E. F. Schubert

Rensselaer Polytechnic Institute, Троя, New York, United States

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Publications (202)433.73 Total impact

  • G.-B. Lin · E. Fred Schubert
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    ABSTRACT: The efficiency droop in GaInN/GaN blue light-emitting diodes (LEDs) usually commences at current density around 10 A/cm2 and the efficiency decreases monotonically after the droop onset. GaN-based LEDs suffer seriously, at typical operating current densities (10-100 A/cm2), by the efficiency droop. Efficiency re-climbing is observed in the typical droop regime at cryogenic temperatures below 125K. The "efficiency re-climbing" coincides with a distinct increase in device conductivity, which is mainly attributed to an enhancement in p-type conductivity due to field ionization of acceptors. The "efficiency re-climbing" phenomenon implies an approach of solving efficiency droop by enhancing hole injection by external electric field.
    No preview · Article · Sep 2015 · International Journal of High Speed Electronics and Systems
  • G.-B. Lin · E. F. Schubert · J. Cho
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    ABSTRACT: Phosphor-converted white light-emitting diodes (pc-WLEDs) inherently have large device-to-device variation in electrical and optical properties compared to conventional light sources. Wider variation means that a smaller proportion of products are appropriate for certain illumination or display applications. We present a method that reduces variation in chromaticity coordinates (x, y), luminous flux, φlum, and forward voltage, V f , simultaneously. In this method, two complementary LEDs form a pair matched quickly by a weighted matching algorithm. The matching results show that the device-to-device variance of x, y, φlum, and V f decrease by 96.0%, 93.7%, 79.3%, and 84.4%, respectively. The complementary devices also maintain low variance for all four properties under different currents ranging from 5 to 40 mA. A simulation reveals that the effect of the matching method is more effective with an increase of LED population.
    No preview · Article · Apr 2015 · Journal of Nanoelectronics and Optoelectronics
  • G.-B. Lin · D.S. Meyaard · E.F. Schubert · J. Cho · J.K. Kim · H. Shim · M.-H. Kim · C. Sone
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    ABSTRACT: An evident correlation between the high injection and the efficiency droop is demonstrated in GaInN light-emitting diodes (LEDs) for temperature ranging from 80K to 450K. For this temperature range, the efficiency droop has the same thermal tendency with the high injection, i.e. the onset shifts to lower voltage and higher current with increasing temperature. The voltage difference between two onsets mainly falls on the resistive p-type region and induces strong electric field to enable electron drift leakage and the resulting efficiency droop.
    No preview · Article · Nov 2014
  • D.S. Meyaard · G.-B. Lin · J. Cho · E.F. Schubert
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    ABSTRACT: The decrease in efficiency at high current densities, i.e. the efficiency droop, is introduced in the context of GaInN/GaN light-emitting diodes (LEDs). We begin with an overview of the ABC model for the recombination of carriers in GaInN/GaN LEDs. The weaknesses of this model are described, and the model is reformulated by including a carrier leakage term. Several explanations for droop are presented and analyzed, including their strengths and weaknesses. Particular emphasis is given to high-level injection and the associated drift-induced electron leakage out of the active region. Finally, several proposed solutions to overcome efficiency droop are presented.
    No preview · Article · Dec 2013
  • I. D. Goepfert · E. F. Schubert · A. Osinsky · P. E. Norris
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    ABSTRACT: Mg-doped superlattices consisting of uniformly doped AlGa{sub 1-x}N and GaN layers are analyzed by Hall-effect measurements. Acceptor activation energies of 70 meV and 58 meV are obtained for superlattice structures with an Al mole fraction of x = 0.10 and 0.20 in the barrier layers, respectively. These energies are significantly lower than the activation energy measured for Mg-doped GaN thin films. At room temperature, the doped superlattices have free hole concentrations of 2 x 10¹ cm³ and 4 x 10¹ cm³ for x = 0.10 and 0.20, respectively. The increase in hole concentration with Al content of the superlattice is consistent with theory. The room temperature conductivity measured for the superlattice structures are 0.27 S/cm and 0.64 S/cm for an Al mole fraction of x = 0.10 and 0.20, respectively.
    No preview · Article · Jan 2012 · MRS Online Proceeding Library
  • J.-q.xi · Manasojha · Woojincho · Th.gessmann · E. F.schubert · J. L.plawsky · W. N.gill
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    ABSTRACT: Triple-layer omni-directional reflectors (ODRs) consisting of a semiconductor, a transparent quarter-wavelength dielectric layer and metal layer have high reflectivities at all angles of incidence. In this paper, triple-layer ODRs are demonstrated that incorporate nanoporous SiO2, a novel low-refractive-index (low-n) material with refractive indices n ≪ 1.46 as well as dense SiO2 (n = 1.46). GaP and Ag serve as the semiconductor and metal layer materials, respectively. An angle-integrated transverse electric (TE) mode reflectivity of Ravg|TE = 99.9 % and transverse magnetic (TM) mode reflectivity Ravg|TM = 98.9 % are calculated for the triple-layer ODRs employing nanoporous SiO2. Reflectivity measurements, including the angular dependence of R, are presented. Novel hybrid ODRs consisting of semiconductor, a several micron thick low-n dielectric material layer, a distributed Bragg reflector (DBR) and metal layer have outstanding reflectivities for all incident angles. GaP and Ag serve as the semiconductor and metal layer, respectively. Nanoporous SiO2 is used as the low-n material. TiO2 and dense SiO2 serve as the DBR materials. The angle-intergrated reflectivities of the TE and TM modes are calculated to be larger than 99.9 % for the hybrid ODRs. The results indicate the great potential of the ODRs for light-emitting diodes with high light extraction efficiency.
    No preview · Article · Nov 2011 · International Journal of High Speed Electronics and Systems
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    ABSTRACT: Room temperature and low temperature photoluminescence studies of AlxGa1-xN/GaN superlattices reveal a red shift of the dominant transition band relative to the bulk GaN bandgap. The shift is attributed to the quantum-confined Stark effect resulting from polarization fields in the superlattices. A theoretical model for the band-to-band transition energies based on perturbation theory and a variational approach is developed. Comparison of the experimental data with this model yields a polarization field of 4.6 × 105 V/cm for room temperature Al0.1Ga0.9N/GaN and 4.5 × 105 V/cm for room temperature Al0.2Ga0.8N/GaN. At low temperatures the model yields 5.3 × 105 V/cm for Al0.1Ga0.9N/GaN and 6.3 × 105 V/cm for Al0.2Ga0.8N/GaN. The emission bands exhibit a blue shift at high excitation densities indicating screening of internal polarization fields by photo-generated free carriers.
    No preview · Article · Jan 2011 · MRS Online Proceeding Library

  • No preview · Article · Jan 2011 · MRS Online Proceeding Library
  • T. H. Chiu · E. F. Schubert · J. E. Cunningham · W. T. Tsang · B. Tell

    No preview · Article · Jan 2011 · MRS Online Proceeding Library
  • E. F. Schubert · T. D. Harris · J. E. Cunningham

    No preview · Article · Jan 2011 · MRS Online Proceeding Library
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    ABSTRACT: The junction temperature of AlGaN/GaN ultraviolet (UV) Light-Emitting Diodes (LEDs) emitting at 295 nm is measured by using the temperature coefficients of the diode forward voltage and emission peak energy. The high-energy slope of the spectrum is explored to measure the carrier temperature. A linear relation between junction temperature and current is found. Analysis of the experimental methods reveals that the diode-forward voltage is the most accurate method (± 3°C). A theoretical model for the dependence of the diode junction voltage (V j) on junction temperature (T) is developed that takes into account the temperature dependence of the energy gap. A thermal resistance of 87.6 K/W is obtained with the AlGaN/GaN LED sample mounted with thermal paste on a heat sink.
    No preview · Article · Jan 2011 · MRS Online Proceeding Library
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    ABSTRACT: Nanoparticle-loaded encapsulants provide unique optical and material properties for the enhancement of light-extraction efficiency in light-emitting diodes (LEDs). We report on the uniform dispersion of TiO2 nanoparticles with average diameter of 40 nm in epoxy, and the demonstration of a refractive index (n) of 1.68 at 400 nm wavelength, higher than that of pure epoxy (n = 1.53). It is found that proper chemical surfactants and nanoparticle preparation are critical to eliminate agglomeration of nanoparticles. Theoretical analysis of optical scattering in nanoparticle-loaded encapsulation materials reveals that although the size and loading factor of nanoparticles greatly influence scattering, specular transparency of the encapsulant film occurs if the thicknesses of the films are kept below the optical scattering length. Furthermore, the encapsulants benefit from an optimized scattering coefficient as demonstrated by three-dimensional ray-tracing simulations showing light-extraction efficiency enhancements greater than 50%.
    No preview · Article · Jan 2011 · MRS Online Proceeding Library

  • No preview · Article · Jan 2011 · MRS Online Proceeding Library
  • Th. Gessmann · Y.-L. Li · J. W. Graff · E. F. Schubert · J. K. Sheu

    No preview · Article · Jan 2011 · MRS Online Proceeding Library
  • I. D. Goepfert · E. F. Schubert · J. M. Redwing

    No preview · Article · Jan 2011 · MRS Online Proceeding Library
  • F.W. Mont · E.F. Schubert
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    ABSTRACT: There is no abstract available for this article.
    No preview · Article · May 2010 · Journal of Applied Physics
  • Source
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    ABSTRACT: We demonstrate GaInN multiple quantum well (MQW) light-emitting diodes (LEDs) having ternary GaInN quantum barriers (QBs) instead of conventional binary GaN QBs for a reduced polarization mismatch between QWs and QBs and an additional separate confinement of carriers to the MQW active region. In comparison with GaInN LEDs with conventional GaN QBs, the GaInN/GaInN LEDs show a reduced blueshift of the peak wavelength with increasing injection current and a reduced forward voltage. In addition, we investigate the density of pits emerging on top of the MQW layer that are correlated with V-defects and act as a path for the reverse leakage current. The GaInN/GaInN MQW structure has a lower pit density than the GaInN/GaN MQW structure as well as a lower reverse leakage current. Finally, the GaInN/GaInN MQW LEDs show higher light output power and external quantum efficiency at high injection currents compared to the conventional GaInN/GaN MQW LEDs. We attribute these results to the reduced polarization mismatch and the reduced lattice mismatch in the GaInN/GaInN MQW active region.
    Preview · Article · Apr 2010 · Journal of Applied Physics
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    ABSTRACT: We compared the phase change behavior of a partially wetting fluid, nonane, on various SiO2 surfaces that had been modified to alter their roughness at the nanoscale. We compared a total of four surfaces: an as-received, smooth surface; a surface roughened by plasma-enhanced chemical vapor deposition (PECVD) of SiO2; and two surfaces where SiO2 nanorods had been deposited using glancing angle deposition (GLAD). Scanning electron microscopy (SEM) and atomic force microscopy (AFM) were used to characterize the surfaces. The topography of the rough surface controlled the wetting characteristics of the fluid that in turn, controlled the change-of-phase heat transfer rate. The measured apparent contact angle characterized the wetting property during the phase change process. Surface roughness promoted wetting in this system, but the direction of heat transfer controlled the topographic design required for enhanced performance. A comparison between two nanorod coatings of differing heights shows that the longer nanorod coating (30 nm high) acted somewhat like a porous surface promoting condensation heat transfer while the shorter nanorod coating (10 nm high) was much more effective at promoting evaporative heat transfer. Surface alteration at the scale over which intermolecular forces dominates the fluid-solid interaction provides a convenient means for probing those interactions.
    No preview · Article · Feb 2010 · International Journal of Heat and Mass Transfer
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    J. Cho · A. Mao · J.K. Kim · J.K. Son · E.F. Schubert
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    ABSTRACT: The characteristics of the reverse leakage current of GaInN/GaN multiple quantum well light-emitting diodes (LEDs) are examined with various n-type GaN doping concentrations and interpreted by using a tunnelling current model. Changing the doping concentration of the n-type GaN influences the tunnelling probability of electrons into the conduction band and thus the reverse leakage current. Reducing the doping concentration of the top 150 nm portion of the n-type GaN layer by half decreases the tunnelling probability, resulting in decrease of the reverse leakage current by 80% at %10%V without deterioration of any forward electrical properties of LEDs.
    Full-text · Article · Jan 2010 · Electronics Letters
  • Source
    J. Cho · D. Zhu · E.F. Schubert · J.K. Kim
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    ABSTRACT: Reverse leakage current characteristics of GaInN/GaN multiple quantum well light-emitting diodes (LEDs) with various chip geometries are examined. The effect of chip geometry on the reverse leakage current is negligible at a low voltage, but becomes apparent at a high voltage. The reverse breakdown voltage of LEDs decreases as the angle of vertex in the chip geometry decreases presumably because of a highly localised electric field strength near the vertex. This suggests that a chip geometry with a rounded vertex is suitable for reliable high-power LEDs.
    Full-text · Article · Aug 2009 · Electronics Letters

Publication Stats

7k Citations
433.73 Total Impact Points

Institutions

  • 2002-2015
    • Rensselaer Polytechnic Institute
      • • Department of Electrical, Computer, and Systems Engineering
      • • Department of Physics, Applied Physics, and Astronomy
      Троя, New York, United States
  • 1996-2012
    • Boston University
      • Department of Electrical and Computer Engineering
      Boston, Massachusetts, United States
  • 1989
    • Massachusetts Institute of Technology
      • Department of Electrical Engineering and Computer Science
      Cambridge, Massachusetts, United States
  • 1988
    • AT&T Labs
      Austin, Texas, United States