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Light Extraction (light + extraction)

Distribution by Scientific Domains
Physics and Astronomy83%

Selected Abstracts

Enhancement of Light Extraction Through the Wave-Guiding Effect of ZnO Sub-microrods in InGaN Blue Light-Emitting Diodes

ADVANCED FUNCTIONAL MATERIALS, Issue 7 2010
Ki Seok Kim
Abstract The improvement of the light extraction efficiency (LEE) of a conventional InGaN blue light-emitting diode (LED) by the incorporation of one-dimensional ZnO sub-microrods is reported. The LEE is improved by 31% through the wave-guiding effect of ZnO sub-microrods compared to LEDs without the sub-microrods. Different types of ZnO microrods/sub-microrods are produced using a simple non-catalytic wet chemical growth method at a low temperature (90,°C) on an indium-tin-oxide (ITO) top contact layer with no seed layer. The crystal morphologies of needle-like or flat-top hexagonal structures, and the ZnO microrods/sub-microrod density and size are easily modified by controlling the pH value and growth time. The wave-guiding phenomenon within the ZnO rods is observed using confocal scanning electroluminescence microscopy and micro-electroluminescence spectra. [source]

340,350 nm GaN-free UV-LEDs

PHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 1 2003
T. Nishida
Abstract Light extraction from AlGaN-based ultraviolet light emitting diodes (UV-LEDs) is described. The device structure is free from a GaN binary layer for the suppression of the reabsorption of the UV emission in the GaN layer. The UV extraction becomes six times higher when compared with that of the normal structure consisting of a GaN buffer layer. The highest external quantum efficiencies of 350- and 340-nm LEDs are 2.2 and 1.7%, respectively. The maximum output powers are 8.6 and 5.5 mW at injection currents of less than 150 mA. We also investigated the potential of combining the 350-nm UV-LED with a mixture of plural fluorescence materials for the application field of lighting. This combination can provide ideal lighting equipment that offers high colour rendering, steady colour, and no glare even with changes of intensity and illumination angle. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Analysis of Improved Efficiency of InGaN Light-Emitting Diode With Bottom Photonic Crystal Fabricated by Anodized Aluminum Oxidxe

ADVANCED FUNCTIONAL MATERIALS, Issue 10 2009
Sang-Wan Ryu
Abstract The improved performance of a bottom photonic crystal (PC) light-emitting diode (LED) is analyzed based on internal quantum efficiency (,int) and light-extraction efficiency (,ex). The bottom PC is fabricated by anodized aluminum oxide nanopatterns and InGaN quantum wells (QWs) are grown over it. Transmission electron microscopy images reveal that threading dislocations are blocked at the nanometer-sized air holes, resulting in improved optical emission efficiency of the QWs. From temperature-dependent photoluminescence measurements, the enhancement of ,int is estimated to be 12%. Moreover, the enhancement of ,ex is simulated to be 7% by the finite-difference time-domain method. The fabricated bottom PC LED shows a 23% higher optical power than a reference, which is close to the summation of enhancements in ,int and ,ex. Therefore, the bottom PC improves LED performance through higher optical quality of QWs as well as increased light extraction. [source]

Polarization properties in deep-ultraviolet AlGaN quantum wells with various substrate orientations

PHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 7 2010
A. Atsushi Yamaguchi
Abstract It is known that emission polarization in AlGaN quantum wells (QWs) on c-plane substrates switches from in-plane polarization to c -axis polarization with increasing Al composition. This behaviour is unfavourable for light extraction from c-plane based light emitting diodes (LEDs). Previously, we proposed theoretically that this unfavourable polarization can be changed into favourable in-plane polarization by decreasing well width and/or introduction of compressive strain in c-oriented AlGaN-QWs. In this work, we have investigated the substrate orientation dependence of polarization properties in such AlGaN QWs by numerical calculation using the 6,×,6 k·p Hamiltonian. It is shown that even small inclination of c-plane substrate makes a drastic change in the polarization characteristics and that the use of the vicinal substrates as well as semipolar and nonpolar substrates could be beneficial in improving optical device performance. [source]

Structural analysis of nitride-based LEDs grown on micro- and nano-scale patterned sapphire substrates

PHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 7-8 2010
Y. K. Su
Abstract The structural properties of nitride-based light-emitting diodes (LEDs) grown on micro- and nano-scale patterned sapphire substrates (PSS) were discussed in detail. The high resolution X-ray diffraction (HRXRD) and etch-pit density (EPD) results reveal that the crystalline quality of the epitaxial GaN film could be effectively improved by using the PSS technique, and depended on the aspect ratio of PSS. The crystalline quality of epitaxial GaN films grown on PSS-2,m and PSS-3,m was better than that of the LED grown on NPSS. The electrical characteristics and junction temperature results of the LED grown on micro-scale PSS were better compared with nano-scale PSS. But the output power of the LED grown on nano-scale PSS was larger than that of the LED grown on micro-scale PSS since the pattern-size of PSS is related to the capability of light extraction. However, when the spacing of PSS is less than 2 ,m, some voids formed at the GaN/sapphire interface may cause the thermal dissipation problem of LEDs. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Realization of extreme light extraction efficiency for moth-eye LEDs on SiC substrate using high-reflection electrode

PHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 7-8 2010
Ryosuke Kawai
Abstract The effect of a high-reflection electrode on the opposite side of a moth-eye structure on the performance of a nitride-based blue LED is described. Because of the interference effect enhanced by multiple reflections with a high-reflectance Ag-Pd-Cu alloy (APC) p-contact, light extraction is markedly improved. For the 600-nm-pitch sample, both APC and conventional Ni/Au contacts were used. Although the light output powers of LEDs with APC and Ni/Au p-contacts without moth-eye structure are almost the same, those with a moth-eye structure are markedly increased and differ from each other. For the APC contact, the light output is improved by a factor of 3.4, whereas that with a Ni/Au contact is increased by a factor of 2.2. In addition, we fabricated moth-eye LEDs with pitches of 300 nm, 400 nm, 500 nm and 600 nm by low-energy electron-beam projection lithography. We found that the sample with a pitch of 500 nm has the highest output power, which is 3.7 times higher than that of the reference sample. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Light extraction enhancement of sapphire-free InGaN LEDs using single- and double-side surface roughening techniques

PHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue S2 2009
Dong-Sing Wuu
Abstract We compare the performance of three types of GaN-based light-emitting diodes (LEDs) epitaxially grown on patterned sapphire substrates (PSSs). For the devices with a single roughened p-GaN surface, the light output power of the original LEDs with the PSS and Ag reflector (Sample A) is lower than that of thin-film LEDs transferred onto Si carriers via glue bonding (Sample B). While the thin GaN LEDs with double-side roughening (both p-GaN and micropillar undoped-GaN surfaces) and glue/Ag omnidirectional reflector (Sample C) shows much higher light output power than that of Sample B. As compared with the Sample A, the Sample C shows a performance enhancement of 50.3% in light output power and 46.3% in power efficiency at 350 mA. These results reflect that the contribution of the micropillar undoped-GaN texturing for the light extraction is dominant. The employment of Si carrier decreases the junction temperature of the Sample C, a 46.6 °C lower than that of the conventional LEDs on PSS. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Luminescence and vibrational properties of erbium-implanted nanoporous GaN

PHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 6 2008
C. B. Soh
Abstract Implantation of erbium (Er) into GaN is useful in creating selected areas to emit at the green, yellow and infrared wavelengths. Enhanced erbium activation is obtained when erbium is implanted into porous GaN formed by electrochemical etching than into as-grown GaN. This is due to the increase in surface areas for light extraction and the availability of more free surfaces to accommodate strain when it is annealed. Furnace annealing at 1100 °C for 30 mins in nitrogen gives rise to higher band-edge photoluminescence intensity. Apart from the host GaN phonon modes, we have also observed disorder-induced lattice vibrations at 170, 200 and 350-365 cm,1 from Er-implanted porous GaN. The E2 (high) mode of GaN also shifts towards higher energy at higher annealing temperatures, indicative of more erbium occupying the VGa site (ionic radii of Er > Ga) and hence increasing the compressive stress in the GaN crystal lattice. The prominent defect-induced local vibrational modes in Er-doped nanoporous GaN are also observed in ultraviolet resonant Raman scattering. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Azimuthal anisotropy of light extraction from photonic crystal light-emitting diodes

PHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 6 2008
Chun-Feng Lai
Abstract Photonic crystal (PhC) light-emitting diodes (LEDs) exhibiting anisotropic light extraction have been investigated experimentally and theoretically. It is found that the anisotropic light extraction strongly depends on the lattice constant and orientation. Optical images of the anisotropy in the azimuthal direction are obtained using annular structure with triangular lattice. 6-fold symmetric light extraction patterns with varying number of petals are observed. More petals in multiple of 6 appear in the observed image with lattice constant increasing. This anisotropic behavior suggests a new means to optimize the PhC design of GaN LED for light extraction. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Spatial mapping on surface light extraction from 2D photonic quasicrystals patterned GaN-based light emitters

PHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 1 2007
ZhenSheng Zhang
Abstract We report the observations of surface light extraction enhancement from two-dimensional (2D) photonic crystal (PC) and Photonic Quasicrystal (PQC) patterns on electrical current injected GaN-based light emitters. Spatial mapping of the surface extraction was acquired by a microscopic electrical luminescence setup and a scanning near-field optical microscope (SNOM). Slight difference of mapping profile between twelve-fold PQC and regular triangular PC was obtained. It is shown that 12PQC was more efficient for light extraction. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

GaN-free transparent structure for ultraviolet light emitting diodes

PHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 6 2004
T. Nishida
Abstract AlGaN-based light emitting diodes (LEDs) are expected to be the next generation of ultraviolet light (UV) sources. With the aim of achieving AlGaN-based UV-LEDs comparable to those of conventional blue and red LEDs, we have investigated the regular epitaxial growth, the fabrication of uniform and abrupt heterointerfaces, the optical characteristics of AlGaN-based nitride quantum structures, the validity of the band engineering, the p-n junction designs, radiative recombination and light extraction. Efficient and transparent UV-LEDs grown on a high-quality AlN-template layer on a sapphire substrate are free from binary GaN, and have great potential for lighting equipment and other applications. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]