Surface Of Light-emitting Diodes Research Articles

On the thermal characterization of an RGB LED-based white light module

Abstract The study aims at evaluating the thermal characteristics of a red-green-blue (RGB) light emitting diode (LED)-based white light module in natural convection through infrared (IR) thermography and forward voltage measurements. The light output and packaging efficiencies of these three color types of LEDs are first characterized using a spherical integrating photometer employing an integrating sphere and an optical analysis program, respectively. These two efficiency data give an estimate of the power fraction not converted into emitting light, by which the temperature distribution of the module is calculated using thermal finite element (FE) modeling. To facilitate the IR thermography measurement, the coefficient of emissivity of the transparent molding compound as the LED optical lens is explored. The results of the measurements are compared with those of the thermal FE modeling and thermocouple measurement. Moreover, the limitation of the IR thermography measurement in LED thermal characterization is addressed, and enhancement of the measurement accuracy is achieved through a proposed temperature correction procedure. Besides, the uncertainty in the forward voltage measurements is also assessed. Results show that the maximum junction temperature of the RGB LED-based white light module can reach about 100 °C even at a low ambient temperature of 25 °C, and the low thermal conductivity of the transparent molding compound is one of the major causes of the poor thermal performance. In addition, IR thermography tends to overrate the surface temperature of the LEDs, and the transparency of the molding compound could be the direct consequence of the overestimate. It also turns out that the IR thermography can be an effective tool for LED surface temperature measurement after modification using the proposed temperature correction procedure. At last, the uncertainty analysis reveals that the uncertainty in the measured junction temperature using the forward voltage method is about 4–8%, depending on the color of the LEDs, and can it be greatly improved by increasing the accuracy of the voltage measurement.

Enhancement in Light Extraction Efficiency of GaN-Based Light-Emitting Diodes Using Double Dielectric Surface Passivation

SiO2Al2O3 double dielectric stack layer was deposited on the surface of the GaN-based light-emitting diode (LED). The double dielectric stack layer enhances both the electrical characteristics and the optical output power of the LED because the first Al2O3 layer plays a role of effectively passivating the p-GaN surface and the second lower index SiO2 layer increases the critical angle of the light emitted from the LED surface. In addition, the effect of the Fresnel reflection is also responsible for the enhancement in output power of the double dielectric passivated LED. The leakage current of the LED passivated with Al2O3 layer was -3.46 × 10-11 A at -5 V, at least two and three orders lower in magnitude compared to that passivated with SiO2 layer (-7.14 × 10-9 A) and that of non-passivated LED (-1.9 × 10-8 A), respectively, which indicates that the Al2O3 layer is very effective in passivating the exposed GaN surface after dry etch and hence reduces nonradiative recombination as well as reabsorption of the emitted light near the etched surface.

Large-area transparent conductive few-layer graphene electrode in GaN-based ultra-violet light-emitting diodes

We report on the development of a large-area few-layer graphene (FLG)—based transparent conductive electrode as a current spreading layer for GaN-based ultraviolet (UV) light-emitting diodes (LEDs). Large-area FLG was deposited on Cu using the chemical vapor deposition (CVD) method and subsequently transferred to the surface of the UV LED. UV light at a peak of 372 nm was emitted through the FLG-based transparent conductive electrode. The current spreading effects of FLG were clearly evident in both the optical images of electroluminescence (EL) and current-voltage (I-V) characteristics. Degradation of the FLG-based transparent conductive electrode could be induced by high power operation. Our results indicate that a large-area FLG-based electrode on GaN offers excellent current spreading and ultra-violet transparency properties when compared to the standard optoelectronic indium tin oxide (ITO) contact layer.

Enhanced light extraction from GaN-based LEDs with a bottom-up assembled photonic crystal

Abstract Photonic crystal (PhC) structure is an efficient tool for light extraction from light-emitting diodes (LEDs). The fabrication of a large area PhC structure on the light output surface of LEDs often involves sophisticated equipments such as nanoimprint lithography machine. In this study a monolayer of polystyrene (PS) microspheres was employed as a template to fabricate a noninvasive photonic crystal of indium tin oxide (ITO) on the surface of GaN-based LED. PS spheres can help to form periodic arrangement of bowl-like holes, a photonic crystal with gradually changed fill factors. Importantly, the electroluminescence intensity of LED with a photonic crystal was significantly enhanced by 1.5 times compared to that of the conventional one under various forward injection currents.

Effect of Surface Texture and Backside Patterned Reflector on the AlGaInP Light-Emitting Diode: High Extraction of Waveguided Light

This paper describes a novel structure of an AlGaInP light-emitting diode (LED) to extract the waveguided light for high-brightness applications. Four devices are considered and compared. They are AlGaInP-sapphire LEDs with: 1) a planar Ag reflector (LED-A); 2) a patterned Ag reflector (LED-B); 3) a planar Ag reflector and surface-roughened features (LED-C); and 4) a patterned Ag reflector and surface-roughened features (LED-D). The patterned Ag reflector can be used to direct some of the waveguided light that bounces within the AlGaInP LED and sapphire substrate to the top escape cone of the LED surface. Additionally, the roughened features, which are randomly distributed on the LED surface, enable the waveguided light that is trapped inside the LED chip to be coupled efficiently into the air. As a result, the external quantum efficiencies of LED-A, LED-B, LED-C, and LED-D measured at I = 350 mA are η = 9.20%, 10.46%, 15.22%, and 16.75%, respectively.

Oblique-angle sputtered AlN nanocolumnar layer as a buffer layer in GaN-based LED

This work presents an aluminum nitride (AlN) nanocolumnar layer sputtered at various oblique angles and its application as a buffer layer for GaN-based light-emitting diodes (LEDs) that are fabricated on sapphire substrates. The OA-AlN nanocolumnar layer has a diameter of about 30–60 nm. The GaN-based LED structure is perpendicularly extended from the OA-AlN nanocolumnar layer. Then, the nanocolumnar structure is merged into p-GaN layer to form a mesa structure with a diameter of about 200–600 nm on the surface of the GaN-based LED. Moreover, optical characteristics of the LED were studied using photoluminescence, along with the blue-shifts observed as well.

Output Power Enhancement of GaN-Based Light-Emitting Diodes Using Circular-Gear Structure

In this study, we investigated the characteristics of circular-gear light-emitting diodes (LEDs) and compared them with those of triangular and rectangular LEDs. The emission optical powers of the circular-gear, triangular, and rectangular LEDs at an injection current of 20 mA are 4.6, 5.9, and 7.9 mW, respectively. The total radiant flux from the circular-gear LED was increased by 73 and 34% compared with those of the rectangular LED and triangular LED, respectively. Then, we fabricated a periodic-textured structure on the surface of a circular-gear LED, and it increased the light output power by 32% at an input current of 20 mA. We discovered a leakage current in the triangular sidewall. Therefore, the circular-gear structure demonstrated better extraction efficiency and an improvement in the leakage current. Furthermore, owing to the circular periphery of the circular-gear sidewall, the current was spread evenly. We also achieved a lower driver voltage than that of the triangular LED, which is about 0.68 V at 20 mA.

Thermal Investigation and Placement Design of High-Brightness LED Array Package on PCB for Uniform Illuminance

This paper reports the thermal performance of a high-brightness light-emitting diode (LED) array package with a novel placement method on a printed circuit board (PCB). The precise heat transfer analysis and modeling using computational fluid dynamics (CFD) were performed according to the practical working conditions of the LED array. Emphasis was placed upon investigating how the temperature of the surface of LEDs changed in accordance with different placement methods. A significant drop in the surface temperature of the LEDs was found when the triangular and arithmetic spacing placement methods were used; hence, the overall heat dissipating capability of the LED array to the PCB was improved. By optimizing the placement design, the average surface temperature of the LED array achieved a decrease of about 20%, from 120°C to 100°C. The illuminance level of each placement design was measured and compared. Both CFD simulation and experimental results are provided to demonstrate the efficacy of the proposed approach for LED array thermal management.

Enhancement of Light Output Power and Modification of Beam Profile in InGaN/GaN Light Emitting Diodes with Micro Polygon Column Shaped Transparent

We report on the fabrication of GaN-based light-emitting diodes (LEDs) with micro-polygon columns (MPC) obtained by selective area growth (SAG) using the technique of metal organic chemical vapor deposition (MOCVD). The SAG makes it possible to control the density, size and position of the micro polygon columns. The facet structure consists of a polygon column covered with {1101} facets. Columns with rough occur on the (0001) surface. The light output power of the MPC-LED was 53% higher than conventional LED. In addition, the light extraction efficiency of InGaN/GaN LEDs is significantly improved owing to the photon path length is shorter. It is possible to modify the beam profile via shaped transparent layer. Thus, this structure can provide photons with multiple chances of escaping from the LEDs surfaces.

Nanoscale ITO/ZnO layer-texturing for high-efficiency InGaN/GaN light emitting diodes

We propose a simple technique to improve the light extraction efficiency of GaN-based light emitting diodes (LEDs) by using nanoscale ITO/ZnO layer-texturing. The surface texturing of the ITO and ZnO layers was performed using a wet chemical etching and a spin-coating process, respectively. It has been found that the light extraction efficiency of the ITO-/ZnO-textured LED was 34.5% greater than that of a conventional LED with a planar ITO, at 20 mA of current injection. A high level of multiple light scattering at the textured surface promoted a high-efficiency in the InGaN/GaN LEDs. In addition, the individual performance of the ITO and ZnO texturing on the LED surface was also investigated. The lowered forward voltage of the ITO/ZnO layer-textured LED indicated this could be a damage-free approach for device fabrication.

Light-extraction enhancement of red AlGaInP light-emitting diodes with antireflective subwavelength structures

We demonstrate the enhancement of light extraction in 633 nm AlGaInP light-emitting diodes (LEDs) with antireflective subwavelength structures (SWS). From the contour plots by the rigorous coupled wave analysis method, it is found that the reduction of the internal reflection strongly depends on the period of SWS. The Ag nanoparticles formed by thermal dewetting were used as an etch mask for dry etch process to fabricate antireflective SWS on the LED surface. The tapered pillars on the GaP were fabricated, on average, with distances below 200 nm, satisfying the required antireflection condition at the emission wavelength. The improvement in light output power by approximately 26.4% was achieved for the fabricated AlGaInP LEDs with SWS compared to the conventional LEDs due to a strongly reduced Fresnel internal reflection at the GaP/air interface. The improved directionality in the far-field pattern was also obtained due to the directional light extraction enhancement.

A numerical study of the emission enhancement of light emitting diodes with an interfacial photonic crystal resonance reflector

In this paper, we propose a vertical structure of light emitting diodes (LEDs) grown on a substrate embedded with a photonic crystal reflector to avoid light absorption in the absorbed substrate. Photonic crystal (PC) patterns are embedded in the interface between the epitaxial layer and substrate. Numerical simulation results prove that for both polarizations, there exists a range of PC lattice period, within which 1D or 2D PC patterns can demonstrate high reflectivity and function as a resonance reflector. LEDs grown on such a high reflective patterned substrate can achieve great omnidirectional emission enhancement compared with the conventional LEDs. In addition, we found that the combining of PCs with the surface of such LEDs can further enhance the emission and redistribute the photon energy. Embedding such an interfacial PC reflector might prove to be a promising method for enhancing the extraction efficiency of high-brightness LEDs.

Enhanced light extraction efficiency of GaN-based light-emitting diodes with ZnO nanorod arrays grown using aqueous solution

We report a dramatic increase in the light extraction efficiency of GaN-based blue light-emitting diodes (LEDs) by ZnO nanorod arrays on a planar indium tin oxide (ITO) transparent electrode. ZnO nanorods were grown into aqueous solution at the low temperature of 90 °C. With 20 mA current injection, the light output efficiency of the LED with ZnO nanorod arrays on ITO was increased by about 57% with no increase in a forward voltage over the conventional LEDs with planar ITO. The increased light extraction by the ZnO nanorod arrays is due to the formation of sidewalls and a rough surface, resulting in a multiple photon scattering at the LED surface.

Ga-Doped ZnO Transparent Conductive Oxide Films Applied to GaN-Based Light-Emitting Diodes for Improving Light Extraction Efficiency

In this study, Ga-doped ZnO (GZO) thin films were deposited on a sapphire substrate utilizing a magnetron sputtering approach. ZnO and Ga2O3 targets were employed as the sputtering sources during a cosputtering deposition. After thermal annealing in nitrogen ambient conditions, the electrical resistivity and optical transparency of the GZO films were analyzed in detail. The GZO films exhibited high transparency (~90%) in visible light and low resistivity (~5.3 x 10-4Omega-cm) when they were annealed at a temperature of 600-800deg C. Although the utilization of indium tin oxide (ITO) serving as the transparent contact layer (TCL) in conventional GaN-based light-emitting diodes (LEDs) is a well accepted technology, ZnO-based TCLs with a high refractive index of around 2.0 would render another advantage when a roughening process is performed on the surface. In other words, since packaged LEDs are generally encapsulated using epoxy with a refractive index of around 1.5, surface roughening performed on ITO TCL would thus result in only a minor improvement in light extraction because the typical refractive index of an ITO film prepared by our e-beam evaporator is around 1.7. In this study, GaN-based LEDs that utilized ITO/GZO composite oxide films as a TCL were also demonstrated. The light output power of an LED (LED-C) with a textured ITO/GZO composite TCL is markedly improved by 42 % and 48 % of magnitude as compared to LEDs with a planar GZO TCL (LED-A) and a ITO/GZO composite TCL (LED-B), respectively. This enhancement is due to the fact that a ZnO-based TCL with a higher refractive index (n~2.0) allows further enhancement of light extraction through the creation of a textured structure on the TCL that is deposited on the top surface of LEDs.

Light Extraction Improvement From GaN-Based Light-Emitting Diodes With Nano-Patterned Surface Using Anodic Aluminum Oxide Template

An improvement of light extraction from GaN-based light-emitting diodes (LEDs) was demonstrated by forming surface nano-structures using an anodic aluminum oxide (AAO) template as a dry etching mask. Nano-pores were simultaneously formed on indium tin oxide and GaN surfaces of LED with different pore depths by the same procedural steps, with a pore density and pore diameter of 1.1×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">9</sup> /cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> and 163 plusmn 31 nm, respectively. The nano-patterned LEDs achieved a top-face light output power enhancemetop-face light output power enhancementnt of 72% compared to the conventional LEDs at 20 mA. The light extraction effects of the nano-structures on the trapped optical modes in LED were revealed by scanning electron microscopy and microscopic electroluminescence.

Improvement of light extraction from high-power flip-chip light-emitting diodes by femtosecond laser direct structuring of the sapphire backside surface

We report on the structuring of the backside surface of sapphire substrates in high-power flip-chip light-emitting diodes (LEDs) by femtosecond laser direct writing. Varying the laser powers has been found to affect the sizes of the inscribed patterns on a submicron scale which facilitates the control of the structure sizes with high precision. Accordingly, since on the one hand the light extraction efficiency reveals a strong dependence on pattern sizes, and on the other hand, femtosecond laser structuring provides a simple opportunity to inscribe diverse structures along the LED surfaces, LEDs with laterally controlled light extraction efficiencies can be fabricated.

Integration of Photonic Crystals on GaN-Based Blue LEDs Using Silicon Mold Substrates

In this paper, we demonstrate a novel method to integrate photonic crystals (PhCs) on GaN-based blue light-emitting diodes (LEDs) using a silicon substrate as a mold for forming the PhCs. This method starts with fabricating a 2D grooved Si substrate as that mold. Subsequently, GaN-based epitaxial layers are grown on the Si mold-substrate, which effectively reduces the dislocation density in GaN by enhanced lateral epitaxial growth. After the epitaxial layers are bonded onto a highly reflective substrate, the Si mold-substrate is removed. This substrate-transfer technique replicates PhC from the mold-substrate on the LED surface free from processing damages. The resultant LEDs with PhC have outperformed the LEDs without PhC in the optical output power by 80%, taking advantage of the enhanced light extraction by PhC.

Improved Light Extraction of GaN-based LEDs with Nano-roughened p-GaN Surfaces

p-GaN surfaces are nano-roughened by plasma etching to improve the optical performance of GaN-based light emitting diodes (LEDs). The nano-roughened GaN present a relaxation of stress. The light extraction of the LEDs with nano-roughened surfaces is greatly improved when compared with that of the conventional LEDs without nano-roughening. PL-mapping intensities of the nano-roughened LED epi-wafers for different roughening times present two to ten orders of enhancement. The light output powers are also higher for the nano-roughened LED devices. This improvement is attributed to that nano-roughened surfaces can provide photons multiple chances to escape from the LED surfaces.

Growth and Characterizations of GaN-Based LEDs Grown on Wet-Etched Stripe-Patterned Sapphire Substrates

The material and electrical properties of GaN-based light-emitting diodes (LEDs) grown on wet-etched stripe-patterned substrates were investigated. Footprint-like patterns, located directly above the inclined groove sidewalls, were found on the as-grown LED surface. Cross-sectional transmission electron microscopy (TEM) showed that ‘tumor’-like structures with poor crystal quality were initiated on the inclined sidewalls, seeding dislocation bundles in the subsequently grown crystal. The high dislocation density slowed down the growth above the inclined sidewall, resulting in the uneven morphology. The fabricated devices showed over 30% enhancement in light output power as a result of improvements in both internal and extraction efficiencies.

Light-extraction enhancement of vertical-injection GaN-based light-emitting diodes fabricated with highly integrated surface textures

We report on the fabrication of high-efficiency vertical-injection GaN-based light-emitting diodes (LEDs) fabricated with integrated surface textures. An optical ray-tracing simulation shows that the high integration of surface textures can effectively enhance the light-extraction efficiency. The integrated surface textures are fabricated on the top surface of LEDs by generating hexagonal cones on the periodically corrugated surfaces of n-GaN. Compared to reference LEDs without textures, LEDs fabricated with integrated surface textures show an enhancement of the output power by a factor of 2.59, which is in agreement with the calculated results.