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Journal of Applied Physics

Issue 2 • Date Jul 2005

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Displaying Results 1 - 25 of 132
  • Issue Cover

    Page(s): c1
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  • Issue Table of Contents

    Page(s): toc1
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  • Fabrication and optical characterization of hexagonal photonic crystal microcavities in InP-based membranes containing InAs/InP quantum dots

    Page(s): 023101 - 023101-5
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    Hexagonal photonic crystal microcavities with missing-hole defects were fabricated in suspended InP membranes. Embedded InAs quantum dots were utilized as broadband emitters to characterize the modes of the cavities. Photoluminescence emission consists of two orthogonally polarized peaks corresponding to the two dipole modes of the hexagonal defect cavity of reduced symmetry. The emission wavelength ranges from 745 to 840 meV, depending on the crystal structure, and quality factors are up to 850. Finite-difference time-domain simulations reproduce the cavity mode energies and the quality factor dependence on the crystal structure, but predict quality factors systematically lower. The experimental quality factors and mode splittings are associated with a slight ellipticity of the lattice holes. View full abstract»

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  • A method to enhance the data transfer rate of eutectic Sb-Te phase-change recording media

    Page(s): 023102 - 023102-5
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    This work describes the effect of nitrogen doping to eutectic Sb-Te phase-change materials in order to enhance the speed of the amorphous-to-crystalline phase transformation. When nitrogen at a sputtering gas flow ratio of N2/Ar=3% was doped in the eutectic Ge-In-Sb-Te recording layer, the data transfer rate was increased up to 1.6 times. When thin GeNx nucleation promotion layers were further added in below and above the recording layer, an overall enhancement up to 3.3 times in data transfer rate was achieved. The nitrogen contents corresponding to the N2/Ar flow ratios (N2/Ar=0%–10%) were calibrated by electron spectroscopy for chemical analysis. Transmission electron microscopy revealed that nitrogen doping was able to promote the phase transformation by generating numerous nucleation sites uniformly distributed in the recording layer and hence increased the recrystallization speed. View full abstract»

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  • Cr3+Nd3+ energy transfer in the YAl3(BO3)4 nonlinear laser crystal

    Page(s): 023103 - 023103-7
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    The energy transfer between Cr3+ and Nd3+ ions in Cr3+, Nd3+:YAl3(BO3)4 (YAB) codoped crystals has been investigated by means of steady-state and time-resolved laser spectroscopies. The mainly nonradiative character of the energy transfer has been evidenced from the comparison between the emission spectra and the donor fluorescence decays. The analysis of the donor decay profiles has been used to determine the main dipole–dipole character of the transfer, in excellent agreement with the predictions made from the Dexter [J. Chem Phys. 21, 836 (1953)] model, as well as with the dependence of the energy-transfer rates on the acceptor concentration. The temperature behavior of the transfer rate has been analyzed considering the thermal-induced population redistribution between the 2E and 4T2 states of Cr3+ ions and the existence of the phonon-assisted energy transfer. We have obtained that 80% of the Cr3+Nd3+ energy transfer in YAB is taking place without the assistance of lattice phonons whereas almost 20% is taking place through multiphonon emission. View full abstract»

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  • Phase-only modulation using a normal-mode liquid-crystal gel

    Page(s): 023104 - 023104-4
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    A normal-mode liquid-crystal (LC) gel for phase-only modulation is demonstrated. In the absence of an electric field, the gel has negligible light scattering. Upon application of an electric field, the gel becomes highly transparent and exhibits phase modulation capability. For a 20-μm transmissive E48 LC gel, the phase shift reaches 0.55π at λ=633 nm and the response time is ∼2 ms. Although the phase change is not very large, it is sufficient for making adaptive microphotonic devices. Such a normal-mode LC gel-based phase modulator is free from scattering and hysteresis and has a fast response time and good long-term operation stability. View full abstract»

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  • Influence of grating characteristics on the operation of circular-grating distributed-feedback polymer lasers

    Page(s): 023105 - 023105-7
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    We explore the influence of grating characteristics on the lasing performance of polymer circular-grating distributed-feedback lasers. A range of circular-grating sizes and profiles were fabricated on a single silica substrate, which was coated with a thin film of the conjugated polymer poly[2-methoxy-5-(2’ethylhexyloxy)-1,4-phenylene vinylene]. Variations in lasing threshold and surface-emitted slope efficiency were determined as a function of grating outer diameter and duty cycle. The experimental lasing results are compared with predictions from a theoretical analysis based on an adaptation of the transfer matrix method. We find that an outer diameter of at least 200 μm is required to minimize the threshold and optimize the surface-emitted slope efficiency. A groove-to-period duty cycle of ∼25% gives the lowest lasing thresholds by optimizing the in-plane feedback. We also find that the structure of the polymer–air surface varies substantially with substrate duty cycle, which has implications for optimum device design. View full abstract»

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  • Parametric study of a highly efficient chemical oxygen-iodine laser with supersonic mixing of iodine and oxygen

    Page(s): 023106 - 023106-9
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    We report on a detailed parametric study of the extremely efficient supersonic chemical oxygen-iodine laser recently developed in our laboratory [V. Rybalkin, A. Katz, B. D. Barmashenko, and S. Rosenwaks, Appl. Phys. Lett. 85, 5851 (2004)]. At the early stage of operation, 40.0% efficiency was measured for 1 s followed by a sustained 35.5% chemical efficiency for 20 s. The power and spatial distributions of the gain and temperature across the flow were measured for different supersonic nozzles with both staggered and nonstaggered iodine injection holes, different injection locations along the flow and nozzle throat heights. The effects of the partial pressure of O2 and the residence time of the flow in the generator, as well as the heating of the nozzle, are discussed and shown to be crucial in attaining this high efficiency. By carefully studying and optimizing the operation of the chemical generator, 0.73 yield of singlet oxygen was obtained for conditions corresponding to the highest efficiency. View full abstract»

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  • Terahertz gain on intersubband transitions in multilayer delta-doped p-Ge structures

    Page(s): 023107 - 023107-7
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    A far-infrared laser concept based on intersubband transitions of holes in p-type periodically delta-doped semiconductor films is studied using numerical Monte Carlo simulation of hot-hole dynamics. The considered device consists of monocrystalline pure Ge layers periodically interleaved with delta-doped layers and operates with vertical hole transport in the presence of an in-plane magnetic field. Population inversion on intersubband transitions arises due to light-hole accumulation in EB fields, as in the bulk p-Ge laser. However, the considered structure achieves spatial separation of hole accumulation regions from the doped layers, which reduces ionized-impurity and carrier-carrier scattering for the majority of light holes. This allows a remarkable increase of the gain in comparison with bulk p-Ge lasers. Population inversion and gain sufficient for laser operation are expected up to 77 K. Test structures grown by chemical-vapor deposition demonstrate feasibility of producing the device with sufficient active thickness to allow quasioptical electrodynamic cavity solutions. View full abstract»

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  • Chromatic dispersion compensators via highly dispersive photonic crystals

    Page(s): 023108 - 023108-4
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    We calculated the phase index np, group index ng, and chromatic dispersion D from photonic band structures in two-dimensional (2D) photonic crystals consisting of triangular-arrayed cylindrical rods. We found that both the np and ng increase rapidly as the frequency of a propagating electromagnetic wave approaches the gap-band edges due to Bragg reflection, thus causing D to be large. The chromatic dispersion D is evaluated to be four to five orders of magnitude greater than the conventional single-mode fiber dispersion and should be useful as dispersion compensation for fiber-optic communications. In addition, the np, ng, and D of the 2D photonic crystals for E- and H-polarized waves are also examined. View full abstract»

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  • Optimization of ultraviolet emission and chemical species generation from a pulsed dielectric barrier discharge at atmospheric pressure

    Page(s): 023301 - 023301-5
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    One of the attractive features of nonthermal atmospheric pressure plasmas is the ability to achieve enhanced gas phase chemistry without the need for elevated gas temperatures. This attractive characteristic recently led to their extensive use in applications that require low temperatures, such as material processing and biomedical applications. The agents responsible for the efficient plasma reactivity are the ultraviolet (UV) photons and the chemically reactive species. In this paper, in order to optimize the UV radiation and reactive species generation efficiency, the plasma was generated by a dielectric barrier discharge driven by unipolar submicrosecond square pulses. To keep the discharge diffuse and to maintain low operating temperatures, helium (He) was used as a carrier gas. Mixed with helium, varying amounts of nitrogen (N2) with the presence of trace amounts of air were used. The gas temperature was determined to be about 350 K at a 1-kHz pulse frequency for all cases and only slightly increased with frequency. The UV emission power density, PUV, reached its highest level when 5% to 10% of N2 is mixed to a balance of He. A maximum PUV of about 0.8 mW/cm2 at 10-kHz pulse frequency for a He(90%)+N2(10%) mixture was measured. This was more than four times higher than that when He or N2 alone was used. Furthermore, the emission spectra showed that most of the UV was emitted by excited NO radicals, where the oxygen atoms came from residual trace amounts of air. In addition to NO, N<- roman>O2, and excited N2, N2+, OH, and He were also present in the plasma. View full abstract»

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  • Calculation of two-dimensional plasma sheath with application to radial dust oscillations

    Page(s): 023302 - 023302-6
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    Dust particles are often confined radially in a plasma potential well above a cylindrical depression in an otherwise flat electrode. The structure of the two-dimensional, time-independent sheath is computed for this geometry using cold, collisionless ions and Boltzmann electrons. A depression with a radius of 16 Debye lengths and a depth of 2 Debye lengths is modeled for negative electrode biases from 6 to 32 times the electron temperature. The normalized radial oscillation frequency for a dust particle in the well is computed from the sheath potential structure. The model results agree qualitatively with the experimental measurements. View full abstract»

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  • A model of the two-stage Hall thruster discharge

    Page(s): 023303 - 023303-11
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    The effect of a third, active electrode placed inside the ceramic chamber of a Hall thruster is analyzed. Both electron-collecting and electron-emitting modes are considered. Significant efficiency enhancement with respect to the single-stage operation can be obtained for a good electron-emitting electrode, placed in an intermediate location of the acceleration region, and for an anode-to-electrode (inner-stage) potential significantly larger than the ionization potential. Optimum values of the electrode location and voltage are determined. The performance improvement is due to a reduction of the energy losses to the chamber walls. This is the consequence of lower Joule heating and thus lower electron temperature in the outer stage. When the ionization process is efficient already in the single-stage operation, (i) two-stage operation does not affect practically the propellant and voltage utilizations and (ii) thrust efficiency decreases when the intermediate electrode works as an electron collector. View full abstract»

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  • Influence of the skin effect and current risetime on the fragmentation of wires by pulsed currents

    Page(s): 023304 - 023304-18
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    This study considers the physical phenomenon whereby wires may fragment in the solid state when subject to a sufficiently high pulsed electric current. A mathematical model is constructed within continuum mechanics which considers both Lorentz force and thermal mechanisms for the creation of stress waves in a wire. Previous studies are extended by including the skin effect, that is allowing for the diffusion of current density across the wire, and also investigating the influence of current risetime. Axisymmetric solutions are sought for rigid-lubricated, clamped, and free wire ends. Analytical solutions are obtained for the case of rigid-lubricated wire ends, while for the other cases the governing equations are solved numerically using an application-specific explicit finite-difference scheme, which is staggered in time and space. The inclusion of the skin effect leads to significant qualitative and quantitative differences in results. For example, in some cases we find tension in the longitudinal zz) stress component, which experiments suggest to be responsible for the fragmentation process, while the uniform-current model predicts compression. In most cases, the inclusion of the skin effect leads to higher peak tensile τzz stresses. Some understanding of the present results is gained with reference to analytical quasistatic solutions. Stresses generated by the Lorentz force mechanism are found to be more sensitive than those generated by the thermal mechanism to the current risetime. In both cases axial stresses increase with decreasing current risetime. Despite the differences in the results obtained with the inclusion of the skin effect, our results support the broad conclusions of the uniform-current model results; the largest stresses are found at the clamps for a wire with clamped ends, while the largest stresses in a wire with free ends are generated by the thermal mechanism and ar- e located at the center of the wire. View full abstract»

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  • Plasma jet emission in fast-pulsed capillary discharges

    Page(s): 023305 - 023305-5
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    The properties of a plasma jet generated in low-pressure pulsed capillary discharge have been measured. The discharge operates in a 5 cm long and 1.6-mm-inner diameter alumina capillary, with argon in a pressure range between 20 and 100 Torr, at 11-kV applied voltage. The temporal and spatial evolutions of the plasma density in the plasma jets are measured close to the capillary end with a Michelson interferometry based on a 10-ns-pulse Nd:yttrium aluminum garnet laser. The maximum on-axis plasma density is of the order of 1017 cm-3 at the capillary output, with an ∼6-ns rise time to reach a significative electron density. At lower pressure the plasma density is seen to decay in a time scale of 50 ns and over an axial distance of the order of 0.5 mm, whereas at the higher pressure, 100 Torr, it remains more or less stationary for up to ∼300 ns. The discharge is characterized by a high efficiency in energy coupling, as the local-stored energy is much less than 1 J per pulse. View full abstract»

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  • Periodic formation and propagation of double layers in the expanding chamber of an inductive discharge operating in Ar/SF6 mixtures

    Page(s): 023306 - 023306-7
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    It has previously been shown [Tuszewski etal, Plasma Sources Sci. Technol. 12, 396 (2003)] that inductive discharges in electronegative gases are subject to two types of instability: the source instability related to the E to H transition and a transport instability, occurring downstream when an expanding chamber is present. These two types of instability are observed in our “helicon” reactor operated without a static magnetic field in low-pressure Ar/SF6 mixtures. Temporally and spatially resolved measurements show that, in our experiment, the downstream instability is a periodic formation and propagation of a double layer. The double layer is born at the end of the source tube and propagates slowly to the end of the expansion region with a velocity of 150 m s-1. View full abstract»

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  • Actinometry of inductively coupled Cl2/N2 plasmas for dry etching of GaAs

    Page(s): 023307 - 023307-5
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    Inductively coupled plasma dry etching of GaAs with Cl2/N2-containing plasmas and investigations of these plasmas with optical emission spectroscopy and actinometry are presented. The results of actinometry were revised to allow the comparison of relative ground-state densities at different pressures. The obtained relative ground-state densities of N2, Cl2, and Cl (I) are presented as functions of the process parameters [rf power, pressure, and Cl2/(Cl2+N2) ratio]. Cl (I) relative ground-state densities were found to be linearly connected to the pressure and the Cl2/(Cl2+N2) ratio. GaAs etch rates up to 3 μm/min were obtained, while etch rates of the photoresist mask did not exceed 0.95 μm/min. The impact of the rf power changes on the etch rates was negligible. Plotting the etch rates against the process parameters revealed a linear relationship between the etch rates and the pressure and between the etch rates and the Cl2/(Cl2+N2) ratio. Therefore a correlation between the Cl (I) relative ground-state density measured in plasmas without wafer and GaAs and the photoresist mask etch rate was found. All GaAs dry etching examined in this investigation was found to take place in a reactant-limited regime with Cl (I) as the rate-limiting species. View full abstract»

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  • Numerical simulation of dual frequency etching reactors: Influence of the external process parameters on the plasma characteristics

    Page(s): 023308 - 023308-13
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    A one-dimensional particle-in-cell/Monte Carlo model is used to investigate Ar/CF4/N2 discharges sustained in capacitively coupled dual frequency reactors, with special emphasis on the influence of the reactor parameters such as applied voltage amplitudes and frequencies of the two voltage sources. The presented calculation results include plasma density, ion current, average sheath potential and width, electron and ion average energies and energy distributions, and ionization rates. The simulations were carried out for high frequencies (HFs) of 27, 40, 60, and 100 MHz and a low frequency (LF) of 1 or 2 MHz, varying the LF voltage and keeping the HF voltage constant and vice versa. It is observed that the decoupling of the two sources is possible by increasing the applied HF to very high values (above 60 MHz) and it is not defined by the frequency ratio. Both voltage sources have influence on the plasma characteristics at a HF of 27 MHz and to some extent at 40 MHz. At HFs of 60 and 100 MHz, the plasma density and ion flux are determined only by the HF voltage source. The ion energy increases and the ion energy distribution function (IEDF) becomes broader with HF or LF voltage amplitude, when the other voltage is kept constant. The IEDF is broader with the increase of HF or the decrease of LF. View full abstract»

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  • Internal electric-field and segregation effects on luminescence properties of quantum wells

    Page(s): 023501 - 023501-4
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    Surface segregation of In atoms during molecular-beam epitaxy and its influence on the energy levels in strained piezoelectric InGaAs/GaAs and InGaN/GaN quantum wells (QWs) are investigated theoretically. It is shown that these effects modify the electronic states in the QW and the emission energy in the photoluminescence (PL) spectra. In this work, we solve analytically the Schrödinger equation in the absence of electric field, taking into account the shape changes in the QWs due to the segregation of In atoms during the growth process of the semiconductor heterostructures. Furthermore, the influence of the built-in electric field due to the piezoelectric effect on the PL emission is calculated by considering a variational electron wave function to calculate the ground-energy transitions inside the active region in the heterostructure. In particular, we apply this model to the case of indium segregation in InGaAs/GaAs for moderate internal electric fields. The transition energy calculations between the confined electron and hole states as a function of the well width for different temperatures and In composition are in agreement with the measured PL energy peaks. View full abstract»

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  • Dislocation-indenter interaction in nanoindentation

    Page(s): 023502 - 023502-6
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    A formulation of dislocation-indenter interaction in two-dimensional, isotropic elasticity is presented. A significant dislocation-indenter interaction is predicted when dislocations are nucleated very close to the indenter. This interaction is expected to have an important influence on dislocation motion and multiplication. Upon nucleation close to the indenter, the dislocations are shown to modify the load, load distribution, and moment acting on the indenter. This effect is seen to vary with the indentation contact length. Further away from the indenter, the indenter-dislocation interaction is shown to be negligible. View full abstract»

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  • Quantitative coverage and stability of hydrogen-passivation layers on HF-etched Si(1-x)Gex surfaces

    Page(s): 023503 - 023503-8
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    This study investigates the stability of surface hydride layers passivating silicon-germanium alloys against contamination in ambient conditions after treatment in dilute hydrofluoric acid (HF), which is of central importance to the fabrication of SiGe-based semiconductor devices. We report quantitative hydrogen coverages on HF-etched Si(1-x)Gex surfaces (x=0,0.30,0.60,0.82), determined after air-to-vacuum transfer by H-specific 1H(15N,αγ)12C nuclear reaction analysis. Combination of this coverage information with analysis of the zero-point vibrational properties and 15N-ion-induced desorption kinetics of the surface H atoms enables the distinction of well-passivated SiGe surfaces terminated exclusively by hydrides of Si and Ge from those partially covered by contaminating adsorbates. It is found that the resistance of HF-etched Si(1-x)Gex alloys against recontamination is drastically reduced at increasing Ge contents. Pure Si(100)–H is stably passivated at least up to 1 week in air by a layer of 1.3±0.1 monolayer total H coverage. Si0.70Ge0.30 initially resembles H-passivated Si(100) but shows indications of moderate contamination after 1 week in air. The HF treatment does not produce stable passivation layers on Ge-rich alloys (x≥0.60), which suffer heavy recontamination within minutes after removal from the HF solution. View full abstract»

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  • Structural, electrical, and optical properties of transparent gallium oxide thin films grown by plasma-enhanced atomic layer deposition

    Page(s): 023504 - 023504-6
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    Gallium oxide (Ga2O3) thin films were deposited on silicon (100) and sapphire (001) substrates using the plasma-enhanced atomic layer deposition (PEALD) technique with an alternating supply of reactant source, [(CH3)2GaNH2]3, and oxygen plasma. The thin films were annealed at different temperatures (500, 700, and 900 °C, respectively) in a rapid thermal annealing system for 1 min. It was found that Ga2O3 thin films deposited by PEALD showed excellent step coverage characteristics. X-ray diffraction measurements showed that the as-deposited thin film was amorphous. However, the thin films annealed at temperatures higher than 700 °C showed a (400) orientation of the monoclinic structure. An atomic force microscope was used to investigate the surface morphologies of the thin films. The thin films showed very smooth surfaces; the roughness of the as-deposited thin film was about 4 Å. With increasing annealing temperature, the thin film became rougher compared with that annealed at lower temperatures. A double-beam spectrophotometer was used to measure the transmittances of the thin films on the sapphire substrates. The thin films showed a very high transmittance (nearly 100%). The band-gap energies of the thin films were determined by a linear fit of the transmittance spectra and were calculated to be between 5.0 and 5.24 eV. The electrical properties of thin films of Pt/film/Si structure were also investigated. It was found that, with increasing annealing temperature, th- e insulating characteristics of the Ga2O3 thin films were significantly improved. Spectroscopic ellipsometry was used to derive the refractive indices and the thicknesses of the thin films. The refractive indices of the thin films showed normal dispersion behavior. The refractive indices of the thin films annealed at low temperatures were smaller than those annealed at high temperatures. View full abstract»

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  • Microdamage in polycrystalline ceramics under dynamic compression and tension

    Page(s): 023505 - 023505-10
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    In-grain microplasticity and intergranular microdamage in polycrystalline hexagonal-structure ceramics subjected to a sequence of dynamic compression and tension are studied computationally using the Voronoi polycrystal model, by which the topological heterogeneity and material anisotropy of the crystals are simulated explicitly. The constitutive modeling considers crystal plasticity by basal slip, intergranular shear damage during compression, and intergranular mode-I cracking during tension. The model parameters are calibrated with the available shock compression and spall strength data on polycrystalline α-6H silicon carbide. The numerical results show that microplasticity is a more plausible micromechanism for the inelastic response of the material under shock compression. On the other hand, the spallation behavior of the shocked material can be well predicted by intergranular mode-I microcracking during load reversal from dynamic compression to tension. The failure process and the resulting spall strength are, however, affected strongly by the intensity of local release heterogeneity induced by heterogeneous microplasticity, and by the grain-boundary shear damage during compression. View full abstract»

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  • Annealing properties of open volumes in HfSiOx and HfAlOx gate dielectrics studied using monoenergetic positron beams

    Page(s): 023506 - 023506-5
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    Thin Hf0.6Si0.4Ox and Hf0.3Al0.7Ox films fabricated by metal-organic chemical-vapor deposition and atomic-layer-deposition techniques were characterized using monoenergetic positron beams. Measurements of the Doppler broadening spectra of annihilation radiation and the lifetime spectra of positions indicated that positrons annihilated from the trapped state by open volumes that exist intrinsically in amorphous structures of the films. For HfSiOx, the mean size of the open volumes and their size distribution decreased with increasing postdeposition annealing (PDA) temperature. For HfAlOx, although the overall behavior of the open volumes in response to annealing was similar to that for HfSiOx, PDA caused a separation of the mean size of the open volumes. When this separation occurred, the value of the line-shape parameter S increased, suggesting an oxygen deficiency in the amorphous matrix. This fragmentation of the amorphous matrix can be suppressed by decreasing the annealing time. View full abstract»

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  • The influence of defects and postdeposition treatments on the free carrier density in lightly phosphorus-doped large-grained polycrystalline silicon films

    Page(s): 023507 - 023507-7
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    The influence of postdeposition treatments (rapid thermal annealing and hydrogenation) on the doping of large-grained polycrystalline silicon p+nn+ thin-film diodes on glass substrates is investigated using resistivity and impedance analysis measurements. Whereas in the lightly phosphorus-doped base region both treatments are found to cause an increase in the active doping concentration, hydrogenation decreases the active doping concentration of both heavily doped layers (Al and P). The different behavior is attributed to acceptorlike defects which are present in the nonhydrogenated base region in a similar concentration as the atomic phosphorus concentration and which are well passivated by hydrogenation. From posthydrogenation annealing experiments and temperature-dependent impedance analysis measurements, different temperature dependences and activation energies (depending on the posthydrogenation annealing temperature) are found for the lightly doped base region. The temperature dependences are quantitatively described using a simple model comprising three energy levels within the silicon band gap: the phosphorus doping level, a shallow donorlike defect level about 0.2 eV below the conduction-band edge, and a midgap acceptorlike defect level. From this model it can be concluded that (i) the density of the acceptorlike defect in our n-type base region is at least 1×1017 cm-3 after the rapid thermal anneal, and (ii) that hydrogenation reduces this defect density by more than a factor of 10. The results of this work demonstrate that accurate control of the doping in large-grained polycrystalline Si films is possible, provided the effects of defects and the necessary postdeposition treatments (such as rapid thermal annealing and hydrogenation) are carefully accounted for. View full abstract»

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

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P. James Viccaro
Argonne National Laboratory