bluuebunny/crossref_metadata_2025 huggingface dataset:
Boltzmann analysis of electron swarm parameters in CF4 using independently assessed electron-collision cross sections
Authors: Bordage M.-C. , Ségur P. , Christophorou L. G. , Olthoff J. K.
Published: July 2002

Using independently assessed electron-collision cross sections, electron swarm parameters were calculated via the solution of the Boltzmann equation under the hydrodynamic regime. The cross sections used for the calculations were from a previously published assessment of electron–CF4-collision cross sections that was recently updated. All of the cross sections used are based on published measurements (except those for direct vibrational excitation), and were not modified during the calculations to improve agreement between the calculated swarm parameters and the experimental values. Agreement between calculated and measured values of the swarm parameters was good for the drift velocity in pure CF4 and in mixtures with argon, for the transverse diffusion coefficient in pure CF4, for the longitudinal diffusion coefficient in pure CF4 and in mixtures with argon, and for the electron attachment coefficient in pure CF4. Agreement is poor for the ionization coefficient in CF4 at most electric field-to-gas density ratios. The mostly reasonable agreement between the measured and calculated electron swarm parameters using the independently assessed cross sections validates the cross sections and the model. The use of independently assessed cross sections removes the potential arbitrariness and lack of uniqueness that often characterize cross section sets derived from Boltzmann analyses. Conjectures as to possible reasons for the lack of agreement between the calculated and measured values of the ionization coefficient are discussed.
Accurate determination of pulsed current waveform in plasma immersion ion implantation processes
Authors: Tian Xiubo , Tang Baoyin , Chu Paul K.
Published: July 2002

This article reports on the measurement of the ion current in plasma immersion ion implantation. Our simulation results indicate that the total current peaks at the end of rise time of the applied voltage. However, our experimental data acquired using a Rogowski coil and digital oscillator show the highest current at the beginning of the voltage pulse. The discrepancy can be explained by a displacement current attributable to the changing voltage, sheath capacitance, circuit loading effects, as well as secondary electron emission.
Analysis of breakdown in ferromagnetic tunnel junctions
Authors: Oepts W. , Verhagen H. J. , Coehoorn R. , de Jonge W. J. M.
Published: July 2002

Due to their very thin tunnel barrier layer, magnetic tunnel junctions show dielectric breakdown at voltages of the order of 1 V. At the moment of breakdown, a highly conductive short is formed in the barrier and is visible as a hot spot. The breakdown effect is investigated by means of voltage ramp experiments on a series of nominally identical Co/Al2O3/Co tunnel junctions. The results are described in terms of a voltage dependent breakdown probability, and are further analyzed within the framework of a general model for the breakdown probability in dielectric materials, within which it is assumed that at any time the breakdown probability is independent of the (possibly time-dependent) voltage that has been previously applied. The experimental data can be described by several specific forms of the voltage breakdown probability function. A comparison with the models commonly used for describing thin film SiO2 breakdown is given, as well as suggestions for future experiments.
Simulation of the dynamics of twisted nematic devices including flow
Authors: Kelly Jack , Jamal Syed , Cui Mingji
Published: July 2002

We have modeled the switching behavior of a twisted nematic cell using the one-dimensional Ericksen–Leslie equations of nematodynamics. We compare the modeling results with experimental data on transmission versus time. Excellent agreement between experiment and model is achieved at all voltages and viewing angles. To achieve this agreement, only two viscosity parameters are required; these are combinations of the Leslie viscosity coefficients, namely, α3−α2 (the rotational viscosity) and α4+α5. A fast and stable adaptive numerical algorithm, based on an effective viscosity parameter, is developed for solving the equations of motion. The viscosity parameters obtained from fitting the flow experiments are in good agreement with those obtained from dynamic light-scattering measurements.
Optical emission study of ablation plasma plume in the preparation of diamond-like carbon films by KrF excimer laser
Authors: Yamagata Y. , Sharma A. , Narayan J. , Mayo R. M. , Newman J. W. , Ebihara K.
Published: July 2002

Optical emission study of the laser ablation plasma plume during the preparation of diamond-like carbon (DLC) films using KrF excimer (248 nm) pulsed laser deposition (PLD) has been carried out by means of a monochromator equipped with an intensified optical multichannel analyzer. In high vacuum (1×10−7 Torr), the emission lines from carbon ions of C+, C2+, and C3+ are observed in addition to atomic carbon emission lines, while no emission from the diatomic carbon molecule (C2) is observed. With increasing background nitrogen pressure up to 500 mTorr, the emission intensities of the C2 Swan band and the carbon nitride (CN) violet band increase. The diamond-like character of deposited DLC film degrades with background nitrogen pressure. The vibrational temperature of C2 and CN molecules decreases with the increasing of nitrogen pressure. The CN vibrational temperature for the first 2 μs after the laser pulse is very high and in agreement with the kinetic energy of monatomic carbon ions. The C2 vibrational temperature is as low as 0.6 eV and is consistent with the electron temperature of about 0.8–3.0 eV. It is conjectured that CN molecules are formed directly in reactions involving energetic ionic monatomic carbon, and that the formation of excited C2 molecules is the result of molecular recombinations of C atoms and ions. From the emission intensity measurements and the estimation of the vibrational temperature, it is suggested that the C2 molecule in the ablated plasma plume is not important, but energetic species, such as C+, are very important for producing high quality DLC films using PLD.
Cavities in helium implanted and annealed silicon characterized by spectroscopic ellipsometry
Authors: Fukarek W. , Kaschny J. R.
Published: July 2002

The formation of helium induced cavities in silicon during short-time annealing is analyzed by spectroscopic ellipsometry. Specimens implanted with 40 keV He+ ions to a dose of 5×1016 cm−2 are heat treated at 800 °C for times of 1–1200 s by rapid thermal annealing. Spectroscopic ellipsometry is employed to obtain quantitative information on the cavity volume depth profiles. A newly developed formula is used to model the optical multilayer depth profiles. The cavity volume is found to increase during annealing for about 300 s and to decrease for longer annealing times. Over this characteristic time a marked change in the He loss occurs, which has been reported only recently. Swelling of the helium implanted and annealed silicon is analyzed using an atomic force microscope. Step heights are consistent with the cavity volume per unit area obtained from spectroscopic ellipsometry data analysis. The number density of cavities after annealing for 600 s is calculated to be ≈1.16±0.27×1017 cm−3 and is found to be largely independent of depth in the central part of the cavity layer.
Nondestructive evaluation of channel carrier concentration in modulation-doped InAlAs/InGaAs field-effect transistor structures by room-temperature photoluminescence
Authors: Watanabe Kazuo , Yokoyama Haruki
Published: July 2002

The sheet carrier concentration (Ns) of channel layers in modulation-doped InAlAs/InGaAs heterostructure field-effect transistor (HFET) structures with n+InGaAs contact layers has been successfully and nondestructively determined using the room-temperature photoluminescence (PL) method. It is found that the spectral energy width between the maximum position of the main PL peak around 0.8 eV and the half-maximum position on the higher energy side has a good positive linear correlation with the Ns of the channel measured by the van der Pauw method. The scattering of the data is less than ±3×1011 cm−2. The determination of Ns is effective even if the HFET structures have not only n+InGaAs contact layers but also layers for InAlAs Schottky level-shift diodes. From a comparison with low-temperature PL spectra, the main PL peak is attributed to the e2h transition in the quantum well of the channel. It is considered that the slope of the peak stretches further to the high energy as the Fermi energy in the channel become higher, i.e., as the Ns becomes larger.
Synchrotron characterization of deep depletion epitaxial GaAs detectors
Authors: Owens Alan , Bavdaz M. , Kraft S. , Peacock A. , Strade R. , Nenonen S. , Andersson H. , Gagliardi M. A. , Gagliardi T. , Graafsma H.
Published: July 2002

We report the results of a series of synchrotron characterizations of two epitaxial GaAs detectors of active areas 2.22 mm2 and thicknesses 40 and 400 μm. In spite of an order of magnitude difference in depletion depths, the detectors were found to have comparable performances at ∼−40 °C, with energy resolutions of ∼1 keV full width at half maximum (FWHM) at 7 keV rising to ∼2 keV FWHM at 200 keV and noise floors in the range of 1–1.5 keV. At the lower energies, the energy resolution was dominated by leakage current and electromagnetic pickup. At the highest energies, however, the measured resolutions appear to approach the expected Fano limit; e.g., ∼950 eV at 200 keV. Both detectors were remarkably linear, with average rms nonlinearities of 0.2% over the energy range of 10–60 keV. By raster scanning the active areas with 20×20 μm2 monoenergetic photon beams, it was found that the nonuniformity in the spatial response of both detectors was less than 1% and independent of energy. The material used to fabricate the detector is extremely pure. For example, low temperature photoluminescence measurements indicate that the density of the As antisite defect (EL2) is of the order of 1012 cm−3, which is ∼2–3 orders of magnitude lower than that generally reported. This indirect measurement of material purity is confirmed by Monte Carlo simulations of the detector x-ray response, which show that in order to reproduce the observed energy-loss spectra, electron and hole trapping cross-section/density products must be ≪1 cm−1.
Optical characterizations of CuInSe2 epitaxial layers grown by molecular beam epitaxy
Authors: Yoshino Kenji , Yokoyama Hirosumi , Maeda Kouji , Ikari Tetsuo , Fukuyama Atsuhiko , Fons Paul. J. , Yamada Akimasa , Niki Shigeru
Published: July 2002

CuInSe 2 (CIS) films with Cu/In ratios of γ=0.82–1.79 have been grown on a GaAs (001) substrate by molecular beam epitaxy. Piezoelectric photoacoustic (PPA) measurements were carried out from liquid helium to room temperature to investigate nonradiative carrier recombination processes in comparison with photoluminescence (PL) measurements which directly detected radiative carrier recombination processes. Three PPA signal peaks which corresponded to band gap energies of the CIS (AB and C bands) and the GaAs substrate, were clearly obtained between liquid helium and room temperatures. A free-exciton emission line was observed up to 200 K in the PL spectra. Two additional peaks on intrinsic defects which are Cu vacancy (VCu) and interstitial In (Ini) were observed in the In-rich CIS samples. The PPA measurements were useful in investigating the defect levels and the band gap energy in the CIS/GaAs thin films.
Lithographically patterned wires of the charge-density-wave conductor Rb0.30MoO3
Authors: Mantel O. C. , Bal C. A. W. , Langezaal C. , Dekker C. , van der Zant H. S. J.
Published: July 2002

We have developed a technology for the patterning of thin films of the charge-density-wave (CDW) conductor Rb0.30MoO3. By means of photolithography and Ar-ion milling, wire structures are defined in the films. The Rb0.30MoO3 wires are contacted by (sub)micron Au contacts, which are fabricated by optical and electron-beam lithographic techniques. Electrical transport measurements clearly demonstrate the CDW state in our patterned structures, including CDW sliding. Patterned wires enable the study of CDWs on mesoscopic length scales.
Electronic transport and 1/f noise studies in 250 MeV 107Ag ion irradiated La0.75Ca0.25MnO3 thin films
Authors: Arora S. K. , Kumar Ravi , Singh Rajendra , Kanjilal D. , Mehta G. K. , Bathe Ravi , Patil S. I. , Ogale S. B.
Published: July 2002

The influence of 250 MeV 107Ag ion irradiation on electrical transport and low frequency conduction noise properties of La0.75Ca0.25MnO3 (LCMO) thin films have been investigated. A systematic variation of metal–insulator transition temperature Tp and resistivity as a function of ion fluence has been observed. It has been noted that for the sample irradiated with 1011 ions/cm2, Tp increased by 10 K and the resistivity is decreased in the ferromagnetic metallic state from that of an unirradiated sample, whereas our noise results show a monotonic increase in the normalized noise Sv/V2, as a function of ion fluence. The observed modifications in the noise properties of the irradiated LCMO thin films are consistent with the general expectation concerning the effect of swift heavy ions irradiation induced lattice defects.
Effect of an electric field on the exciton linewidth due to scattering of excitons by ionized impurities in semiconducting quantum well structures
Authors: Feng Yuan-ping , Spector Harold N.
Published: July 2002

We have calculated the effect of an electric field applied along the direction of carrier confinement on the exciton linewidth due to the scattering of excitons by ionized impurities in semiconducting quantum well structures. The contributions to the linewidth due to the elastic and inelastic scattering of the excitons by the ionized impurities have both been taken into account. Both these contributions to the linewidth are found to increase with the applied electric field except for the narrowest wells. However, the linewidth is found to initially decrease with increasing well width for all electric fields and then increase again with increasing well widths for finite electric field for the case where the exciton is scattered elastically. The minimum in the linewidth as a function of well width shifts to smaller well widths with increasing electric field. In the absence of an electric field, the linewidth continues to decrease with increasing well width. When the exciton is ionized as a result of the scattering, the linewidth increases with well width for all nonzero electric fields. However, in the absence of the electric field, the linewidth decreases with increasing well width. This reflects the effect of decreasing confinement on the scattering by ionized impurities. In the case of elastic scattering of the exciton by the ionized impurities, the linewidth has a maximum as a function of the exciton energy with the maximum shifting to lower energies as the well width decreases. For scattering which results in the ionization of the exciton, there is a threshold in the linewidth as a function of the exciton’s center of mass energy with the threshold being larger for smaller well widths. The contribution to the linewidth due to elastic scattering of the exciton is many orders of magnitude larger than that due to scattering in which the exciton is ionized.
Exciton binding energy in spherical quantum dots in a magnetic field
Authors: Xiao Zhigang
Published: July 2002

The binding energy of an exciton in spherical GaAs quantum dots in a magnetic field is calculated by a variational approach within the effective-mass approximation. The quantum dot is modeled by an isotropic parabolic potential. The magnetic effects on exciton binding energies and exciton energy levels in spherical GaAs quantum dots with different parabolic potentials are studied. The calculated results indicate that the interplay of the spatial confinement and the magnetic confinement on electrons and holes in the quantum dots leads to complicated exciton binding energies, and high magnetic fields obviously enhance exciton binding energies in the case of the weak spatial confinement.
The effect of boron on the uniaxial magnetic anisotropy energy and the polar Kerr rotation in amorphous NdFe films
Authors: Kim Jai-Young , Hoffmann Horst , Schmidt Thomas , Hampel Franz
Published: July 2002

Amorphous light rare earth–transition metal alloys are candidates for magneto-optical recording materials in the wavelength region of a blue laser beam (λ≈400 nm) due to the large Kerr rotation angle θK. However, the large demagnetizing energy (shape anisotropy) prevents the films from developing an effective perpendicular anisotropy. The addition of boron to the amorphous Nd Fe matrix reduces the magnetization, enhances the effective perpendicular anisotropy, and enhances the Kerr rotation angle θK.
Growth, microstructure and electronic properties of amorphous carbon nitride films investigated by plasma diagnostics
Authors: Bhattacharyya Somnath , Granier A. , Turban G.
Published: July 2002

Amorphous nitrogenated carbon films are prepared in a dual electron cyclotron resonance-radio frequency plasma from a mixture of methane and nitrogen gas. A marked variation of electronic properties and microstructure of the films as a function of nitrogen concentration is observed from Fourier transform infrared (FTIR) spectra, electron energy loss spectra, optical absorption spectra, x-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy and electrical conductivity. From the variation of intensity of different positive ions and neutral radicals, using quadrupole mass spectroscopy and optical emission spectroscopy, the growth rate, structure and properties of the films are investigated. The density of methyl (CH3) radicals and the film growth rate are found to decrease with the increase of nitrogen concentration. A correlation between the C/N atomic ratio in the films and CH/CN and also CH/N ratio in the plasma is noticed. Also, the CH radical intensity in the plasma and the amount of CH bonds in the films, observed from FTIR spectra, vary in a similar fashion as a function of nitrogen concentration. A model describing film growth and nitrogen incorporation in the films is proposed.
Growth and photoluminescence study of ZnTe quantum dots
Authors: Liao M. C. Harris , Change Y. H. , Tsai C. C. , Chieng M. H. , Chen Y. F.
Published: July 2002

ZnTe quantum dots embedded in ZnS were grown successfully by controlling the flow duration in a metalorganic chemical vapor deposition system. Blueshift as large as 250 meV was observed in photoluminescence measurement, and the emission persists up to room temperature. The amount of blueshift decreases with increasing quantum dot size and for large quantum dots, no photoluminescence could be detected. From studying the temperature-dependent integrated intensity of the emission spectra, it is found that the activation energy for the quenching of photoluminescence increases with decreasing quantum dot size, and is identified as the binding energy of exciton in ZnTe quantum dot.
Influence of neodymium concentration on the cw laser properties of Nd doped Ca3Ga2Ge3O12 laser garnet crystal
Authors: Jaque D. , Caldiño U. , Romero J. J. , Garcı́a Solé J.
Published: July 2002

Spectral and laser properties of Nd3+ doped Ca3Ga2Ge3O12 have been investigated for different Nd concentrations ranging from 0.1 to 16 at %. The F3/24→4I11/2 emission spectrum is dominated by two main peaks at 1060 and 1065 nm whose stimulated emission cross sections change with Nd concentration. For concentrations above 0.5% luminescence quenching begins. Then, continuous wave laser gain experiments have been used to investigate the influence of these effects on the laser properties. The different factors (crystal losses and saturation intensity factor) contributing to the increase observed in the laser pump threshold with Nd concentration are discussed. Analysis of the laser spectra shows a double laser oscillation around 1060 and 1065 nm, which tends to disappear for high Nd concentrations.
Experimental investigation of structures of interior interfaces in GaAs
Authors: Bernatz Georg , Nau Siegfried , Rettig Rasmus , Jänsch Heinz , Stolz Wolfgang
Published: July 2002

A method for the structural investigation of interior inverted GaAs/AlAs interfaces is presented which combines highly selective etching and subsequent atomic force microscopy. It provides three-dimensional mappings of interior GaAs interfaces on a lateral scale on the order of micrometers with angstrom z resolution. The perfection of this method is demonstrated, which allows the observation of the real interface monolayer island and terrace structure. Potential aluminum residues on the uncovered interfaces are below the detection limit of Auger electron spectroscopy, which is estimated to 16% of aluminum in a single monolayer on a GaAs crystal. The structure of an interior interface can differ significantly from that of a corresponding surface layer after cooling down from the growth temperature. The substantial restructuring of the interface morphology caused by growth interruptions is investigated in detail for metalorganic vapor phase epitaxy.
The electro-optical behavior of SrS:Ce electroluminescent devices under photonic excitation
Authors: Benoit J. , Barthou C. , Benalloul P. , Polamo K.
Published: July 2002

The electro-optical behavior of the SrS:Ce electroluminescent devices under pulsed photonic excitation in the lower energy absorption band of Ce3+ was analyzed below the electroluminescence threshold voltage for a rectangular electric pulse. The photoluminescence quenching due to the ionization of the Ce3+ ions under the electrical field increases with the applied voltage (40% at the threshold). Delocalization of involved electrons is responsible for emissions at the trailing edge of the electric pulse and for emissions during the following pulse. These emissions do not restore the level of the photoluminescence without applied voltage. These different emissions allow detailed study of energy trap levels for each insulator/SrS interface. An interpretation of the photoluminescence quenching is proposed
Field-emission properties of diamond grains grown on textured Fe/Si substrates
Authors: Hirakuri K. K. , Kurata T. , Mutsukura N. , Friedbacher G. , Ohuchi M.
Published: July 2002

Diamond grains were produced by the hot-filament chemical-vapor deposition technique using a mixture of CH4/H2 as the synthesis gas. The Si substrates were textured (random pyramids) by chemical etching before growth of diamond grains, because emission of electrons is facilitated at the top of the pyramids. In order to increase the diamond nucleation density, an iron thin film (Fe) was deposited on the textured Si substrate by a radio-frequency plasma sputtering technique. Moreover, partial iron thin films (20% surface coverage) were fabricated for application as field emitter displays using a metal mask. The relationship between the nucleation density and the thickness of the iron films has been investigated by scanning electron microscopy (SEM). The samples were placed in a high-vacuum chamber with a pumping system and the emission current was measured as a function of the applied voltage. The voltage–current (V–I) characteristics were estimated for the field emitter. In order to etch nondiamond components, these samples were etched by O2 and CF4 plasmas. SEM observation revealed that diamond growth is concentrated at the textured Fe/Si area. The nucleation density was 107 cm−2 on the textured Fe/Si area. On the plain textured Si substrate diamond growth could not be observed. Numberless tips were observed on the Fe/Si area after etching. The V–I characteristics revealed a current 10−8 Å at an applied voltage of 1.8 kV. The V–I characteristics could be enhanced by using the textured Fe/Si substrate.
A Monte-Carlo model of xenon resonance radiation transport in a plasma display panel cell: Transition from optically thick to thin regimes
Authors: van der Straaten Trudy , Kushner Mark J.
Published: July 2002

Plasma display panels (PDPs) are a promising technology for large-area flat panel color displays. The operation of PDPs relies on UV radiation from the Xe(5p56s→5s25p6) and Xe(5p56s′→5s25p6) resonance transitions, and the radiative relaxation of Xe2*, to excite phosphors, thereby producing visible radiation. The particular combination of device dimensions, gas composition, and operating conditions typically used in PDP cells is such that the resonance UV radiation is in a quasi-optically thick regime. In this article, results from a radiation transport model which accounts for the frequency redistribution resulting from multiple resonance absorption and re-emission is discussed. The model employs Monte-Carlo photon transport and frequency redistribution algorithms to treat the nonlocal nature of resonance radiation transport. Using results from this model, the transition from optically thin to optically thick transport is discussed for conditions typically used in conventional PDPs.
Effects of donor concentration on transient enhanced diffusion of boron in silicon
Authors: Solmi S. , Bersani M.
Published: July 2002

The effects of substrate donor concentration on transient enhanced diffusion (TED) of boron implanted into silicon wafers predoped with arsenic or phosphorus have been investigated by using secondary ion mass spectroscopy and theoretical simulations. Boron ions have been implanted at low energy (10–20 keV) and successively annealed in the temperature range 700–1000 °C. No detectable boron TED (TED<10×) has been observed in samples uniformly predoped with P at the concentration of 1.2×1020 cm−3. The B TED is strongly reduced even in the samples predoped with As. However, for the highest As concentration (1.3×1020 cm−3) an exponential high diffusivity tail has been detected in the B profiles at all the examined annealing temperatures. This anomalous effect has been attributed to the interstitials produced by an As clustering reaction occurring during the low temperature annealing. The main effects responsible for the vanishing of the B TED, i.e., ionization of the interstitials (Fermi level effect), competitive behavior of P in the interaction with the interstitials, and donor-acceptor pairing have been examined and discussed.
Strain and shape analysis of multilayer surface gratings by coplanar and by grazing-incidence x-ray diffraction
Authors: Baumbach T. , Lübbert D. , Gailhanou M.
Published: July 2002

The surface shape and the spatial distribution of strain in GaInAs/InP multilayer gratings is experimentally determined by combining high-resolution x-ray diffraction and grazing-incidence diffraction. Dramatic deformations of the diffraction patterns in the measured reciprocal space maps of such gratings indicate a strongly nonuniform character of lattice distortions in the layers, caused by elastic strain relaxation. By coupling x-ray diffraction theory and elasticity theory within one single evaluation formalism, an experimental strain and shape analysis could be performed which was not yet available in this form by other methods. The different components of the strain tensor are determined by recording diffraction patterns around different reciprocal lattice points. We study how the strain relaxation of the multilayer grating evolves towards the free surface, and how the local strain distribution is modulated due to the compositional profile of the layered setup. Furthermore, the article describes in detail the effects of the grating shape, the multilayer morphology, and the lattice strain field on the diffraction patterns of multilayered gratings.
Interdiffusion mechanisms in CdTe/CdMgZnTe:N modulation-doped heterostructures
Authors: Arnoult A. , Cibert J. , Tatarenko S. , Wasiela A.
Published: July 2002

p-type modulation doping of CdTe–CdMgZnTe heterostructures, using nitrogen as a dopant, induces a strong interdiffusion of the quantum wells. Photoluminescence, secondary ion mass spectrometry, and x-ray diffraction measurements give a coherent description of the interdiffusion process: the destruction of the quantum wells occurs essentially by exchange of Cd and Mg atoms across the interface, while the Zn atoms stay in their lattice sites. We show that the presence of nitrogen at the interface, which enhances the interdiffusion, is related to the diffusion of nitrogen and not to segregation. Additional mechanisms are observed at higher doping.
Magnetic trapping of neutral particles: Classical and quantum-mechanical study of a Ioffe–Pritchard type trap
Authors: Gov S. , Shtrikman S. , Thomas H.
Published: July 2002

Recently, we developed a method for calculating the lifetime of a particle inside a magnetic trap with respect to spin flips, as a first step in our efforts to understand the quantum mechanics of magnetic traps. The one-dimensional toy model that was used in this study was physically unrealistic because the magnetic field was not curl free. Here, we study, both classically and quantum mechanically, the problem of a neutral particle with spin S, mass m, and magnetic moment μ, moving in three dimensions in an inhomogeneous magnetic field corresponding to traps of the Ioffe–Pritchard “clover-leaf” and “baseball” type. Defining by ωp, ωz, and ωr the precessional, the axial, and the lateral vibrational frequencies, respectively, of the particle in the adiabatic potential Veff =μ|B|, we find classically the region in the (ωr/ωp)−(ωz/ωp) plane where the particle is trapped. Quantum mechanically, we study the problem of a spin-one particle in the same field. Treating ωr/ωp and ωz/ωp as small parameters for the perturbation from the adiabatic Hamiltonian, we derive a closed-form expression for the transition rate 1/Tesc of the particle from its trapped ground state. In the extreme cases, the expression for 1/Tesc reduces to 4πωr exp(−2ωp/ωr) for ωp≫ωr≫ωz, to 82πωpωi exp(−2ωp/ωi) for ωp≫ωr=ωz≡ωi and to π/2 ωr(ωz/ωp)3/2exp(−2ωp/ωz) for ωp≫ωz≫ωr.
Ultraviolet light assisted oxidation for magnetic tunnel junctions
Authors: Rottländer P. , Kohlstedt H. , Grünberg P. , Girgis E.
Published: July 2002

Recently, ferromagnetic tunnel junctions attracted much interest because they exhibit large magnetoresistance effects up to ΔR/R=40% at room temperature. In this paper, an alternative approach will be presented, i.e., exposing the aluminum film to ultraviolet light during oxidation. The underlying mechanism can be explained in the framework of the Cabrera–Mott theory of oxidation. With UV assisted oxidation, magnetoresistance ratios of up to 22.5% at room temperature have been obtained in Ni80Fe20–Al(ox)–Co junctions. The resistivities were in the 1 kΩ μm2 range for junctions of 25–400 μm2. Barrier heights obtained by fitting to different theories will be discussed.
Perpendicular magnetic recording thin film media using Co/Pd superlattice on ultrathin indium–tin–oxide seed layers
Authors: Peng W. , Victora R. H. , Judy J. H. , Gao K. , Sivertsen J. M.
Published: July 2002

Co/Pd superlattices (13 bilayers) have been fabricated on ultrathin (2 nm) indium–tin–oxide (ITO) seed layers for use as perpendicular magnetic recording media. The ITO seed layers have significantly increased the coercivity from 1 to 5 kOe as well as the crystalline orientation texture. It was also found that excellent magnetic properties could be obtained by using Ar instead of Kr as sputtering gas. The sheared hysteresis loop and high squareness suggest weak intergranular coupling and adequate perpendicular magnetic anisotropy which make Co/Pd supelattice a promising candidate for ultrahigh recording densities with thermal stability.
PtMn spin valve with synthetic ferrimagnet free and pinned layers
Authors: Saito Masamichi , Hasegawa Naoya , Tanaka Kenichi , Ide Yosuke , Koike Fumihito , Kuriyama Toshihiro
Published: July 2002

Spin valve films with a laminated antiparallel Co/NiFe/Ru/NiFe synthetic ferrimagnet (SF) free layer combined with PtMn/Co/Ru/Co SF pinned layers were demonstrated to maintain large magnetoresistance ratio of ∼8.0% even below ∼10 Å of effective thickness of the Co/NiFe/Ru/NiFe SF free layers, defined as the thickness difference of the two ferromagnetic layers antiferromagnetically coupled through Ru. Spin flopping field Hsf and saturation field Hs of the Co/NiFe/Ru (8 Å)/NiFe SF free layers slightly increased after annealing at 250 °C for 4 h, and then the Hsf of the Co (3 Å)/NiFe(30 Å)/Ru(8 Å)/NiFe(20 Å) SF free layer after annealing remained 600 Oe, even at 200 °C. Further, antiferromagnetic coupling interlayer materials and their thickness dependence of Hsf and Hs were investigated in the Co(3 Å)/NiFe(40 Å)/X/NiFe(25 Å), X=Ru, Cr, Ir, and Rh, sandwich stacks. The oscillatory antiferromagnetic exchange coupling was clearly observed, and the large Hsf and Hs exceeding 2000 Oe and 7 kOe, respectively, were obtained both at Ir 3.8 Å and Rh 6.1 Å, which corresponded to the oscillatory first peak in the Co/NiFe/Ir/NiFe and Co/NiFe/Rh/NiFe sandwiches.
Magnetic anisotropy and rotational hysteresis loss in exchange coupled Ni–Fe/Mn–Ir films
Authors: Tsunoda Masakiyo , Tsuchiya Yoshihiro , Hashimoto Takahiko , Takahashi Migaku
Published: July 2002

The magnetic anisotropy and rotational hysteresis loss in Ni–Fe/Mn–Ir bilayers were investigated for films prepared by an ultraclean sputtering deposition process. An in-plane field of 30 Oe during deposition served to define the antiferromagnetic (AF) alignment axis for the Mn–Ir layer. The Ni–Fe layer thickness was maintained at 50 Å and the Mn–Ir layer thickness ranged from 20 to 200 Å. Room temperature magnetization and torque measurements were made as a function of the Mn–Ir layer thickness dAF and the applied field. The magnetization data were obtained for fields applied in the same direction as during deposition. The magnetization data indicate a critical dAF value of 37 Å, taken as dAFcr. For dAF>dAFcr, the data show hysteresis loops which are displaced along the field axis. The torque response and rotational hysteresis characteristics are sensitive to both dAF and the measuring field. (1) When dAF is much less than dAFcr, the torque curves have a sin θ characteristic at fields below 30–40 Oe or so which suddenly changes to a sin 2θ characteristic at higher fields. With the onset of the sin 2θ torque response, rotational hysteresis loss also appears but then vanishes for fields above 100 Oe or so. (2) As dAF approaches dAFcr from below, the torque response is the same as above. Here, however, the rotational hysteresis appears for fields well below the field at which the torque response assumes a sin 2θ character and persists to the maximum available measuring field of 15 kOe or so. (3) When dAF exceeds dAFcr, the torque has a predominant sin θ character at all fields and a small sin 2θ component and rotational hysteresis which only around a field of 400 Oe or so. These results, while somewhat complicated, are in accord with responses evaluated from the simple exchange anisotropy model of W. H. Meiklejohn and C. P. Bean [Phys. Rev. 102, 1413 (1956); 105, 904 (1957)]. Among other things, one may conclude that a rotational hysteresis which persists to high field is not intrinsic to exchange anisotropy.
Magnetic properties of self-assembled Co nanowires of varying length and diameter
Authors: Zeng H. , Zheng M. , Skomski R. , Sellmyer D. J. , Liu Y. , Menon L. , Bandyopadhyay S.
Published: July 2002

Ferromagnetic Co nanowires have been electrodeposited into self-assembled porous anodic alumina arrays. Due to their cylindrical shape, the nanowires exhibit perpendicular anisotropy. The coercivity, remanence ratio, and activation volumes of Co nanowires depend strongly on the length, diameter, and spacing of the nanowires. Both coercivity and thermal activation volume increase with increasing wire length, while for constant center-to-center spacing, coercivity decreases and thermal activation volume increases with increasing wire diameter. The behavior of the nanowires is explained qualitatively in terms of localized magnetization reversal.
Interactions and switching field distributions of nanoscale magnetic elements
Authors: Kirk K. J. , Chapman J. N. , McVitie S. , Aitchison P. R. , Wilkinson C. D. W.
Published: July 2002

Magnetic nano-elements made from NiFe and Co have been investigated using magnetic imaging in the transmission electron microscope. Nano-elements like these have possible uses for in-plane patterned media or solid state memory. In both cases the elements will need to be patterned into closely spaced arrays and magnetostatic interactions between the elements will begin to become significant. Arrays must therefore be designed so that an element’s interactions with its neighbors will be small compared to its coercivity. Arrays of NiFe elements 300 nm long, 50–100 nm wide, and 26 nm thick, were fabricated by electron beam lithography and lift-off patterning. Their switching behavior and the interactions between them were studied in detail. Magnetization sequences were recorded and hysteresis loops constructed. For rows of NiFe elements with the gap between elements the same as the element width or larger, the interactions turn out to be small, suggesting that denser arrays would be possible.
Slow magnetization dynamics of small permalloy islands
Authors: Koo H. , Luu T. V. , Gomez R. D. , Metlushko V. V.
Published: July 2002

The conditions that lead to specific domain configurations and the associated switching characteristics of small permalloy islands were studied by using magnetic force microscopy. By measuring a large number of particles, it was established that islands that have nonzero remanent moments (nonsolenoidal) exist in one of three distinct configurations, namely: (a) true single domain, (b) quasisingle domain with edge closure patterns, and (c) multidomain with nonuniform internal magnetization. The configuration depended upon the island width as well as the aspect ratio. Islands that are 310 nm wide or less are true single domain particles at low aspect ratios (∼1.87) and higher, while islands wider than 500 nm always exhibited edge closure domains even for very large aspect ratios. In the range between 310 and 500 nm, the onset of single domain behavior was a function of the aspect ratio and thickness. Our studies involving in situ applied field similarly revealed the mechanisms of the reversal processes for each of the configurations, which correlated quite well with the values of the switching fields.
Coercivity in mechanically milled Pr–Co–Zr powders with TbCu7 structure
Authors: Chen Zhongmin , Meng-Burany X. , Hadjipanayis G. C.
Published: July 2002

The structural, microstructural, and magnetic properties of the PrCo7−xZrx (x=0, 0.2, 0.3, and 0.4) powders produced by mechanical milling have been studied with the aim of developing fine microstructure and high coercivity in these powders. X-ray diffraction, transmission electron microscope, and magnetic measurements show that a single phase with TbCu7-type structure and with high anisotropy of up to 100 kOe is obtained in as-cast PrCo6.7Zr0.3 alloy. Magnetic hardening is developed in the mechanically milled powders. An optimum coercivity of 5.3 kOe has been obtained in PrCo6.7Zr0.3 powders milled for 2 h and annealed at 800 °C for 1 min, which shows a uniform microstructure of the 1:7 phase with an average grain size of about 10–15 nm. The observed magnetic hardening is believed to arise from the high anisotropy field of the Pr(Co, Zr)7 phase and the uniform nanoscale microstructure developed by mechanical milling and subsequent annealing.
Determination of the phase distribution in nitrogen-implanted iron from conversion electron Mössbauer spectroscopy data
Authors: Jagielski J. , Kopcewicz M.
Published: July 2002

A new deconvolution procedure of conversion electron Mössbauer spectroscopy (CEMS) data based on the computer simulations of nitride phase transformations is described. The method uses the depth distribution of impurity atoms extracted from the Monte Carlo simulations and the phase contributions to the CEMS spectra to calculate the phase distribution profiles. The procedure has been applied to nitrogen implanted iron and provided a significantly extended amount of information on the phase transformations in the FeN system available from the analysis of the Mössbauer data.
Photoluminescence spectra of Zn1−xCdxAl2S4 (0.0⩽x⩽0.2;0.8⩽x⩽1.0) single crystals
Authors: Hyun Seung-Cheol , Kim Chang-Dae , Park Tae-Young , Kim Hyung-Gon , Jin Moon-Seog , Lee Choong-Il , Goh Jae-Mo , Kim Wha-Tek
Published: July 2002

The photoluminescence spectra as well as the lattice constants and band gaps for the mixed single crystals Zn1−xCdxAl2S4 with 0.0⩽x⩽0.2 and 0.8⩽x⩽1.0 grown by the chemical transport reaction method were investigated. The Zn1−xCdxAl2S4 crystals were a cubic spinel phase α in the range of 0.0⩽x⩽0.2 and a defect chalcopyrite in the range of 0.8⩽x⩽1.0, and showed a miscibility range from x=0.2 to x=0.8 in the composition dependence of the lattice constants and band gaps. We observed two emission bands consisting of a strong blue emission band and a weak broad emission band due to donor-acceptor pair recombinations in the crystals with a cubic spinel and a defect chalcopyrite structure. These emission bands showed a different behavior in their temperature and composition dependence. An energy band scheme for the radiative mechanism of the Zn1−xCdxAl2S4 was proposed on the basis of our experimental results along with the measurements of photoinduced current transient spectroscopy and thermoluminescence.
Effects of quantum mechanical coupling on the optical properties of vertically stacked V-groove quantum wires
Authors: De Giorgi M. , Rinaldi R. , Passaseo A. , Lomascolo M. , Cingolani R. , Ferreira R. , Bastard G. , Taurino A. , Catalano M.
Published: July 2002

The effects of the quantum mechanical coupling on the optical properties of vertically stacked InGaAs/GaAs V-shaped quantum wires have been studied by means of photoluminescence and photoluminescence-excitation spectroscopy. The experimental results have been analyzed by a simple theoretical model based on an analytical procedure. We found that by decreasing the barrier thickness (Lb) between the wires, the vertical coupling induces a splitting of the single wire levels into symmetric and antisymmetric states characterized by a polarization anisotropy. Furthermore, a clear shift of the coupled levels and a narrowing of the spectral linewidth are observed with a decrease in Lb. These findings are consistent with the theoretical predictions.
Imaging the variation in band bending across a silicon pn junction surface using spectromicroscopy
Authors: Phaneuf R. J. , Kan H.-C. , Marsi M. , Gregoratti L. , Günther S. , Kiskinova M.
Published: July 2002

We present a characterization of lateral silicon pn junction arrays fabricated on a Si(001) surface using a synchrotron-based scanning photoelectron microscope (SPEM). The Si 2p images show energy dependent contrast which varies continuously across the space charge region between regions of different doping. Combined with measurements of the changes in the Si 2p spectra across the pn junction, we demonstrate the capacity of SPEM in imaging variations in dopant concentration, the width of the charge depletion zone, and variations in band bending with oxide preparation.
Deconvolution of time of flight photocurrent transients and electronic response time in columnar liquid crystals
Authors: Donovan K. J. , Kreouzis T.
Published: July 2002

The attempts to create thinner samples of columnar discotic liquid crystals in order to avoid stack defects and measure charge carrier motion on low defect density stacks by the time of flight technique, inevitably leads to problems. The time of flight becomes faster while the electronic response time becomes slower as a result of the increasing sample capacitance. Eventually the time of flight transit time becomes indiscernible due to the system response distortion. This article demonstrates the recovery of the time of flight signal from such distorted data by analyzing the nature of the signal and using this analysis to deconvolve a digitally acquired signal.
Modeling of laser-induced breakdown in dielectrics with subpicosecond pulses
Authors: Apostolova T. , Hahn Y.
Published: July 2002

Theoretical study of ultrafast laser induced damage by short pulses (τ<1 ps) is carried out on large-band-gap dielectric in an effort to understand the complex physical processes involved. The numerical method of solving a general time-dependent Fokker–Planck type equation for free electron production is discussed in detail. The calculation shows that the collisional avalanche ionization competes with the multiphoton ionization even for pulse length shorter than 25 fs. Sensitivity tests of all the rates in the equation are performed and the most critical ones are identified. From these tests we obtain valuable information in developing new materials that have the desired damage fluence for specific applications. To describe the relaxation of electron plasma, a three body recombination rate is included. Thus, the temporal behavior of the electron density due to a single pulse is treated, as well as the case of exposure to two laser pulses with a time delay between them. The model is only partially successful in reproducing the recent experimental data. Effect of the presence of a linear decay term and optical defects on the damage threshold is considered in the context of the rate equation input.
Decrease of channel conductivity with increasing sheet electron concentration in modulation-doped heterostructures
Authors: Požela J. , Požela K. , Namajūnas A. , Jucienė V. , Mokerov V. , Fedorov Yu. , Hook A.
Published: July 2002

The great decrease of electron mobility with increasing sheet electron concentration nS>5×1015 m−2 in modulation-doped Al0.25Ga0.75As/GaAs/Al0.25Ga0.75As and Al0.25Ga0.75As/In0.19Ga0.81As/GaAs quantum wells is observed experimentally. At nS>1016 m−2 a conductivity decreases with increasing the sheet electron concentration. The calculations of electron mobility limited by polar optical (PO) phonon scattering show that the great increase of electron intrasubband scattering by emission of PO phonons when nS exceeds 1015 m−2 is responsible for the mobility and conductivity decrease. When nS changes in the range of 1015−1017 m−2, the alternate increase and decrease of channel conductivity is observed.
AlGaN/GaN heterojunction bipolar transistor structures-design considerations
Authors: Zhang Yumin , Cai Cheng , Ruden P. Paul
Published: July 2002

The potential of III-nitride materials for the fabrication of bipolar transistors is investigated theoretically. Several different AlGaN/GaN n–p–n heterojunction bipolar transistor structures are examined through calculations of their band profiles and majority carrier distributions in equilibrium and in forward active mode. Spontaneous and piezoelectric polarization charges are utilized to create large hole sheet carrier densities in the base layer, thus minimizing the base spreading resistance. At the same time, a large accelerating field in the base can help reduce the base transit time of the electrons and, hence, increase the current gains of these devices. The temperature dependence of the hole concentration in the base is also investigated.