E. F. Schubert

Rensselaer Polytechnic Institute, Troy, New York, United States

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Publications (184)433.14 Total impact

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    ABSTRACT: The dependence of the polarization-induced electric field in GaInN/GaN multiple-quantum-well light-emitting diodes (LEDs) on the GaN quantum barrier (QB) thickness is investigated. Electrostatic arguments and simulations predict that a thin QB thickness reduces the electric field in the quantum wells (QWs) and also improves the LED efficiency. We experimentally demonstrate that the QW electric field decreases with decreasing QB thickness. The lower electric field results in a better overlap of electron and hole wave functions and better carrier confinement in the QWs. A reduced efficiency droop and enhanced internal quantum efficiency is demonstrated for GaInN/GaN LEDs when the QB thickness is reduced from 24.5 to 9.1 nm.
    IEEE Photonics Journal 08/2013; 5(4):1600207-1600207. · 2.36 Impact Factor
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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.
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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).
  • I. D. Goepfert, E. F. Schubert, J. M. Redwing
    MRS Online Proceeding Library 01/2011; 482.
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    ABSTRACT: The thermal properties, including thermal time constants, of GaInN light-emitting diodes LEDs and laser diodes LDs are analyzed. The thermal properties of unpackaged LED chips are described by a single time constant, that is, the thermal time constant associated with the substrate. For unpackaged LD chips, we introduce a heat-spreading volume. The thermal properties of unpackaged LD chips are described by a single time constant, that is, the thermal time constant associated with the heat spreading volume. Furthermore, we develop a multistage R th C th thermal model for packaged LEDs. The model shows that the transient response of the junction temperature of LEDs can be described by a multiexponential function. Each time constant of this function is approximately the product of a thermal resistance, R th , and a thermal capacitance, C th . The transient response of the junction temperature is measured for a high-power flip-chip LED, emitting at 395 nm, by the forward-voltage method. A two stage R th C th model is used to analyze the thermal properties of the packaged LED. Two time constants, 2.72 ms and 18.8 ms are extracted from the junction temperature decay measurement and attributed to the thermal time constant of the LED GaInN/sapphire chip and LED Si submount, respectively. © 2010 American Institute of Physics.
    Journal of Applied Physics 10/2010; 108(8). · 2.21 Impact Factor
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    ABSTRACT: We model the carrier recombination mechanisms in GaInN/GaN light-emitting diodes as R=An+Bn<sup>2</sup>+Cn<sup>3</sup>+f(n) , where f(n) represents carrier leakage out of the active region. The term f(n) is expanded into a power series and shown to have higher-than-third-order contributions to the recombination. The total third-order nonradiative coefficient (which may include an f(n) leakage contribution and an Auger contribution) is found to be 8×10<sup>-29</sup> cm <sup>6</sup>  s <sup>-1</sup> . Comparison of the theoretical ABC+f(n) model with experimental data shows that a good fit requires the inclusion of the f(n) term.
    Applied Physics Letters 10/2010; · 3.52 Impact Factor
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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; · 2.21 Impact Factor
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    ABSTRACT: There are eight invited papers and 12 contributed papers in this special issue on light-emitting diodes.
    IEEE Transactions on Electron Devices 02/2010; · 2.06 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. · 2.52 Impact Factor
  • F.W. Mont, E.F. Schubert
    Journal of Applied Physics 01/2010; 107(10):109901-109901-1. · 2.21 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). · 1.04 Impact Factor
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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; · 4.08 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; · 1.04 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. · 3.39 Impact Factor
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    ABSTRACT: Room-temperature photoluminescence (PL) measurements are performed on GaInN/GaN multiple-quantum-well heterostructures grown on GaN-on-sapphire templates with different threading-dislocation densities. The selective optical excitation of quantum wells and the dependence of integrated PL intensity on excitation power allow us to determine the internal quantum efficiency (IQE) as a function of carrier concentration. The measured IQE of the sample with the lowest dislocation density (5.3×108 cm−2) is as high as 64%. The measured nonradiative coefficient A varies from 6×107 to 2×108 s−1 as the dislocation density increases from 5.3×108 to 5.7×109 cm−2, respectively.
    Applied Physics Letters 03/2009; 94(11). · 3.52 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 01/2009; 311(10):2864-2866. · 1.55 Impact Factor
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    ABSTRACT: The effect of edge and screw dislocations on the electrical and optical properties of n -type Al <sub>0.34</sub> Ga <sub>0.66</sub> N is investigated. It is found that edge dislocations strongly affect the electrical properties of n -type Al <sub>0.34</sub> Ga <sub>0.66</sub> N . Both free carrier concentration and mobility decrease with increasing edge dislocation density. Edge dislocations also enhance nonradiative recombination, which is indicated by decreasing near-band-edge UV as well as parasitic blue photoluminescence. The UV/blue ratio is found to be independent of the edge dislocation density but strongly depends on the Si doping concentration.
    Applied Physics Letters 12/2008; · 3.52 Impact Factor
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    ABSTRACT: We propose an analytic model that accurately predicts the porosity and deposition rate of nanoporous films grown by oblique-angle deposition. The model employs a single fitting parameter and takes into account geometrical factors as well as surface diffusion. We have determined the porosity and deposition rate from the measured refractive index and thickness of Si O <sub>2</sub> and indium tin oxide nanoporous films deposited at various incident angles. Comparison of experimental data with the model reveals excellent agreement. The theoretical model allows for the predictive control of refractive index, porosity, and deposition rate for a wide range of deposition angles and materials.
    Applied Physics Letters 10/2008; · 3.52 Impact Factor
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    E.F. Schubert, Jong Kyu Kim
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    ABSTRACT: The refractive index, a most fundamental quantity in optics and optoelectronics, determines many figures of merit of optical components such as reflectors, filters, and resonators. Here we present a new class of optical thin-film materials that have a very low refractive index. Specular films of high optical quality with refractive indices as low as 1.05 are demonstrated. Applications of the material in optoelectronics and solid-state lighting will also be discussed.
    Numerical Simulation of Optoelectronic Devices, 2007. NUSOD '07. International Conference on; 10/2007
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    ABSTRACT: What we believe to be a new principle is introduced for the design and selection of gradient-index antireflection profiles that are effective over a wide range of incident angles as well as wavelengths at a given physical film thickness. It is shown that at oblique incidence the smoothness of the optical path of incident light inside a gradient-index film has a crucial effect on the overall reflection. Thus the smoothness of variations in refractive angle (rather than that of the index profile itself) needs to be maximized for wide-angle operation. As an example, the performance of Gaussian and Quintic profiles at large incident angles are considered in light of this point of view.
    Applied Optics 10/2007; 46(26):6533-8. · 1.69 Impact Factor

Publication Stats

3k Citations
433.14 Total Impact Points

Institutions

  • 2003–2010
    • Rensselaer Polytechnic Institute
      • • Department of Electrical, Computer, and Systems Engineering
      • • Department of Physics, Applied Physics, and Astronomy
      Troy, New York, United States
  • 2007
    • National Taiwan University
      • Department of Electrical Engineering
      Taipei, Taipei, Taiwan
  • 1996–2002
    • Boston University
      • • Department of Electrical and Computer Engineering
      • • Department of Physics
      Boston, MA, United States
  • 2001
    • University of Central Florida
      • Department of Physics
      Orlando, FL, United States
  • 1997
    • National Sun Yat-sen University
      • Department of Physics
      Kaohsiung, Kaohsiung, Taiwan
  • 1988–1996
    • AT&T Labs
      Austin, Texas, United States
    • Massachusetts Institute of Technology
      Cambridge, Massachusetts, United States