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Applied Physics Letters

Issue 9 • Date Aug 2001

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Displaying Results 1 - 25 of 63
  • Issue Cover

    Page(s): c1
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    Freely Available from IEEE
  • Issue Table of Contents

    Page(s): toc1
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    Freely Available from IEEE
  • Midinfrared surface-emitting PbSe/PbEuTe quantum-dot lasers

    Page(s): 1225 - 1227
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    Midinfrared laser emission from self-organized PbSe quantum dots in a high-finesse vertical-cavity surface-emitting laser structure is reported. The structure was grown by molecular-beam epitaxy and consists of high reflectivity epitaxial EuTe/PbEuTe Bragg mirrors with a PbSe/Pb1-xEuxTe quantum-dot superlattice as the active region. Narrow laser emission at 4.2–3.9 μm induced by optical pumping is achieved at temperatures up to 90 K. The observation of simultaneous two-mode emission indicates a width of the inhomogeneously broadened PbSe dot gain spectrum of about 18 meV. © 2001 American Institute of Physics. View full abstract»

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  • Periodic nanostructure array in crossed holographic gratings on silica glass by two interfered infrared-femtosecond laser pulses

    Page(s): 1228 - 1230
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    Two cross-superposed holographic gratings were encoded on silica glass by femtosecond laser pulses (wavelength ∼800 nm). A variety of periodic nanostructures from a one-dimensional wire array to two-dimensional arrays of holes or islands were formed by changing the energy density and the incidence angle of the irradiation laser beams. The smallest dimensions were a width of ∼15 nm for wires and a diameter of ∼20 nm for holes. Laser-driven microexplosions occurring within the microcylindrical-lens array created by the first laser pulse are suggested as a mechanism for the formation of these structures. Only two pulses are required to encode these periodic structures, which are applicable to emerging nanostructured devices such as photonic crystals and quantum dot or wire arrays. © 2001 American Institute of Physics. View full abstract»

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  • Development of high-performance blue–violet-emitting organic electroluminescent devices

    Page(s): 1231 - 1233
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    Two high-performance blue-violet-emitting organic electroluminescent devices with electroluminescence peaking at 404 and 415 nm were developed using a hole-blocking amorphous molecular material, 1,3,5-tris(4-fluorobiphenyl-4-yl)benzene, and emitting amorphous molecular materials, N,N-bis(3-methylphenyl)-N,N-diphenyl-[1,1-biphenyl]-4,4-diamine and N,N-di(4-biphenylyl)-N,N-diphenyl-[1,1-biphenyl]-4,4-diamine. The devices show maximum luminaces of 3960 cd m-2 at 15 V and 2550 cd m-2 at 12.0 V, having turn-on voltages of 4.0 V and external quantum efficiencies of 1.40% and 1.25%. © 2001 American Institute of Physics. View full abstract»

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  • Production of relativistic electrons by irradiation of 43-fs-laser pulses on copper film

    Page(s): 1234 - 1236
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    The energy spectra of fast electrons produced by ultrashort, high-intensity laser pulses were directly measured using a magnetic spectrometer with an imaging plate. The typical temperature was 350 keV for irradiation on 30 μm thick copper film by pulses of width 43 fs, intensity 2.7×1018W/cm2, repetition rate 10 Hz without artificial prepulses and was found to be close to the ponderomotive potential. In addition, the energy spectra of high-energy photons, which are expected to be produced from the electrons, were calculated. © 2001 American Institute of Physics. View full abstract»

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  • Observation of pinching in the compact toroid injection experiment: Implications for a plasma opening switch

    Page(s): 1237 - 1239
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    A compact toroid inductively stores the energy released by a capacitor bank as it is being accelerated. This energy can be stored for a period of more than ten microseconds and then transferred to a load on a much shorter time scale. This article presents framing camera images of the radial compression of plasma trailing behind a compact toroid as the compact toroid leaves its inner electrode. This compression illustrates the basic principles of a compact toroid plasma opening switch which could be used to drive fast z pinches. © 2001 American Institute of Physics. View full abstract»

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  • Xenon excimer emission from pulsed microhollow cathode discharges

    Page(s): 1240 - 1242
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    By applying electrical pulses of 20 ns duration to xenon microplasmas, generated by direct current microhollow cathode discharges, we were able to increase the xenon excimer emission by more than an order of magnitude over direct current discharge excimer emission. For pulsed voltages in excess of 500 V, the optical power at 172 nm was found to increase exponentially with voltage. Largest values obtained were 2.75 W of vacuum-ultraviolet optical power emitted from a single microhollow cathode discharge in 400 Torr xenon with a 750 V pulse applied to a discharge. Highest radiative emittance was 15.2 W/cm2. The efficiency for excimer emission was found to increase linearly with pulsed voltages above 500 V reaching values of 20% at 750 V. © 2001 American Institute of Physics. View full abstract»

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  • Photoluminescence from quantum dots in cubic GaN/InGaN/GaN double heterostructures

    Page(s): 1243 - 1245
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    We have measured photoluminescence spectra of molecular-beam-epitaxy-grown cubic GaN/InxGa1-xN/GaN double heterostructures with x between 0.09 and 0.33. We observe a luminescence peak at about 2.3–2.4 eV which is almost independent of the InGaN layer composition. High-resolution x-ray diffraction measurements revealed a pseudomorphic In-rich phase with x=0.56±0.02 embedded in the InGaN layers. Including strain effects we calculate a gap energy Eg=2.13 eV of this phase. In cubic InGaN, spontaneous polarization and strain-induced piezoelectric fields are negligible. Therefore, the observed difference between the luminescence energy and the gap of the In-rich phase is assumed to be due to the localization of excitons at quantum-dot-like structures with a size of about 15 nm. © 2001 American Institute of Physics. View full abstract»

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  • Pulsed liquid-injection metalorganic chemical vapor deposition of (La0.7Sr0.3MnO3/SrTiO3)15 superlattices

    Page(s): 1246 - 1248
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    (La0.7Sr0.3MnO3/SrTiO3)15 superlattices have been grown by pulsed liquid-injection metalorganic chemical vapor deposition on monocrystalline substrates such as LaAlO3, SrTiO3, and MgO. The pulsed-injection technique allows one to control precisely the amount of precursors delivered to the deposition chamber and thus the thickness of each individual layer. The period of the superlattices depends indeed linearly on the number of injected droplets. In our deposition conditions, the average growth rates are ∼0.130 nm/injection for La0.7Sr0.3MnO3 and 0.042 nm/injection for SrTiO3, with no significant difference as regard to the substrate used. The strain’s state of the superlattices depends on the relative thicknesses of the La0.7Sr0.3MnO3 and SrTiO3 layers and also on the substrate used. Finally, the deposition of superlattices with ultrathin interlayers of few unit cells has been demonstrated. © 2001 American Institute of Physics. View full abstract»

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  • Optical gain in Si/SiO2 lattice: Experimental evidence with nanosecond pulses

    Page(s): 1249 - 1251
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    Experimental evidence of population inversion and amplified spontaneous emission was found for Si nanocrystallites embedded in SiO2 surrounding under pumping with 5 ns light pulses at 380, 400, and 500 nm. As an important property, our experiments show a short lifetime of the population inversion allowing a generation of short (a few nanosecond) amplified light pulses in the Si/SiO2 lattice. The estimate for optical gain in the present samples is 6 cm-1 at 720 nm. © 2001 American Institute of Physics. View full abstract»

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  • Grapevine-like growth of single walled carbon nanotubes among vertically aligned multiwalled nanotube arrays

    Page(s): 1252 - 1254
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    Large arrays of self-oriented, multiwalled carbon nanotubes (MWNTs) have been obtained by chemical vapor deposition. It has so far been impossible to translate this idea for growing single walled nanotubes (SWNTs) but we show here that oriented growth of SWNTs among vertically aligned MWNT arrays can be realized by the catalytic pyrolysis of ferrocene and xylene. The MWNTs act as the support structures for SWNTs, forcing them to grow upward steadily, like grapevines. The growth of vertically aligned SWNT over large areas brings about the possibility of exploring their properties in select configurations. © 2001 American Institute of Physics. View full abstract»

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  • CoPt/Ag nanocomposites with (001) texture

    Page(s): 1255 - 1257
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    CoPt/Ag nanocomposites with the tetragonal (L10) structure have been prepared by magnetron sputtering. The dependence of texture on film thickness, bilayer thickness, CoPt volume fraction, and annealing conditions is investigated. Films with a thickness below 15 nm consist of islands with (001) texture while as the thickness increases, the islands coalesce into a continuous film and the (111) texture appears. Microstrain is minimized in the range of film thickness where the (001) texturing is optimum indicating that strain energy provides the driving force of (001) growth texturing. The (001) texture improves with CoPt volume fraction for all annealing times but disappears above 95 vol % indicating that the existence of the Ag plays an important role in the development of the (001) texture. © 2001 American Institute of Physics. View full abstract»

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  • Annealing effects on the microstructure of Ge/Si(001) quantum dots

    Page(s): 1258 - 1260
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    Ge/Si(001) multilayer islands produced by gas-source molecular-beam epitaxy at 575 °C were investigated using energy-filtering transmission electron microscopy. Results show, for as-grown samples, not only a continuous enlargement of island size in upper layers but also a continuous increase of Ge concentration within islands in upper layers. As a result of the increasing island size and Ge concentration within the islands, the island density in upper layers decreases. For samples annealed at 900 °C for 5 min, the aspect ratio of buried islands increases significantly, and the average Ge concentration within islands of different layers becomes uniform. © 2001 American Institute of Physics. View full abstract»

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  • Selective area metalorganic vapor phase epitaxy of thick crack-free GaN films on trenched SiC substrates

    Page(s): 1261 - 1263
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    Crack-free GaN films up to 11-μm-thick have been grown by using trenched SiC substrates and selective area metalorganic vapor phase epitaxy. These crack-free GaN films have hexagonal shapes and are surrounded by trenches. 97% of the hexagonal GaN films with side lengths of 100 μm and thickness of 11 μm was crack-free. The GaN films do not crack because the lateral propagation of cracks stops at the trenches and strain is relaxed in the small-area hexagonal GaN. This strain relaxation is confirmed by micro-Raman scattering measurements and agrees well with theoretical predictions. © 2001 American Institute of Physics. View full abstract»

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  • Void formation by thermal stress concentration at twin interfaces in Cu thin films

    Page(s): 1264 - 1266
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    A void formation mechanism was investigated in an electroplated copper thin film on Ta/SiO2/Si. Microstructural observation after thermal cycling indicated that void formation occurred at intersecting points or terminating corners of annealing twins. The calculated stress distribution was compared with experimental results of the void formation tendency. An excellent correlation was found between void formation sites and stress concentration sites. Electron diffraction analysis revealed that most twin interfaces in Cu thin films are incoherent {322} planes. The stress concentration drives diffusion along incoherent twin interfaces of {322} and leads to void formation at twin interfaces and corners. © 2001 American Institute of Physics. View full abstract»

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  • Molecular dynamics simulation of the meniscus formation between two surfaces

    Page(s): 1267 - 1269
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    The molecular dynamics computational method is used to simulate meniscus formation around an asperity in a rough surface represented as a sinusoidal wave. Simulation results show that the meniscus formation depends on the interaction potential between the solid wall and the liquid atoms. For completely and partially dry substrates a meniscus cannot form around an asperity. For partially and completely wetting substrates the asperity helps to adsorb the fluid atoms and form a meniscus. These simulation results confirm that if the film thickness exceeds a critical value, the capillary pressure contributes strongly to stiction. © 2001 American Institute of Physics. View full abstract»

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  • Self-doping near the seed/layer interface in conformal GaAs layers grown on Si

    Page(s): 1270 - 1272
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    Undoped GaAs layers grown on Si substrates by the conformal method were studied by micro-Raman spectroscopy, cathodoluminescence, and diluted Sirtl solution with light (DSL) etching. The results show that nonintentional doping of conformal layers can take place near the seed/layer interface. The self-doped area presents a bright luminescence emission and shows longitudinal optic-plasmon coupled Raman modes. The nonintentional dopants were n type as deduced from Raman spectroscopy and DSL selective etching. The doped region extends only 2–3 μm from the seed and was tentatively associated with enhanced diffusion of Si in the presence of dislocations at the interface between the seed and the conformal layer. © 2001 American Institute of Physics. View full abstract»

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  • Binding energy of vacancy clusters generated by high-energy ion implantation and annealing of silicon

    Page(s): 1273 - 1275
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    We have measured the evolution of the excess-vacancy region created by a 2 MeV, 1016/cm2 Si implant in the silicon surface layer of silicon-on-insulator substrates. Free vacancy supersaturations were measured with Sb dopant diffusion markers during postimplant annealing at 700, 800, and 900 °C, while vacancy clusters were detected by Au labeling. We demonstrate that a large free vacancy supersaturation exists for short times, during the very early stages of annealing between the surface and the buried oxide (1 μm below). Afterwards, the free vacancy concentration returns to equilibrium in the presence of vacancy clusters. These vacancy clusters form at low temperatures and are stable to high temperatures, i.e., they have a low formation energy and high binding energy. © 2001 American Institute of Physics. View full abstract»

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  • Conductive phosphate and fluorophosphate glasses with fullerene doping

    Page(s): 1276 - 1278
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    The appropriate amount of fullerene doping in phosphate and fluorophosphate optical glasses modifies the structures of glass matrices by bonding the nearby -[PO4]- tetrahedra with nonbridging oxygen anions, and consequently building up observable conductivity at room temperature. Non-Arrhenius ionic conductivity is observed, which is interpreted as a result of the temperature dependence of the activation energies of the mobile cations. The variation of microphotoluminescence induced by the electric field presents us with an indirect way to characterize the random activation, percolation migration, and retrap of metal cations near fullerene-related amorphous islands. © 2001 American Institute of Physics. View full abstract»

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  • Inter-conduction band electron relaxation dynamics in 6H–SiC

    Page(s): 1279 - 1281
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    The ultrafast inter-conduction band carrier dynamics in 6H–SiC was observed by using pump and probe transient absorption technique. Probe wavelength dependence of the bleaching was compared with the steady-state absorption profiles for polarizations parallel and perpendicular to the c axis, and these bleachings were ascribed to the decrease of electron populations in the lowest conduction band. The relaxation time from the higher to the lowest conduction band due to the inter-conduction band electron–phonon scattering is 1.25 ps. © 2001 American Institute of Physics. View full abstract»

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  • Strain effects on exciton resonance energies of ZnO epitaxial layers

    Page(s): 1282 - 1284
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    Magnitudes of strain in ZnO epitaxial layers grown on sapphire(0001) substrates under various growth conditions were experimentally determined by x-ray diffraction. We discuss the strain-induced energy shift on the exciton resonances, the results of which were analyzed theoretically using the Hamiltonian for the valence bands under in-plain biaxial strain. Comparative studies with GaN evidenced the advantages of ZnO in terms of sensitivity of the strain-induced energy shift and of piezoelectric effect in heterostructures. © 2001 American Institute of Physics. View full abstract»

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  • Study of exciton–longitudinal optical phonon coupling in quantum wells for optoelectronic applications

    Page(s): 1285 - 1287
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    The study of exciton–longitudinal optical (LO) phonon coupling in semiconductor quantum well (QW) structures and their corresponding bulk materials reveals four distinct features: a large difference between (i) III–V and II–VI QW structures, (ii) multiple QW and single QW structures, as well as (iii) QW structures and their corresponding bulk materials, and its linear dependence on well width in QW structures. A quantitatively theoretical approach is presented, which can explain well all the experimental observations and can clarify the controversy in the literature. The effects of alloy disorder and strain in QW structures on exciton–LO phonon coupling are also discussed. © 2001 American Institute of Physics. View full abstract»

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  • Effect of surface roughness on field emission from chemical vapor deposited polycrystalline diamond

    Page(s): 1288 - 1290
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    The effect of surface roughness on electron emission from hydrogenated polycrystalline diamond films is reported. Field emission measurements were performed with both millimeter and nanometer spatial resolution using scanning probe techniques. Surface asperities were removed by ion beam treatment, which resulted in a reduction of the rms roughness from 198 to 94 nm, leading to an increase in the threshold field required for electron emission by about a factor of 2. These results suggest that surface asperities, rather than grain boundaries, are the dominant influence on electron emission in polycrystalline diamond films. © 2001 American Institute of Physics. View full abstract»

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  • Ultrafast carrier trapping in microcrystalline silicon observed in optical pump–terahertz probe measurements

    Page(s): 1291 - 1293
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    We report on direct evidence of ultrafast carrier dynamics displaying features on the picosecond time scale in microcrystalline silicon (μc-Si:H). The dynamics of photogenerated carriers is studied by using above-band-gap optical excitation and probing the instantaneous carrier mobility and density with a THz pulse. Within the first picoseconds after excitation, the THz transmission transients show a fast initial decay of the photoinduced absorption followed by a slower decrease due to carrier recombination. We propose that the initial fast decay in the THz transients is due to carrier capture in the trapping states. © 2001 American Institute of Physics. View full abstract»

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Applied Physics Letters, published by the American Institute of Physics, features concise, up-to-date reports on significant new findings in applied physics.

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Nghi Q. Lam
Argonne National Laboratory