Novel patterned sapphire substrates for enhancing the efficiency of GaN-based light-emitting diodes.

We demonstrate high light extraction efficiency of GaN-based light-emitting diodes (LEDs) using the selective growth of ZnO nanorods (NRs). The chemical wet-etched hexagonal patterns of ZnO transparent conductive film were used to achieve the selective growth of ZnO NRs, which could be grown at the non-etched hexagonal pattern regions due to the wet-etching effect. The optical transmittance of GaN-based LEDs was decreased by forming additional different types of ZnO transparent conductive films, such as the hexagonal ZnO patterns and the ZnO NRs selectively grown on hexagonal ZnO patterns. However, the light output power of GaN-based LEDs increased on decreasing the optical transmittance by employing a hexagonal ZnO transparent conductive film and selectively grown ZnO NRs. Based on these results, we suggest that the combined structure of the surface hexagonal patterns and the selectively grown ZnO NRs decreases the optical transmittance, but is more effective in improving the light extraction efficiency in the GaN-based LEDs.

We have proposed reticulated photonic crystals (PhCs) on the top surface of conventional light-emitting diodes (LEDs) to increase the light extraction efficiency (LEE). The rigorous electromagnetic simulations based on the three-dimensional finite-difference time-domain method were carried out to investigate the optical properties of reticulated PhCs LEDs and mechanisms of reticulated PhCs improving the LEE of LEDs. Results from these simulations showed that more than 115% enhancement of LEE had been achieved for LEDs with reticulated PhCs period of 200 nm and depth of 200 nm. This study provides an innovative approach to the creation of high-efficiency LEDs of the upcoming generation.

We demonstrated that amorphous photonic crystals can effectively improve the light extraction efficiency of GaN-based light-emitting diodes (LEDs) by the Anderson localization effect. The finite-difference time-domain method was employed to analyze the frequency dependence of the light localization characteristics in the amorphous photonic crystals structure. Numerical studies revealed that the localization modes become most efficient at certain frequencies, owing to the confinement by short-range order. With the action of these strong localization modes, the light that propagated in the horizontal direction was redirected and propagated in the <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${\rm z}$</tex></formula> -direction. Compared with conventional LEDs, significant enhancement of the light extraction efficiency (5.83 times) was achievable from amorphous photonic crystals LEDs with optimized structural parameters. Finally, we analyzed the physical mechanisms by which amorphous photonic crystals can effectively improve light extraction.

The improvement of light extraction efficiency of InGaN light-emitting diodes (LEDs) using microsphere arrays with various refractive indices was analyzed. Finite-difference time-domain (FDTD) simulations show that the use of microsphere (dmicrosphere = 500 nm) arrays with refractive indices of 1.8 and 2.5 led to increase in light extraction efficiency of InGaN LEDs by 1.9 times and 2.2 times, respectively. The enhancement in light extraction efficiency is attributed to the decrease in the Fresnel reflection and increase in effective photon escape cone due to graded refractive index and curvature formed between microsphere and free space. The maximum enhancement of light extraction efficiency of InGaN quantum well LEDs was achieved by employing the refractive index matched anatase-TiO2 microsphere arrays. The effects of microsphere diameters on the light extraction efficiency were also investigated and 2.4 times enhancement was achieved by employing 400-nm refractive index matched TiO2 sphere arrays.

Internal quantum efficiency of GaN-based light-emitting diodes grown on silicon substrates determined from rate equation analyses

Data and analysis are presented employing several new methods to address carrier dynamics and photon management issues in order to improve internal quantum efficiency and light-extraction efficiency of GaN-based light-emitting diodes. For the carrier dynamics, studied are the effects of a newly-developed InAlN electron blocking layer and a p-InGaN:Mg layer on electron blocking and hole transport through multiple-quantum well active region. Also, a new technique of direct surface patterning is demonstrated using three-beam interference laser ablation for efficient photon management.

High light extraction efficiency (LEE) of 75.86% and 77% for light-emitting diodes (LEDs) grown on free-standing GaN and patterned sapphire substrates, respectively, are demonstrated using novel vertical transparent LED (VT-LED) package. Compared to conventional silver header package with consistent external quantum efficiency (EQE) and LEE, enhancement of ~14% and ~12% for GaN and sapphire chips, respectively, are also demonstrated up to 100 A/cm2 under DC operations without any current crowding issues. Also, using high refractive index materials (n~2.1) between LED and ZnO stand, LEE of 81% and 82% for blue and green GaN chips can be achieved.

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).

In Ga N ∕ Ga N blue light-emitting diode (LED) structures are grown on nanoepitaxial lateral overgrown (NELO) GaN template on sapphire substrates by metal organic chemical vapor deposition. Nanopore arrays in SiO2 film are fabricated on the underlying GaN using anodic alumina as etch masks, resulting in a nanoporous SiO2 mask layer with an average nanopore diameter and interpore distance of 60 and 110nm, respectively. The LED on NELO GaN demonstrates low defect density, which is revealed by cross-sectional transmission electron microscopy. More importantly, the LED on NELO GaN demonstrates a higher output power, one order of magnitude higher at 80mA, and an enhancement of the emission efficiency compared to that of the LED on planar GaN. This is mainly attributed to the improvement of light extraction efficiency by random scattering at the interface of the nanoporous SiO2 mask. The nanoporous SiO2 mask acts as both a threading dislocation reduction layer and a light scattering layer to enhance both internal quantum efficiency and external light extraction efficiency. The use of NELO method improves both internal quantum efficiency and external quantum efficiency for III-nitrides based optoelectronic devices.

We present a first-principle method for quantitative modeling of optical energy flow and dissipation in thin-film (TF) light-emitting diodes (LEDs) based on highly general Green's function method. Unlike conventional models, the presented model simultaneously accounts for interference, near-field effects, and internal absorption in determining the radiance generated by a LED. We show that these effects have a profound influence on the efficiency of LEDs and strongly affect the light extraction efficiency (LEE) and the internal quantum efficiency. According to our results, the LEE of an InGaN-GaN TF-LED with untextured surfaces and typical active region (AR) thickness on the order of 10 nm is 67% while a LED with a thin AR exhibits only a LEE of 29%. Based on the numerical results, we discuss the factors that affect the overall efficiency and design considerations to optimize the structure of thin-film LEDs.

This paper demonstrates the strong enhancement of light extraction efficiency of light-emitting diodes (LEDs) by a novel three-dimensionally arranged micro-cavity. There are several optimal designed parameters, including chip dimensions, absorption coefficients, the shape of the micro-cavity and package are analyzed on the basis of a Monte-Carlo ray tracing simulation. The most important that studying includes GaN LEDs which are applied to various applications, including traffic signals, backlight system for LCD and outdoor illumination by white light LEDs. The functional of the three-dimensionally arranged micro-cavity is to make the light extraction from LED with high efficiency. The shape of micro-cavities are making like hexagon solids on the top view. The structure were evaluated and simulated by TracePro software respectively. The light extraction efficiency of LED can be greatly improved by three-dimensionally arranged micro-cavity. This study shows that the micro-cavities induced on the surface rather than that inside the LED greatly enhances the light extraction efficiency. This stipulation holds for both sapphire-based and Thin-GaN LEDs. The results indeed identify the attributes of the LED, which make it possible to achieve excellent luminance performance using a GaN-based approach from the LED of "three-dimensionally arranged micro-cavity". This structure was stringent expected to allow a high-efficiency LED, since the illumination systems needed for higher luminance energy can be added independently of the effects.

The carrier recombination ABC model is a powerful tool to understand the performance of Light-Emitting Diodes (LEDs) such as Light Extraction Efficiency (LEE), Internal Quantum Efficiency (IQE), and lifetime. Starting from the ABC model we demonstrate that the optical-output-power junction-temperature dependence can be used to obtain an LED’s LEE and IQE. Using this analysis to a high power Deep Ultraviolet (DUV) LED and a state-of-the-art blue LED we obtain LEE values of 16% and 80% for the DUV and blue LEDs, respectively. This points LEE to be the main factor for efficiency improvement of DUV LEDs. Also, if IQE (not LEE) is responsible for the optical power decay, an LED lifetime model based on the ABC carrier recombination can be established to predict LED’s lifetime, and the model shows that the LED lifetime is mainly determined by the initial defect density and the defect generation interest rate. We present room temperature lifetime measurement data up to 3,000 hours for some 267 nm DUV LEDs, and the lifetime to maintain 70% of the initial optical power (L70) according to the lifetime model can be extrapolated to be of 20,000 to 120,000 hours. In view of the data presented in this paper, high-efficiency, long-lifetime DUV LEDs can be realized if the light extraction efficiency, initial defect density and uniform current spreading can be optimized.

We report on the fabrication of TiO2 nanosphere (NS) monolayer arrays for the enhancement of light extraction quantum efficiency of GaN-based light-emitting diodes (LEDs). The fabricated TiO2 NSs monolayer arrays were composed of different phases of anatase (An-) and amorphous (Am-) TiO2. The arrays were transferred onto the topmost layer of LED chips via the facile icing transfer method. The LED chips covered with Am-TiO2 NS monolayer arrays showed 3.0- times enhanced light output power intensity compared with reference LED chips at a fixed injection current of 100 mA. The enhanced light extraction of LED chips by an Am-TiO2 NS monolayer can be attributed to a high transmittance (91.1%) in visible and increased light extraction probability of photons generated in LEDs, resulting from the enhanced light coupling efficiency by reduced total internal reflection (TIR). Finite-difference time-domain (FDTD) simulation results also agreed well with the experimentally observed results. Based on the experimental and theoretical results, our suggested Am- and An-TiO2 NS arrays can be considered a very facile and effective method to improve the device performance of various visible LED chips.

In this study, we investigate AlGaN metasurface increasing the light-extraction efficiency (LEE) of AlGaN-based deepultraviolet light-emitting diodes (DUV-LEDs) by utilizing the finite-difference time-domain (FDTD) method. As a first step, a unit cell of metasurface structure adopting the AlGaN cylindrical resonator, which highly increases the transmittance near 280 nm wavelength before the critical angle, was searched numerically. A selected unit cell structure with the resonator was lattice constant a =110 nm with square lattice, the height of cylinder h = 45 nm, and radius r = 40 nm. Transmittance map was constructed for the optimized unit cell as functions of wavelength and incidence angle of plane waves for TE and TM polarized sources, respectively. The map showed perfect transmittance near 280 nm wavelength with normal incidence. Angle-dependent transmittance slowly decreases as the incidence angle increases, but as the incidence angle positions near the critical angle, the decrease of transmittance is gradually accelerated. As a next step, the extracted AlGaN metasurface structure is uniformly deployed to a flip chip LED, and light-extraction efficiencies are calculated as a function of p-GaN thickness for TE and TM mode sources, respectively. The dimension of LED considered in this study was about 2μm× 2μm×1μm. Calculated LEE values clearly showed that by adopting the designed AlGaN metasurface, LEE always increases regardless of p-GaN thickness based on the fundamental increment of transmittance.

We propose a method to quantitatively analyze the internal quantum efficiency (IQE) as well as the efficiencies of non-radiative recombination in the active region (NRA) and carrier escape out of the active region (ESC) by comparing open-circuit (OC) to short-circuit (SC) conditions of InGaN-based light-emitting diodes (LEDs). First, the IQE was extracted from excitation-power dependent photoluminescence at low temperature, and the electron-hole wavefunction overlaps were calculated under OC and SC conditions. Then, the NRA and ESC efficiencies were quantitatively deduced and also compared with photocurrent data. The proposed method would be useful for assessing and designing quantum barriers and analyzing leakage current in LEDs.