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Journal of Applied Physics

Issue 12 • Date Jun 2009

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Displaying Results 1 - 25 of 182
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  • Electrons in quantum dots: One by one

    Page(s): 122401 - 122401-4
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    A quantum point contact placed close to a quantum dot can be used as a charge detector with time resolution to monitor the charge flow on the level of individual electrons. The current through the quantum point contact may take two possible values corresponding to the situation of an additional electron being on or off the quantum dot. Time traces of such two-level behavior allow to measure the average current, the tunnel rates in and out of the quantum dot, the time-dependent fluctuations of the current (noise), as well as higher-order current correlations. This high-sensitivity method to measure charge flow can also be used to detect time-resolved single-electron interference. View full abstract»

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  • Electrical manipulation of spins in the Rashba two dimensional electron gas systems

    Page(s): 122402 - 122402-8
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    We present our theoretical and experimental studies on manipulation of electron spins based on the Rashba spin-orbit interaction (SOI) in semiconductor heterostructures. Quantum well (QW) thickness dependence of the Rashba SOI strength α is investigated in InP/InGaAs/InAlAs asymmetric QWs by analyzing weak antilocalization. Two different QW thicknesses show inverse Ns dependence of |α| in the same heterostructures. This inverse Ns dependence of |α| is explained by the kp perturbation theory. We confirm that narrow wires are effective to suppress the spin relaxation. Spin interference effects due to spin precession are experimentally studied in small array of mesoscopic InGaAs rings. This is an experimental demonstration of a time reversal Aharonov–Casher effect, which shows that the spin precession angle in an InGaAs channel can be controlled by an electrostatic gate. View full abstract»

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  • Experimental observation of bulk band dispersions in the oxide semiconductor ZnO using soft x-ray angle-resolved photoemission spectroscopy

    Page(s): 122403 - 122403-4
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    The electronic structure of the oxide semiconductor ZnO has been investigated using soft x-ray angle-resolved photoemission spectroscopy (ARPES). The obtained band dispersions within the kx-ky planes reflect the symmetry of the Brillouin zone and show no surface-state-derived flat bands. Band dispersions along the kz direction have also been observed. The obtained band dispersions qualitatively agree with band-structure calculations except for the bandwidth. The observations provide experimental evidence that soft x-ray ARPES enables us to study the bulk band structure of semiconductors. View full abstract»

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  • Photonic crystal mode terahertz lasers

    Page(s): 122404 - 122404-4
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    We present the design and fabrication of photonic crystal (PhC) based resonators for terahertz quantum-cascade laser with a gain maximum at 2.7 THz. The PhC provides the confinement in lateral direction, for the vertical confinement a double-metal waveguide is used. We show theoretical and experimental analyses of devices based on a central gain region, which is surrounded by a PhC mirror. The devices are lasing in the bandgaps or at high-symmetry points of the PhC, depending on the period. These concepts allow us to tune the emission of the lasers in the range of a few hundred gigahertz. View full abstract»

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  • Evolution and stability of ordered SiGe islands grown on patterned Si(100) substrates

    Page(s): 122405 - 122405-4
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    SiGe quantum dots are proposed as building blocks for future Si device technology. However, in order to exploit the full potential of SiGe islands, it is necessary to control their positioning and size on a nanometer length. This is achieved by templated self-assembly, which combines substrate patterning and subsequent epitaxy. In this paper we report on the evolution of SiGe islands on patterned substrates under consideration of small template variations and postgrowth annealing. The impact of the structural variations on the optical properties of the islands is investigated by photoluminescence measurements. View full abstract»

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  • Multiexciton complexes in InAs self-assembled quantum dots

    Page(s): 122406 - 122406-6
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    We review our recent work on multiexciton complexes in InAs self-assembled quantum dots using a combination of effective mass, k∙p, and atomistic sp3s*d5 tight-binding approaches. The single-particle levels from effective mass, k∙p, and atomistic tight-binding models are used as input into configuration-interaction calculation of multiexciton spectra. We describe the principles of the atomistic approach and apply all these computational tools to illustrate the concept of hidden symmetry as underlying principle in energy levels of multiexciton complexes, optical detection of electron spin polarization, tunneling of holes in quantum dot molecules, and tuning of multiexciton spectra with lateral electric fields for entangled photon pair generation. View full abstract»

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  • Unconventional roles of metal catalysts in chemical-vapor syntheses of single-crystalline nanowires

    Page(s): 122407 - 122407-5
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    In this invited contribution at the 29th International Conference on the Physics of semiconductors (ICPS 2008), we review two examples of solid-catalytic nanowire (NW) growth in parallel comparisons to the NW growth from the eutectic liquid catalyst. First, we demonstrated the Cu-catalyzed Ge NW growth using GeH4 vapor precursor at 200 °C, which is far below the Cu–Ge eutectic temperature of 644 °C, with a relatively uniform diameter distribution directly templated from that of the catalysts. We provide evidence that the formation of solid Cu3Ge catalysts and Ge diffusion across the catalysts are responsible for such low-temperature growth of Ge NWs in a size-deterministic manner. Second, we show the spontaneous silicidation of NiSix NWs on continuous Ni bulks using SiH4 vapor precursor at 400 °C. This growth is particularly marked in that NiSix NWs are formed in a self-organized manner without employing the nanocluster catalysts. We discuss this spontaneous growth of NiSix NWs within the frame of the nucleation kinetics in the low supersaturation limit in analogous with the earlier examples of the vapor-condensation at the low vapor pressures. View full abstract»

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  • Polarization fine structure and enhanced single-photon emission of self-assembled lateral InGaAs quantum dot molecules embedded in a planar microcavity

    Page(s): 122408 - 122408-5
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    Single lateral InGaAs quantum dot molecules have been embedded in a planar microcavity in order to increase the luminescence extraction efficiency. Using a combination of metal-organic vapor phase and molecular beam epitaxy samples could be produced that exhibit a 30 times enhanced single-photon emission rate. We also show that the single-photon emission is fully switchable between two different molecular excitonic recombination energies by applying a lateral electric field. Furthermore, the presence of a polarization fine structure splitting of the molecular neutral excitonic states is reported which leads to two polarization split classically correlated biexciton-exciton cascades. The fine structure splitting is found to be on the order of 10 μeV. View full abstract»

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  • Two-dimensional electron gases: Theory of ultrafast dynamics of electron-phonon interactions in graphene, surfaces, and quantum wells

    Page(s): 122409 - 122409-7
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    Many-particle electron-phonon interaction effects in two-dimensional electron gases are investigated within a Born–Markov approach. We calculate the electron-phonon interaction on a microscopic level to describe relaxation processes of quantum confined electrons on ultrafast time scales. Typical examples, where two-dimensional electron gases play a role, are surfaces and two-dimensional nanostructures such as graphene and quantum wells. In graphene, we find nonequilibrium phonon generation and ultrafast cooling processes after optical excitation. Electron relaxation dynamics at the silicon (001) 2×1 surface exhibits two time scales, corresponding to intrasurface and inside bulk-scattering processes. For GaAs quantum wells, we present broad emission spectra in the terahertz range assisted by LO-phonons of the barrier material. View full abstract»

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  • Quantum control and manipulation of donor electrons in Si-based quantum computing

    Page(s): 122410 - 122410-7
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    Doped Si is a promising candidate for quantum computing due to its scalability properties, long spin coherence times, and the astonishing progress on Si technology and miniaturization in the past few decades. This proposal for a quantum computer ultimately relies on the quantum control of electrons bound to donors near a Si/barrier (e.g., SiO2) interface. We address here several important issues and define critical parameters that establish the conditions that allow the manipulation of donor electrons in Si by means of external electric and magnetic fields. View full abstract»

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  • Midinfrared semiconductor optical metamaterials

    Page(s): 122411 - 122411-7
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    We report on a novel class of semiconductor metamaterials that employ a strongly anisotropic dielectric function to achieve negative refraction in the midinfrared region of the spectrum, ∼8.5–13 μm. We present two types of metamaterials, layered highly doped/undoped heterostructures and quantum well superlattices that are highly anisotropic. Contrary to other optical metamaterials these heterostructure systems are optically thick (up to 20 μm thick), planar, and require no additional fabrication steps beyond the initial growth. Using transmission and reflection measurements and modeling of the highly doped heterostructures, we demonstrate that these materials exhibit negative refraction. For the highly doped quantum well superlattices, we demonstrate anomalous reflection due to the strong anisotropy of the material but a determination of the sign of refraction is still difficult. This new class of semiconductor metamaterials has great potential for waveguiding and imaging applications in the long-wave infrared. View full abstract»

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  • Decoherence effects in the intraband and interband optical transitions in InAs/GaAs quantum dots

    Page(s): 122412 - 122412-7
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    We present a review of coherence properties of interband and intraband optical transitions in self assembled InAs/GaAs quantun dots. Indeed, recent experimental and theoretical investigations of the optical transitions in both spectral domains have allowed a better understanding of the different phenomena that affects the interaction of confined carriers with light. These studies point out the many different ways the electron-phonon interactions play a role on the optical response of quantum dots. They also stress the primary role of the close environment on the coherence characteristics of quantum dots. View full abstract»

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  • ZnO grown by atomic layer deposition: A material for transparent electronics and organic heterojunctions

    Page(s): 122413 - 122413-5
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    We report on zinc oxide thin films grown by atomic layer deposition at a low temperature, which is compatible with a low thermal budget required for some novel electronic devices. By selecting appropriate precursors and process parameters, we were able to obtain films with controllable electrical parameters, from heavily n-type to the resistive ones. Optimization of the growth process together with the low temperature deposition led to ZnO thin films, in which no defect-related photoluminescence bands are observed. Such films show anticorrelation between mobility and free-electron concentration, which indicates that low n electron concentration is a result of lower number of defects rather than the self-compensation effect. View full abstract»

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  • Lasing and polariton condensation: Two distinct transitions in GaAs microcavities with stress traps

    Page(s): 122414 - 122414-5
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    We have used stress to create a harmonic potential for polaritons in GaAs microcavities and have previously reported that the polaritons undergo spontaneous coherence in the trap. In this paper we present results for both trapped conditions and resonant, nontrapped conditions in the same sample. We find that the results are qualitatively different with two distinct types of transitions. At low density in the trap, the polaritons remain in the strong coupling regime while going through the threshold for onset of coherence; at higher density, there is a different threshold behavior, which occurs with weak coupling and can be identified with lasing; this transition occurs both with and without a trap. The transition at lower density can therefore be identified as a type of nonequilibrium Bose–Einstein condensation. View full abstract»

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  • Evolution of electroluminescence from multiple Si-implanted silicon nitride films with thermal annealing

    Page(s): 123101 - 123101-5
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    Influence of thermal annealing on electroluminescence (EL) from multiple-Si-implanted silicon nitride films has been investigated. A reduced injection current and an enhanced EL intensity have been obtained simultaneously by increasing the annealing temperature, which results in a higher EL quantum efficiency. In addition, four emission bands are identified, and their peak energies, intensities, and full widths at half maxima are found to change with annealing temperature. A model is proposed to illustrate the carrier transport, the mechanisms of the four emission bands, and the evolution of the EL bands with annealing as well. The two major bands and the minor ultraviolet band are explained in terms of the recombination of the injected electrons in either the silicon dangling-bond (≡Si0) states or the nitride conduction band with the injected holes in either the band tail states above the nitride valence band or the valence band itself, while the minor near infrared band is attributed to the Si nanocrystals formed in the thin film. View full abstract»

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  • Monte-Carlo-based spectral gain analysis for terahertz quantum cascade lasers

    Page(s): 123102 - 123102-5
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    Employing an ensemble Monte Carlo transport simulation, we self-consistently analyze the spectral gain for different terahertz quantum cascade laser structures, considering bound-to-continuum as well as resonant phonon depopulation designs. In this context, we investigate temperature dependent gain broadening, affecting the temperature performance of terahertz structures. Furthermore, we discuss the influence of the individual scattering mechanisms, such as electron-electron, impurity and interface roughness scattering. A comparison of the simulation results to experimental data yields good agreement. View full abstract»

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  • Upconversion and anomalous power dependence in Ca12Al14O33:Er3+/Yb3+ single phase nanophosphor

    Page(s): 123103 - 123103-7
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    Ca12Al14O33 (Ca12Al7) phase nanophosphor material codoped with Er3+/Yb3+ ions has been synthesized through combustion route, and its UV/visible upconversion luminescence excited by infrared of 976 nm radiation has been monitored. Different batches of samples were calcinated in air at different temperatures, and then changes in the upconversion luminescence have been studied. Freshly synthesized sample shows low upconversion efficiency; however the sample calcinated at 1473 K shows a bright upconverted emission. The Yb3+ and Yb2+ ionic states of Yb have been observed in the samples, and the enhancement in upconversion intensities has been attributed to the conversion of Yb2+ and Yb3+. The upconversion emissions from Er3+ ions have been observed at 335, 366, 380, 408, 475, 523, 548, 661, and 848 nm wavelengths and have been assigned to the transitions 4G9/24I13/2, 2G9/24I15/2, 4G11/24I15/2, 2P3/24I13/2(2H9/24I15/2), 4F3/24I15/2, 2H11/24I15/2, 4S3/24I15/2, 4F9/24I15/2, and 4I9/24I15/2, respectively. The bands at 408, 523, 548, and 661 nm are very intense and can be detected even with the eye using suitable filters at pump power of as low as 50 mW. The pump power dependence of the emission intensities for different bands has been investigated and shows a strange behavior. View full abstract»

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  • A stereo vision method for tracking particle flow on the weld pool surface

    Page(s): 123104 - 123104-8
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    The oscillation of a weld pool surface makes the fluid flow motion quite complex. Two-dimensional results cannot reflect enough information to quantitatively describe the fluid flow in the weld pool; however, there are few direct three-dimensional results available. In this paper, we describe a three-dimensional reconstruction method to measure weld pool surface features based on a single high-speed camera. A stereo adapter was added in front of the high-speed camera lens to obtain two images in the same frame from different view points at the same time. According to machine vision theory, three-dimensional parameters can be reconstructed based on two such images. In this work, three-dimensional velocity fields have been obtained using this method. Based on the calibration technique employed, the associated error is estimated to be less than 11.4%. Quantitative experimental results are useful for understanding the flow pattern, and possibly for controlling the flow of liquid in the weld pool. View full abstract»

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  • Kinetics of photodarkening in a-As2Se3 thin films

    Page(s): 123105 - 123105-6
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    We report kinetics of photodarkening of a-As2Se3 films in terms of changes in absorption coefficient and optical band gap. These are estimated by in situ measurements of transmission spectra at intervals of 10 ms under laser illumination. The absorption coefficient increase with increasing time of laser illumination before saturating. The change in optical band gap, estimated from transmission spectra are studied as a function of time and intensity. A theoretical fitting of the change in optical parameters is done using photon-assisted site switching model, yielding the time constants for the changes in optical absorption and optical band gap. The change in absorption coefficient and optical band gap with incident laser intensity is also examined to investigate the saturation phenomenon. View full abstract»

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  • Optical and ultrasonic signatures of femtosecond pulse filamentation in fused silica

    Page(s): 123106 - 123106-9
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    Millimeter-long filaments and accompanying luminous plasma and defect channels created in fused silica by single, moderately focused femtosecond laser pulses with supercritical powers were probed in situ using optical imaging and contact ultrasonic techniques. Above the threshold pulse energy Eopt=5 μJ corresponding to a few megawatt power, the pulses collapse due to self-focusing and the nonlinear focus moves upstream with increasing pulse energy. Behind the focus, elongated, gradually narrowing awl-shaped channels of electron-hole plasma and luminescent defects are produced. In the channels, whose dimensions generally depend on the pulse energy, supercontinuum emission propagating downstream the channels occurs, although its observation requires elevated pulse energies above 25 μJ in order to compensate energy dissipation in the channels. Ultrasonic side-view imaging of the channels, conducted from a few millimeters distance, reveals predominantly compressive pressure transients. The compressive signals are observed above the same threshold pulse energy Eopt, and their amplitude increases linearly with the laser pulse energy, simultaneously exhibiting significant temporal broadening of the corresponding pulsewidths, reflecting square root dependence of the channel length and sublinear (∝E3/4) dependence of the source pressure on the pulse energy. Altogether, these optical and ultrasonic studies demonstrate filamentary pulse propagation with considerable dissipation (∼10 cm-1) in the awl-shaped subcritical plasma channels rich with generated point defects and optical damage sites. View full abstract»

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  • Laser patterning of diamond. Part I. Characterization of surface morphology

    Page(s): 123107 - 123107-5
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    As the use of diamond as a material for electronic and research applications increases, methods of patterning diamond will be required. In this work, single- and polycrystalline synthetic diamond samples were exposed to laser beams of different energies, wavelengths, and pulse durations. The effects of this exposure were characterized using optical microscopy, scanning electron microscopy, and atomic force microscopy. The threshold ablation energy density for 266 nm radiation with ∼30 ps pulse duration was measured to be ∼14 J/cm2. The threshold for ∼10 ns pulses at the same wavelength was similar, but the ablated area displayed larger surface damage. The surface damage and the threshold energy increase significantly for 532 and 1064 nm radiations. Ablation performed using 213 nm radiation produced the most uniform surface. Changes in the ablated surface are presented in detail. View full abstract»

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  • Laser patterning of diamond. Part II. Surface nondiamond carbon formation and its removal

    Page(s): 123108 - 123108-5
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    As diamond becomes more prevalent for electronic and research applications, methods of patterning diamond will be required. One such method, laser ablation, has been investigated in a related work. We report on the formation of surface nondiamond carbon during laser ablation of both polycrystalline and single-crystal synthetic diamonds. Near edge x-ray absorption fine structure spectroscopy was used to confirm that the nondiamond carbon layer formed during the ablation was amorphous, and Fourier transform infrared absorption spectroscopy (FTIR) was used to estimate the thickness of this layer to be ∼60 nm. Ozone cleaning was used to remove the nondiamond carbon layer. View full abstract»

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  • Microphotoluminescence investigation on single ZnO microrods with different morphologies

    Page(s): 123109 - 123109-4
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    Spatially resolved microphotoluminescence -PL) was employed to investigate the photoluminescent properties of single ZnO microrods with three morphologies: fusiform, straight, and dumbbell. The morphology of ZnO microrods as well as the measurement region, both had great influence on the observed μ-PL. These were analyzed in terms of the defect density, the ionization effect of surface charges, and the thermal effect of laser. It was found that crystal defects favored the formation of bound excitons, which resulted in the redshift of ultraviolet bands in μ-PL. This redshift effect, however, could be submerged by the ionization of the bound excitons under the surface electric field, especially at the large surface-to-volume regions. The thermal effect of laser, an important factor for traditional photoluminescence characterization, can be neglected in the case of single rod μ-PL measurement. View full abstract»

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Aims & Scope

Journal of Applied Physics is the American Institute of Physics' (AIP) archival journal for significant new results in applied physics

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Editor
P. James Viccaro
Argonne National Laboratory