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Journal of Applied Physics

Issue 6 • Date Mar 2004

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Displaying Results 1 - 25 of 47
  • Issue Cover

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

    Page(s): toc1
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  • Optical studies of metallodielectric photonic crystals: Bismuth and gallium infiltrated opals

    Page(s): 2947 - 2951
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    We have fabricated three-dimensional metallodielectric photonic crystals that consist of bismuth and gallium metals infiltrated into porous synthetic opals that have 300 nm diam silica balls. The specular reflectivity was measured in a broad spectral range from 0.3 to 25 μm using a variety of light sources, detectors and spectrometers. In addition to broadening of the original Bragg stop bands in the visible spectral range that give rise to iridescence colors, we also found a dramatic decrease of reflectivity in the visible/near-infrared spectral range. For frequencies below a cut-off frequency in the mid-infrared range the reflectivity increases to its normal bulk value in the respective metal, as predicted by theory and fitted by a model calculation. © 2004 American Institute of Physics. View full abstract»

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  • Laser-induced cavitation bubbles for cleaning of solid surfaces

    Page(s): 2952 - 2956
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    When a high-power laser beam is focused into liquid, it results in a shock wave emission and cavitation bubble generation. Upon inserting a rigid substrate into the liquid, the bubbles migrate towards the substrate due to the Bjerknes attractive force. Due to bubble–substrate and/or bubble–free-surface interaction, a high-speed liquid jet is formed during bubble collapse, and a collapse shock wave is generated at the moment of bubble collapse near the substrate. These shock waves and liquid jet induce large forces acting on the substrate to remove particles from it. For a substrate several millimeters away from the laser focus point, the collapse shock wave and liquid jet play key roles in removal of particles. The cleaning efficiency increases with an increase of laser fluence and decreases with an increase of distance between substrate surface and laser beam focus point or depth below liquid surface. In a case of bubbles close to substrate and liquid-surface boundaries, implosion of the bubbles will give rise to shock waves and liquid jets oblique to the substrate surface with the parallel and perpendicular components of the forces onto the particles. These oblique liquid jets and shock waves result in high cleaning efficiency. A liquid, such as alcohol and commercial washing solution, as the surrounding medium, rather than air or vacuum, can reduce adhesion force and enhance cleaning efficiency. © 2004 American Institute of Physics. View full abstract»

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  • Infrared-laser induced photon avalanche upconversion in Ho3+Yb3+ codoped fluoroindate glasses

    Page(s): 2957 - 2962
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    An intense green upconversion emission due to a photon avalanche mechanism has been observed in Ho3+Yb3+ codoped fluroindate glasses, measured at 200 K, under excitation at around 750 nm. The influence of the Yb3+ concentration on the photon avalanche process has been studied. The pump power threshold of the avalanche mechanism decreases as the Yb3+ concentration is increased. A reduction by about a factor of 3 is observed from the 2.25 mol % Ho3+ single doped glass to the 2.25 mol % Ho3+–2.25 mol % Yb3+ codoped glass. This effect has been related to an extra feeding of the 5I7 intermediate level of Ho3+ ions via Ho3+Yb3+ and Yb3+Ho3+ energy transfer and back transfer processes, respectively. © 2004 American Institute of Physics. View full abstract»

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  • Three regimes of growth of a Gaussian ripple on a uniform plane electromagnetic wave front in a plasma

    Page(s): 2963 - 2968
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    Starting with the scalar wave equation and relevant expressions for nonlinear dielectric constant of plasma, the propagation of a Gaussian ripple on a plane wave front of uniform intensity has been studied in the paraxial approximation. In the plane of the ripple width (along the y axis) and the ratio of the electric field intensities of the ripple and the main beam at r=0, z=0, (along the x axis) three distinct regions (for a given intensity of the main beam) can be identified which correspond to steady divergence, oscillatory divergence, and self-focusing of the ripple. The variation of the ripple width with distance of propagation has also been obtained for typical points in the three regions. © 2004 American Institute of Physics. View full abstract»

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  • Macroparticle rotation in the vacuum arc plasma jet

    Page(s): 2969 - 2974
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    The motion of macroparticles (MPs) emitted from the cathode into the vacuum arc plasma jet is considered taking into account asymmetric ion bombardment of the MP surface. The MP velocity is increased up to 0.17 of the plasma jet velocity for an initial MP radius R0=0.1 μm and up to 0.3 of the plasma jet velocity for R0=0.02 μm. The electric current density distribution within the MP caused by the asymmetric plasma bombardment was calculated assuming homogeneous internal conductivity. The electric potential difference between the apex and antiapex points is ≈10-8V for an MP radius of R0=0.1 μm, while the average MP electric potential relative to the surrounding plasma is V≈-11.27 V. Interaction between the internal electric current and the magnetic field used to collimate the plasma jet rotates the MPs with a frequency of ≈105s-1. © 2004 American Institute of Physics. View full abstract»

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  • Comparative study of soft x-ray emission characteristics in a low energy dense plasma focus device

    Page(s): 2975 - 2981
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    An investigation on the soft x rays emitted in a 2.2 kJ Mather-type dense plasma focus device using a multichannel diode spectrometer and a simple pinhole camera is reported. Emitted x rays associated with different shapes (hollow, solid, and hemispherical) of anode and in hydrogen/nitrogen gas medium are compared. The structure of x-ray emitting sites as well as x-ray yields were found to be strongly influenced by the shape of the anode and the filling gas pressure. The maximum yield of 2.2 J into 4π sr was obtained in the case of hemispherical anode in hydrogen gas medium. The x-ray pinhole images of the collapsed plasma with the hemispherical anode indicated spot-like structure having 500–800 μm in diameter. On the contrary, other anode shapes showed columnar pinched structure of 8–10 mm in length and 1–2 mm in diameter. Results indicated that an appropriate design of the anode could enhance the x-ray yield by more than tenfold in a conventional low energy dense plasma focus device. © 2004 American Institute of Physics. View full abstract»

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  • Analytical fit of the I–V probe characteristic for finite ion temperature values: Justification of the radial model applicability

    Page(s): 2982 - 2990
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    This article deals with an extension of the radial model for the sheath that surrounds a cylindrical or spherical Langmuir probe immersed in a plasma in which positive ion thermal motion is taken into account. The dependence of the electric potential profile and of the I–V characteristic on the positive ion temperature is obtained. Moreover, a different parameterization is established from a numerical fit of the I–V characteristic as a function of the probe radius, the biasing potential, and the ratio between the positive ion temperature and the electron temperature. This parameterization allows us to obtain an analytical approximation to the potential profile and the sheath edge. Finally, it also gives us a means of justifying under what conditions the radial model can be applied in plasmas with finite positive ion temperature. © 2004 American Institute of Physics. View full abstract»

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  • The yield strength of thin copper films on Kapton

    Page(s): 2991 - 2997
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    Thin films of copper, with thickness between 0.1 and 3 μm, were vapor-deposited on 12.7 or 7.6-μm-thick polyimide (Kapton) substrates. They were tested in a microtensile tester in which the strain is measured by optical diffraction from a microlithographically applied grid. The Young modulus is independent of film thickness and is about 20% below the value calculated from single-crystal elastic constants. The yield stress depends strongly on the film thickness and is fit by σy=116+355(t)-0.473, where t is the thickness in μm and σy is in MPa. The microstructure of the films was studied by focused ion-beam microscopy. The grains are heavily twinned and the microstructural lengths (grain size, twin spacing, twin width) depend only weakly on the film thickness. A substantial part of the yield stress is therefore attributable to an effect of the film thickness, such as that predicted by strain gradient plasticity theory. The lower limit and some estimates of the thickness contribution to the yield stress are calculated. The estimated characteristic length of strain gradient plasticity theory is 0.6 μm for these materials. © 2004 American Institute of Physics. View full abstract»

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  • Competition between strain-induced and temperature-controlled nucleation of InAs/GaAs quantum dots

    Page(s): 2998 - 3004
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    Atomic force microscopy and photoluminescence spectroscopy (PL) have been used to study asymmetric bilayer InAs quantum dot (QD) structures grown by molecular-beam epitaxy on GaAs(001) substrates. The two QD layers were separated by a GaAs spacer layer (SL) of varying thickness and were grown at different substrate temperatures. Grown independently, these two layers would exhibit a widely different QD number density, and this technique therefore enables us to assess the influence of the strain fields created by the dots in the first layer on the second-layer QD nucleation and characteristics. For very large SLs (≫40 nm), total strain relief causes the QD nucleation to be controlled exclusively by the substrate temperature, which influences the migration of In adatoms. In this case, the optical and morphological properties of the second QD layer are identical to a structure with a single QD layer grown at the same temperature. In structures with a much smaller SL, strain effects dominate over the effect of temperature in controlling the nucleation of the QDs, thereby fixing the second-layer QD number density to that of the first (templating effect). There is also evidence that strain relaxation is present in the QDs of the second layer and that this is crucial for extending their emission wavelength. The optimum SL thickness is shown to be 11 nm, for which low-temperature PL emission peaks at 1.26 μm, with a full width at half-maximum of only 15 meV. Intermediate SL thicknesses exhibit broad QD size distributions, with strain effects only partly influencing the QD growth in the second layer. © 2004 American Institute of Physics. View full abstract»

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  • Infrared 45° reflectometry of Li doped ZnO films

    Page(s): 3005 - 3009
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    Lithium doped (0–10 at. % Li) ZnO films were grown in the wurtzite structure on sapphire (001) substrates and investigated in the 200–1200 cm-1 frequency range at 300 K by far-infrared reflectivity spectroscopy using polarized oblique (45°) incidence. This technique has enabled us to determine the longitudinal optical phonon frequency E1(LO) at 576 cm-1 of the fundamental lattice vibration at the center of the Brillouin zone, as well as to investigate the LO phonon-plasmon coupling in the low carrier density (N≤1018cm-3) ZnO films. The energy shift and halfwidth broadening of the LO phonon band in comparison with the uncoupled mode in high-ohmic ZnO:0.8 at. % Li dc=0.6×106 Ω cm) film have been analyzed to get the concentration and “optical” mobility of charge carriers in the Li doped ZnO films. The results of optical, x-ray diffraction and dc resistivity measurements are discussed. © 2004 American Institute of Physics. View full abstract»

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  • Optical properties of Cd1-xZnxTe films in a device structure using variable angle spectroscopic ellipsometry

    Page(s): 3010 - 3019
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    The optical properties of polycrystalline Cd1-xZnxTe alloy thin films in device structures are reported for energy from 0.8 to 4.6 eV. Cd1-xZnxTe alloy thin films with x from 0 to 1 were deposited on glass/indium-tin-oxide (ITO)/CdS substrates by coevaporation from CdTe and ZnTe sources and were characterized by variable angle spectroscopic ellipsometry, energy dispersive x-ray spectroscopy, and x-ray diffractometry. The Cd1-xZnxTe films are single phase with a zincblende crystal structure over the entire alloy range. The Cd1-xZnxTe optical constants were determined using a multilayer optical model incorporating the optical constants of glass, ITO and CdS, determined independently from glass, glass/ITO, and glass/ITO/CdS specimens. The optical constants of the Cd1-xZnxTe thin films are comparable to literature values reported for single crystals, indicating that the polycrystalline nature of the films does not measurably alter the optical constants. A semiconductor alloy model for determining the composition of CdxZn1-xTe alloy films is developed using the optical data obtained from the analysis. This alloy model can be used to evaluate compositional grading and the effects of Cd1-xZnxTe film processing. © 2004 American Institute of Physics. View full abstract»

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  • Optical transitions and upconversion luminescence of Er3+/Yb3+-codoped halide modified tellurite glasses

    Page(s): 3020 - 3026
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    Er3+-doped halide modified tellurite glasses were synthesized by conventional melting and quenching method. The Judd–Ofelt analysis was performed on the absorption spectra and the transition probabilities, excited state lifetimes, and the branching ratios were calculated and discussed. The intense infrared and visible fluorescence spectra under 980 nm excitation were obtained. Strong upconversion signal was observed at pumping power as low as 30 mW in the glasses with halide ions. The upconversion mechanisms and power dependent intensities were discussed, which showed two-photon process are involved for the green and red emissions. The decay times of the emitting states and the corresponding quantum efficiency were determined and explained. © 2004 American Institute of Physics. View full abstract»

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  • Cathodoluminescence of defects in sintered tin oxide

    Page(s): 3027 - 3030
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    Cathodoluminescence (CL) in the scanning electron microscope (SEM) has been used to investigate the luminescence mechanisms in tin oxide. Sintered material prepared from high purity powder has been found to show a strong dependence of the CL emission on the thermal treatments applied during sample preparation. SEM images show the presence of nano and microcrystalline grains. The correlation of the grain size and morphology with the optical emission is analyzed by CL microscopy and spectroscopy. The main emission bands appear centered at about 2.58, 2.25, and 1.94 eV depending on the sintering treatment. CL images reveal that the 2.25 and the 2.58 eV bands are associated at specific crystal faces. The evolution of the luminescence bands with mechanical milling shows a complex evolution of the 1.94 and 2.58 eV emissions which is explained by formation and recovery of defects during milling. © 2004 American Institute of Physics. View full abstract»

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  • Organic molecules and nanoparticles in inorganic crystals: Vitamin C in CaCO3 as an ultraviolet absorber

    Page(s): 3031 - 3036
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    Organic molecules and nanoparticles embedded in inorganic crystalline lattices have been studied to add different properties and functions to composite materials. Calcium carbonate was precipitated by dropping an aqueous solution of CaCl2 into that of Na2CO3 containing dissolved vitamin C (ascorbic acid). The optical absorption ascribed to divalent ascorbate anions in the lattice was observed in the ultraviolet B (wavelength: 280–315 nm) region, while solid vitamin C exhibited absorption in the ultraviolet C (100–280 nm) region. The divalent ascorbate anion is only stable in CaCO3 due to the absence of oxygen molecules. Doping CaCO3 with nanoparticles of ZnO also enhanced the absorption in the ultraviolet A (315–380 nm) region. These composite materials are suggested for use as UV absorbers. © 2004 American Institute of Physics. View full abstract»

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  • Interdiffusion and thermal stability in magnetic tunnel junction ferromagnet/insulator/ferromagnet trilayer structures

    Page(s): 3037 - 3040
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    We used molecular beam epitaxy grown magnetic tunnel junction structures for thermal stability studies using x-ray photoelectron spectroscopy (XPS) and in situ scanning tunneling microscopy (STM). Unannealed trilayers show no oxidation of the bottom magnetic layer for barriers thicker than 1.1 nm. However, upon annealing, we find that the metallic XPS peak of the top layers decrease dramatically, while that of the bottom and barrier layers increases. STM images of the Co top layer on the Al–oxide barrier show that the Co does not agglomerate upon annealing up to 500 °C, which suggests interdiffusion is the most likely explanation for the XPS data. © 2004 American Institute of Physics. View full abstract»

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  • Epitaxial PbTiO3 thin films grown on (100) MgO by pulsed-laser deposition for optical waveguiding properties

    Page(s): 3041 - 3047
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    Perovskite single phased PbTiO3 thin films have been grown by pulsed-laser deposition on (100) MgO single crystals in order to prepare high quality optical waveguides. The substrate temperature and the oxygen pressure during the deposition have been optimized to improve crystalline quality and to grow epitaxial thin films. The films display two main axis orientations according to the MgO (100) basal plane. The waveguiding properties and the optical losses characterized by using m-lines spectroscopy and charge-coupled device camera measurement, respectively, are correlated to the structural properties of the films. Such textured and epitaxied films display very good optical properties, as the optical losses are found to be lower than 1 dB cm-1. © 2004 American Institute of Physics. View full abstract»

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  • Dynamic annealing in III-nitrides under ion bombardment

    Page(s): 3048 - 3054
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    We study the evolution of structural defects in AlxGa1-xN films (with x=0.0–0.6) bombarded with kilo-electron-volt heavy ions at 77 and 300 K. We use a combination of Rutherford backscattering/channeling spectrometry and cross-sectional transmission electron microscopy. Results show that an increase in Al content not only strongly enhances dynamic annealing processes but can also change the main features of the amorphization behavior. In particular, the damage buildup behavior at 300 K is essentially similar for all the AlGaN films studied. Ion-beam-produced disorder at 300 K accumulates preferentially in the crystal bulk region up to a certain saturation level (∼50%–60% relative disorder). Bombardment at 300 K above a critical fluence results in a rapid increase in damage from the saturation level up to complete disordering, with a buried amorphous layer nucleating in the crystal bulk. However, at 77 K, the saturation effect of lattice disorder in the bulk occurs only for x≳0.1. Based on the analysis of these results for AlGaN and previously reported data for InGaN, we discuss physical mechanisms of the susceptibility of group-III nitrides to ion-beam-induced disordering and to the crystalline-to-amorphous phase transition. © 2004 American Institute of Physics. View full abstract»

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  • Simulating structure and optical response of vacuum evaporated porous rugate filters

    Page(s): 3055 - 3062
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    The drive to develop better thin film devices stimulates great interest in the understanding and control of the properties and morphology of films. In this article we present a study of the optical response of thin film interference filters, utilizing both experimental tools and computer simulation. The filters were deposited onto flat substrates in high vacuum with the technique of glancing angle deposition, which produces complex film structures with nanometer scale pores. A three-dimensional Monte Carlo simulator accurately reproduced this complicated morphology, while also providing information about nucleation, structure evolution, and packing density. Finally, the results of the computer modeling were used to optimize the effects of process parameters to minimize the difference between the design and the observed optical responses, thus providing a powerful tool for improving the performance of optical devices. © 2004 American Institute of Physics. View full abstract»

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  • Growth and charge transfer luminescence of Yb3+-doped YAlO3 single crystals

    Page(s): 3063 - 3068
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    Yb3+-doped YAlO3 single crystals have been grown by the Czochralski method with a radio-frequency heating system. Starting melt compositions of Y1-xYbxAlO3 were varied with x=0.02, 0.1, 0.2, 0.3, and 0.45. The best Yb3+-doped YAlO3 single crystals were obtained for a growth rate of 1.0 mm/h. The grown crystals were transparent and almost colorless. To investigate the homogeneity, the effective segregation coefficient of the Yb ion was estimated. The absorption, photoluminescence, and luminescence decay kinetics of Yb3+-doped YAlO3 were investigated for the temperature range 4–300 K. Very fast charge transfer luminescence of Yb3+ from the near ultraviolet to visible spectral range and the high density of the Yb-rich YAlO3 makes this material a promising candidate for fast scintillators. © 2004 American Institute of Physics. View full abstract»

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  • Model of work function of tungsten cathodes with barium oxide coating

    Page(s): 3069 - 3074
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    Using a full-potential band structure approach, we have investigated the work function of barium oxide coated tungsten cathodes in low pressure discharge lamps. The main objective of this work is to understand why the work function for such cathodes is lower than that of the uncoated tungsten. The model studied in this work is based on a well known supposition that the source of thermionic electrons is the barium atoms released from the barium oxide coating due to a chemical reaction with the underlying metallic tungsten. For the unrelaxed seven-layer model of (100) surface of barium on barium oxide, the work function is calculated to be 2.22 eV, which is lower than that of BaO, Ba, and W metals separately. For a fully relaxed nine-layer surface, it becomes 1.36 eV. Although this value of the work function is lower than those estimated for the fluorescent cathodes by electrical measurements, which averages contributions from surfaces in all possible random orientations, this model provides a satisfactory explanation of the lowering of the work function of tungsten based cathodes in low pressure fluorescent lamps. © 2004 American Institute of Physics. View full abstract»

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  • Spectroscopic, topological, and electronic characterization of ultrathin a-CdTe:O tunnel barriers

    Page(s): 3075 - 3080
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    Ultrathin oxygenated amorphous CdTe (a-CdTe:O) films are prepared by rf sputtering of CdTe in a background of argon or argon/nitrogen/oxygen mixtures. Atomic force microscopy (AFM) is used to characterize the films and shows that they have an island structure typical of most sputtered thin films. However, when sufficiently low powers and deposition rates are employed during sputtering, the resulting films are remarkably smooth and sufficiently thin for use as barrier layers in inelastic electron tunneling (IET) junctions. Four terminal current–voltage data are recorded for Al/a-CdTe:O/Pb tunnel junctions and conductance–voltage curves are derived numerically. WKB fits to the conductance–voltage curves are obtained using a two-component trapezoidal plus square (TRAPSQR) model barrier potential to determine values for the tunnel barrier parameters (height, shape, and width); these parameters are consistent with AFM topological measurements and values from similar devices reported in the literature. IET spectra are presented which confirm that electrons tunnel through ultrathin regions of the a-CdTe:O films, which contain aluminum oxide subregions in a manner consistent with the TRAPSQR barrier model. Because tunneling occurs predominantly through these ultrathin regions, IET spectroscopic data obtained are representative of states at, or within a few tenths of nanometers from, the surface and confirm that the a-CdTe:O surface stoichiometry is very sensitive to changes in the argon/oxygen/nitrogen concentration ratios during film growth. Full IET spectra, current–voltage, and conductance–voltage data are presented together with tunnel barrier parameters derived from (WKB) fits to the data. The results presented here indicate that inelastic electron tunneling spectroscopy is a usefu- l tool for characterizing the surface states of a-CdTe:O and possibly other photovoltaic materials. © 2004 American Institute of Physics. View full abstract»

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  • The Schrödinger–Poisson self-consistency in layered quantum semiconductor structures

    Page(s): 3081 - 3092
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    We develop a self-consistent solution of the Schrödinger and Poisson equations in semiconductor heterostructures with arbitrary doping profiles and layer geometries. An algorithm for this non-linear problem is presented in a multiband k∙P framework for the electronic band structure using the finite element method. The discretized functional integrals associated with the Schrödinger and Poisson equations are used in a variational approach. The finite element formulation allows us to evaluate functional derivatives needed to linearize Poisson’s equation in a natural manner. Illustrative examples are presented using a number of heterostructures including single quantum wells, an asymmetric double quantum well, p-i-n-i superlattices, and trilayer superlattices. © 2004 American Institute of Physics. View full abstract»

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  • An active tunneling model for the magnetotransport in mixed-phase polycrystalline manganites

    Page(s): 3093 - 3098
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    Based on phase separation between metallic and insulating domains, the evolution of magnetoresistance in mixed-phase polycrystalline manganites, as a function of temperature, has been modeled via a bond-disordered random resistor network. The tunneling bond is introduced in the resistor network to account for the contribution from grain boundaries, where the energy barrier is formed. The model is analytically worked out by means of the effective-medium approximation. Compared with the transport in single-phase ferromagnetic-ordered polycrystal, the transport in mixed-phase polycrystal is governed by not only the properties of grain boundaries but also the competing metallic and insulating phases. The fraction of tunneling bonds strongly depends on the temperature and external magnetic field. An active tunneling model is thus established in mixed-phase polycrystalline manganites. Within our model, the effect of crystallinity on the temperature dependence of magnetoresistance in mixed-phase polycrystalline manganites is well explained. © 2004 American Institute of Physics. 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