bluuebunny/crossref_metadata_2025 huggingface dataset:
A general centroid determination methodology, with application to multilayer dielectric structures and thermally stimulated current measurements
Authors: Miller S. L. , Fleetwood D. M. , McWhorter P. J. , Reber R. A. , Murray J. R.
Published: July 2002

A general methodology is developed to experimentally characterize the spatial distribution of occupied traps in dielectric films on a semiconductor. The effects of parasitics such as leakage, charge transport through more than one interface, and interface trap charge are quantitatively addressed. Charge transport with contributions from multiple charge species is rigorously treated. The methodology is independent of the charge transport mechanism(s), and is directly applicable to multilayer dielectric structures. The centroid capacitance, rather than the centroid itself, is introduced as the fundamental quantity that permits the generic analysis of multilayer structures. In particular, the form of many equations describing stacked dielectric structures becomes independent of the number of layers comprising the stack if they are expressed in terms of the centroid capacitance and/or the flatband voltage. The experimental methodology is illustrated with an application using thermally stimulated current (TSC) measurements. The centroid of changes (via thermal emission) in the amount of trapped charge was determined for two different samples of a triple-layer dielectric structure. A direct consequence of the TSC analyses is the rigorous proof that changes in interface trap charge can contribute, though typically not significantly, to thermally stimulated current.
Mode profiles in waveguide-coupled resonators
Authors: Hunt William D. , Cameron Tom , Saw John C. B. , Kim Yoonkee , Suthers Mark S.
Published: July 2002

Surface acoustic wave (SAW) waveguide-coupled resonators are of considerable interest for narrow-band filter applications, though to date there has been very little published on the acoustic details of their operation. As in any resonator, one must fully understand its mode structure and herein we study the SAW mode profiles in these devices. Transverse mode profiles in the resonant cavity of the device were measured at various frequencies of interest using a knife-edge laser probe. In addition we predict the mode profiles for the device structure by two independent methods. One is a stack-matrix approach adapted from integrated optics and the other is a conventional analytical eigenmode analysis of the Helmholtz equation. Both modeling techniques are in good agreement with the measured results.
Epitaxial growth of Al films on modified AlAs(001) surfaces
Authors: Maeda Narihiko , Kawashima Minoru , Horikoshi Yoshiji
Published: July 2002

In order to understand the physical principles which underlie the formation of metal/semiconductor interfaces, epitaxial Al films are grown by molecular beam epitaxy on As-stabilized and Al-stabilized AlAs(001) surfaces some of which were coated with a single monolayer (1 ML) of Ga or In. Reflection high-energy electron diffraction and x-ray diffraction measurements reveal that the growth direction and epitaxial relationship of the Al films are drastically changed by the surface modification by 1 ML Ga or In. A variety of Al film configurations are obtained, of which the Al(001)/Ga(or In)/AlAs(001) heterostructure is technologically important because it has an ideally lattice matched metal/semiconductor heterostructure. We propose structural models for three Al phases, i.e., Al(001) and two kinds of Al(110), and discuss the physical origin of the variation of Al phases. It is shown that the changes observed in the epitaxial relationships of Al films can be systematically explained in terms of bond strengths at the interface. In the growth of metals on semiconductors, interface modification is shown to be effective for controlling the growth.
Structure and property of polycrystalline (Fe0.99Mo0.01)78Si9B13 alloys
Authors: Liu X. D. , Wang J. T. , Ping D. H. , Li D. X.
Published: July 2002

Polycrystalline Fe-Mo-Si-B alloys with grain sizes of 15–200 nm were synthesized via crystallization of the amorphous alloy. Positron lifetime results show that there are two existing types of interfacial defects in nanocrystalline Fe-Mo-Si-B alloys, namely free-volume-sized defects and nanovoids. The free-volume-sized defect shows almost no change in size or density during grain growth, and has a mean lifetime smaller than that of its amorphous counterpart as a result of structural relaxation in the process of crystallization. However, abnormal changes in size and density of nanovoids with grain growth were noticed. It is of great interest that the variations of intermediate lifetime τ2 and intensity ratio I1/I2 with the average grain size D̄ are exactly compatible with those of microhardness. A lower density of nanovoids corresponds to a larger microhardness in the case of the present alloy system.
Comparison of electron-density measurements made using a Langmuir probe and microwave interferometer in the Gaseous Electronics Conference reference reactor
Authors: Overzet Lawrence J. , Hopkins Michael B.
Published: July 2002

A Langmuir probe and a microwave interferometer have been combined to measure the electron density of argon glow discharges in the Gaseous Electronics Conference reference reactor [Bull. Am. Phys. Soc. 36, 207 (1991)]. The two techniques indicate the same charge density at 100 mTorr to within 30%. This 30% difference is easily explained by the experimental peculiarities. While the predicted charge densities obtained from the two techniques track one another as the applied rf voltage is varied at constant pressure, they do not track one another as the pressure is varied at constant rf voltage. In fact, the charge densities predicted from the Langmuir probe using Laframboise’s theory (Tech. Rep. 100, Univ. Toronto Inst. Aerospace Study, 1966) are factors of 2 and 4 times lower than those from the interferometer at 250 and 500 mTorr, respectively. It appears that the probe alters the charge density in its vicinity when the probe radius becomes greater than the ion mean free path. The interferometer has also been used to investigate the macroscopic perturbation of the plasma electron density caused by the Langmuir probe. It was found that presence of the probe in a radio-frequency discharge does not appreciably alter the electron density measured by the interferometer at a set rf voltage irrespective of the potential placed upon it. This was not the case for dc discharges in the same apparatus where the volume-averaged electron density could be depleted by as much as 70% when the probe was biased even a few volts above plasma potential.
A new stable ferromagnetic phase in Nd-Fe-B magnets and its effect on coercivity
Authors: Chen Zhongmin , Wang Naxin , Song Xiaoping , Wang Xiaotian
Published: July 2002

The microstructure and coercivity of NdxFe93−xB7 (x=16–28) sintered magnets were systematically studied. A new stable ferromagnetic phase with a composition of NdFe2Ox (x≊0.3) and a Curie temperature of 145 °C is found in the intergranular regions of the magnets, owing to the introduction of oxygen in the magnet processing. It forms at ∼650 °C and its volume fraction reaches a maximum of ∼4% in the Nd22Fe71B7 magnets. The coercivity is considerably enhanced by the appearance of this new phase.
Thermal conductivity of the molecular crystal (–)2–(α–methylbenzylamino)-5-nitropyridine
Authors: Bailey R. T. , Cruickshank F. R. , Kerkoc P. , Pugh D. , Sherwood J. N.
Published: July 2002

Thermal diffusivities of high quality (-)2-(α-methylbenzylamino)-5-nitropyridine (MBANP) single crystals were measured in different directions by the flash diffusivity method. The complete thermal conductivity tensor at temperatures of 295, 323, and 349 K was then obtained for this monoclinic crystal from the thermal diffusivity, specific heat, and density data. For instance, the thermal conductivity coefficients at 295 K are k1=0.27 W/mK, k2=0.29 W/mK, and k3=0.20 W/mK. The peak melting point of MBANP lies at 357.3 K, whereas the corresponding enthalpy of melting ΔHm has a value 67.2 J/g.
Effect of graphitic carbon films on diamond nucleation by microwave-plasma-enhanced chemical-vapor deposition
Authors: Feng Z. , Komvopoulos K. , Brown I. G. , Bogy D. B.
Published: July 2002

Diamond nucleation on very smooth (100) silicon substrates coated with thin films of a colloidal graphite suspension was investigated with a microwave-plasma-enhanced chemical-vapor-deposition system. Nucleation densities of the order of 106 cm−2 were obtained by coating the substrates with carbon films of thicknesses less than 1 μm. However, very low nucleation densities were obtained with carbon film thicknesses greater than 1 μm. The effect of the carbon film thickness on diamond nucleation was examined by measuring the etching rate of carbon films exposed to a hydrogen plasma and was further interpreted on the basis of scanning electron microscopy and Raman spectroscopy results. Etching of the original carbon may lead to the formation of a thin residual carbon film when the initial film thickness is less than a critical value. Results demonstrated that the high nucleation densities of good quality cubo-octahedral diamond crystals obtained with relatively thin carbon films were primarily due to the formation of a porous ultrathin residual carbon film. The critical initial film thickness was a function of the plasma etching and deposition rates of carbon which, in turn, affected the effective local carbon concentration. Thick carbon films yielded insignificant nucleation densities and poor quality diamond because of the high local carbon content resulting from the partial etching of carbon and the increased carbon concentration in the plasma. The local carbon concentration and the residual carbon film are the proposed principal factors for the obtained high diamond nucleation densities on unscratched silicon substrates.
Ferrimagnetic resonance lineshape asymmetry due to Suhl instabilities
Authors: Dorsey P. , Sokoloff J. B. , Vittoria C.
Published: July 2002

We present calculations of the ferrimagnetic resonance lineshapes resulting from second order Suhl instabilities for thin films and spheres. We find that whereas a spherical sample has a lineshape which is symmetrical around the resonant frequency, a thin film has an asymmetrical lineshape. The calculations are in agreement with measurements that we have performed of the lineshape as a function of input power for thin film samples of both barium ferrite and yttrium iron garnet. When the magnetic field direction is changed from perpendicular to parallel to the film plane, the asymmetry of the lineshape at magnetic resonance changes in opposite sense relative to the resonant field. Theoretical estimates of the critical microwave field necessary for second order Suhl instabilities to occur are in agreement with measured critical fields.
Variation of trapping/detrapping properties as a function of the insulator size
Authors: Oh K. H. , Ong C. K. , Tan B. T. G. , Gressus C. Le , Blaise G.
Published: July 2002

Using the scanning electron microscope we have investigated the physical parameters determining the size effect of various dielectric samples submitted to a surface electric field. It is shown that the size effect is a function of the static permittivity and of the space charge distribution. The results are explained by the consideration of charge diffusion and polarization relaxation processes resulting from the space charge formation. A one-dimensional mathematical model has also been used to describe space charge distribution. The findings were consistent with the experimental observations.
Study of ion activation in the in situ low-temperature laser deposition of superconducting YBa2Cu3O7−δ films
Authors: Mukherjee P. , Sakthivel P. , Ahmed K. , Witanachchi S.
Published: July 2002

The presence of a pulsed discharge during the reactive deposition of high Tc thin films allows the lowering of substrate temperatures to ∼475 °C for in situ superconducting film deposition. We present the first ion probe analysis to study the role of this pulsed discharge on the excimer laser ablated YBa2Cu3O7−δ plume. The ionic enhancement in the laser-ablated plume, during reactive deposition in the presence of a positively biased ring electrode, is studied both in terms of its extent and temporal characteristics. A significant increase in the forward-directed incidence of oxygen ions on the depositing substrate, following in the wake of the highly directional plume, is demonstrated. The dependence of this ionic enhancement on ambient oxygen pressure and the bias voltage on the ring electrode is discussed.
Photodoping dynamics in the Ag/As-S system
Authors: Tanaka Keiji , Sanjoh Hiroaki
Published: July 2002

The photodoping process in Ag/As30S70 layers excited by pulsed light has been studied. The photodoping begins within 0.1 μs after the pulse excitation, and the response amplitude and relaxation time exhibit similar characteristics to those of the transient photoconduction in amorphous As30S70 and AgAsS2. These observations suggest that the photodoping phenomenon is closely related to photoconduction. A model assuming Ag+ counterflow to the photogenerated hole motion is proposed.
Electric conduction of single crystal and glass of Li2B4O7
Authors: Matsuo T. , Yagami T. , Katsumata T.
Published: July 2002

The dc and ac conductivity of single crystal and glass of Li2B4O7 have been measured at various temperatures. It has been shown that these materials are mixed conductors in which electronic and ionic conduction coexist. In the single crystals, both the electronic conduction and ionic conduction along the c axis are much smaller than that perpendicular to the c axis. The anisotropy in the electronic conduction is considered to be due to the conduction by the π electron of the BO3 layer located on the (001) crystallographic plane. The anisotropy in the ionic conduction is also considered to be due to the diffusion of Li ions and/or protons between the BO3 layers. The thermal activation energies of the electronic conduction were 0.65 eV for glass, 1.61 eV for the crystal parallel to the c axis, and 0.78 eV for the crystal perpendicular to the c axis. The activation energy of the ionic conduction was 0.46 eV for the crystal perpendicular to the c axis.
Phase decomposition and superconductivity in Bi2Sr2CaCu2Oy single crystals
Authors: Wu Wenbin , Wang Liangbin , Li Xiao-Guang , Zhou Guien , Qian Yitai , Qin Qiaonan , Zhang Yuheng
Published: July 2002

The phase decomposition in high quality Bi2Sr2CaCu2Oy(2212) single crystals annealed in air at 400–750 °C is studied by x-ray diffraction and scanning electron microscopy. It is found that the Bi2Sr2CaCu2Oy phase of the as-grown single crystals readily decomposes even at temperatures as low as 450 °C. With the annealing temperature increasing, phases of bismuth oxides, strontium bismuth oxide, and the Bi2Sr2CuOx(2201) are segregated successively from the air annealed single crystals and are found to be temperature dependent. Combining with the enhancement of the superconducting onset temperatures and the very narrowness of the diamagnetic transitions observed for the annealed crystals, it is suggested that the structural change indicated by the surface phase decomposition may play an important role in the variations of superconductivity in Bi2Sr2CaCu2Oy single crystals.
A Monte Carlo model for trapped charge distribution in electron-irradiated α-quartz
Authors: Oh K. H. , Ong C. K. , Tan B. T. G. , Le Gressus G.
Published: July 2002

The space dependence of charge carriers trapped in α-quartz under electron-beam bombardment is investigated using a Monte Carlo algorithm. The average energy of the electron after being detrapped from a trap site is first calculated by considering both the polar and nonpolar phonon scatterings. Later, the detrapping and trapping rates are also included in the model to obtain a stable trapped charge distribution, which is found to be dependent on the size as well as the temperature of the sample. Comparisons with experimental results of the size effect on the dielectric strength obtained from a scanning electron microscope are also made.
An ordered Ga2Te3 phase in the ZnTe/GaSb interface
Authors: Chou C. T. , Hutchison J. L. , Cherns D. , Casanove M.-J. , Steeds J. W. , Vincent R. , Lunn B. , Ashenford D. A.
Published: July 2002

A Ga2Te3 interfacial phase has been observed in a ZnTe/(001)GaSb heterostructure by high resolution electron microscopy under special imaging conditions. This phase exists in domains 5–10 nm in size on the ZnTe side of, and usually 2–4 nm away from, the interface. A structural model has been proposed for this phase that is derived from the sphalerite cell with cation sites occupied either fully (occupancy 1) or partially (occupancy 5/9) by Ga atoms. The fully occupied Ga sites form a regular array of uninterrupted chains along the [110] direction of the sphalerite unit cell. The partially occupied Ga sites can also be considered as forming chains containing both Ga atoms and vacancies along the [110] direction. Within these chains vacancies are highly mobile, resulting in an average Ga occupancy of 5/9. The unit cell of Ga2Te3 is orthorhombic with the space group Amm2. The lattice parameters of the unit cell have been derived from electron diffraction data.
Minority carrier mobility of Czochralski-grown silicon by microwave-detected photoconductance decay
Authors: Stephens A. W. , Green M. A.
Published: July 2002

In this article the contactless microwave detected photoconductance decay technique (MW-PCD) is used to examine two important issues relating to carrier mobility. One is whether or not minority and majority carrier lattice scattering mobilities are equal, and the other whether or not mobilities depend on growth method of the semiconductor material. To answer these two questions, the minority carrier mobility for Czochralski-grown silicon has been measured by the MW-PCD method. The results are compared with both majority carrier mobility measurements for this material and minority carrier mobility values for float-zone material. The method employed by the MW-PCD analysis also yields bulk lifetime estimates. The results confirm earlier suspicions that differences in material quality can be the reason for differences between minority and majority carrier mobility measurements at doping levels below 1017 cm−3 rather than any fundamental difference in the scattering mechanisms of the two types of carriers.
SmCo (2:17-type) magnets with high contents of Fe and light rare earths
Authors: Huang M. Q. , Zheng Y. , Wallace W. E.
Published: July 2002

The magnetic properties of SmCo (2:17-type) magnets with high contents of Fe (v=Fe/3d≳0.34) and/or light rare earths (up to 0.4) were investigated. For magnets with compositions of Sm(FevCobalZr0.02Cu0.06{Ga}x)7.72, v=0.35, x=0–0.06, both iHc and (BH)max can be improved significantly by Ga doping, from 1.0 kOe and 5.4 MG Oe for x=0 to 6.4 kOe and 15.4 MG Oe for x=0.06. For magnets with compositions of Sm0.8Pr0.2(FevCobalZr0.02Cu0.06)7.67, v=0.30–0.31, iHc can be increased strikingly by a special heat treatment, from 2.5 to over 20 kOe for v=0.30 and from 0.5 to 15.5 kOe for v=0.31. Examining magnets with compositions of Sm0.59Pr0.41(Co0.68Fe0.20Cu0.10Zr0.02)z (z=6.68–7.75) shows two stages of the magnetization reversal process in the range of 5.5 to 9.5 kOe and 16 to 19 kOe. Observed (BH)max ranged from 17 to 22.5 MG Oe.
Deposition and properties of NiFe2O4 thin films (abstract)
Authors: van Dover R. B. , Gyorgy E. M. , Phillips J. M. , Marshall J. H. , Felder R. J. , Fleming R. M. , O’Bryan H.
Published: July 2002

We have investigated the properties of NiFe2O4 thin films prepared by laser ablation of a stoichiometric NiFe2O4 target. Textured polycrystalline films were obtained on a-SiO2 as well as on various substrates with Au, Ag, Pt, and MgF2 buffer layers. Epitaxially oriented films were obtained on MgO, (11̄02)-oriented Al2O3, (112̄0)-oriented Al2O3, Y-stabilized ZrO2 (YSZ), and SrTiO3, although the crystalline quality of the films varied. Contamination by diffusion from the substrate and strains induced by both lattice constant mismatch and differential thermal expansion degraded the magnetic properties of the films, and in some cases decreased the electrical resistivity as well. By choosing the right substrate (YSZ), temperature (600 °C), and PO2 (0.01 mT), we are able to prepare epitaxial films with bulk saturation magnetization (Ms=270 G) and fairly low anisotropy (K∼105 erg/cm3) as inferred by torque magnetometry. These films and bilayers are expected to be useful in a variety of fundamental investigations as well as having the potential for technological applications.
Scaling behavior of the homogeneous magnetization dynamics in the ferromagnetic state of EuS
Authors: Dombrowski R. , Görlitz D. , Kötzler J. , Marx Chr.
Published: July 2002

Using a broadband vectorial microwave reflectometer, we measured the longitudinal dynamic susceptibility of a EuS sphere between 0.1 and 20 GHz in the temperature interval 4.2 K≤T≤Tc=16.5 K. For low internal fields, the shape corresponds to a heavily overdamped Lorentzian resonance, χ−1z(ω)= χ−1z(0)iω/Lz−(ω/Ωz)2 where Lz≤Ωz. The field and temperature dependence of the intrinsic kinetic coefficient and resonance is fully explained by that of the static susceptibility χz(0). Surprisingly, the resulting scaling function, Lz[χz(0)], agrees exactly with that observed earlier above Tc, where Lz[χz(0)] could quantitatively be explained by a crossover from a van Hove behavior for large χz(0), due to dipolar anisotropic fluctuations, to a critical speeding-up for χz(0)<1, where isotropic spin diffusion prevails. This finding suggests that the dipolar anisotropy dominates the magnetization dynamics also below Tc of EuS, which seems to be supported by the fact that the correlation frequency of the torques acting on the magnetization, ωc=Ω2z/Lz, agrees with the relaxation rate of the longitudinal magnetization fluctuations, δMq∥q.
Low-temperature magnetic relaxation of organic coated NiFe2O4 particles
Authors: Kodama R. H. , Seaman C. L. , Berkowitz A. E. , Maple M. B.
Published: July 2002

We report measurements of relaxation rate of the remanent magnetization versus temperature for NiFe2O4 particles coated with the surfactant oleic acid. It has been reported that the organic coating induces a strong surface anisotropy resulting in surface spin canting and anomalous hysteresis properties at low temperature as compared to the bulk material. We have measured, at temperatures down to 400 mK, relaxation of remanent magnetization which extrapolates to a nonzero value at zero temperature. Macroscopic quantum tunneling at these temperatures has been predicted and observed only for materials with high magnetocrystalline anisotropy, so it is particularly interesting to observe it in NiFe2O4, which has a relatively low bulk anisotropy. It is suggested that the strong surface anisotropy induced by the organic coating may be responsible for tunneling in these particles.
Order by disorder in an anisotropic pyrochlore lattice antiferromagnet
Authors: Bramwell S. T. , Gingras M. J. P. , Reimers J. N.
Published: July 2002

The properties of the classical Heisenberg pyrochlore lattice antiferromagnet with local planar single ion anisotropy are discussed. The zero temperature ground state is found to be macroscopically degenerate and without long range order. Monte Carlo simulations are used to investigate the properties of the system at finite temperature. We find that thermal fluctuations select a subset of the ground state manifold, and induce a first-order phase transition to a conventional Néel ground state.
Initial stages of AlN thin-film growth on alumina using trimethylamine alane and ammonia precursors
Authors: Bertolet D. C. , Liu Herng , Rogers J. W.
Published: July 2002

A novel precursor combination, trimethylamine alane (TMAA) and ammonia (NH3), has been investigated for the low-temperature chemical vapor-deposition of AlN thin films. The initial stages of AlN growth on alumina powder substrates were studied by Fourier transform infrared spectroscopy and x-ray photoelectron spectroscopy. Upon exposure of TMAA to the alumina surface at 300 K, infrared data show the presence of molecular trimethylamine alane on the surface. NH3 reacts with the TMAA derivatized surface at 300 K, removing all of the trimethylamine, and leaving a four-coordinate -NH2- species bound to aluminum in extended networks. Concurrently, the population of alane on the surface is greatly reduced as a result of reaction with NH3 to form -NH2- and liberate H2. Subsequent exposure of TMAA leads to reaction with the surface -NH2- species and the further adsorption of TMAA. These reactions continue to propagate following additional alternating exposures of NH3 and TMAA, although with decreasing efficiency. The efficiency is increased if the processing is carried out at 400 K. X-ray photoelectron spectroscopy results confirm the formation of AlN after repeated cyclic exposure/processing at substrate temperatures as low as 400 K but not at 300 K.
Monte Carlo simulations of remanent magnetization decay driven by interactions
Authors: Matson M. E. , Lottis D. K. , Dahlberg E. Dan
Published: July 2002

Many physical systems, including ferromagnets, exhibit slow relaxation behavior. The usual explanation of this time dependence is to assume a distribution of energy barriers, i.e., disorder. A recent model [Phys. Rev. Lett. 67, 362 (1991)] relies on a mean field approximation of magnetostatic interactions to be the driving force responsible for the decay. Following this work, we have simulated the dynamics interacting Ising spin system key features found in the original model. In addition, it is shown that the interaction driven relaxation results in a faster decay of the magnetization; a result reflecting the model system is being driven towards its equilibrium state by the interactions.
Magnetic and magneto-optical properties of amorphous rare-earth–transition-metal alloys containing Pr, Nd, Fe, Co
Authors: Hansen P. , Raasch D. , Mergel D.
Published: July 2002

Amorphous rare-earth–transition-metal alloys of composition RE1−xFex and RE1−xCox with RE=Pr, Nd and 0<x<1 were prepared by coevaporation. Also disks for magneto-optical recording were fabricated using TbNd-FeCo or TbPr-FeCo layers. The magnetization, Curie temperature, uniaxial anisotropy, coercivity, and Faraday and Kerr rotation were investigated as a function of composition and temperature. The spectral variation of the Kerr rotation was measured for some alloys. The magnetization data indicate a strong dispersion of the Pr and Nd subnetworks and of the Fe subnetwork for Fe-rich alloys. The latter gives rise to a maximum of the Curie temperature TC around x=0.7 and to very low TC values for Fe-based alloys with x≳0.9. The magneto-optical effects result from both the light rare earths and the transition-metal alloys giving rise to significantly larger rotations in the visible as compared to the alloys containing heavy rare earths. Recording experiments on standard disks with a TbNd-FeCo layer indeed reveal good write/erase and read performances. The carrier-to-noise ratio ranges between 49 and 56 dB for wavelengths between 458 and 820 nm. Disks based on RE-TM layers containing Pr reveal a lower write/read performance and appear less suitable for magneto-optical recording.
Direct carbide synthesis by multipulse excimer laser treatment of Ti samples in ambient CH4 gas at superatmospheric pressure
Authors: Mihailescu I. N. , Chitica N. , Teodorescu V. S. , Popescu M. , De Giorgi M. L. , Luches A. , Perrone A. , Boulmer-Leborgne Ch. , Hermann J. , Dubreuil B. , Udrea S. , Barborica A. , Iova I.
Published: July 2002

Successful carbidation of Ti in a layer forming on the surface of a Ti sample submitted to multipulse excimer (λ=308 nm) laser treatment in CH4 at a slightly superatmospheric pressure is reported. The layer is only surface contaminated with oxygen while its main part consists of fcc TiC. The layer apparently ends with a tail of carbides with low C content, extending deeper into the sample’s bulk. The characteristics of the synthesized layer are suggested to be related to the peculiarities of the chemical synthesis which are enhanced by gas propulsion into a melted layer under the recoil action of a plasma evolving in front of the sample. A cavitation mechanism inside the melted surface layer in order to account for plasma initiation is proposed. This mechanism also facilitates the strong substance propulsion into the sample’s bulk.
Metallurgical study of Ni/GaSb (111) and (001) contacts: Comparison with the Ni/GaAs and Ni/AlAs systems
Authors: Guivarc’h A. , Caulet J. , Minier M. , Le Clanche M. C. , Deputier S. , Guérin R.
Published: July 2002

Solid-state interdiffusions at the Ni/GaSb(111) and (001) interfaces were investigated in the temperature range 200–600 °C using 1.8 MeV He+ ion backscattering and channeling and also using x-ray diffraction. The starting point was the experimental determination of the Ni-Ga-Sb phase diagram. The steps of the interaction were the same for GaSb(111) and GaSb(001). Three various phases were observed: the ternary A phase with the Ni2.5GaSb atomic composition, Ni2Ga3, and Ni2Ga0.25Sb1.75, the Ga-rich limit of the substituted NiSb binary compound. All these phases had a hexagonal pseudocubic structure. First, an epitaxial Ni2.5GaSb A phase reacted layer was obtained. This phase was not in thermodynamical equilibrium with GaSb and the final product was a mixture of Ni2Ga3 and Ni2Ga0.25Sb1.75 grains with an average atomic composition equal to Ni1.6GaSb. The metallurgical behavior of the Ni/GaSb contacts was compared to that of the Ni/GaAs and Ni/AlAs contacts. It was concluded that the experimental ternary diagram allowed a correct estimation of the sequence of phase formation in metal/compound semiconductor contact. In contrast, due to major differences in the ternary phase diagrams, the steps of the solid phase interdiffusion cannot be deduced from the results obtained on a priori equivalent contacts.
Optical properties and crystallization of amorphous Si:Sb alloy thin films
Authors: Li X.-H. , Carlsson J. R. A. , Järrendahl K. , Gong S. F. , Hentzell H. T. G.
Published: July 2002

Amorphous Si:Sb alloy thin films with various Sb concentrations were deposited on preoxidized Si substrates in an electron-beam evaporation system. The optical properties and the crystallization of the amorphous alloy as a function of Sb concentration were investigated with spectroscopic ellipsometry and x-ray diffraction (XRD), respectively. It has been found that the optical energy band gap decreases from 1.6 to 0.35 eV with increasing Sb concentration from 0 to 50 at. %, and that the index of refraction decreases overall with the increase of Sb concentration. From the crystallization study, it is seen that the crystallization temperature of the alloy decreases with increasing Sb concentration. From a detailed XRD examination, three different crystallization products from the amorphous alloys have been observed in low, medium, and high Sb concentration regions. The structure of the amorphous alloys and the correlation between the optical properties and the structures of the amorphous alloys are discussed. It is suggested that the amorphous alloys have differing favorable bonding structures, depending on the Sb concentration, that correspond to the bonding structures of the crystallization products.
Internal state populations and the time-of-flight of ground-state species ejected after the 193 nm excimer laser ablation of CuO, BaO2, and Y2O3
Authors: Fried Daniel , Jodeh Shehdeh , Reck Gene P. , Rothe Erhard W. , Kushida Toshimoto
Published: July 2002

Laser-induced fluorescence is used to measure time-of-flight (TOF) and internal state distributions of atoms, ions, and diatomics ejected after the 193 nm ablation of Y2O3, BaO2, and CuO. These measurements indicate that the bulk of material is ejected with speeds in the range of 104–105 cm/s while particle speeds in the emitting component of the plume exceed 106 cm/s. The TOF profiles of all the species were non-Maxwellian, containing extended low-velocity ‘‘tails.’’ It is postulated that these tails arise from the evaporation and/or sputtering of target material that occurs after the laser ablation pulse. This particle emission is caused either by residual energy deposited in the target after the explosive vaporization/ablation pulse or from radiation exchange and/or ion bombardment from the expanding plasma plume. The extent of these ‘‘tails’’ increases with increasing fluence, generating oscillations in the TOF distributions of Cu atoms from CuO at high fluence. Rotational and vibrational temperatures were estimated for YO and BaO molecules in the plume, and no CuO was detected.
Transverse Shapiro steps in a radio frequency driven superconducting Wheatstone bridge
Authors: Qian G. , Cahay M. , Kothari R.
Published: July 2002

We study the current-voltage characteristics of a superconducting Wheatstone bridge in the presence of radio frequency current sources. Besides the regular integer Shapiro steps in the current-voltage characteristics observed when the voltage is measured between the input and output leads of the dc biasing current, we also found additional integer steps in the transverse voltage for large value of the dc biasing current. An explanation for these steps is given in terms of competing circulating currents in the bridge.
A generalized view of the correlation factor in solid-state diffusion
Authors: Akbar Sheikh A.
Published: July 2002

The correlation factor is commonly used to determine the mechanism of diffusion in solids. Although originally thought to be associated only with tracer diffusion, the concept of the correlation factor has broadened considerably over the last couple of decades. In light of these developments, it is important to generalize the concept. This article attempts to present a simple picture of an integrated view of the correlation factor in a way accessible to a wider audience. Some areas where the generalized correlation factor plays important roles are also highlighted.
Determination of interface coherency by x-ray double-crystal diffraction
Authors: Zhu Nanchang , Li Runshen , Chen Jingyi , Xu Shunsheng
Published: July 2002

The degree of coherency at the interface between an epitaxial layer and a substrate or between two different epilayers is defined as a coherency factor fCOH. By performing just three diffraction measurements on an x-ray double-crystal diffractometer and calculating the deformations of the corresponding epilayer according to the peak separations, this coherency factor can be determined together with the composition of the epilayer on any orientation substrate. The experimental results show that the coherency factor at the interface varies significantly with the composition and thickness of the epilayer for a system with large misfit, so the determination of the coherency factor is necessary for the calculation of the epilayer composition via the relaxed lattice constant.
Raman scattering from LO phonon-plasmon coupled modes in gallium nitride
Authors: Kozawa T. , Kachi T. , Kano H. , Taga Y. , Hashimoto M. , Koide N. , Manabe K.
Published: July 2002

Raman spectra of n-type gallium nitride with different carrier concentrations have been measured. The LO phonon band shifted towards the high-frequency side and broadened with an increase in carrier concentration. Results showed that the LO phonon was coupled to the overdamped plasmon in gallium nitride. The carrier concentrations and damping constants were determined by line-shape fitting of the coupled modes and compared to values obtained from Hall measurements. The carrier concentrations obtained from the two methods agree well. As a result, the dominant scattering mechanisms in gallium nitride are deformation-potential and electro-optic mechanisms.
Ballistic phonon transport in denser gases
Authors: Dharmadurai G.
Published: July 2002

The salient features of thermal transport by phonons generated in pulse-heated gas columns are examined in view of the oscillatory and translatory degrees of freedom enjoyed by the gas molecules. The ratio between the transport coefficients for energy transport occurring at acoustic and mean molecular velocity is found to exceed unity when the gas becomes denser, suggesting that ballistic phonons dominate heat propagation in denser gases in accord with some exciting experimental observations.
Nonmobile water quantified in fully saturated porous materials by magnetic resonance relaxation and electrical resistivity measurements
Authors: Borgia G. C. , Fantazzini P.
Published: July 2002

Irreducible fluid saturation in porous media is the fraction of the wetting phase which remains trapped in the solid matrix after displacement with a nonwetting phase. Methods have been proposed to estimate irreducible saturation in fully water-saturated porous samples by means of proton nuclear magnetic resonance relaxation times, but the results are quite varied. Some features are discussed of the trapped fluid and it is shown that the fraction of nonmobile water can be well estimated for a varied suite of clean sandstones, using only measurements on the fully saturated samples. This is done by combining relaxation measurements with those of electrical resistivity factor F. One of several simple correlations is Swi ∝√F/T1s, where T1s is the stretched-exponential relaxation time.
Magnetic force microscopy of single crystal magnetite (Fe3O4)(abstract)
Authors: Proksch R. , Foss S. , Orme C. , Sahu S. , Moskowitz B.
Published: July 2002

The micromagnetic domain structure of a magnetite (Fe3O4) single crystal has been studied using a magnetic force microscope (MFM). The MFM responds to the perpendicular component of the stray field above the magnetite surface. The sample was polished in the (011) plane. In this case, there are two easy magnetic axes parallel to the surface. Surface domains observed near cracks and edges have a complex closure structure (see Fig. 1), while walls seen far from such boundaries have a sinusoidal structure. Of particular interest is the presence of walls with either even or odd symmetry of the perpendicular stray field component across the transition. These can be conventionally modeled as Bloch or Neel walls, respectively. Both types of walls have been modeled and compared with the experimentally observed structures. We find the Bloch domain walls to be about 300 nm wide, nearly twice the value expected from bulk wall calculations. This distinction is consistent with a surface broadening of the domain wall due to magnetostatic effects.
Mean interaction field in magnetic recording media
Authors: Tagawa Ikuya , Takeo Akihiko , Nakamura Yoshihisa
Published: July 2002

The mean interaction field was calculated for geometrical arrangements of isolated particles representing a longitudinally oriented Ba-ferrite and metal particulate medium. The effective demagnetizing factor was also calculated for a perpendicular Co-Cr recording medium. A positive mean field was obtained for the Ba-ferrite medium because the distance between particles placed in series to the magnetization direction is much shorter than that between parallel particles. When the orientation distribution became broad, the mean interaction field decreased because the location of magnetic particles changed. On the other hand, a negative mean field was obtained for the metal particulate medium. The results suggested that a positive mean field may act in the perpendicular Co-Cr medium.
Transport and magnetic properties of polycrystalline Sm2−xCexCuO4−y
Authors: Jardim R. F. , Westphal C. H. , Cohenca C. H. , Ben-Dor L. , Maple M. B.
Published: July 2002

A systematic study of transport and magnetic properties of polycrystalline Sm2−xCexCuO4−y (0.15≤x≪0.18) specimens obtained from a sol-gel precursor reveals a striking double resistive superconducting transition. One of the resistive transitions takes place at a temperature Tci and is almost insensitive to Ce concentration, applied magnetic field, and current density, while the other transition occurs at a lower temperature Tcj and is very sensitive to Ce content, applied magnetic field, and current density. Alternating-current magnetic susceptibility χac measurements reveal two interesting features. There is only one contribution to the diamagnetic signal below ∼20 K, which is particularly evident in the resistive component χ″ of χac where a peak at Tcj can be identified. The application of an applied magnetic field shifts the χ″ peak to lower temperatures and reveals a spin-glass-like behavior. All the results can be satisfactorily explained in terms of weakly linked Josephson junctions between small superconducting islands. The small islands have a superconducting transition at a higher temperature Tci, and the long-range phase coherence is achieved through Josephson coupling at lower temperatures Tcj. All the results suggest a superconducting glass state in a disordered Josephson network.
Mechanically alloyed nanocomposite magnets
Authors: Gong Wei , Hadjipanayis G. C. , Krause R. F.
Published: July 2002

The magnetic and structural properties of mechanically alloyed Nd2Fe14B+ (Fe, FeCo) powders have been investigated over a wide range of temperatures. X-ray diffraction patterns on samples milled for 48 h showed that the powders had a two-phase structure consisting of a disordered Nd-Fe-B alloy and α-Fe with a fine grain size about 140 Å. Enhanced magnetic remanence in the range of 95–105 emu/g with reduced remanence α=Mr/Ms=0.60–0.68 and a coercivity Hc=4–6 kOe have been obtained after annealing at 650–750 °C. This behavior is the result of an exchange coupling between the hard and soft phases with small grain sizes, about 500 and 200 Å, respectively.
Magnetic properties of antiferromagnetic superlattices
Authors: Zhang Shufeng , Zhang Guihua
Published: July 2002

The temperature dependence of the magnetic properties of magnetic superlattices by using the Bethe approximation to the inhomogeneous Ising model is studied. A self-consistent equation which determines the variation of the magnetic moment for any layer thicknesses is derived. In particular, the crossover from bulk-dominated behavior for large thicknesses of layers to interface-dominated behavior for small thicknesses is discussed. The results both qualitatively and quantitatively differ from mean-field theory; the experimental observation for FeF2/CoF2 and CoO/NiO superlattices is explained.
Magnetic properties of FeRhP: Influence of metallic clusters on ferromagnetism
Authors: Guillot M. , Artigas M. , Bacman M. , Fruchart D. , Boursier D. , Fruchart R.
Published: July 2002

Synthesis of compounds belonging to the FeRhP system reveal the existence of three main structures: hexagonal H3-Fe2P type, orthorhombic O10-NdCoB type, and hexagonal H12-FePdP type described as polytypes resulting from different packings of a pseudorhombohedral FeRhP subcell. High magnetic field magnetic measurements were carried out in the 4.2–300 K temperature range in fields up to 200 kOe. Very different magnetic properties were observed for the three phases. To explain the low saturation magnetization of both O10 and H12 phases compared to that of the H3 phase, a structural magnetic framework that leads to the formation of diamagnetic clusters within the ferromagnetic matrix is proposed.
Temperature effect on magnetoresistance in Co/Ru sandwiches
Authors: Dinia A. , Zoll S. , Ounadjela K.
Published: July 2002

Evidence is given of a strong dependence of magnetoresistance on temperature in Co/Ru sandwiches. This dependence while weak for thin Ru layers is much stronger for thicker Ru layers. In fact, three oscillations in saturation magnetoresistance and saturation field as a function of the thickness of the Ru layer tRu are observed at 4.2 K while only one oscillation is found at 300 K. In addition, for thin Ru layers, tRu≤11 Å, the saturation magnetoresistance is found to decrease between 4.2 and 300 K. In contrast, for Ru thickness in the range 11 Å≤tRu≤14 Å, the behavior is reversed, leading to the increase of the magnetoresistance at 300 K. This behavior is attributed to the spin disorder occurring at the interfaces at room temperature. Finally, for tRu≥14 Å the magnetoresistance completely vanishes at 300 K while a relatively high value is still observed for large Ru thickness, tRu=80 Å, at 4.2 K. This indicates that the mean free path of the conduction electrons in the Ru layer is strongly reduced from about 100 Å at 4.2 K to approximately 14 Å at room temperature.
Theory of Brillouin light scattering from dipole-exchange spin waves in magnetic double layers with interlayer exchange coupling
Authors: Slavin A. N. , Rojdestvenski I. V. , Cottam M. G.
Published: July 2002

The analytic theory of Brillouin light scattering from dipole-exchange spin waves (SW) in single magnetic films is extended to the case of perpendicularly magnetized exchange-coupled magnetic double layers. It is shown that the influence of the interlayer exchange coupling on the SW spectrum of a symmetric double layer can be taken into account by renormalizing the pinning parameter at the interface between the magnetic film and the spacer for the antisymmetric SW modes (the symmetric modes are unaffected). Using a Green’s-function formalism for light scattering from magnetic films, we calculate the light-scattering cross sections from the dipole-exchange discrete SW branches of the double layer. Numerical examples are presented.
Effects of ion beam and electron cyclotron resonance etch-induced damage on the optical properties of multiple quantum well structures
Authors: Bensaoula A. H. , Bensaoula A. , Freundlich A.
Published: July 2002

In this study we have utilized photoluminescence spectroscopy (PL) on GaAs/AlGaAs multiple quantum well (MQW) structures to assess and compare the relative damage induced by the standard ion beam etch technique and the ‘‘less damaging’’ newer electron cyclotron resonance (ECR) etch technique. The PL intensity is shown to correlate well with the ion dose and energy. At an equivalent total surface erosion under Ar ion irradiation, the ECR process is shown to cause significantly less deterioration on QW PL yield. Furthermore, after a short anneal the ECR-irradiated structures’ PL showed total recovery, whereas PL of structures exposed to an Ar+ ion beam exhibited only partial improvement.
Transient diffusion of Ga in amorphous silicon
Authors: Zagwijn P. M. , Huisman W. J. , Polman A. , Vlieg E. , Reader A. H. , Gravesteijn D. J.
Published: July 2002

The redistribution of Ga in amorphous silicon (a-Si) in the temperature range of 560–830 K by means of medium-energy ion scattering has been studied. During the initial 10 s of the annealing the diffusivity shows a transient behavior that is attributed to the change in the relaxation state of the amorphous matrix. From 560 to 830 K the diffusivity during relaxation is enhanced by seven to two orders of magnitude compared to the value for bulk a-Si. Possible models that show the observed transient diffusion behavior are discussed.
Photovoltaic activity of electrodeposited p-CuInSe2/electrolyte junction
Authors: Guillén C. , Herrero J. , Lincot D.
Published: July 2002

The photovoltaic activity of electrodeposited CuInSe2 thin films in an aqueous acidic solution has been determined by photoelectrochemical measurements. We used the model of Gärtner to extract values of effective optoelectronic parameters of the CuInSe2 films. These values suggest the presence of a high density of bulk or surface defects in the electrodeposited layers.
Parametrization of the electron beam induced energy spread in free electron laser dynamics
Authors: Dattoli G. , Giannessi L. , Ottaviani P. L. , Torre A.
Published: July 2002

We discuss a formula providing a parametrization of the energy spread induced on the electron beam in a free electron laser (FEL) oscillator. The formula we propose depends on the intracavity laser intensity and on the saturation intensity of the system. We show that such a parametrization can usefully be exploited in the analysis of FEL storage ring dynamics.