the EBL.

Enhancement of light output power on GaN‐based light‐emitting diodes using two‐direction stripe‐patterned sapphire substrate

AbstractTo achieve both the reduction of threading dislocation density (TDD) and the enhancement of light extraction efficiency (LEE) of light‐emitting diodes (LEDs), a two‐directional stripe‐patterned sapphire substrate (TPSS) was fabricated. First, a stripe pattern parallel to the a‐axis of Al2O3 was formed on an as‐received c ‐sapphire wafer and then a second stripe pattern rotated around 3° from the m‐axis of Al2O3 was formed on the first stripe PSS. The light output power of TPSS LEDs was around 1.25 times higher than that of LEDs on a flat sapphire substrate. This enhancement is caused by the increase of both the internal quantum efficiency (IQE) and the LEE. On the other hand, although the LED on TPSS with the second stripe just parallel to the m‐axis of Al2O3 showed a similar enhancement ratio of the light output power, the current leakage of this LED increased with the concentration of threading dislocations on the surface of LEDs. Stripes rotated 3° from the m‐axis of Al2O3 changed the growth mode and realized GaN films with a smooth surface without pit formation. Therefore, the misalignment of the stripe from the m‐axis of Al2O3 will become a new design parameter for realizing high‐efficiency reliable LEDs. (© 2014 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Efficiency improvement and droop behavior in nanospherical-lens lithographically patterned bottom and top photonic crystal InGaN/GaN light-emitting diodes

Large-scale SiO2 nanodisk arrays fabricated by nanospherical-lens lithography are embedded in the n-GaN and p-GaN layer of an InGaN/GaN light-emitting diode (LED) to produce photonic crystal (PC) structures for efficiency improvement. Following the obvious reduction of view angle, the light output power of bottom, top, and double PC LEDs is enhanced by 74.5%, 60.1%, and 88.2% compared to that of a conventional LED at 350 mA current, respectively. Despite the enhanced external quantum efficiency due to improved crystalline quality and light extraction, these PC LEDs exhibit lower peak efficiency current density and more serious efficiency droop than conventional LEDs. Combined with the rate equation, the droop mechanisms of PC LEDs have also been investigated experimentally and by simulation.

Enhancement of Optical Output Power of Light-Emitting Diodes with Photonic Crystals in InGaN/GaN Multiple Quantum Wells by Sulfide Passivation

We investigate blue light-emitting diodes (LEDs) containing photonic crystals (PhCs) that are fabricated in InGaN/GaN multiple quantum wells by plasma etching. Sulfide passivation of the etched surfaces in the LEDs using thioacetamide/ammonium solution effectively reduces the content of surface defects that induce defect-related recombination and the reverse leakage current through the sidewall surface of PhCs. Sulfide passivation enhances the optical output power of the PhC LEDs by 71% at 20 mA compared with that of planar LEDs. The large enhancement of optical power is attributed to out-coupling of confined light by the PhCs and the effective recovery of internal quantum efficiency by sulfide passivation.

GaN-Based LEDs With Rough Surface and Selective KOH Etching

The authors propose a simple method to enhance the performances of GaN-based light-emitting diodes (LEDs) with rough surface. By using KOH to selectively smooth the area beneath the p-contact pad, it was found that we could reduce the forward voltage and enhance output power of the GaN-based LEDs. It was also found that wall-plug efficiency (WPE) of the proposed LEDs was 10.3% larger than that of the conventional LEDs with rough surface and 33.9% larger than that of the conventional LEDs with flat surface. Furthermore, it was found that the use of selective KOH etching will not degrade drooping effect of the LEDs.

InGaN Flip-Chip Light-Emitting Diodes With Embedded Air Voids as Light-Scattering Layer

The performance of GaN-based flip-chip light-emitting diodes (LEDs) with embedded air voids grown on a selective-area Ar-implanted sapphire (SAS) substrate was demonstrated in this letter. The GaN-based epitaxial layers grown on Ar-implanted regions exhibited lower growth rates compared with those grown on implantation-free regions. Accordingly, air voids formed over the implanted regions after merging laterally grown GaN facet fronts. The light-output power of LEDs grown on SAS was greater than that of LEDs grown on implantation-free sapphire substrates. The output power of LEDs grown on SAS was enhanced by 20% at an injection current of 700 mA. The increase in output power was mainly attributed to the scattering of light around the air voids, which increased the probability of photons escaping from the LEDs.

Light transmission enhancement from hybrid ZnO micro-mesh and nanorod arrays with application to GaN-based light-emitting diodes

A hybrid ZnO micro-mesh and nanorod arrays (MMNR) was fabricated as a light output window for GaN-based light-emitting diodes (LEDs) to enhance the light extraction efficiency. The light output power of GaN-based LEDs with the ZnO MMNR is improved by 95% compared to the original planar LEDs. The ZnO MMNR is manufactured by photolithography techniques and a two-step wet chemical growth process. The incident angle-resolved light transmission of the ZnO MMNR beyond the critical angle of total internal reflection is greatly enhanced. The light diffraction pattern of the ZnO MMNR shows that it possesses both the two-dimensional diffraction grating effect of a ZnO micro-mesh and the light scattering effect of a ZnO nanorod array. LEDs with the ZnO MMNR have greater light extraction efficiency than those with only a ZnO micro-mesh or a ZnO nanorod array. The local optical field patterns of the ZnO micro-mesh and the ZnO MMNR are investigated using confocal scanning electroluminescence microscopy. The microscopic light extraction mechanism of the ZnO MMNR is analyzed in-depth.

GaN-Based Ultraviolet Light Emitting Diodes With Ex Situ Sputtered AlN Nucleation Layer

We demonstrated the electro-optical characteristics of gallium nitride (GaN)-based ultraviolet (UV) light emitting diodes (LEDs) with sputtered aluminum nitride (AlN) nucleation layer. The introduction of the ex situ sputtered AlN nucleation layer improved the crystal quality of the GaN and the n-AlGaN layer of the GaN-based UV LEDs. Hence, the 20-mA output power of UV LEDs with ex situ AlN nucleation layers is higher than that of UV LEDs with GaN nucleation layers. In addition, the enhanced power output of UV LEDs with ex situ AlN nucleation could reach around 52% in magnitude at peak emission wavelengths of 370 nm compared with power outputs of UV LEDs with GaN nucleation layers. Furthermore, UV LEDs with ex situ AlN nucleation show improved reliability. The UV LEDs with ex situ AlN nucleation layer revealed a power output drop of around 9% within 168 hours , which is less than the around 14% power drop of UV LEDs with GaN nucleation layer.

GaN-Based Light-Emitting Diodes With AlGaN Strain Compensation Buffer Layer

An AlGaN strain compensation layer (SCL) was proposed to modulate the strain and thus alleviate the polarization of GaN-based light-emitting diodes (LEDs). With the SCL, it was found that the 350 mA LED output power could be enhanced from 258 to 285 mW. It was also found that the SCL could alleviate the efficiency droop and reduce the forward voltage of the LEDs. These improvements could all be attributed to in-plane tensile strain induced by the AlGaN layer which could effectively compensate the compressive strain induced by the InGaN well layers. From micro-Raman spectra measurement, it was found that in-plane biaxial stresses in the reference and SCL samples were 0.30 and 0.07 GPa, respectively.

Effects of current crowding on light extraction efficiency of conventional GaN-based light-emitting diodes

Current crowding effects (CCEs) on light extraction efficiency (LEE) of conventional GaN-based light-emitting diodes (LEDs) are analyzed through Monte Carlo ray-tracing simulation. The non-uniform radiative power distribution of the active layer of the Monte Carlo model is obtained based on the current spreading theory and rate equation. The simulation results illustrate that CCE around n-pad (n-CCE) has little effect on LEE, while CCE around p-pad (p-CCE) results in a notable LEE droop due to the significant absorption of photons emitted under p-pad. LEE droop is alleviated by a SiO₂ current blocking layer (CBL) and reflective p-pad. Compared to the conventional LEDs without CBL, the simulated LEE of LEDs with CBL at 20 A/cm² and 70 A/cm² is enhanced by 7.7% and 19.0%, respectively. It is further enhanced by 7.6% and 11.4% after employing a reflective p-pad due to decreased absorption. These enhancements are in accordance with the experimental results. Output power of LEDs with CBL is enhanced by 8.7% and 18.2% at 20 A/cm² and 70 A/cm², respectively. And the reflective p-pad results in a further enhancement of 8.9% and 12.7%.

Highly bright broadband red light produced by fluorescence polymer/InGaN hybrid light-emitting diodes

The fabrication of fluorescence polymer/InGaN hybrid light-emitting diodes (LEDs) that emit highly bright broadband red light is presented in this Letter. The absorption peak of the fluorescence polymer was 455nm, and the emission peak was 640nm. The light output power and external quantum efficiency of hybrid LEDs at a driving current of 100mA were 46.6mW and 24.1%, respectively. The emission spectrum of hybrid LEDs was located at a wavelength of 641nm, with a broadband FWHM of 106nm. Thus this study offers potential methods for enhancing the output power of commercial white-light-emitting devices.

Breaking the Carrier Injection Bottleneck of Phosphor-Free Nanowire White Light-Emitting Diodes

We have examined the carrier injection process of axial nanowire light-emitting diode (LED) structures and identified that poor carrier injection efficiency, due to the large surface recombination, is the primary cause for the extremely low output power of phosphor-free nanowire white LEDs. We have further developed InGaN/GaN/AlGaN dot-in-a-wire core-shell white LEDs on Si substrate, which can break the carrier injection efficiency bottleneck, leading to a massive enhancement in the output power. At room temperature, the devices can exhibit an output power of ~1.5 mW, which is more than 2 orders of magnitude stronger than nanowire LEDs without shell coverage. Additionally, such phosphor-free nanowire white LEDs can deliver an unprecedentedly high color rendering index of ~92-98 in both the warm and cool white regions, with the color rendering capability approaching that of an ideal light source, i.e. a blackbody.