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Display Technology, Journal of

Issue 4 • Date April 2013

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Displaying Results 1 - 24 of 24
  • Front Cover

    Page(s): C1
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  • Journal of Display Technology publication information

    Page(s): C2
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  • Table of contents

    Page(s): 185 - 186
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  • Guest Editorial Recent Advances in Solid State Lighting

    Page(s): 187 - 189
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  • Semipolar ({\hbox {20}}\bar{{\hbox {2}}}\bar{{\hbox {1}}}) InGaN/GaN Light-Emitting Diodes for High-Efficiency Solid-State Lighting

    Page(s): 190 - 198
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (1560 KB) |  | HTML iconHTML  

    This work examines the effects of polarization-related electric fields on the energy band diagrams, wavelength shift, wave function overlap, and efficiency droop for InGaN quantum wells on various crystal orientations, including polar (0001) (c -plane), semipolar (202̅1̅), semipolar (202̅1̅), and nonpolar (101̅0̅) (m-plane). Based on simulations, we show that the semipolar (202̅1̅) orientation exhibits excellent potential for the development of high-efficiency, low-droop light-emitting diodes (LEDs). We then present recent advancements in crystal growth, optical performance, and thermal performance of semipolar (202̅1̅) LEDs. Finally, we demonstrate a low-droop, high-efficiency single-quantum-well blue semipolar (202̅1̅) LED with an external quantum efficiency of more than 50% at 100 A/cm2. View full abstract»

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  • Analysis of Interdiffused InGaN Quantum Wells for Visible Light-Emitting Diodes

    Page(s): 199 - 205
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (1483 KB) |  | HTML iconHTML  

    Interdiffused InGaN quantum wells (QWs) with various interdiffusion lengths (Ld) are comprehensively studied as the improved active region for Light-Emitting Diodes (LEDs) emitting in the blue and green spectral regime. The electron-hole wavefunction overlap (Γe_hh), spontaneous emission spectra, and spontaneous emission radiative recombination rate (Rsp) for the interdiffused InGaN QWs are calculated and compared to that of the conventional InGaN QWs emitting in the similar wavelengths. The calculations of band structure, confined energy levels, electron and hole wavefunctions, and spontaneous emission radiative recombination rate (Rsp) are based on the self-consistent 6-band k·p method, taking into account the valence band mixing, strain effect, spontaneous and piezoelectric polarizations and carrier screening effect. Studies indicate a significant enhancement of the electron-hole wavefunction overlap (Γe_hh) and the spontaneous emission radiative recombination rate (Rsp) for the interdiffused InGaN QWs. The improved performance for the interdiffused InGaN QWs is due to the modification of the band lineups at the InGaN-GaN interfaces, which leads to the enhancement of the electron-hole wavefunction overlap significantly. View full abstract»

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  • Investigating the Effect of Piezoelectric Polarization on GaN-Based LEDs With Different Quantum Barrier Thickness

    Page(s): 206 - 211
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (1131 KB) |  | HTML iconHTML  

    The effect of temperature-dependent electroluminescence (EL) on nitride-based light-emitting diodes (LEDs) with different thicknesses of quantum barrier are studied and demonstrated. It was found that quantum confined stark effect (QCSE) of 6-nm thick barrier was more slightly than that of 9- and 12-nm thick barrier. The results indicated that the polarization field is independent of ambient temperature due to no clearly change of blue-shift value. The results also pointed out that the polarization field within the active region of 12-nm thick barrier was stronger than the others due to larger variation of the wavelength transition position (i.e. blue-shift change to red-shift) from 300 to 350 K, and thus it needed more injection carriers to complete the screening of QCSE. In this study, we reported a simple method to provide useful comparison of electrostatic fields within active region in nitride-based LEDs, specifically for structures consisting of identical active regions with different barrier thicknesses. View full abstract»

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  • Analysis of Internal Quantum Efficiency and Current Injection Efficiency in III-Nitride Light-Emitting Diodes

    Page(s): 212 - 225
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (3557 KB) |  | HTML iconHTML  

    Current injection efficiency and internal quantum efficiency (IQE) in InGaN quantum well (QW) based light emitting diodes (LEDs) are investigated. The analysis is based on current continuity relation for drift and diffusion carrier transport across the QW-barrier systems. A self-consistent 6-band k ·p method is used to calculate the band structure for InGaN QW structure. Carrier-photon rate equations are utilized to describe radiative and non-radiative recombination in the QW and the barrier regions, carrier transport and capture time, and thermionic emission leading to carrier leakage out of the QW. Our model indicates that the IQE in the conventional 24-Å In0.28Ga0.72 N -GaN QW structure reaches its peak at low injection current density and reduces gradually with further increase in current due to the large thermionic carrier leakage. The efficiency droop phenomenon at high current density in III-nitride LEDs is thus consistent with the high-driving-current induced quenching in current injection efficiency predicted by our model. The effects of the monomolecular recombination coefficient, Auger recombination coefficient and GaN hole mobility on the current injection efficiency and IQE are studied. Structures combining InGaN QW with thin larger energy bandgap barriers such as AlxGa1-xN, lattice-matched AlxIn1-xN, and lattice-matched AlxInyGa1-x-y N have been analyzed to improve current injection efficiency and thus minimize droop at high current injection in III-nitride LEDs. Effect of the thickness of the larger energy bandgap barriers (AlGaN, AlInN and AlInGaN) on injection efficiency and IQE are investigated. The use of thin AlGaN barriers shows slight reduction of quenching of the injection efficiency as the current density increases. The use of thin lattice-matched AlInN or AlInGaN barriers shows significant suppression of efficien- y-droop in nitride LEDs. View full abstract»

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  • On the Effect of Step-Doped Quantum Barriers in InGaN/GaN Light Emitting Diodes

    Page(s): 226 - 233
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (1175 KB) |  | HTML iconHTML  

    InGaN/GaN light-emitting diodes (LEDs) make an important class of optoelectronic devices, increasingly used in lighting and displays. Conventional InGaN/GaN LEDs of c-orientation exhibit strong internal polarization fields and suffer from significantly reduced radiative recombination rates. A reduced polarization within the device can improve the optical matrix element, thereby enhancing the optical output power and efficiency. Here, we have demonstrated computationally that the step-doping in the quantum barriers is effective in reducing the polarization-induced fields and lowering the energy barrier for hole transport. Also, we have proven experimentally that such InGaN/GaN LEDs with Si step-doped quantum barriers indeed outperform LEDs with wholly Si-doped barriers and those without doped barriers in terms of output power and external quantum efficiency. The consistency of our numerical simulation and experimental results indicate the effects of Si step-doping in suppressing quantum-confined stark effect and enhancing the hole injection, and is promising in improving the InGaN/GaN LED performance. View full abstract»

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  • Effects of {\hbox {H}}_{{2}} in GaN Barrier Spacer Layer of InGaN/GaN Multiple Quantum-Well Light-Emitting Diodes

    Page(s): 234 - 238
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (749 KB) |  | HTML iconHTML  

    We demonstrate the optoelectrical characteristics of thick well short-period InGaN/GaN multiple-quantum-well (MQW) light-emitting diodes (LEDs) with H2 in GaN barrier spacer layer. Introducing ramped H2 in the GaN barrier spacer layer creates a wide range of severe well thickness variation randomly distributed in the thick InGaN well. The thickness-fluctuated InGaN well would effectively increase the carrier concentration in the region of the thick InGaN well region during the current injection. Moreover, the ramped H2 in GaN barrier spacer layer would improve the interface and crystal quality of thick well short-period InGaN/GaN MQWs LEDs. Therefore, compared with traditional long-period InGaN/GaN MQW LEDs, thick well short-period InGaN/GaN MQW LEDs with fluctuated InGaN well thickness enhance output power (25.6% at 20 mA) and improve efficiency droop from 55.0% to 36.7%. View full abstract»

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  • Improved Carrier Distributions by Varying Barrier Thickness for InGaN/GaN LEDs

    Page(s): 239 - 243
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (905 KB) |  | HTML iconHTML  

    In this paper, we minimized efficiency droop by varying barrier thickness for InGaN/GaN multiple quantum wells (MWQs) featuring narrow quantum barriers (NQBs). The external quantum efficiency (EQE) for a light-emitting diode (LED) possessing NQBs improved by 18% at a current density of 200 A·cm-2, compared to that of a conventional LED incorporating a 12-nm-thick barrier. The enhanced carrier distribution resulting from the presence of NQBs was practically approved from another experimental design in this study. We suggest that the NQBs displayed uniform carrier distribution in active layer and decreased the carrier density in the active layer at a critical current density. View full abstract»

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  • Effect of Polarization-Matched n-Type AlGaInN Electron-Blocking Layer on the Optoelectronic Properties of Blue InGaN Light-Emitting Diodes

    Page(s): 244 - 248
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (833 KB) |  | HTML iconHTML  

    A design approach is proposed to improve the performances of blue InGaN light-emitting diodes (LEDs) at high current by using of the polarization-matched n-type AlGaInN electron-blocking layer (EBL) instead of conventional EBLs, owing to the more uniform carrier distribution across the multiply quantum well (MQW) active regions. And the response parameters of the blue InGaN LEDs, such as the radiative recombination rate, the internal quantum efficiency and output power, are calculated by the advanced physical model of semiconductor devices (APSYS) software. The simulation results reveal that the total power and the radiative recombination rates of the blue InGaN LEDs with the n-type AlGaInN EBL have been greatly promoted, and their efficiency droop has also been evidently moderated, compared with that of the LEDs with three types of EBL which are p-type AlGaN EBL, p-type AlGaInN EBL and n-type AlGaN EBL. View full abstract»

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  • Effect of Si Doping Level in n-Cladding Layer on the Performance of InGaN-Based Light-Emitting Diodes

    Page(s): 249 - 254
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (1101 KB) |  | HTML iconHTML  

    In this study, we have systematically investigated the effect of Si doping level in the n-cladding layer on the performance of InGaN/GaN-based light-emitting diodes (LEDs). Detailed structural, optical and electrical properties of the sample with different Si doping level were studied. Based on these investigations, it was found that with a low level of Si doping (0.738 nmol/min, ~ 5.4×1017 cm3), the subsequent multiple-quantum-well (MQW) structure showed superior crystalline quality with low-density threading dislocations (TDs); however, the operation voltage of the LED chips became unbearable. On the other hand, the highly-doped (14.40 nmol/min, ~ 1.1×1019 cm3) samples showed a substantially lower operation voltage, but the MQW quality deteriorated with much higher TD density. Finally, the effect of inserting a 100-nm-thick undoped spacer GaN layer between the n-cladding layer and active layers was also investigated. The electrical and emission properties both deteriorated while the crystalline quality improved for LEDs with this structure, proving the importance of electron injection on the performance of LEDs. View full abstract»

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  • Performance Improvement of Nitride-Based Light-Emitting Diode With a Thin Mg-Delta-Doped Hole Injection Layer

    Page(s): 255 - 259
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (545 KB) |  | HTML iconHTML  

    The performance of InGaN/GaN multiple quantum wells (MQWs) blue light-emitting diodes (LEDs) was improved by inserting a thin Mg-delta-doped hole injection layer at the end of the MQWs. The forward- and reverse-leakage currents were significantly reduced compared with those of the LEDs without the inserting layer. The light output power was enhanced by 13% at a 350 mA injection current. The improved performance could be ascribed to the dislocation suppression and hole concentration enhancement in the p-type GaN by inserting the Mg-delta-doped structure. View full abstract»

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  • Observation of Electroluminescence From Quantum Wells Far From p-GaN Layer in Nitride-Based Light-Emitting Diodes

    Page(s): 260 - 265
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (990 KB) |  | HTML iconHTML  

    We report the observation of electroluminescence from the first to fourth quantum wells (QWs) from the p -GaN layer in InGaN/GaN multiple-QW light-emitting diodes (LEDs) with various indium contents (4%-16%) in each QW. The investigated LED sample showed a lower turn-on voltage and ideality factor as well as a reduction of etching pit density compared with the reference sample. Also, the X-ray reciprocal space maps revealed a partial strain relaxation in the active region. The enhanced hole injection efficiency was attributed to the weakening of strain-induced polarization field in the QWs and the good crystalline quality. View full abstract»

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  • Lateral Current Spreading Effect on the Efficiency Droop in GaN Based Light-Emitting Diodes

    Page(s): 266 - 271
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    The lateral current spreading (CS) effect on the efficiency droop in GaN-based LEDs has been studied in terms of the CS distance (LCS) using a designed pattern with the 2-D current spreading profile. The correlations of CS effect with the electrical, luminescent and electric-thermal properties of the LEDs have been discussed. LEDs with the LCS longer than the theoretically calculated effective CS length (Leff) suffer from more serious efficiency droop and the degradation of luminescent properties. However, the influence of CS effect on the ideality factors of LEDs is not obvious. Higher chip temperature caused by poor CS was observed and may further enlarge the efficiency droop. View full abstract»

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  • First-Principle Electronic Properties of Dilute-As GaNAs Alloy for Visible Light Emitters

    Page(s): 272 - 279
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    The band structure of dilute-As GaNAs alloy with the As composition range from 0% to 12.5% is studied by using First-Principle density-functional calculation. Our analysis shows that the dilute-As GaNAs alloy exhibits the direct band gap properties. The dilute-As GaNAs alloy shows a band gap range from 3.645 eV down to 2.232 eV with As content varying from 0% to 12.5%, which covers the blue and green spectral regime. This finding indicates the alloy as a potential candidate for photonic devices applications. The bowing parameter of 14.5 eV ±0.5 eV is also obtained using line fitting with the First-Principle and experimental data. The effective masses for electrons and holes in dilute-As GaNAs alloy, as well as the split-off energy parameters, were also presented. Minimal interband Auger recombination is also suggested for the dilute-As GaNAs alloy attributing to the off-resonance condition for this process. View full abstract»

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  • Efficiency and Droop Improvement in Hybrid Warm White LEDs Using InGaN and AlGaInP High-Voltage LEDs

    Page(s): 280 - 284
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (609 KB) |  | HTML iconHTML  

    This study investigates the optical and electrical characteristics in hybrid warm white high-voltage light-emitting diodes (HV-LEDs). The luminous efficiency of the hybrid warm white LED in this study improved by 11% and 51%, compared to conventional cool and warm LEDs, respectively, solving the warm white gap in white LEDs. The efficiency droop of the hybrid warm white LED was reduced to 21.8% from 26.8% for the conventional cool white LED, and from 26.3% in the conventional warm white LED at 40 mA (35 A/cm2) the operated current. Furthermore, the color rendering index (CRI) and angular correlated color temperature (CCT) were analyzed, indicating a significant improvement in hybrid warm white HV-LEDs. View full abstract»

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  • Enhanced Light Output Power and Growth Mechanism of GaN-Based Light-Emitting Diodes Grown on Cone-Shaped {\hbox {SiO}}_{2} Patterned Template

    Page(s): 285 - 291
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    In this study, we successfully transferred the patterns of a cone-shaped patterned sapphire substrate (CPSS) into SiO2 layer to fabricate a cone-shaped SiO2 patterned template by using nanoimprint lithography (NIL). The GaN-based light-emitting diodes (LEDs) were grown on this template by metal-organic chemical vapor deposition (MOCVD). The transmission electron microscopy (TEM) images suggest that the stacking faults formed near the cone-shaped SiO2 patterns during the epitaxial lateral overgrowth (ELOG) can effectively suppress the threading dislocations, which results in an enhancement of internal quantum efficiency. The Monte Carlo ray-tracing simulation reveals that the light extraction efficiency of the LED grown on cone-shaped SiO2 patterned template can be enhanced as compared with the LED grown on CPSS. As a result, the light output power of the LED grown on cone-shaped SiO2 patterned template outperformed the LED grown on CPSS. View full abstract»

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  • Effects of Initial GaN Growth Mode on Patterned Sapphire on the Opto-Electrical Characteristics of GaN-Based Light-Emitting Diodes

    Page(s): 292 - 296
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (1448 KB) |  | HTML iconHTML  

    We have studied the initial growth modes of GaN on patterned sapphire substrate (PSS) with different initial TMGa flow rates. The FWHM of the (102) XRD spectrum of GaN on PSS increased from 470 to 580 arcsec when the initial TMGa flow rate was increased from 80 to 200 sccm. A low TMGa flow rate sufficiently suppresses GaN island growth on the top of the pattern and hence improves GaN crystal quality. The electrical and optical characteristics of GaN-based LEDs on PSS with low initial TMGa were also improved. More than 90% of the GaN LED chips with low initial GaN growth rate can hold the 1-kV machine-mode electrostatic discharge level. View full abstract»

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  • [Blank page]

    Page(s): 297 - 298
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  • IEEE Copyright Form

    Page(s): 299 - 300
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  • Journal of Display Technology information for authors

    Page(s): C3
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  • [Blank page - back cover]

    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.