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Applied Physics Letters

Issue 13 • Date Sep 2002

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Displaying Results 1 - 25 of 57
  • 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
  • Efficient polarized light-emitting diodes utilizing ultrathin photoaddressable alignment layers

    Page(s): 2319 - 2321
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    We demonstrate that an ultrathin photoaddressable polymer (PAP) layer with a thickness as small as 5 nm can be utilized for the monodomain alignment of thermotropic liquid crystalline polyfluorene. The optical anisotropies in absorption and emission are found to be independent of the PAP layer thickness within a range of 5 to 30 nm. On the other hand, decreasing the PAP layer thickness greatly improves the performance of polarized blue light-emitting diodes: With a PAP layer thickness of only 10 nm, the device turns on at 5 V and reaches a brightness of 100 cd/m2 at 8 V with an efficiency of 0.66 cd/A. © 2002 American Institute of Physics. View full abstract»

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  • Third-order nonlinear optical properties of in-backbone substituted conjugated polymers

    Page(s): 2322 - 2324
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    We present an alternative approach to tune the third-order nonlinear optical properties of organic molecules by inserting a functional group into the path of the π-electron conjugation instead of at its ends. We show that this scheme has a beneficial influence on the second-order hyperpolarizability for short molecules, but that in two instances where such molecules were polymerized into longer molecules the overall hyperpolarizability is reduced. © 2002 American Institute of Physics. View full abstract»

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  • Resolution of photonic crystal superprism

    Page(s): 2325 - 2327
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    We theoretically investigated the performance of the photonic crystal superprism, that is, the propagating beam quality, the wavelength sensitivity, and the resolution as a narrow band filter at 1.5-μm-wavelength range. First, we defined the equi-incident-angle curve in the Brillouin zone. Next, we mapped three parameters that represented the abovementioned performance over the Brillouin zone. As a result, we found a narrow design window that allows a high resolution of 0.4 nm along an equi-incident-angle curve but requires an incident beam width of over 100 μm and a device length of centimeter order. It can be an essential high efficiency filter if the input end of the crystal is optimized and the propagation loss is suppressed. © 2002 American Institute of Physics. View full abstract»

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  • High-repetition rate femtosecond laser-driven hard-x-ray source

    Page(s): 2328 - 2330
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    A compact, high-repetition rate, ultrashort-pulse laser-driven hard-x-ray source based on the combination of a femtosecond laser system with an x-ray diode is demonstrated. A comparison with available laser-plasma sources is presented. Numerical modeling is performed which proves that picosecond and subpicosecond hard-x-ray pulses can be produced with this source. © 2002 American Institute of Physics. View full abstract»

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  • Enhanced blue and green emission in rare-earth-doped GaN electroluminescent devices by optical photopumping

    Page(s): 2331 - 2333
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    Electroluminescence (EL) from rare-earth-doped GaN (GaN:RE) EL devices (ELD) emission has been observed to be greatly enhanced by ultraviolet (UV) photopumping. With radiation from a HeCd laser (325 nm) both blue (from GaN:Tm) and green (from GaN:Er) EL brightness have been enhanced up to 2 orders of magnitude, depending on bias conditions. We explain the luminescence increase by the following mechanism: photoelectrons generated by above GaN band-gap excitation are accelerated by the electric field along with electrically injected electrons and both types of carriers contribute to EL emission through RE impact excitation. The EL intensity increases monotonically with increasing applied bias and with photopumping power. The photopumped-induced EL gain is most efficient at relatively low bias, reaching values of 50–100×. This increase in EL emission can be applied to flat panel displays with enhanced brightness, especially blue, and with improved color balance. Other applications include UV indicators and detectors, and infrared emitters. © 2002 American Institute of Physics. View full abstract»

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  • Properties of highly strained InGaAs/GaAs quantum wells for 1.2-μm laser diodes

    Page(s): 2334 - 2336
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    The properties of 1.2-μm highly strained InGaAs quantum wells (QWs) grown on GaAs substrates have been analyzed. Optical gain spectra versus injection current and temperature, transparency current density, as well as other figures of merit were assessed from measurements on broad-area and ridge-waveguide lasers based on these QWs. Such active regions are of interest for a range of applications, including GaAs-based high-power lasers and vertical-cavity lasers for wavelengths beyond 1.2 μm. © 2002 American Institute of Physics. View full abstract»

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  • Field-controlled optical profile of a waveguide having a liquid-crystalline core

    Page(s): 2337 - 2339
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    The optical properties of a nematic liquid-crystal waveguide in a hybrid configuration have been dynamically reshaped in a straightforward and controlled way by applying an external electric field. The anisotropic refractive index profile of the waveguide has been determined for different electric-field strengths by means of m-lines spectroscopy. The experimental results are in very good agreement with the values predicted by a simple model that takes into account the dielectric, optical, and elastic properties of the liquid-crystal guiding layer. © 2002 American Institute of Physics. View full abstract»

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  • Transmission spectroscopy of photonic crystals in a silicon-on-insulator waveguide structure

    Page(s): 2340 - 2342
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    Optical properties of two-dimensional photonic crystals (PCs) integrated in a silicon-on-insulator stripe waveguide are reported. Transmission measurements are done on a wide wavelength range (1.1–1.7 μm) by coupling a white-light source in the cleaved edge of the sample and collecting light at the other cleaved side of the waveguide. PC mirrors and one-dimensional (1D) Fabry–Pérot cavities formed between two PC reflectors have been fabricated and characterized. The photonic band gap of the mirror is observed in the Γ–M direction for the transverse electric like guided modes. For the waveguide with the 1D Fabry–Pérot cavity, resonant modes appear within the gap. It is found that the experimental spectra are in very good agreement with both the theoretical photonic band structure and the theoretical transmission calculated by finite difference time domain. © 2002 American Institute of Physics. View full abstract»

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  • Perveance of a planar diode with explosive emission finite-diameter cathodes

    Page(s): 2343 - 2345
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    Perveance of a planar diode with explosive emission velvet cathodes of 20–100 mm diameters in a range of diode gaps of 0.5–5 cm is investigated experimentally. An empirical universal curve linking the diode perveance and the cathode radius-to-diode gap ratio is determined thus allowing application of a Child–Langmuir model to diodes of finite dimensions. Comparison of experimental results with two-dimensional (2D) computational models is made and a simple method to translate 2D simulation results to the three-dimensional diode configurations is suggested. View full abstract»

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  • Evidence of a ternary Co1-xFexSi2 phase with a CaF2-type structure: High-resolution transmission electron microscopy and diffraction anomalous fine structure study

    Page(s): 2346 - 2348
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    A ternary silicide phase (Co0.6Fe0.4Si2) with a cubic CaF2-type structure, grown in epitaxy on Si(111), is investigated by using diffraction anomalous fine structure and high-resolution transmission electron microscopy. This cubic phase is formed by annealing at 930 K a room-temperature-grown epitaxial ternary layer with ratios of 0.7:2 for Fe:Si and 0.3:2 for Co:Si. This phase is mixed with a tetragonal α-FeSi2-type phase, but both phases are ternary. It is shown that a large amount of Fe (about 40%) substitutes for Co atoms on the metallic sites of the CaF2-type lattice of the cubic silicide phase. © 2002 American Institute of Physics. View full abstract»

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  • Microsecond carrier recombination times in InAs/AlAs quantum dots

    Page(s): 2349 - 2351
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    We report the measurement of microsecond decay times of the photoluminescence from self-assembled InAs/AlAs quantum dots. The long decay times are attributed to recombination involving electrons and holes localized in spatially separated quantum dots. The spatial separation occurs during the relaxation process where electrons can scatter to adjacent quantum dots via the AlAs X states. © 2002 American Institute of Physics. View full abstract»

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  • All-angle negative refraction in a three-dimensionally periodic photonic crystal

    Page(s): 2352 - 2354
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    We introduce a photonic crystal that has a large range of effective negative refractive index in three dimensions (3D) and demonstrate its negative-refraction property by numerical simulations. With slight modifications, our design is also amenable to layer-by-layer fabrication. This work should enable experimental observation of negative refraction and related phenomena in 3D at optical frequencies. © 2002 American Institute of Physics. View full abstract»

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  • Radiative recombination of electron–hole pairs spatially separated due to quantum-confined Stark and Franz–Keldish effects in ZnO/Mg0.27Zn0.73O quantum wells

    Page(s): 2355 - 2357
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    We studied photoluminescence (PL) properties of eighteen samples of wurtzite ZnO/MgxZn1-xO multiple quantum wells (x=0.12 and 0.27) with various well widths (Lw) of 0.7–4.65 nm. Radiative recombination of the electron–hole pairs that are spatially separated due to the quantum-confined Stark (QCS) and Franz–Keldish (QCFK) effects was observed in two thicker samples at 5 K. This PL band is located ≈ 40 meV in energy below the emission band of the localized excitons and ≈ 60 meV below the absorption energy of the free exciton transition. One can not observe such kind of luminescence unless both of the following conditions are accomplished: (1) higher Mg concentration (x=0.27) and (2) Lw≥4.23 nm. These experimental findings do not contradict the following two characteristic features for the QCS and QCFK effects; the magnitude of the electric field due to spontaneous and piezoelectric polarizations and the depth of the triangle-shaped potential wells are the monotonically increasing functions of Mg concentration and the Lw, respectively. The coupling strength with longitudinal-optical phonons, which is determined from the relative luminescence intensities of the phonon replicas, is significantly larger than that between the localized excitons and phonons. It is considered that the strong electric field increases the distance between electron and hole charge distributions from that determined by the Coulomb force and leads to the enhancement in the phonon interaction. © 2002 American Institute of Physics. View full abstract»

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  • Grazing-incidence small-angle scattering measurement of Ge islands capped with a Si layer

    Page(s): 2358 - 2360
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    Grazing-incidence small-angle scattering (GI-SAXS) technique was applied to self-organized Ge islands capped with Si. The average size and height as well as the average distance between Ge islands were obtained. The present results demonstrate that GI-SAXS is a useful tool to examine the structure of self-organized semiconducting nanodots even if they are covered with a cap layer, and a simple kinematical approach can be safely used under some conditions. © 2002 American Institute of Physics. View full abstract»

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  • Self-formation of microdomains by the topographical and fringe field effects in a liquid crystal display with dielectric surface gratings

    Page(s): 2361 - 2363
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    We report on the self-formation of microdomains by the topographical and fringe field effects in a twisted nematic liquid crystal display (LCD) with dielectric surface gratings (DSG). A regular array of the DSG produces periodically aligned microdomains in each pixel because of the topographical alignment of the LC and the spatial variations of the effective voltage across the LC layer. The mutual optical compensation within each pixel is naturally achieved and thus the range of viewing in the LCD is significantly extended without complex surface treatments. © 2002 American Institute of Physics. View full abstract»

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  • Thermally stabilized photoinduced Bragg gratings

    Page(s): 2364 - 2366
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    Bragg gratings were printed in Ge–SiO2 and Ge–B–SiO2 thin glass films by KrF excimer laser irradiation through a phase mask. The diffraction efficiency of the grating printed in the Ge–B–SiO2 film was approximately one order of magnitude greater than that of the Ge–SiO2 film. Although the gratings were almost erased upon annealing at temperatures lower than 500 °C, a grating with much higher diffraction efficiency than before annealing was formed exclusively in the Ge–B–SiO2 film after annealing at 600 °C. The diffraction efficiency of the grating was unchanged upon repeated heating between room temperature and 600 °C. © 2002 American Institute of Physics. View full abstract»

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  • Pseudodielectric functions of InGaAs alloy films grown on InP

    Page(s): 2367 - 2369
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    We present room-temperature pseudodielectric function spectra <ε> of InxGa1-xAs films grown on (100) InP by solid-source molecular-beam epitaxy. A wet-chemical etching procedure is used to remove overlayers and obtain the best approximation to the bulk dielectric responses ε of the films. By line shape fitting, we determined the x dependences of the E1 and E11 critical-point energies and that of the Δ1 bowing parameter. The results are in good agreement with the predictions of a universal tight-binding model. © 2002 American Institute of Physics. View full abstract»

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  • Raman mapping investigations and finite element analysis of double epitaxial lateral overgrown GaN on sapphire substrates

    Page(s): 2370 - 2372
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    Double epitaxial lateral overgrown (D–ELO) GaN grown by metalorganic vapor phase epitaxy on sapphire substrates was characterized using Raman mapping and finite element analysis. Reductions in stress variations at the D–ELO top surface with respect to single ELO GaN were achieved. Stress near the top surface was mainly attributed to the presence of voids on top of the upper dielectric mask. © 2002 American Institute of Physics. View full abstract»

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  • Room-temperature epitaxial growth of AlN films

    Page(s): 2373 - 2375
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    We have grown AlN films on nearly-lattice-matched (Mn,Zn)Fe2O4 (111) substrates by pulsed-laser deposition at room temperature and investigated their structural properties using reflection high-energy electron diffraction (RHEED), grazing incidence angle x-ray reflectivity (GIXR), grazing incidence angle x-ray diffraction (GIXD), and atomic force microscope. We have found that AlN grows epitaxially even at room temperature. In situ RHEED observations have shown that the room-temperature growth of AlN starts with the two-dimensional mode followed by a transition to the three-dimensional mode at the film thickness of 2 nm. GIXR and GIXD measurements have revealed that the heterointerface between AlN and (Mn,Zn)Fe2O4 is abrupt and approximately 90% of the lattice mismatch is released at the interface due to the introduction of crystalline defects such as misfit dislocations. These results indicate that the present technique solves one of the two major problems with heteroepitaxial growths of group III nitrides (mismatch in the thermal expansion coefficients) and alleviates the other problem (mismatch in the lattice constants). © 2002 American Institute of Physics. View full abstract»

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  • Infrared dielectric functions and phonon modes of wurtzite MgxZn1-xO (x≤0.2)

    Page(s): 2376 - 2378
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    Infrared dielectric function spectra and phonon modes with polarization parallel and perpendicular to the c axis of high quality, highly relaxed, and single crystalline wurtzite MgxZn1-xO films with 0≤x≤0.2 prepared by pulsed-laser deposition on c-plane sapphire substrates were obtained from infrared spectroscopic ellipsometry (380–1200 cm-1) and Raman scattering studies. A two-mode behavior is found for the modes with E1 symmetry, a lattice mode and an impurity-type mode are obtained for the A1 symmetry phonons. Model dielectric function spectra will become useful for future infrared ellipsometry analysis of complex MgxZn1-xO-based heterostructures. © 2002 American Institute of Physics. View full abstract»

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  • Thermoelectric properties of Bi2Sr2Co2O9 whiskers under hydrostatic pressure

    Page(s): 2379 - 2381
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    We report the temperature dependence of thermopower (S) in the ab plane of Bi2Sr2Co2O9 (BC-222) single crystalline whiskers under high pressure up to 1.2 GPa. Special modifications were made to our high-pressure transport measurement system to improve the measurement accuracy of S for whiskers with high resistance. Pressure improved the contact resistance dramatically. We observed a slight decrease of S and four-wire electrical conductivity (σ) under high pressure. As a result, the power factor (S2σ) was decreased less than 20% by pressure. From the temperature dependence of σ, the band gap of BC-222 whiskers increase very slightly under high pressure. © 2002 American Institute of Physics. View full abstract»

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  • Surface Fermi-level position and gap state distribution of InGaP surface grown by metalorganic vapor-phase epitaxy

    Page(s): 2382 - 2384
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    Electronic properties of “free” n-In0.49Ga0.51P surfaces grown by metalorganic vapor-phase epitaxy were directly characterized using the contactless capacitance–voltage technique. The HCl-treated surface showed a wide and continuous distribution of surface state density (Dss) in energy with relatively low densities, leading to no pronounced Fermi-level pinning effect on the surface. The minimum Dss value was determined to be 8×1011cm-2eV-1. The surface Fermi-level position was found at 1.2 eV above the valence band maximum, consistent with the x-ray photoelectron spectroscopy results. © 2002 American Institute of Physics. View full abstract»

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  • High energy proton irradiation effects on SiC Schottky rectifiers

    Page(s): 2385 - 2387
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    4H-SiC Schottky rectifiers with dielectric overlap edge termination were exposed to 40 MeV protons at fluences from 5×107–5×109cm-2. The reverse breakdown voltage decreased from ∼500 V in unirradiated devices to ∼-450 V after the highest proton dose. The reverse leakage current at -250 V was approximately doubled under these conditions. The forward current at -2 V decreased by ∼1% (fluence of 5×107cm-2) to ∼42% (fluence of 5×109cm-2), while the current at lower biases was increased due to the introduction of defect centers. The ideality factor, on-state resistance, and forward turn-on voltage showed modest increases for fluences of ≤5×108cm-2, but were more strongly affected (increase of 40%–75%) at the highest dose employed. © 2002 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