Light Extraction Efficiency Of Light-emitting Diodes Research Articles

FDTD Simulation to Investigate the Effect of Nanopattern on Light Extraction Distribution of ZnS:Cu LED

Light Emitting Diode (LED) technology is very common in everyday life. Due to the large refractive index difference between LED material and air, many excited photons from the active layer cannot be passed on to the air, thereby Total Internal Reflection (TIR) will occur and reducing the Light Extraction Efficiency (LEE) of LED. The use of nanopatterns on the LED surface can help trapped photons to get out of the LED by changing the direction and angle of photons’ propagation. Because the shape of the nanopattern will affect the increase in the LEE of LEDs and its fabrication is expensive, simulations are needed to design and study the effect of the shape and size of the nanopattern on the LEE of LEDs. In this study, FDTD simulations will be carried out on nano-sized grating patterns on the ZnS:Cu LED surface and then compared with experimental results to see the accuracy of the simulation results using Ansys Lumerical FDTD. The simulation results obtained in light emission distribution are consistent with the real measurement results. The simulation results are also used to evaluate the performance of the LEDs with and without the nanopattern on the surface.

Improvement of the Optical and Electrical Performance of GaN-Based Light-Emitting Diodes (LEDs) Using Transferrable ZnSnO3 (ZTO) Microsphere Monolayers

We report on the formation of transferable microspheres monolayers consisting of ZnSnO3 (ZTO) synthesized via fast ethanol precipitation and their application to GaN-based light-emitting diodes (LEDs) to improve device performance. ZTO microspheres with diameters of 820 ± 20 nm (ZTO-1), 910 ± 10 nm (ZTO-2), and 1200 ± 10 nm (ZTO-3) were synthesized and arrayed to form a monolayer using polydimethylsiloxane (PDMS) and a unidirectional rubbing method. ZTO-1, ZTO-2, and ZTO-3 monolayers exhibited optical transmittance percentages of 94.4%, 92.98%, and 87.09% at 450 nm, respectively. Effects of ZTO monolayers on the light extraction efficiency of LEDs were evaluated using LED chips sized 800 μm × 800 μm. LEDs with a ZTO-3 monolayer as a top layer had a light extraction efficiency of ∼144%, relative to regular LEDs, as well as improved electrical properties, as evidenced by a decrease in Vturn on of ∼0.6 V and a decrease in Ron by 0.036 kΩ. XPS analysis indicated that the improvement in the electrical properti...

Enhancement of the light-extraction efficiency of light-emitting diodes with SiO2 photonic crystals

In this paper, we proposed a method of using SiO2 photonic crystals (PhC) to improve the light-extraction efficiency (LEE) of light-emitting diodes (LEDs). Numerical simulations based on the finite-difference time-domain (FDTD) algorithm were performed to reveal the mechanism of using SiO2 PhC to improve the light extraction of LEDs. The effects of several critical parameters, including the depth, the filling factor, and the radius of SiO2 PhC on the enhancement of LEE were investigated. The rigorous coupled-wave analysis (RCWA) method was utilized to verify the variation trends we got from the FDTD simulations. The LEE of the LEDs with normal air-hole PhC structures and the conventional planar LEDs were also studied for comparison. According to our calculations, more than 37% improvement has been achieved in the LEE of the SiO2 PhC LEDs in comparison to that of the conventional planar LEDs. And SiO2 PhC LEDs obtained more light extraction than normal air-hole PhC LEDs. The influences of the SiO2 PhC on the current density and the temperature distributions in the active layer of LEDs have also been discussed in this paper. Results demonstrated no noticeable degradation in electrical and thermal characteristics under high current injections.

Analysis of the light-extraction efficiency of SiC substrate-based flip-chip vertical light-emitting diodes with embedded photonic crystals

To improve the light-extraction efficiency (LEE) of flip-chip vertical light-emitting diodes (LEDs) grown on silicon carbide (SiC) substrate, embedded photonic crystals (PhCs) were alternatively introduced into the n-GaN layer of LEDs, since etching of the SiC substrate was very difficult. The finite-difference time-domain (FDTD) method was employed to investigate the combination effects of the micro-cavity and the embedded PhCs. The influences of the PhCs configurations on the LEE of LEDs were also examined to get an optimal structure. With the optimized parameters, about 20% enhancement of LEE was achieved comparing to planar SiC substrate-based flip-chip vertical LEDs. The LEE of conventional surface PhCs LEDs and double layer PhCs LEDs were also investigated for comparison. The results indicated that LEDs with carefully designed embedded PhCs could provide more LEE than surface PhCs LEDs. The structures proposed here offered scopes for the design of high-efficiency, high-power LEDs.

GaN-based light emitting diodes on nano-hole patterned sapphire substrate prepared by three-beam laser interference lithography

Nano-hole patterned sapphire substrates (NHPSSs) were successfully prepared using a low-cost and high-efficiency approach, which is the laser interference lithography (LIL) combined with reactive ion etching (RIE) and inductively coupled plasma (ICP) techniques. Gallium nitride (GaN)-based light emitting diode (LED) structure was grown on NHPSS by metal organic chemical vapor deposition (MOCVD). Photoluminescence (PL) measurement was conducted to compare the luminescence efficiency of the GaN-based LED structure grown on NHPSS (NHPSS-LED) and that on unpatterned sapphire substrates (UPSS-LED). Electroluminescence (EL) measurement shows that the output power of NHPSS-LED is 2.3 times as high as that of UPSS-LED with an injection current of 150 mA. Both PL and EL results imply that NHPSS has an advantage in improving the crystalline quality of GaN epilayer and light extraction efficiency of LEDs at the same time.

Light extraction efficiency enhancement using surface-structured light-emitting diodes with a subwavelength coating

The results of our analysis, which was done by using the finite-difference, time-domain (FDTD) methods suggest that surface-structured light-emitting diodes (LEDs) with a subwavelength aluminum-nitride coating will have an enhanced light extraction efficiency. The use of such a LED structure will reduce both the internal reflection and the impedance mismatch, leading to a 3.2-fold enhancement in the light extraction efficiency compared to that for bare planar LEDs. We also present an improved method of modeling the light emission from the LED’s active layers to numerically calculate the light extraction efficiency of LEDs.

Improving the extraction efficiency of planar GaN-based blue light-emitting diodes via optimizing indium tin oxide nanodisc arrays

We report an enhancement in light emission of a GaN-based blue light-emitting diode (LED) with periodic indium tin oxide (ITO) nanodisc arrays. The periodic nanodisc arrays were elaborately designed by the Bragg diffraction and the maximum transmittance of a model of bilayer film on a GaN epitaxial layer. In the experiment, the periodic ITO nanodisc arrays have been fabricated by polystyrene nanosphere lithography. The electroluminescence intensity of the LEDs with periodic ITO nanodisc arrays is better than that of conventional planar GaN-based LED, and the greatest enhancement could increase the intensity by a factor of 2.08 at an operating current of 20 mA compared with the conventional planar GaN-based LED. The simulation results based on three-dimensional finite difference time-domain method showed that the LED with periodic ITO nanodisc arrays can improve the light-extraction efficiency of LED by enhancing the critical angle of light output. This method would be valuable for the fabrication of high-efficiency planar GaN-based LEDs.

Enhancement of light extraction efficiency in InGaN/GaN vertical blue light emitting diodes by surface patterning: Design and simulation

Light extraction of InGaN/GaN based surface-patterned vertical blue light emitting diodes (LEDs) is investigated by the use of ray-tracing approach. To optimize the light extraction efficiency of LEDs, various types of facet-patterning have been introduced on the free surfaces of the n-GaN epitaxial layer. A drastic increase in light extraction efficiency of the LED is observed due to increase of random and multiple scattering of light through patterned surfaces. The improvement in overall light extraction efficiency for the LEDs having rectangular and hexagonal patterning in free surfaces have been estimated around 2.25 and 3.39 times of that of the LED structure with smooth free surface (i.e., without any patterning), respectively.

Improved light extraction efficiency and leakage characteristics in light-emitting diodes by nanorod arrays formed on hexagonal pits

We report the enhancement of light extraction efficiency (LEE) and electrical performance in GaN-based green light-emitting diodes (LEDs) using ZnO nanorods formed on the etched surface of p-GaN. Green LEDs with hybrid ZnO nanorod structures grown on the hexagonally etched topmost layer of the LEDs, show an improvement in electroluminescence intensity that is 3.5 times higher than LEDs without any other surface treatments. The improvement in LEE in LEDs with nanohybrid structures was confirmed by finite-difference time-domain simulation analysis. Besides LEE enhancement, the surface etching effects on the reduction of leakage current of fabricated LEDs were also investigated.

Enhanced optical power of GaN-based light-emitting diode with compound photonic crystals by multiple-exposure nanosphere-lens lithography

The light-emitting diodes (LEDs) with single, twin, triple, and quadruple photonic crystals (PCs) on p-GaN are fabricated by multiple-exposure nanosphere-lens lithography (MENLL) process utilizing the focusing behavior of polystyrene spheres. Such a technique is easy and economical for use in fabricating compound nano-patterns. The optimized tilted angle is decided to be 26.6° through mathematic calculation to try to avoid the overlay of patterns. The results of scanning electron microscopy and simulations reveal that the pattern produced by MENLL is a combination of multiple ovals. Compared to planar-LED, the light output power of LEDs with single, twin, triple, and quadruple PCs is increased by 14.78%, 36.03%, 53.68%, and 44.85% under a drive current 350 mA, respectively. Furthermore, all PC-structures result in no degradation of the electrical properties. The stimulated results indicate that the highest light extraction efficiency of LED with the clover-shape triple PC is due to the largest scattering effect on propagation of light from GaN into air.

Effects of TiO2-doped silicone encapsulation material on the light extraction efficiency of GaN-based blue light-emitting diodes

The effects of silicone encapsulation material doped with TiO2 nanoparticles (NPs) on the light extraction efficiency (LEE) of GaN-based blue light-emitting diodes (LEDs) were investigated. The LED samples were encapsulated with pure silicone, uniform TiO2 NP-doped silicone, TiO2 NP-doped silicone around the chip only, and TiO2 NP-doped silicone encapsulation dome. The average refractive index of silicone was increased by doping with TiO2 NPs with an average diameter of 21nm, and the LEE of GaN-based blue LEDs was also enhanced. Methyl and phenyl silicone were used as encapsulation materials. TiO2 NP-doped methyl silicone encapsulation dome increased the LEE by 10.9%, whereas TiO2 NP-doped phenyl silicone encapsulation dome increased the LEE by 12.9%. The thermal effect, reliability test, and LEE of the LEDs were also analyzed. TiO2 NP-doped silicone encapsulation materials could better enhance LEE compared with pure silicone encapsulation material because of the increased average refractive index, which reduced the total internal reflection at the encapsulation at the air interface.

Design of an LED chip structure with an integrated two-dimensional photonic crystal to enhance the light-extraction efficiency

We numerically simulated the light-extraction efficiency of light-emitting diodes (LEDs) with an integrated two-dimensional photonic crystal (PC) structure on the top surface in order to enhance light extraction. We considered InGaN-based LED chips with a typical emission wavelength of λo = 460 nm and an emission wavelength inside the LED chip of λ = λo/nGaN, where nGaN is the refractive index of GaN. We used positive (relief) and negative (intaglio) patterns for the PC structures with square arrangements. The pattern period (Λ), width (d), and height (h) of the PC structure were varied systematically in the PC-LEDs; then the light-extraction efficiency of each PC-LED was simulated numerically using a three-dimensional finite-difference time-domain method to optimize the PC structure in terms of light extraction. The PC LED with a square pilla pattern with Λ ≃ 1.4λ, d ≃ 0.75Λ, and h ≃ 0.75Λ had the maximum light-extraction efficiency for positive patterns while the cylindrical hole pattern with Λ ≃ 1.2λ, d ≃ 0.5Λ, and h ≃ 0.5Λ had the maximum light-extraction efficiency for negative patterns.

Fabrication of large-scale spherical-cap structure and the application on n-GaN based light-emitting diodes

An under exposure method based on Fresnel diffraction effect in a conventional optical lithography system is used to fabricate large-scale, uniform spherical-cap structures. This method provides an effective roughening technology on the top surface of light-emitting diodes (LEDs) to improve the light extraction efficiency of LEDs. LEDs with high duty cycle spherical-cap structures showed enhanced light output power by 130%–160% compared with the LED with a flat surface. This simple and easy shape control method has potential applications in other optical devices such as organic LEDs, inorganic solar cells, and laser diodes.

Design of Wide-Bottomed Patterned Sapphire Substrates for Performance Improvement of GaN-Based Light-Emitting Diodes

We propose patterned sapphire substrates (PSSs) with wide-bottomed cone-shaped patterns to enhance the light extraction efficiency of GaN-based light-emitting diodes (LEDs). Simulation reveals that PSS with wide-bottomed cone-shaped patterns is of higher yield in LED light extraction efficiency. The subsequent fabrication of and epitaxial growth of LED structure on the proposed wide-bottomed PSS verify the simulation results experimentally. It is demonstrated that wide-bottomed patterns with narrow distance between patterns can effectively release stress, avoid the decline in crystal quality and improve light extraction efficiency. By comparison with the popular commercially available PSS, LED grown on the proposed PSS whose wide-bottomed patterns are of 3.91 μm in width, 2.37 μm in height and 0.09 μm in distance is proven to be of as high as 16.7% improvement in luminous efficacy and 33.8% drop in dislocation density, which shows the promising future for its application in the highly efficient LEDs.

Laser-induced periodic structures for light extraction efficiency enhancement of GaN-based light emitting diodes

The laser-induced periodic surface structure technique was used to form simultaneously dual-scale rough structures (DSRS) with spiral-shaped nanoscale structure inside semi-spherical microscale holes on p-GaN surface to improve the light-extraction efficiency of light-emitting diodes (LEDs). The light output power of DSRS-LEDs was 30% higher than that of conventional LEDs at an injection current of 20 mA. The enhancement in the light output power could be attributed to the increase in the probability of photons to escape from the increased surface area of textured p-GaN surface.

Study on the light extraction efficiency of GaN-based light emitting diode by using the defects of the photonic crystals

Many studies have shown that the photonic crystals and surface nanostructures can improve the light extraction efficiency of light emitting diode (LED). The defects and disorders exist in photonic crystals inevitably as the manufacturing technology limitations, and so the light extraction efficiency of LED will be changed correspondingly. In this work we perform a simulation study on defects and disorder of photonic crystals by the finite-difference time-domain (FDTD) method, and we speed up the FDTD by GPU acceleration technology. Simulation results show that a small number of defects in photonic crystals LED do not reduce the LED light extraction efficiency. Instead, the light extraction efficiency of LED will be increased by adding some defects into the photonic crystals. We give a detailed analysis of its physical mechanism. We design a kind of photonic crystals with defects, and its light extraction efficiency achieves 1.6 times that of the perfect photonic crystals.

Double-Grating Displacement Structure for Improving the Light Extraction Efficiency of LEDs

To improve the light extraction efficiency of light-emitting diodes (LEDs), grating patterns were etched on GaN and silver film surfaces. The grating-patterned surface etching enabled the establishment of an LED model with a double-grating displacement structure that is based on the surface plasmon resonance principle. A numerical simulation was conducted using the finite difference time domain method. The influence of different grating periods for GaN surface and silver film thickness on light extraction efficiency was analyzed. The light extraction efficiency of LEDs was highest when the grating period satisfied grating coupling conditions. The wavelength of the highest value was also close to the light wavelength of the medium. The plasmon resonance frequencies on both sides of the silver film were affected by silver film thickness. With increasing film thickness, plasmon resonance frequency tended toward the same value and light extraction efficiency reached its maximum. When the grating period for the GaN surface was 365 nm and the silver film thickness was 390 nm, light extraction efficiency reached a maximum of 55%.

Effects of the refractive index of the encapsulant on the light-extraction efficiency of light-emitting diodes

We investigate the effects of the refractive index of the encapsulant on the light-extraction efficiency (LEE) of light-emitting diodes (LEDs) for GaN LEDs (n ≈ 2.5) and AlGaInP LEDs (n ≈ 3.0). For non-absorbing rectangular parallelepiped LED chips, as the refractive index of the encapsulant increases, the LEE first increases quasi-linearly, then increases sub-linearly, and finally a saturation is reached. Furthermore, LEDs with a dual-layer graded-refractive-index (GRIN) encapsulant (n(encapsulant 1) = 1.57 and n(encapsulant 2) = 1.41) is fabricated through a two-step curing process. We demonstrate that such an LED further enhances the LEE by reducing Fresnel reflection loss at the encapsulant/air interface by 35% compared with an LED encapsulated with a single-layer encapsulant (n(encapsulant) = 1.57).

Comparison of various surface textured layer in InGaN LEDs for high light extraction efficiency

The various surface texturing effects of InGaN light emitting diodes (LEDs) have been investigated by comparison of experimented data and simulated data. The single-layer and double-layer texturing were performed with the help of ITO nanospheres using wet etching, where the ITO ohmic contact layer and the p-GaN layer are textured using ITO nanospheres as an etch mask. In case of single-layer texturing, p-type GaN layer texturing was more effective than ITO ohmic contact layer texturing. The maximum enhancement of wall-plug efficiency of double-layered textured LEDs is 40% more than conventional LEDs, after packaging at an injected current of 20 mA. The increase of light scattering at the textured GaN surfaces is a major reason for increasing the light extraction efficiency of LEDs.

Analysis of light extraction efficiency of GaN-based light-emitting diodes

GaN-based light emitting diodes (LEDs) as one of the most important light source in next-generation solid-state lighting have been extensively studied and remarkable progress has been obtained. However, the light extraction efficiency (LEE) is not sufficient to satisfy application requirements. Most of the photons generated in multiple quantum well (MQW) always are trapped inside the semiconductor because both the reflection index of GaN and InGaN are higher than that of air, which results in total internal reflection (TIR). Great efforts were made in enhancing light extraction of LEDs experimentally in previous investigation. However, detailed theoretical studies in predicting the LEE of different types of LEDs are not available. In this paper the light extraction efficiency(LEE) of conventional chip (CC), flip chip (FC) and vertical chip (VC) is investigated using Monte Carlo ray tracing method is presented in conventional chip (CC). Monte Carlo ray tracing simulation based on statistics is known to be one of the most suitable methods to analyze the dependence of the light extraction efficiency of LEDs on the variety of the device parameters. Diffused bottom surface was found to be better for improvement of LEE than the perfect mirror surface. When there is a material with higher refraction index on the top surface the VC structure has the highest LEE no matter the bottom surface is perfect mirror or diffuse surface, which is above 80%. It is indicated that VC is potential in high-power GaN-based LEDs from the results.