GaN-based Green Light-emitting Diodes Research Articles

Enhancement of the light extraction of GaN-based green light emitting diodes via nanohybrid structures

Improvement in the light extraction efficiency (LEE) of GaN-based green light emitting diodes (LEDs) with ZnO nanostructures synthesized by a hydrothermal method is reported. Formation of ZnO nanorods, hemispheres, and cones was controlled by varying the pH of the aqueous synthesis solution. The shape of the ZnO nanostructures integrated onto the LEDs shows a strong relationship with the LEE characteristics of GaN-based green LEDs. The electroluminescence (EL) intensity of LEDs covered by ZnO nanostructures increased compared to conventional LEDs. In terms of LEE, LEDs with surface-textured ZnO hemispheres showed the highest EL intensity, which can be attributed to an increase in the effective critical angle, the escape cone, and multiple scattering. Finite difference time domain (FDTD) simulation was conducted to theoretically confirm the experimental results.

Light extraction efficiency of GaN-based LEDs with non-periodic and periodic sub-wavelength structures

Antireflective non-periodic and periodic sub-wavelength structures (SWSs) are fabricated on indium tin oxide (ITO) layers deposited on p-GaN layers of GaN-based green light-emitting diodes (LEDs). The light output powers of the LEDs using non-periodic and periodic SWSs are increased by 18% and 39% at a 20-mA input current, respectively, compared to LEDs with flat ITO layers. Periodic ITO SWSs show a larger enhancement of output power because they have more effective profiles of the graded refractive index. The output power of periodic SWS LEDs is improved in all angular directions, indicating that the improved output power is attributed to lower total internal reflection and lower Fresnel reflection.

Enhanced light extraction from a GaN-based green light-emitting diode with hemicylindrical linear grating structure

Significant enhancement in the light output from GaN-based green light-emitting diodes (LEDs) was achieved with a hemicylindrical grating structure on the top layer of the diodes. The grating structure was first optimized by the finite-difference time-domain (FDTD) method, which showed that the profile of the grating structure was critical for light extraction efficiency. It was found that the transmission efficiency of the 530 nm light emitted from the inside of the GaN LED increased for incidence angles between 23.58° and 60°. Such a structure was fabricated by electron-beam lithography and an etching method. The light output power from the LED was increased approximately 4.7 times compared with that from a conventional LED. The structure optimization is the key to the great increase in transmission efficiency. Furthermore, the light emitted from the edge of the LED units could be collected and extracted by the grating structures in adjacent LED units, thus enhancing the performance of the whole LED chip.

Short-wavelength light beam in situ monitoring growth of InGaN/GaN green LEDs by MOCVD

In this paper, five-period InGaN/GaN multiple quantum well green light-emitting diodes (LEDs) were grown by metal organic chemical vapor deposition with 405-nm light beam in situ monitoring system. Based on the signal of 405-nm in situ monitoring system, the related information of growth rate, indium composition and interfacial quality of each InGaN/GaN QW were obtained, and thus, the growth conditions and structural parameters were optimized to grow high-quality InGaN/GaN green LED structure. Finally, a green LED with a wavelength of 509 nm was fabricated under the optimal parameters, which was also proved by ex situ characterization such as high-resolution X-ray diffraction, photoluminescence, and electroluminescence. The results demonstrated that short-wavelength in situ monitoring system was a quick and non-destroyed tool to provide the growth information on InGaN/GaN, which would accelerate the research and development of GaN-based green LEDs.

Enhanced Light Extraction of GaN-Based Green Light-Emitting Diodes With GaOOH Rods

We reported the enhanced light extraction efficiency in InGaN/GaN multiple quantum well green light-emitting diodes (LEDs) with gallium oxide hydroxide (GaOOH) rods. The GaOOH rods were prepared by an aqueous gallium nitrate solution at 80°C and then coated on the surface of indium tin oxide electrodes of LEDs by a simple drop coating process. The synthesized GaOOH rods indicated a rhombus-shaped rod structure with average lengths of 2 μm and lateral dimensions of 50-500 nm. For LEDs with GaOOH rods, the light output powers were increased by 24.3% and 26.4% compared to the conventional LED on patterned sapphire substrate at 20 mA and 100 mA, respectively. Also, there was no distinct degradation in electrical characteristics of LEDs with GaOOH rods.

Effect of the graded electron blocking layer on the emission properties of GaN-based green light-emitting diodes

We report on the effect of a graded AlGaN electron blocking layer (GEBL) on the emission properties of InGaN/GaN multiple quantum wells light-emitting diode (LED). The adoption of GEBL in the LED enhances the electroluminescence intensity and reduces the wavelength blue-shift with increasing injection current. The light output power of the GEBL LED is enhanced by 163% and 415% at 20 and 350 mA, respectively. Moreover, the forward voltage of the GEBL LED is reduced by 0.38 V at the forward current of 20 mA.

Investigation of the Carrier Dynamic in GaN-Based Cascade Green Light-Emitting Diodes Using the Very Fast Electrical–Optical Pump–Probe Technique

For the first time, the internal carrier dynamic inside GaN-based green light-emitting diodes (LEDs) during operation has been directly observed using the demonstrated electrical-optical pump-probe technique. Short electrical pulses (~100 ps) were pumped into high-speed cascade green LEDs, and the output optical pulses were probed using high-speed photoreceiver circuits. Using such a method, the recombination time constant of the carriers can be directly measured without any assumption about the recombination process. A high-speed cascade LED structure was adopted in the experiments to eliminate the influence of the RC delay time on the measured responses. Our measurement results indicate that both single- and three-LED cascade structures have the same internal response time due to current continuity. Furthermore, based on responses measured under different temperatures (from 25°C to 200°C), the origin of the efficiency droop in GaN-based green LEDs under a high bias current density may be attributed to the strong nonradiative Auger effect rather than device heating or carrier overflow. The demonstrated measurement scheme and high-speed cascade device structure offer a novel and simple way to straightforwardly investigate the internal carrier dynamic inside the active layers of the LED during forward-bias operation.

Improved Light Extraction of GaN-Based Green Light-Emitting Diodes with an Antireflection Layer of ZnO Nanorod Arrays

We investigate the effect of double ZnO nanorod layers on the light extraction of green light-emitting diodes LEDs . The double ZnO nanorod layers with a step gradient of the refractive index are formed as antireflection layers on the transparent electrode of LEDs. The light output power of the green LEDs with ZnO nanorods is increased by 42.9% at an injection current of 20 mA compared to those without ZnO nanorods. The increase in light output power is mostly attributed to a reduction in Fresnel reflection by antireflection layers of ZnO nanorods on green LEDs. © 2010 The Electrochemical Society. DOI: 10.1149/1.3526959 All rights reserved.

Full wafer scale nanoimprint lithography for GaN-based light-emitting diodes

A UV-imprinting process for a full wafer was developed to enhance the light extraction of GaN-based green light-emitting diodes (LEDs). A polyvinyl chloride flexible stamp was used in the imprinting process to compensate for the poor flatness of the LED wafer. Two-dimensional photonic crystal patterns with pitches ranging from 600 to 900 nm were formed on the p-GaN top cladding layer of a 2 inch diameter wafer using nanoimprint and reactive ion etching processes. As a result, the optical output power of the patterned LED device was increased by up to 44% at a driving current of 20 mA by suppressing the total internal reflection and enhancing the irregular scattering of photons at the patterned p-GaN surface.

A nondamaging electron microscopy approach to map In distribution in InGaN light-emitting diodes

Dark-field inline electron holography and, for comparison, high-resolution transmission electron microscopy are used to investigate the distribution of indium in GaN-based commercial high-efficiency green light-emitting diodes consisting of InGaN multiquantum wells (QWs). Owing to the low electron doses used in inline holography measurements; this technique allows to map the indium distribution without introducing any noticeable electron beam-induced damage which is hardly avoidable in other quantitative transmission electron microscopy methods. Combining the large field of view with a spatial resolution better than 1 nm, we show that the InGaN QWs exhibit random alloy nature without any evidence of nanometer scale gross indium clustering in the whole active region.

Enhancement of light extraction from GaN-based green light-emitting diodes using selective area photonic crystal

We report the development of a GaN-based green light-emitting diode (LED) with a selective area photonic crystal (SPC) structure, which was formed outside the p-bonding electrode on p-GaN. As a result, the optical output power of LEDs with SPC was enhanced by 78% compared to that without PC. In addition, the forward voltage, series resistance, and leakage current of LEDs with SPC were remarkably improved. These results show that the light extraction efficiency of green LEDs can be greatly increased using the SPC structure, with no degradation of electrical properties.

Fabrication of moth-eye structure on p-GaN layer of GaN-based LEDs for improvement of light extraction

Moth-eye structures were produced on a p-GaN top cladding layer by UV imprint and inductively coupled plasma (ICP) etch processes in order to improve the light extraction efficiency of GaN-based green light-emitting diodes (LEDs). The height and shape of moth-eye structures were adjusted by controlling the thickness of Cr mask layer and ICP etching time. The transmittance of LED device stacks with moth-eye structure was increased up to 1.5–2.5 times, compared to identical LED sample without moth-eye structure and the intensity of photoluminescence from the InGaN multi-quantum well layer of LED sample with moth-eye structure was 5–7 times higher than that of the LED sample without the moth-eye structure.

Low-resistance and transparent Ni–Co solid solution/Au ohmic contacts to p-type GaN for green GaN-based light-emitting diodes

We report on the formation of low-resistance and transparent p-type ohmic contact for green GaN-based light-emitting diodes (LEDs) using Ni–Co solid solution(5 nm)/Au(5 nm) scheme. The as-deposited Ni–Co solid solution(5 nm)/Au(5 nm) contact shows nonlinear current–voltage ( I – V ) characteristics. However, annealing the sample at 550 ∘C in air results in the formation of ohmic contacts with specific contact resistance of 6.5×10 −4 Ω cm 2. The annealed sample shows light transmittance of 71.7% at a wavelength of 530 nm, which is better than that (64.9%) of oxidized Ni/Au contacts. Based on Auger electron spectroscopy and x-ray photoemission spectroscopy results, the annealing-induced improvement of the contacts is described and discussed.

High-Speed GaN-Based Green Light-Emitting Diodes With Partially n-Doped Active Layers and Current-Confined Apertures

We demonstrate a high-speed GaN-based green light-emitting diode for plastic optical fiber (POF) communication applications. By using a combination of n-type doping and undoped In <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> N/GaN based multiple quantum wells (MQWs), and a 76-mum-diameter current-confined aperture structure, we can obtain an extremely high electrical-to-optical (E-O) 3 dB bandwidth (~330 MHz), which is limited by the spontaneous recombination lifetime of the MQWs. A reasonable coupled power (-264 muW) can be simultaneously achieved for a 2 mm in diameter POF with a 0.5 numerical aperture (NA).

Enhanced light extraction from GaN-based green light-emitting diode with photonic crystal

This letter reports the properties of GaN-based green light-emitting diodes (LEDs) having a p-GaN photonic crystal layer with a photonic bandgap (PCWG) and without a photonic bandgap (PCOG). With decreasing the photoluminescence (PL) detection angle from 140° to 60°, the enhancement of PL intensity of LED with PCWG was largely increased from 9 to 25 times, compared to that of LEDs without a patterned structure, while the PL intensity of LED with PCOG was increased from 4.6 to 5.6 times. The electroluminescence output power of green LEDs with a PCWG was enhanced about two times compared to LEDs with a PCOG. These results suggest that the light extraction of green LEDs can be greatly increased by using PCWG instead of PCOG.

Linear Cascade Arrays of GaN-Based Green Light-Emitting Diodes for High-Speed and High-Power Performance

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> In the following study, we demonstrated linear cascade GaN-based light-emitting-diode (LED) arrays at a wavelength of approximately 520 nm. Experimental LEDs were analyzed with the goal to improve the output power and differential efficiency of a single LED. The study shows that using arrays with up to four LEDs connected in series, we can achieve four times the improvement in output power (differential quantum efficiency) under the same bias current as compared to a single LED apparatus. We have also measured the modulation-speed performance of experimental LEDs, and both devices exhibit similar 3-dB bandwidth (90 MHz) under the same bias currents. Experimental results indicate that the cascade connection offers the advantages of significantly enhanced external differential efficiency and provision of a method to use a constant-voltage power supply. The current crowding problem and resistance– capacitance- limited bandwidth degradation issues in a large active area LED can also be minimized using the connection demonstrated in our experiment. </para>

The improvement in modulation speed of GaN-based Green light-emitting diode (LED) by use of n-type barrier doping for plastic optical fiber (POF) communication

We demonstrate a high-speed GaN-based light-emitting diode at a wavelength of around 500 nm for the application to plastic optical fiber communication. By use of the n-type doping in the GaN barrier layers of the In <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> N-GaN-based multiple-quantum-well (MQW), superior performance of modulation-speed (120 versus 40 MHz) and output power to the undoped control under the same bias current has been observed. According to the measured electrical-to-optical bandwidths and extracted RC-limited bandwidths of both devices, the superior speed performance can be attributed to higher electron/hole radiative recombination rate in the n-doped MQW than that of undoped MQW