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Applied Physics Letters

Issue 21 • Date May 2004

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Displaying Results 1 - 25 of 71
  • Issue Cover

    Page(s): c1
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  • Issue Table of Contents

    Page(s): toc1
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  • Visible quantum cutting through downconversion in Eu3+-doped KGd3F10 and KGd2F7 crystals

    Page(s): 4141 - 4143
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    Visible quantum cutting (visible two-photon luminescence) through downconversion is observed for the Gd3+Eu3+ couple in Eu3+-doped KGd3F10 (KG3F) and KGd2F7 (KG2F). The dependence of the efficiency of cross-relaxation required for visible quantum cutting on the Eu3+-doping concentration and crystal structure is discussed in terms of the probability of energy transfer as a function of the rare-earth interatomic distance. Eu3+-doped KG3F exhibits a higher efficiency in the cross-relaxation step than Eu3+-doped KG2F. The highest efficiency of the cross-relaxation step, 0.65, is obtained for the KG3F crystal doped with 2 at. % Eu3+. It is also suggested from dependence of the lifetime of 5D0 emission of Eu3+ in KG3F on Eu3+ concentration that interactions between rare-earth ions are strong at greater than 2 at. % Eu3+. © 2004 American Institute of Physics. View full abstract»

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  • Vacuum response and gas leak detection in piezoelectrically driven sound-resonance cavity

    Page(s): 4144 - 4146
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    It has been found that a piezoelectrically driven sound-resonance cavity (PSRC) is quite sensitive to low-pressure changes: a change from ambient to vacuum (67 Pa) conditions results in a 89° phase shift and a -81% voltage decrease. Under vacuum conditions, the PSRC is highly sensitive (∼7 ppm) to leakage of H2 gas. © 2004 American Institute of Physics. View full abstract»

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  • Nonlinear microwave properties of ferroelectric thin films

    Page(s): 4147 - 4149
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    The nonlinear microwave properties of ferroelectric SrTiO3 thin films are characterized via complex analysis of the intermodulation distortion (IMD) signals up to high microwave power. The measurements reveal an unusual dependence of the IMD signals on the input power, which indicates the presence of two different nonlinear properties being responsible for the generation of IMD, i.e., the nonlinear permittivity and conductivity at low and high rf power levels, respectively. The IMD signal strength cannot be explained in terms of the classical description based on a Taylor expansion of the nonlinear parameter. In contrast, simulations of the frequency spectra using more appropriate descriptions of two nonlinear parameters yield an excellent agreement between theory and experiment, and, thus, demonstrate that the IMD experiments together with the simulation might provide further insight into the mechanism of nonlinear behavior of these materials. © 2004 American Institute of Physics. View full abstract»

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  • Investigation of forward and reverse current conduction in GaN films by conductive atomic force microscopy

    Page(s): 4150 - 4152
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    We have used conductive atomic force microscopy (C–AFM) to investigate the forward and reverse bias current conduction of homo- and heteroepitaxial GaN-based films grown by molecular beam epitaxy. In the case of homoepitaxy, C–AFM shows enhanced current conduction at the centers of ∼30% of spiral hillocks, which are associated with screw dislocations. Local current–voltage spectra taken by C–AFM on and off such hillocks indicate Frenkel–Poole and field emission mechanisms, respectively, for low current levels in forward conduction. In the case of heteroepitaxial GaN films grown on sapphire, the correlation between conduction pathways and topography is more complex. We do observe, however, that films with more rectifying nominal Schottky behavior (less reverse leakage current) produce forward and reverse bias C–AFM images with strong asymmetry. © 2004 American Institute of Physics. View full abstract»

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  • Conformal sensor skin approach to the safety-monitoring of H2 fuel tanks

    Page(s): 4153 - 4154
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    A conformal sensor skin approach has been developed for safety monitoring of H2 fuel tanks. Small piezoelectrically driven sound resonance cavities were embedded in a porous polymer. When placed on a structural composite plate, it was found feasible to detect the leakage of small concentrations of H2 in real time. © 2004 American Institute of Physics. View full abstract»

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  • Electrically injected InGaAs/GaAs quantum-dot microcavity light-emitting diode operating at 1.3 μm and grown by metalorganic chemical vapor deposition

    Page(s): 4155 - 4157
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    We present a quantum-dot microcavity light-emitting diode emitting at 1.3 μm at room temperature. The long wavelength emission is achieved by using InGaAs quantum dots directly grown on GaAs, by metalorganic chemical vapor deposition. The device exhibits electroluminescence bright emission, peaked at 1298 nm and with a full width at half maximum of 6.5 meV. © 2004 American Institute of Physics. View full abstract»

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  • Unpinning of Fermi level in nanocrystalline semiconductors

    Page(s): 4158 - 4160
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    A theoretical model has been developed to interpret the size dependent behavior of nanostructured metal-oxide semiconductors. It is based on the determination of the surface-state density, which pins the Fermi level of the semiconductor, thus removing the linear relationship between the work function and the Schottky barrier. To provide the model with numerical input, measurements of the Schottky barrier height were performed at different temperatures on nanocrystalline SnO2 and TiO2 films. The obtained solution predicts the unpinning of the Fermi level when the material can be considered as nanostructured, and the evidence confirms it. © 2004 American Institute of Physics. View full abstract»

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  • Reduced dielectric dispersion in ferroelectric (Pb,La)TiO3/(Pb,Ca)TiO3 thin-film multilayer heterostructures due to a mechanical stress relaxation mechanism

    Page(s): 4161 - 4163
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    The dielectric dispersion of (Pb,Ca)TiO3/(Pb,La)TiO3/(Pb,Ca)TiO3 and (Pb,La)TiO3/(Pb,Ca)TiO3/(Pb,La)TiO3 ferroelectric thin-film multilayer heterostructures onto Si-based substrates has been studied and compared with that of identically prepared (Pb,La)TiO3 and (Pb,Ca)TiO3 films. Grazing incidence x-ray diffraction analysis reveals that the tetragonal distortion of the heterostructures is higher than the ones for the single-component films, which evidences a mechanical stress relaxation mechanism in the heterostructures. A lower dielectric dispersion has been obtained in the heterostructures with respect to the single-component films. This is also a consequence of the stress relaxation by plastic deformation through vacancy diffusion present in the former. Vacancy diffusion decreases the number of VPb–VO defect-dipoles, which could contribute to the dielectric permittivity at low frequencies in the heterostructures. © 2004 American Institute of Physics. View full abstract»

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  • Lasing mode pattern of a quantum cascade photonic crystal surface-emitting microcavity laser

    Page(s): 4164 - 4166
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    The identification of the lasing mode within a quantum cascade photonic crystal microcavity laser emitting at λ∼8 μm is presented. The symmetry of the lasing mode is determined by the position of nodal lines within micro-bolometer camera measurements of its polarized spatial distribution. Full three-dimensional finite-difference time-domain simulations are also performed, and the resulting vertically emitted radiation field pattern is seen to follow the experimental results closely. © 2004 American Institute of Physics. View full abstract»

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  • Control of surface roughness in amorphous thin-film growth

    Page(s): 4167 - 4169
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    The growth of optical layers of SiO2 and Nb2O5 on amorphous substrates is investigated. We develop a kinetic Monte Carlo model which mimics the amorphous structure by randomly fluctuating binding energies. The resulting surface profiles are characterized by their root-mean-square roughness, height–height correlation functions, and growth exponents. For strong random fluctuations, the growth exponents exceed the value of 0.5, in good agreement with experiment. © 2004 American Institute of Physics. View full abstract»

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  • First-principles study of magnetism in (112¯0) Zn1-xMnxO thin film

    Page(s): 4170 - 4172
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    First-principles calculations of total energies and magnetism of Zn1-xMnxO thin film are performed by simulating it with a slab consisting of seven layers along (112¯0). It is shown that a single Mn atom shows very little preference for the site it occupies. This is consistent with the experimental finding that Mn atoms are homogeneously distributed in ZnO films. As the concentration of Mn atoms increases, antiferromagnetic coupling between Mn atoms becomes more favorable, and there is a tendency for Mn atoms to form clusters around oxygen, in agreement with recent experiments. © 2004 American Institute of Physics. View full abstract»

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  • Role of intericosahedral chains on the hardness of sputtered boron carbide films

    Page(s): 4173 - 4175
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    The relationship between the structure and mechanical properties of sputter-deposited boron carbide films was investigated. Changes in the structure induced by annealing were characterized in terms of chemical composition, chemical bonding, and concentrations of defects and trapped impurities. The creation of intericosahedral chains for higher annealing temperatures was revealed by infrared and Raman measurements, and the intensity of the infrared band at 1500 cm-1 was found to be related to the hardness. The presence of residual trapped Ar atoms and of open-volume defects is insensitive to relatively high annealing temperatures and does not influence the recovery of the hardness. Our results suggest postdeposition annealing as a pathway to enhance the mechanical properties of boron carbide films. © 2004 American Institute of Physics. View full abstract»

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  • Growth and transport properties of complementary germanium nanowire field-effect transistors

    Page(s): 4176 - 4178
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    n- and p-type Ge nanowires were synthesized by a multistep process in which axial elongation, via vapor–liquid–solid (VLS) growth, and doping were accomplished in separate chemical vapor deposition steps. Intrinsic, single-crystal, Ge nanowires prepared by Au nanocluster-mediated VLS growth were surface-doped in situ using diborane or phosphine, and then radial growth of an epitaxial Ge shell was used to cap the dopant layer. Field-effect transistors prepared from these Ge nanowires exhibited on currents and transconductances up to 850 μA/μm and 4.9 μA/V, respectively, with device yields of ≫85%. © 2004 American Institute of Physics. View full abstract»

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  • Cold electron Josephson transistor

    Page(s): 4179 - 4181
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    A superconductor-normal metal-superconductor mesoscopic Josephson junction has been realized in which the critical current is tuned through normal current injection using a symmetric electron cooler directly connected to the weak link. Both enhancement of the critical current by more than a factor of two, and supercurrent suppression have been achieved by varying the cooler bias. Furthermore, this transistor-like device demonstrates large current gain (∼20) and low power dissipation. © 2004 American Institute of Physics. View full abstract»

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  • Brillouin observation of bulk and confined acoustic waves in silica microspheres

    Page(s): 4182 - 4184
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    Brillouin scattering from loose silica microspheres has been investigated as a function of their diameter (140 nm to 4 μm). The measured linear dependence of the confined acoustic mode frequencies on the inverse sphere diameters accords well with Lamb’s theory. Bulk acoustic modes are also observed in the larger microspheres, suggesting that the limiting size of a particle for these modes to be observable is about three times their wavelength. Internal consistency in the analyses of the bulk and confined mode data shows the validity of this Brillouin technique in the evaluation of the elastic properties of microspheres. Additionally it also affords a means of sizing the diameters of microspheres. © 2004 American Institute of Physics. View full abstract»

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  • High-speed 1.55 μm operation of low-temperature-grown GaAs-based resonant-cavity-enhanced p–i–n photodiodes

    Page(s): 4185 - 4187
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    We report the design, growth, fabrication, and characterization of GaAs-based high-speed p–i–n photodiodes operating at 1.55 μm. A low-temperature-grown GaAs (LT-GaAs) layer was used as the absorption layer and the photoresponse was selectively enhanced at 1.55 μm using a resonant-cavity-detector structure. The bottom mirror of the resonant cavity was formed by a highly reflecting 15-pair GaAs/AlAs Bragg mirror. Molecular-beam epitaxy was used for wafer growth, where the active LT-GaAs layer was grown at a substrate temperature of 200 °C. The fabricated devices exhibited a resonance around 1548 nm. When compared to the efficiency of a conventional single-pass detector, an enhancement factor of 7.5 was achieved. Temporal pulse-response measurements were carried out at 1.55 μm. Fast pulse responses with 30 ps pulse-width and a corresponding 3 dB bandwidth of 11.2 GHz was measured. © 2004 American Institute of Physics. View full abstract»

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  • Voltage gain effect in a ring-type magnetoelectric laminate

    Page(s): 4188 - 4190
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    It has been observed that a ring-type magnetoelectric laminate composite of circumferentially magnetized magnetostrictive Tb1-xDyxFe2 and circumferentially polarized piezoelectric Pb(Zr,Ti)O3 layers has a large magnetoelectric voltage gain effect, offering potential in high-power miniature transformer applications. © 2004 American Institute of Physics. View full abstract»

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  • Observation of photoluminescence emission in ferromagnetic semiconductor GaCrN

    Page(s): 4191 - 4193
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    Optical properties of GaN-based diluted magnetic semiconductor GaCrN were studied. The GaCrN layers were grown by electron-cyclotron-resonance plasma-assisted molecular-beam epitaxy. They exhibited ferromagnetic behavior at room temperature and strong photoluminescence (PL) emission at 3.29 eV (10 K). The PL emission peak energy of the GaCrN decreases with increasing temperature in accordance with the Varshini formula similar to the GaN excitonic transition peak. The 3.29 eV PL emission was assigned to be a band-to-band transition in GaCrN from the temperature- and the excitation-power-density dependences of the PL. © 2004 American Institute of Physics. View full abstract»

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  • Fluidic adaptive lens of transformable lens type

    Page(s): 4194 - 4196
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    Fluidic adaptive lenses with a transformable lens type were demonstrated. By adjusting the fluidic pressure, not only can the lens properties, such as the focal distance and numerical aperture, be tuned dynamically but also different lens types, such as planoconvex, planoconcave, biconvex, biconcave, positive meniscus, and negative meniscus lenses, can be formed. The shortest focal length for a 20 mm aperture adaptive lens is 14.3 mm when the device is transformed into a positive lens, and -6.3 mm when transformed into a negative lens. The maximum resolution of the fluidic lens is better than 40 line pairs/mm. © 2004 American Institute of Physics. View full abstract»

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  • Magneto-optical Kerr effect in Fe21Ni79 films on Si(100): Quantum behavior for film thicknesses below ∼6 nm

    Page(s): 4197 - 4199
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    The magneto-optical Kerr effect (MOKE) has been observed and characterized in 1–80 nm thick Fe21Ni79 films deposited onto Si(100), for an external magnetic field (variable in strength up to 400 G) oriented parallel or orthogonal to the magnetization axis of the film. A measurable response is observed for film thicknesses (d) as small as 2 nm and, if the external magnetic field lies in the plane of the film, two-dimensional quantum behavior is evident for d>~6 nm. A precipitous decline in the magnitude of the MOKE response is accompanied by an increase in the coercivity and, when the external field is perpendicular to the film magnetization axis, a rapid rise in the saturation field. Experiments also confirm the existence of a component of the film magnetization oriented out of the plane of the film, a result consistent with the prediction of computational studies [T. Trunk etal, J. Appl. Phys. 89, 7606 (2001)] that the transition between Bloch and Néel wall domain structure occurs in FeNi films for film thicknesses of ≈30 nm. © 2004 American Institute of Physics. View full abstract»

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  • Dip-pen nanopatterning of photosensitive conducting polymer using a monomer ink

    Page(s): 4200 - 4202
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    Controlled patterning of conducting polymers at a micro- or nanoscale is the first step towards the fabrication of miniaturized functional devices. Here, we introduce an approach for the nanopatterning of conducting polymers using an improved monomer “ink” in dip-pen nanolithography (DPN). The nominal monomer “ink” is converted, in situ, to its conducting solid-state polymeric form after patterned. Proof-of-concept experiments have been performed with acid-promoted polymerization of pyrrole in a less reactive environment (tetrahydrofuran). The ratios of reactants are optimized to give an appropriate rate to match the operation of DPN. A similar synthesis process for the same polymer in its bulk form shows a high conductance and crystalline structure. The miniaturized conducting polymer sensors with light detection ability are fabricated by DPN using the improved ink formula, and exhibit excellent response, recovery, and sensitivity parameters. © 2004 American Institute of Physics. View full abstract»

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  • Effects of finite deformed length in carbon nanotubes

    Page(s): 4203 - 4205
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    The effect of finite deformed length is demonstrated by squashing an armchair (10,10) single-walled carbon nanotube with two finite tips. Only when the deformed length is long enough, an effectual metal–semiconductor–metal heterojunction can be formed in the metallic tube. The effect of finite deformed length is explained by the quantum tunneling effect. Furthermore, some conceptual designs of nanoscale devices are proposed from the metal–semiconductor–metal heterojunction. © 2004 American Institute of Physics. View full abstract»

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  • Characterization and reduction of unexplained noise in superconducting transition-edge sensors

    Page(s): 4206 - 4208
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    The noise in superconducting transition-edge sensors (TESs) commonly exceeds simple theoretical predictions. The reason for this discrepancy is presently unexplained. We have measured the amplitude and frequency dependence of the noise in TES sensors with eight different geometries. In addition, we have measured the dependence of the noise on operating resistance, perpendicular magnetic field, and bath temperature. We find that the unexplained noise contribution is inversely correlated with the temperature width of the superconducting-to-normal transition and is reduced by a perpendicular field and in certain geometries. These results suggest paths to improved sensor performance. © 2004 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