E. F. Schubert

Boston University, Boston, Massachusetts, United States

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Publications (198)436.57 Total impact

  • 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.
    MRS Online Proceeding Library 01/2012; 595. DOI:10.1557/PROC-595-F99W3.85
  • Y.xi · E. F.schubert ·
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    ABSTRACT: A theoretical model for the dependence of the diode forward voltage (Vf) on junction temperature (T) is developed. A new expression for dVf/dT is derived that takes into account all relevant contributions to the temperature dependence of the forward voltage including the intrinsic carrier concentration, the bandgap energy, and the effective density of states. Experimental results on the junction temperature of GaN UV LEDs are presented. Excellent agreement between the theoretical and experimental temperature coefficient of the forward voltage (dVf/dT) is found. The experimentally found linear dependence of the junction temperature on forward current is explained by a thermal conduction model. A thermal resistivity of 342.2 K/W is found for the UV LED.
    International Journal of High Speed Electronics and Systems 11/2011; 14(03). DOI:10.1142/S0129156404002715
  • 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.
    International Journal of High Speed Electronics and Systems 11/2011; 14(03). DOI:10.1142/S0129156404002740
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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.
    MRS Online Proceeding Library 01/2011; 639. DOI:10.1557/PROC-639-G11.1

  • MRS Online Proceeding Library 01/2011; 102. DOI:10.1557/PROC-102-509
  • T. H. Chiu · E. F. Schubert · J. E. Cunningham · W. T. Tsang · B. Tell ·

    MRS Online Proceeding Library 01/2011; 102. DOI:10.1557/PROC-102-475
  • E. F. Schubert · T. D. Harris · J. E. Cunningham ·

    MRS Online Proceeding Library 01/2011; 145. DOI:10.1557/PROC-145-21
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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.
    MRS Online Proceeding Library 01/2011; 831. DOI:10.1557/PROC-831-E1.7
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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%.
    MRS Online Proceeding Library 01/2011; 955. DOI:10.1557/PROC-0955-I13-02

  • MRS Online Proceeding Library 01/2011; 482. DOI:10.1557/PROC-482-1009
  • Th. Gessmann · Y.-L. Li · J. W. Graff · E. F. Schubert · J. K. Sheu ·

    MRS Online Proceeding Library 01/2011; 693. DOI:10.1557/PROC-693-I11.35.1
  • I. D. Goepfert · E. F. Schubert · J. M. Redwing ·

    MRS Online Proceeding Library 01/2011; 482. DOI:10.1557/PROC-482-679
  • F.W. Mont · E.F. Schubert ·

    Journal of Applied Physics 05/2010; 107(10):109901-109901-1. DOI:10.1063/1.3425782 · 2.18 Impact Factor
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    Wonseok Lee · Min-Ho Kim · Di Zhu · Ahmed N. Noemaun · Jong Kyu Kim · E. F. Schubert ·
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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.
    Journal of Applied Physics 04/2010; 107(6-107):063102 - 063102-6. DOI:10.1063/1.3327425 · 2.18 Impact Factor
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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.
    International Journal of Heat and Mass Transfer 02/2010; 53(s 5–6):910–922. DOI:10.1016/j.ijheatmasstransfer.2009.11.033 · 2.38 Impact Factor
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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.
    Electronics Letters 01/2010; 46(2). DOI:10.1049/el.2010.3236 · 0.93 Impact Factor
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    Min-Ho Kim · Wonseok Lee · Di Zhu · M.F. Schubert · Jong Kyu Kim · E.F. Schubert · Yongjo Park ·
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    ABSTRACT: GaInN-based multiple-quantum-well (MQW) blue LEDs with ternary GaInN barriers polarization-matched to GaInN wells are fabricated. Single-layered Ga<sub>0.9</sub>In<sub>0.1</sub>N and Ga<sub>0.9</sub>In<sub>0.1</sub>N/GaN multiple-layered quantum barriers (MLQBs) are used for 50% polarization matching. Compared to conventional GaInN/GaN MQW LEDs, the polarization-matched LED with GaInN/GaN MLQBs shows a higher light output power in a high injection current regime, resulting in reduced efficiency droop, along with a minimal blue-shift of emission with injection current, reduced ideality factor, and reduced forward voltage. These results are attributed to a reduced magnitude of polarization sheet charges at heterointerfaces between the GaInN well and the GaInN barrier, and the resultant reduced internal polarization field in the MQWs, thereby minimizing electron leakage current and efficiency droop.
    IEEE Journal of Selected Topics in Quantum Electronics 09/2009; 15(4-15):1122 - 1127. DOI:10.1109/JSTQE.2009.2014395 · 2.83 Impact Factor
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    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.
    Electronics Letters 08/2009; 45(14-45):755 - 756. DOI:10.1049/el.2009.0470 · 0.93 Impact Factor
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    ABSTRACT: Recently it has been discovered that when growing AlxGa1−xN on low-defect-density bulk AlN substrates pseudomorphic layers can be achieved with a thickness far exceeding the critical thickness as given by the Matthews and Blakeslee model. For instance, the critical thickness of an AlxGa1−xN layer (with x=0.6) is about 40 nm thick. However we have been able to grow layers with this composition that are pseudomorphic with a thickness exceeding the critical thickness by more than an order of magnitude. This work defines the limits of pseudomorphic growth on low defect density, bulk AlN substrates to obtain low defect density, high-power UV LEDs.
    Journal of Crystal Growth 05/2009; 311(10):2864-2866. DOI:10.1016/j.jcrysgro.2009.01.101 · 1.70 Impact Factor
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    ABSTRACT: An optimized graded-refractive-index (GRIN) antireflection (AR) coating with broadband and omnidirectional characteristics--as desired for solar cell applications--designed by a genetic algorithm is presented. The optimized three-layer GRIN AR coating consists of a dense TiO2 and two nanoporous SiO2 layers fabricated using oblique-angle deposition. The normal incidence reflectance of the three-layer GRIN AR coating averaged between 400 and 700 nm is 3.9%, which is 37% lower than that of a conventional single-layer Si3N4 coating. Furthermore, measured reflection over the 410-740 nm range and wide incident angles 40 degrees -80 degrees is reduced by 73% in comparison with the single-layer Si3N4 coating, clearly showing enhanced omnidirectionality and broadband characteristics of the optimized three-layer GRIN AR coating.
    Optics Letters 04/2009; 34(6):728-30. DOI:10.1364/OL.34.000728 · 3.29 Impact Factor

Publication Stats

6k Citations
436.57 Total Impact Points


  • 1996-2012
    • Boston University
      • Department of Electrical and Computer Engineering
      Boston, Massachusetts, United States
  • 2002-2011
    • Rensselaer Polytechnic Institute
      • • Department of Electrical, Computer, and Systems Engineering
      • • Department of Physics, Applied Physics, and Astronomy
      Троя, New York, United States
  • 1988-1992
    • AT&T Labs
      Austin, Texas, United States