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Display Technology, Journal of

Issue 5 • Date May 2013

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Displaying Results 1 - 21 of 21
  • [Front cover]

    Publication Year: 2013 , Page(s): C1
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  • Journal of Display Technology publication information

    Publication Year: 2013 , Page(s): C2
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  • Table of contents

    Publication Year: 2013 , Page(s): 299 - 300
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  • Surface-Roughened Light-Emitting Diodes: An Accurate Model

    Publication Year: 2013 , Page(s): 301 - 316
    Cited by:  Papers (5)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (2777 KB) |  | HTML iconHTML  

    Surface roughening is frequently employed to increase light extraction from light-emitting diodes (LEDs), especially in the important case of III-Nitride LEDs. We explore the physics governing this scheme. We introduce a numerical model, based on solving Maxwell's equations, to accurately describe scattering by a roughened semiconductor interface. This model reveals the complex angular dependence of the scattering properties. We then couple this approach to an LED light extraction model and predict how surface roughness impacts light extraction. We focus on two important cases, thin-film LEDs and volumetric LEDs. We show that optical losses in the LED dictate light extraction, and that volumetric LEDs offer an opportunity for ultimate efficiency. View full abstract»

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  • FDTD Analysis on Extraction Efficiency of GaN Light-Emitting Diodes With Microsphere Arrays

    Publication Year: 2013 , Page(s): 317 - 323
    Cited by:  Papers (21)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (1161 KB) |  | HTML iconHTML  

    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 ( {d}_{\rm \\musphere} = 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-TiO _{2} 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 TiO _{2} sphere arrays. View full abstract»

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  • Light Extraction Efficiency Enhancement of III-Nitride Light-Emitting Diodes by Using 2-D Close-Packed {\hbox {TiO}}_{2} Microsphere Arrays

    Publication Year: 2013 , Page(s): 324 - 332
    Cited by:  Papers (18)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (1304 KB) |  | HTML iconHTML  

    The enhancement of light extraction efficiency of InGaN quantum well light emitting diodes (LEDs) was achieved by employing the refractive index matched TiO2 microsphere arrays. The optimization studies of the dipping method and rapid convective deposition (RCD) method were carried out for the deposition of TiO2 microsphere arrays onto LEDs. The two-dimensional (2D) close-packed TiO2 microsphere arrays were deposited by the using optimized conditions of the dipping and RCD methods, respectively. The light extraction efficiencies of LEDs under electrical injection were enhanced by 1.8-1.9 times by utilizing 520-nm diameter amorphous and anatase TiO2 microspheres via the two deposition methods. View full abstract»

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  • Light Extracting Properties of Buried Photonic Quasi-Crystal Slabs in InGaN/GaN LEDs

    Publication Year: 2013 , Page(s): 333 - 338
    Cited by:  Papers (2)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (835 KB) |  | HTML iconHTML  

    Detailed analysis of experimentally observed far-field diffraction patterns from LEDs containing a buried photonic quasi-crystals (PQC) slab are presented for devices with PQC layer depths of 300 to 1300 nm and pitch 400 nm. Ewald constructions allowed identification of the features in experimental data to be correlated with identified waveguide modes from which light is extracted. Effective indices for the slab waveguide modes calculated by a modal expansion method are found to be in close agreement with values extracted from the experimental results, allowing validation of both an effective medium model for the PQC slabs and a design and analysis method for LED structures with diffractive buried photonic crystals. View full abstract»

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  • Design and Analysis of “Chess Board” Like Photonic Crystal Structure for Improved Light Extraction in GaN/InGaN LEDs

    Publication Year: 2013 , Page(s): 339 - 345
    Cited by:  Papers (2)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (1269 KB) |  | HTML iconHTML  

    In this paper, we have proposed a “Chess board” like photonic crystal (PhC) structure on top surface of {\rm p-GaN/SiO}_{2} layer in conventional LED, on top surface of n-GaN layer in vertical LED and an embedded PhC structure in n-GaN layer for achieving an improved light extraction in GaN/InGaN LEDs. The results are compared with that of the LED structures with conventional 2-D PhC structures and of the conventional LEDs. Results from these simulations show that the maximum light extraction for conventional LED having “Chess board” like structure occurs for a grating period of 0.6 \mu{\hbox {m}} with a grating-depth of 0.18 \mu{\hbox {m}} , which gives \sim 4 times enhancement compared to that of conventional LED and 1.2–1.4 times compared to that of LED with conventional 2-D PhCs. In case of a vertical LED, the maximum enhancement in light extraction occurs for the same grating-period with a depth of 0.5 \mu{\hbox {m}} , which is \sim 3.5 times compared to that of the conventional vertical LEDs. We have also simulated our proposed structure on top of {\hbox {SiO}}_{2} -on-p-GaN layer in order to avoid the etching of p-GaN layer, which shows \sim 2.2 times enhancement in comparison to that of conventional LED. For the proposed embedded PhC structure in n-GaN layer, the light extraction is enhanced by a factor of 2.8–3.5 as compared to the conventional LED. View full abstract»

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  • Improved Light Extraction Efficiency of Nonpolar a-Plane GaN-Based LEDs Based on Embedded Pyramid-Shape Air-Gap Structure

    Publication Year: 2013 , Page(s): 346 - 352
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (879 KB) |  | HTML iconHTML  

    A significant improvement of output power and external quantum efficiency (EQE) for nonpolar a-plane (11-20) GaN light-emitting diodes (LEDs) on r-plane sapphire substrates has been demonstrated by using embedded pyramid-shape air-gap arrays on hexagonally patterned SiO2 layer. The air-gap structure was realized based on asymmetrical growth behavior of a-plane epitaxial lateral overgrowth (ELO) GaN along +c-axis and -c-axis. The output power and EQE of the a-plane LEDs with the air-gaps on patterned SiO2 have increased by more than 50% when compared to those of the conventional a-plane LEDs. Theoretical fit to the measured EQE suggested that the significant improvement of EQE was mainly attributed to the increase in light extraction efficiency (EXE). Light emission pattern analysis and ray-tracing simulation revealed that the air-gap arrays on top of patterned SiO2 enlarged the guided-light scattering at the air-gap surface due to the significant refractive index contrast with the GaN layer, followed by the improvement of EXE of nonpolar a-plane LEDs. View full abstract»

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  • GaN-Based Light-Emitting Diodes With Step Graded-Refractive Index ({\hbox {ZnO}}) _{x}({\hbox {SiO}}_{2})_{1-x} Micropillar Array

    Publication Year: 2013 , Page(s): 353 - 358
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    Step graded-refractive index (SGRI) (ZnO)x(SiO2)1-x micropillar multilayers have been introduced and demonstrated on GaN-based LEDs combined with the mesh ITO. The SGRI (ZnO)x(SiO2)1-x micropillars were produced by controlling the Zn/Zn+Si ratio of co-sputtered ZnO and SiO2. The introduced three-layered SGRI (ZnO)x(SiO2)1-x micropillars improved both critical angle inside GaN LEDs and Fresnel transmittance coefficient (ηFr) as well as had better light coupling into the micropillars. Moreover, a high number of layers of the SGRI micropillars would aid the light coupled in the pillars to escape from the side wall of the pillar. LEDs with three-layered SGRI (ZnO)x(SiO2)1-x micropillars exhibited output power enhancements of 12.2% with a 20 mA Vf of 3.19 V. The output power of the mesh ITO LEDs with SGRI (ZnO)x(SiO2)1-x micropillars was further enhanced to 15.3% by improving the current spreading. View full abstract»

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  • Enhancing LED Light Extraction by Optimizing Cavity and Waveguide Modes in Grating Structures

    Publication Year: 2013 , Page(s): 359 - 364
    Cited by:  Papers (2)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (1121 KB) |  | HTML iconHTML  

    An optimization of light extraction efficiency (LEE) from light-emitting diodes (LEDs) is provided. The optimization is achieved when both the cavity and waveguide modes are in their best conditions and their emitted waves interfere constructively in the grating-assisted structures. In other words, high LEE appears when the structural parameters results in good cavity modes in both planar and grating structures, and also when the propagation direction of the fundamental waveguide mode is significantly converted to the direction normal to the grating surface by the gratings. In this paper, we use two simple equations, one for cavity modes and the other for waveguide modes, to predict the loci of the optimal structural parameters of gratings. The prediction is verified with the rigorous coupled wave analysis (RCWA). We systematically present an investigation of the various structural effects of gratings on the LEE of the thin film vertical gallium-nitride (GaN) based LEDs. The results show that ultra-high LEE can be achieved when the equivalent cavity thickness is good for fundamental cavity mode, the source is positioned around antinodes for both cavity and waveguide modes, there is significant diffraction, and the fundamental waveguide mode is largely converted to the normal direction to the air, and the emitted waves from fundamental cavity and waveguide modes interfere constructively. View full abstract»

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  • InGaN-Based Resonant-Cavity Light-Emitting Diodes Fabricated With a {\hbox {Ta}}_{2}{\hbox {O}}_{5}/{\hbox {SiO}}_{2} Distributed Bragg Reflector and Metal Reflector for Visible Light Communications

    Publication Year: 2013 , Page(s): 365 - 370
    Cited by:  Papers (1)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (1313 KB) |  | HTML iconHTML  

    The potential of visible light communications based upon phosphor-converted white resonant-cavity light-emitting diodes (RCLEDs) is investigated experimentally. To fabricate a blue InGaN RCLED, a λ/4 -thick Ta2O5/SiO2 distributed Bragg reflector, along with a metallic Ag layer, were respectively coated onto the top and bottom of normal LEDs to form an optical cavity. As evaluated from the emission spectrum of blue RCLEDs, the discrepancy of the expected cavity length from the measurements suggests that cavity oscillation may mostly occur in the GaN-based epistructures. In addition to the presence of the optical cavity effect, the incorporation of a bottom reflector is useful to increase the light extraction efficiency of the RCLEDs. As a result, these RCLEDs exhibit improved operational characteristics over normal LEDs in terms of light output power, external quantum efficiency, spectral purity, and directionality. With an increase in injection current, the enhancement of the spontaneous emission rate is responsible for the improved quality of eye patterns in blue RCLEDs operating at a transmission rate of 100 Mbit/s and 175 mA . After encapsulating the blue RCLEDs with a phosphor layer, we found that white RCLEDs have the capacity for free-space optical communication with a data rate of 12 Mbit/s. View full abstract»

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  • A Stress Analysis of Transferred Thin-GaN Light-Emitting Diodes Fabricated by Au-Si Wafer Bonding

    Publication Year: 2013 , Page(s): 371 - 376
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    Thin-GaN light-emitting diodes were fabricated by Au-Si wafer bonding and laser lift-off. The relaxation process of the thermal strain in the transferred GaN films on a Si substrate was studied by varying the bonding film thickness of the Au over a wide range from 7 μm to 40 μm. The transferred GaN films were found to be strained by the biaxial compressive stress. A 10 μm Au bonding layer thickness was proven to have the lowest residual compressive stress, and the complete compressive stress variation throughout the entire thin-GaN fabrication process is discussed. Finally, we changed the biaxial in-plane stress of the transferred GaN thin film by controlling the bonding conditions, including the bonding layer thickness and the bonding temperature. View full abstract»

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  • A GaN-Based LED With Perpendicular Structure Fabricated on a ZnO Substrate by MOCVD

    Publication Year: 2013 , Page(s): 377 - 381
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    A perpendicular InGaN/GaN multiple-quantum- wells structure on ZnO substrate for blue light emitting diode (LED) was successfully fabricated by use of Metal-organic Chemical Vapor Deposition (MOCVD). During the growing process of GaN-based materials on ZnO substrates, the low-temperature-grown GaN buffer layer, inserted between ZnO substrate and undoped GaN layer, prevented the Zn and O from diffusing from ZnO substrate into the n-GaN layer. This thin GaN buffer layer, mainly as a insulating layer, was grown at relatively low temperature of 530 °C. By using our method, an integrated LED with ZnO substrate can be fabricated with a crack-free GaN film on (0001) ZnO substrate by MOCVD using this method. The epilayer crystalline structure has been measured by atomic force microscopy (AFM), and the optical properties of the LED were also characterized by photoluminescence and Current-Voltage characteristic curve. View full abstract»

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  • A Highly Power-Efficient LED Back-Light Power Supply for LCD Display

    Publication Year: 2013 , Page(s): 382 - 387
    Cited by:  Papers (3)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (1194 KB) |  | HTML iconHTML  

    This paper proposes a highly power-efficient light-emitting diode (LED) back-light power supply for liquid crystal display (LCD). The proposed converter integrates the power factor correction (PFC) circuit with the dc-dc converter. It provides an isolated dc voltage without using any full-bridge diode rectifier. Conduction losses are reduced by eliminating the full-bridge diode rectifier. Switching power losses are also reduced by achieving zero-voltage switching (ZVS) operation of power switches. By reducing power losses, the power efficiency of the LED back-light power supply can be improved. The experimental results are presented to verify the feasibility of the proposed converter for a 100 W LED back-light power. View full abstract»

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  • Analysis and Suppression of Overcurrent in Boost LED Drivers

    Publication Year: 2013 , Page(s): 388 - 395
    Cited by:  Papers (1)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (2525 KB) |  | HTML iconHTML  

    Light-emitting diodes (LEDs) are the major backlighting source used in liquid crystal displays and dimming the LEDs is a common approach to increase the contrast ratio or save power. Pulse-width modulation (PWM) is more popular than analog methods of dimming, because it produces a less pronounced shift in chromaticity. However, in PWM dimming mode, overcurrent can shorten the lifespan of the LEDs. This paper proposes a technique to overcome the drawbacks inherent in conventional approaches to the suppression of overcurrent in LED devices. The design was implemented using the TSMC 0.25- μm 60-V bipolar-CMOS-DMOS process, resulting in a chip area of 2.2 mm2. A comparison with two commercial chips demonstrates the effectiveness of the proposed design in the suppression of LED overcurrent and the subsequent extension of the lifespan. View full abstract»

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  • LED Junction Temperature Measurement Using Generated Photocurrent

    Publication Year: 2013 , Page(s): 396 - 401
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    LED-based lamps that are currently on the market are expensive due to the complex packaging required to dissipate the heat generated. This also limits their performance and lifetime due to the degradation of the phosphor or individual LED chips, in the case of RGB sources. There is a strong commercial imperative to develop in situ technology to measure and ultimately compensate for the thermal environment of a luminaire. Utilizing the large Stoke's shift in InGaN green and blue emitting LEDs, a blue LED emitter pump can induce a photocurrent within devices which emit in either the blue or green region. Measurements have shown that green and blue emitters may be excited on the absorption edge in an effect which results in a rise in the open circuit voltage with increasing temperature. From these measurements the junction temperature of a device operating in quasi-cw mode at 80 mA is shown to result in a junction temperature of 86 (±2) °C which agrees well with a junction temperature of 87 (±2) °C measured using the more conventional forward voltage technique. This paper describes the technique utilized and the results achieved in driving a green emitting LED with a blue emitting pump LED and furthermore it discusses some of the benefits and issues associated with this technique for determining the junction temperature. View full abstract»

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  • [Blank page]

    Publication Year: 2013 , Page(s): 402
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  • IEEE Copyright Form

    Publication Year: 2013 , Page(s): 403 - 404
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  • Journal of Display Technology information for authors

    Publication Year: 2013 , Page(s): C3
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  • [Blank page - back cover]

    Publication Year: 2013 , Page(s): C4
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Aims & Scope

This publication covers the theory, design, fabrication, manufacturing and application of information displays and aspects of display technology.

Full Aims & Scope

Meet Our Editors

Editor-in-Chief
Arokia Nathan
University of Cambridge
Cambridge, U.K.