J V Foreman

Publications (5)3.84 Total impact

• Source

  Available from: Henry O Everitt

  Article: Effects of reabsorption and spatial trap distributions on the radiative quantum efficiencies of ZnO

  J. V. Foreman, H. O. Everitt, J Yang, T. McNicholas, J Liu
  [show abstract] [hide abstract]
  ABSTRACT: Ultrafast time-resolved photoluminescence spectroscopy following one- and two-photon excitation of ZnO powder is used to gain unprecedented insight into the surprisingly high external quantum efficiency of its "green" defect emission band. The role of exciton diffusion, the effects of reabsorption, and the spatial distributions of radiative and nonradiative traps are comparatively elucidated for the ultraviolet excitonic and "green" defect emission bands in both unannealed, nanometer-sized ZnO powders and annealed, micrometer-sized ZnO:Zn powders. We find that the primary mechanism limiting quantum efficiency is surface recombination because of the high density of nonradiative surface traps in these powders. It is found that unannealed ZnO has a high density of bulk nonradiative traps as well, but the annealing process reduces the density of these bulk traps while simultaneously creating a high density of green-emitting defects near the particle surface. The data are discussed in the context of a simple rate equation model that accounts for the quantum efficiencies of both emission bands. The results indicate how defect engineering could improve the efficiency of ultraviolet-excited ZnO:Zn-based white light phosphors. Comment: 25 pages, 8 figures, submitted to Phys. Rev. B on August 3, 2009
  08/2009;
• Article: A visible transparent electroluminescent europium doped gallium oxide device

  P Wellenius, A Suresh, J V Foreman, H O Everitt, J F Muth
  [show abstract] [hide abstract]
  ABSTRACT: Beta phase gallium oxide thin films deposited by pulsed laser deposition are efficient hosts for rare earth metals such as europium. In this study europium doped gallium oxide deposited on glass substrates is used to make red (611 nm) electroluminescent devices that are transparent to the visible spectrum. The conducting electrodes used are indium tin oxide (ITO), and a novel indium gallium zinc oxide (IGZO) layer also deposited by pulsed laser deposition. The origin of the red emission is the D-5(0) to F-7(2) transition and is consistent with photoluminescence and cathodoluminescence results. The turn on voltage of the device is about 45 V ac, and the device appears to be robust, operating at elevated voltages without degradation. (C) 2007 Elsevier B.V. All rights reserved.
  Materials Science and Engineering B-Solid State Materials for Advanced Technology. 01/2008; 146(1-3):252-255.
• Source

  Available from: Henry O Everitt

  Article: Low dislocation densities and long carrier lifetimes in GaN thin films grown on a SiNx nanonetwork

  J. Xie, U. Ozgur, Y. Fu, X. Ni, H. Morkoc, C. K. Inoki, T. S. Kuan, J. V. Foreman, H. O. Everitt
  [show abstract] [hide abstract]
  ABSTRACT: Significant improvement of structural and optical qualities of GaN thin films on sapphire substrates was achieved by metal organic chemical vapor deposition with in situ Si N _x nanonetwork. Transmission electron microscope (TEM) studies revealed that screw- and edge-type dislocations were reduced to 4.4×10⁷ and 1.7×10⁷ cm ^-2 , respectively, for a ∼5.5-μ m -thick layer. Furthermore, room temperature carrier lifetimes of 2.22 and 2.49 ns were measured by time-resolved photoluminescence (TRPL) for samples containing single and double Si N _x network layers, respectively, representing a significant improvement over the previous studies. The consistent trends among the TEM, x-ray diffraction, and TRPL measurements suggest that in situ Si N _x network reduces line defects effectively as well as the point-defect-related nonradiative centers.
  Applied Physics Letters 02/2007; · 3.84 Impact Factor
• Article: Photoluminescence study of ZnO films codoped with nitrogen and tellurium

  H L Porter, J F Muth, J Narayan, J V Foreman, H O Everitt
  [show abstract] [hide abstract]
  ABSTRACT: Epitaxial ZnO films codoped with tellurium and nitrogen were grown by pulsed laser deposition on c-axis oriented sapphire substrates. The codoping strategy allowed the resistivity of the films to be controlled over several orders of magnitude and may prove useful in the development of ZnO based light emitters. Photoluminescence studies of tellurium-doped, nitrogen-doped, tellurium and nitrogen codoped, and undoped ZnO films were conducted. Strong room temperature photoluminescence and stimulated emission were observed in the undoped and Te-doped films, but not in codoped films. Time-resolved photoluminescence measurements indicated that carrier lifetime was significantly reduced in doped ZnO as compared to undoped ZnO. (c) 2006 American Institute of Physics.
  Journal of Applied Physics. 01/2006; 100(12).
• Article: Effects of reabsorption and spatial trap distributions on the radiative quantum efficiencies of ZnO

  J. V. Foreman
  [show abstract] [hide abstract]
  ABSTRACT: Ultrafast time-resolved photoluminescence spectroscopy following one- and two-photon excitations of ZnO powder is used to gain unprecedented insight into the surprisingly high external quantum efficiency of its "green" defect emission band. The role of exciton diffusion, the effects of reabsorption, and the spatial distributions of radiative and nonradiative traps are comparatively elucidated for the ultraviolet excitonic and "green" defect emission bands in both unannealed nanometer-sized ZnO powders and annealed micrometer-sized ZnO:Zn powders. We find that the primary mechanism limiting quantum efficiency is surface recombination because of the high density of nonradiative surface traps in these powders. It is found that unannealed ZnO has a high density of bulk nonradiative traps as well, but the annealing process reduces the density of these bulk traps while simultaneously creating a high density of green-emitting defects near the particle surface. The data are discussed in the context of a simple rate equation model that accounts for the quantum efficiencies of both emission bands. The results indicate how defect engineering could improve the efficiency of ultraviolet-excited ZnO:Zn-based white light phosphors.