bluuebunny/crossref_metadata_2025 huggingface dataset:
Magnetic and magneto-optical properties of Mn5(Ge1−xMx)3 alloys with M=Sn, Pb
Authors: Zhang Y. , Runge A. P. , Shan Z. S. , Sellmyer D. J.
Published: July 2002

We report on the magnetic and magneto-optical properties of the intermetallic alloys Mn5(Ge1−xMx)3, where M=Sn, Pb. Generally, these alloys were ordered ferromagnetically at T≥300 K for x<0.3. The structure of Mn5(Ge1−xSnx)3 remains in the hexagonal Mn5Si3(hP16) phase up to x=0.25 and then changes to Ni2In (hP6) as x approaches 1. The magnetization and Curie temperature of these alloys is largest for small x and reach a minimum value near x=0.5. The room temperature Polar Kerr rotation, however, increases from −0.06° at x=0 to maximum value of −0.12° at x=0.1 before decreasing toward zero for x≳0.3. The Mn5(Ge1−xPbx)3 samples show a Mn5Si3 structure with ferromagnetic behavior for x≤0.3, and varying structure with rapidly decreasing magnetization above x=0.3. The magnetic behavior observed was complex for x≥0.3. The Kerr rotation of the Pb-doped alloys in the ferromagnetic region monotonically decreases from −0.06° to zero at x≊0.3.
Photoluminescence excitation interpreted by photon recycling in GaAs/GaAlAs multiple-quantum-well structure
Authors: Goldys E. M. , Chin V. W. L. , Tansley T. L. , Vaughan M. R.
Published: July 2002

The photoluminescence spectra of GaAs/GaAlAs multiple quantum wells on GaAs buffer show a band related to confined quantum-well states as well as the emission from electron-hole recombination in the buried GaAs layer. The power dependence of the GaAs emission intensity indicates that the nonequilibrium minority-carrier concentration in GaAs is large due to substantial photon recycling. The principal features of the photoluminescence excitation data are discussed in the context of the photon recycling mechanism and a quantitative calculation of the photoluminescence excitation spectrum is presented.
Spectral characteristics of visible light emission from porous Si: Quantum confinement or impurity effect?
Authors: Bondarenko V. P. , Borisenko V. E. , Dorofeev A. M. , Germanenko I. N. , Gaponenko S. V.
Published: July 2002

Emission and excitation spectra of visibly luminescing porous Si structures subjected to a long-term air impregnation are analyzed. A red shift of emission spectrum with increasing initial porosity is reported. A correlation of excitation spectra with the energy of Γ′25-Γ15 transition in Si monocrystal is found in the samples differing in the initial porosity. The results seem to be hard to explain in terms of the quantum-confinement models in their present state. An alternative model based on band-gap widening in heavily doped silicon structures is considered as well.
Magnetic properties of (Nd0.9R0.1)5Fe17 with R=Sm, Gd, and Y
Authors: Liu Cong-Xiao , Sun Yun-Xi , Liu Zun-Xiao , Lin Chin
Published: July 2002

Magnetic properties of (Nd0.9R0.1)5Fe17 have been investigated. Samples were prepared by arc melting and annealed at 600 °C for 48 days. X-ray diffraction and thermomagnetic analyses showed that the samples are almost single phase with the Nd5Fe17 structure when R=Sm, Gd, and Y, but this structure cannot be formed when R=Pr. The effects of R substitution on saturation magnetization, Curie temperature, and lattice parameters are reported. X-ray diffraction analyses of aligned powders suggests that these samples have basal plane anisotropy. The magnetization does not fully saturate along the field in a field up to 70 kOe although the magnetization curves show the characteristic behavior of a soft ferromagnetic material.
Thermal equilibrium noise with 1/f spectrum in a ferromagnetic alloy: Anomalous temperature dependence
Authors: Vitale S. , Cavalleri A. , Cerdonio M. , Maraner A. , Prodi G. A.
Published: July 2002

We report on the magnetization noise in a soft ferromagnetic alloy for frequencies f<10 Hz, temperatures T<4.2 K, and excitation field amplitudes <150 μA/m. The spectra agree, without any adjustable parameter, with the fluctuation dissipation formula and the measured complex susceptibility of the material. The spectra show a 1/f shape coming from a frequency-independent imaginary susceptibility χ″0. χ″0 is found to be proportional to T above ≊2.5 K but tends to be independent of T below this value. After annealing the sample to relieve the internal stresses, χ″0 is found nearly temperature independent above ≊2.5 K and to sharply increase by decreasing the temperature below that value. We discuss these findings in terms of the hopping of the magnetization by activation or tunneling.
Stationary lattice mobility of holes in gallium arsenide
Authors: Scholz Reinhard
Published: July 2002

The stationary lattice mobility of holes in bulk GaAs is investigated within a hydrodynamic model, including balance equations for density, current density, and energy density in each hole subband. Generalized hot displaced Maxwellians are used to calculate the input parameters of this model, which are average transport masses for each subband, velocity and energy relaxation rates for the different hole-phonon scattering channels, and transfer rates between different subbbands. The nonparabolicity of the light hole band produces a strong dependence of its transport mass on the temperature of the light hole distribution. Even for low temperatures, this transport mass is much higher than the parabolic band mass. The velocity and energy relaxation rates are extracted from the phonon scattering rates calculated in the preceeding paper. The contributions of different scattering channels to velocity relaxation depend not only on the scattering rates, but also on the average velocity of the final states after scattering. It is shown that the finite average velocities after interband scattering lead to an intrinsic coupling of the mobilities of light and heavy holes. This makes it impossible to determine the mobilities in each subband separately. Instead, the coupled mobilities are extracted from the stationary solution of the equations of motion of the hydrodynamic model. The resulting hole mobility is in good agreement with measured data up to E=40 kV cm−1 if the deformation potential d0 between holes and optical phonons is extracted from the stationary drift mobility at low field: μ0=400±40 cm2/V s and a heavy hole mass of mh*=0.50±0.02 lead to d0=27.4±5.2(μ0)±3.9(mh*) eV, where the first error is related to the measured mobility and the latter to the dependence of the calculated mobility on the heavy hole mass.
Electron trapping kinetics at dislocations in relaxed Ge0.3Si0.7/Si heterostructures
Authors: Grillot P. N. , Ringel S. A. , Fitzgerald E. A. , Watson G. P. , Xie Y. H.
Published: July 2002

The capture kinetics and trapping properties of a dislocation related electron trap detected in strain-relaxed, compositionally graded Ge0.3Si0.7/Si grown by rapid thermal chemical-vapor deposition are investigated by deep-level transient spectroscopy (DLTS). The volume DLTS trap concentration scales linearly with the areal threading dislocation density, as determined by electron-beam-induced current measurements on samples with different compositional grading rates, indicating that the detected trap is most likely associated with dislocation core states in these graded structures. The dislocation related trap exhibits both the logarithmic dependence of DLTS peak height on fill pulse time tp, and broadened DLTS peaks which typically characterize carrier trapping at dislocations. These effects are quantified and analyzed to gain insight into the trapping properties of dislocations in GeSi/Si heterostructures and to investigate the effects of dislocation related carrier trapping on DLTS measurements. It is demonstrated that the peak broadening, as characterized by the dimensionless broadening parameter FWHM/Tp, where FWHM and Tp are the full width at half-maximum of the DLTS peak and the DLTS peak temperature, respectively, monotonically decreases with decreasing fill pulse duration, and approaches point-defectlike behavior for tp<100 μs.

The observed broadening is asymmetric about Tp, and occurs predominantly on the low-temperature side of the DLTS peak. This asymmetric broadening is shown to shift the ‘‘apparent’’ trap activation energy, as determined by Arrhenius analysis, from EC−0.6 eV to EC−0.9 eV (relative to the bulk conduction-band edge) as tp decreases from 5 ms to 50 μs. These observations are explained by the presence of a dislocation related distribution of energy levels within the GeSi band gap and the consequent fill-pulse-dependent local band bending. The lowest-energy states within this distribution are preferentially filled with electrons for short fill pulse times. The Arrhenius-determined ‘‘apparent’’ activation energy is hence interpreted as being a measure of the average energy of the filled defect states, weighted by the density of states distribution in this energy band and by the related fill-pulse-dependent local band bending. It is further demonstrated that the minority-carrier capture cross section may be enhanced by the presence of an attractive coulombic barrier for minority carriers at the dislocations, and we use the logarithmic capture equations to derive a value of 4×10−12 cm2 for this ‘‘effective’’ capture cross section.
Analysis of Franz–Keldysh oscillations in photoreflectance spectra of a AlGaAs/GaAs single-quantum well structure
Authors: Hughes P. J. , Weiss B. L. , Hosea T. J. C.
Published: July 2002

Experimental results are presented for the physical origins of room-temperature photoreflectance features of a AlGaAs/GaAs single-quantum well structure. The spectra exhibit well-defined Franz–Keldysh oscillations which overlap with photoreflectance features due to the quantum well and complicate the determination of the energies of the transitions within the quantum well. The origin of the Franz–Keldysh oscillations are determined using wet chemical etching to selectively remove grown layers down to the substrate. The resulting spectra are presented as a function of etch depth which allows the magnitude of the built-in electric fields to be determined and reveals the location within the quantum well structure where the Franz–Keldysh oscillations originate.
Junction characteristics of Ga0.5In0.5P n+p diodes and solar cells
Authors: Reinhardt K. C. , Yeo Y. K. , Hengehold R. L.
Published: July 2002

Forward and reverse bias dark current characteristics of n+p mesa diodes and solar cells fabricated from metalorganic chemical vapor deposition grown Ga0.5In0.5P have been examined, and dark current behavior has been correlated with Ga0.5In0.5P solar cell conversion efficiency. Dependencies of dominant dark currents on voltage, temperature, and doping density were found to be consistent with recombination theory. Recombination dark current with an ideality factor of A=1.9–2.1 was dominant in the voltage range of ∼0.5 to 1.5 V and ∼0.8 to 1.2 V for devices with a p-base doping density of ∼1016 and 1017 cm−3, respectively. Reverse current-voltage-temperature measurements identified two dominant deep levels at 0.10 and ∼0.45 eV relative to either the valence- or conduction-band minimum. The carrier recombination was found to occur through these deep level centers in the n+p junction.
Effect of inductance in externally shunted Josephson tunnel junctions
Authors: Whan C. B. , Lobb C. J. , Forrester M. G.
Published: July 2002

We report measurements on current-voltage (I-V) characteristics of externally shunted Josephson tunnel junctions where the external shunts have nonzero inductance. At low temperatures (T<6.6 K), we observe standard I-V curves, but as temperature is increased above 6.6 K we observe an anomaly in the I-V curves. We attribute this anomaly to the nonzero inductance in the shunt loop. Numerical simulations which properly take into account the effect of inductance show that the dynamics in the anomalous region of the I-V curve are dominated by subharmonic relaxation oscillations. We use a load-line analysis model to study the transition between the relaxation oscillations and the Josephson oscillations. The analysis is in good agreement with our data. Our analysis further predicts, and our data confirms, that in order for the relaxation oscillations to occur it is necessary to have βC=2πIcR2sC/Φ0<(4/π)2, regardless of the value of the inductance.
Fabrication and characterization of pyrophosphoric acid proton exchanged lithium tantalate waveguides
Authors: Davis Gillian M. , Lindop Nathan A.
Published: July 2002

Planar waveguides in z-cut LiTaO3 have been fabricated by ion exchange in pyrophosphoric acid. The dependence of the refractive index change and waveguide depth on the ion-exchange time and temperature, and the anneal time and temperature has been investigated. In addition, waveguide transmission and inversion depths have been measured. The ion exchange temperature and duration were varied between 230 and 293 °C and 15 and 90 min, respectively. The anneal temperature and duration were varied between 300 and 594 °C, and 16 and 840 s, respectively. Under certain conditions the depth of the waveguide was shown to have a simple dependence on the processing conditions. The minimum waveguide coupling and propagation loss was measured to be 0.4 dB. Inversion was observed for anneals of 180 s and temperatures of 525–594 °C.
Time-sliced Mössbauer absorption spectroscopy using synchrotron radiation and a resonant Bragg monochromator
Authors: Tischler J. Z. , Larson B. C. , Boatner L. A. , Alp E. E. , Mooney T.
Published: July 2002

Doppler absorption measurements have been used to investigate time-slice Mössbauer absorption spectroscopy for synchrotron radiation and a mosaic resonant Bragg monochromator. The energy widths for wide time intervals and the time-dependent narrowing of the spectral width of resonant Bragg scattering from a mosaic 57Fe2O3 crystal have been measured using a powdered 9.7 mg/cm2 57Fe2O3 absorber. The measurements are compared with calculations for a thick (9.7 mg/cm2) and a thin (0.5 mg/cm2) absorber, and the feasibility of high angular collimation neV range x-ray scattering spectroscopy using synchrotron radiation and time windowing is assessed.
The nitrogen acceptor energy in ZnTe measured by Hall effect and optical spectroscopy
Authors: Grün M. , Haury A. , Cibert J. , Wasiela A.
Published: July 2002

The binding energy of the nitrogen acceptor in zinc telluride is measured. The zinc telluride samples were grown by molecular beam epitaxy using a plasma source for nitrogen doping. The acceptor ionization energy at infinite dilution limit is deduced from Hall effect analysis to be 53.2±1.5 meV. This value agrees with that of the acceptor ground state energy obtained from photoluminescence spectra, 53.6±0.7 meV. The acceptor energy in strain-free material may be slightly higher due to some residual tensile strain in the samples.
Phase change recording using a scanning near-field optical microscope
Authors: Hosaka Sumio , Shintani Toshimichi , Miyamoto Mitsuhide , Kikukawa Atsushi , Hirotsune Akemi , Terao Motoyasu , Yoshida Masaru , Fujita Kouichi , Kämmer Stefan
Published: July 2002

The formation and observation, with reflected light, of 60-nm-diam phase-changed domains in a thin GeSbTe film using a scanning near-field optical microscope with a 785 nm wavelength laser diode is demonstrated. The dependence of the domain size on incident laser power was obtained, and the size changed from 150 to 60 nm in diameter with incident power of 8.4–7.3 mW in the probe. At the threshold power of 7.3 mW, the film temperature rose to around 180 °C to partially phase change the local area of the film from amorphous to crystalline. A detected reflectivity increase due to phase change in the formed domain was 8%–2%. The observing (reading) was performed with an incident laser power of 0.2 mW, which corresponds to 10−2–10−3 times less than in a magneto-optical recording. The incident laser power shows that the phase change reading using the reflection scanning near-field optical microscope has the potential to read the recorded bit at a speed over 10 MHz.
Analysis and experimental study of surface transverse wave resonators on quartz
Authors: Gavignet E. , Ballandras S. , Bigler E. , Bonjour C. , Renaud J.-C. , Daniau W.
Published: July 2002

Since surface transverse wave resonators are becoming increasingly used in high-frequency electrical component applications, there is a pressing need for reliable simulation tools in this field. We develop analytical techniques for prediction of the resonant peaks of practical devices and apply them to the results of experiments in this laboratory and in others. The devices investigated are three-grating structures composed of three arrays of metal strips or grooves on quartz substrates, which support a shear horizontal surface wave polarization. The outer gratings act as mirrors for the cavity resonance. The influence of the main design parameters on the resonator response is studied.
Transversely varying thickness modes in trapped energy resonators with shallow and beveled contours
Authors: Tiersten H. F. , Lwo B. J. , Dulmet B.
Published: July 2002

The equation for transversely varying thickness modes in doubly rotated quartz resonators is applied in the analysis of contoured resonators with rectangular electrodes. The influence of both the contouring and the continuity conditions at the edges of the electrodes are included in the analysis. The steady-state forced vibrations of contoured trapped energy resonators is treated and a lumped parameter representation of the admittance, which is valid in the vicinity of a resonance, is obtained. Calculated results are presented for a number of trapped energy resonators with shallow and beveled contours.
γ and fission-reactor radiation effects on the visible-range transparency of aluminum-jacketed, all-silica optical fibers
Authors: Griscom David L.
Published: July 2002

Four aluminum-jacketed, fluorine-doped silica clad optical fibers with silica core materials fabricated by differing technologies were subjected to sequential 60Co-γ ray and fission-reactor irradiations (at ∼20 and 40 °C, respectively), an intervening isothermal anneal (∼20 °C), and a final isochronal anneal (to 600 °C) while monitoring the radiation-induced absorption spectra in the range ∼400–1000 nm. The two low-OH/low-chloride core fibers (one of which was doped with 0.5 mass % fluorine) both developed bands at 660 and 760 nm which exceeded 10 000 dB/km for doses in the range ∼102–106 Gy(Si); however, these bands declined to <1000 dB/km by the end of the γ-irradiation phase [12 MGy(Si) at 5.6 Gy(Si)/s]. All fibers displayed an ‘‘UV band tail,’’ which was the strongest in the high-chloride core fiber, as well as bands in the range ∼600–630 nm generally attributed to nonbridging-oxygen hole centers (NBOHCs). During the γ-irradiation phase the strengths of the NBOHC bands proved to be strongly dependent on the method of core material manufacture. Contrary to previous results for acrylate-jacketed fibers, no substantial bleaching of the UV-tail or NBOHC bands took place during γ irradiation despite the continuous propagation of white light powers ∼5–50 μW. The incremental induced absorption spectra consequent to the reactor-irradiation [∼4 MGy(Si) γ-ray dose at 70 Gy(Si)/s, plus a fluence of ≳2.8-MeV neutrons ∼2×1016 cm−2] were much less sensitive to fiber core material. The prospects for developing rad hard optical fibers for fusion reactor diagnostics are discussed in light of these findings.
Material parameters for electrostriction
Authors: Shkel Yuri M. , Klingenberg Daniel J.
Published: July 2002

Electrostriction is often described by a phenomenological tensor relating a material’s deformation to an applied electric field. However, this tensor is not a material parameter; for deformable, weakly compressible materials (e.g., elastomers), the field-induced deformations depend strongly upon boundary conditions. A different approach that relates the deformation to material properties as well as boundary conditions is required. In this article, we describe a linear theory which introduces five material parameters governing electrostriction: the relative dielectric constant, ε0, two derivatives of the dielectric constant tensor, a1 and a2, Young’s modulus, Ey and Poisson’s ratio, ν. Knowledge of these parameters and appropriate boundary conditions allow one to predict field-induced deformations for arbitrary configurations. We demonstrate an experimental procedure for measuring deformations and permittivity changes, from which the parameters a1 and a2 may be extracted (ε0, ν, and Ey can be measured by a variety of established methods). The linear theory reproduces experimental results for two polyurethane films at small to moderate electric field strengths. We find that the electrostatic force associated with the parameters a1 and a2 is at least ten times larger than the Coulombic attractive force between the electrodes.
Microstructural origin of current localization and “puncture’’ failure in varistor ceramics
Authors: Vojta Agnes , Clarke David R.
Published: July 2002

The effect of microstructural disorder, such as variations in grain size and in the height of the electrostatic potential at grain boundaries, on current flow in varistors has been simulated by computing the currents in a network of grain boundaries each of which can be described by nonlinear electrical characteristics. Microstructural disorder causes current localization, which is analyzed in terms of two parameters, the infinite moment and the second moment of the current distribution. These parameters enable quantitative comparison of different microstructures. By incorporating the effect of Joule heating the temperature increases due to current localization can be simulated and the effect of short electrical pulses of different energies investigated. On the basis of the simulations a mechanism for current localization, due to microstructural variations, leading to thermal runaway and “puncture’’ failure by melting is proposed. The results suggest that electrode protrusions and large grains at the electrodes are likely sources for failure, and that some degree of microstructural disorder is required to resist thermal runaway for all but the most energetic pulses.
Superconducting quantum interference device-based magnetic nanoparticle relaxation measurement as a novel tool for the binding specific detection of biological binding reactions (abstract)
Authors: Kötitz R. , Bunte T. , Weitschies W. , Trahms L.
Published: July 2002

Biological binding reactions play a key role in biology and medicine. Their detection is usually achieved by labeling one of the reaction components with radioisotopes, enzymes, or fluorescence dyes. In particular assays, using the specificity of the reaction between antibodies and antigens are of outstanding importance. Most of these assays are hampered by the drawback that the label generates a signal that is not influenced by the binding reaction. Therefore, separation procedures between bound and unbound reaction components are mostly inevitable. Here, we present the use of superconducting quantum interference device (SQUID)-based magnetic nanoparticle relaxation measurement as a novel tool for the quantitative determination of biological binding reactions, where magnetic nanoparticles are used as labels to antibodies. The rotational diffusion of the label is hindered by the binding of the antibody to the antigen, which is adsorbed to the sample tube wall. As a result, the observed relaxation of its magnetization is driven by the internal reorientation of the magnetic moment of the nanoparticle (Néel relaxation). By this, the measured signal is specific for the magnetically labeled antibody bound to the antigen. We could show that already at a very early stage of technical development SQUID-based magnetic nanoparticle relaxation shows a higher sensitivity as a comparable standard assay technique (enzyme linked immunosorbent assay), without the need of any separation step between bound and unbound reaction components. Magnetic nanoparticle relaxation amplitudes as small as 500 fT were detectable. This corresponds to a magnetic moment of m=1.6×10−10 A m2 or, e.g., some 5×106 spherical magnetic iron oxide nanoparticles of 50 nm diameter.
Effect of substrate materials on the electron field emission characteristics of chemical vapor deposited diamond films
Authors: Lee Jau-Sung , Liu Kuo-Shung , Lin I-Nan
Published: July 2002

Substrate materials used for growing diamond films were observed to modify thin films’ electron field emission properties significantly. Using heavily doped silicon (LR-Si) as a substrate lowered the turn-on field from (E0)Si=14.4 V/μm to (E0)LR-Si=9.7 V/μm and increased the emission current density from (Je)Si=4 μA/cm2 to (Je)LR-Si=40 μA/cm2 (at 16 V/μm). However, electron field emission properties can be further improved only by using Au precoatings to modify the characteristics of interfacial layer. The turn-on field was lowered further to (E0)Au–Si=8.7 V/μm and emission current density was increased further to (Je)Au–Si=400 μA/cm2 (at 16 V/μm). Secondary ion mass spectroscopic examination indicated that the main interaction is the outward diffusion of Au species into amorphous carbon layer, lowering the resistivity of this interfacial layer. The electrons can therefore be transported easily from Si substrate across the interfacial layer to the diamonds and subsequently field emitted.
Deep level transient spectroscopy of CdS/CdTe thin film solar cells
Authors: Lourenço M. A. , Yew Yip Kum , Homewood K. P. , Durose K. , Richter H. , Bonnet D.
Published: July 2002

Deep levels in polycrystalline n-CdS/p-CdTe photovoltaic structures have been studied by deep level transient spectroscopy (DLTS). The results were obtained from cells which have undergone different post-deposition treatment (as-deposited, heat treated, and heat treated in the presence of CdCl2). The DLTS results showed a deep level distribution independent of the post-deposition treatment. For all samples the spectra were dominated by a hole trap, localized at EV+0.48 eV. Metastable hole and electron traps were also observed, mainly in the heat treated (with and without a CdCl2 layer) devices.
Thermal degradation mechanism of Ti/Pt/Au Schottky contact to n-type GaAs
Authors: Lee Jong-Lam , Mun Jae Kyoung , Lee Byung-Teak
Published: July 2002

The thermal stability of Ti/Pt/Au Schottky gates on high-low doped GaAs metal–semiconductor field-effect transistors (MESFETs) was investigated in the temperature range of 300–500 °C, using current–voltage and capacitance–voltage measurements and cross-sectional transmission electron microscopy with energy dispersive x-ray spectroscopy. At annealing temperatures >350 °C, the interfacial reactions cause the formation of a layered structure of Ti/Ti–Ga/Ti–As/GaAs. The depth distribution of electron concentration moves toward the Ti/GaAs interface as the annealing temperature increases. This is due to the growth of Ti–As, followed by the reduction of the channel thickness. The activation energy for the growth of Ti–As is determined to be 1.74 eV. The electron concentration in the channel layer decreases with the increase of the annealing temperature. This is due to the out-diffusion of Ga to the Ti film, resulting in the production of acceptor-type Ga vacancies and thereby the decrease of electron concentration in the channel. The activation energy for the reduction of electron concentration, equal to the formation energy of the acceptor-type Ga vacancies, is determined to be 1.42 eV. This is closed to the activation energy of 1.3 eV for the device failure obtained at annealing temperatures ranged from 300 to 350 °C. From these observations, it is proposed that the thermal degradation of GaAs MESFETs at temperatures <350 °C, mainly proceeds by the electron compensation with acceptor-type Ga vacancies in the channel, whereas it occurs by the growth of Ti–As below the original Ti/GaAs interface at temperatures >350 °C.
Optical properties of PbS
Authors: Kanazawa Hideyuki , Adachi Sadao
Published: July 2002

Spectroscopic ellipsometry has been used to measure the complex dielectric function ε(E)=ε1(E)+iε2(E) of PbS single crystals. The measurements are made on cleaved PbS surfaces in the 1.5–5.3 eV photon-energy range at room temperature. It is found that the optical properties of PbS are very sensitive to air exposure. The observed ε(E) spectra reveal distinct structures at energies of the E1, E2, and E3 critical points (CPs). These data and the literature values are used for the modeling of the optical constants of PbS over the 0.3–5.3 eV photon-energy range, including the band-gap (E0∼0.4 eV) region. It is shown that the three-dimensional M0 CP (E0) and damped harmonic-oscillator model (E1, E2, and E3) provide satisfactory fits to the experimental optical-constant data.
Passivation properties of the local oxidation of silicon–oxide/Si interface defects
Authors: Fujieda Shinji , Nobusawa Hajime , Hamada Masayuki , Tanigawa Takaho
Published: July 2002

The passivation properties of the local oxidation of silicon (LOCOS) oxide/Si interface defects were investigated by reverse current measurement and capacitance transient spectroscopy of pn junction diodes that had a large LOCOS-defined perimeter. The LOCOS/Si interface defects had some properties similar to those of the SiO2/Si(100) interface states of metal–oxide–silicon (MOS) diodes. However, there was a significant difference between the two interfaces in the rate of unpassivated defects remaining after H2 annealing: this rate was higher for the LOCOS/Si interface than for the MOS interface.
Dielectric tensor formulation of magnetic dichroism sum rules
Authors: Zhao Dai , Höchst Hartmut , Huber David L.
Published: July 2002

A formalism is developed to interpret the magnetic circular dichroism (MCD) sum rules in terms of the complex dielectric tensor ε(ω). Utilizing classical Maxwell–Fresnel theory, the formalism is applied to the reflection spectra of Fe at the M2,3 transition to calculate the orbital moments 〈Lz〉. The analysis shows that due to the energy dependence of the unpolarized normalization integral and the absence of a well-defined cut-off energy across the transition edge, large errors can be introduced. Uncertainties in 〈Lz〉 by more than 100% are possible which makes a comparison with other literature values questionable. Due to the arbitrary energy cut-off, the application of the sum rules as a general tool to unambiguously decompose MCD spectra into orbital and spin components should be practiced with great caution.
Epitaxial growth of GaN thin film on sapphire with a thin ZnO buffer layer by liquid target pulsed laser deposition
Authors: Sun X. W. , Xiao R. F. , Kwok H. S.
Published: July 2002

Epitaxially grown gallium nitride thin films were deposited on sapphire(0001) substrates with a thin zinc oxide buffer layer by a liquid target pulsed laser deposition technique. The GaN thin film optimized at a substrate temperature of 600 °C has an epitaxial relationship with ZnO buffered sapphire(0001) of (0001)GaN//(0001)ZnO//(0001)sapphire, and (101̄0)GaN//(101̄0)ZnO//(112̄0)sapphire. The surface morphology was also improved by applying a ZnO buffer layer shown by scanning electron microscopy. Although the as-grown GaN thin film showed no band edge or yellow band photoluminescence at room temperature, a weak band edge luminescence of 3.42 eV could be seen at 20 K.
Experimentally diagnosed transient behavior of pulse modulated inductively coupled thermal plasma
Authors: Sakuta T. , Paul K. C. , Katsuki M. , Ishigaki T.
Published: July 2002

An argon/hydrogen (argon: 89% molar) pulse-modulated radio-frequency inductively coupled thermal plasma was generated at adequate power level (active plasma power up to about 15 kW) and has features such as control of thermal flux in time domain and introduction of nonequilibrium phenomenon. These characteristic features have ample attraction for thermal plasma materials processing. A solid state power source, which supplies the electric power at a nominal frequency of 1 MHz with high matching efficiency of 90%, was used as the pulsing generator to generate the induction plasma. Experimental measurements were carried out to analyze the transient responses of ArI (at 751 nm wavelength) line and current intensities for the imposed pulsing signal. Results were obtained for pressure range from 200 to 760 Torr, with a power on time in the range from 2 to 20 ms while the power off time ranged from 5 to 12 ms. The plate power level was varied from 11 to 17 kW though the results presented in the present article are mainly for the 11 kW (and 200 Torr) case. During the pulse modulated operation where the nonequilibrium exists more or less, the average exciting temperature of Ar atom was estimated to be varied between 5000 and 14 000 K, for pulsation of 5 ms on time with 50% duty factor, corresponding to the on and off of the pulsing signal. At different on times, ArI line intensity changed considerably; with 2 ms on time, steady-state of ICP was not achieved. For higher pressure, the intensities increased obviously and response or relaxation time was longer.
Ellipsometric characterization of anisotropic porous silicon Fabry–Pérot filters and investigation of temperature effects on capillary condensation efficiency
Authors: Zangooie S. , Jansson R. , Arwin H.
Published: July 2002

Porous silicon Fabry-Pérot optical filters with reflectivity peaks in the visible and near infrared spectral range have been manufactured electrochemically and characterized with variable angle spectroscopic ellipsometry. Generalized ellipsometry and the Bruggeman effective medium approximation were employed to study the anisotropy of the samples including determination of the tilt of the optic axis relative to the sample normal. At a wavelength of 700 nm, the difference between the real parts of the ordinary and extraordinary indexes of refraction of the low and high index sublayers were 0.05 and 0.07, respectively. In addition, the effect of temperature on the spectral redshifts caused by capillary condensation of water and toluene vapors in the porous material are investigated. It is demonstrated that a temperature decrease causes the spectral shifts to occur at lower partial pressures, indicating a more effective vapor capture at lower temperatures. Finally, it is shown that this phenomenon can be used to increase the ellipsometric response to very low vapor concentrations and thus improve the sensitivity of the technique in gas sensor applications.
Tuning of colossal magnetoresistance via grain size change in La0.67Ca0.33MnO3
Authors: Hueso L. E. , Rivas J. , Rivadulla F. , López-Quintela M. A.
Published: July 2002

In this article, we show how colossal magnetoresistance effect (CMR) can be tuned in polycrystalline mixed valence manganite La0.67Ca0.33MnO3 via changing grain size by means of a sol-gel method. Below a critical diameter (150 nm), CMR disappears, but large intergrain MR remains even well above Tc (1.2Tc for ≈95 nm particles). Possible explanation for this effect involves single magnetic domain behavior in samples annealed at low temperature.
Abnormal temperature dependence of band-gap energies observed in (InAs)/(GaAs) and (InP)/(GaP) superlattices with strong lateral composition modulation
Authors: Chou Shu-Tsun
Published: July 2002

An abnormal variation of the band-gap energy with temperature was observed in both the (InAs)2.2/(GaAs)2 and (InP)2/(GaP)2 short-period-superlattice (SPS) structures. Strong lateral composition modulation induced two horizontally adjacent regions with different In content in the SPS regions. Due to the difference of thermal expansion coefficients among the substrate and the two adjacent regions in the SPS structures, it is proposed that a thermal strain has been generated and accommodated by altering the lattice constant in the growth direction. The magnitude of lattice variation depends on the relative thermal expansion and compressibility of the two adjacent materials. The net modification of the lattice thermal expansion in the In-rich region results in the abnormal variation of band-gap energies with temperature.
Effect of mixing enthalpy on relaxed and strained growth of III–VyIV1−yII compound alloys using molecular-beam epitaxy
Authors: Genoe J. , Németh Š. , Grietens B. , Behet M. , Vounckx R. , Borghs G.
Published: July 2002

The strict virtual crystal model has been shown to describe accurately the mixing enthalpy of III–V ternary alloys. We allocate the different components of this model to both subreactions, i.e., the reactions forming III–VI and III–VII, present in the molecular-beam epitaxial growth of III–VIVII compound alloys, and derive thereof a model that describes accurately the composition obtained. A good correspondence with experimental growth data is obtained. We apply this model to both relaxed and strained layer growth and the evolution in the composition is given when a relaxed layer is grown on a substrate with a different lattice constant.
Interplay of antiferromagnetic coupling in copper/permalloy combination multilayers
Authors: Heitmann S. , Hütten A. , Hempel T. , Schepper W. , Reiss G. , Alof C.
Published: July 2002

The evolution of the giant magnetoresistance (GMR) effect in sputtered combination multilayers (CMLs) of type Py1.8nm//{(Cu1.8nm/Py1.6nm)N/(Cu0.9nm/Py1.6nm)N}y with Py (permalloy=Ni81Fe19) has been investigated at room temperature. It is shown that the GMR characteristic of these CMLs can be phenomenologically predicted, if the physical properties are known, i.e., the GMR effect amplitude, double layer conductance, and bilinear and biquadratic antiferromagnetic exchange coupling constants of the two underlying {Cu/Py}N base systems at the first (Cu0.9nm) and second (Cu1.8nm) antiferromagnetic coupling maximum (AFCM). The GMR characteristic of the simplest CML with N=1 is, e.g., determined by averaging the bilinear and biquadratic exchange coupling constants of the two base systems. The GMR characteristics of CML with N⩾2 are a superposition of that of the underlying {Cu/Py}N base systems weighted by the fraction of the corresponding double layer conductance. Furthermore, it is demonstrated that the CMLs are interesting from an application point of view since they combine the temperature stability of the underlying {Cu/Py}N base system at the second AFCM with a larger GMR effect amplitude.
The spin flop of synthetic antiferromagnetic films
Authors: Tong H. C. , Qian C. , Miloslavsky L. , Funada S. , Shi X. , Liu F. , Dey S.
Published: July 2002

The spin flop of synthetic antiferromagnetic pinned layers (SAF), under a magnetic field has been theoretically predicted and recently reported [J. G. Zhu and Y. Zheng, IEEE Trans. Magn. 34, 1063 (1998); J. G. Zhu, IEEE Trans. Magn. 35, 655 (1999)]. However, no experimental data have yet being reported to confirm the theoretical prediction. This article will provide direct experimental evidence to confirm the spin flop phenomenon in SAF layers. A spin valve, [CoFe/NiFe]/Cu/[CoFe(II)/Ru/CoFe(I)]/IrMn, was used to verify the spin flop in SAF layers. The exchange bias direction of CoFe(I)/IrMn was introduced by a magnetic annealing process at 225 °C with a field strength of Han(10 kOe) and the exchange bias direction was found parallel to the magnetic field. These samples serve as the reference for the remaining experiments. By magnetic annealing the reference samples at 225 °C with lower magnetic fields, we found that the magnetic field threshold for SAF spin flop is about 1 kOe. When the field is further increased, the spins of CoFe(I) and CoFe(II) scissor toward the applied magnetic field axis and the moment of the SAF reaches saturation when the applied field is equal to or greater than 10 kOe. Thus the above experiments demonstrate the flop behavior of the SAF. It also provides direct evidence that the exchange bias direction between a ferro/antiferromagnetic interface is determined by the magnetization of the ferromagnetic layer, not by the magnetic annealing field.
Oscillator strengths, Huang–Rhys parameters, and vibrational quantum energies of cerium-doped gadolinium oxyorthosilicate
Authors: Cooke D. W. , Bennett B. L. , McClellan K. J. , Roper J. M. , Whittaker M. T.
Published: July 2002

Temperature-dependent optical absorption of cerium-doped gadolinium oxyorthosilicate (Gd2SiO5:Ce) has been measured and analyzed for impurity-ion-lattice coupling parameters and oscillator strengths. Although the spectrum consists of overlapping Ce3+ bands and Gd3+ lines, two well-resolved Ce3+ bands with 10 K maxima at 3.32 eV (peak a) and 3.61 eV (peak b) are amenable to spectral analysis. These bands, previously assigned to Ce3+ ions occupying crystallographically inequivalent substitutional sites, are characterized by Gaussian line shapes and temperature-dependent half widths that are well described by the linear model of impurity-ion-lattice coupling. Huang–Rhys [Proc. R. Soc. A 204, 404 (1950)] parameters of peaks a and b are 22.7 and 5.7, respectively, indicating strong ion-lattice coupling, with vibrational frequencies 1.83×1013 s−1 (peak a) and 5.07×1013 s−1 (peak b). Peak b centroid is approximately temperature independent, but peak a centroid shifts to higher energy with increasing temperature. This dependence is adequately described by including higher-order coupling terms in the ion-lattice interaction, although crystal-field contributions cannot be excluded. Absorption band oscillator strengths, f, are calculated from Smakula’s [Z. Phys. 59, 603 (1930)] formula and knowledge of cerium concentration for the two inequivalent sites. In the interval 10–300 K, peak a f values range from (9.8 to 26.8)×10−4 and peak b f values vary from (7.8 to 5.8)×10−3. From the known correlation between oscillator strength and metal-ion-ligand separation, we identify peaks a and b as the seven- and nine-oxygen-coordinated sites, respectively.
Dynamics of simple gas–surface interaction. II. Rotationally inelastic collisions at rigid and moving surfaces
Authors: Polanyi J. C. , Wolf R. J.
Published: July 2002

We have used the classical trajectory method to investigate rotationally inelastic encounters between diatomic molecules and a hierarchy of model surfaces: a rigid surface (RS), a simple harmonic oscillator (SHO), and a generalized Langevin oscillator (GLO). The diatom masses correspond to NO throughout, and the gas–surface interaction potential was invariant, with an attractive potential of ε=0.58 or 0.2 eV. Collision energies were 0.3 or 0.7 eV. Encounters were classified as ‘‘direct,’’ ‘‘indirect,’’ and ‘‘adsorbed.’’ Change from the RS to the SHO surface markedly increased the percentage of indirect encounters; change from SHO to GLO introduced adsorbed trajectories. Rainbow structure in the product rotational distribution, clearly evident on the RS, was obscured by the surface motion in the SHO and GLO models, remaining evident nonetheless for the higher collision energy. Sticking on the GLO surface decreased with increasing initial rotation, particularly for the weaker attractive potential. Consequently application of time-reversal symmetry led to a yield of desorbing molecules weighted toward lower final rotation, i.e., to a rotational temperature TROT<TS (TS=surface temperature). The need for detailed experimental studies to establish the dynamics is evidenced by the fact that the characteristic bimodal distribution over final rotational states is obtained for all three model surfaces—RS, SHO, and GLO.
A Wiener integral model for stiff polymer chains
Authors: Bawendi M. G. , Freed Karl F.
Published: July 2002

A Wiener integral model for stiff polymer chains is developed following the approach of Bixon and Zwanzig. The model is mathematically tractable yet retains the general physics of the crossover between a flexible chain and a rod. Previous approaches to this problem are reviewed and compared to this model in the context of their applicability to the study of nonideal stiff polymer chains.
Nitrogen quadrupole coupling constants for HCN and H2CN+: Explanation of the absence of fine structure in the microwave spectrum of interstellar H2CN+
Authors: Scuseria Gustavo E. , Lee Timothy J. , Saykally Richard J. , Schaefer Henry F.
Published: July 2002

Nitrogen 14 quadrupole coupling constants for H2CN+ and HCN are predicted via ab initio self-consistent-field and configuration interaction theory. Effects of electron correlation, basis set completeness, and geometrical structure on the predicted electric field gradients are analyzed. The quadrupole coupling constant obtained for H2CN+ is one order of magnitude less than in HCN, providing an explanation for the experimental fact that the fine structure of the microwave spectrum of H2CN+ has not been resolved. This research also allows a reliable prediction of the nuclear quadrupole moment of 14N, namely Q(14N)=2.00×10−26 cm2.
The photoelectron spectrum of water in the 30 to 140 eV photon energy range
Authors: Banna M. S. , McQuaide B. H. , Malutzki R. , Schmidt V.
Published: July 2002

Synchrotron radiation from the German storage ring BESSY has been used to study the photoelectron spectrum of the water molecule. Asymmetry parameters and relative photoionization cross sections have been measured for all four molecular orbitals (2a1, 1b2, 3a1, and 1b1) in the 30–140 eV photon energy range. The results have been compared with the Xα calculations of Roche, Salahub, and Messmer and good agreement was found. Comparison was also made with a number of previous measurements, including those obtained by (e,2e) spectroscopy. Finally, the inner valence (2a1) region has been studied at 60 and 100 eV photon energies and the relative intensities of some of the satellites were found to change as the exciting energy was varied.
Pattern formation in precipitation processes. II. A postnucleation theory of Liesegang bands
Authors: Venzl Gerd
Published: July 2002

A theory is presented which explains recurrent precipitation bands (‘‘Liesegang rings’’) as dissolution patterns in a previously produced essentially homogeneous and continuous colloid. This picture is consistent with recent experimental evidence that the formation of Liesegang bands is a postnucleation phenomenon. Mathematically we formulate a model which contains a suitable diffusion equation plus a threshold condition for band formation and repetitive boundary conditions at the location of the previous band. The theory predicts the familiar exponential (spacing) law for the band positions but is sufficiently flexible to account for deviations from this law if additional physico-chemical details are taken into account.
Theoretical studies of the low-lying states of ScO, ScS, VO, and VS
Authors: Bauschlicher Charles W. , Langhoff Stephen R.
Published: July 2002

A theoretical study of the lowest three (X 2Σ+, A′ 2Δ, A 2Π) states of ScO and ScS is presented. The A′ 2Δ and A 2Π states lie considerably closer to the ground state in ScS than in ScO, and the unobserved A′ 2Δ state of ScS is predicted to lie at 9000±1000 cm−1. An analogous comparison is presented for the low-lying X 4Σ−, A′ 4Φ, A 4Π, and (1)2Δ states of VO and VS. Extended one-particle Gaussian basis sets are employed, and electron correlation is included using the singles-plus-doubles configuration interaction and the coupled pair functional formalisms. For the ground states of all four systems there is some triple-bond character arising from 3dσ and 3dπ bonds, combined with the donation of the Oπ lone pair into an empty 3dπ orbital. The ionic component of the bonding is reduced for the sulfides, owing to the smaller electronegativity of S. The low-lying states generally arise from excitation of the metal 4s electron into the nonbonding δ or π orbitals. However, the A′ 4Φ and A 4Π states of VO arise from an excitation of the nonbonding 3dδ electron into a π orbital. From the trends in the theoretical dissociation energies for ScO, ScS, and VO, we suspect that the new experimental values for the D0 of VS may be too high, and that the older value of 4.67±0.15 eV is more accurate.