Showing papers in "Journal of Applied Physics in 1990"
TL;DR: In this article, the authors studied the physical processes involved in laser-produced plasma in confined geometry, and showed that a large fraction of the absorbed laser energy (80% to 90%) is used for the ionization of the medium in these conditions of irradiation.
Abstract: We study in this paper the different physical processes involved in laser‐produced plasma in confined geometry. With this technique, a laser irradiates a target at an intensity of a few GW/cm2, and the produced plasma is confined by a transparent overlay to the laser which covers this target. This configuration has appeared necessary for example for metallurgical applications where, for a given laser energy, enhanced pressures must be realized in order to achieve high shock pressures. Therefore, a physical study of this method is useful in order to optimize this technique. We have first developed an analytical model which describes the different steps involved in this process, points out the interest of this technique, and compares it to the direct ablation regime. In the first stage, during the laser heating, the generated pressure is typically 4–10 times greater than the corresponding one obtained in direct ablation. The second step begins after the switch‐off of the laser and is characterized by an adiabatic cooling of the plasma which maintains the applied pressure over a period which is about 2 times the laser‐pulse duration. Finally, the third stage concerns also the adiabatic cooling of the recombined plasma, but during this period the exerted pressure is too small to realize a plastic deformation of the material. We show that the impulse momentum given to the target is mainly generated during this step. This model allows us to also determine the velocities of thin foils accelerated with confined plasmas, and we show that very high hydrodynamic efficiencies can be achieved by this technique. Experimentally, we measured with quartz gauges, the pressures obtained in confined geometry, for 30‐, 3‐, and 0.6‐ns laser‐pulse duration. This study shows that short pulse durations are sensitive to the initial roughness of the interface, and such an effect should be suppressed by using a liquid confinement. Then, we conclude that a large fraction of the absorbed laser energy (80%–90%) is used for the ionization of the medium in these conditions of irradiation. Finally, we experimentally point out that the laser‐induced breakdown of the confining medium is the main mechanism which limits the generated pressure and show the influence of the laser‐pulse duration on this effect.
1,188 citations
TL;DR: In this article, the dielectric relaxation of a solid solution of lead titanate in lead magnesium niobate is found to be similar to the magnetic relaxation in spin-glass systems.
Abstract: The dielectric relaxation of a solid solution of 10‐mol % lead titanate in lead magnesium niobate is found to be similar to the magnetic relaxation in spin‐glass systems.1–3 Based on this analogy, it is proposed that the relaxor ferroelectric is a polar‐glassy system which has thermally activated polarization fluctuations above a static freezing temperature. An activation energy and freezing temperature of 0.0407 eV and 291.5 K, respectively, were found by analyzing the frequency dependence of the temperature of the dielectric maximum using the Vogel–Fulcher relationship.4,5 It has also been shown that a macroscopic polarization is sustained on heating up to this freezing temperature. A coupling between nanometer scale clusters is believed to control the kinetics of the fluctuations and the development of a frustration as the system freezes into states of local equilibrium. The possibility of an orientational freezing associated with the ferroelastic nature of the nanosized polar regions in the rhombohedr...
1,148 citations
TL;DR: In this article, the control factors controlling the growth of native silicon oxide on silicon (Si) surfaces have been identified, and the chemical bond structures for native oxide films grown in air and in ultrapure water are also discussed.
Abstract: The control factors controlling the growth of native silicon oxide on silicon (Si) surfaces have been identified. The coexistence of oxygen and water or moisture is required for growth of native oxide both in air and in ultrapure water at room temperature. Layer‐by‐layer growth of native oxide films occurs on Si surfaces exposed to air. Growth of native oxides on n‐Si in ultrapure water is described by a parabolic law, while the native oxide film thickness on n +‐Si in ultrapure water saturates at 10 A. The native oxide growth on n‐Si in ultrapure water is continuously accompanied by a dissolution of Si into the water and degrades the atomic flatness at the oxide‐Si interface, producing a rough oxide surface. A dissolution of Si into the water has not been observed for the Si wafer having surface covered by the native oxide grown in air. Native oxides grown in air and in ultrapure de‐ionized water have been demonstrated experimentally to exhibit remarkable differences such as contact angles of ultrapure waterdrops and chemical binding energy. These chemical bond structures for native oxide filmsgrown in air and in ultrapure water are also discussed.
803 citations
TL;DR: In this paper, a selfconsistent, one-dimensional solution of the Schrodinger and Poisson equations is obtained using the finite-difference method with a nonuniform mesh size.
Abstract: A self‐consistent, one‐dimensional solution of the Schrodinger and Poisson equations is obtained using the finite‐difference method with a nonuniform mesh size. The use of the proper matrix transformation allows preservation of the symmetry of the discretized Schrodinger equation, even with the use of a nonuniform mesh size, therefore reducing the computation time. This method is very efficient in finding eigenstates extending over relatively large spatial areas without loss of accuracy. For confirmation of the accuracy of this method, a comparison is made with the exactly calculated eigenstates of GaAs/AlGaAs rectangular wells. An example of the solution of the conduction band and the electron density distribution of a single‐heterostructure GaAs/AlGaAs is also presented.
674 citations
TL;DR: In this article, it was shown that the properties of the deep level when resonant with the conduction band are similar to its characteristics when it is the stable state of the donor.
Abstract: D X centers, deep levels associated with donors in III‐V semiconductors, have been extensively studied, not only because of their peculiar and interesting properties, but also because an understanding of the physics of these deep levels is necessary in order to determine the usefulness of III‐V semiconductors for heterojunction device structures. Much progress has been made in our understanding of the electrical and optical characteristics of D X centers as well as their effects on the behavior of various device structures through systematic studies in alloys of various composition and with applied hydrostatic pressure. It is now generally believed that the D X level is a state of the isolated substitutional donor atom. The variation of the transport properties and capture and emission kinetics of the D X level with the conduction‐band structure is now well understood. It has been found that the properties of the deep level when it is resonant with the conduction band, and is thus a metastable state, are similar to its characteristics when it is the stable state of the donor. And it has been consistently found that there is a large energy difference between the optical and thermal ionization energies, implying that this deep state is strongly coupled to the crystal lattice. The shifts in the emission kinetics due to the variation in the local environment of the donor atom suggest that the lattice relaxation involves the motion of an atom (the donor or a neighboring atom) from the group‐III lattice site toward the interstitial site. Total energy calculations show that such a configuration is stable provided that the donor traps two electrons, i.e., has negative U. Verification of the charge state of the occupied D X level is needed as well as direct evidence for its microscopic structure.
673 citations
TL;DR: In this paper, the authors used the ac detection method which senses the force gradient acting on a small magnetic tip due to fields emanating from the domain structure in the sample, and showed that the final 20 μm is essentially single domain with magnetization approximately parallel with the tip axis.
Abstract: This paper discusses the principles of magnetic force microscopy (MFM) and its application to magnetic recording studies. We use the ac detection method which senses the force gradient acting on a small magnetic tip due to fields emanating from the domain structure in the sample. Tip fabrication procedures are described for two types of magnetic tips: etched tungsten wires with a sputter‐deposited magnetic coating and etched nickel wires. The etched nickel wires are shown to have an apex radius on the order of 30 nm and a taper half‐angle of approximately 3°. Lorentz‐mode transmission electron microscopy of the nickel tips reveals that the final 20 μm is essentially single domain with magnetization approximately parallel with the tip axis. Images of written bit transitions are presented for several types of magnetic media, including CoPtCr, CoSm, and CoCr thin films, as well as γ‐Fe2O3 particulate media. In general, the written magnetization patterns are seen with high contrast and with resolution better ...
606 citations
TL;DR: In this paper, an inconsistency between commonly used values of the silicon intrinsic carrier concentration, the effective densities of states in the conduction and valence bands, and the silicon band gap is resolved by critically assessing the relevant literature.
Abstract: An inconsistency between commonly used values of the silicon intrinsic carrier concentration, the effective densities of states in the conduction and valence bands, and the silicon band gap is resolved by critically assessing the relevant literature. As a result of this assessment, experimentally based values for the valence‐band ‘‘densities‐of‐states’’ effective mass are determined in the 300–500 K range and are shown to be in good agreement with recent theoretical calculations. At 300 K, experimentally based values of 3.1×1019 cm−3 for the valence‐band effective densities of states and 1.08×1010 cm−3 for the intrinsic carrier concentration are determined. Although in good agreement with theoretical calculations, these are significantly higher and lower, respectively, than commonly used values in the past. These results have important implications in the calculation of other silicon material and device parameters.
600 citations
TL;DR: In this article, the physics of the growth mechanisms, characterization of epitaxial structures and device properties of GaAs and other compound semiconductors on Si are reviewed, and the nontrivial problems associated with the heteroepitaxial growth schemes and methods that are generally applied in the growth of lattice mismatched and polar on nonpolar material systems are described in detail.
Abstract: The physics of the growth mechanisms, characterization of epitaxial structures and device properties of GaAs and other compound semiconductors on Si are reviewed in this paper. The nontrivial problems associated with the heteroepitaxial growth schemes and methods that are generally applied in the growth of lattice mismatched and polar on nonpolar material systems are described in detail. The properties of devices fabricated in GaAs and other compound semiconductors grown on Si substrates are discussed in comparison with those grown on GaAs substrates. The advantages of GaAs and other compound semiconductors on Si, namely, the low cost, superior mechanical strength, and thermal conductivity, increased wafer area, and the possibility of monolithic integration of electronic and optical devices are also discussed.
542 citations
TL;DR: In this article, the authors used dc magnetron sputter deposition in mixed Ar+N2 discharges with an applied negative substrate bias of either 0 or 150 V. The oxide overlayers consisted of two partially crystalline sublayers, the upper one Al-rich and the lower one Ti-rich.
Abstract: Metastable single‐phase, NaCl‐structure, polycrystalline Ti0.5Al0.5N alloy films have been shown to exhibit much better high‐temperature (750–900 °C) oxidation resistance than polycrystalline TiN films grown under similar conditions. The Ti0.5Al0.5N alloys, ≂3 μm thick, were deposited at temperatures between 400 and 500 °C on stainless‐steel substrates by dc magnetron sputter deposition in mixed Ar+N2 discharges with an applied negative substrate bias Vs of either 0 or 150 V. Oxidation in pure O2 initially occurred at a rate that varied parabolically with time. The oxide overlayers consisted of two partially crystalline sublayers, the upper one Al‐rich and the lower one Ti‐rich, with no measurable N concentrations in either. Inert‐marker transport experiments showed that oxidation proceeded by the simultaneous outward diffusion of Al to the oxide/vapor interface and inward diffusion of O to the oxide/nitride interface. The oxidation rate constant K increased with oxidation temperature Tox at a rate much h...
494 citations
TL;DR: In this article, the advantages of using multi-quantum-well or superlattice systems as the absorbers in concentrator solar cells are discussed, and an effective band-gap variation that covers the high-efficiency region of the solar spectrum can be obtained.
Abstract: The advantages of using multi‐quantum‐well or superlattice systems as the absorbers in concentrator solar cells are discussed. By adjusting the quantum‐well width, an effective band‐gap variation that covers the high‐efficiency region of the solar spectrum can be obtained. Higher efficiencies should result from the ability to optimize separately current and voltage generating factors. Suitable structures to ensure good carrier separation and collection and to obtain higher open‐circuit voltages are presented using the (AlGa)As/GaAs/(InGa)As system. Efficiencies above existing single‐band‐gap limits should be achievable, with upper limits in excess of 40%.
491 citations
TL;DR: In this article, a numerical algorithm for the solution of the two-dimensional effective mass Schrodinger equation for current-carrying states is developed and boundary conditions appropriate for such states are developed and a solution algorithm constructed that is based on the finite element method.
Abstract: A numerical algorithm for the solution of the two‐dimensional effective mass Schrodinger equation for current‐carrying states is developed. Boundary conditions appropriate for such states are developed and a solution algorithm constructed that is based on the finite element method. The utility of the technique is illustrated by solving problems relevant to submicron semiconductor quantum device structures.
TL;DR: In this article, Fourier transform infrared absorption spectroscopy has been used to characterize the structure of porous silica gel films, both deposited on c-Si substrates and free standing.
Abstract: Fourier transform infrared absorption spectroscopy has been utilized to characterize the structure of porous silica gel films, both deposited on c‐Si substrates and free standing. The films were either dried at room temperature or subjected to partial densification at 400–450 °C. The spectra of the gel films are compared to those of thermal SiO2 grown on c‐Si and to Kramers–Kronig analysis of the reflection spectra of bulk SiO2 gels and v‐SiO2. The gel films show small frequency shifts compared to the latter spectra and they also exhibit new bands due to the presence of OH groups, although very little molecular water or residual organic species were found. The results are interpreted in terms of the gel structure. Compared to the thermal oxide, the sharp peak near 1070 cm−1 is narrower for the gels and the spread in intertetrahedral angles is estimated at 24° and 27° for room temperature dried and partially densified gels, respectively, compared to 33° for the thermal oxide. This is in agreement with a st...
TL;DR: In this article, a theoretical model of fatigue in ferroelectric thin-film memories based upon impact ionization (e.g., Ti+4 to Ti+3 conversion in PbZr1−xTixO3), resulting in dendritic growth of oxygen-deficient filaments, is presented.
Abstract: A theoretical model of fatigue in ferroelectric thin‐film memories based upon impact ionization (e.g., Ti+4 to Ti+3 conversion in PbZr1−xTixO3), resulting in dendritic growth of oxygen‐deficient filaments, is presented. The predictions of spontaneous polarization versus switching cycles Ps(N) are compared with both Monte Carlo simulations for a two‐dimensional Ising model and with experimental data on small‐grain (40 nm) sol‐gel PZT films. Excellent agreement between theory and experiment is obtained. In addition to modeling the Ps(N) curves, the theory developed explains the observed linear proportionality between switching time ts(N) and polarization Ps(N) during fatigue; other models of aging do not account for this. Earlier theories of switching are also extended to include finite grain sizes, surface nucleation, triangular drive pulses, and dipolar forces. Good agreement with sol‐gel PZT switching data is obtained.
TL;DR: A physicochemical model based on the modified classical heterogeneous nucleation theory was proposed to analyze the ice formation inside biological cells during freezing, and a method to estimate the nucleation frequency from the observed probability of nucleation was suggested.
Abstract: A physicochemical model based on the modified classical heterogeneous nucleation theory was proposed to analyze the ice formation inside biological cells during freezing. According to this model, intracellular ice formation (IIF) may be catalyzed either by the plasma membrane via the effects of the external ice on the plasma membrane, called surface‐catalyzed nucleation (SCN), or by the intracellular particles, called volume‐catalyzed nucleation (VCN), depending on the freezing conditions. The model for IIF was coupled with the model describing the kinetics of water transport to predict the thermodynamic state of the cytoplasm.A method to estimate the nucleation frequency from the observed probability of nucleation was suggested. This method was based on the assumption that each cell has the same heteronucleating particles with identical properties to alter the nucleation kinetics in an identical way. This allowed the correlation of the experimental probability of IIF with the nucleation rate. It was sugg...
TL;DR: In this article, a fast (5 min) essentially preparation-free large area (25 cm2 ) technique is presented to determine the Fe concentration in boron-doped silicon with a sensitivity of 2.5×1011 cm−3.
Abstract: Iron is one of the most important impurities in silicon integrated‐circuit technology. We present a fast (5 min), essentially preparation‐free large‐area (25 cm2 ) technique to determine the Fe concentration in boron‐doped silicon with a sensitivity of 2–5×1011 cm−3. The principle of the method is based on the fact that interstitially dissolved Fe undergoes a reversible pairing reaction with boron and that the minority‐carrier diffusion length—as measured by the surface photovoltage method—is modified by this reaction. The method has been calibrated by deep‐level transient spectroscopy and is also suitable to measure a surface Fe contamination in combination with a rapid thermal annealing diffusion step.
TL;DR: In this article, a physically based methodology is developed for modeling the behavior of electrical circuits containing nonideal ferroelectric capacitors, illustrated by modeling the discrete capacitors as a stacked dielectric structure.
Abstract: A physically based methodology is developed for modeling the behavior of electrical circuits containing nonideal ferroelectric capacitors. The methodology is illustrated by modeling the discrete ferroelectric capacitor as a stacked dielectric structure, with switching ferroelectric and nonswitching dielectric layers. Electrical properties of a modified Sawyer–Tower circuit are predicted by the model. Distortions of hysteresis loops due to resistive losses as a function of input signal frequency are accurately predicted by the model. The effect of signal amplitude variations predicted by the model also agree with experimental data. The model is used as a diagnostic tool to demonstrate that cycling degradation, at least for the sample investigated, cannot be modeled by the formation of nonswitching dielectric layer(s) or the formation of conductive regions near the electrodes, but is consistent with a spatially uniform reduction in the number of switching dipoles.
TL;DR: In this article, the authors describe the electrical admittance of a plane-parallel resonator loaded on one face with a viscoelastic medium, including the piezoelectric effect of the quartz transducer and the shear modulus and viscosity of the overlayer.
Abstract: In this work, the electrical admittance of this plane‐parallel resonator loaded on one face with a viscoelastic medium is described directly in terms of the physical properties of the system. This description is based on the detailed matrix equations for the piezoelectric quartz and includes the piezoelectric effect of the quartz transducer and the shear modulus and viscosity of the overlayer. This physically based analysis has proven invaluable in relating the admittance behavior directly to the materials’ properties. Initial comparisons of the theory with experimental data are given, including the appearance of even harmonics as described by Benes [E. Benes, J. Appl. Phys. 56, 608 (1984)] for the asymmetrically loaded resonator. Additionally, measurements on a series of perfluoropolyether fluids of differing molecular weights indicate that they cannot be described as simple viscous liquids in the MHz range. The assignment of a frequency‐dependent viscosity with a relaxation time proportional to the mole...
TL;DR: In this paper, the structure and morphology of polycrystalline diamond films prepared by chemical vapor deposition (CVD) have been studied using x-ray texture analysis, angle-resolved optical reflection, and scanning electron microscopy.
Abstract: Structure and morphology of polycrystalline diamond films prepared by chemical vapor deposition (CVD) have been studied using x‐ray texture analysis, angle‐resolved optical reflection, and scanning electron microscopy. The films under investigation exhibit a pronounced 110 fiber texture, i.e., a preferential alignment of {110} planes perpendicular to the growth direction. By thinning a 180‐μm‐thick CVD diamond film in an oxygen discharge the dependence of the degree of 110 texture on the film thickness has been investigated. It was found that the crystals formed at the beginning of the film growth are randomly oriented, and that a preferential orientation of {110} planes develops with increasing film thickness. Computer simulations show that this behavior can be explained by evolutionary selection, i.e., competing growth of differently oriented crystals, which implies that 〈110〉 is the direction of fastest growth. In addition, angle‐resolved optical reflection and scanning electron micrographs show that t...
TL;DR: Theoretical efficiencies for two-junction, series-connected solar cells using air mass 1.5 global and direct irradiance spectra were derived in this article, where thinning of the top cell was shown to result in significant increases in the theoretical efficiencies.
Abstract: Theoretical efficiencies are calculated for two‐junction, series‐connected solar cells using air mass 1.5 global and direct irradiance spectra. For band‐gap combinations previously limited by a low bottom‐cell current, thinning of the top cell is shown to result in significant increases in the theoretical efficiencies. The increases are primarily due to increased short‐circuit currents, since current matching is achievable. Smaller gains are also seen in the open‐circuit voltages of the thinner cells when a low surface‐recombination velocity is assumed. Thus, a number of material combinations which previously could only be used in four‐terminal configurations can now be considered for series‐connected two‐junction solar cells.
TL;DR: In this paper, the infrared photoluminescence (PL) from InSb, InAs, and InAs1−xSbx (x < 0.3) epitaxial layers grown by atmospheric pressure organometallic vapor phase epitaxy has been investigated for the first time over an extended temperature range.
Abstract: Infrared photoluminescence (PL) from InSb, InAs, and InAs1−xSbx (x<0.3) epitaxial layers grown by atmospheric pressure organometallic vapor phase epitaxy has been investigated for the first time over an extended temperature range. The values of full width at half maximum of the PL peaks show that the epitaxial layer quality is comparable to that grown by molecular‐beam epitaxy. The observed small peak shift with temperature for most InAs1−xSbx epilayers may be explained by wave‐vector‐nonconserving transitions involved in the PL emission. For comparison, PL spectra from InSb/InSb and InAs/InAs show that the wave‐vector‐conserving mechanism is responsible for the PL emission. The temperature dependence of the energy band gaps, Eg, in InSb and InAs is shown to follow Varshni’s equation Eg(T)=Eg0−αT2/ (T+β). The empirical constants are calculated to be Eg0=235 meV, α=0.270 meV/K, and β=106 K for InSb and Eg0=415 meV, α=0.276 meV/K, and β=83 K for InAs.
TL;DR: In this paper, the Barkhausen effect in metallic ferromagnetic systems is theoretically investigated by a Langevin description of the stochastic motion of a domain wall in a randomly perturbed medium.
Abstract: The Barkhausen effect (BE) in metallic ferromagnetic systems is theoretically investigated by a Langevin description of the stochastic motion of a domain wall in a randomly perturbed medium. BE statistical properties are calculated from approximate analytical solutions of the Fokker–Planck equation associated with the Langevin model, and from computer simulations of domain‐wall motion. It is predicted that the amplitude probability distribution P0(Φ) of the B flux rate Φ should obey the equation P0(Φ)∝Φc−1 exp(−cΦ/〈Φ〉), with c>0. This result implies scaling properties in the intermittent behavior of BE at low magnetization rates, which are described in terms of a fractal structure of fractal dimension D<1. Analytical expressions for the B power spectrum are also derived. Finally, the extension of the theory to the case where many domain walls participate in the magnetization process is discussed.
TL;DR: In this article, a spherical reflector illuminated by a point source is found to be capable of launching an electromagnetic missile, but the surfaces of the reflector are not complete spherical surfaces, and the problem cannot be solved by simple boundary matching.
Abstract: A spherical reflector illuminated by a point source is found to be capable of launching an electromagnetic missile. Since the surfaces of the reflector are not complete spherical surfaces, the problem cannot be solved by simple boundary matching. The electromagnetic fields have to be solved in three regions separately. The general solutions are matched on the boundaries to obtain a set of coefficient equations. Under the conditions for an electromagnetic missile, the coefficient equations are solved asymptotically. The evaluation of the Poynting vector shows that the energy radiated from the reflector has the same slow rate of decay as for the spherical lens.
TL;DR: Anisotropic gels were produced by photopolymerization of an oriented liquid-crystalline (LC) mixture containing LC diacrylates and conventional LC molecules as mentioned in this paper.
Abstract: Anisotropic gels were produced by photopolymerization of an oriented liquid‐crystalline (LC) mixture containing LC diacrylates and conventional LC molecules. Gels produced in this way were clear and did not give rise to excess scattering of light. Upon application of an electric field across the gel, the system turned translucent. Light scattered by the gel was found to be highly dependent on the direction of polarization and wavelength of the incident light.
TL;DR: In this paper, a superconducting thin film of Ba2YCu3O7x was prepared on (100) SrTiO3 substrates by metalorganic deposition (MOD) of trifluoroacetate precursors.
Abstract: Superconducting thin films of Ba2YCu3O7‐x were prepared on (100) SrTiO3 substrates by metalorganic deposition (MOD) of trifluoroacetate precursors The best electrical transport properties were measured in films annealed at 750 °C in a humid, low PO2 gas mixture followed by slow cooling in oxygen These specimens had sharp resistive transitions with Tc above 90 K and zero‐field critical current densities in excess of 106 A/cm2 at 77 K Critical current densities of this magnitude have not previously been reported in films produced by MOD The highest Jc obtained in films fired only in humid oxygen was 3×105 A/cm2 Annealing at high temperature in the low PO2 atmosphere also resulted in a smoother surface morphology than was observed in the oxygen‐fired films Use of the low PO2 furnace gas appeared to suppress the formation of b‐axis normal oriented grains in the superconducting films and to strengthen c‐axis normal texture X‐ray powder diffraction indicated the presence of a‐axis normal textured material in the films, although it was not present as separate microstructural features which could be identified by scanning electron microscopySuperconducting thin films of Ba2YCu3O7‐x were prepared on (100) SrTiO3 substrates by metalorganic deposition (MOD) of trifluoroacetate precursors The best electrical transport properties were measured in films annealed at 750 °C in a humid, low PO2 gas mixture followed by slow cooling in oxygen These specimens had sharp resistive transitions with Tc above 90 K and zero‐field critical current densities in excess of 106 A/cm2 at 77 K Critical current densities of this magnitude have not previously been reported in films produced by MOD The highest Jc obtained in films fired only in humid oxygen was 3×105 A/cm2 Annealing at high temperature in the low PO2 atmosphere also resulted in a smoother surface morphology than was observed in the oxygen‐fired films Use of the low PO2 furnace gas appeared to suppress the formation of b‐axis normal oriented grains in the superconducting films and to strengthen c‐axis normal texture X‐ray powder diffraction indicated the presence of a‐axis normal textured materia
TL;DR: In this article, the structures and grain sizes of the particles were examined by means of x-ray diffraction and both transmission and scanning electron microscopy, and the structure of these nanocrystalline materials was also described.
Abstract: Magnetron sputtering has been used for the production of nanoscale particles of pure metals, binary alloys, intermetallics, and ceramics. Al, Mo, Cu91Mn9, Al52Ti48, and ZrO2 particles with diameters of 7–50 nm were synthesized. The structures and grain sizes of the particles were examined by means of x‐ray diffraction and both transmission and scanning electron microscopy. The particles were collected and compacted in situ in the sputtering chamber. The structure of these nanocrystalline materials is also described. This novel application of magnetron sputtering at high gas pressures is of interest for both the production of nanocrystalline materials and isolated clusters.
TL;DR: In this article, the role of the InAs/AlSb interface in determining the electron transport in AlSb/InAs/alSb quantum wells grown by molecular-beam epitaxy was investigated.
Abstract: Data are presented on the role of the InAs/AlSb interface in determining the electron transport in AlSb/InAs/AlSb quantum wells grown by molecular‐beam epitaxy. Because both anion and cation change across an InAs/AlSb interface, it is possible to grow such wells with two different types of interfaces, one with an InSb‐like bond configuration, the other AlAs‐like. Electron mobility and concentration were found to depend very strongly on the manner in which the quantum well’s interfaces were grown, indicating that high mobilities are seen only if the bottom interface is InSb‐like. An As‐on‐Al sites antisite defect model is postulated for bottom AlAs‐like interfaces. Such antisites were used in subsequent samples as donors in modulation‐doped high‐mobility InAs/AlSb quantum wells.
TL;DR: In this paper, the role of the azimuthal quadrupole electric field in signal pickup, and its effects on ion motion at the sum frequency of the cyclotron and magnetron motions, have been identified.
Abstract: Ion motion in a Penning trap and the electrical signals it can produce have been analyzed for the purpose of identifying the important causes of uncertainty in high‐accuracy mass measurements of heavy ions. The role of the azimuthal quadrupole electric field in signal pickup, and its effects on ion motion at the sum frequency of the cyclotron and magnetron motions, have been identified. A useful scheme for calculating the signal strength and strength of the interaction between an applied field and the ion motion has been developed. The important sources of uncertainty in using the sum frequency of the cyclotron and magnetron motions for determining the ion mass are discussed. Particular application is made to the case of cyclotron resonance detection by observation of the time of flight of ejected ions.
TL;DR: In this article, a model for organic MISFETs, derived by changing the classical equations according to this particular operating mode, has been proposed, and the ohmic current, parallel to the channel current, has also been taken into account.
Abstract: Metal‐insulator‐semiconductor field‐effect transistors (MISFETs) based on organic semiconductors, mainly conjugated organic polymers and oligomers, have been reported recently. Unlike conventional MISFETs, these devices work through the modulation of an accumulation layer at the semiconductor‐insulator interface. A model for organic MISFETs, derived by changing the classical equations according to this particular operating mode is proposed. The ohmic current, parallel to the channel current, and due to the nonrectifying character of source and drain contacts, has also been taken into account. According to this model, the characteristics of these organic devices can be improved by decreasing the doping level and the thickness of the semiconducting layer. Simple rules are deduced and applied to devices based on α‐conjugated sexithienyl.
TL;DR: In this paper, the Mott elastic scattering cross section of electrons for most elements of the periodic table up to element number 94 in the energy range 20 eV-20 keV have been performed.
Abstract: Calculations of Mott elastic scattering cross section of electrons for most elements of the periodic table up to element number 94 in the energy range 20 eV–20 keV have been performed. The Dirac equation transformed to a first‐order differential equation was solved numerically. The influence of the choice of atomic potential on the scattering factor was studied in comparison to a simple muffin‐tin approximation of the atomic potential in solids. The application of calculated cross sections to a conventional Monte Carlo model for electron scattering using modified Bethe equation is described and results concerning the electron backscattering for different atomic potentials are compared.
TL;DR: In this paper, a theoretical model for simulating the pulsed laser evaporation (PLE) process has been developed, which considers an anisotropic three-dimensional expansion of the laser generated plasma, initially at high temperature and pressure.
Abstract: We have theoretically and experimentally analyzed the laser‐induced evaporation process for deposition of superconducting thin films from bulk targets. The spatial thickness variations have been found to be significantly different from a conventional thermal deposition process. Unlike a cos θ thickness variation expected from a thermal evaporation process, the laser evaporation process is characterized by a forward‐directed deposit with a sharp variation in its thickness as a function of distance from the center of the deposit. We have studied in detail the interactions of nanosecond excimer laser pulses with bulk YBa2Cu3O7 targets leading to evaporation, plasma formation, and subsequent deposition of thin films. A theoretical model for simulating the pulsed laser evaporation (PLE) process has been developed. This model considers an anisotropic three‐dimensional expansion of the laser‐generated plasma, initially at high temperature and pressure. The forward‐directed nature of laser deposition has been found to result from anisotropic expansion velocities of the plasma edges arising due to the density gradients in the gaseous plasma.The physical process of the laser ablation technique for deposition of thin films can be classified into three separate interaction regimes: (i) interaction of the laser beam with the bulk target, (ii) plasma formation and initial isothermal expansion, and (iii) adiabatic expansion leading to deposition of thin films. The first two regimes occur during the time interval of the laser pulse, while the last regime initiates after the laser pulse terminates. Under PLE conditions, the evaporation of the target is assumed to be thermal in nature, while the plasma expansion dynamics is nonthermal as a result of interaction of the laser beam with the evaporated material. The expansion velocities of the plasma edges are related to the initial dimensions and temperature of the plasma, and the atomic weight of the respective species present in it. Preliminary calculations have been carried out on spatial thickness variations as a function of various parameters in PLE deposited thin films. The effects of the various beam and substrate parameters including energy density and substrate‐target distance affecting the nature of deposition of superconducting thin films have been theoretically examined. Experimental results have been obtained from thin films deposited on silicon substrates by XeCl pulsed excimer laser (λ=308 nm, τ=45×10−9 s) irradiation. The spatial thickness and compositional variations in thin films have been determined using Rutherford backscattering technique and the results compared with the theoretical calculations.