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Journal of Applied Physics

Issue 7 • Date Apr 1992

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Displaying Results 1 - 25 of 96
  • Issue Table of Contents

    Page(s): toc1
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    Freely Available from IEEE
  • Self‐consistent analysis in the presence of phase‐randomizing processes for double‐barrier structures

    Page(s): 3077 - 3090
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    We present a model, based on the nonequilibrium retarded Green’s function method of the quantum kinetic (Keldysh) theory, that describes carrier transport in three‐dimensional quantum structures with translational invariance in the transverse direction. The transport equations include inelastic phase‐breaking processes and describe the transport of both the coherent and incoherent electrons within the same framework with a set of first‐order coupled linear differential equations. These equations can be solved without resorting to evaluating the Green’s function. The model accounts for local space charges in Poisson’s equation and is suitable for modeling the steady‐state current‐voltage characteristics of double‐barrier structures. A realistic model for these devices should include the effects of inelastic processes and space charge simultaneously. However, as an illustration, we present numerical results for double‐barrier devices by assuming that the electrons undergo elastic phase‐breaking collisions only. Our simulation results show that the accumulated space charge is a function of phase‐breaking collision and that the presence of dissipation within the contacts is partly responsible for the low observed peak‐to‐valley current ratio. View full abstract»

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  • The beam breakup instability in quadrupole and solenoidal electron‐beam transport systems

    Page(s): 3091 - 3102
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    Dispersion relations are derived to determine the growth rate, dominant wavelength, and group velocity of disturbances caused by the beam breakup instability. Considerations include weak and strong focusing, x‐y coupling in solenoidal transport, the spacing of accelerator cavities, and periodically pulsed beams. Beam breakup growth is minimum when the cavity spacing equals an integral number of half‐betatron wavelengths for quadrupole focusing, and an integral number of betatron wavelengths for solenoidal focusing. Minimum growth is also found for periodic pulses separated by an integral number of half‐periods of the TM110 cavity mode. Expressions for beam breakup growth at the minima are obtained. View full abstract»

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  • Unified theory of the backscattering of electromagnetic missiles by a perfectly conducting target

    Page(s): 3103 - 3106
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    It has been shown recently that when an electromagnetic (EM) missile with an energy decreasing as r is incident upon a perfectly conducting plate the reflected energy will decrease as r with the same exponent as the incident EM missile. It has also been shown that when the same EM missile is incident upon a perfectly conducting sphere the rate of the reflected energy decay will be proportional to r-(2+ϵ). In this paper we present a generalized study of the backscattering of EM missile by a perfectly conducting target of arbitrary shape. Utilizing the tangent‐plane approximation, we find that the rate of energy decay of the backscattered field of an EM missile whose energy decreases as r is proportional to rR1R2/(R1+2r)(R2+2r), where R1 and R2 are the two principal radii of curvature of the surface at the specular point. View full abstract»

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  • A theory of the magnetic field from current monopoles

    Page(s): 3107 - 3113
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    A method for calculating the magnetic fields of volume currents by using point current sources (monopoles) is derived. A model is developed for a point source that is consistent with Maxwell’s equations. An arbitrary arrangement of point sources in a conducting volume is shown to generate no magnetic field outside the conductor if the volume surface coincides with an equipotential surface of the sources. This allows such sources to be added to any problem without affecting the magnetic field, and simplifies the calculation of the magnetic field generated by an arbitrary point source in both spherical and semi‐infinite volume conductors. The magnetic field generated by current monopoles located within these volume conductors is then derived. The formulation greatly simplifies the analysis of the magnetic field generated by higher‐order current sources, notably dipoles, in these volumes. View full abstract»

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  • Optimization of injection current and feedback phase of an optically self‐locked laser diode

    Page(s): 3114 - 3117
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    The injection current and feedback phase of an optically self‐locked laser diode system are controlled simultaneously to the optimum condition by modulating the injection current and using phase sensitive detection. This simple method requires no special devices and techniques. A small modulation index, less than 0.2, was achieved and this method is expected to be applicable in many fields. View full abstract»

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  • Control and conversion of the polarization of the electromagnetic wave

    Page(s): 3118 - 3122
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    Treatments of the three reflector polarizer and polarization control by selective transmission are given. A twist reflector consists of a unidirectionally conducting surface with a parallel, perfectly conducting plane placed behind it. The twist reflector can be used in the development of a device that can rotate the plane of polarization of the incident wave by the rotation of the twist reflector about an axis normal to its surface. The polarization angle of the emerging wave is determined as a function of the orientation angle of the twist reflector. The polarizer design can be modified to enable it to convert the incident linear polarization into either right or left circular polarization. View full abstract»

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  • Opto‐optical switching in ion‐implanted poly(methyl methacrylate)‐waveguides

    Page(s): 3123 - 3126
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    Implantation of 1‐MeV and 2‐MeV He+ in bulk poly‐(methyl methacrylate) (PMMA) leads to the formation of monomode and multimode lightguides in the range of 1012–1014 ions/cm2. The waveguide profiles are measured in pure PMMA and in PMMA doped with some wt% of the azo dye Methyl‐Red. Although parts of the dye molecules are destroyed in the implanted region, a residual dye concentration is maintained to obtain photoinduced TRANS‐CIS isomerization, a change between two spatial forms of the molecule, in the guide area. This is demonstrated by switching the coupling intensity of the guided light by an exposure to blue light in the coupling zone of a prism coupler. View full abstract»

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  • Negative ion assisted preionization for excimer lasers

    Page(s): 3127 - 3133
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    This article describes a novel preionization technique, negative ion assisted preionization (NIAP), in which electrons liberated from a reservoir of negative halide ions by photoionization are used to provide preionization for excimer lasers. The NIAP technique is shown to be capable of greatly reducing the technologically demanding requirements associated with conventional flash preionizers, a factor of particular importance for the high average power laser systems currently under development. The efficiency of the NIAP scheme is described with reference to both experimental measurements of the ion reservoir lifetime, which are found to agree with theoretical models, and to theoretical predictions of the electron liberation efficiency, which are found to differ significantly from some previous treatments. The wide range of possible implementations of the NIAP technique is split into three operational regimes that have quite different characteristics. The benefits of improved preionization control and reduced ionization source intensity requirements are described for each regime. The practical limitations of the technique are discussed, and two experimental demonstrations are presented. View full abstract»

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  • Acoustic field study in layered structures by means of x‐ray diffraction

    Page(s): 3134 - 3137
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    A new approach for probing acoustic field in acousto‐optical devices based on dynamical x‐ray diffraction is demonstrated. The measurements were carried out on the ZnO‐SiO2‐Si system. The experimental results as well as the advantages of the method are discussed. View full abstract»

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  • Analytical relations between the phase of the photothermal signal and the thermal wavelength

    Page(s): 3138 - 3142
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    Analytical expressions are derived for the photothermal signal, as the probe beam explores the thermal wave generated by the pump beam in the sample, grazing along its surface at a distance x from the heating spot. The relation between the phase of the signal and the thermal wavelength λth is pointed out, providing an analytical foundation to a refined photothermal technique for measuring the thermal diffusivity α. In particular, the analysis shows that both λth and α can be measured using arbitrary phase shifts relative to the heating spot center, which improves the measurement accuracy and the resolution in the application of this technique to imaging. View full abstract»

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  • The thermal conductivity of chemical‐vapor‐deposited diamond films on silicon

    Page(s): 3143 - 3146
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    The thermal conductivity of chemical‐vapor‐deposited diamond films on silicon is measured for the case of heat flow parallel to the plane of the film. A new technique uses thin‐film heaters and thermometers on a portion of the film which is made to be free standing by etching away the substrate. Effects of thermal radiation are carefully avoided by choosing the length scale properly. Data for several films yield thermal conductivities in the range 2–6 W/cm °C. This is comparable to copper (4 W/cm °C) and is in a range that would be useful as a thin‐film dielectric material, provided that the interface thermal resistance can be minimized. The conductivity varies inversely with the growth rate and the Raman linewidth. View full abstract»

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  • Peculiarities of pulse laser alloying: Influence of spatial distribution of the beam

    Page(s): 3147 - 3158
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    This paper deals with the influence of the spatial distribution of laser beam intensity on the dynamic of laser melting, the surface tension driven convection, and the distribution of alloying elements in the molten pool. The shape of the melting front boundary, the flow pattern, as well as the concentration fields of alloying elements are evaluated for different pulse laser actions operated by different types of nonmonotonic spatial distributions of energy density flow. The relationships between the shape of the laser beam and the structures of the alloyed zones are determined. View full abstract»

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  • Maximum entropy method in inverse black body radiation problem

    Page(s): 3159 - 3163
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    The maximum entropy method is applied to the inverse black body problem, as an alternative procedure for the determination of the temperature distribution of a black body from the measured power spectrum. Starting with the classical moment problem and the generalized moment problem, the maximum entropy method is developed to solve the Fredholm equation of the first kind by introducing a set of transformation functions. This procedure is then used to solve the inverse black body radiation problem by formulating it as a minimization problem. A few examples are studied where exact analytical results are known. The results are very good, and the procedure offers advantages over previous approaches. View full abstract»

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  • A dynamic simulation of particle rearrangement in powder packings with realistic interactions

    Page(s): 3164 - 3173
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    A computer simulation of evolution of random structures of spherical particles has been performed by solving Newton’s equations of motion. The forces considered are gravity, Hertz contact force, frictional force, and van der Waals interaction (VDWI). 948 monosized particles were placed without overlapping inside a rigid cylindrical container by generating the coordinates of the centers of the particles with the help of a random number generator. The initial packing density was only 0.363. The particles were allowed to settle under gravity. When the coefficient of friction (μ) among the particles and between the particle and the wall of the container was 0.3, the packing density reached a value of 0.578. If there is no friction, the density reached 0.633 which is comparable to the random close‐packed density obtained in the random structures of steel ball bearings. For small particles, VDWI can reduce the packing density by agglomeration of particles into local clusters. For example, with VDWI and μ of 0.3, the random structures of particles with diameter 100 and 50 μm had packing densities of 0.528 and 0.420, respectively. When μ was increased to 0.7, the packing density of 100‐μm particles with VDWI was 0.505, compared to 0.528 in the case of μ equal to 0.3. The average velocity of the particles initially increased, and after reaching a maximum, started to decrease due to collisions amongst the particles. During the dynamic simulation, the trajectories of a few selected particles were traced. To study the rearrangement process, the average displacement of a few particles in a direction normal to the direction of gravity and the average angle of rotation of the straight line joining the centers of a few selected pairs of particles were calculated at various instants of time. It is seen that the rearrangement of 50‐μm particles was smaller than that of 100‐μm particles bec- ause of VDWI. The radial distribution functions of the computer‐generated close‐packed structure is similar to that of Finney’s close‐packed structure of steel balls. In the case of 50‐μm particles with VDWI, there were additional peaks close to the first peak, due to agglomerating effect of VDWI. The cumulative number of contacts was computed as a function of the radial distance and, in the case of 100‐μm particles without friction and VDWI, it agreed with Mason’s data for steel balls. View full abstract»

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  • On flow through porous material using a generalized Schwarz–Christoffel theory

    Page(s): 3174 - 3180
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    In the present paper we extend the work of Richardson [J. Fluid Mech. 49, 327 (1971)], on the analysis of a model flow through a porous material, to include grooves of arbitrary shape using a generalized Schwarz–Christoffel formula. The analysis reveals that the findings of Taylor [J. Fluid Mech. 49, 319 (1971)] are still true. View full abstract»

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  • Temporal behavior of population densities of V atoms in an optically pumped V vapor laser

    Page(s): 3181 - 3185
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    The time variation of population densities of V atoms in absorption levels pertinent to an optically pumped atomic V vapor laser is studied experimentally. V vapor is generated from a V metal by irradiation of a pulsed Nd:YAG (λ=1.06 μm) laser of 2 J/pulse energy and 1.4 ms pulse duration. The population densities of the ground‐state and excited‐state (E∼2400 cm-1) V atoms have been measured by the interferometric method, and the temperature in the vapor is estimated from these populations. It has been experimentally observed that the populations of the ground state and the excited state are about 1×1014/cm3 and 4×1013/cm3, respectively. The temperature in the vapor increased from about 1800 to 4000 K with time during the YAG laser irradiation. View full abstract»

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  • CF and CF2 actinometry in a CF4/Ar plasma

    Page(s): 3186 - 3192
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    Relative ground state CF and CF2 concentrations have been measured in a 13.56‐MHz rf CF4/Ar discharge using both laser‐induced fluorescence (LIF) and actinometric techniques to assess the validity of actinometry for CF and CF2 over a limited parameter space of pressure and power. Relative measurements of the CF ( A2Σ - X2Π) system and the CF2 ( A1B1 - X1A1) system were made by LIF. Actinometric values were calculated from relative measures of the plasma‐induced emission (PIE) intensity of the CF* ( B2Δ- X2Π) at 202.4 nm, CF2* ( A1B1 - X1A1) at 251.9 nm, and Ar* [4s’(1/2)°-4p’(1/2)] at 750.4 nm. Both LIF and PIE signals were spatially averaged over the bulk of the plasma. Steady‐state actinometric and LIF measurements were compared for CF4/5% Ar discharges at pressures in the range of 500 to 1000 mTorr and nominal powers in the range of 20 to 100 W. Dynamic actinometric and LIF measurements of CF were made by modulating the discharge power and monitoring the CF transient at one set of conditions, 500‐mTorr pressure and 70‐W nominal power. Our results indicate that actinometric measurements of CF and CF2 correlate well with relative CF and CF2 LIF measurements under the studied conditions. View full abstract»

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  • Effects of the energy distribution of fast electrons on H2 vibrational excitation in a tandem negative ion source

    Page(s): 3193 - 3196
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    The population distribution of vibrationally excited hydrogen molecules H2(v‘), and the H- production are investigated theoretically by solving numerically a set of particle balance equations in a steady‐state pure hydrogen plasma. In particular, the enhancement of the H2(v‘) distribution is discussed for different energy distributions of fast electrons ef. Whether the energy distribution for ef is a delta function type or a plateau type, collisional excitation of H2(v‘) caused by ef is very effective to enhance the H- production and a resultant vibrational distribution becomes the plateau distribution. Besides these, if ef with energies higher than 30–40 eV are present, both H2(v‘) excitation and then H- yield hardly depend on the form of the energy distribution of ef. View full abstract»

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  • Electrode shapes for high‐power diodes with non‐space‐charge‐limited flow

    Page(s): 3197 - 3200
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    Electrode shapes appropriate to diodes with non‐space‐charge‐limited flow (e.g., laser‐irradiated photocathodes) and high‐power (eV0 ≳ mc2) are derived. The electrode shapes are designed to keep the electron beam rectilinear, and generalize the shapes derived by Pierce for space‐charge‐limited cathodes in low‐power diodes. View full abstract»

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  • The influence of argon ion bombardment on form birefringence in thin films of titania

    Page(s): 3201 - 3203
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    Thin films of titanium oxide deposited in the presence of oxygen at oblique incidence under argon ion bombardment exhibit birefringence that depends on the energy and relative direction of the ion beam and material flux. View full abstract»

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  • Measurement of azimuthal anchoring energy in nematic liquid crystals by transmitted and reflected light methods

    Page(s): 3204 - 3215
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    In a recent paper Yokoyama [Mol. Cryst. Liq. Cryst. 165, 265 (1988)] showed that a correct thermodynamic definition of the anchoring energy at the interface between a nematic liquid crystal (NLC) and another medium requires the preliminary choice of a ‘‘Gibbs dividing surface.’’ As a consequence of this, the measured value of the anchoring energy coefficient can depend on which method is used to perform the experiment. This dependence probably explains the large discrepancies that are often reported in the literature between anchoring energy coefficients measured on the same substrate by different authors. To test this important point the anchoring energy coefficient at a SiOx nematic interface has been measured by using two different methods at the same time: a reflection light method and a transmitted light method. Both these methods have been already used in the literature. In this paper the latter method is analyzed in detail and it is shown that its accuracy is greatly reduced by the presence of spurious bulk contributions. To increase accuracy, a new experimental procedure is proposed that exploits the differing dependence of surface and bulk contributions on the intensity of the magnetic field. This new method allows one to separate bulk and surface contributions without using fitting procedures. Therefore both the anchoring energy and information on the bulk director distortion can be directly obtained from the experiment. The transmitted light and the reflected light methods are used simultaneously to measure the azimuthal anchoring energy coefficient at the SiOx nematic interface on the same NLC sample at the same temperatures. A satisfactory agreement between the anchoring energy coefficients measured by the two methods is found. View full abstract»

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  • Interdiffusion in polystyrene crosslinked by keV ion irradiation

    Page(s): 3216 - 3219
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    Mass transport properties of polymers are deeply influenced by the presence of crosslinks between chains, induced by high‐energy ion irradiation. Diffusion coefficients in polystyrene, measured after irradiation with 300‐keV protons in the fluence range 1012–1014 ions/cm2, show strong variation up to two orders of magnitude. Diffusion process has been studied in deuterated/hydrogenated polystyrene bilayer by using the forward recoil analysis technique with 2.0‐MeV He beam. The adopted experimental procedure allows us to detect diffusion coefficients in the range 5×10-13–10-15 cm2/s. Two molecular weights have been chosen for the hydrogenated polystyrene (7500 and 575 000 amu), in both experiments the molecular weight of deuterated polystyrene has been maintained constant (27 000 amu). Combining diffusion, molecular weight distribution, and solubility measurements performed in the same irradiation conditions, we have tested that diffusion process is slowed down when branching of chains or formation of a large unsoluble fraction occurs in the original polymer structure. View full abstract»

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  • Hall mobility and carrier concentration versus temperature for type IIa natural insulating diamond doped with boron by ion implantation

    Page(s): 3220 - 3224
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    A natural IIa diamond was implanted with boron ions at a substrate temperature of 80 K. Ohmic contacts (Mo/Au) were formed using a solid‐state reaction process. Van der Pauw resistivity/Hall measurements were taken as a function of temperature from room temperature to nearly 600 °C. Heating to approximately 350 °C was necessary to stabilize resistance values to over 3.4 MΩ/sq and 1.3 MΩ/sq for the unimplanted and implanted sides, respectively, indicating possible hydrogen incorporation into the diamond during annealing of the metallized sample in hydrogen. Comparison of the electrical properties of the implanted and unimplanted sides showed that the boron implant introduced electrically active acceptors. View full abstract»

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  • Plasma etch effects on low‐temperature selective epitaxial growth of silicon

    Page(s): 3225 - 3230
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    The quality of epitaxial silicon grown on regions exposed to plasma etching using He, CHF3, and CF4 etching gases has been studied. Plasma‐etch‐induced surface damage leads to defects in the epilayers. Dislocation loops and precipitates at the epi/substrate interface are always observed. High‐temperature prebake can further lead to microtwins and threading dislocations. The formation of defects in epilayers is attributed to the ion bombardment induced surface/subsurface defects and the reactive‐ion‐induced contaminants on exposed Si areas. A 900 °C grown sacrificial oxide film can efficiently remove both near‐surface defects and contaminants to get a clean SiO2/Si interface, resulting in high quality Si epilayers. A 5‐nm sacrificial oxide is adequate. An air exposure between several plasma etch cycles is also important in suppressing polymer accumulation on substrates. 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