ALBERT

Notizen: The effects of heating on both the reflectivity and layered structures of Mo/Si and Mo-silicide/Si (MoSi2/Si and Mo5Si3/Si) multilayers were evaluated. The Cu-Kα x-ray reflectivity and the periodic length of the Mo/Si multilayer markedly decreased after heating at above 300 °C, while those of the Mo-silicide/Si multilayers changed only slightly even after heating up to 600 °C. Transmission electron microscopy observations revealed markedly less thermally induced deterioration in the Mo-silicide/Si multilayers than in the Mo/Si multilayer, indicating that Mo-silicide/Si multilayers have better heat resistance than Mo/Si multilayers. © 1995 American Institute of Physics.

Notizen: We report on the optical reflective properties of the planar texture of cholesteric liquid crystal reflective displays. The reflecting liquid-crystal medium is partitioned into many distinguishable domains, each of which is characterized by its local dielectric anisotropy. Such a multidomain cholesteric liquid crystal is made bistable by either dispersing a low concentration of polymer or by treating the cell substrate surfaces, both of which affect the domain alignment. Both types of cells result in a bistable colorful reflective display. We determine the role that the polymer network and surface alignment has on the reflective properties in regard to their photometric and colorimetric properties as a function of viewing angle using both collimated and diffuse illumination. Both the polymer network and surface anchoring have the effect of distributing the orientation of the cholesteric helix axes about the cell normal. Theoretically we characterize these cells by this distribution. Our goal is to properly model both types of display cells so as to quantitatively elucidate their distinguishing features. © 1995 American Institute of Physics.

Notizen: The statistics of microdischarges crossing the air gap between conductors and insulating polymers have been measured. Long tails of pulse-amplitude and pulse-time-separation distributions have been observed and their nature examined. The long tails of time distributions followed quasiexponential behavior while the long tails of amplitude distributions satisfied a fractal power law. The transformation relations between both sets of statistics have been determined and their consequences discussed. The clearly resolved fractal long tails of amplitude distributions have not been observed so far and represent a new result in the statistical studies of microdischarges. Also the determined transformation relations between time and amplitude statistics bring a new insight into the studied phenomenon. © 1995 American Institute of Physics.

Notizen: Cross-sectional transmission electron microscopy, selected area electron diffraction, and small-angle x-ray diffraction have been used to observe the systematic deviations of period in W/Si and W/C multilayers. The mean periods decreased gradually as the detected regions were located farther away from the substrate. To avoid destroying the multilayer, a method is suggested to measure the systematic deviation of period by using soft x rays with wavelengths below and above an absorption edge and hard x rays such as Cu Kα1. © 1995 American Institute of Physics.

Notizen: This work summarizes results of a simple procedure to incorporate dopants into the near surface region of single-crystal sapphire. We demonstrate the formation of iron-doped and chromium-doped sapphire thin films by solid-phase epitaxial growth. Amorphous alumina films of about 200–350 nm thickness were deposited onto single-crystal sapphire substrates. Fe or Cr ions were introduced into the films during deposition. A post-deposition thermal process was performed in oxidizing ambients at 800–1400 °C to induce epitaxial growth and to incorporate dopants. The epitaxial relationship of the grown film with the substrate was confirmed by both ion channeling and cross-sectional transmission electron microscopy. The growth kinetics were determined by time-resolved reflectivity measurements for different dopant concentrations. Ion channeling angular scans revealed that the Fe or Cr ions are incorporated onto octahedral sites (Al3+ sites) in the corundum structure as expected in equilibrium. External optical transmittance measurements exhibited absorption in the near ultraviolet range associated with the Fe3+ state. The substitution of Cr for Al3+ was also confirmed by the observation of R1 and R2 luminescence lines characteristic of ruby. The doping procedure has potential applications in the fabrication of thin film planar optical waveguides and thin film stress sensors. © 1995 American Institute of Physics.

Notizen: Epitaxial Fe3−xSi1+x films have been grown on Si(111) by codeposition at room temperature. Their structural and electronic properties have been investigated by means of low-energy electron diffraction (LEED), x-ray photoelectron diffraction (XPD), and x-ray photoemission spectroscopy (XPS). These films, with compositions ranging from Fe3Si to FeSi, exhibit a (1×1) LEED pattern. Both XPD and core level binding energy measurements indicate that single Fe3−xSi1+x phases (with 0〈x〈1), without bulk counterpart, can be stabilized by epitaxy on Si(111). The XPD experiment clearly shows that these Fe3−xSi1+x (0≤x≤1) films adopt the same cubic structure. Furthermore, the Si 2p, Fe 2p3/2, and Fe 3s core levels are slightly shifted to higher binding energies resulting from chemical effects and differences in local coordination when going from Fe3Si (DO3) to FeSi (CsCl). Multiplet splittings ΔE3s are observed in Fe 3s core-level XPS spectra for all Fe3−xSi1+x compounds except the FeSi (CsCl) one. © 1995 American Institute of Physics.

Notizen: A simple dispersion equation for surface thermal waves propagating along a solid surface covered with a thin film of higher thermal conductivity is presented. It is shown to describe well phase measurements with a photothermal microscope carried out on metal films on glass substrates. © 1995 American Institute of Physics.

Notizen: Atmospheric pressure plasmas can be used to provide a vacuum-atmosphere interface as an alternative to differential pumping. Vacuum-atmosphere interface utilizing a cascade arc discharge was successfully demonstrated and a 175 keV electron beam was successfully propagated from vacuum through such a plasma interface and out into atmospheric pressure. Included in the article are a theoretical framework, experimental results, and possible applications for this novel interface. © 1995 American Institute of Physics.

Notizen: Diamondlike hydrogenated carbon films were deposited using saddle-field glow discharge in pure methane. The structure of the films was studied using x-ray photoelectron spectroscopy, x-ray stimulated Auger electron spectroscopy, and Raman spectroscopy. It was found that for appropriate conditions of pressure and substrate bias a very high percentage of sp3 bonding could be achieved. © 1995 American Institute of Physics.

Notizen: Current-voltage, small-signal measurements, and deep-level transient spectroscopy (DLTS) spectra of p-n junctions made by Mg implantation into undoped InP are described. The I-V characteristics show that the dominant conduction mechanism at forward bias is recombination in the space-charge zone, whereas a thermally activated tunneling mechanism involving a trap at 0.32 eV dominates at reverse bias. Five deep levels located in the upper-half of the band gap were detected in the junctions by DLTS measurements, three of which (at 0.6, 0.45, and 0.425 eV) were found to appear due to rapid thermal annealing. The origin of the other two levels, at 0.31 and 0.285 eV, can be ascribed to implantation damage. Admittance spectroscopy measurements showed the presence of three levels at 0.44, 0.415, and 0.30 eV, all in agreement with those found by DLTS. The DLTS measurements showed that the concentration of deep levels decreased after longer annealing times, and that the concentration of deep levels due to the implantation increased after additional P or Si implantations. This explains the influence of annealing time and additional implantations on the I-V characteristics of the junctions. © 1995 American Institute of Physics.

Notizen: The effect of 3.5 keV electron irradiation on adhesion and contact resistivity of the Ag/YBa2Cu3O7 interface has been studied using an evaporated silver layer on c-axis oriented superconducting YBa2Cu3O7 thin films. Electron doses ranged between 1016 and 1018 electrons/cm2. The Q-tip method of adhesion testing showed that even at the lowest electron dose adhesion is significantly improved. The contact resistivity of the interface was measured using a cross-junction four-point probe. Contact resistivity was unchanged at the lowest electron dose but increased as the electron dose increased. A theoretical model involving an electron irradiation damaged layer at the interface has been developed to explain measured contact resistivity changes. © 1995 American Institute of Physics.

Notizen: We investigate by photoacoustic spectroscopy the optical band-gap energy of mercuric iodide, α-HgI2, grown by sublimation in a sealed ampoule. Due to its importance as a detector material operating at ambient temperature, the physical properties of α-HgI2 have been recently studied. We found, by two different methods, the band-gap energies EG=2.32 and 2.39 eV, respectively. These results are in good agreement with recent measurements based on reflection and absorption spectra. © 1995 American Institute of Physics.

Notizen: We report that organic electroluminescence devices with a hole transporting emitting layer composed of distyrylarylene derivatives realized highly efficient and bright emission in the blue-green region. Luminous efficiency was obtained to be 2.1 lm/W in the low-luminance region (135 cd/m2) using an indium tin oxide/emitting layer/electron transporting layer/Mg:Ag structure. The external quantum efficiency was estimated to be about 1.5%. The highest luminance was obtained to be 4000 cd/m2 for the device. © 1995 American Institute of Physics.

Notizen: The formation of amorphous diamondlike films by deposition of energetic carbon atoms is modeled. Formation of diamondlike bonding is attributed to transient high pressure created by the ion impact. The relaxation to energetically favored graphitic state is assumed to proceed through succession of metastable states, taking place until the glass transition point is reached. The model illustrates from the new point of view the processes involved in the formation of diamondlike films. It predicts qualitatively similar dependence of film properties on the energy of the incoming carbon atoms as observed in experiments. © 1995 American Institute of Physics.

Notizen: An analytical approximation for the electron density in the conduction band of the entire Hg1−xCdxTe (MCT) alloy system (0〈x〈1) as a function of the composition, temperature and Fermi energy location, is proposed. A hyperbolic expression for the conduction band is shown to yield an error which is practically not larger than Kane's model in the entire composition range of MCT. The analytical approximation is compared with a numeric calculation of the Fermi-Dirac integral using this hyperbolic band approximation, and shows a deviation of a few percents for temperatures in the range 2〈T〈300[K], compositions in the entire range 0〈x〈1 and electron densities up to n=1020[cm−3]. This analytical approximation can be extremely useful for numerical band diagram and transport simulations of graded and abrupt MCT heterojunctions and devices. © 1995 American Institute of Physics.

Notizen: This reply reinforces the need for examining the internal consistency of longitudinal and lateral stress data under plane shock wave propagation. This was pointed out in an article by Dandekar, Abbate, and Frankel [J. Appl. Phys. 76, 4077 (1994)] to determine the shear strength of a solid. It also shows that shear strength of a crystalline solid under plane shock wave compression can be determined accurately from its reliable Hugoniot and applicable hydrodynamic compression of the solid.

Notizen: By use of the linear-response dielectric theory and local electron density approximation, we calculate the energy straggling of protons in solids. The correlation and exchange interaction of electron gas in solids is taken into consideration by introducing a static local-field-correction function in the dielectric function. The theoretical results are compared with experimental and empirical data, as well as with other previous theoretical results. © 1995 American Institute of Physics.

Notizen: The initial holographic recording rate has been analytically studied for a photorefractive thin slab. The finite thickness is explicitly taken into account by using a two-dimensional formulation (coordinates x and z parallel and perpendicular to the slab faces, respectively). It is shown that for a thickness comparable to the grating period, the solution appreciably departs from the usual one-dimensional result, i.e., edge effects are remarkable. In particular, significant space-charge fields perpendicular to the slab faces are generated. These effects may substantially modify the diffraction properties of the recorded photorefractive gratings, as exemplified for a GaAs film. © 1995 American Institute of Physics.

Notizen: Ion energy distribution functions (IEDF) were measured at the surface of a grounded or a rf-powered electrode exposed to a microwave (MW, 2.45 GHz) or a rf (13.56 MHz) discharge in argon. The IEDFs measured on the grounded electrode in both the rf and the MW modes show a higher contribution of low-energy ions when the pressure is raised. The maximum ion energy Em decreased from 12.0 to 8.5 eV in the MW plasma, but remained at 26±0.5 eV in the rf plasma. The mean ion energy E¯ decreased from 6.1 to 3.8 eV (MW) and from 16.0 to 12.0 eV (rf) when the pressure was raised from 30 to 210 mTorr. The IEDFs in the rf mode exhibit a single peak, while a bimodal structure is observed in the MW mode. When the electrode is rf powered to achieve a negative dc bias voltage VB, the Em values reach ∼1.2eVB (rf) and ∼1.1eVB (MW), respectively. The IEDFs in the rf mode display a multiple peaked structure which is associated with a rf-modulated sheath. In the dual-frequency MW/rf mode the IEDF is single peaked, narrow, and centered around eVB. A nearly tenfold increase in the ion flux is observed when increasing the MW input power, while the ion energy remains unchanged for a constant VB value. This allows an independent control of ion energy and ion flux, suitable for the control of material properties in plasma processing. © 1995 American Institute of Physics.

Notizen: In a previous article, we proved and characterized a phase transition of an order-disorder type, below room temperature, in the Pb1−3X/2LaX(D'Alembertian)X/2(Zr0.95Ti0.05)O3 (X=0.5–4 wt % La) ceramics. In the present article, the analysis of this order-disorder transition is continued on the basis of the comparative study of Raman scattering, as a function of the Nb or La dopant. The parallel analysis and the interpretation of the Raman spectra of the nondoped, Nb-doped, and La-doped Pb(Zr0.95Ti0.05)O3 ceramics lead to new information concerning the identification of the vibrational modes involved. In addition, the correlation between the phase transitions and the nature of the dopants is established. New information concerning the cell doubling, during the ferroelectric low temperature to ferroelectric high temperature phase transition, is obtained as well. The Raman study of the lead-lanthanum zirconate-titanate ceramics allowed us to identify for the first time the so-called "antiferroelectric mode'' and "ferroelectric mode'' which had been theoretically predicted. © 1995 American Institute of Physics.

Notizen: We performed imaging of semiconductor nanostructures (quantum wells and quantum wires) by atomic force microscopy of the cleaved edge of the samples under ambient conditions. Selective etching was used to enhance the composition contrast of the semiconductor heterostructure layers, and a procedure for retrieving the nanostructure dimensions from the scanned image is presented. The simple sample preparation and the relatively large (up to ∼100×100 μm2) imaged areas offer advantages over more conventional nanometer resolution techniques such as transmission electron microscopy. © 1995 American Institute of Physics.

Notizen: The critical thickness of an epilayer on a substrate with different elastic constants is investigated by following Stroh's treatment of anisotropic elasticity [Philos. Mag. 3, 625 (1958)]. A closed formula is derived to calculate the critical thickness and an exact solution may involve numerical evaluation of the equation. The results indicate that the self-energy of the dislocation is controlled by the soft phase between the epilayer and the substrate, while the interaction energy depends only on the elastic constants of the thin film. It is easier for a dislocation to be formed if the substrate is softer than the film, and consequently the critical thickness is smaller. On the other hand, a soft epilayer can have a large thickness without any mismatch dislocation. Explicit equations are given here for the {100}, {110}, and {111} epitaxial planes. The system of a GexSi1−x epilayer on a Si substrate was taken as an example to demonstrate the influence of the difference in elastic constants on the critical thickness. Even though the difference between the elastic constants of the epilayer and the substrate is not very large, ignoring this difference can cause a relative error over 20% in calculation of the critical thickness. For this system, a simplified equation yields sufficiently accurate results. © 1995 American Institute of Physics.

Notizen: The interfacial reactions in the Zr–Si system have been studied by in situ cross-section transmission electron microscopy (TEM) including high-resolution-mode energy-dispersive spectroscopy (EDS) and nanobeam electron diffraction (nanodiffraction). The as-deposited Zr film has a columnar structure and an amorphous interlayer is observed at the Zr/Si interface. The amorphous layer is found to grow during annealing at 400 °C. The growth of the amorphous layer consists of three stages: a rapid increase in the early stage, a gradual increase in the intermediate metastable stage, and saturation in the final stage. The kinetics at each stage are discussed with in situ TEM observation and ex situ EDS analysis. Annealing at 500 °C creates a ZrSi2 layer at the amorphous layer/Si interface. The phase and orientation relationship are determined from the nanodiffraction patterns. The ZrSi2 is found to grow layer by layer into the Si substrate via a ledge mechanism. © 1995 American Institute of Physics.

Notizen: The propagation of surface acoustic waves (SAWs) in AlGaAs/GaAs and InGaAs/InP quantum well structures is modeled using a Laguerre polynomial approach to determine the effects of the quantum well parameters (layer thickness, layer composition, and number of layers) on the induced potential, electric field, particle displacement, and strain. These characteristics show that variations of the electric field within regions of the structure of interest for device applications can be brought about, and are of use for the design of devices based on SAW interactions in quantum well structures. © 1995 American Institute of Physics.

Notizen: A theory has been developed describing x-ray diffuse scattering from misfit dislocations in epitaxial layers. This approach has been used for explaining the origins of diffuse x-ray scattering from SiGe layers with linearly graded Ge content. The distribution of the diffusely scattered intensity in reciprocal plane measured by triple-axis x-ray diffractometry has been compared with theoretical predictions and a good agreement has been achieved. It is demonstrated that the main part of the diffusely scattered intensity originates from random strains caused by misfit dislocations at the substrate–epilayer interface or in the relaxed part of the compositionally graded layers. The contribution of the threading dislocation segments to the diffuse scattering is rather small. © 1995 American Institute of Physics.

Notizen: The thermopower η of Sr1−xLaxTiO3 ceramics was investigated up to x=0.5 and in the temperature range between 150 K and 1200 K. In addition, the carrier concentration n was determined by Hall measurements and by a chemical Ti3+-analysis. For low temperatures and high n, η depends linearly on temperature and on n−2/3, as expected from a degenerate quasi free electron gas. In the case of high temperatures and low n, the absolute value of η rises with 1.5⋅ln10⋅k/e per decade of temperature and with ln10⋅k/e per decade of carrier concentration, as expected from a classical broad-band semiconductor obeying the Boltzmann statistics. In the range of degeneration an effective mass meff of 4.2 electron masses can be deduced without the assumption of a transport factor Ae. In the classical range Ae=3 can be evaluated, requiring only a temperature and lanthanum independent meff. Thus, the thermopower of Sr1−xLaxTiO3 ceramics can be described by a constant effective mass and a constant transport factor within a wide range of temperature and lanthanum content. Furthermore, the transition from degeneration to classical behavior can be described as a function of temperature and electron density, e.g., at room temperature it takes place at about x≈0.2 (i.e., n≈3.4⋅1021/cm3). © 1995 American Institute of Physics.

Notizen: The effects of Al as an isoelectronic dopant have been investigated on the deep level defects in GaAs grown by molecular-beam epitaxy (MBE), using deep level transient spectroscopy. Two different compositions containing 0.1% Al (Al0.001Ga0.999As) and 1% Al (Al0.01Ga0.99As) have been studied. At least nine different deep levels have been detected. Their detailed characteristics consisting of emission rate signatures, capture cross sections, concentrations, and junction depth profiles have been determined. The deep levels observed have been compared with the M levels normally found in MBE n-GaAs. The emission rates of deep levels have been found to shift to higher values with decrease in Al concentration. This fact has been attributed to lattice strain and random alloy effects. The relative concentrations of deep levels are seen to undergo large changes as the Al concentration is increased from 0.1% to 1%. Al doping upto 0.1% does not seem to reduce the deep level concentration, unlike the case of other isovalent dopants such as In and Sb in MBE GaAs. Further increase in the Al doping to 1% is found to lead to a pronounced increase in the overall deep level concentration. These data along with the other measured characteristics are interpreted in terms of the possible models for the various defects. © 1995 American Institute of Physics.

Notizen: The degradation (fatigue) of dielectric properties of ferroelectric Pb(ZrxTi1−x)O3 (PZT) and SrBi2Ta2O9 thin films during cycling was investigated. PZT and SrBi2Ta2O9 thin films were fabricated by metalorganic decomposition and pulsed laser deposition, respectively. Samples with electrodes of platinum (Pt) and ruthenium oxide (RuO2) were studied. The interfacial capacitance (if any) at the Pt/PZT, RuO2/PZT, and Pt/SrBi2Ta2O9 interfaces was determined from the thickness dependence of low-field dielectric permittivity (εr) measurements. It was observed that a low εr layer existed at the Pt/PZT interface but not at the RuO2/PZT and Pt/SrBi2Ta2O9 interfaces. In the case of Pt/PZT, the capacitance of this interfacial layer decreases with increasing fatigue while the εr of the bulk PZT film remains constant. This indicates that fatigue increases the interfacial layer thickness and/or decreases interfacial layer permittivity, but does not change the bulk properties. For the capacitors with RuO2/PZT/RuO2 and Pt/SrBi2Ta2O9/Pt structures, however, the εr does not change with ferroelectric film thickness or fatigue cycling. This implies no interfacial layer exists at the interfaces and which can be correlated to the observed nonfatigue effect. Additionally, the equivalent energy-band diagrams of these different capacitor structures were proposed to complement the proposed fatigue mechanism. © 1995 American Institute of Physics.

Notizen: Unexpected periodic variations of the magnetic field were recently found along the axes of superconducting accelerator magnets. This modulation, which reduces the field quality of the magnets, shows a complex space and time dependence containing very long time constants. We show in this article that the variation of the field rate dB/dt along the length of a superconducting cable induces superposed coupling currents which flow over a long length. The space and time dependence of these "supercurrents'' for a two-wire cable model is obtained by the solution of the diffusion equation with the diffusivity given by the cable parameters. The existence of supercurrents explains the observed effects in accelerator magnets. It is furthermore shown that supercurrents can lead to a highly inhomogeneous current distribution over the cable cross section and to additional coupling losses, even in sections of the magnet where dB/dt=0. Both these effects can reduce the stability of magnets, which may explain the ramp rate limitation found in accelerator magnets as well as in large magnets for fusion research. © 1995 American Institute of Physics.

Notizen: The current amplitudes of Shapiro steps in large-area metallic-barrier Josephson junctions, both with and without a ground plane, are investigated with the goal of optimizing junction parameters for programmable voltage standards. Using the resistively shunted junction model without capacitance, we calculate maximum step amplitudes as a function of reduced frequency and junction dimension for both one- and two-dimensional junctions. For junctions without a ground plane, we conclude that step amplitudes of order 10 mA are practical, but significantly larger amplitudes require excessive microwave power.

Notizen: The behavior of Ar plasma-induced defects deactivating Si donors in GaAs has been studied. We have applied photoreflectance spectroscopic analysis combined with stepwise wet etching to the depth profiling of defects and succeeded in determining the concentrations and profiles of defects in the sub-surface layer for the first time. We found that the point defects responsible for deactivation undergo electronically enhanced diffusion under the circumstance of photoexcited carriers, demonstrating that the ultraviolet light from plasma is a cause of the deep penetration of defects far beyond the stopping range of ions. We also found that the generation of these point defects is enhanced by photoexcited carriers and that diffusing point defects are trapped by the background defects or impurities, forming immobile complexes. We propose a model in which self-interstitials are the most probable point defects responsible for deactivation. © 1995 American Institute of Physics.

Notizen: We have carried out a comprehensive study of the Raman spectra of a-Si:H films produced by the glow discharge (GD) and radio frequency sputtering (RFS) deposition techniques. The results show that the short-range disorder (bond-angle deviation), as measured by the width of the TO band (ΓTO), is larger in RFS than in GD a-Si:H films. The intermediate-range disorder (dihedral angle deviation), as measured by the ratio of the intensity of the TA band to that of the TO band (ITA/ITO), is generally larger in RFS than in GD a-Si:H films. However, while the ITA/ITO values of RFS films remain relatively close to those of GD films when the interior is probed, the near surface of RFS films shows much larger values evidencing the existence of a significant disorder gradient along the growth axis. Together, these results indicate that the network order and homogeneity of RFS amorphous silicon is lower than those of GD for substrate temperatures that produce the hydrogenated material. These structural differences are interpreted in terms of the differences between the two film growth processes and are believed to be the reason for the poorer transport properties of RFS a-Si:H films. © 1995 American Institute of Physics.

Notizen: An approach for the dipole relaxation in polymers is treated considering the dipole–dipole interaction between the molecules. The thermally stimulated depolarization peak parameters, i.e., the activation energy E, the pre-exponential factor τ0, and the dipole interaction strength parameter q, were evaluated using the iterative technique. Good results are obtained in comparison with other methods. A linear relationship between the activation energy and the logarithm of the pre-exponential factor of the relaxation times is existing confirming the operation of the compensation law. © 1995 American Institute of Physics.

Notizen: Photoreflectance spectra have recently been obtained for a range of single strained Si1−xGex epilayers for 0.12〈x〈0.24, which are buried under a Si cap. Spectra measured at different positions on a sample wafer showed pronounced changes in the SiGe line shape. Here these changes are shown to be due to phase shifts arising from changing optical interference effects caused by variations in the Si cap thickness. The phase shifts are determined accurately using a Kramers–Kronig analysis and are interpreted in terms of a multiple-reflection treatment incorporating a calculation of the Seraphin coefficients. This allows the Si cap thiakness changes to be determined and compared to the0results of speatroscopic ellipsometry measurements. © 1995 American Institute of Physics.

Notizen: Silicon carbide (SiC) films have been grown on 4 in. diam (100) silicon wafers by atmospheric pressure chemical vapor deposition, using propane, silane, and hydrogen. X-ray photoelectron spectroscopy data confirm that the films are stoichiometric SiC, with no major impurities. X-ray diffraction and transmission electron microscopy (TEM) data indicate that the films are single-crystalline cubic polytype (3C) across the 4 in. substrates. With the exception of slip lines near the edge of the wafers, the films appear featureless when observed optically. The nitrogen concentration, as determined by secondary ion mass spectroscopy, is 4×1018 cm3. Cross-sectional TEM images show a fairly rough, void-free interface. © 1995 American Institute of Physics.

Notizen: Compositional distributions of films laser deposited in vacuo at energy densities 〈1 J/cm2 were found to be partly different from previous observations and theories. Analyzing them, we have inferred that evaporation processes at low energy densities contain decomposition of the target materials and evaporation of the decomposed materials. Based on these analyses, we have concluded that pulsed laser codeposition was one of best pulsed laser deposition methods. To realize this using one laser source, a modified version of pulsed-laser-deposition rapid-sequential-pulsed-laser deposition, is introduced. The dependence of YBa2Cu3O7 films properties on deposition conditions is discussed. Elimination of particulates is demonstrated and good electrical and crystallographic properties as well as suppression of precipitates were achieved in films having the correct stoichiometric composition. © 1995 American Institute of Physics.

Notizen: A scanning tunneling microscope in ultrahigh vacuum has been used to investigate the growth, morphology, and surface atomic structure of ultrathin titanium silicide films on Si(111) substrates. Microstructural considerations have been used to identify various stages of the silicide growth. Atomic resolution images of a titanium silicide crystallite facet, formed at 850 °C, have been identified as a 2×2 silicon termination of a C54-TiSi2(010) surface. Possible epitaxial silicide/silicon relationships are provided. Theoretical consideration has been given to the interatomic bonding in the C54-TiSi2 lattice and the dangling bond density of ideally terminated silicide planes has been calculated. The highly reconstructed atomically flat surface of a large crystallite, formed at 1200 °C, has been assigned as a C54-TiSi2(311) plane giving the epitaxial relation C54-TiSi2(311)(parallel)Si(111). The presence of pairs and linear chains of defects, with common orientations, is attributed to the decomposition of a diatomic gas on the facet, producing sites of preferential adsorption on the silicide surface. © 1995 American Institute of Physics.

Notizen: Photoluminescence has been used to investigate the behavior of Si-implanted InP, submitted to a postimplantation rapid thermal annealing. Compared with the non-implanted material, the implanted crystal displays a new broad band around 1.382 eV, in the low temperature (2 K) spectra. This band appears to be made of two unresolved recombination paths ascribed to the electron–acceptor (e,A°) and donor–acceptor pair recombinations of the silicon acceptor impurity substituted on phosphorus site. Besides the Zn-related transitions to s and p excited states, the selective excitation of the donor–acceptor pairs reveals additional recombination paths, ascribed to transitions to 2s3/2 and 2p5/2 (Γ7 and Γ8) excited states of silicon. The acceptor behavior of Si in InP is hence given a definite support by this work. © 1995 American Institute of Physics.

Notizen: Tortuosity, i.e., the high-frequency limit of the squared propagation index, has been evaluated with narrow-band piezoelectric transducers on reticulated plastic foams. The basic procedure was recently reported by J. F. Allard, B. Castagnède, M. Henry and W. Lauriks [Rev. Sci. Instrum. 65, 754 (1994)]. Further experimental work related to the anisotropic nature of these materials has been done by probing the propagation index versus angle. A simple numerical routine has been implemented in order to recover the propagation index along principal directions. These predicted values compare well with measurements taken directly along the principal geometric axes of the samples. Slight angular deviations of the principal axes themselves have been observed. This is the very first account of the anisotropy of tortuosity measured by ultrasonic methods in air-saturated porous materials. © 1995 American Institute of Physics.

Notizen: Thin films of the nonlinear optical material 4′-nitrobenzylidene-3-acetamino-4-methoxy-aniline have been prepared using the organic molecular beam deposition technique. High quality homoepitaxial layers have been grown at substrate temperatures of 80 °C and moderate growth rates of 0.1–0.5 A(ring)/s. The samples have been characterized by optical polarization and interference microscopy as well as atomic force microscopy. Growth experiments on inorganic substrates, including silicon and glass, have been performed in a substrate temperature range of −190 to 100 °C and led to amorphous films at low temperatures and polycrystalline films at temperatures above 50 °C. © 1995 American Institute of Physics.

Notizen: Magnetoimpedance in as-cast, nonmagnetostrictive CoFeBSi amorphous ferromagnetic wires, submitted to an ac electric current of 5 mA in the frequency range 100 Hz–100 kHz, is analyzed in terms of equivalent circuits. It is shown that the equivalent circuit representing the wire frequency behavior can be approximated by a parallel LpRp arrangement with elements Rs and Ls in series; Lp and Rp are associated with circumferential domain wall permeability and wall damping, respectively; Ls is related to the circumferential rotational permeability of the wire, and Rs accounts for the dc resistance in the circuit. When the wire is submitted to a longitudinal dc field high enough to approach saturation (Hdc=3600 A/m), the circuit becomes simply a series RsLs circuit. The various contributions from basic magnetization processes to magnetoimpedance are discussed, as well as deviations from this idealized model. © 1995 American Institute of Physics.

Notizen: We have found evidence through optical spectroscopy that AgO is indeed generated in the laser plume during pulsed laser deposition of YBa2Cu3O7−δ (YBCO) thin films using Ag-doped YBCO targets. This supports our earlier conjecture that formation of AgO in the plume and its subsequent dissociation at the elevated substrate temperature (since AgO is unstable above 350 °C) provides active oxygen to the YBCO lattice, thereby increasing oxygen incorporation during growth of YBCO thin films. © 1995 American Institute of Physics.

Notizen: Measurements of the temperature dependence of the normal state resistivity, ρn (T), for (R0.8Pr0.2)Ba2Cu3O7−δ (R=Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, and Yb) are reported. We found that ρn is linearly dependent on the temperature (except R=Nd). At a constant temperature, ρn is linearly dependent on the ionic radius of R. The results are interpreted in terms of the hybridization between the local states of the Pr ion and the valence-band states of the CuO2 planes. © 1995 American Institute of Physics.

Notizen: When annealed, thin amorphous deposits of highly lattice mismatched materials provide specular crystalline surfaces for epitaxy. Mismatch strain is predominantly relieved by misfit dislocations propagating in the plane of the interface, so that resulting films have low threading dislocation densities. We demonstrate the application of this concept to the growth of (001) oriented CdTe epitaxy on (001) GaAs. © 1995 American Institute of Physics.

Notizen: We have found a hole trap related to hydrogen and carbon in p-type crystalline silicon after hydrogen and deuterium injection by chemical etching and plasma exposure. It was found from deep-level transient spectroscopy that this center is located at 0.33 eV above the valence band and shows no Poole–Frenkel effect in electric fields lower than 6×103 V/cm. The depth profiling technique using deep-level transient spectroscopy indicated that this center is distributed over the range 1–7 μm from the surface with densities of 1011–1013 cm−3, depending on the hydrogenation method. On the other hand, secondary ion mass spectroscopy revealed that the majority of deuterium injected into silicon exists within a much shallower region less than 60 nm from the surface with higher densities of 1018–1020 cm−3. We have therefore concluded that the majority of injected hydrogen stays in the near-surface region probably in the form of a molecule and larger clusters and only the minority diffuses into the bulk in an atomic form to form an electrically active complex with carbon. We performed annealing experiments to investigate the thermal stability of the complex. It was stable in the dark up to 100 °C, above which it was completely annihilated in first-order kinetics with an activation energy of about 1.7 eV. The illumination of band gap light with and without a reverse bias at room temperature and at 50 °C induced no effect on the stability of the trap. This is contrast to the photoinduced annihilation of a recently observed electron trap related also to hydrogen and carbon and with comparable thermal stability in n-type silicon.These similarities and differences between the two traps and the comparison of the present results with the recently published theoretical calculations of the total energy of hydrogen configurations in the hydrogen-carbon complex suggest that the previously observed electron trap and the presently observed hole trap arise from two different defects with similar origins and structures and are tentatively ascribed to the electronic states of "bond-centered'' and "anti-bonding of carbon'' configurations of hydrogen in the hydrogen-carbon complex, respectively. © 1995 American Institute of Physics.

Notizen: The electronegativity difference between silicon and SiO2 produces a dipole layer at the Si–SiO2 interface which determines the barrier height between the silicon and SiO2 conduction bands. Because thermally grown SiO2 is amorphous, the alignment of these dipoles with respect to each other fluctuates resulting in a barrier height distribution. Photon-assisted injection of electrons into the thermally grown gate oxide of a metal-oxide-semiconductor field-effect transistor is used to extract this distribution by experiment. The ratio of the injected gate current to the short-circuit source–drain photocurrent collected under the gate is shown to be the Laplace transform of the barrier height distribution. By inverting the Laplace transform, measured under the specific experimental conditions described, the barrier height distribution is found to be Gaussian. The average zero-field barrier height is found to be 3.5 eV with a standard deviation of 0.64 eV measured in the oxide over a plane parallel to the Si–SiO2 interface. The relation is given between the average barrier height and the high-field extrapolated barrier height of 3.1 eV, and it is argued that the standard deviation is a measure of the degree of disorder in the SiO2. © 1995 American Institute of Physics.

Notizen: Ideal threshold current densities of 2.1–4.1 μm IR lasers are calculated for active layers composed of InAs/InGaSb superlattices, InGaAsSb quantum well quaternaries, InAsSb bulk ternaries, and HgCdTe superlattices. The fully K-dependent band structure and momentum matrix elements, obtained from a superlattice K⋅p calculation, are used to calculate the limiting Auger and radiative recombination rates and the threshold current density. InGaAsSb quantum wells and InAs/InGaSb superlattices are found to be more promising laser candidates than HgCdTe superlattices and InAsSb bulk ternaries. The calculated threshold current densities of InAs/InGaSb superlattices are similar to those of InGaAsSb active layers at 2.1 μm, but are significantly lower at longer wavelengths. Comparison with experiment indicates that the threshold current densities of InGaAsSb-based devices are about three times greater than those calculated for 25 cm−1 gain. The threshold current densities of present InAs/InGaSb superlattices are about 100 times above their theoretical limit. © 1995 American Institute of Physics.

Notizen: Alternating current (ac) losses in silver sheathed mono- and multicored (Bi,Pb)2Sr2Ca2Cu3O10 superconducting tapes were investigated at liquid neon temperature (27 K) under various direct current (dc) background magnetic fields. The loss behavior was well accounted for by the Bean critical state model with the inclusion of eddy current loss in the silver sheath. The interfilamentary coupling loss was found to be negligibly small for the present orientation of the tape. In addition, a severe distortion of the loss signal and the B-H hysteresis curve was observed at 27 K when a high dc magnetic field was applied simultaneously with ac fields. The losses of the tapes were also measured at 77 K and compared with the 27 K values. © 1995 American Institute of Physics.

Notizen: This article presents a comparison of one-, two-, and three-dimensional models for domain wall motion coercivity. The effect of wall bending in two-dimensional models as compared to one-dimensional models and the additional effect of wall flexing in three-dimensional models is studied. The models are implemented numerically and calculations are carried out for soft magnetic materials. A discussion of how parameters must be chosen in lower dimensional models to obtain the correct magnitude of coercivity is included. © 1995 American Institute of Physics.

Notizen: Work has been carried out to optimize the operation of a high-voltage trigatron switch. It is demonstrated here that the trigatron can operate in two different modes depending on the route of the initial breakdown. This initial breakdown can occur either to the adjacent electrode or to the opposite electrode. It is shown here that, for a given switch, the mode of operation depends only on the ratio of trigger voltage to working voltage. The optimization was based upon the suggestion that the optimum operation of a trigatron would occur when the trigger pin breaks down simultaneously to both the adjacent and the opposite electrodes. This occurs for the trigger to working voltage ratio, which results in equal mean electric fields across the main gap and the trigger gap. The experiments were carried out with working voltages, Vg, between −1 and −2 MV and with trigger voltage to working voltage ratios of between −2% and −8%. It is shown that the minimum delay and jitter figures are indeed obtained with the trigger voltage closest to this optimum value, Vt*. For the switch used here, this corresponded to a ratio of Vt*/Vg=−3.4%. A single switch was operated for two hundred shots with a working voltage of −2 MV and the optimum trigger voltage and gave an average delay of 44 ns with an overall jitter of 4.4 ns. © 1995 American Institute of Physics.

Notizen: The deep level transient spectroscopy technique has been employed to follow closely the effect of 1–300 Mrad 60Co γ irradiation on the deep electron traps in undoped vapor-phase-epitaxy n-type GaAs. The 1 Mrad γ-irradiated Schottky device was identical to the as-grown or control device, with only two electron traps EL2 (Ec−0.820 eV) and EL3 (Ec−0.408 eV) detected. At a γ dose of 5 Mrad, two additional electron traps EL6 (Ec−0.336 eV) and E2 (Ec−0.128 eV) were observed. As the γ doses were increased to ≥10 Mrad, a third electron trap E1 (Ec−0.033 eV) was observed, and the single exponential EL2 capacitance transient became a double exponential, indicating two deep levels lying at Ec−0.820 eV (EL2/EL2-A) and Ec−0.843 eV (EL2-B). The trap concentration of EL2-A remained unchanged up to a γ dose of 50 Mrad before starting to increase slowly as the γ dose was increased to ≥100 Mrad. In contrast, the EL2-B trap concentration was found to increase by 32 times, reaching 2.6×1014 cm−3 at 300 Mrad from a low 8.0×1012 cm−3 at 10 Mrad when it was first observed, whereas for the 1 MeV electron irradiation with low electron fluence of 1014 e cm−2, the EL6, E2, E1, and the double exponential EL2 were detected at the same time. There was no sign of EL2-B, EL3, EL6, E2, or E1, but an additional broad U band was observed after irradiation with 1 MeV neutrons. The results of the γ and neutron irradiation suggest that the presence of the double exponential EL2 transients is not related to either EL6, E2, E1, or the U band, and is unlikely to be due AsGa→VGa+Asi but is probably caused by the AsGa complex defects involving an irradiation defect. The defect concentration of trap E1 increased strongly from 5.4×1013 cm−3 at 10 Mrad to 9.3×1014 cm−3 at 100 Mrad, and E2 increased from 2.1×1013 cm−3 at 5 Mrad to 6.7×1014 cm−3 at 100 Mrad. © 1995 American Institute of Physics.

Notizen: The scanning electron microscope mirror image method is developed to measure the charge distribution volume in insulators. An electrostatic potential expression is derived by assuming the dipolar approximation and hemispheroid distribution. Dielectric samples with different relative permittivities are employed in charging experiments to justify our approach. The proposed method is employed to measure the radius of the charge distribution volume in polymethylmethacrylate samples irradiated by electron beams with energy ranging from 25 to 39 keV. Experimental results achieved are in good agreement with those obtained through the use of other experimental techniques and Monte Carlo simulation. The strength of the present method is in its ability to quantitatively give the total trapped charge and its distribution in the electron irradiated insulators in a single experiment. © 1995 American Institute of Physics.

Notizen: Ferroelectric Pb(Zr0.52Ti0.48)O3 (PZT) (001) thin films, integrated to high Tc superconducting YBa2Cu3O7−x (YBCO) films used as electrodes, have been studied for their photocurrent response and permittivity as a function of temperature. A stable photocurrent was observed to increase with increasing the temperature over the range of 25–350 °C. This increase was found to be strongly polarization dependent and due to the change of the pyroelectric coefficient of PZT thin film with temperature. The pyroelectric coefficient for a PZT sample was measured as ∼30 nC/cm2 K at room temperature, and ∼80 nC/cm2 K at 320 °C. The YBCO electrode showed a stable metallic resistance behavior in this temperature range. There was no detectable photocurrent from YBCO layer. No poling is required until 350 °C for the PZT/YBCO heterostructure detector because the PZT film is oriented when grown on c-axis oriented YBCO. © 1995 American Institute of Physics.

Notizen: Conducting tubes filled with neutral gas at pressures between 0.001 and 0.1 Torr can be used to transport, to center, and to reduce the transverse oscillations of high current ((approximately-greater-than)10 kA) electron beams. Electron impact ionization of the gas leads to partial neutralization of the beam space charge allowing self-focused beam transport and phase-mix damping of injected beam oscillations. In addition, the presence of conducting walls helps center the beam in the transport tube. High current beams, transported through a 1.3 m long tube, were centered to within one-tenth of the beam radius and input transverse oscillations were damped to submillimeter values without significant current loss or emittance growth. Beam transport properties are examined as a function of injected current, gas pressure, and cell geometry. Experimental results are compared with a theoretical model.

Notizen: We describe our measurements of linewidths of the two J=2-1 transitions (232.24 and 236.25 A(ring)) and the J=0-1 transition (196.20 A(ring)) of Ne-like germanium soft x-ray laser in slab targets. The high resolution spectral measurement was performed using a grazing incidence spectrometer with an x-ray CCD camera. Time integrated linewidths, the full width at half maximum, of the two J=2-1 lasing lines are about 20(±4) mA(ring) when the gain-length product (GL) is ∼8. The linewidth of the J=0-1 line is measured to be 25 mA(ring) as the GL is ∼5. The measured linewidths of the 236.25 A(ring) line are slightly narrower than those of 232.24 A(ring) in the same shot, reflecting their different intrinsic linewidths. For the J=2-1 transitions, the thermal Doppler broadening (43 mA(ring) inhomogeneous) has been obtained by one-dimensional hydrodynamic simulation of the plasma expansion, and the collisional broadening (15 mA(ring) homogeneous) has been considered with all inelastic electron collisional rates among the transition levels in the plasma. The predicted 52 mA(ring) Voigt profile intrinsic linewidth for the 236.25 A(ring) line has been used to calculate linewidth narrowing by the one-dimensional model of amplified spontaneous emission. The experimental data are in agreement with the model calculation in the short targets. Spectral narrowing is not evident in the long targets. We examined soft x-ray propagation in the plasma by ray-trace calculation. It is shown that refraction is an important factor affecting the spectral narrowing of soft x-ray lasers. © 1995 American Institute of Physics.

Notizen: A transient model of a low-pressure Cs-Ba diode which operates in a collisionless Knudsen mode is developed and benchmarked with experimental measurements. The model couples the external circuit to the plasma discharge in the diode. The model calculates the electron energy, plasma density, densities of ground state and first four excited states of neutral Cs atoms, sheath potentials, Cs coverage on the electrode surfaces, forward voltage drop, and discharge current as functions of time, as well as the current-voltage (I-V) characteristic of the diode. Calculated terminal voltage and current characteristics are in good agreement with experimental values. © 1995 American Institute of Physics.

Notizen: We propose a model for point-defect-assisted transient diffusion and activation of high-dose boron implants in silicon. To model transient diffusion, a nonlinear equilibrium clustering model for point defects is used. The activation of boron is modeled as a Fermi-level-dependent transformation of inactive dopant clusters to substitutional atoms. Comparison with experimental data shows that this approach can provide a description of both rapid thermal annealing and long-time furnace annealing steps, with an excellent predictive capability for both chemical and electrically active profiles. © 1995 American Institute of Physics.

Notizen: Using first-principles band-structure theory we have systematically calculated the (i) alloy bowing coefficients, (ii) alloy mixing enthalpies, and (iii) interfacial valence- and conduction-band offsets for three mixed-anion (CuInX2, X=S, Se, Te) and three mixed-cation (CuMSe2, M=Al, Ga, In) chalcopyrite systems. The random chalcopyrite alloys are represented by special quasirandom structures (SQS). The calculated bowing coefficients are in good agreement with the most recent experimental data for stoichiometric alloys. Results for the mixing enthalpies and the band offsets are provided as predictions to be tested experimentally. Comparing our calculated bowing and band offsets for the mixed-anion chalcopyrite alloys with those of the corresponding Zn chalcogenide alloys (ZnX, X=S, Se, Te), we find that the larger p−d coupling in chalcopyrite alloys reduces their band offsets and optical bowing. Bowing parameters for ordered, Zn-based II-VI alloys in the CuAu, CuPt, and chalcopyrite structures are presented: we find that ordered Zn2SeTe has bowing coefficients of 1.44 and 3.15 eV in the CuAu and CuPt structures, while the random ZnSexTe1−x alloy has a bowing of 1.14 eV. The band alignment between CuInSe2 and CuInSe2-derived ordered vacancy compounds are also presented. © 1995 American Institute of Physics.

Notizen: We have studied the effect of erbium-impurity interactions on the 1.54 μm luminescence of Er3+ in crystalline Si. Float-zone and Czochralski-grown (100) oriented Si wafers were implanted with Er at a total dose of ∼1×1015/cm2. Some samples were also coimplanted with O, C, and F to realize uniform concentrations (up to 1020/cm3) of these impurities in the Er-doped region. Samples were analyzed by photoluminescence spectroscopy (PL) and electron paramagnetic resonance (EPR). Deep-level transient spectroscopy (DLTS) was also performed on p-n diodes implanted with Er at a dose of 6×1011/cm2 and codoped with impurities at a constant concentration of 1×1018/cm3. It was found that impurity codoping reduces the temperature quenching of the PL yield and that this reduction is more marked when the impurity concentration is increased. An EPR spectrum of sharp, anisotropic, lines is obtained for the sample codoped with 1020 O/cm3 but no clear EPR signal is observed without this codoping. The spectrum for the magnetic field B parallel to the [100] direction is similar to that expected for Er3+ in an approximately octahedral crystal field. DLTS analyses confirmed the formation of new Er3+ sites in the presence of the codoping impurities. In particular, a reduction in the density of the deepest levels has been observed and an impurity+Er-related level at ∼0.15 eV below the conduction band has been identified.This level is present in Er+O-, Er+F-, and Er+C-doped Si samples while it is not observed in samples solely doped with Er or with the codoping impurity only. We suggest that this new level causes efficient excitation of Er through the recombination of e-h pairs bound to this level. Temperature quenching is ascribed to the thermalization of bound electrons to the conduction band. We show that the attainment of well-defined impurity-related luminescent Er centers is responsible for both the luminescence enhancement at low temperatures and for the reduction of the temperature quenching of the luminescence. A quantitative model for the excitation and deexcitation processes of Er in Si is also proposed and shows good agreement with the experimental results. © 1995 American Institute of Physics.

Notizen: Metastable thin-film alloys of tin and silicon have been grown on room temperature substrates by pulsed laser deposition. The composition of the targets, made by pressing pellets of a mixture of tin and silicon powders, was maintained in the deposited films. The granular films consisted of tin nano-crystallites surrounded by an amorphous matrix. Deposited films with tin concentration greater than 15% showed metallic behavior, optically as well as electrically, while films with tin contents less than 15% displayed optical bandgaps ranging from 100 to 300 meV. Charge transport in the semiconducting films can be modeled by a combination of conduction in extended states and hopping at the Fermi level. © 1995 American Institute of Physics.

Notizen: A model for the leakage current of silicon oxide-silicon nitride-silicon oxide (ONO) dielectrics at low electric fields ((approximately-equal-to)2 MV/cm) was successfully developed. It is proposed that two transition mechanisms occur simultaneously. One is the detrapping of electrons from the silicon dangling bond in amorphous silicon nitride (SiN), which corresponds with the transition of dangling bonds among three possible charge states. The second is the direct tunneling of the detrapped electrons from the SiN to the gate through the thin silicon oxide. Both the location and the energy levels of the defect state are taken into account. The energy level, and the intrinsic time constant of the Si dangling bond and the uniform trap density in SiN, can be obtained by comparing the experimental results of the ONO discharge current with the calculated ones based on the above model. It can be found that the energy levels for negatively charged and neutral Si dangling bonds (E− and E0), with respect to the SiN conduction band, are 1.2 and 2.0 eV, respectively, the intrinsic time constants t− and t0 are 1.0×10−14 and 4.0×10−13 s, respectively, and the uniform trap density is 4.0×1019/cm3. From the energy level difference between E− and E0, we can conclude that the effective correlation energy of the Si dangling bonds in SiN is 0.8 eV, which is consistent with Robertson's results based on a tight binding calculation. © 1995 American Institute of Physics.

Notizen: The mechanism of carrier transport through a thin silicon-oxide layer for 〈spray-deposited indium-tin-oxide (ITO)/silicon-oxide/Si〉 solar cells has been studied by measurements of the dark current density as a function of the thickness of the silicon-oxide layer, together with the observation of transmission electron micrographs. Cross-sectional transmission electron micrography shows that a uniform silicon-oxide layer with the thickness of ∼2 nm is present between ITO and Si when the ITO film is deposited on a flat Si(100) surface at 450 °C. The dark current density under a depletion condition strongly depends on the thickness of the silicon-oxide layer. It is concluded from these results that quantum mechanical tunneling is the dominant mechanism for the charge carrier transport through the silicon-oxide layer. On the other hand, when the ITO film is deposited on a mat-textured Si surface at the same temperature, a nonuniform silicon-oxide layer is formed, with ITO penetrating into the silicon-oxide layer in the top and valley regions of the pyramidal structure. By raising the deposition temperature of the ITO film on the flat Si(100) surface to 500 °C, the silicon-oxide layer becomes also nonuniform. For these diodes with the nonuniform silicon-oxide layer, the carrier transfer probability is less dependent on the thickness of the silicon-oxide layer, leading to the conclusion that minute channels of ITO are present in the silicon-oxide layer and charge carriers transfer through the channels. The photovoltage is decreased by the presence of the minute channels, with its magnitude depending on the density of the channels. The conversion efficiency of the 〈ITO/silicon-oxide/n-Si(100)〉 solar cells is unchanged upon illumination for 1000 h. The good cell stability is attributed to the well-crystallized ITO film which effectively suppresses diffusion of oxygen from the air and to low reactivity of ITO with Si at room temperature. © 1995 American Institute of Physics.

Notizen: In this paper we study the magnetic behavior of amorphous ferromagnetic ribbons having helical anisotropy with the helix axis perpendicular to the sample surface. It is shown that the magnetization processes under the action of a longitudinal magnetic field can be controlled by the nonuniform field produced by an alternating current, of the same frequency than the exciting field, flowing through the sample. Hysteresis loops with very different susceptibilities and coercive fields have been obtained by varying the amplitude of the current intensity and the phase differences between the exciting magnetic field and the current through the sample. © 1995 American Institute of Physics.

Notizen: Photoreflectance(PR) spectra of the graded InAlAs/InGaAs heterojunction bipolar transistor layers were investigated at various temperatures between 8 K and 300 K. The energy features of the PR spectra were fitted and identified as band-to-band transitions in the graded layers which were grown by pulsed molecular beam epitaxy (pulsed-MBE) and InGaAs as well as InAlAs layers. The temperature variation of energy gaps can be described by the Varshni and Bose-Einstein equations. A linear variation relationship of band gaps with Al composition (z) was observed and approximated to be E0(z)=0.809+0.769z eV at T=0 K. However, the parameters aB and aitch-thetaB derived from the Bose-Einstein expression do not change meaningfully in the whole range of Al composition. From the observed Franz-Keldysh oscillations (FKOs) we have evaluated the built-in dc electric fields in the i-InGaAs collector, i-InGaAs spacer and n-InAlAs emitter regions. The electric fields are in good agreement with the continuity condition of electric displacements in the interfaces between emitter and base. © 1995 American Institute of Physics.

Notizen: Shoulders have been observed in the measured semilogarithmic current-voltage (I–V) characteristics of high-efficiency passivated emitter and rear locally diffused silicon (Si) solar cells. An improved understanding is given of the mechanism proposed to cause these nonideal I–V curves. It is shown that this mechanism is due to the electrostatic behavior of free carriers at the Si/SiO2 interface of oxidized Si devices in which the Si adjacent to the oxide is depleted (or in some cases, inverted) at equilibrium, and results in saturation of the surface recombination rate. Two-dimensional numerical computer simulations are used to investigate this mechanism and its effect on cell performance. In addition, the simulations provide a means of estimating the extent to which lateral conduction in the rear surface channel also contributes to the observed recombination saturation in these cells. It is shown that ohmic limitation of lateral conduction occurs, however, the lateral current flows are negligible in comparison to the recombination currents due to the former mechanism. © 1995 American Institute of Physics.

Notizen: Copper nitride thin films were obtained by the reactive sputtering method. A metallic copper target was sputtered in nitrogen gas with radio-frequency (rf) magnetron sputtering equipment. Highly [100]-oriented polycrystalline films of the cubic anti-ReO3 structure were obtained. Films with a lattice constant above 3.868 A(ring) were conductors, while films with a lattice constant below 3.868 A(ring) were insulators. The resistivity of conducting films was 0.5–3×10−2 Ω cm. The insulating films showed an optical energy gap of 1.3 eV, while the conducting films showed a smaller value which decreased with decreasing resistivity. © 1995 American Institute of Physics.

Notizen: The initial stages of oxide formation on atomically flat, monohydride terminated Si(111) surfaces by oxidation in 30% hydrogen peroxide solution (H2O2) were investigated by scanning tunneling microscopy and x-ray photoelectron spectroscopy. The reaction proceeds via homogeneous nucleation of small oxide clusters on the surface and subsequent lateral growth of these clusters within the surface bilayer. The oxidation of the topmost Si(111) bilayer in H2O2 solution is completed after 30 min, leading to a SiO1.2 average layer composition. For the next bilayer the oxidation rate decreases drastically—after 2 months only 60% of the second bilayer are oxidized. An inverse logarithmic rate of the second bilayer oxidation is consistent with a field assisted growth mechanism. The significant differences in the oxidation rates between the first and the second bilayer allow to produce well defined oxide layers of about 5 A(ring) thickness. © 1995 American Institute of Physics.

Notizen: The qualitative and structural modifications of very thin SiO2 films with thicknesses between 80 and 500 A(ring), caused by the oxidation procedure, were studied with spectroscopic ellipsometry (SE). Analysis of the experimental dielectric function obtained by SE provides the structural characteristics and the thickness of the oxide in fairly good agreement with electron microscopy results in cross-sectional geometry. The calculated voids volume fraction was found to drop below 3% (2%) for oxides thicker than 250 (400) A(ring) grown at 900 (1000) °C. The densification of thermal oxides grown at low oxidation temperature predicted by Fourier transform infrared is discussed and compared to the SE results, whereas a relation between the final oxidation time and the viscoelastic relaxation time was found. Furthermore, it is shown that in situ SE can be used to monitor the process of oxide removal with very low-energy Ar+ ions and to control the conditions in order to avoid oxide and Si substrate damage. It is found that Ar+ ions with energy of about 10 eV are required to avoid oxide damage and Si substrate amorphization at a depth below 6 A(ring). The latter finding is also corroborated by atomic force microscopy images obtained from Si substrates after oxide etching. Finally, the influence of oxide thickness and the oxidation procedure on the E1 structure of c-Si was studied and from the results the stress applied by the oxide on the Si substrate was determined. From this study it was found that the stress depends strongly on the oxidation duration as well as on the oxidation procedure and the type of Si substrate and is minimized at an oxidation temperature 900 °C for oxidation duration longer than 70 min. © 1995 American Institute of Physics.

Notizen: The improvements of the AlGaAs solar cell grown on the Si substrate and the AlGaAs/Si tandem solar cell by metalorganic chemical vapor deposition have been investigated. The active-area conversion efficiency of the Al0.1Ga0.9As solar cell on the Si substrate as high as 12.9% has been obtained by improving the growth sequence and adopting an Al compositionally graded band emitter layer. A high efficiency monolithic AlGaAs/Si tandem solar cell with the active-area conversion efficiency of 19.9% and 20.6% (AM0 and 1 sun at 27 °C) under two-terminal and four-terminal configurations, respectively, is demonstrated. © 1995 American Institute of Physics.

Notizen: We present a device model to describe polymer light-emitting diodes (PLEDs) under bias conditions for which strong electrical injection does not occur (i.e., reverse, zero, and weak forward bias). The model is useful to interpret: capacitance–voltage measurements, which probe the charged trap density in the PLEDs; electroabsorption measurements on PLEDs, which probe the built-in electric field in the device; and internal photoemission measurements, which probe the effective Schottky barriers at the contacts of the PLED. The device model is based on the low-density nondegenerate continuum model for the electronic structure of polymers. Polarons and bipolarons are the principal charged excitations in this model. Polarons are singly charged excitations which play the primary role in charge injection and in experiments such as internal photoemission which probe single particle interface properties. Bipolarons are doubly charged excitations which can play an important role in establishing Schottky barriers at metal/polymer interfaces. In the device model, the region of the polymer near each contact is assumed to be in quasiequilibrium with that contact. The charge density as a function of position is found from the electrostatic potential and equilibrium statistics. Poisson's equation is integrated to determine the electrostatic potential. We find that a large charge density is transferred into the polymer if the chemical potential of a contact is higher than the negative bipolaron formation energy per particle or lower than the positive bipolaron formation energy per particle. The transferred charge pins the Fermi level and establishes the effective Schottky barrier. If the contact chemical potential is between the formation energy per particle of the two types of charged bipolarons, there is little charge transfer into the polymer and the Fermi level is not pinned. The electric field in the device is found for different contacts and bias conditions. Capacitance as a function of voltage is calculated for various trap binding energies and densities. The calculated results are used to interpret recent measurements on PLEDs. © 1995 American Institute of Physics.

Notizen: The magnitude and relative phase of the rf magnetic flux density has been measured as a function of radial position using a magnetic (B-dot) probe in an inductively coupled low pressure argon discharge driven at 13.56 MHz. The spatial variation of the rf electric field and the discharge current density were determined from the probe measurements and are compared at gas pressures of 3, 30, and 300 mTorr for a fixed discharge power of 50 W and at discharge powers of 21, 50, and 103 W for a fixed gas pressure of 30 mTorr. In the active zone near the induction coil where the rf field is strong, the electron rf drift velocity derived from the B-dot data and Langmuir probe measurement was found to be considerably less than the electron thermal velocity even at the lowest argon pressure. For an axially symmetric inductive discharge, as is found in many applications, a two-dimensional magnetic probe measurement with space and phase resolution is necessary to correctly infer the rf electric field and the discharge current density distribution. © 1995 American Institute of Physics.

Notizen: Actinide carbides encapsulated with multilayered graphitic capsules have been produced by the arc burning of a composite carbon rod containing thoriated tungsten or uranium ore. The morphology of these filled nanoparticles was observed by transmission electron microscopy. The preferential encapsulation in the constituent elements of the core materials surrounded by the graphite cage was demonstrated. Furthermore, it was found that these carbon nanoparticles filled with radioactive materials have been quite stable for 1 year or more. © 1995 American Institute of Physics.

Notizen: The diffusion of tin in intrinsic germanium has been studied in the temperature range from 555 to 930 °C by secondary ion mass spectrometry. Tin has been indiffused under vacuum conditions both from the gas phase and from thin films. In both cases, pure metal was used as a source of tin. In the studied temperature range the diffusion coefficient of tin as a function of temperature can be expressed as D(cm2/s)=8.4×102 exp[(−3.26 eV)/kT]. Based on the similarities between tin diffusion and germanium self-diffusion, it is concluded that tin diffuses in germanium via the monovacancy mechanism. © 1995 American Institute of Physics.

Notizen: We investigated the in-plane compositional modulation of InGaAs epilayers in selective area metalorganic vapor phase epitaxy using a stagnant layer model. The growth pressure dependence of selective area growth of InAs and GaAs constituents using trimethylindium and triethylgallium revealed that the origin of the compositional modulation is in the mask region. A larger escaping probability of desorbed Ga source materials from the mask surface to the fluid layer and a slightly larger sticking coefficient defined at the mask surface of Ga source materials enrich the In composition of InGaAs epilayers near the mask edge. The escape probability is determined by the length ratio of the vapor phase mean-free path of source materials to the stagnant layer thickness. The finite sticking coefficient at the mask surface corresponds to a chemical reaction which produces nonreactive species. With the larger escape probability of Ga source materials and the introduction of finite sticking coefficients at the mask surface, we successfully quantitatively predicted the experimental results. © 1995 American Institute of Physics.

Notizen: The effects of the incident ion/metal flux ratio (1≤Ji /JTi≤15), with the N+2 ion energy Ei constant at (approximately-equal-to)20 eV ((approximately-equal-to)10 eV per incident accelerated N), on the microstructure, texture, and stoichiometry of polycrystalline TiN films grown by ultrahigh-vacuum reactive-magnetron sputtering have been investigated. The layers were deposited in pure N2 discharges on thermally oxidized Si(001) substrates at 350 °C. All films were slightly overstoichiometric with a N/Ti ratio of 1.02±0.03 and a lattice constant of 0.4240±0.0005 nm, equal to that of unstrained bulk TiN. Films deposited with Ji/JTi=1 initially exhibit a mixed texture—predominately (111), (002), and (022)—with competitive columnar growth which slowly evolves into a pure (111) texture containing a network of both inter- and intracolumn porosity with an average column size of (approximately-equal-to)50 nm at t=1.6 μm. In contrast, films grown with Ji/JTi≥5 do not exhibit competitive growth. While still columnar, the layers are dense with an essentially complete (002) preferred orientation and an average column size of (approximately-equal-to)55 nm from the earliest observable stages. The normalized x-ray diffraction (002) intensity ratio in thick layers increased from (approximately-equal-to)0 to 1 as Ji/JTi was varied from 1 to ≥5. Both 111 and 001 interplanar spacings remained constant as a function of film thickness for all Ji/JTi. Thus, contrary to previous models, strain is not the dominant factor in controlling the development of preferred orientation in these films. Moreover, once film texture is fully evolved—whether it be (002) or (111)—during deposition, changing Ji/JTi has little effect as preferred orientation becomes controlled by pseudomorphic forces. Film porosity, however, can be abruptly and reversibly switched by increasing or decreasing Ji/JTi. © 1995 American Institute of Physics.

Notizen: The crystallization of amorphous carbon films, under inert atmospheres, occurs at annealing temperatures above 800 °C. In this work we have found that when the annealing of carbon films is performed under atmospheric conditions, crystallization occurs at temperatures as low as 200 °C. The catalytic effect of oxygen in the crystallization process is understood in terms of the generation of a porous structure in the carbon film due to the vaporization of carbon oxides. © 1995 American Institute of Physics.

Notizen: Raman spectroscopy has been used to investigate the effects of dynamic and postimplantation annealing on glassy carbon implanted with 50 keV C ions to a dose of 5×1016 ions/cm2. The postimplantation annealing of damage in the ion-beam modified material was found to occur in two stages as a function of postimplantation annealing temperature Ta. These occur for 500〈Ta〈800 K and Ta(approximately-greater-than)1300 K and correspond to the thermal energy required to activate C interstitials and vacancies, respectively. Once mobile these defects diffuse through the implanted layer, reducing bond angle disorder which leads to an increase in graphitic order as interstitial-vacancy recombination occurs. The effects of the ion-beam irradiation on the final structure of glassy carbon were found to be a strong function of the temperature of the sample during the irradiation, Ti. This dependence is interpreted in terms of dynamic annealing and radiation-enhanced diffusion. Three temperature regimes were found to be important. For Ti〈300 K defect motion during irradiation is suppressed. For 300〈Ti〈600 K, the mobility of C interstitials during irradiation results in dynamic annealing which prevents amorphization, with the result that the ion irradiation creates a highly disordered, but essentially graphitically bonded carbon. For Ti(approximately-greater-than)600 K, vacancy mobilities are sufficiently high such that most ion-beam-induced defects are dynamically annealed and, for Ti(approximately-greater-than)800 K the unimplanted glassy carbon microstructure is retained following the ion-beam irradiation. Finally, activation energies for interstitial and vacancy mobilities were determined and found to compare favorably with those found in other forms of carbon. © 1995 American Institute of Physics.

Notizen: Buried boron layers were epitaxially grown on single crystal silicon substrates and subjected to steam oxidation at 650–750 °C and pressures of 1, 5, and 15 atm. The layers were approximately 200 nm thick and capped by 400 nm of undoped silicon. The boron concentration varied from 8×1017 to 4×1018 atoms/cm3. The ensuing enhanced boron diffusion was modeled on the assumption that the oxidation maintained a supersaturation of interstitials at the surface which was proportional to the square root of the oxidation rate. Fully coupled dopant-defect diffusion equations were necessary to accurately model the oxidation enhanced diffusion. © 1995 American Institute of Physics.

Notizen: In liquid-lubricated bearings used in experimental near-contact magnetic recording devices, rheological properties of lubricant films at very high shear rates (up to about 109 s−1) and at near-ambient pressure are required. In this extremely high shear rate regime, shear thinning (non-Newtonian behavior) becomes extremely important. A rheometer is designed and constructed that makes it possible to study the rheology of lubricants at extremely high shear rates. Since traditional rheometers are limited to moderate shear rates due to viscous heating, the design used in this study is based on a magnetic disk drive configuration in which an ultrathin film of lubricant is sheared between a magnetic disk and a slider at relatively low sliding speeds. By monitoring the film thickness between the slider and the disk using a capacitive method, it is possible to accurately measure the shear rate. Two different types of perfluoropolyether lubricants have been studied up to shear rates of 107 s−1, and these data have been compared to previously published data. © 1995 American Institute of Physics.

Notizen: Light emitted from metal/oxide/metal tunnel junctions can originate from the slow-mode surface plasmon polariton supported in the oxide interface region. The effective radiative decay of this mode is constrained by competition with heavy intrinsic damping and by the need to scatter from very small scale surface roughness; the latter requirement arises from the mode's low phase velocity and the usual momentum conservation condition in the scattering process. Computational analysis of conventional devices shows that the desirable goals of decreased intrinsic damping and increased phase velocity are influenced, in order of priority, by the thickness and dielectric function of the oxide layer, the type of metal chosen for each conducting electrode, and temperature. Realizable devices supporting an optimized slow-mode plasmon polariton are suggested. Essentially these consist of thin metal electrodes separated by a dielectric layer which acts as a very thin (a few nm) electron tunneling barrier but a relatively thick (several 10's of nm) optically lossless region. © 1995 American Institute of Physics.

Notizen: The in-plane uniaxial anisotropy and perpendicular anisotropy of sputtered Co/Ag(111) multilayers have been investigated by magnetic measurements and the in-plane ferromagnetic resonance (FMR) technique. The data-fitting analysis for the FMR experimental results about the angular dependence of the resonance field shows that the in-plane anisotropy cannot be fully described by only using the first-order term, and the second-order term must be included. Furthermore, the interface-induced anisotropy has been obtained by FMR. Both ferromagnetic and antiferromagnetic couplings have been revealed by hysteresis loop measurement. Specifically, a spin-flip phase transition has been observed in the sputtered Co/Ag(111) multilayers, which is attributed to the effects of the AF-coupling and in-plane uniaxial anisotropy. © 1995 American Institute of Physics.

Notizen: The magnetic properties of a series of Ce2Fe17−xSix solid solutions with x equal to 0.0, 0.23, 0.4, 0.6, 0.8, 1.02, 1.98, and 3.20 have been studied by magnetic measurements, neutron diffraction, and Mössbauer spectroscopy. An x-ray-diffraction study indicates that the compounds adopt the rhombohedral Th2Zn17-type structure. The substitution of silicon for iron in Ce2Fe17 leads to a contraction of the a axis by 0.2%, an expansion of the c axis by 0.2%, and a consequent reduction of the unit-cell volume by about 0.2% per substituted silicon. Magnetization studies indicate that the Curie temperature increases uniformly from 238 K for Ce2Fe17 to 455 K for Ce2Fe14Si2. Powder neutron-diffraction results, obtained at 295 K, indicate both that the silicon atoms preferentially occupy the 18h sites and that the iron moments increase with increasing silicon content, an increase which is related to the increase in Curie temperature. The Mössbauer spectra have been fit with a binomial distribution of the near-neighbor environments in terms of a maximum hyperfine field Hmax for an iron with zero silicon near neighbors, and a decremental field ΔH per silicon near neighbor. The compositional independence of both the weighted average maximum hyperfine field and of the decremental field indicates that the silicon acts as a magnetic hole, a hole which does not perturb the magnetic moments at the iron sites. The compositional dependence of the weighted average isomer shift is explained in terms of an interband mixing of the iron 4s and silicon 2p bands, due to the reduction of the iron 18h bond lengths. This interband mixing affects the charge but not the spin distribution at the iron sites. © 1995 American Institute of Physics.

Notizen: The surface morphology and the dislocation structure of thin and thick strained InxGa1−xAs heterostructures grown on (001) InP substrates have been investigated using atomic force microscopy and transmission electron microscopy. Long linear ridges, correlated to underlying dislocations, are detected in thin partially relaxed In0.82Ga0.18As layers. The ridges (with increased height along the [110] direction comparing to along the [1-10] direction) are associated with strong inhomogeneous local growth rates induced by elastic strain relief and controlled by kinetic factors. The crosshatched surface morphology in thick In0.65Ga0.35As layers is explained using the same mechanism with multiplication and interaction of dislocations and ridges during growth. In contrast, the development of isolated "V-shaped'' grooves parallel to [1-10] in lattice-matched capping layers grown on partially relaxed In0.82Ga0.18As quantum wells is explained by the generation of surface corrugations in tensile strained regions appearing just above the elongated relaxed region of In0.82Ga0.18As. © 1995 American Institute of Physics.

Notizen: An isotropic two band model is proposed for electrons in silicon, that has the same density of states and magnitude of group velocity as those of the full band structure based on empirical pseudopotential method calculations. The band model and transport parameters are calibrated through extensive comparisons with Monte Carlo simulation results and various experiments related to electron transport in silicon. Specifically, the drift velocity, impact ionization coefficient, quantum yield, 2p core level line intensity and broadening from x-ray photoemission spectroscopy have all been used in calibration. Through the study of electron emission at the Si/SiO2 interface in metal-oxide-semiconductor structures using the Monte Carlo method, it is demonstrated that the model has good accuracy in modeling high field transport phenomena in silicon. © 1995 American Institute of Physics.

Notizen: The effect of interelectrode spacing on the properties of hydrogenated amorphous silicon (a-Si:H) films grown at high radio-frequency (rf) power density by rf plasma enhanced chemical vapor deposition method, with control of dusty plasma conditions by heating both the electrodes, was investigated. The formation of precursors responsible for gas phase polymerization itself was sought to be controlled by preheating of the source gas mixture. Optimization of the interelectrode spacing for film characteristics was carried out for this novel deposition technique combining cathode heating and preheating of the source gases. The films were characterized by infrared vibrational spectroscopy, absorption and reflection measurements in the visible and near infrared regions, measurements of dark and photo-conductivity (with light induced degradation), and electron spin resonance spectroscopy. © 1995 American Institute of Physics.

Notizen: The crystallographic microstructure and electrical characteristics of Au/Pt/Ti/Ni ohmic contacts on p-type (001) ZnTe layers are investigated as a function of annealing temperature, by using the transmission line model method, cross-sectional transmission electron microscopy, and Auger electron spectroscopy. The specific contact resistance decreases when the annealing temperature is increased and reaches a minimum at 300 °C. A minimum value of 1.1×10−6 Ω cm2 is obtained for a hole concentration of 3×1019 cm−3. The epitaxial NiTe2 that formed at the metal/semiconductor interface due to annealing is considered to play an important role in lowering the contact resistance. The excess Zn atoms created by the reaction between Ni and ZnTe are found to diffuse upward and to segregate at the Pt/Ni interface. A contact stability test performed at 102 °C suggests that these ohmic contact structures are stable even under high-current injection. © 1995 American Institute of Physics.

Notizen: We describe a new and efficient method for the numerical study of the dynamics and statistics of single electron systems presenting arbitrary combinations of small tunnel junctions, capacitances, and voltage sources. The method is based on numerical solution of a linear matrix equation for the vector of probabilities of various electric charge states of the system, with iterative refining of the operational set of states. The method is able to describe very small deviations from the "classical'' behavior of a system, due to the finite speed of applied signals, thermal activation, and macroscopic quantum tunneling of charge (cotunneling). As an illustration, probability of dynamic and static errors in two single electron memory cells with 6 and 8 tunnel junctions have been calculated as a function of bias voltage, temperature, and switching speed. © 1995 American Institute of Physics.

Notizen: We measured by internal photoemission the conduction-band discontinuity ΔEc in p-In0.53Ga0.47As/n-InP heterojunctions with a Si δ layer (1×1012 cm−2) inserted in InP at 10 A(ring) from the interface. The n-type Si δ doping induced an inhomogeneous and temperature-dependent conduction-band offset reduction as revealed by two onsets in the spectral response. The first one was absent in room-temperature data and was due to the Si intralayer presence. The second correlated with the conduction-band discontinuity value for heterojunctions without δ doping and its presence served as an indication of the inhomogeneity of the Si δ layer. The measured value of the modification was 0.11±0.04 eV in good agreement with the calculated one. Current-voltage measurements confirmed that the Si δ layer modified the transport parameters of the heterojunction only at low temperature. © 1995 American Institute of Physics.

Notizen: In grain oriented 3% Si–Fe, the magnetization curve from saturation to remanence is mainly determined by the crystallographic texture. The saturation approach law describes the rotation of magnetization vector towards the magnetic field, which increases in strength in this region of the hysteresis curve. The orientation of the magnetization vector with respect to the easy magnetization direction can be experimentally determined measuring the torque produced by the magnetic field in a disk-shaped specimen. This torque is related directly to the spread of grain orientations and its measure allows us to evaluate different texture qualities. © 1995 American Institute of Physics.

Notizen: Magnetic measurements have been performed on a GdBa2Cu3O6+x superconducting thin film. The paramagnetism carried by rare earth Gd3+ ions in the film tilts the magnetic hysteresis loop and broadens the width of the magnetic hysteresis ΔM, then the magnetization critical current density and the volume flux pinning force density based on the Bean critical state model deviate from intrinsic values. Therefore, in order to get useful information on the pinning mechanism, correction for the paramagnetism is essential. And after correction for the paramagnetism, a scaling law of the volume flux pinning force density is obtained as f(b)=(b/4)0.5(1−b/4)1.5, based on which the possible pinning mechanism in the film is discussed. In the end, deviation from the scaling law at high fields is interpreted by the collective pinning theory. © 1995 American Institute of Physics.

Notizen: The space-charge limiting current for an intense, magnetized, relativistic electron beam injected into a grounded metallic pipe is investigated with a 2(1/2)-dimensional particle-in-cell code. Comparisons between the simulation results, the well known Bogdankevich-Rukhadze limiting current, and more recent theoretical estimates of the limiting current are presented. Transmitted currents (approximately-greater-than)15% above those predicted by the Bogdankevich–Rukhadze and other limiting current estimates are observed. However, good agreement between the simulation results and the analytic estimates of Uhm [Phys. Fluids B 5, 1919 (1993)] and Fessenden [Lawrence Livermore Lab. Rep. No. UCID-16527 (1974)] is found. For an injected current above the limiting value, a virtual cathode is formed which alters the transmitted current density profile of the beam. A theoretical estimate of the magnitude of the transmitted current under this condition is compared with simulation results. The predicted transmitted current is found to be valid only for injected currents slightly above the limiting current. In addition, the transition between vacuum and ion-focused-regime transport, with and without an applied axial magnetic field is simulated. For ion-focus-regime densities (np ∼ nb), the effect of the virtual cathode in limiting the beam transmission is greatly diminished as expected. © 1995 American Institute of Physics.

Notizen: We have studied the oxidation in water vapor of thick (0.5–2 μm) AlGaAs layers with aluminum contents ranging from 48% to 78% grown by molecular beam epitaxy. The oxidation rate was measured as a function of the aluminum content, the temperature and the flow rate of the N2 carrier gas supplying the water vapor. The oxide films proved to be mechanically very stable and the growth was found to be linear with time also in the case of an excessive supply of water vapor. The activation energies for the wet oxidation of AlGaAs were determined to increase from 1.1 to 1.8 eV for the Al contents decreasing from 78% to 48%. © 1995 American Institute of Physics.

Notizen: A simple Carnot-like irreversible power cycle is modeled with two isothermal and two adiabatic, irreversible processes. The generic source of internal irreversibility, deduced from the Clausius inequality, produces entropy at a rate proportional to the external heat conductance and the engine temperature ratio. This cycle is optimized for maximum power and maximum efficiency, and its performances compared to those of the endoreversible cycle, based on typical heat source and heat sink temperatures. Both cycles produce maximum power at the same engine temperature ratio, but the irreversible cycle prediction of maximum efficiency and heat conductance allocation between steam boiler and condenser, appear to be not only more realistic, but also more relevant to actual design considerations of power plants. © 1995 American Institute of Physics.

Notizen: Scanning optical microscopy with uncoated dielectric silica probe is used in the near field to investigate the propagation of optical modes along tapered integrated semiconductor optical amplifier devices and at larger working distances to study the electromagnetic intensity profile in the focal plane of various microlensed fibers. We show how this technique provides images of the mode structure of optoelectronic devices and profiled optical fibers with typical sizes in the range 2–10 μm, with an accuracy of 0.2 μm in beamwaist measurements. © 1995 American Institute of Physics.

Notizen: Ion-beam-induced epitaxial crystallization (IBIEC) of Fe-implanted Si(001) was studied by transmission electron microscopy and Rutherford backscattering spectrometry. For sufficiently high Fe doses, it was found that IBIEC at 320 °C results in sequential epitaxy of Fe silicide phases in Si, with a sequence of γ-FeSi2, α-FeSi2, and β-FeSi2 with increasing Fe concentration along the implantation profile. The critical concentrations for the γ-α and α-β phase transitions were determined as ≈11 and 21 at. % Fe, respectively. The observed sequential phase formation can be correlated to the degree of lattice mismatch with the Si matrix and the stoichiometry of the silicide phases. © 1995 American Institute of Physics.

Notizen: We investigate how the mobility and carrier density of AlGaAs/GaAs two-dimensional electron gas are influenced by the fabrication process with plasma-excited chemical vapor deposition (plasma-CVD) SiN film. These properties are greatly reduced by annealing with plasma-CVD SiN film as a cap but are restored by reannealing after removing the SiN film. We further use capacitance-voltage measurements to investigate the influence of this same process on a more simplified structure, Si-doped GaAs layer. Annealing with a plasma-CVD SiN film changes the defect density of Si-doped GaAs in two, temperature dependent ways: annealing below 380 °C reduces deposition damage, and annealing above 300 °C produces new defects, which might be caused by the film stress. These new defects can be reduced by reannealing after removing the SiN film. © 1995 American Institute of Physics.

Notizen: We have studied the soft laser sputtering of (100)GaAs with 337 nm photons, starting from the threshold for particle emission (a few tens of mJ/cm2) to some 300 mJ/cm2 fluences. Atoms and molecules sputtered from the irradiated surface are detected, their relative number measured, and their time of flight determined using laser resonant ionization mass spectrometry. The surface after laser irradiation is examined by scanning electron microscopy and electron microprobe analysis.One observes a significant preferential emission of arsenic in the form of As2. This leads to the formation of perturbed Ga-rich surface structure which appears even at low fluence and after a few tens of laser shots on the same spot. This initial transformation seems to determine the further evolution of the irradiated surface. First, Ga atoms aggregate to form Ga islands on the surface; after a sufficient number of shots, micrometric structures are produced which finally behave as pure Ga metal. This evolution of the surface state after multipulse irradiation appears practically the same for low and medium laser fluences, the only difference being in the number of shots required to obtain the same microscopic structure. The velocity distribution of Ga atoms and As2 molecules is well fitted by half-space Maxwellian distributions. The kinetic temperatures are in broad agreement with the results obtained from a model of laser heating of the surface. The gross features of the experimental results can be interpreted from the particular thermodynamics properties of GaAs which exhibits very large As2 pressure above the solid as soon as the temperature exceeds 950 K. After a few laser shots, corresponding to particle emission from defect sites, the thermodynamics of GaAs appears to govern the further evolution of the laser-sputtered surface. Two sputtering regimes are evidenced: In the low-fluence regime (from threshold to 90 mJ/cm2) sputtering appears to be dominated by surface defect emission, whereas for higher fluences emission is more characteristic of thermal process accompanied by preferential sputtering of arsenide. According to these experimental results, a simple analytical model was developed which relates the quantitative surface to the quantitative sputtered cloud compositions. © 1995 American Institute of Physics.

Notizen: Filling of the pore network of porous silicon layers with Ge and Si has been demonstrated using a chemical vapor deposition (CVD) technique. It is shown that at low growth rate the species are deposited throughout the whole layer which can be completely filled. At high growth rate, the deposition takes place mainly on the top surface leading to pore mouth bridging. Investigation of the experimental data through a model shows that pore filling is a powerful tool for the study of the mechanisms involved in CVD processes at low temperature. © 1995 American Institute of Physics.

Notizen: We have presented in this paper a semi-numerical simulation of dispersive transport under one-dimensional nonuniform electric field which could be used to study dispersive transport of carriers in the oxide. Our simulation is based on continuous time random walk principles (CTRW). Previous formulations using CTRW were derived under the assumption of uniform electric field. Comparison of our simulation results with the equivalent uniform field case shows that the treatment of nonuniform electric field is necessary to correctly predict events linked to dispersive transport, such as the growth of trapped oxide charge due to hole trapping or interface states due to H+ ions reaching oxide semiconductor interface of metal-oxide-semiconductor devices. © 1995 American Institute of Physics.