bluuebunny/crossref_metadata_2025 huggingface dataset:
Effect of beam dwell time on surface changes during laser scribing
Authors: Rauch G. C. , Krause R. F.
Published: July 2002

Laser scribing can be used to reduce the ferromagnetic domain size, and thus core loss, in grain-oriented electrical steels. The localized stress required for domain-size control can be produced either by shock deformation, optimally associated with beam dwell times (pulse lengths) less than 2×10−8 sec, or by thermal stressing occurring with dwell times optimally longer than 1×10−4 sec. In this paper we show that comparable core loss improvements can be obtained at greatly different dwell times, but that the associated changes in the surface condition of coated silicon steel laser scribed for maximum core loss reduction vary dramatically as dwell time varies. At the very short times characterizing shock deformation, achieved using a Q-switched solid-state laser, the coating is almost completely removed, but there is little effect on the steel itself, and recoating has been used to restore coating integrity. Under optimum dwell times for thermal expansion effects, achieved using continuous-wave radiation, there is no significant effect on the coating, and no postscribing treatment is required. At intermediate dwell times, however, significant core loss reduction is associated with extensive disruption of the coating and with melting of the steel. At dwell times of approximately 1×10−6 sec using CO2 laser radiation, there is an unstable transition between the shock (vaporization) and heating modes, with intermittent melting.
New platinum silicide formation method using reaction between platinum and silane
Authors: Takahashi Yasuo , Ishii Hiromu , Murota Junichi
Published: July 2002

The new Pt silicide formation method using the reaction between Pt film and SiH4 has been proposed. It has been found that Pt silicide is formed by the reaction with SiH4 at a low temperature range of 250–400 °C. Parabolic relationships of silicide growth using the reaction with SiH4 as well as the growth using the reaction between Pt and substrate Si are confirmed. Pt silicide formation with SiH4 is less influenced by oxygen contamination than the formation by the reaction between Pt and substrate Si.
Arc resistance of laser-triggered spark gaps
Authors: Kushner M. J. , Kimura W. D. , Byron S. R.
Published: July 2002

In the use of spark gaps as switching devices, it is desirable to maximize the power delivered to the load and to minimize the power deposited in the switch; that is, it is desirable for the resistance of the switch to be negligible as compared to the load. The hydrodynamic time scale for expansion of the arc in a spark gap and hence for the reduction in its resistance to a small value is tens to hundreds of nanoseconds. Therefore, with current pulses of duration of a few hundred nanoseconds or less, the resistance of the spark gap may be a significant fraction of that of the load. In this paper, we report on measurements that determine the resistance of the arc in a fully diagnosed laser-triggered spark gap. The spark gap switches a 100-ns, 1.5-Ω waterline into a 1.5-Ω load resistor. A capacitive voltage divider housed within the switch enclosure measures the voltage drop across the switch, a current-viewing resistor measures the current, and an interferometer measures the diameter of the plasma column, a value required to calculate its inductance. The resistance of the arc is found to remain in excess of 0.1–0.2 Ω for the duration of the current pulse for a variety of switch gas mixtures. The resistance decreases with increasing charging voltage on the waterline at the time of triggering and decreases with decreasing average molecular weight of the gas mixture in which the arc is sustained.
Application of digital image analysis to pattern formation in polymer systems
Authors: Tanaka Hajime , Hayashi Takafumi , Nishi Toshio
Published: July 2002

Digital image analysis (DIA) is applied to extract various types of physical information from an image of high-order structures in polymer systems observed under an optical microscope. DIA can be applied to analyze the pattern formation phenomena in general. Various numerical operations in real and wave-number space make it possible to extract physical information unobtainable by other methods, such as the use of light scattering. The possibility of valuable operations in DIA are presented with examples. The technique itself is useful not only in polymer systems but also in various fields such as studies of liquid crystals, biological systems, metals, and pattern formation in solution or chemical reaction.
Generation of lithium negative ions in a volume source with optical pumping
Authors: McGeoch M. W. , Schlier R. E.
Published: July 2002

An experiment is described in which a high density of lithium negative ions (1×1010 cm−3) is generated by dissociative attachment of electrons to optically pumped lithium molecules in a supersonic expansion. During a 3-μs period up to 7% of electrons are attached. A measurement of the lithium vibrational temperature is also reported. Theoretical aspects of Li− production in both a static multicusp source and a supersonic beam source are discussed. The optical pumping requirements for a continuous Li− supersonic beam source are derived.
Grain growth and alignment in hot deformed Nd-Fe-B magnets
Authors: Mishra Raja K. , Brewer Earl G. , Lee Robert W.
Published: July 2002

Formation of grain oriented Nd-Fe-B magnets from melt-spun ribbons by hot deformation has been studied using electron microscopy. It is shown that deformation and alignment of Nd2Fe14B magnets result from a combination of plastic deformation, grain boundary migration, and grain boundary sliding. Due to a limited number of available slip systems in this material, samples with large grains deform less easily. Small grain size materials, as encountered in the melt-spun ribbons, are well suited for die-upsetting to produce oriented magnets.
Effective second-order elastic constants of a strained cubic crystal in the finite strain theory
Authors: Rao R. Ramji , Padmaja A.
Published: July 2002

Expressions for the effective second-order elastic constants of a strained cubic crystal have been obtained using the finite strain theory of Murnaghan, in terms of the higher-order elastic constants of its natural state. These formulae are found to be different from those obtained by the earlier workers. These expressions are used to obtain the pressure derivatives of the effective second-order elastic constants of some cubic crystals for which experimental second- and third-order elastic constants are available. The second-order anharmonic contribution to the pressure derivatives of C111 and C112 of copper, silver, and gold is calculated from a knowledge of the estimated values of the fourth-order elastic constants of these metals.
Anomalous behavior of magnetization for PtMnSb/CuMnSb multilayer films
Authors: Takanashi K. , Watanabe M. , Fujimori H.
Published: July 2002

Metallic multilayer films consisting of ferromagnetic PtMnSb and antiferromagnetic CuMnSb were prepared by an rf sputtering method. From the x-ray diffraction data, all the films were found to have a [111] texture of the C1b -type Heusler structure. The magnetization measured in PtMnSb/CuMnSb multilayers was larger than that in a PtMnSb compound. The temperature dependence of magnetization shows a bend at about 50 K near the Néel temperature of CuMnSb. This bend is explained qualitatively by using a molecular field model and assuming a trapezoidal modulation of exchange interaction.
Oxidation of HF-treated Si wafer surfaces in air
Authors: Gräf D. , Grundner M. , Schulz R. , Mühlhoff L.
Published: July 2002

The change in the chemical surface state of polished Si wafers [p-type, (100) oriented] during storage in air at room temperature was investigated for storage times up to half a year. Measurements were performed by x-ray Photoelectron Spectroscopy (XPS) and High Resolution Electron Energy Loss Spectroscopy (HREELS). Immediately after the HF treatment (1 min 5% HF, 2 min water rinse) vibrational spectroscopy (HREELS) shows a predominant coverage of the surface with hydride groups (80%–90% of a ML), which can be inferred from the presence of the stretching (2100 cm−1), scissor (900 cm−1) and bending (640 cm−1) vibrations in the spectra. A slight additional coverage with oxygen is proved by XPS and originates from Si-OH groups (3670 cm−1) and oxygen-related hydrocarbon groups (XPS). These Si-OH groups result from an exchange reaction of Si-F with water during the two-minute water rinse. The development of an oxygen coverage during subsequent storage in air occurs extremely slowly and shows a logarithmic behavior. A monolayer coverage of oxygen (7×1014/cm2) is reached after approximately 7 days of storage in air. HREELS spectra exhibit the concurrent development of the asymmetric Si-O-Si vibration, which indicates that oxygen penetrates the lattice and breaks Si—Si bonds. During this period the Si-O-Si frequency shifts from about 1060 to 1100 cm−1.

The penetration of backbonds of Si—H gets evident by broadening of the Si-H stretching vibration and finally by a shift to higher wavenumbers. Chemically shifted components of the Si 2p line (partially oxidized Si) are present with the SiO2−x component (chemical shift ≳3.4 eV) becoming dominant after roughly a week. Further oxidation proceeds essentially by an increase of the SiO2 peak in combination with a steeper slope of the logarithmic growth curve. The SiO2 thickness after half a year is about 8 Å. The frequency of the Si-O-Si vibration shifts up to 1120 cm−1, which can be related to a growing angle of the Si-O-Si bridge. Si—H groups are still present, the final peak position is about 2220 cm−1. The measurements show an extended induction period until the monolayer range of oxide coverage is attained. We ascribe this to the passivation of the surface by hydrogen and to a HF treatment according to Very-Large-Scale-Integration standards.
Optically detected magnetic resonance studies of neutron-transmutation-doped GaP
Authors: Liesert B. J. Heijmink , Godlewski M. , Gregorkiewicz T. , Ammerlaan C. A. J.
Published: July 2002

A direct proof of neutron transmutation doping (NTD) of GaP is presented on the basis of optically detected magnetic resonance (ODMR). GaP:S samples grown by the liquid- encapsulated Czochralski method were irradiated with thermal neutrons and subsequently annealed at 800 °C. In the ODMR experiments the transmuted Ge substitutional on Ga sites was detected. The NTD process was also found to create deep acceptors; these are tentatively identified as associates of gallium vacancies (VGa) and germanium donors on gallium sites (GeGa). Such identification requires that some of the structural defects (vacancies) created by β and γ recoil during transmutation are stabilized by forming VGa- GeGa complexes.
Magnetic properties of high-density Mn-Zn ferrites
Authors: Pannaparayil T. , Marande R. , Komarneni S.
Published: July 2002

Subdomain size superparamagnetic particles of Mn-Zn ferrites, having a narrow particle size distribution (peaked around 10 nm) were prepared under mild hydrothermal conditions. High densities (≳99.5%) have been obtained upon sintering these green ferrite powders at relatively low temperatures (∼1000 °C), without the use of any sintering aids. Permeability spectra and other magnetic and electrical properties of the sintered ferrites were studied.
Elastic scattering of electrons and positrons by bound gallium and arsenic atoms
Authors: Öztürk N. , Williamson W. , Antolak A. J.
Published: July 2002

Elastic differential and total cross sections for positrons, from 5 eV to 10 keV, and for electrons, from 50 eV to 10 keV, scattered by bound gallium and arsenic atoms have been calculated using the method of partial waves. The optical potential consists of a static potential, a Buckingham-type polarization potential, and the Mittleman–Watson exchange potential (for electrons). Low-order partial wave phase shifts are computed by numerical integration of the radial part of the Schrödinger equation. For higher orders, a closed analytic expression is obtained in the first Born approximation for the phase shifts. The calculated total cross sections are parametrized in terms of the screened Rutherford cross section to be used in Monte Carlo transport codes.
Intrinsic stress in hydrogenated amorphous silicon deposited with a remote hydrogen plasma
Authors: Stevens K. S. , Johnson N. M.
Published: July 2002

In hydrogenated amorphous silicon it is demonstrated that the maximum compressive intrinsic stress correlates with the optimum electronic properties. Undoped films were deposited over a range of temperatures in a remote hydrogen plasma (RHP) reactor and, for comparison, in a rf glow discharge (GD) system. The dependence of the stress on deposition temperature is qualitatively identical for the two reactors. Quantitatively, both the maximum compressive stress and the optimized electronic properties (e.g., minimum defect density) are obtained at 400 °C for the RHP films and near 250 °C for the GD films. Additionally, it is demonstrated that the transition from amorphous to microcrystalline silicon, induced by high hydrogen dilution, is accompanied by a reduction in compressive stress. Formation of compressive stress during RHP growth is ascribed to the insertion of hydrogen into the rigid silicon network immediately beneath the growing surface.
Thickness and sound velocity measurement in thin transparent films with laser picosecond acoustics
Authors: Wright O. B.
Published: July 2002

A new thin-film optical testing method is described for both thickness and sound velocity of transparent films on opaque substrates with laser picosecond acoustics using the optical pump-probe technique. The theory of excitation and detection of ultrasonic stress pulses for this geometry is presented in detail together with experimental results for sputtered thin films of silica of thickness 200 nm–2 μm on amorphous germanium substrates. Reflectance variations, measured as a function of pump-probe delay time, are characterized by echoes and beating oscillations superimposed on periodic steplike changes. These effects are modeled as a sum of an echo contribution from the stress-induced modulation of the substrate reflectance, an interference contribution from the light reflected by the moving stress pulse in the transparent film, a contribution from the modulation of the light on transmission through this stress pulse, and a contribution from the stress-induced ultrafast vibrations of the film interfaces of order 10−3 nm. The latter contribution arises from thin-film interference effects that represent a novel detection mechanism for surface vibrations in the picosecond regime. Sound velocity and thickness are derived from the data to an accuracy of a few percent, and the photoelastic constant of the transparent film is determined.
The elastic interaction between a moving screw dislocation and a crack
Authors: Tsai Yu-Zen , Lee Sanboh
Published: July 2002

We investigate the elastic interaction between a moving screw dislocation and a crack. The stress intensity factor at the crack tip decreases with the increasing velocity of the screw dislocation. This implies that the shielding effect of the moving screw dislocation on crack propagation is reduced. The image force on the moving screw dislocation arising from the crack surface decreases with the increasing velocity of the screw dislocation itself. Finally, Newton’s third law is not valid in the system.
Application of permittivity tensor for accurate interpretation of magneto-optical spectra
Authors: Allen Gary A. , Dionne Gerald F.
Published: July 2002

The inaccuracies of widely used approximations to the theory of the off-diagonal elements ε1= ε′1 + iε″1 of the permittivity tensor are examined. Errors occurring in the ‘‘diamagnetic’’ case can become important when curve fitting is employed to extract parameter values from Bi-substituted magnetic garnets. A discrepancy between the exact and approximate forms of the ε′1 relation (which is responsible for Faraday rotation and Kerr ellipticity at lower energies) as a function of transition energy is found in the low-energy (wavelength λ∼1 μm) tails of the lines and originates from the dropping of a nonresonant term from the complete theory. Errors also occur in the line shapes near resonance where the ratio of the excited-state splitting to the Lorentzian linewidth approaches unity (Δ/Γ→1). A family of curves with Δ/Γ as a parameter is presented, and an exact expression relating Δ/Γ to the resonance value of ε′1/ε″0 (where ε″0 is the imaginary part of the diagonal element ε0) is derived.
Hydrogen-atom spectroscopy of the ionizing plasma containing molecular hydrogen: Line intensities and ionization rate
Authors: Sawada Keiji , Eriguchi Kouji , Fujimoto Takashi
Published: July 2002

Our previous calculation of the excited atom population [T. Fujimoto, K. Sawada, and K. Takahata, J. Appl. Phys. 66, 2315 (1989)] is revised on the basis of new assessment of the cross sections for excitation from atomic hydrogen and dissociative excitation from molecular hydrogen. The effective ionization rate of molecular hydrogen is also calculated by the method of the collisional-radiative model. Its ratio to the Balmer α photon emission rate is higher than that for the atomic hydrogen case by more than one order of magnitude.
Comment on ‘‘An ecological optimization criterion for finite-time heat engines’’ [J. Appl. Phys. 69, 7465 (1991)]
Authors: Yan Zijun
Published: July 2002

It is pointed out that the loss power is T0σ such that the ecological function is E=P−T0σ for a heat engine.
Tribological studies of silicon for magnetic recording applications (invited)
Authors: Bhushan Bharat , Koinkar Vilas N.
Published: July 2002

In the present study our objective is to investigate whether the friction and wear performance of bare silicon is adequate for disk drive application or whether certain coatings/treatments are necessary for low friction and wear. Macrotribological experiments have been performed with various pin/slider materials and magnetic disks in a modified disk drive. Microtribological studies have also been conducted on silicon using a friction force microscope. Based on macrotests, we found that the friction and wear performance of bare silicon is not adequate. With single and polycrystalline silicon, transfer of amorphous carbon from the disk to the pin/slider and oxidation-enhanced fracture of pin/slider material followed by oxidation of the transfer coating (tribochemical oxidation) is responsible for degradation of the sliding interface and consequent friction increase in ambient air. With dry-oxidized or plasma-enhanced chemical-vapor deposition-SiO2-coated silicon, no significant friction increase or interfacial degradation was observed in ambient air. In the absence of an oxidizing environment (in dry nitrogen), the coefficient of friction decreased from 0.2 to 0.05 following amorphous carbon transfer for the materials tested. Nanoscratching/nanowear and nanoindentation studies also indicate that coated silicon is superior to bare silicon. Macro- and microcoefficient of friction values of all samples is found to be about the same with the microvalues lower than the macrovalues. Based on this study, we conclude that coated silicon is an excellent candidate for the construction of magnetic head sliders.
Plate impact response of ceramics and glasses
Authors: Raiser G. F. , Wise J. L. , Clifton R. J. , Grady D. E. , Cox D. E.
Published: July 2002

Soft-recovery plate impact experiments have been conducted to study the evolution of damage in polycrystalline Al2O3 samples. Examination of the recovered samples by means of scanning electron microscopy and transmission electron microscopy has revealed that microcracking occurs along grain boundaries; the cracks appear to emanate from grain-boundary triple points. Velocity-time profiles measured at the rear surface of the momentum trap indicate that the compressive pulse is not fully elastic even when the maximum amplitude of the pulse is significantly less than the Hugoniot elastic limit. Attempts to explain this seemingly anomalous behavior are summarized. Primary attention is given to the role of the intergranular glassy phase which arises from sintering aids and which is ultimately forced into the interfaces and voids between the ceramic grains. Experiments are reported on the effects of grain size and glass content on the resistance of the sample to damage during the initial compressive pulse. To further understand the role of the glass, plate impact experiments were conducted on glass with chemical composition comparable to that which is present in the ceramic. These experiments were designed to gain further insight into the possibility of ‘‘failure waves’’ in glasses under compressive loading.
Modeling electronegative plasma discharges
Authors: Lichtenberg A. J. , Vahedi V. , Lieberman M. A. , Rognlien T.
Published: July 2002

A macroscopic analytic model for a three-component electronegative plasma has been developed. Assuming the negative ions to be in Boltzmann equilibrium, a positive ion ambipolar diffusion equation is found. The electron density is nearly uniform, allowing a parabolic approximation to the plasma profile to be employed. The resulting equilibrium equations are solved analytically and matched to an electropositive edge plasma. The solutions are compared to a simulation of a parallel-plane rf driven oxygen plasma for two cases: (1) p=50 mTorr, ne0=2.4×1015 m−3, and (2) 10 mTorr, ne0=1.0×1016 m−3. In the simulation, for the low power case (1), the ratio of negative ion to electron density was found to be α0≊8, while in the higher power case α0≊1.3. Using an electron energy distribution that approximates the simulation distribution by a two-temperature Maxwellian, the analytic values of α0 are found to be close to, but somewhat larger than, the simulation values. The average electron temperature found self-cosistently in the model is close to that in the simulation. The results indicate the need for determining a two-temperature electron distribution self-consistently within the model.
Shock-induced and shock-assisted solid-state chemical reactions in powder mixtures
Authors: Thadhani N. N.
Published: July 2002

Shock compression of powder mixtures can lead to chemical reactions, resulting in the formation of equilibrium as well as nonequilibrium compounds, and rapid increases in temperature. The reactions occur as manifestations of enhanced solid-state chemical reactivity of powders, caused by configurational changes and defect states introduced during shock compression. Two types of reactions are possible and can be distinguished on the basis of their respective process mechanisms and kinetics. Shock-induced chemical reactions occur during the shock-compression state, before unloading to ambient pressure, and in time scales of mechanical equilibrium. In contrast, shock-assisted reactions occur after unloading to ambient pressure, in an essentially shock-modified material, in time scales of temperature equilibration. The mechanisms of shock-assisted reactions include solid-state defect-enhanced diffusional processes. Shock-induced reactions, on the other hand, require mechanisms different from conventional solid-state nucleation and growth processes. The complex nature of deformation of powders has precluded a detailed understanding of the reaction mechanisms of such high-rate reaction processes. Results of controlled experiments, however, suggest that shock-induced chemical reactions involve nondiffusional processes giving rise to mechanochemical effects and solid-state structural rearrangements. Mechanistic concepts that distinguish between shock-induced and shock-assisted chemical reactions are described. The effects of configurational changes introduced during shock compression, and the influence of material properties and shock-loading characteristics on such effects, are analyzed to identify the mechanisms of complex processes leading to chemical reaction initiation and compound formation.
Spectroscopy and green upconversion laser emission of Er3+-doped crystals at room temperature
Authors: Danger T. , Koetke J. , Brede R. , Heumann E. , Huber G. , Chai B. H. T.
Published: July 2002

The spectroscopic parameters of Er3+-doped crystals were determined with regard to the upconversion laser parameters of the green transition 4S3/2→4I15/2. The influence of excited-state absorption on this laser channel was determined. Furthermore, upconversion pump mechanisms using ground-state and excited-state absorption around 810 and 970 nm were investigated by direct measurements of excited-state absorption. The spectroscopic results confirm the pulsed room-temperature laser experiments on the 4S3/2→4I15/2 transition. The lasers based on Er:LiYF4, Er:Y3Al5O12, and Er:Lu3Al5O12 were directly excited into the upper laser level by an excimer laser pumped dye laser in the blue spectral range. In Er:LiYF4, Er:KYF4, and Er:Y3Al5O12, laser action was achieved with two-step upconversion pumping by a Ti:sapphire laser and a krypton ion laser. In the case of the fluorides, the additional pumping with the krypton ion laser was not necessary. The laser emission wavelengths were 551 nm for Er:LiYF4, 561 nm for Er:Y3Al5O12 and Er:Lu3Al5O12, and 562 nm for Er:KYF4. In addition, green quasi-cw laser emission of Er:LiYF4 pumped with an argon-ion laser was realized at room temperature.
Mössbauer spectrometry study of magnesioferrite particles
Authors: Hamdeh H. H. , Xia Z. , Foehrweiser R. , McCormick B. J. , Willey R. J. , Busca G.
Published: July 2002

Fine particles of ferrimagnetic MgFe2O4 were synthesized through a sol-gel supercritical drying procedure, then cold-pressed and heated at 500 and 800 °C. These materials were characterized by Mössbauer spectrometry in the temperature range 25–298 K, and in the presence of a small applied magnetic field. The Mössbauer measurements show that heat treatment results in the formation of a small percentage of α-Fe2O3, an increase in the mean particle size and hyperfine magnetic field, and a decrease in the inversion parameter of the magnesioferrite. An average anisotropy constant equivalent to the bulk magnetocrystalline anisotropy is estimated for particles with diameter D=14 nm.
Epitaxial growth and dielectric properties of BaTiO3 films on Pt electrodes by reactive evaporation
Authors: Yano Y. , Iijima K. , Daitoh Y. , Terashima T. , Bando Y. , Watanabe Y. , Kasatani H. , Terauchi H.
Published: July 2002

Thin films of BaTiO3 have been epitaxially grown on Pt(001)/MgO(100) substrates by reactive evaporation. Structural and electrical properties were investigated as a function of film thickness. In situ reflection high-energy electron diffraction and cross-sectional transmission electron microscope observations have revealed that the BaTiO3 films are epitaxially grown on Pt/MgO substrates from the initial stage without any other phase formation. From the images of an atomic force microscope, it has been found that islands of BaTiO3 are present on the bare Pt surface at the initial stage of deposition; the island structure changes to a continuous layer above 1.2 nm in thickness and BaTiO3 grows in a two-dimensional mode. The lattice parameters and the dielectric properties are dependent on the film thickness. Thermodynamic theory was introduced to explain the thickness dependence of the relative dielectric constant εr. Good agreement between the experimental results and the theoretical calculations leads to the conclusion that the thickness dependencies of the lattice parameters and the dielectric constants are caused by the two-dimensional stress due to the lattice mismatch between Pt and BaTiO3 and/or the difference in the thermal expansion coefficients of BaTiO3 and the MgO substrate.
A scanning tunneling microscope study of a palladium sphere in hydrogen gas: Expansion and surface topology
Authors: Blackford B. L. , Arnold C. S. , Mulhern P. J. , Jericho M. H.
Published: July 2002

A scanning tunneling microscope monitored the time dependence of both the expansion and surface morphology changes of 1–2 mm palladium spheres exposed to 0.5 atm hydrogen gas. The surfaces were initially smooth except for 2–10 nm high steps separating flat terraces 50–150 nm wide. Spheres exposed to hydrogen expanded, during which time the surface buckled forming 30–50-nm-deep and 1000-nm-wide features and the step structures vanished. There were two distinct regimes in the expansion: 25%–30% of the total expansion occurred in the first regime, which only lasted about 5% of the time required to reach 90% of the total expansion; and in the second regime the sphere asymptotically approached the final size. Some samples took ∼500 h to expand, but other samples with only slightly different surface morphology expanded in as little as 8 h. We developed a quantitative model describing the time dependence of the observed expansion in the asymptotic regime by assuming the eventual formation of β-Pd hydride and the elastic properties of the material. The model also suggested the cause of the surface buckling.
Structure and thermal stability of sputtered metal/oxide multilayers: The case of Co/Al2O3
Authors: Morawe Ch. , Zabel H.
Published: July 2002

Co/Al2O3 multilayers have been grown on sapphire (112̄0) substrates by rf sputtering techniques. In situ resistance measurements reveal percolation thresholds of Co between 1.5 and 2.5 nm. Continuous metallic layers appear at thicknesses beyond 3.0 nm. Co layers grow in a polycrystalline manner, whereas Al2O3 layers are amorphous, as inferred from x-ray and transmission electron microscope studies. By fitting reflectivity data, interface roughnesses of only 0.25 nm were obtained. Annealing experiments up to 800 °C show a gradual internal oxidation at the Co/Al2O3 interface, while the overall structure and the multilayer period remain stable with constant interface roughnesses and reflectivity values. At 900 °C the layered structure is partly destroyed and Co forms crystallites thicker than one former Co layer.
Anomalously high entropy change in FeRh alloy
Authors: Annaorazov M. P. , Nikitin S. A. , Tyurin A. L. , Asatryan K. A. , Dovletov A. Kh.
Published: July 2002

The temperature dependences of heat expansion, elastocaloric effect, magnetocaloric effect, and the shift in the critical temperature of the transition due to tensile stress have been measured using samples of the same concentration Fe49Rh51 in the antiferromagnetic-ferromagnetic (AF-F) transition range. Using data on specific heat and magnetocaloric effects, entropy-temperature diagrams for the alloy were made for various magnetic fields. The ratios ΔS(TcH)/TcH, where ΔS(TcH) is the entropy change during transition and TcH is the transition’s critical temperature, were found to be ∼3.98×102 erg/g K2 which is close to the value of the change in the electronic specific heat coefficient Δγ obtained by other researchers. It has been concluded that the change in the electronic part of entropy is the main mechanism of the transition. The phenomenological model is proposed, taking into account the electronic entropy change during the transition. Calculations using the model give values for the main thermodynamic parameters of the transition (free energy change ΔF=1.91×106 erg/g, the critical temperature shift ∂Tc/∂P=3.08×10−9 K cm2/Dyn and ∂Tc/∂H=0.788 K/kOe due to hydrostatic pressure and magnetic field respectively), which are in agreement with experimental data.
Semiconductor ultraviolet detectors
Authors: Razeghi M. , Rogalski A.
Published: July 2002

In this review article a comprehensive analysis of the developments in ultraviolet (UV) detector technology is described. At the beginning, the classification of UV detectors and general requirements imposed on these detectors are presented. Further considerations are restricted to modern semiconductor UV detectors, so the basic theory of photoconductive and photovoltaic detectors is presented in a uniform way convenient for various detector materials. Next, the current state of the art of different types of semiconductor UV detectors is presented. Hitherto, the semiconductor UV detectors have been mainly fabricated using Si. Industries such as the aerospace, automotive, petroleum, and others have continuously provided the impetus pushing the development of fringe technologies which are tolerant of increasingly high temperatures and hostile environments. As a result, the main efforts are currently directed to a new generation of UV detectors fabricated from wide band-gap semiconductors the most promising of which are diamond and AlGaN. The latest progress in development of AlGaN UV detectors is finally described in detail.
A semi-kinematic approach to x-ray diffraction of real crystals with small defects
Authors: Li M. , Ress H. , Gerhard T. , Landwehr G. , Cui S. F. , Mai Z. H.
Published: July 2002

A semi-kinematic approach to x-ray diffraction of real crystals with small defects is proposed by incorporating an extinction function into the kinematic diffraction formula. The effect of multiple reflections is described by the extinction coefficient which can be calculated by Darwin’s recursion relations. In this approach the intensity of the coherent scattering is related to the static Debye–Waller factor E, while the intensity of diffuse scattering is determined by the correlation function of the distortion field. When E=1, i.e., for perfect crystals, it gives exactly the same result as Darwin’s theory does. The feasibility of this approach is demonstrated by investigating the reciprocal space mapping and the crystal truncation rod of a GaAs crystal.
Light emitting mechanism of porous silicon
Authors: Chang I. M. , Chen Y. F.
Published: July 2002

We present photoluminescence (PL) and infrared absorption on sets of porous silicon with peak energy ranging from 1.5 to 1.9 eV prepared at different anodization current density. The effects of the as-grown samples under several different treatments have been investigated. Quite surprisingly, the PL peak positions of all the samples studied shift toward 1.7 eV upon the introduction of more oxygen by annealing in a low-temperature H2 environment then exposing to the air. This behavior indicates that the 1.7 eV emission plays a unique role in the PL spectra of porous silicon. However, samples rinsed in water show only blueshift. From our measurements, we conclude that PL emission in porous silicon contains two components. One is the above-mentioned 1.7 eV peak, which can be correlated to the absorption of O–Si–H bonds. The other component of PL is shown to be sensitive to the strength of the Si–O–Si bond related absorption. Based on the previous reports and the results shown here, a possible PL mechanism in porous silicon is emerging. We propose that PL originates from quantum confinement effect for samples with peak energy lower than 1.7 eV. For samples with PL peak larger than 1.7 eV, the PL results from the fact that after generated in the nanocrystalline silicon by photon pumping, the carriers relax into the surface states and then make a radiative recombination. We studied the evolution of the PL spectra under laser illumination and found that the anomalous behavior in the evolution process can also be explained with our model.
Excited state dynamics and energy transfer processes in YVO4:Er3+ crystals
Authors: Kabro P. , Capobianco J. A. , Ermeneux F. S. , Moncorgé R. , Bettinelli M. , Cavalli E.
Published: July 2002

An extensive investigation of the fluorescence dynamics of the (2H11/2,4S3/2), 4F9/2, 4I9/2, 4I11/2, and 4I13/2 states was carried out for 0.1%, 1%, 2.5%, and 10% Er3+ doped in YVO4. The decay profiles of the luminescence of the 4S3/2 level of Er3+ (1.0%) obey the Inokuti–Hirayama model for energy transfer for an electric dipole–electric dipole interaction in the absence of diffusion among the donors. From the analysis we were able to calculate the critical distance, R0, and it was found to be 5.7 Å: The dipole–dipole coupling parameter was estimated to be 2.9×10−51 m6 s−1 and compared well to other well known systems.
Optimum performance of a regenerative Brayton thermal cycle
Authors: Roco J. M. M. , Velasco S. , Medina A. , Hernández A. Calvo
Published: July 2002

The optimum performance of a regenerative Brayton cycle was analyzed. The model includes external and internal irreversibilities coming from four main sources: coupling to external heat reservoirs, turbine and compressor nonisentropic processes, pressure losses in the heater and the cooler, and the regenerator. In terms of the parameters accounting for each type of irreversibility, explicit numerical results are presented for the maximum efficiency, maximum power output, efficiency at maximum power output, power output at maximum efficiency, as well as for the pressure ratios required for maximum efficiency and maximum power. This analysis could provide a general theoretical tool for the optimal design and operation of real regenerative gas turbine power plants.
Effects of deposition temperature on the properties of hydrogenated tetrahedral amorphous carbon
Authors: Sattel S. , Robertson J. , Ehrhardt H.
Published: July 2002

The properties of hydrogenated carbon films deposited from a highly ionized hydrocarbon plasma beam are studied as a function of deposition temperature. At low temperatures, the films have high sp3 bonding, density, and compressive stress and are very smooth. Two transition temperatures are observed, a lower transition T1 around 250 °C, dependent on ion energy, due to graphitization of C–C bonds, and a higher one T2 at about 450 °C due to the loss of hydrogen. The roughness rises at T1 and falls above T2. These transitions are used to understand the subplantation deposition mechanism. The optical gap varies differently, decreasing gradually across T1 due to ordering of sp2 sites. We also report the temperature dependence of the x-ray diffraction, Raman spectrum, elastic modulus, hardness, substrate adhesion, friction coefficient, refractive index, and paramagnetic defect density. The friction coefficient of ta-C:H is low (0.05–0.1), and is maintained at ambient humidities, unlike for a-C:H. The friction mechanism is attributed to shear-induced graphitization. The spin density is found to decline with deposition temperature and the spin resonance line is argued to be exchange narrowed.
In situ measurement of radiation induced conductivity in oxide insulators during neutron irradiation
Authors: White D. P. , Snead L. L. , Zinkle S. J. , Eatherly W. S.
Published: July 2002

An experimental investigation of the in situ electrical conductivity of Wesgo Al995 polycrystalline alumina at approximately 450 °C has been performed at the high flux beam reactor at Brookhaven National Laboratory. The measured radiation induced conductivity (RIC) was about 10−8 S/m at an ionizing dose rate of 6000 Gy/s. No evidence for permanent radiation induced electrical degradation was observed for an applied electric field of 147 V/mm up to a dose level of ≈1.8 displacements per atom. The effect of neutron irradiation on the electrical properties of two mineral insulated cables was also investigated. The RIC in the MgO insulation of a coaxial and a triaxial cable was measured to be in the range of 6–20 ×10−8 S/m at an ionizing dose rate of ≈ 6000 Gy/s.
Characterization of the local structure of amorphous GaAs produced by ion implantation
Authors: Ridgway M. C. , Glover C. J. , Foran G. J. , Yu K. M.
Published: July 2002

The first report of the structural parameters of amorphous GaAs produced by ion implantation, as determined with extended x-ray absorption fine structure measurements, is presented herein. Relative to a crystalline sample, the nearest-neighbor bond length and Debye–Waller factor both increased for amorphized material. In contrast, the coordination numbers about both Ga and As atoms in the amorphous phase decreased to ∼3.85 atoms from the crystalline value of four. All structural parameters were independent of both implant temperature and ion dose, the latter extending two orders of magnitude beyond that required for amorphization, and as a consequence, were considered representative of intrinsic, amorphous GaAs as opposed to an implantation-induced extrinsic structure.
X-ray diffraction, extended x-ray absorption fine structure and Raman spectroscopy studies of WO3 powders and (1−x)WO3−y⋅xReO2 mixtures
Authors: Kuzmin A. , Purans J. , Cazzanelli E. , Vinegoni C. , Mariotto G.
Published: July 2002

Pure ground tungsten trioxide WO3 and (1−x)WO3−y⋅xReO2 mixtures were studied by x-ray absorption spectroscopy, x-ray powder diffraction and Raman spectroscopy in comparison with hydrogen bronzes HxWO3 and hydrogenated calcium tungstate CaWO4:H. It was found that a grinding of pure WO3 leads to a decrease of the crystallites size and a development of the bluish coloration. The color change was found to be reversible under moderate heat treatment or after storage in oxidizing atmosphere and is attributed to the reduced W5+ ions, located at the surface of freshly ground powder. The (1−x)WO3−y⋅xReO2 mixtures were found to be composed of monoclinic/orthorhombic WO3 and orthorhombic ReO2 phases with a grain boundary containing reduced W5.7+ ions which are mainly responsible for the compound color at low rhenium ion concentrations. In both cases, the W(6−z)+ (0<z⩽1) color centers are responsible for strong optical absorption resulting in the dramatic decrease of the total Raman intensity. The structural models of free surface in pure ground WO3 and bulk WO3/ReO2 intragrain boundary in (1−x)WO3−y⋅xReO2 mixtures are proposed and discussed.
Influence of 90 MeV oxygen ion induced disorder on the magnetotransport in epitaxial La0.7Ca0.3MnO3 thin films
Authors: Ogale S. B. , Ghosh K. , Gu J. Y. , Shreekala R. , Shinde S. R. , Downes M. , Rajeswari M. , Sharma R. P. , Greene R. L. , Venkatesan T. , Ramesh R. , Bathe Ravi , Patil S. I. , Ravikumar R. , Arora S. K. , Mehta G. K.
Published: July 2002

Epitaxial films of La0.7Ca0.3MnO3 have been irradiated with 90 MeV oxygen ions at different dose values ranging from 1011–1014 ions/cm2. The structural, magnetization, and magnetotransport properties have been studied as a function of the ion dose. It is found that the properties change gradually up to a dose of 1013 ions/cm2; however, drastic changes occur when the sample is irradiated at the higher dose of 1014 ions/cm2. Specifically, this sample exhibits a large, nearly temperature independent magnetoresistance in the low temperature regime. The Rutherford backscattering channeling data bring out the presence of defects in the irradiated films. The x-ray diffraction data, the temperature dependence of resistivity and magnetization, and the low temperature magnetic hysterisis data collectively indicate the presence of two different phases in the sample irradiated at 1014 ions/cm2. The surface morphology of this film, observed by atomic force microscopy, exhibits swelling, presumably due to subsurface clustering of point defects. The observed results are analyzed in terms of point defect induced random spin disorder and its effect on the magnetotransport properties.
Model of magnetorheological elastomers
Authors: Davis L. C.
Published: July 2002

Magnetorheological elastomers consist of natural or synthetic rubber filled with micron-sized magnetizable particles. During curing of the elastomer, an applied magnetic field aligns the particles into chains. The shear modulus of the resulting cured material is sensitive to magnetic fields of several kOe magnitude. Such sensitivity to magnetic field makes these materials attractive for applications in automotive mounting components. At large fields (magnetic induction B>1 T), the Fe particles are completely magnetized or saturated. Calculations using finite element analysis show that for typical elastomers the increase in shear modulus due to interparticle magnetic forces at saturation is about 50% of the zero-field modulus. The optimum particle volume fraction for the largest fractional change in modulus at saturation is predicted to be 27%. Calculations of the zero-field shear modulus perpendicular to the chain axis indicate that it does not exceed the modulus of a filled elastomer with randomly dispersed particles of the same concentration.
Coefficient of thermal expansion and elastic modulus of thin films
Authors: de Lima M. M. , Lacerda R. G. , Vilcarromero J. , Marques F. C.
Published: July 2002

The coefficient of thermal expansion (CTE), biaxial modulus, and stress of some amorphous semiconductors (a-Si:H, a-C:H, a-Ge:H, and a-GeCx:H) and metallic (Ag and Al) thin films were studied. The thermal expansion and the biaxial modulus were measured by the thermally induced bending technique. The stress of the metallic films, deposited by thermal evaporation (Ag and Al), is tensile, while that of the amorphous films deposited by sputtering (a-Si:H, a-Ge:H, and a-GeCx:H) and by glow discharge (a-C:H) is compressive. We observed that the coefficient of thermal expansion of the tetrahedral amorphous thin films prepared in this work, as well as that of the films reported in literature, depend on the network strain. The CTE of tensile films is smaller than that of their corresponding crystalline semiconductors, but it is higher for compressive films. On the other hand, we found out that the elastic biaxial modulus of the amorphous and metallic films is systematically smaller than that of their crystalline counterparts. This behavior stands for other films reported in the literature that were prepared by different techniques and deposition conditions. These differences were attributed to the reduction of the coordination number and to the presence of defects, such as voids and dangling bonds, in amorphous films. On the other hand, columnar structure and microcrystallinity account for the reduced elasticity of the metallic films.
Empirical low-field mobility model for III–V compounds applicable in device simulation codes
Authors: Sotoodeh M. , Khalid A. H. , Rezazadeh A. A.
Published: July 2002

A Caughey–Thomas-like mobility model with temperature and composition dependent coefficients is used in this work to describe the dependence of electron and hole mobilities on temperature, doping concentration, and alloy composition. Appropriate parameter sets are given for a large number of III–V binary and ternary compounds, including: GaAs, InP, InAs, AlAs, GaP, Al0.3Ga0.7As, In0.52Al0.48As, In0.53Ga0.47As, and In0.49Ga0.51P. Additionally, physically justifiable interpolation schemes are suggested to find the mobilities of various ternary and quaternary compounds (such as AlxGa1−xAs, In1−xGaxP, In1−xGaxAs, In1−xAlxAs, and In1−xGaxAsyP1−y) in the entire range of composition. The models are compared with numerous measured Hall data in the literature and very good agreement is observed. The limitations of the present model are also discussed. The results of this work should be extremely useful in device simulation packages, which are currently lacking a reliable mobility model for the above materials.
Femtosecond spin dynamics of epitaxial Cu(111)/Ni/Cu wedges
Authors: Koopmans B. , van Kampen M. , Kohlhepp J. T. , de Jonge W. J. M.
Published: July 2002

The femtosecond spin dynamics in ferromagnetic metals is studied using time-resolved magneto-optics on an epitaxial Cu(111)/Ni/Cu wedge. A novel and sensitive double modulation scheme is introduced. The induced Kerr rotation and ellipticity are separately measured, and display a contrasting behavior during the first 500 fs after excitation. This proves the absence of a direct relation between the magneto-optical signal and the magnetization. A systematic study of the spin dynamics along the nickel wedge shows a surprisingly small influence of interface scattering, magnetic anisotropy, and morphology on the dynamics.