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Journal of Applied Physics

Issue 10 • Date Nov 1992

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Displaying Results 1 - 25 of 90
  • Issue Table of Contents

    Page(s): toc1
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    Freely Available from IEEE
  • Optical properties of the magnetic monopole field applied to electron microscopy and spectroscopy

    Page(s): 4505 - 4513
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    An analytical treatment of the electron’s motion in a magnetic monopole field results in useful expressions for both the lens action and the mirror action of the field. Using an appropriate definition of the magnetic moment of the electron, it is shown that there is an exact conservation of this parameter in the monopole field, implying that the motion is perfectly adiabatic. This property is important when the field is used for directing Auger electrons from a target to a detector; that is, when it is used as a parallelizer in a through‐the‐lens detection scheme. Regarding the monopole field as an electron lens, the image position and magnification are derived for an arbitrary object position. Expressions for both the axial aberrations (chromatic and spherical) and the image aberrations (coma, field curvature, astigmatism, distortion, and transverse chromatic) are derived for an arbitrary number of intermediate images between object and final image. The chromatic aberration turns out to be independent of the number of intermediate images and the spherical aberration decreases slightly with this number. This property is important when an electron beam must be focused to a small probe in a strong magnetic field. It is shown that if a certain combination of deflectors is used in conjunction with the monopole field, an ideal swinging objective lens is obtained: All image aberrations except field curvature disappear. Designs are presented in which the monopole field is used in the objective lenses of a transmission electron microscope and a scanning electron microscope.   View full abstract»

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  • Three‐dimensional quasistationary approach to Cherenkov‐type optical harmonic generation using a unidirectional‐radiation model

    Page(s): 4514 - 4520
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    A three‐dimensional calculation of Cherenkov‐type frequency doubling in an optical channel waveguide is implemented by slicing the lateral profile of the interacting fields into a number of small segments. In each segment, lateral field variations of the harmonic signal are neglected. This method is more simple and requires less computational effort in comparison with a direct three‐dimensional beam simulation. With the unidirectional‐radiation model one can predict an optimum configuration and the conversion efficiency of miniaturized frequency doublers that utilize guided–to–radiation‐mode interactions in nonlinear channel waveguides. Typical results are shown for blue‐light generation through frequency doubling in a proton‐exchanged MgO‐doped lithium niobate channel waveguide. Analyses of green‐light generation in organic channel waveguides are reported as well. View full abstract»

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  • An optical waveguide with a nonlinear optical susceptibility inversion structure in the thickness direction

    Page(s): 4521 - 4528
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    A nonlinear optical susceptibility (χ(2)) inversion structure in the thickness direction (NIST) is proposed for an optical waveguiding layer to achieve efficient second‐harmonic generation (SHG) in the form of Cerenkov radiation. Using a coupled‐mode analysis, it is found that the SHG efficiency of the Langmuir–Blodgett (LB) film waveguide incorporating NIST is several tens of times larger than that of the conventional LB film waveguide. 2‐docosylamino‐5‐nitropyridine (DCANP) was used as the LB film material. High precision is not so strongly required for the inversion position in the waveguiding layer thickness as for the total thickness of the waveguiding layer, if the refractive indices are almost uniform in the layer. A guiding principle is shown for selecting suitable materials for the substrate and the clad to achieve the highest efficiency with Cerenkov angle approaching zero. A waveguide structure was designed using a kind of perfluoropolymer for the clad. Several fabrication methods are proposed for the NIST waveguide. An enhancement of 20–30 times in the efficiency of Cerenkov radiation SHG due to the NIST is also experimentally demonstrated using DCANP‐LB film as the waveguiding layer. View full abstract»

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  • Nearly diffraction‐free CO2 laser beam

    Page(s): 4529 - 4532
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    Nearly diffraction‐free beams have been generated by a CO2 laser with a new type of cavity composed of a concave mirror and an output mirror with high reflectivity (99.4%) in the marginal part and low reflectivity (94.5%) in the central part. The beam propagation behavior is reproduced fairly well by the Fresnel–Kirchhoff integral of the Bessel–Gauss beam. View full abstract»

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  • Analytical formulation of ionization source term for discharge models in argon, helium, nitrogen, and silane

    Page(s): 4533 - 4537
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    Simple analytical representations of the ionization source terms in argon, helium, nitrogen, and silane dc glow discharges for steady‐state and quasisteady‐state conditions are presented. These analytical forms express well the highly nonequilibrium nature of the ionization in the cathode fall and negative glow regions which cannot be described by a Townsend ionization coefficient depending on the local value of the reduced electric field. These source terms can be easily incorporated into fluid models of gas discharges. View full abstract»

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  • Modeling of the initiation and evolution of a laser‐ionized column in the lower atmosphere: 314.5 nm wavelength resonant multiphoton ionization of naturally occurring argon

    Page(s): 4538 - 4547
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    A model has been developed to examine the effects of a 314.5 nm wavelength laser pulse directed vertically through the atmosphere on the local ion and electron concentrations in the beam path. The 314.5 nm wavelength was selected to exploit a three‐photon excitation resonance with the 3p54s 1P10 excited state in argon. Absorption of a fourth photon of the same wavelength will ionize the excited atom. Using a rate equation formalism and a detailed collection of atmospheric chemistry reactions the model provides the concentrations of electrons and ions as functions of altitude, time, laser energy, relative humidity, focal characteristics of the pulse, and ambient atmospheric electric‐field conditions. Both linear and nonlinear effects on the propagation of the laser pulse have been taken into account. The calculated charged particle concentrations are used to estimate the conductivity of the ionized column. Results presented indicate peak electron densities up to 108 cm-3 can be created using laser intensities on the order of 108 W/cm2. Electron lifetimes in the column are typically 50–200 ns, however, the lifetime of the conductive channel is long in comparison (100 μs). Long‐lived ions created by charge transfer and attachment reactions between laser‐produced argon ions and electrons and other atmospheric species provide a persistent enhanced conductivity in the beam path. Model results demonstrate that laser pulses having peak powers attainable with current technology can perturb the conductivity of an ionized column by as much as seven orders of magnitude in comparison to ambient conditions. A distinct tradeoff between column length and the extent of ionization is evident in the results presented. The tradeoff is a result of the competing mechanisms of multiphoton ionization and stimulated Raman scattering and is discussed. View full abstract»

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  • Origins of charged particles in vapor generated by electron‐beam evaporation

    Page(s): 4548 - 4555
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    The origins of electrons and ions in uranium vapor generated by electron‐beam evaporation have been determined. Measurements were made for the electron emission current due to high‐energy electron‐beam irradiation on a uranium surface (backscattered electrons, etc.), thermionic emission current from the melt surface, and electron current due to vapor ionization. Comparison of these currents confirmed that vapor ionization was the main electron generation process at evaporation surface temperatures above 2200 K. The ionized vapor formed a weakly ionized plasma of very low electron temperature: The degree of ionization ≤1%, electron temperature ≤0.3 eV. The electron‐impact ionization process contributed mainly to plasma formation. Beam electrons, their backscattered electrons, and secondary electrons from the beam‐irradiated uranium surface were the source electrons for this process. Thermal ionization was the next major process. In addition to the plasma formation model, plasma behavior in vapor was described by a one‐dimensional symmetric expansion model. The calculated degree of ionization was in good agreement with the measured value over a wide range of evaporation temperatures and electron‐beam currents. View full abstract»

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  • Ignition and extinguishing characteristics of Cs‐Ba tacitron

    Page(s): 4556 - 4565
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    The operation characteristics of the Cs‐Ba tacitron as a switch are investigated experimentally in three modes: (a) breakdown mode; (b) I‐V mode; and (c) current modulation mode. The switching frequency, grid potentials for ignition and extinguishing of discharge, and the Cs pressure and emission conditions (Ba pressure and emitter temperature) for stable current modulation are determined. The experimental data is also used to determine the off time required for successful ignition, and the effects of the aforementioned operation parameters on the ignition duty cycle threshold for stable modulation. Operation parameters measured include switching frequency up to 8 kHz, hold‐off voltage up to 180 V, current densities in excess of 15 A/cm2, switch power density of 1 kW/cm2, and a switching efficiency in excess of 90% at collector voltages greater than 30 V. The voltage drop strongly depends on the Cs pressure and to a lesser extent on the emission conditions. Increasing the Cs pressure and/or the emission current lowers the voltage drop, however, for the same initial Cs pressure and emission conditions, the voltage drop in the I‐V mode is usually lower than that during current modulation. As long as the discharge current is kept lower than the emission current, the voltage drop during stable current modulation could be as low as 3 V. View full abstract»

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  • Ion kinetic‐energy distributions in argon rf glow discharges

    Page(s): 4566 - 4574
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    Kinetic‐energy distributions have been measured for different mass‐selected ions sampled from 13.56 MHz rf glow discharges in argon inside a ‘‘GEC rf reference cell.’’ The electrode geometry of this cell produces an asymmetric discharge and the cell is operated in a pressure regime where ion‐molecule collisions in the sheath region of the discharge are significant. Ions are sampled from the side of the plasma perpendicular to the interelectrode axis using an electrostatic energy analyzer coupled to a quadrupole mass spectrometer. Kinetic‐energy distributions for Ar+, Ar2+, Ar++, and ArH+ are presented as functions of applied rf voltage, gas pressure, and distance of the mass spectrometer entrance aperture from the edge of the electrodes. The distributions obtained for the sampling orifice placed close enough to the electrodes to allow formation of a sheath in front of the orifice exhibit features similar to those observed previously when sampling ions through the grounded electrode of a parallel‐plate reactor. The Ar+ and Ar++ distributions exhibit secondary maxima predicted to result from the formation of low‐energy (thermal) ions in the sheath region, such as by charge‐exchange and high‐energy electron collisions. Kinetic‐energy distributions for Ar2+ and ArH+ exhibit no secondary maxima and are peaked at high energies indicative of the sheath potential, and consistent with a formation mechanism involving relatively low‐energy collisions in the bulk plasma (glow region). View full abstract»

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  • Frequency shifting of electromagnetic radiation by sudden creation of a plasma slab

    Page(s): 4575 - 4580
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    The effect of switching a plasma medium of finite width (slab) on a time‐harmonic plane electromagnetic wave propagating in free space is considered. The transient solution is obtained through the use of Laplace transforms. The solution is broken into two components: A component which in steady state has the same frequency as the incident wave frequency (ω0) and a B component which has an upshifted frequency ω1 =√ω202p. Here ωp is the plasma frequency of the switched plasma slab. Numerical results are presented to show the effect of the slab width on the reflected and transmitted waves. A proper choice of ω0, ωp and the slab width will yield a transmitted electromagnetic pulse of significant strength at the upshifted frequency. View full abstract»

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  • Development of wall‐stabilized z discharges for intense ion‐beam transport in inertial confinement fusion facilities

    Page(s): 4581 - 4596
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    The wall‐stabilized z discharge has been scaled successfully to the parameters required for a light‐ion‐beam‐driven inertial confinement fusion facility. The electrical behavior of discharges with various gas species, pressures, lengths, and currents has been investigated. These investigations identify the required dielectric strength of the discharge channel wall. A low‐mass, low‐Z wall construction with sufficient dielectric strength is demonstrated. The discharge internal dynamics have been studied using temporal and imaging interferometry, framing photography, magnetic‐field measurement, and spectroscopy. The discharge current radial profile, and its dependence on discharge parameters, has been diagnosed. The discharge consists of a magnetohydrodynamically stable, imploding thick annulus. The observed radial profile explains data from previous transport experiments. Contamination of the discharge by wall material is found to be negligible during the times of interest. These observations motivate a zero‐dimensional model of discharge behavior. This model reproduces approximately both the discharge dynamics and the electrical characteristics over a range of parameters. Calculations indicate that the beam ions will lose only 10% of their energy during transport through the discharge in a fusion facility. A conceptual design for a z‐discharge transport system is presented. The results of this work confirm that wall‐stabilized z‐discharge transport is a viable, backup approach to transport in a light‐ion‐beam‐driven inertial confinement fusion facility. View full abstract»

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  • Factors affecting the Cl atom density in a chlorine discharge

    Page(s): 4597 - 4607
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    A mathematical model was developed for the bulk plasma of an electrodeless chlorine discharge sustained in a tubular reactor. The model was used to investigate the factors affecting the Cl atom density in the plasma. Rate coefficients for electron‐particle reactions in the Cl2/Cl mixture were obtained by solving the Boltzmann transport equation for the electron‐energy distribution function. These rate coefficients were then used in a plasma model to calculate the self‐sustaining electric field, electron density, and atomic chlorine density in the plasma. The effect of frequency, power, gas flow rate, neutral density, tube radius, and wall recombination coefficient was examined. For otherwise identical conditions, nearly the same atom density was obtained in 13.56 MHz and 2.45 GHz discharges. It was found that very high degrees of molecular dissociation are possible with only a few W/cm3 in the plasma. Despite the fact that the atom density decreased with increasing feed gas flow rate, the atom flux increased with flow rate. In the parameter range investigated, lower pressures and larger tube radii favored higher atom density in the plasma. The model is useful for optimizing source efficiency and for use as a ‘‘module’’ in multidimensional radical transport and reaction models of remote plasma processing reactors. View full abstract»

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  • Cation distribution and magnetization of BaFe12-2xCoxSnxO19 (x=0.9,1.28) single crystals

    Page(s): 4608 - 4614
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    The distribution of Sn4+ cations within the five crystallographic sites of the magnetoplumbite (M) ‐like compound BaFe12-2xCoxSnxO19 has been analyzed using single‐crystal x‐ray‐diffraction data. The species Fe3+ and Co2+ cannot be distinguished using x rays because of their very similar atomic numbers; however, the calculation of the apparent valencies for the different sites allows an insight into the Co2+ cation segregation. The use of previous data from neutron powder diffraction allows a precise picture of the cation distribution, which indicates a pronounced site selectivity for both Sn4+ and Co2+ cations. The Sn4+ cations prefer the 4f2 sites and to a much lower extent the 12k sites, while they do not enter the octahedral 2a sites at all. Co2+ cations are distributed among tetrahedral and octahedral sites displaying a clear preference for the tetrahedral 4f1 sites. Magnetic measurements indicate that the compound still exhibits uniaxial anisotropy with the easy direction parallel to the c axis. Nevertheless, the magnetic structure shows a considerable degree of noncolinearity. A strong reduction of the magnetic anisotropy regarding that of the undoped compound is also detected. View full abstract»

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  • Interstitial oxygen reduction in silicon at thermal donor temperatures

    Page(s): 4615 - 4618
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    Silicon wafers with a range of initial oxygen and carbon concentration were annealed at 450, 475, and 500 °C for up to 500 h with the interstitial oxygen concentration being monitored every 20 h. It is found that the rate of oxygen loss scales as the fifth power of oxygen concentration for wafers annealed below 500 °C. View full abstract»

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  • Formation of cobalt‐silicided p+n junctions using implant through silicide technology

    Page(s): 4619 - 4626
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    This paper investigates electrical and material properties of cobalt silicided p+n junctions fabricated using implant through silicide (ITS) technology. The annealing procedure was carried out in an open‐tube furnace with flowing nitrogen. To prevent residual oxygen in the furnace from reacting with the cobalt, a passivating film of molybdenum was used during the initial stage of annealing. BF2+ ion implantation was employed for the p+n junction formation. The ITS scheme and the subsequent annealing conditions were evaluated by analysis of the material properties and investigation of the electrical characteristics of the silicided junctions. During high‐temperature annealing (≥900 °C), Co silicide releases its high surface energy via silicon precipitation and film agglomeration. High‐temperature stability of the Co silicide can be improved by BF2+ ion implantation, as indicated by the retardation of film agglomeration and decreased degradation of sheet resistance. Cobalt‐silicided p+n junction diodes with 0.1 μm junction depth measured from the silicide/silicon interface were fabricated at a 700 °C annealing and shown to possess excellent electrical properties. The leakage current density measured at -5 V was 0.5 nA/cm2 and a forward ideality factor of 1.006 was obtained. It was found that Co silicide is suitable to serve as an energy barrier and dopant diffusion source for the ITS scheme at low‐temperature annealing. For elevated temperatures (≥900 °C), the use of cobalt silicide as a dopant diffusion source becomes impractical because the dopant evaporation and silicide agglomeration severely degrade junction performance. Nevertheless, the silicide layer can be used as an energy barrier and implantation damage basin for high‐temperature processes. No evidence of the - electrically activated trapping centers related to the cobalt atoms was observed with BF2+ implantation at energies up to 100 keV. Thus, there is no fundamental limitation prohibiting the formation of cobalt‐silicided shallow junctions being fabricated using ITS technology. View full abstract»

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  • Diffraction from multilayer films with partially correlated interfacial roughness

    Page(s): 4627 - 4633
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    The interfacial roughness in multilayer films is described by a model in which the parameters are directly related to those of microscopic growth processes. The height‐height correlation function is extracted from this model. Stationary and nonstationary roughness are discussed. The diffuse intensity in diffraction from multilayer films that have various degrees of correlation in roughness between interfaces is calculated. The presence of perfect or partial interfacial roughness correlation can be distinguished unambiguously in the diffusely scattered intensity. View full abstract»

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  • Electron microscopic study of partial dislocations in textured Y‐Ba‐Cu‐O superconductors

    Page(s): 4634 - 4637
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    Electron microscopy shows that textured Y‐Ba‐Cu‐O superconductors contain many partial dislocations associated with stacking faults in a‐b planes. Two types of partial dislocations parallel to the [100] and [010] direction have been identified with Burgers vectors [a/2, 0, c/6] and [0, -b/2, c/6], respectively, which are the same as the displacement vectors of the associated stacking faults. View full abstract»

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  • Deformation and thermal oxidation of GaAsP wafers locally heated by a Nd:Y3Al5O12 (yttrium aluminum garnet) laser beam

    Page(s): 4638 - 4642
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    A Ga0.6As0.4P epitaxial wafer, deposited on a GaAs substrate with a compositional graded layer placed between these two substances, was locally heated to temperatures ranging from 600 to 1300 °C (melting point) in air by a Nd:Y3Al5O12 (yttrium aluminum garnet) laser beam. The characteristics of the wafer were determined by three different x‐ray diffraction methods. When the temperature gradient was very steep, large disordering took place in the irradiated region of the wafer. This resulted in the formation of explosive disordered GaAsP, fibrous β‐Ga2O3 on the epitaxial‐layer side, and GaPO4 polycrystals on the graded‐layer side. The characteristic formation mechanisms for these by‐products are discussed. View full abstract»

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  • Carbon diffusion in uncoated and titanium nitride coated iron substrates during microwave plasma assisted chemical vapor deposition of diamond

    Page(s): 4643 - 4647
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    Auger electron spectroscopy has been employed to investigate the effectiveness of thin films of TiN as barriers to carbon diffusion during chemical vapor deposition (CVD) of diamond onto Fe substrates. Auger depth profiling was used to monitor the C concentration in the TiN layer, through the interface and into the substrate both before and after CVD diamond deposition. The results show that a layer of TiN only 250 Å thick is sufficient to inhibit soot formation on the Fe surface and C diffusion into the Fe bulk. View full abstract»

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  • Silicon‐doping level dependent diffusion of Be in AlGaAs/GaAs quantum well lasers

    Page(s): 4648 - 4654
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    We have investigated Be diffusion during molecular beam epitaxial growth of GaAs/AlGaAs graded index separate confinement heterostructure laser structures using secondary ion mass spectrometry (SIMS). For growth at 700 °C we find that Be from the p‐type AlGaAs cladding layer diffuses into the quantum well and beyond. As a result, the p‐n junction is displaced from the heterojunction. The extent of Be diffusion is found to depend on the dopants in the graded index (GRIN) regions adjoining the GaAs active layer. When the GRIN segments are left intentionally undoped, Be diffuses through the entire p‐side GRIN, the quantum well active and a significant portion of the n‐side GRIN. However, when the GRIN regions are doped, respectively, with Be and Si on the p and n sides, the displacement of the p‐n junction caused by Be diffusion is significantly reduced. Assuming that Be diffuses from a constant source at the surface into a n‐type layer as a singly charged interstitial donor, our analysis predicts that increasing the doping of the n layer retards the diffusion of Be while that of the p layer enhances it. Further, including the electric field of the p‐n junction in the model leads to peaks and inflections resembling those observed in the experimental SIMS profiles. In view of Be‐related oxygen contamination and Be diffusion on the p‐side GRIN region, Be should be dispensed with on the p side, however, Si addition on the n side is beneficial as it minimizes Be diffusion and p‐n junction displacement. View full abstract»

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  • Crystallinity of ZnS:Tb,F thin films and characteristics of green‐color thin‐film electroluminescent devices prepared by rf‐magnetron sputtering

    Page(s): 4655 - 4659
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    In order to fabricate high‐brightness thin‐film electroluminescent devices, the dependence of crystallinity on deposition conditions of the ZnS:Tb,F thin films deposited by rf‐magnetron sputtering system have been studied. The optimal deposition conditions to get the best crystallinity are obtained as rf power density of 4.39 W/cm2, substrate temperature 150 °C, and post‐anneal at 550 °C for 1 h. The (111) plane spacing and lattice constant are 3.1238 and 5.411 Å, respectively. The green electroluminescent devices with the structure of glass/indium‐tin‐oxide/SiO2/HfO2/ZnS:Tb,F/HfO2/SiO2/Al have the highest brightness and luminous efficiency hη of 830 cd/m2 and 0.8 lm/W, respectively, under 1 kHz sinusoidal wave voltage excitation and the Commission Internationale de l’Eclairage chromaticity is x=0.3096, y=0.5998, respectively. View full abstract»

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  • Titanium disilicide formation by interdiffusion of titanium/amorphous silicon multilayers: Influence of the bilayer silicon to titanium thickness ratio on the film properties

    Page(s): 4660 - 4668
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    Titanium disilicide is formed by thermal annealing of amorphous Si/Ti multilayers, deposited on monocrystalline silicon by alternate electron‐gun evaporation of Si and Ti. The bilayer periodicity was equal to 4. The bilayer Si/Ti thickness ratio was varied between 2 and 3 and its influence on the film properties (stoichiometry, surface and interface roughness and resistivity) was studied and compared to the properties of a silicide of the same thickness, formed by annealing of a single deposited Ti layer on monocrystalline silicon. The characterization techniques used are: x‐ray‐diffraction, Rutherford backscattering, scanning electron microscopy, surface profilometry, and electrical measurements. The reaction rate of deposited Ti with monocrystalline silicon for the formation of TiSi2 is also compared in a special experiment to that of deposited Ti with amorphous Si (α‐Si). View full abstract»

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  • Density and defects in thin metal films using x‐ray reflectivity and variable‐energy positrons

    Page(s): 4669 - 4673
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    X‐ray reflectivity has been used to determine the absolute metal density for both metals in bilayers of Al on top of Co, Cr, Cu, Mn, Ni, and Pd. A large variation in density is found with an observed range of 0.87–1.0 of bulk values. The results can be correlated with changes in the defect character as determined by variable‐energy positron measurements. The size of the open volume defects systematically increases as the metal density decreases. A distinct densification of the Co layer was observed after annealing, and was accompanied by a corresponding reduction in the average size of the defects. There seems to be at least a partial correlation of the density with the melting point of the metals, although other factors such as the crystal structure are likely important. These results also demonstrate the application of x‐ray reflectivity and variable‐energy positrons to studies of thin metal films, and a discussion of their potential utility is included. View full abstract»

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  • One‐dimensional diffusion into a multilayer structure: An exact solution for a bilayer

    Page(s): 4674 - 4676
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    An exact solution to the problem of time‐dependent, one‐dimensional diffusion in two parallel layers backed by an impenetrable wall is presented, together with graphical results for some numerical examples. The solution can be generalized to more than two layers, and it is expected to be useful to several areas of applied science. This includes (a) ion diffusion in multilayer thin‐film solid‐state ionic devices such as electrochromic windows and rechargeable batteries, (b) mass diffusion of neutral species (e.g., dopants) encountered in the fabrication of multilayer thin‐film integrated electronics and integrated optics circuits, and (c) heat diffusion through a multiplicity of parallel layers, each of which have different diffusion coefficients.   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