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Applied Physics Letters

Issue 5 • Date Jul 2001

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Displaying Results 1 - 25 of 51
  • 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
  • Hole injection barriers at polymer anode/small molecule interfaces

    Page(s): 557 - 559
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    A photoemission study of the interface between spin-cast films of a conducting polymer blend consisting of poly(3,4-ethylenedioxythiophene) (PEDOT), poly(4-styrenesulfonate) (PSS) and glycerol as an additive, and vacuum-evaporated hole transport layers (HTL) of 4,4-bis(carbazol-9-yl)biphenyl, N,N-diphenyl-N,N-bis(1-naphthyl)-1-1biphenyl-4,4-diamine and N,N-diphenyl-N,N-bis(3-methylphenyl)-1,1- biphenyl-4,4-diamine reveals a hole injection barrier between 0.5 and 0.9 eV at the glycerol-modified PEDOT-PSS/HTL interface. The measured energy barriers imply a reasonable charge injection, which is very encouraging for further development of the novel anode structures based on a conducting polymer/small molecule interface to be utilized in electro-optic applications such as organic light-emitting devices. © 2001 American Institute of Physics. View full abstract»

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  • White light emission from blends of blue-emitting organic molecules: A general route to the white organic light-emitting diode?

    Page(s): 560 - 562
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    We show that all possible binary combinations of molecules from four different families of organics—a diamine derivative, N,N-bis(3methylphenyl)-N,N-diphenylbenzidine, an oxidiazole derivative, 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole, a substituted thiophene dioxide, 2,5-bis(trimethylsilyl thiophene)-1,1-dioxide, and poly(9-vinylcarbazole)—produce white or near-white emission. We suggest that this is due to exciplex formation, and that this is likely to be a general phenomenon for blends of blue-emitting aromatic organics. This implies that films of spin-coated blends of blue-emitting organics represent a general, simple, and cheap route to white-emitting organic light-emitting diodes. © 2001 American Institute of Physics. View full abstract»

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  • Efficient polymer light emitting diodes with metal fluoride/Al cathodes

    Page(s): 563 - 565
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    We reported efficient polymer light emitting diodes with alkali and alkaline earth metal fluoride Al cathodes. The quantum efficiencies of these devices are more than two orders of magnitude higher than those in devices without fluoride layer, comparable to the best data reported with similar electroluminescent polymers. The mechanism of the metal fluoride/Al bilayer cathodes was also discussed based on photovoltaic, ac impedance, and photoluminescence efficiency measurement. © 2001 American Institute of Physics. View full abstract»

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  • Broad-band fluorescence upconversion for femtosecond spectroscopy

    Page(s): 566 - 568
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    With near-infrared gating and improved light collection geometry, the entire fluorescence band can be upconverted in a broad range of 10 000 cm-1 without readjusting optical elements, thus allowing measurements with a single pump-gate scan. Monitoring of the pump-induced white light continuum provides for the time correction of the up-converted fluorescence spectra. The overall time resolution is then limited by the pump-gate cross correlation. The technique is illustrated with the femtosecond evolution of fluorescence from two molecular probes in solution. © 2001 American Institute of Physics. View full abstract»

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  • Cavity-enhanced and quasiphase-matched multi-order reflection-second-harmonic generation from GaAs/AlAs and GaAs/AlGaAs multilayers

    Page(s): 569 - 571
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    We have observed quasiphase-matched second-harmonic generation in the reflection geometry from GaAs/AlAs multilayers. By using GaAs/AlGaAs multilayers and Fresnel reflection as a cavity, we have also achieved cavity-enhanced nonphase-matched second-harmonic generation from GaAs/AlAs multilayers. The linewidth for the first order reflection-second-harmonic generation is limited only by wave-vector mismatch. In addition, we have demonstrated two-order-of-magnitude enhancement on the conversion efficiency by using the cavity. © 2001 American Institute of Physics. View full abstract»

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  • Quantum cascade lasers with a heterogeneous cascade: Two-wavelength operation

    Page(s): 572 - 574
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    A quantum cascade (QC) laser with a heterogeneous cascade containing two substacks previously optimized to emit at 5.2 μm and 8.0 μm wavelengths, respectively, is presented. The low-temperature performance of the two-wavelength laser is comparable to the respective homogeneous stack lasers, indicating no penalty from the heterogeneity of the cascade. Each substack is apportioned the optimum fraction of the applied bias. This demonstrates the general applicability of this scheme. In addition, an etch-stop layer inserted between the two substacks allowed fabrication of a “tap” into the cascade. The latter was used to selectively manipulate the laser threshold of one substack, turning the 8.0 μm laser on and off while the adjacent 5.2 μm QC laser was operating undisturbed. © 2001 American Institute of Physics. View full abstract»

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  • Resonantly coupled surface plasmon polaritons in the grooves of very deep highly blazed zero-order metallic gratings at microwave frequencies

    Page(s): 575 - 577
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    Sharp features are observed in the wavelength-dependent reflectivity of very deep highly blazed zero-order metallic gratings at microwave frequencies. These features are attributed to the resonant excitation of self-coupled surface plasmon polaritons in the cavities of such samples. The slat structures may, in a simplistic model, be treated as a class of Fabry–Pérot cavity resonator. © 2001 American Institute of Physics. View full abstract»

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  • Improving the performance of polymer light-emitting diodes using polymer solid solutions

    Page(s): 578 - 580
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    Optical and electronic properties of conjugated polymers can be modified significantly by arranging the polymer chains differently. For example, it is well known that polymer solutions of lower concentrations have different photoluminescence spectra from solutions of higher concentrations. In this manuscript, we demonstrated that such effects can also be achieved in solid states, i.e., in polymer solid solutions consisting of a narrower band-gap material—poly(2-methoxy-5(2-ethyl-hexyloxy)-1,4-phenylene vinylene (MEH-PPV) with a wider band-gap polymer–poly(9,9-dioctylfluorene). At very low MEH-PPV concentrations, emission spectra of these solid solutions are similar to the photoluminescence spectrum of a diluted MEH-PPV solution in an ordinary organic solvent, suggesting the absence of a significant amount of interchain species. In addition, the electroluminescence efficiency is significant enhanced. Furthermore, the full width at half maximum (FWHM) is dramatically reduced at lower MEH-PPV concentrations, as a result of suppressed interchain interactions. This approach is particularly important for the application to display technology, in which well-defined narrow FWHM emission spectra of red, green, and blue colors are required. © 2001 American Institute of Physics. View full abstract»

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  • Terahertz field characterization using Fabry–Perot-like cantilevers

    Page(s): 581 - 583
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    We show that a Fabry–Perot-like tunneling cantilever consisting of two parallel thin metallic cantilevers joined at their free ends is able to sense the power of an incoming terahertz (THz) field. An array of such microstructures, with linearly varying distances between the two parallel cantilevers, can measure simultaneously the power and frequency of the THz field in the bandwidth 1.2–58 THz. © 2001 American Institute of Physics. View full abstract»

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  • Optical limiting behavior of a glass–ceramic containing sodium niobate crystallites

    Page(s): 584 - 586
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    The nonlinear absorption properties of a glass–ceramic containing ferroelectric sodium niobate (NaNbO3) crystallites were investigated. Two- and three-photon absorption coefficients were determined using nanosecond pulsed lasers. The optical power-limiting performance for light beams at 1064 and 532 nm was dependent on the volume fraction of NaNbO3 crystallites formed during the heat-treatment process. © 2001 American Institute of Physics. View full abstract»

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  • Traveling wave electro-optic phase modulators based on intrinsically polar self-assembled chromophoric superlattices

    Page(s): 587 - 589
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    Traveling-wave electro-optic modulators based on chromophoric self-assembled superlattices (SASs) possessing intrinsically polar microstructures have been designed and fabricated. Although the thickness of the SAS layer is only ∼150 nm, a π-phase shift is clearly observed. From the measured Vπ value, the effective electro-optic coefficient of the SAS film is determined to be ∼21.8 pm/V at an input wavelength of 1064 nm. © 2001 American Institute of Physics. View full abstract»

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  • Laser emission under resonant pump in the emitting level of concentrated Nd:YAG ceramics

    Page(s): 590 - 592
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    The possibility of using the thermally activated optical absorption bands for a resonant pump in the emitting level of highly doped Nd laser materials is discussed. This potential is demonstrated by the continuous wave 1064 nm laser emission under 885 nm pump in concentrated (up to 6.8 at. %) Nd:YAG ceramics. © 2001 American Institute of Physics. View full abstract»

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  • Maskless etching of silicon using patterned microdischarges

    Page(s): 593 - 595
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    Microdischarges in flexible copper-polyimide structures with hole diameters of 200 μm have been used as stencil masks to pattern bare silicon in CF4/Ar chemistry. The discharges were operated at 20 Torr using the substrate as the cathode, achieving etch rates greater than 7 μm/min. Optical emission spectroscopy provides evidence of excited fluorine atoms. The etch profiles show a peculiar shape attributed to plasma expansion into the etched void. Forming discharges in multiple hole and line shapes permits direct pattern transfer in silicon and could be an alternative to ultrasonic milling and laser drilling. © 2001 American Institute of Physics. View full abstract»

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  • Strong optical transitions via surface states on Si(001)2×1:H ultrathin films: A theoretical study

    Page(s): 596 - 598
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    Optical transitions via localized surface states on Si(001) ultrathin films terminated by monohydride dimers on both surfaces with 2×1 periodicity are studied by use of the extended Hückel-type nonorthogonal tight-binding method. The calculated oscillator strength between the bottom of the empty surface states and the valence-band maximum is much larger than that for direct band-to-band transitions in films with an ideal dihydride termination on both surfaces. An analysis shows that this strong optical coupling is attributed to a significant s-like character introduced into the surface-state bottom by the formation of the monohydride dimers on the film surface. © 2001 American Institute of Physics. View full abstract»

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  • Temperature dependence of photoluminescence of InGaN films containing In-rich quantum dots

    Page(s): 599 - 601
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    The temperature dependence of the photoluminescence (PL) of InGaN films, grown by metalorganic chemical vapor deposition, has been investigated. A strained InGaN thin film which contains composition-fluctuated regions shows the so-called S-shaped temperature dependence of the dominant PL peak energy. However, an InGaN thick film which contains quantum dot-like In-rich regions shows a sigmoidal temperature dependence of the dominant PL peak energy, as the result of a transfer of carriers from the band-edge related luminescent centers to quantum dot-like In-rich regions. It is also found that the activation energy for the thermal quenching of PL intensity in the InGaN thick film which contains quantum dot-like In-rich regions is larger than that in the strained InGaN thin film which contains composition-fluctuated regions. © 2001 American Institute of Physics. View full abstract»

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  • Implantation-produced structural damage in InxGa1-xN

    Page(s): 602 - 604
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    The influence of In content on the accumulation of structural damage in InxGa1-xN films (with x=0.0–0.2) under heavy-ion bombardment is studied by a combination of Rutherford backscattering/channeling spectrometry and transmission electron microscopy. Results show that an increase in In concentration strongly suppresses dynamic annealing processes and, hence, enhances the buildup of stable lattice disorder in InGaN under ion bombardment, A comparison of the damage buildup behavior and defect microstructure in InGaN with those in GaN is presented. Results of this study may have significant technological implications for estimation and control of implantation-produced damage in InGaN/GaN heterostructures. © 2001 American Institute of Physics. View full abstract»

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  • Electron-stimulated surface stress relaxation of Si

    Page(s): 605 - 607
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    We have observed the nonthermal relaxation of surface stress in Si induced by electron irradiation at room temperature. An atomically thin disordered layer was introduced by Ar ion bombardment. The surface stress change during ion bombardment and the following electron irradiation of Si(100) was measured by means of an optical microcantilever technique. We have found that the compressive stress in the Si surface due to disorder induced by ion bombardment was completely relaxed by electron irradiation at low energy. The criterion for complete relaxation is found not to be total energy deposition, but the number of irradiated electrons. © 2001 American Institute of Physics. View full abstract»

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  • Static analysis of off-axis crystal film growth onto a lattice-mismatched substrate

    Page(s): 608 - 610
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    The interface construction of tilt growth that is sometimes observed for epilayer growth on a lattice mismatched substrate is modeled on the basis that tilt relieves misfit strain in the epilayer. In this model off-axis misfit accommodation is assumed to be due to only tilt dislocations or tilt dislocations combined with misfit dislocations. The average interval between successive dislocations which are formed along the interface can be estimated using the lattice units of both materials and that of the epilayer in another principal axis direction; the tilt angle can also be calculated geometrically. The tilt angle predicted by this model agrees well with experimental results for several examples of mismatched epilayer growth by molecular-beam epitaxy. The model suggests a method to grow a single domain of stress-free epilayer by using a substrate cut to an angle that allows off-axis fit to the lattice unit of the epilayer. © 2001 American Institute of Physics. View full abstract»

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  • Deformation behavior and plastic instabilities of ultrafine-grained titanium

    Page(s): 611 - 613
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    Ultrafine-grained (UFG) Ti samples have been prepared using equal channel angular pressing followed by cold rolling and annealing. The deformation behavior of these materials, including strain hardening, strain rate dependence of flow stress, deformation/failure mode, and tensile necking instability, have been systematically characterized. The findings are compared with those for conventional coarse-grained Ti and used to explain the limited tensile ductility observed so far for UFG or nanocrystalline metals. © 2001 American Institute of Physics. View full abstract»

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  • Epitaxial nanocrystalline tin dioxide thin films grown on (0001) sapphire by femtosecond pulsed laser deposition

    Page(s): 614 - 616
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    Nanocrystalline tin dioxide (SnO2) thin films of different thicknesses were fabricated on the (0001) surface of α-Al2O3 (sapphire) using femtosecond pulsed laser deposition. X-ray diffraction and transmission electron microscopy (TEM) analysis revealed that the microstructure of the films strongly depends on the film thickness. The films with a small thickness (≪30 nm) are composed of nanosized columnar (100) oriented grains (3–5 nm in diameter) which grow epitaxially on the substrate with three different in-plane grain orientations. The (101) oriented grains (25 nm in diameter) appear when the film thickness becomes larger than a critical value (about 60 nm). The volume fraction of the (101) grains increases with film thickness. Cross-section TEM studies indicated that the (101) oriented grains nucleate on the top of the (100) oriented nanosized grains and show abnormal grain growth driven by surface energy minimization. As a result, the electrical transport properties are strongly dependent on the film thickness. © 2001 American Institute of Physics. View full abstract»

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  • Self-limiting atomic-layer deposition of Si on SiO2 by alternate supply of Si2H6 and SiCl4

    Page(s): 617 - 619
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    Atomic-layer deposition of Si on SiO2 with a self-limiting growth mode was achieved at substrate temperatures between 355 and 385 °C by means of alternate supply of Si2H6 and SiCl4 gas sources. The growth rate was saturated at 2 ML per cycle at these temperatures and for Si2H6 exposure time over 120 s. The smooth surface (∼0.26 nm in arithmetic average roughness) was obtained under the self-limiting condition irrespective of a film thickness up to 6.5 nm. © 2001 American Institute of Physics. View full abstract»

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  • Sputtered silver oxide layers for surface-enhanced Raman spectroscopy

    Page(s): 620 - 622
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    We present results of reactively sputtered silver oxide thin films as a substrate material for surface-enhanced Raman spectroscopy (SERS). Herein, we show that deposited layers develop an increasingly strong SERS activity upon photoactivation at 488 nm. A benzoic acid/2-propanol solution was used to demonstrate that the bonding of molecules to SERS active sites at the surface can be followed by investigating temporal changes of the corresponding Raman intensities. Furthermore, the laser-induced structural changes in the silver oxide layers lead to a fluctuating SERS activity at high laser intensities which also affects the spectral features of amorphous carbon impurities. © 2001 American Institute of Physics. View full abstract»

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  • Experimental identification of nitrogen-vacancy complexes in nitrogen implanted silicon

    Page(s): 623 - 625
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    Nitrogen implantation is commonly used in multigate oxide thickness processing for mixed signal complementary metal-oxide-semiconductor and System on a Chip technologies. Current experiments and diffusion models indicate that upon annealing, implanted nitrogen diffuses towards the surface. The mechanism proposed for nitrogen diffusion is the formation of nitrogen-vacancy complexes in silicon, as indicated by ab initio studies by J. S. Nelson, P. A. Schultz, and A. F. Wright [Appl. Phys. Lett. 73, 247 (1998)]. However, to date, there does not exist any experimental evidence of nitrogen-vacancy formation in silicon. This letter provides experimental evidence through positron annihilation spectroscopy that nitrogen-vacancy complexes indeed form in nitrogen implanted silicon, and compares the experimental results to the ab initio studies, providing qualitative support for the same. © 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