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Applied Physics Letters

Issue 17 • Date Oct 2005

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Displaying Results 1 - 25 of 90
  • Issue Cover

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

    Page(s): toc1
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  • Efficiency limits of photovoltaic fluorescent collectors

    Page(s): 171101 - 171101-3
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    This paper examines the thermodynamic limits of photovoltaic solar energy conversion by fluorescent collectors. The maximum efficiency of a fluorescent collector corresponds to the Shockley–Queisser limit for a nonconcentrating solar cell with a single bandgap energy. To achieve this efficiency, the collector requires a photonic structure at its surface that acts as an omnidirectional spectral band stop filter. The large potential of photonic structures for the efficiency enhancement of idealized as well as real fluorescent collectors is highlighted. View full abstract»

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  • Ultraslow light (≪200 m/s) propagation in a semiconductor nanostructure

    Page(s): 171102 - 171102-3
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    We report time-domain measurements of ultraslow light propagation in a semiconductor quantum-well structure using coherent population oscillation. Delays greater than 1 ns are achieved for an amplitude-modulated optical beam propagating through a 195-nm-long active region, corresponding to group velocities less than 200 m/s. Delays can be easily varied by adjusting the intensity of the control laser. The bandwidth is suitable to delay sub-GHz modulated optical signals. View full abstract»

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  • Self-fabrication of void array in fused silica by femtosecond laser processing

    Page(s): 171103 - 171103-3
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    We demonstrate self-fabrication of a submicrometer-sized void array in fused silica using a 100 fs 0.2–3 μJ Ti:Sapphire femtosecond laser and a high 0.9 numerical aperture (NA) objective lens. The effect of the focusing conditions of NA, laser energy, and pulse number on the shape of the fabricated void was investigated. The void has a linearly drawn shape in the direction of the laser irradiation when a single pulse is irradiated and an increasing number of incident pulses resulted in the break up of the long void into multiple spherical ones, leading to a periodically aligned void array. The void shape also varied with the depth of the focus point beneath the fused silica surface, because the amount of self-focusing has a significant effect on the generation of the voids. The void shape was narrower and longer when the laser pulse was focused with the higher NA (up to 0.9) objective lens in the deeper position (up to 70 μm) in the fused silica. View full abstract»

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  • Adiabatic matching stage for coupling of light to extended Bloch modes of photonic crystals

    Page(s): 171104 - 171104-3
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    In this letter, we present a matching stage for reflection reduction based on the principle of gradual change to efficiently couple light to propagating modes of photonic crystals (PCs). Basic physical considerations in designing these matching stages are investigated and a systematic yet simple design procedure is suggested. We show that matching stages obtained using this method are wideband in frequency, have a wide acceptance angle, and are robust against fabrication imperfections. Therefore, they are the preferred choice in general-purpose matching stages to be used along with dispersion-based PC devices. View full abstract»

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  • High-performance photorefractive polymer operating at 1550 nm with near-video-rate response time

    Page(s): 171105 - 171105-3
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    The development of a high-performance photorefractive polymer composite operating at 1550 nm is reported. We show 40% internal diffraction efficiency with response time of 35 ms and a net gain of 20 cm-1 in four-wave mixing and two-beam coupling experiments, respectively. This is more than an order of magnitude improvement in the diffraction efficiency and net two beam coupling gain and two orders of magnitude in the response time than the previously reported photorefractive polymer operating at this technologically important wavelength. The improvement in photorefractive characteristics is accomplished by an enhanced orientation of the nonlinear optical chromophore in the present composite. View full abstract»

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  • Experimental demonstration of Fano-type resonance in photoluminescence of ZnS:Mn/SiO2 one-dimensional photonic crystals

    Page(s): 171106 - 171106-3
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    We investigated the photoluminescence (PL) properties of ZnS:Mn/SiO2 one-dimensional photonic-crystal structures. PL spectra were measured from two opposite directions perpendicular to the sample. The cavity mode emission measured from the sample surface showed asymmetric spectral shape; measurements from the back side of the sample showed a symmetric spectral shape. The experimental spectra were analyzed by a simple model calculation based on the transfer matrix method. From the model calculation, it was found that the asymmetric shape observed in cavity mode emission is caused by Fano-type resonance which is the coupling effect between discrete emission from the ZnS:Mn cavity layer and continuous background emission from all ZnS:Mn layers except the cavity layer. View full abstract»

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  • III-nitride integration on ferroelectric materials of lithium niobate by molecular beam epitaxy

    Page(s): 171107 - 171107-3
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    Integration of III-nitride electrical devices on the ferroelectric material lithium niobate (LiNbO3) has been demonstrated. As a ferroelectric material, lithium niobate has a polarization which may provide excellent control of the polarity of III-nitrides. However, while high temperature, 1000 °C, thermal treatments produce atomically smooth surfaces, improving adhesion of GaN epitaxial layers on lithium niobate, repolarization of the substrate in local domains occurs. These effects result in multi domains of mixed polarization in LiNbO3, producing inversion domains in subsequent GaN epilayers. However, it is found that AlN buffer layers suppress inversion domains of III-nitrides. Therefore, two-dimensional electron gases in AlGaN/GaN heterojunction structures are obtained. Herein, the demonstration of the monolithic integration of high power devices with ferroelectric materials presents possibilities to control LiNbO3 modulators on compact optoelectronic/electronic chips. View full abstract»

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  • Second-harmonic performance of a-axis-oriented ZnO nanolayers on sapphire substrates

    Page(s): 171108 - 171108-3
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    We report on the nonlinear optical performance of sub-μm ZnO films grown by metal organic aerosol deposition on r-plane sapphire substrates. These films display scale-like nanocrystalline structures. Layers of different crystallite sizes and shapes are studied. Both, x-ray diffractometry and the characteristic angular and polarization dependence of the second harmonic generation, indicate a strongly uniform a-axis orientation of the crystallites. Using 35-fs Ti:sapphire laser pulses, we demonstrate much higher conversion efficiencies for ZnO layers than previously reported. The robust performance at normal incidence makes this device suitable for advanced pulse characterization techniques. View full abstract»

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  • Decomposition in as-grown (Ga,In)(N,As) quantum wells

    Page(s): 171901 - 171901-3
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    We report on the investigation of the local element distribution in as-grown (Ga,In)(N,As) quantum wells with high In and N contents by using low-loss electron energy-loss spectroscopy combined with dark-field transmission electron microscopy. The (Ga,In)(N,As) quantum wells were grown on GaAs(001) substrates at different growth temperatures by molecular-beam epitaxy. Lateral modulations on the nanometer scale were detected with reversal In and N distributions pointing to the existence of regions with a more favorable Ga–N and In–As bond configurations, respectively. These composition fluctuations are the driving force for the morphological instabilities at the interfaces. Lowering the growth temperature of the quantum well results in a more homogeneous element distribution of the quaternary compound. This result is discussed with regard to the influence of the epitaxial strain and cohesive bond energy on the alloy formation during epitaxial growth. View full abstract»

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  • Patterning of sub-10-nm Ge islands on Si(100) by directed self-assembly

    Page(s): 171902 - 171902-3
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    A process is reported for creating arbitrary patterns of sub-10-nm Ge islands on a Si(100) substrate by directed self-assembly. Carbon-based templates are created on Si substrates by electron-beam-induced deposition using high-resolution electron beam lithography. Ozone etching, followed by annealing in ultra-high vacuum, yields small (≪4 nm) SiC nucleation sites for subsequently deposited Ge. Quantitative analysis of atomic force microscope images reveals templated Ge islands with mean diameter d∼8 nm, averaging 2000±500 atoms per island, with controlled spacings as small as 35 nm, and 2 nm absolute positional accuracy. The Ge/Si nanostructures reported here may find use in end-of-scaling classical computing and single-electron devices and spin-based quantum computing architectures. View full abstract»

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  • Macroscopic thermoelectric inhomogeneities in (AgSbTe2)x(PbTe)1-x

    Page(s): 171903 - 171903-3
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    Exceptionally high thermoelectric figure of merit (zT≫2), has been reported for (Ag1-ySbTe2)0.05(PbTe)0.95, which may involve the nanoscale microstructure. However, conflicting reports on the same materials claim only zT of 1 or less. Here we show that (Ag1-ySbTe2)0.05(PbTe)0.95 materials are multiphase on the scale of millimeters despite appearing homogeneous by x-ray diffraction and routine electron microscopy. Using a scanning Seebeck microprobe, we find significant variation of Seebeck coefficient (including both n-type and p-type behavior in the same sample) that can explain the discrepancy in reported zT. More homogeneous samples can be prepared with faster cooling rates. View full abstract»

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  • Dielectric mismatch effects on the electronic and optical properties of GaN/HfO2 quantum wells

    Page(s): 171904 - 171904-3
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    In this work, we demonstrate that the image charges effect induced by the dielectric mismatch strongly modifies the electronic structure of GaN/HfO2 quantum wells (QWs) in such a way that the ideal QW confinement model is no longer suitable for the description of these systems. Particularly, two different confinement regimes were observed for narrow and wide QWs. In the former, electrons, light and heavy holes are spatially localized in the same region. In wide QWs, heavy holes are confined in the interfacial regions due to the strong attraction of the image charges, which does not occur for electrons and light holes. As a consequence, optical transitions involving electrons and heavy holes become less efficient in wide QWs. View full abstract»

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  • Strong anisotropic spin dynamics in narrow n-InGaAs/AlGaAs (110) quantum wells

    Page(s): 171905 - 171905-3
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    Anisotropic spin dynamics of two-dimensional electrons in strained n-InGaAs/AlGaAs (110) quantum wells (QWs) is investigated by a time-resolved Faraday rotation technique. Strong anisotropy of the relaxation time for the electron spins in parallel ||) and perpendicular ) to the QWs is observed ||∼60) at 150 K as a result of the enhanced D'yakonov–Perel' (DP) spin relaxation mechanism. At 5 K, an anisotropic feature of the spin relaxation time is also observed in the presence of in-plane magnetic field, suggesting that the DP mechanism is effective for low-temperature spin relaxation. View full abstract»

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  • Chemical passivity of III-VI bilayer terminated Si(111)

    Page(s): 171906 - 171906-3
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    The chemical stability of Si(111), terminated with bilayer AlSe and GaSe, upon exposure to atmosphere, N2 and O2, was investigated with core-level and valence band photoelectron spectroscopy. Si(111):GaSe and Si(111):AlSe both form stable, unreconstructed surfaces with no states in the silicon energy gap; their atomic structures are nearly identical. However, similarities in surface electronic and atomic structure do not imply similar chemical passivity. While Si(111):GaSe is largely unaffected by the exposures, Si(111):AlSe reacts irreversibly with both pure O2 and atmosphere, removing over 13 of the Se and permanently destroying long-range order. View full abstract»

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  • Thermally induced second-order nonlinearity in silica-based glasses

    Page(s): 171907 - 171907-3
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    This letter presents the observation of second-harmonic generation in pure silica and germanium-doped glass samples after heat treatment at elevated temperatures followed by rapid quenching to room temperature. No applied voltage has been used and the samples do not contain crystals. The induced nonlinearity is located near the surfaces of the sample. The possible origin of the second-order nonlinearity is discussed. View full abstract»

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  • Work function tuning of nickel silicide by co-sputtering nickel and silicon

    Page(s): 171908 - 171908-3
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    Co-sputtered nickel silicide films were evaluated on thin layers of SiO2 gate dielectrics. Work function values ranging from 4.86 eV for Ni rich films to 4.3 eV were observed at 400 °C and were found to be a strong function of the Ni and Si ratio in the films. Phase analysis indicated the presence of different phases of NixSiy for varying concentrations of Ni and Si. High-temperature characteristics, leakage, and change in equivalent oxide thickness values were also evaluated for selected conditions. Rutherford backscattering, x-ray diffraction, Auger electron spectroscopy and high-resolution transmission electron microscopy were used for material analyses. View full abstract»

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  • Atomically flat aluminum-oxide barrier layers constituting magnetic tunnel junctions observed by in situ scanning tunneling microscopy

    Page(s): 171909 - 171909-3
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    Observation using in situ scanning tunneling microscopy of the layers constituting a magnetic tunnel junction with a naturally oxidized aluminum barrier layer revealed an extremely flat aluminum-oxide surface. It was clarified from line-scan images that the aluminum-oxide barrier layer has atomic steps. This flatness, which is surprising given that the aluminum-oxide film is amorphous, reduced electron scattering within the barrier, leading to momentum-dependent tunneling, which should enable the fabrication of advanced devices, such as spin-polarized resonant tunneling transistors. View full abstract»

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  • Electroluminescence from two fluorinated organic emitters embedded in polyvinylcarbazole

    Page(s): 171910 - 171910-3
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    Electroluminescence and photoluminescence of two fluorinated dyes emitting in the blue and in the green, blended with polyvinylcarbazole and an oxadiazole compound, are reported. Organic light-emitting diodes realized with about 0.1 wt % dye concentration show interesting performances. Excited state complexes reduce the effectiveness of energy transfer in the blue-emitting device while, for the green-emitting device, the main mechanism of exciton generation is based on charge trapping at the emissive dye. View full abstract»

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  • 980 nm laser-diode-excited intense blue upconversion in Tm3+/Yb3+-codoped gallate–bismuth–lead glasses

    Page(s): 171911 - 171911-3
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    Intense blue-upconversion in Tm3+/Yb3+-codoped gallate–bismuth–lead glasses has been achieved under an excitation from a commercially available 980 nm laser diode. Energy transfer processes and excited-state absorption account for the population of the 1G4 emitting level of the Tm3+. Although the addition of GeO2 has enhanced the glass thermal stability, the phonon mode associated with vibration of GeO2 has almost no influence on the blue-upconversion intensity and the radiative lifetime of 3H4 level. The dependence of the phonon energy of the host on contributions from multiphonon decay on the fluorescence has been discussed. Significant enhancement of the blue-upconversion has also been observed in gallate–bismuth–lead glasses with the incorporation of PbF2 content. View full abstract»

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  • Thermoelectric properties and microstructure of c-axis-oriented Ca3Co4O9 thin films on glass substrates

    Page(s): 171912 - 171912-3
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    c-axis-oriented Ca3Co4O9 thin films have been grown directly on glass (fused silica) substrate by pulsed laser deposition. Detailed microstructure analysis showed stacking faults abundant throughout the films. However, the Seebeck coefficient (∼130 μV/K) and resistivity (∼4.3 mΩ cm) of these films on glass substrate at room temperature were found comparable to those of the single-crystal samples. The presence of these structural defects could reduce thermal conductivity, and thus enhance the overall performance of cobaltate films to be potentially used in the thermoelectric devices. View full abstract»

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  • Relaxation of biaxial tensile strain in ultrathin metallic films: Ductile void growth versus nanocrystalline domain formation

    Page(s): 171913 - 171913-3
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    We report a computational analysis of the atomistic mechanisms of relaxation of biaxially applied tensile strains over a range of strain levels up to 17% in free-standing ultrathin metallic films with the film plane oriented normal to the [111] crystallographic direction. The analysis is based on molecular-dynamics simulations using slab supercells that contain millions of atoms to model single-crystalline thin films without and with cylindrical voids oriented normal to the film plane and penetrating through the film thickness. At high levels of applied strain (≫8%), a strain relaxation regime other than the ductile void growth is revealed that gives rise to a practically uniform distribution of dislocations in the film and subsequent formation of nanometer-scale face-centered-cubic crystalline domains, i.e., a single-to-polycrystalline structural transition. It is demonstrated that in this strain relaxation regime, void growth is inhibited as the dislocations emitted from the void surface are pinned by their interaction with the simultaneously generated network of defects in the nanocrystalline material. View full abstract»

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  • Bulkier glass formability enhanced by minor alloying additions

    Page(s): 171914 - 171914-3
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    In this study, we present a computational thermodynamic strategy to obtain a minor but optimum amount of additional element into a base alloy to improve its glass-forming ability, through thermodynamically calculating the maximum liquidus depressions caused by various alloying addition (or replacement) schemes. We demonstrate the successful use of Zr56.2Cu31.3Ni4.0Al8.5 as the base alloy with the addition of 4.9% Ti, by observing a significant increase in the glass-forming ability of more than 100% in terms of the diameter of the glass formed from the base alloy to the one with the addition of 4.9% Ti. The approach presented here can be considered as a universal method to synthesize novel and bulkier metallic glasses not only of scientific interest but also potential technological applications. View full abstract»

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  • Epitaxial growth of multiferroic YMnO3 on GaN

    Page(s): 171915 - 171915-3
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    In this work, we report on the epitaxial growth of multiferroic YMnO3 on GaN. Both materials are hexagonal with a nominal lattice mismatch of 4%, yet x-ray diffraction reveals an unexpected 30° rotation between the unit cells of YMnO3 and GaN that results in a much larger lattice mismatch (10%) compared to the unrotated case. Estimates based on first principles calculations show that the bonding energy gained from the rotated atomic arrangement compensates for the increase in strain energy due to the larger lattice mismatch. Understanding the energy competition between chemical bonding energy and strain energy provides insight into the heteroepitaxial growth mechanisms of complex oxide-semiconductor systems. 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