bluuebunny/crossref_metadata_2025 huggingface dataset:
Improving the performance of a thermomagnetic generator by cycling the magnetic field
Authors: Solomon Dan
Published: July 2002

A new design for a thermomagnetic generator is described where the magnetic field that is applied to the working magnetic material is cycled as the working magnetic material is thermally cycled. The magnetic field is produced by a superconducting coil. The energy generated and the efficiency of the thermomagnetic generator are given for various operating conditions using gadolinium as the working magnetic material. Hysteresis losses in the superconductor are considered as well as thermal transfer rates between a working thermal fluid and the magnetic material.
Computer simulation model of the effects of interface states on high-performance amorphous silicon solar cells
Authors: Tasaki H. , Kim W. Y. , Hallerdt M. , Konagai M. , Takahashi K.
Published: July 2002

A computer simulation model of amorphous silicon solar cells using a Scharfetter and Gummel solution of Poisson’s equation and Taylor and Simmons occupancy statistics for the dangling bond gap states (D states), has been developed. With a suitable choice of parameters, the numerical results for solar cell collection efficiency and dark and illuminated I-V characteristics agree well with experimental values. The model has been used specifically to study the influence of interface states at the TCO-p (transparent conductive oxide), p-i, i-n, and n-metal interfaces and to explain the beneficial role of a graded-band-gap layer at the p-i interface.
Optical properties of molecular beam epitaxially grown GaAs1−xSbx (0<x<0.5) on GaAs and InP substrates
Authors: Huang D. , Chyi J. , Klem J. , Morkoç Hadis
Published: July 2002

Undoped and lightly Si doped GaAs1−xSbx bulk layers have been grown on GaAs substrates with Sb composition as high as 0.5 by molecular beam epitaxy. Low temperature (4 K) photoluminescence (PL) and absorption measurements were carried out. For samples with x values smaller than 0.06, PL spectra were dominated by two bands, the emission associated with the bound excitons and with the electron (or donor) to acceptor recombinations. However, the free exciton peak was clearly observable in absorption spectra. For higher Sb compositions (x>0.06), only one peak associated with the impurity or defect related transitions dominates PL spectra. We found that the energy and the line shape of this main peak are closely related to the sample quality for the same x value. The lower peak energy than the band gap, the low signal intensity and the large line broadening are due to the emissions associated with the defect band below the ideal (perfect crystal) energy gap introduced by the lattice mismatch near the heterointerface of GaAs1−xSbx and GaAs. As expected, higher quality epilayers provide a stronger PL intensity, smaller line width, as well as a higher peak energy.
Unusual magnetic and lattice transformation in UNiSn, a possible half-metallic ferromagnetic system
Authors: Bykovetz N. , Herman Warren N. , Yuen T. , Jee Chan-Soo , Lin C. L. , Crow J. E.
Published: July 2002

UNiSn, as well as certain other ternary actinide compounds, displays very anomalous behavior in the temperature dependence of the electrical resistivity, ρ(T). In UNiSn this unusual ρ(T) dependence has been attributed to half-metallic behavior with a semiconducting gap in the minority spin band and metallic behavior in the majority spin band with a ferromagnetic transition in the vicinity of 50 K. We have studied UNiSn using 119Sn Mössbauer spectroscopy and observed that annealing procedures influence the microscopic properties of the sample in a profound way. As-cast samples are nonmagnetic. Annealing in the neighbor of 800 °C results in the formation of a magnetic component. The conversion to a magnetic phase is incomplete even after annealing for 210 days at 800 °C. The magnetic component goes through two transformations: (1) a region below approximately 60 K where there is a coexistence and gradual transformation of paramagnetic to magnetically ordered material and (2) a region below 43 K where the magnetic component is completely ordered. In the region between 43 and 60 K, the hyperfine field shows a sequence of first-order transitions. The temperature dependence of the 119Sn hyperfine field for UNiSn is presented and discussed along with measurements of the temperature dependence of the specific heat, electrical resistivity, and the temperature and field dependence of the magnetization.
In situ secondary ion mass spectrometry study of the surface oxidation of silicon using 18O tracer
Authors: Littlewood S. D. , Kilner J. A.
Published: July 2002

The formation and growth of SiO2, pertaining to normal incidence oxygen bombardment of silicon in secondary ion mass spectrometry analysis, has been studied using in situ implantation of 18O+ tracer. At the chosen energy of 15-keV O+, buried layer formation has been shown to occur prior to the formation of a continuous surface oxide layer. Supersaturation of oxygen within the oxide layer was found to give rise to a high oxygen mobility, with a diffusion coefficient of the order of 10−14 cm2/s, resulting in a large desorption flux at the front surface. From preferential sputtering considerations, the surface concentration was found to be close to that of SiO2.
Thermally stimulated current studies of hole transport in a layered organic photoconductor
Authors: Sanda P. N. , Takamori T. , Dove D. B.
Published: July 2002

Thermally stimulated current (TSC) measurements have been peformed on a layered organic photoconductor based on p-diethylaminobenzaldehyde diphenylhydrazone (DEH) dispersed in bisphenol A polycarbonate. Well-defined transient current peaks were observed, indicative of a hole transport process having an activation energy of ∼0.5 eV at an applied electric field of 10 V/μm. This TSC feature is attributed to carriers which are thermally activated from electron donor states associated with the DEH molecules responsible for transport in this medium. This transport peak is clearly distinguished from the higher temperature secondary structure which is attributed to trap states. Measurements of the steady-state photoconductivity over a range of temperatures also yield approximately the same activation energy for the transport process.
Revised role for the Poole–Frenkel effect in deep-level characterization
Authors: Buchwald W. R. , Johnson N. M.
Published: July 2002

The Poole–Frenkel effect is commonly used to decide between donorlike and acceptorlike electronic character for deep-level defects in semiconductors. However, there exists at least one defect, the EL2 center in GaAs, which is experimentally established to be a deep donor and yet does not exhibit the classical Poole–Frenkel effect for thermal emission of electrons. In this communication it is proposed that the existence of another well-documented deep-level phenomenon can suppress the Poole–Frenkel effect. Namely, a thermally activated capture cross section, which identifies an energy barrier to carrier capture and is commonly ascribed to a multiphonon emission process, introduces additional mechanisms which can alter the predominance of the Coulombic potential of the emitted carrier so as to suppress the electric-field-induced barrier lowering. A simple one-dimensional model is analyzed to qualitatively illustrate the combined phenomena.
Comparison of partition function calculations for metal plasmas
Authors: Rolader Glenn E. , Batteh Jad H. , Desai Prateen V.
Published: July 2002

Three methods are commonly used to handle the unobserved energy levels in calculating the properties of plasmas. The contribution of the levels may be simply ignored, or the energies of the levels may be estimated by either the hydrogenic or the Rydberg–Ritz approximation. In this paper we compare the predictions of plasma properties obtained with each of these methods for Cu and Al plasmas for temperatures of 10 000–50 000 K and pressures from 0.1 to 600 atm. The validity of simplifying assumptions used to handle the unobserved levels has been documented, and regimes where each method is most appropriate have been identified.
Study of interdiffusion in a Te-doped AlAs-GaAs superlattice
Authors: Mei P. , Schwarz S. A. , Venkatesan T. , Schwartz C. L. , Colas E.
Published: July 2002

The enhanced layer interdiffusion in Te-doped AlAs/GaAs superlattices has been studied by secondary ion mass spectrometry. The superlattice sample was grown by organometallic chemical vapor deposition with Te doping at concentrations of 2×1017–3×1018 cm−3 during the growth process. In the temperature range from 800 to 1000 °C, the Al diffusion coefficient has an activation energy of 3.0 eV and is approximately proportional to the Te concentration. These results contrast sharply with Si-induced mixing which, in an analogous experiment, yielded an activation energy of 4.1 eV for the Al diffusion coefficient with a high power law dependence on the Si concentration.
Model calculations of the temperature distribution in the laser-heated diamond cell
Authors: Bodea Sorin , Jeanloz Raymond
Published: July 2002

Numerical calculations are used to simulate the steady-state temperature distribution achieved during cw laser heating of a dielectric sample inside the high-pressure diamond cell. According to the results, the temperature field is controlled in the radial direction by the dimension (profile) of the laser beam; in the axial direction it is controlled by heat loss to the diamond anvils. Thus, the peak temperature is approximately proportional to sample thickness. The temperature distribution is nearly Gaussian in both the radial and axial directions, especially near the center of the hot zone in the sample. For typical experimental conditions, peak temperatures exceeding 5000 K and temperature gradients of ∼108–109 K/m are expected, with radial and axial dimensions of the hot zone being ∼20 and ∼10 μm, respectively. These values are in accord with experimental measurements, and they imply that despite the high temperatures achieved inside the sample volume the surrounding diamond anvils remain essentially at ambient temperatures.
Shear horizontal acoustic waves at the periodically corrugated interface between two isotropic elastic media
Authors: Djafari-Rouhani B. , Maradudin A. A.
Published: July 2002

On the basis of the Rayleigh method we have obtained two equivalent exact dispersion relations for acoustic waves of shear horizontal polarization propagating across the periodically corrugated interface between two different isotropic elastic media. These dispersion relations have been solved numerically, and the dispersion curve of the corresponding surface acoustic waves obtained. These waves exist for only a limited range of wave vectors in the immediate vicinity of the boundary of the one-dimensional first Brillouin zone of the periodically corrugated interface, in agreement with the results of earlier, perturbative studies of this problem. By continuing the solutions into the radiative region of the frequency-wave vector plane, we also calculate the attenuation of these shear horizontal interface acoustic waves.
Metastable defects in GaAs grown by metalorganic chemical vapor deposition: Dependence on the V/III ratio
Authors: Tabata A. S. , Pudensi M. A. A. , Machado A. M.
Published: July 2002

Metastable defects in GaAs samples grown by metalorganic chemical vapor deposition were analyzed as a function of arsine to trimethylgalium ratio, using deep-level transient spectroscopy and isochronal annealing. The data have shown that metastability is related to the presence of an electron level 0.33 eV below the conduction band which is independent on the As/Ga ratio. Arsenic interstitials could be responsible for this level.
Theory of ion-hose instability
Authors: O’Brien K. J.
Published: July 2002

Solutions are derived for the partial-differential equations of the rigid model of ion hose with and without an applied axial magnetic field. An upper bound on the amplitude of the beam response, derived from these exact solutions, is compared with the upper bound derived by means of an asymptotic saddle-point calculation. In the exact solutions, explicit boundary conditions are retained. The details of the boundary conditions are significant in understanding the driven ion-hose instability in the experiments. Consideration of the effects of a magnetic field is important because in the ion-hose experiments at Sandia National Laboratories the beam is immersed in an applied axial-magnetic field.
Estimate of the escape time of resonant tunneling electrons from a quantum well in double-barrier heterostructures
Authors: Gu Benyuan , Gu Lei , He Miao
Published: July 2002

An estimate of the escape time of a resonant tunneling wave packet from a quantum well (QW) of double-barrier heterostructures (DBH) is presented by using the properties of the propagator of the system. For large times, regardless of the detailed structure and composition of the system, the probability density of finding electrons inside a QW decays exponentially with time. The escape time is proportional to the reciprocal of energy width Γn of the quasibound states at resonance. The full width at half maximum of the transmission peak is just equal to the energy width of the quasibound states. However, the dwell time of electrons depends on the electron incident direction on the DBH for an asymmetric system. In the symmetric potential the dwell time is equal to twice the escape time.
Photochemical hole burning of tetraphenylporphin in phenoxy resin at 4.2–80 K
Authors: Furusawa Akira , Horie Kazuyuki , Kuroki Kazuo , Mita Itaru
Published: July 2002

Photochemical hole burning was performed with tetraphenylporphin doped in phenoxy resin (aromatic polyhydroxyether) at 4.2–80 K. Thermal stability of a hole burnt at a low temperature (4.2 K) and hole formation at high temperatures are studied for this system in order to obtain information on the dephasing and structural relaxation in the system.
Modeling of elongated fine ferromagnetic particles
Authors: Yan Ying Dong , Della Torre Edward
Published: July 2002

This paper presents some results of a numerical micromagnetic calculation for ferromagnetic fine particles. Two basic shapes are discussed: elongated parallelopipeds and parallelopipeds with pointed ends. The variation of coercivity, switching field, and reversal modes with particle size, orientation, and material parameters are discussed. Equilibrium magnetization structures suggest that particles may have remanent states that are substantially nonuniform. The flipping mode, in which the magnetization reversal starts from the longitudinal ends of the particles, is found to be the energetically favorable switching mechanism. Small particles are switched by the quasi-coherent flipping mode, while large particles are reversed by the quasi-curling flipping mode. The angular dependence of the coercive and switching fields for three particle sizes is found to agree qualitatively with classical results, but both fields are quantitatively smaller. The nucleation field is compared with the Stoner–Wohlfarth model as well as with the curling mode, and is found to be about half of their value.
Experimental characterization of ultrasonic phenomena by a learning system
Authors: Grabec Igor , Sachse Wolfgang
Published: July 2002

This paper describes the application of an adaptive learning system comprising an associative memory to the characterization of ultrasonic phenomena. The mapping of source and waveform data and vice versa can be performed by utilizing the off-diagonal cross-correlation portions of the associative memory. It is suggested that a more general description of the ultrasonic phenomena can be obtained if the diagonal, autocorrelation portions of the memory are also utilized. In this case, the memory is applicable for the autoassociative optimal filtering of experimental data. Experiments are described which utilize such an adaptive system, running on a laboratory minicomputer, to process the signals from a transient ultrasonic source in a plate specimen. It is shown how the system learns from the experimental pattern vectors, formed from the ultrasonic waveforms and encoded information about the source. The source characteristics are recovered by the recall procedure from the detected ultrasonic signals and vice versa. Also, the changes in the wave phenomenon corresponding to changes in the boundary conditions of the specimen can be detected from the discrepancy between the presented and the learned signals.
Interfacial reactions of titanium thin films on BF+2 -implanted (001)Si
Authors: Lur W. , Chen L. J.
Published: July 2002

Interfacial reactions of titanium thin films on BF+2 -implanted (001)Si have been studied by both cross-sectional and planview transmission electron microscopy as well as by sheet resistivity measurement. Amorphous Ti-Si interlayers were observed in Ti films on blank and implanted samples following 400 °C annealing. The presence of the doping impurities was found to promote the early growth of the C54-TiSi2 phase. 100% surface coverage of TiSi2 on silicon was found in implantation amorphous samples annealed at 1000 °C for 1 h. The C54-TiSi2/Si interface energy and/or the silicide surface energy are thought to be lowered by the presence of the doping impurities to improve the surface morphology at high temperatures. The distribution of fluorine bubbles in doped samples with Ti overlayers annealed at 500–1000 °C indicated that (1) the solubilities of F in Ti silicides are very low, (2) the diffusivities of F in both C49- and C54-TiSi2 are rather high, and (3) most of fluorine atoms in TiSi2 layer escaped into ambient during 900–1000 °C annealing. The end-of-range defects were found to be not significantly affected by the TiSi2 formation. In samples annealed at 700–800 °C, the sheet resistance values were found to be the same in spite of the difference in distribution of doping impurities and defect structure.
Thermal donor annihilation and defect production in n-type silicon by rapid thermal annealing
Authors: Tokuda Yutaka , Kobayashi Nobuji , Usami Akira , Inoue Yajiro , Imura Makoto
Published: July 2002

Thermal donor (TD) introduction and annihilation in magnetic-field-applied Czochralski-grown (100) n-type silicon with an oxygen concentration around 8.5×1017 cm−3 have been studied with deep-level transient spectroscopy (DLTS), capacitance-voltage (C-V), and resistivity measurements. The introduction of TDs has been performed with heat treatment at 450 °C, and the annihilation of TDs has been investigated with rapid thermal annealing (RTA) in the temperature range 600–1080 °C. The change of the resistivity with heat-treatment time at 450 °C corresponds to that of the DLTS signal observed around 55 K, identified with TDs. C-V measurements show the decreasing thermal-donor depth profile with shallower depth. TDs are found to disappear by RTA around 800 °C. However, four electron traps, A1 (Ec −0.18 eV), A2 (Ec −0.25 eV), A3 (Ec −0.36 eV), and A4 (Ec −0.52 eV), are induced by RTA above around 700 °C. These traps are induced only by RTA without thermal donor introduction. The significant feature of these RTA-induced defects shows the spatial distributions across the wafer. Traps A2, A3, and A4 have W-shaped patterns, and a trap A1 has an M-shaped pattern. It is suggested that RTA-induced defects are caused by the thermal stress due to the temperature nonuniformity across the wafer during RTA.
Optimum cavity length for high conversion efficiency quantum well diode lasers
Authors: Bour D. P. , Rosen A.
Published: July 2002

The cavity length which maximizes the peak power conversion efficiency is determined for quantum well diode lasers. These calculations are based upon simple models of the diode injection laser’s electrical and optical behaviors, including saturation in the quantum well gain-current characteristic. Here the influences of the distributed optical cavity loss, electrical resistivity, and facet reflectivity on the optimum cavity length are described. Although a lower facet reflectivity results in increased threshold current, there are advantages to longer devices, as the peak conversion efficiency is not reduced. Since the optimum cavity length is greater for low reflectivity, the diode series resistance is smaller. Furthermore, when operating at the point where conversion efficiency is a maximum, the power output of the device with low facet reflectivity exceeds that of the device with higher facet reflectivity. Therein lies the principle advantage of reduced front-facet reflectivities in high power, high efficiency quantum well diode lasers. Good agreement results when these predictions are applied to a strained InGaAs/AlGaAs single quantum well laser (λ=0.93 μm).
Magnetic and structural properties of iron/amorphous iron alloy multilayers
Authors: de Wit H. J. , Dirne F. W. A. , Witmer C. H. M.
Published: July 2002

The magnetic and structural properties of ion-beam and rf sputtered Fe/amorphous FeCrB multilayers were studied as a function of Fe and FeCrB layer thicknesses. The material is magnetically soft [μ (10 MHz)≊2000, Hc<100 A/m] and has a saturation magnetization ≊1.8 T if the FeCrB layer thickness is larger than 1 nm and the Fe thickness substantially lower than the domain wall thickness in Fe, 55 nm. The magnetostriction is dependent on the ratio of Fe and FeCrB layer thicknesses. Magnetostriction close to zero can be achieved, even after annealing to 350 °C. Domain patterns show the presence of a weak magnetic anisotropy.
Effect of laser-irradiation on structural and electrical properties of CdS thin films
Authors: Dawar A. L. , Shishodia P. K. , Chauhan Gayatri , Kumar Anil , Mathur P. C.
Published: July 2002

CdS films were deposited by a resistive heating technique onto glass substrates kept at 150 °C. The films were irradiated with laser pulses of various energy densities. A pulsed laser (Nd-doped yttrium aluminum garnet) capable of producing a 20-ns pulse of 0.53 μm wavelength (frequency doubled) with varying energy densities (2–50 mJ/cm2 ) was employed. X-ray diffraction studies showed that the crystallinity of the films improved with laser irradiation. dc conductivity and Hall coefficient measurements were made on the films in the temperature range 77–300 K for both as-grown and laser-irradiated films. It was observed that both the Hall coefficient and mobility increased with an increase of energy density as well as the number of pulses. Typically, the mobility increased from 71 to 121 cm2 V−1 s−1 after irradiation with 50 laser pulses of energy density 28 mJ/cm2 .
Measurements of electrode temperature evolution by laser light reflection
Authors: Kempkens H. , Byszewski W. W. , Gregor P. D. , Lapatovich W. P.
Published: July 2002

The electrode temperature rise during the starting of an arc discharge lamp has been measured using the temperature-dependent reflection coefficient of He-Ne laser light from metal surfaces. Lock-in techniques permit temporal resolution of about 3 ms, which is adequate to resolve 60-Hz phenomena such as electrode heating during the ac cycle. Calibration curves obtained by pyrometric measurements of constant-current tungsten ribbon lamps were used to relate the observed signal during electrode heating to its temperature. The technique described affords a nonintrusive method for measuring transient electrode temperature in an optically clustered environment with an accuracy of about 20%.
NL10 defects formed in Czochralski silicon crystals
Authors: Hara Akito , Hirai Iesada , Ohsawa Akira
Published: July 2002

The electron spin resonance of defects formed in high-resistivity Czochralski silicon crystals annealed at 470 °C were observed. Defects with C2v symmetry in nitrogen in-diffused crystals annealed for less than about 50 h were observed. With annealing for more than about 50 h, new electron-spin-resonance (ESR) spectra made up of C2v and nearly isotropic defects were evident. Only nearly isotropic defects were observed in non-nitrogen-doped crystals. It is believed that defects with C2v symmetry are oxygen-nitrogen complexes. Strong similarities between oxygen-nitrogen complex and previously reported NL10, in terms of C2v symmetry, g values, the dependence of g values on annealing time, the temperatures at which complexes are formed, the absence of hyperfine interactions in ESR measurement, and the character of shallow energy levels suggest that they may be the same defect.
Island growth and coarsening in thin films—conservative and nonconservative systems
Authors: Atwater Harry A. , Yang Chih M.
Published: July 2002

A model is developed for time-dependent growth and coarsening of islands in conservative and nonconservative thin-film systems with substrate coverages in the range 0<Φ<0.5. Solution of the continuity equation, for both matter-conservative and -nonconservative systems, coupled with mean-field or effective-medium rate laws for island coarsening, allows the evolution of arbitrary island size distributions to be modeled. For matter conservative systems with substrate coverage of Φ=0, all distributions investigated approach the dynamically scaling distribution and the steady-state power-law coarsening rate predicted by analytic theory following from the work of Lifschitz and Slyozov [J. Phys. Chem. Solids 19, 35 (1961)] and Wagner [Z. Electrochem. 65, 581 (1961)]. When it is assumed that Φ>0 but constant during coarsening, a dynamically scaling distribution and a steady state are also approached asymptotically. The form of the initial distribution has a significant effect on the coarsening kinetics in the transient regime before a dynamically scaling distribution is achieved. When the substrate coverage is allowed to vary for a conservative system, the coverage decreases monotonically with time. Simultaneous thin-film growth and coarsening in nonconservative systems (e.g., by vapor deposition, low-energy ion bombardment) is also examined. In particular, the relationship between growth and coarsening kinetics and mechanisms for ion-surface interactions is discussed.
A Hugoniot theory for solid and powder mixtures
Authors: Krueger Barry R. , Vreeland Thad
Published: July 2002

A model is presented from which one can calculate the Hugoniot of solid and porous two-component mixtures up to moderate pressures using only static thermodynamic properties of the components. The model does not presuppose either the relative magnitude of the thermal and elastic energies or temperature equilibrium between the two components. It is shown that for a mixture, the conservation equations define a Hugoniot surface and that the ratio of the thermal energy of the components determines where the shocked state of the mixture lies on this surface. This ratio, which may strongly affect shock-initiated chemical reactions and the properties of consolidated powder mixtures, is found to have only a minor effect on the Hugoniot of a mixture. It is also found that the Hugoniot of solids and solid mixtures is sensitive to the pressure derivative of the isentropic bulk modulus at constant entropy.
Modeling the dc superconducting quantum interference device coupled to the multiturn input coil
Authors: Enpuku K. , Yoshida K.
Published: July 2002

In order to study the effect of a multiturn input coil on superconducting quantum interference device (SQUID) characteristics, the radio frequency (rf) properties of the coupling circuit between the SQUID coil and the input coil are studied. For the measurement of rf properties of the coupling circuit, the so-called expanded model of the coupling circuit using normal metals was adopted, which is shown to be very useful for this purpose. It is shown that the SQUID coil cannot be expressed by a simple inductance when the SQUID coil is coupled to the input coil, i.e., many resonant structures are observed in rf properties of the coupling circuit. It is also shown that the damping resistance is useful in suppressing the resonant structures. Using the experimental results, a circuit model of the SQUID coupled to the multiturn input coil is proposed.
Positron beam defect profiling of silicon epitaxial layers
Authors: Schut H. , van Veen A. , van de Walle G. F. A. , Gorkum A. A. van
Published: July 2002

Epitaxial layers of silicon grown on a Si(100) substrate by molecular-beam epitaxy (MBE) and solid-phase epitaxy (SPE) have been investigated by slow positron beam analysis methods. Results of Doppler broadening measurements revealed that the S parameter of the SPE material is considerably higher than the value measured for the MBE layer, indicative of a higher concentration of open-volume defects in the former material. This was confirmed by measurements of the positronium fraction at elevated temperatures.
Three-dimensional theory to study photothermal phenomena of semiconductors. I. Modulated optical reflectance
Authors: Cheng Jian-chun , Zhang Shu-yi
Published: July 2002

A thorough theoretical study of the modulated optical reflectance (MOR) response of semiconductor samples has been presented. The three-dimensional plasma and thermal equations are solved analytically, and then numerical calculations are given for the high-frequency MOR signal. The influences of the optical, thermal, and electronic parameters on the MOR signal are discussed in detail. The generation mechanisms of the MOR signal will be clarified. On the other hand, the mathematical analyses also provide a quantitative tool by which one can fit the theoretical curves to experimental data to determine several material parameters, such as the surface recombination velocity, the lifetime of photogenerated carriers, and thermal conductivity of the materials.
An analysis of the Pr contribution to the spin-reorientation phenomenon in PrCo5
Authors: Ma B. M. , Kawai T. , Sankar S. G. , Wallace W. E.
Published: July 2002

Spin-reorientation phenomenon in the PrCo5 compound has been analyzed over the temperature range 0 to 300 K using a Hamiltonian consisting of the crystal-field term and the 4f–3d exchange interaction. We have performed a careful analysis by including the second- and the fourth-order interaction terms to the crystal-field Hamiltonian. In general, the crystal-field B02 term favors the planar anisotropy while B04 term promotes an axial anisotropy. The combination of these two terms yield the off-axial anisotropy of the Pr sublattice. The magnitudes of B02 and B04 contribution are relatively strong at low temperatures and decrease considerably with increasing temperature. The contribution of the B02 term is more pronounced than that of the B04 term throughout the temperature range investigated. The competition between the off-axial anisotropy of Pr sublattice and the axial anisotropy of Co sublattice yields the spin-reorientation transition phenomenon in PrCo5 at 105 K.
Magnetic properties of ultrafine nickel particles
Authors: Du You-wei , Xu Ming-xiang , Wu Jian , Shi Ying-bing , Lu Huai-xian , Xue Rong-hua
Published: July 2002

Ultrafine nickel particles have intriguing physical and chemical properties, which are interesting both in fundamental and applied research. The size of the particle was controlled by gas pressure. X-ray diffraction studies showed that fine Ni particles have fcc crystal structure and are coated with thin Ni oxide on the surface. Electron micrographs showed a spherical particle shape, forming a long chain. Size dependence of magnetic properties were studied. The specific magnetic moment drastically decreases when reducing the diameter d of particles <15 nm. The coercivity Hc also approaches zero when d is about 15 nm. Therefore, we can suppose that the critical size for superparamagnetism at room temperature is about 15 nm. According to the superparamagnetic formula KV=25 kT, the value of the magnetic anisotropy constant can be determined, K = −5.8 × 105 (erg/cc). It is found to be larger than bulk Ni [K=−3.4–5.1 × 104 (erg/cc)], the same as Fe particles. The maximum of coercivity at room temperature is about 250 Oe, that is less than the theory value for the coherent rotation model, Hc = 4 K/3Ms=1600 Oe. The mechanism of reversal magnetization can be understood by the sphere chain model. The critical diameter of the single domain is about 65 nm. Curie temperature Tc obviously decreases for 9-nm average diameter particles rather than bulk. It may be in connection with the lattice contraction.
Magnetic properties of U(Ru1−xRhx)2Si2 single crystals (0≤x≤1)
Authors: Miyako Y. , Kawarazaki S. , Amitsuka H. , Paulsen C. C. , Hasselbach K.
Published: July 2002

The temperature dependence of the nonlinear susceptibility of URu2Si2 has been studied in order to understand the properties of the phase transition at 17.5 K. We have also studied the various magnetic phases of U(Ru1−xRhx)2Si2 as a function of x by means of specific heat, susceptibility, and neutron scattering experiments. Special attention has been paid to x=0.3 where two successive phase transitions occur.
Monte Carlo simulation of ion-channeling spectra from partially damaged crystals
Authors: Nakano Hirohiko , Kido Yoshiaki
Published: July 2002

We have elaborated a new Monte Carlo program to simulate ion-channeling spectra from partially damaged crystals. This program provides the transverse-flux distribution together with the fractions of channeled and dechanneled ions, their average energies, and energy fluctuations at any depth by calculating channeled-ion trajectories under a multistring potential. Thus we can determine the depth profiles of defects and dopant atoms, including preferential lattice positions as well as atomic configurations at interfaces. The present program is applied to deriving the depth profiles of defects induced by 100-, 150-, and 200-keV Ar+ irradiation upon GaP(111) crystals. Overall agreement is obtained between the present results and those determined from an analytic manner and calculated from a modified trim program, which simulates collision cascades in solids.
A comprehensive defect model for amorphous silicon
Authors: Hata Nobuhiro , Wagner Sigurd
Published: July 2002

The density and distribution of dangling-bond defects in a given sample of hydrogenated amorphous silicon (a-Si:H) depends on thermal history and on electron and hole densities. As a consequence, the defect density of a-Si:H devices changes during their operation. Optoelectronic device modeling therefore must be coupled with the modeling of the density and distribution of dangling-bond defects. In this paper such a defect model is presented. The model accounts for the effects of the temperature of film growth, the rate of film growth, the film thickness, light-soaking intensity, time, and temperature, and the temperature and duration of thermal annealing. The model is based on the assumption of a limited pool of defects with a Gaussian distribution of thermal annealing energies. This is the most simple model that agrees reasonably well with a variety of experimental data. The applicability of the model is demonstrated on many examples drawn from measurements of dark and photoconductivity, transmission spectroscopy, and subgap optical absorption.
Separation of the bulk and surface components of recombination lifetime obtained with a single laser/microwave photoconductance technique
Authors: Buczkowski A. , Radzimski Z. J. , Rozgonyi G. A. , Shimura F.
Published: July 2002

An algorithm for separating the bulk and surface components of recombination lifetime obtained via a contactless single laser excitation/microwave reflection decay measurement is presented. The surface recombination component of lifetime is determined by extrapolating the tail portion of the carrier decay curve to the carrier axis. Although the slope of this curve depends on both surface and bulk properties, it is shown that the y intercept depends only on the surface component of lifetime. A wide range of surface lifetimes, corresponding to surface recombination velocities from 102 to 105 cm/s, and bulk lifetimes from a few microseconds to several hundred microseconds can be measured. An experimental verification of the analysis is presented using microwave absorption/reflection measurements on silicon wafers representing a wide variety of bulk and surface lifetime components.
Possible mechanism of proximity effect coupled to spin fluctuation in YBa2Cu3Oy/magnetic manganese oxide/YBa2Cu3Oy junctions
Authors: Kasai Masahiro , Kanke Yoko , Ohno Toshiyuki , Kozono Yuzoo
Published: July 2002

Current-voltage characteristics were investigated for YBa2Cu3Oy/La1−xCaxMnOz or La1−xSrxMnOz(200 nm)/YBa2Cu3Oy junctions. The results are discussed together with electrical and magnetic properties of the barrier materials. Supercurrents passed through a 200-nm-thick magnetic barrier for certain values of x. The barrier materials did not necessarily have high electric conductivity in this region, but the ferromagnetism nearly disappeared. The results suggested the possible mechanism of a novel kind of proximity effect concerned with magnetism of the barrier.
Electronic structures of BaB2O4 and LiB3O5
Authors: Hsu W. Y. , Kasowski R. V.
Published: July 2002

The electronic bands of BaB2O4 (BBO) and LiB3O5 (LBO) have been computed from first principles using the ab initio pseudofunction method. Optical conductivities and densities of states in good agreement with optical measurements and x-ray photoemission are obtained. The gap in BBO results from the borate group to the Ba. In LBO, the gap is from a fourfold coordinated borate to a threefold coordinated borate. This suggests that cluster calculations based on the borate groups alone is a reasonable approximation for LBO, but not for BBO.
Dislocation imaging using transmission ion channeling
Authors: Breese M. B. H. , King P. J. C. , Whitehurst J. , Booker G. R. , Grime G. W. , Watt F. , Romano L. T. , Parker E. H. C.
Published: July 2002

Interface dislocations present in a Si0.85Ge0.15/Si sample have been imaged using the channeling scanning transmission ion microscopy (CSTIM) method with a 2 MeV proton beam 200 nm across. Groups of parallel dislocations gave dark bands of contrast down to ∼1.5 μm across, the contrast arising from dechanneling of the beam by the bent lattice planes. Tilting of the sample caused the band contrast to change and gave quantitative data concerning the local bending of the lattice planes. A low-angle boundary model was developed to describe the effect of the groups of dislocations on the channeling contrast. Channeling and topography contrast were obtained from mesa structures present on the sample. Improvements in the sensitivity of the CSTIM method are discussed. The dislocations in the sample were initially characterized by transmission electron microscopy.
Doping and pressure studies on YbBiPt
Authors: Lacerda A. , Movshovich R. , Hundley M. F. , Canfield P. C. , Arms D. , Sparn G. , Thompson J. D. , Fisk Z. , Fisher R. A. , Phillips N. E. , Ott H.-R.
Published: July 2002

The compound YbBiPt exhibits an extremely large low-temperature C/T (γ∼8 J K−2 mol−1/Yb) which, if due solely to a renormalized effective mass, would make this material the heaviest correlated electron system known to date. In the Kondo model, the very large γ corresponds to a small characteristic energy scale that is expected to be pressure dependent. We have studied the effect of chemical pressure on YbBiPt single crystals by heat-capacity measurements on Y and Lu-doped samples. We have also made preliminary low-temperature measurements under hydrostatic pressure of the heat capacity (300 mK≤T≤2 K, up to 8 kbar) and resistance (30 mK≤T≤1 K, at 16 kbar).
Structural quality and the growth mode in epitaxial ZnSe/GaAs(100)
Authors: Guha S. , Munekata H. , Chang L. L.
Published: July 2002

We have investigated the effect of the initial growth mode on the structural quality and dislocation configuration in epitaxial ZnSe/GaAs(100). We find that a three-dimensional initial growth mode strongly degrades the crystal quality and results in a high density of threading dislocations and short misfit dislocation segments. On the other hand a two-dimensional growth mode, achieved by a Zn exposure treatment of GaAs surfaces, results in few threading dislocations and long misfit dislocations. These differences are explained by postulation of new dislocation generation sites created by island coalescence.
Tunneling emitter undoped quantum-well infrared photodetector
Authors: Bandara K. M. S. V. , Levine B. F. , Asom M. T.
Published: July 2002

An extensive series of measurements and theoretical analysis are presented on an undoped single-quantum-well infrared photodetector. The well is filled by electron tunneling through the thin emitter barrier resulting in several novel characteristics compared with the usual directly well-doped detectors. These include a unity optical gain and a dramatic drop in the well carrier density (and responsivity) at high bias when the bound state in the well drops below the emitter conduction-band edge.
Hygrothermal stresses in a solid: Constant surface stress
Authors: Chu J. L. , Lee Sanboh
Published: July 2002

The hygrothermal stresses in a solid with a boundary condition of constant surface stress have been investigated. Three simple geometries: thin slab, sphere, and cylinder, have been considered. The effect of geometry on temperature distribution and moisture absorption inside the solid from small to large follows the sequence: thin slab, cylinder, and sphere. If the temperature on the surface is a function of time whereas the moisture on the surface remains constant, the location of plastic deformation appears in the interior region during the coupled transfer process. On the other hand, when the moisture absorption is a function of time whereas the temperature is kept constant, the plastic deformation always occurs on the surface if possible. The maximum shear stress (σz−σr)/2 of cylinder is similar to the maximum shear stress σyy/2 of thin slab, and the maximum shear stress (σr−σt)/2 of sphere is similar to the maximum shear stress (σr−σθ)/2 of cylinder.
Scaling laws for diamond chemical-vapor deposition. II. Atomic hydrogen transport
Authors: Goodwin D. G.
Published: July 2002

Scaling relations are developed to allow estimation of the atomic hydrogen concentration at the substrate during diamond chemical-vapor deposition for both diffusion-dominated and convection-dominated reactors. In the convection-dominated case, it is shown that there exists an optimal Mach number which maximizes the H concentration delivered to the substrate. In addition, when homogeneous recombination is taken into account, there exists an optimal operating pressure. This analysis shows that a sonic flow of highly dissociated hydrogen at a pressure near atmospheric is optimal for rapid growth of high-quality diamond.
Structure and magnetic properties of mechanically alloyed SmFe2
Authors: Milham Clive D.
Published: July 2002

Mechanical alloying has been used to prepare SmFe2 from powdered Sm and Fe. A maximum remanence Mri of 40.9 emu/g and a maximum coercivity MHc of 3.09 kOe were measured for samples annealed at 500 and 600 °C, respectively. Above these temperatures both Mri and MHc decrease monotonically with increasing grain size. Annealing at temperatures between 500 and 800 °C lead to the formation of second-phase SmFe3 affecting saturation magnetization Ms, which ranges from 55.7 to 61.9 emu/g, and resulted in constriction of the hysteresis curves. Activation volumes v were determined from the results of magnetic viscosity measurements and range from 3.3×10−18 to 10.5×10−18 cm3 for samples annealed at 500 and 800 °C, respectively. An estimate of the anisotropy energy coefficient K of 5.6×106 ergs/cm for mechanically alloyed SmFe2 was calculated using an estimate of the exchange energy coefficient A and the experimentally determined value of v.
The role of Cu distribution and Al2Cu precipitation on the electromigration reliability of submicrometer Al(Cu) lines
Authors: Colgan E. G. , Rodbell K. P.
Published: July 2002

The microstructure and Cu distribution were determined for blanket and patterned Al(≤4 wt % Cu) thin films as a function of annealing. The growth of Θ-phase (Al2Cu) precipitates in blanket and patterned submicrometer-wide lines was quantified along with the Cu concentrations within the Al grains. The reliability of 0.5-μm-wide lines was found to be strongly influenced by the details of the annealing sequence; however, 1-μm-wide lines were less affected. The difference in the electromigration behavior of 0.5-μm-wide lines is shown to be due to the different film microstructures formed in the patterned lines as a function of annealing history. Specifically, the amount of Cu in the grains and the size and distribution of the Θ-phase precipitates were found to be a strong function of both the annealing conditions and the linewidth in 0.5–1-μm-thick Al(Cu) films.
Thermal waves in materials with linearly inhomogeneous thermal conductivity
Authors: Fivez Jan , Thoen Jan
Published: July 2002

The problem of plane thermal waves in media with linearly varying thermal conductivity κ in the absence of internal heat sources is rigorously solved in terms of Bessel functions. An exact solution in the presence of an internal heat source arising from optical absorption is also presented. Focus is on the semi-infinite solid. In particular, an inverse calculation procedure is outlined that is able to yield, in a very fast way, a linear κ profile that reveals the most important aspects of the actual profile.
Two-photon absorption laser-induced fluorescence of H atoms: A probe for heterogeneous processes in hydrogen plasmas
Authors: Tserepi Angeliki D. , Miller Terry A.
Published: July 2002

Two-photon absorption laser-induced fluorescence has been used to obtain the spatial distribution of H atoms in the interelectrode space of a parallel plate rf reactor. Continuous and pulsed operation of the discharge allows the monitoring of H atoms, which are produced at the sheath boundary and are removed on the confining surfaces, at rates that depend on the conditions of the discharge and the nature of the surfaces. A variety of metallic and semiconducting surfaces have been loaded on the ground electrode resulting in widely varying H concentration decay rates and concentration gradients characteristic of the surfaces, which range from significantly absorbing to nearly reflecting. A simple analytical model that simulates spatial and temporal profiles determines from the experimental data surface loss coefficients γ in the range 0.1%–20% for H atoms. Finally, we demonstrate the sensitivity of the method in probing, in situ and in real-time, surface modifications induced by energetic plasmas.
Relation between electrical properties and microstructure of YBa2Cu3O7−x thin films deposited by single-target off-axis sputtering
Authors: Westerheim A. C. , Anderson Alfredo C. , Oates D. E. , Basu S. N. , Bhatt D. , Cima M. J.
Published: July 2002

The relationship between the deposition conditions and the structural and electrical properties of in situ superconducting YBa2Cu3O7−x thin films deposited by off-axis magnetron sputtering has been investigated. High-quality films have been produced with a transition temperature TC (R=0) of 92 K, a critical current density JC (zero field) of 3.3×107 A/cm2 at 4.2 K and 4.8×106 A/cm2 at 77 K, and a microwave surface resistance RS of 2.6×10−6 Ω at 1.5 GHz and 4.2 K which rises to 8.3×10−6 Ω at 77 K. Among the deposition conditions explored, substrate temperature was identified as the most influential in producing these high-quality films. A quantitative relationship was established between substrate temperature and TC, normal-state resistivity ρ, JC, orientation distribution, x-ray-diffraction peak broadening, lattice expansion, RS, and penetration depth λ. Increasing substrate temperature results in an increase in TC, a decrease in ρ, an increase in JC, an increase in grain size, an increase in the ratio of c-axis- to a-axis-oriented grains, and a decrease in λ. The deposition conditions of high substrate temperature and oxygen pressure, used to form films of the highest electrical and structural quality, also promote the formation of CuO precipitates of about 1 μm in dimension, resulting from a slightly copper-rich stoichiometry.