bluuebunny/crossref_metadata_2025 huggingface dataset:
Switching in single domain nanostructures at finite temperatures
Authors: Chui S. T. , Tian De-Cheng
Published: July 2002

We study the finite temperature magnetization reversal of single domain nanostructures (particles and wires) of different materials with Monte Carlo and analytic techniques. The temperature, angular, and shape dependences of the switching field are calculated. For large structure diameters, there are different reversal mechanisms at different orientations of the external field. For small structure diameters growth usually starts with the nucleation and a subsequent depinning of domain walls at the end(s) of the structure. The nucleation energy of the domain wall in a magnetic field approaches zero near the coherent rotation limit at small aspect ratios and at fields less than the coherent rotation limit at large aspect ratios. As the domain wall energy approaches zero the domain wall width can remain finite. For small diameters there is a significant temperature dependence in the coercive field. For structures whose easy crystalline anisotropy axis is not along the major axis, a variety of switching scenario is observed.
Generation and dissociation of complexes of iron and phosphorus atoms in silicon
Authors: Takahashi Hideki , Suezawa Masashi , Sumino Koji
Published: July 2002

To examine the possibility of generating complexes of iron and phosphorus atoms in silicon, neutral interstitial iron in phosphorus doped silicon crystals with various concentrations were measured with the electron spin resonance (ESR) method after cooling from high temperature at various cooling rates. This concentration was found to depend on the cooling rate when the concentration of phosphorus was high: The concentration of interstitial iron was low only when the concentration of phosphorus was high and the cooling rate was slow. This result shows that some iron atoms form complexes with phosphorus atoms during cooling and become ESR-inactive. Annealing behaviors of these complexes monitored as to the concentration of neutral interstitial iron are described as a combination of two processes, namely, the release of iron atoms from iron-phosphorus complexes and the diffusion of iron to quenched-in defects which are concluded to exist based on the dependence of annealing behavior on the cooling rate. From the analysis of annealing behaviors, the binding energy between iron and phosphorus atoms and the activation energy of diffusion of iron were determined to be 0.90 and 0.80 eV, respectively.
An eigenvalue method for open-boundary quantum transmission problems
Authors: Shao Zhi-an , Porod Wolfgang , Lent Craig S. , Kirkner David J.
Published: July 2002

We present a numerical technique for open-boundary quantum transmission problems which yields, as the direct solutions of appropriate eigenvalue problems, the energies of (i) quasi-bound states and transmission poles, (ii) transmission ones, and (iii) transmission zeros. The eigenvalue problem results from reducing the inhomogeneous transmission problem to a homogeneous problem by forcing the in-coming source term to zero. This homogeneous problem can be transformed to a standard linear eigenvalue problem. By treating either the transmission amplitude t(E) or the reflection amplitude r(E) as the known source term, this method also can be used to calculate the positions of transmission zeros and ones. We demonstrate the utility of this technique with several examples, such as single- and double-barrier resonant tunneling and quantum waveguide systems, including t-stubs and loops.
Understanding hot-electron transport in silicon devices: Is there a shortcut?
Authors: Fischetti M. V. , Laux S. E. , Crabbé E.
Published: July 2002

Results of a Monte Carlo study of carrier multiplication in silicon bipolar and field-effect transistors and of electron injection into silicon dioxide are presented. Qualitative and, in most cases, quantitative agreement is obtained only by accounting for the correct band structure, all relevant scattering processes (phonons, Coulomb, impact ionization), and the highly nonlocal properties of electron transport in small silicon devices. In addition, it is shown that quantization effects in inversion layers cause a shift of the threshold energy for impact ionization which is very significant for the calculation of the substrate current in field-effect transistors. Conservation of parallel momentum, image-force corrections, dynamic screening of the interparticle Coulomb interaction, and improvements to the WKB approximation are necessary to treat correctly the injection of electrons from silicon into silicon dioxide. The validity of models—analytic or Monte Carlo—which treat hot-electron transport with oversimplified physical approximations is argued against. In a few words, there is no shortcut.
Epitaxial Tl2Ba2CaCu2O8 superconducting thin film on Sr2(AlTa)O6 buffer layer
Authors: Tang Y. Q. , Chen K. Y. , Afonso S. , Xu X. L. , Xiong Q. , Salamo G. , Chan F. T. , Guo R. , Bhalla A.
Published: July 2002

Epitaxial Tl2Ba2CaCu2O8 superconducting films have been successfully grown on the dielectric Sr2(AlTa)O6 (SAT) buffer layers. X-ray diffraction data showed that the films were highly c-axis oriented with a rocking curve full width half maximum as narrow as 0.3°. The films also had an excellent in-plane epitaxy with Tl2Ba2CaCu2O8[100] aligned with SAT[100] and MgO[100] of the substrate. The zero resistance temperature Tc of the superconducting films ranged from 95 to 103 K and the transport critical current density Jc in zero field was 3×105 A/cm2 at 77 K.
GaxIn1−xAs quantum wire heterostructures formed by strain-induced lateral-layer ordering
Authors: Chou S. T. , Cheng K. Y. , Chou L. J. , Hsieh K. C.
Published: July 2002

GaxIn1−xAs quantum wire (QWR) arrays were grown on (100) on-axis InP substrates by single-step molecular-beam epitaxy. The QWRs were formed in situ in (GaAs)2/(InAs)2.2 short-period-superlattice (SPS) layers by the strain-induced lateral-layer ordering (SILO) process. An analysis of the cross-sectional and plan-view transmission electron microscopy images, photoluminescence peak energies, and polarization anisotropy has confirmed the QWR nature of these heterostructures. The SILO process occurs over a wide growth temperature range near 500 °C. However, both high and low growth temperatures result in a weaker lateral composition modulation. The strength of the lateral composition modulation is proportional to the total thickness of the SPS quantum-well layers, regardless of the thickness of the individual quantum well. In other words, the magnitude of composition modulation accumulates when growth proceeds. A strain-driven bulk solid-state diffusion model has been proven to be part of the driving force of the SILO process, in addition to a dynamic surface diffusion during growth.
Optical determination of the heavy-hole effective mass and exciton binding energy for a Si0.83Ge0.17/Si quantum well
Authors: Lee Hosun , Jones E. D. , Kurtz S. R. , Schmiedel T. , Houghton D. C. , Lee K.-S.
Published: July 2002

We have measured the diamagnetic shift of band-edge luminescence, no-phonon peak, and phonon sideband from a pseudomorphic, undoped Si0.83Ge0.17/Si quantum well using high-field magnetoluminescence spectroscopy. The quadratic dependence of the diamagnetic shift of the no-phonon luminescence peak suggests that the luminescence originates from excitons. Using a variational calculation, we determine the effective heavy-hole mass and the exciton binding energy to be 0.27m0 and 14.8 meV, respectively, and compare these results with reported values obtained from other measurement techniques.
Thickness dependence of the preferred orientation of Mn–Zn ferrite thin films deposited by ion-beam sputtering
Authors: Cho Hae Seok , Kim Hyeong Joon
Published: July 2002

The thickness dependence of the preferred orientation of the Cu or Ti added Mn–Zn ferrite thin films deposited on SiO2(1000 Å)/Si(100) at 350 °C by ion-beam sputtering was investigated. A mosaic target, consisting of a single-crystal (110) Mn–Zn ferrite with a metal strip on it, was employed as the target. The (hhh) preferred orientation, formed at the initial growth stage, of the Cu added Mn–Zn ferrite film changed to the (h00) preferred orientation with increasing film thickness, while the initially formed (h00) preferred orientation of the Ti added one was enhanced with increasing film thickness. Such different behaviors were discussed in terms of the surface energy and the preferred growth orientation of the ferrite film. The thickness dependence of magnetic properties of the ferrite films was also investigated.
Improvement of crystallographic and electroluminescent characteristics of SrS:Ce thin film devices by post-deposition annealing in Ar-S atmosphere
Authors: Ohmi Koutoku , Fujimoto Kazushi , Tanaka Shosaku , Kobayashi Hiroshi
Published: July 2002

Post-deposition annealing in an Ar-S atmosphere at atmospheric pressure has been demonstrated to improve crystallographic properties and electroluminescent (EL) characteristics of SrS:Ce thin film EL devices. Crystallinity and degree of the orientation of the SrS:Ce thin films with polycrystalline columnar grains are improved by the annealing. It seems that recrystallization takes place between SrS grains. For the annealed devices, charge generation in the SrS:Ce layer is suppressed and relaxation of the phosphor field is decreased. As a result, an instantaneous EL efficiency at the leading edge emission is enhanced. The EL luminance and the average EL efficiency are increased. The annealed SrS:Ce thin-film EL device showed a luminance of 800 cd/m2 and an efficiency of 0.42 lm/W at 1 kHz drive.
Copious electron emission from triglycine sulfate ferroelectric crystals
Authors: Kugel V. D. , Rosenman G. , Shur D. , Krasik Ya. E.
Published: July 2002

It is shown experimentally that the electron charge emitted from triglycine sulfate pulse ferroelectric cathodes can be as large as 129 μC/cm2. This charge considerably exceeds the measured value of spontaneous polarization, Ps=2.8 μC/cm2. A bipolar voltage is found to facilitate the appearance of the electron emission. It is proposed that the source of the emission current is the plasma of uncompleted surface discharges. This plasma is initiated at the metal-vacuum-dielectric triple points both by the field electron emission and the electron emission stimulated by polarization switching.
Growth of diamond on silicon during the bias pretreatment in chemical vapor deposition of polycrystalline diamond films
Authors: Stöckel R. , Janischowsky K. , Rohmfeld S. , Ristein J. , Hundhausen M. , Ley L.
Published: July 2002

The processes that occur on the silicon substrate during the bias pretreatment in a microwave plasma chemical vapor deposition system for the growth of diamond are investigated using a variety of techniques. The direct current during the bias pretreatment is correlated with changes in the shape of the plasma and with the amount and chemical and crystalline nature of the deposit on the substrate as well as with the nucleation density for subsequent diamond growth. The spatio-temporal nature of the nucleation process explains the evolution of the bias current. It is shown that the bias pretreatment alone already yields a closed layer of nanocrystalline diamond and a strong etching of the silicon substrate. An analysis of the nucleation conditions necessary for the formation of oriented and textured diamond crystallites leads to a choice of parameters during bias pretreatment that yield homogeneous layers of highly oriented and textured diamond crystals over an area of 100 mm2.
Au/ZnSe contacts characterized by ballistic electron emission microscopy
Authors: Morgan Brent A. , Ring Ken M. , Kavanagh Karen L. , Talin A. Alec , Williams R. Stanley , Yasuda Takashi , Yasui Takanari , Segawa Yusaburo
Published: July 2002

Ballistic electron emission microscopy (BEEM) has been performed on Au/ZnSe (001) diodes prepared in ultra high vacuum. An average barrier height (BH) of 1.37 eV is found for Au/n-ZnSe in close agreement with previously published values for diodes measured by conventional techniques. The BH distribution is relatively narrow, from 1.32 to 1.43 eV, consistent with cross-sectional transmission electron microscopy which indicates that the interface is abrupt, and without reaction products. These results differ from those reported for BEEM measurements on chemically etched Au/ZnSe diodes. [R. Coratger et al., Phys. Rev. B. 51, 2357 (1995)].
Resonant photorefractive AlGaAs/GaAs multiple quantum wells grown by molecular beam epitaxy at low temperature
Authors: Feng W. , Zhang Z. G. , Yu Y. , Huang Q. , Fu P. M. , Zhou J. M.
Published: July 2002

The resonant photorefractive devices using low temperature AlGaAs/GaAs multiple-quantum-well structures in a parallel field geometry are demonstrated. The samples are semi-insulating as grown. The AsGa-related defects incorporated into the samples during low temperature growth provide the required deep centers. No proton implantation, Cr doping, or annealing is needed for device fabrication. In the photorefractive wave mixing experiment, an output diffraction efficiency higher than 0.84% and a two-wave-mixing gain of more than 3000 cm−1 are obtained under a dc electric field of 15 kV/cm.
Surface photoabsorption study of the effects of growth temperature and V/III ratio on ordering in GaInP
Authors: Murata H. , Ho I. H. , Su L. C. , Hosokawa Y. , Stringfellow G. B.
Published: July 2002

Surface photoabsorption (SPA) measurements were used to clarify the Cu–Pt ordering mechanism in Ga0.5In0.5P layers grown by organometallic vapor phase epitaxy. The Cu–Pt ordering is strongly affected by the growth temperature and the input partial pressure of the phosphorus precursor, i.e., the V/III ratio. SPA was used to measure the concentration of [1̄10]-oriented phosphorus dimers on the surface, which are characteristics of the (2×4) reconstruction, as a function of the growth temperature and V/III ratio. The degree of order decreases markedly with increasing growth temperature above 620 °C at a constant V/III ratio of 40 [tertiarybutylphosphine (TBP) partial pressure of 50 Pa]. This corresponds directly to a decrease of the P-dimer concentration on the surface. Below 620 °C, the degree of order decreases as the growth temperature decreases, even though the concentration of P dimers increases. This is most likely due to the slow migration of adatoms on the surface during growth. The degree of order is found to decrease monotonically with decreasing V/III ratio in the range from 160 to 8 at 670 °C. This corresponds directly to the decrease of the P-dimer concentration on the surface. The direct correlation of the [1̄10]-oriented phosphorus dimer concentration and the degree of order with changes in both temperature (≥620 °C) and V/III ratio suggests that the (2×4) surface reconstruction is necessary to form the Cu–Pt structure, in agreement with published theoretical studies. The physical structure of the surface of these Ga0.5In0.5P layers was also characterized, using atomic force microscopy. For growth at 670 °C and a V/III ratio of 160, the structure of the layers growth on exactly (001) oriented GaAs substrates consists of islands surrounded mainly by bilayer (approximately 6 Å) steps. As the V/III ratio is reduced, the step height transforms to one monolayer. Exclusively monolayer steps are formed at a V/III ratio of 8. This is interpreted in terms of the stabilization of the bilayers by formation of the (2×2) reconstruction on the (111)B step face at high V/III ratios.
Magnetization processes of free- and fixed-single-crystal antiferromagnets
Authors: Zhao Z. G. , Châtel P. F. de , de Boer F. R. , Buschow K. H. J.
Published: July 2002

A simple two-sublattice model is used to analyze the magnetization process of anisotropic antiferromagnets at T=0. The magnetization is calculated for samples free to be oriented by the applied field (free single crystals or powders consisting of single-grain particles). We find that the anisotropy does not necessarily lead to a nonlinear M(B) relation, but that the magnetic isotherms are broken into two linear sections separated by a discontinuity. The difference between the slopes in the two regions enables a determination of the anisotropy constant K1. The results will be discussed in relation to our findings for the ferrimagnetic case and to the behavior of oriented antiferromagnetic crystals.
Laser-induced etching of Si surfaces: The effect of weak background light
Authors: Grebel H. , Gayen T. , Wu H-W.
Published: July 2002

Two different lasers were used to etch patterns on Si surfaces employing a thin film cell configuration. A strong, pulsed, 20 W KrF excimer laser was used for etching. A weak, cw, 5 mW HeNe laser provided background light. This laser, by itself, was incapable of etching the Si surface. A substantial enhancement of the laser etching process with background light was observed either when using many pulses or only one UV laser pulse. An even bigger change was observed as a function of the etchant concentration.
Pulsed laser induced epitaxial crystallization of carbon–silicon alloys
Authors: Kramer K. M. , Thompson M. O.
Published: July 2002

Nonequilibrium Si1−xCx alloys produced by pulsed laser induced epitaxy from ion implanted Si have been studied in the concentration range from 0.35 to 3.8 at. % C. Films were formed by multiple energy ion implantation of carbon into {001} Si to produce nearly uniform composition profiles, followed by irradiation with a 308 nm, 30 ns excimer laser pulse. Heteroepitaxy proceeded from the underlying {001} Si through the carbon containing layer at approximately 5 m/s. Transmission electron microscopy, Fourier transform infrared spectroscopy, high resolution x-ray diffraction, ion channeling, and secondary-ion mass spectrometry were used to characterize the structure and composition. At low concentrations, the films are fully strained and the carbon is substitutional. At concentrations exceeding 1.4 at. % C, SiC precipitates were observed and the substitutional carbon concentration decreased.
A 57Fe Mössbauer study of Gd2Fe17−xGaxC2 (x=0−6)
Authors: Hu Bo-Ping , Li Hong-Shuo , Shen Bao-Gen , Wang Fang-Wei , Cadogan J. M. , Zhan Wen-Shan
Published: July 2002

A 57Fe Mössbauer study of Gd2Fe17−xGaxC2 (x=0−6) has been carried out in order to reveal the effects of Ga substitution on the Fe-sublattice magnetic properties. We have used a simplified fitting model which uses six broadened sextets in the theoretical fit to the spectra of Gd2Fe17−xGaxC2. Our results suggest that the electronic factor plays an important role in determining the Fe magnetism in the substituted R2Fe17-xTx (T=Al,Ga,Si,Ti,...) compounds and their carbides. The average Fe magnetic moment in Gd2Fe17−xGaxC2 varies from 1.99μB for x=0 to 0.94μB for x=5 at room temperature. Therefore, the optimum substituent concentration would be 1≤x≤3 for the substituted R2Fe17−xTx compounds as potential permanent magnet materials.
X-ray-diffraction investigation of the anodic oxidation of porous silicon
Authors: Buttard D. , Bellet D. , Dolino G.
Published: July 2002

A systematic study of porous silicon anodic oxidation is reported. We have studied the evolution of the lattice parameter versus oxidation levels for p- and p+-type materials by using high-resolution x-ray diffraction, which gives a determination of the strains of the porous layer. The analysis of an asymmetric reflection enables us to measure the lattice parameter parallel to the surface. Moreover a diffuse scattering, observed at the bottom of the Bragg peaks, comes from the porous structure. The study of this diffuse scattering with reciprocal space maps gives information about the pore size and shape. It is shown that the evolution of the lattice mismatch parameter is related to the surface stress variations. After a review of previous works on porous silicon strains and on surface stresses of silicon we discuss the origin of the strains of as-formed and oxidized porous silicon.
Switching characteristics of submicron cobalt islands
Authors: Gomez R. D. , Shih M. C. , New R. M. H. , Pease R. F. W. , White R. L.
Published: July 2002

The magnetic characteristics of 0.2×0.4×0.02 μm3 cobalt islands were investigated using magnetic force microscopy in the presence of an applied field. The islands were noninteracting and showed a wide variety of single and multidomain configurations. The distribution of magnetization directions supports earlier models which suggest that crystalline anisotropy plays a dominant role in establishing a dispersion of easy axis directions about the long axis of the particles. The magnetic evolution, involving rotation and switching of individual islands, was observed at various points along the microscopic magnetization curve. A magnetization curve of an ensemble of islands was derived from the images and compares remarkably well with macroscopic M–H measurements.
Simulation of the capacitance–voltage characteristics of a single-quantum-well structure based on the self-consistent solution of the Schrödinger and Poisson equations
Authors: Brounkov P. N. , Benyattou T. , Guillot G.
Published: July 2002

The numerical self-consistent solution of the coupled Schrödinger and Poisson equations is used to simulate the C–V characteristic of Schottky barrier heterostructures with a single quantum well (SQW). This model is applied to study n-type SQW structures based on InGaAs/InAlAs. It has been shown from analysis of the C–V characteristics of a SQW structure that it is possible to extract information about the energy position of subband levels and the distribution of electron density in the QW. We have demonstrated that due to the two-dimensional distribution of electron gas in the QW the apparent concentration profile NC–V–W derived from the C–V characteristic fails to describe the free electron density distribution in the QW layer. However, the number of the NC–V–W peaks indicates the quantity of electron subband levels in the QW situated below the Fermi level at zero reverse bias.
Two-dimensional laser-induced fluorescence imaging of a pulsed-laser deposition process of YBa2Cu3O7−x
Authors: Nakata Y. , Kaibara H. , Okada T. , Maeda M.
Published: July 2002

Two-dimensional time-resolved density distributions of ground state barium (Ba) and copper (Cu) atoms as well as ground state yttrium oxide (YO) molecules have been measured by two-dimensional laser-induced fluorescence during a pulsed-laser deposition (PLD) process of YBa2Cu3O7−x. The gas phase of PLD has been investigated at ambient oxygen gas pressures between vacuum and 1 Torr. Characteristic behaviors have been observed for each of the measured ground state species. This is due to different oxidation schemes with the ambient oxygen gas. Whereas YO molecules mainly formed near the expanding front of the ablation plume, Ba atoms immediately oxidized at the plume front. Cu atoms, however, did not react during the gas phase of PLD. In addition, a well defined boundary of the expanding front of the ablation plume has been observed at oxygen gas pressures above 100 mTorr. Measurements of density distributions of emissive Ba, Cu, and YO are also discussed.
Determination of atomic hydrogen densities in gases by a deuterium based Lyman-alpha absorption method
Authors: Laimer J. , Misslinger G. , Schwärzler C. G. , Störi H.
Published: July 2002

We discuss the variant of a method for the determination of absolute hydrogen atom densities based on the absorption of the hydrogen Lyman-alpha line. Previously, we demonstrated that by using a simple vacuum ultraviolet spectrometer with low resolution an accuracy of better than 50% for the determined hydrogen atom densities can be obtained for transmissions ranging from 10% to 90%. For transmissions outside of this range excessive errors occur, thus, limiting the usefulness of the method to a certain range of hydrogen atom densities, depending on absorber length and temperature. This range of atomic hydrogen densities accessible to the measurement can be extended by the new method that consists of using well-defined mixtures of hydrogen and deuterium in the absorber. Using a source of either hydrogen or deuterium Lyman-alpha radiation, only one sort of atoms contributes to the absorption. Thus, by selecting an appropriate mixture of hydrogen and deuterium, the range of atom densities accessible to the measurement can be extended to higher densities. Using well-defined mixtures ranging from 1% hydrogen in deuterium to 2% deuterium in hydrogen as absorbers, we were able to determine hydrogen atom densities up to a factor 100 higher than those measured previously in pure hydrogen. Using mixtures down to the natural abundance of deuterium in hydrogen, the measurement of even higher atom densities seems possible. The effect of the difference of the energies of dissociation of hydrogen and deuterium has been investigated using a computer model of relevant chemical processes. In most situations, this effect is smaller than the experimental error. The main limitation of the method is its sensitivity to absorption by impurities, as many molecular gases absorb Lyman-alpha radiation.
Performance study of polycapillary optics for hard x rays
Authors: Wang Lei , Rath B. K. , Gibson W. M. , Kimball J. C. , MacDonald C. A.
Published: July 2002

In order to investigate the feasibility of using Kumakhov capillary x-ray optics for high energy x-ray applications, measurements have been performed on the behavior of capillary optics from 10 to 80 keV. Transmission efficiencies of straight polycapillary fibers of different types have been measured as a function of source location and x-ray energy. The measurements are analyzed using a geometrical optics simulation program, which includes roughness and waviness effects. Despite the low critical angle for total external reflection at high energies, capillary x-ray optics appear promising for many hard x-ray applications. Transmission measurements at high energies have also proven to be a very sensitive tool in capillary quality analysis.
Properties of ultra-thin lead zirconate titanate thin films prepared by ozone jet reactive evaporation
Authors: Torii Kazuyoshi , Kawakami Hirosi , Miki Hiroshi , Kushida Keiko , Fujisaki Yoshihisa
Published: July 2002

Lead zirconate titanate (PZT) thin films were prepared by reactive coevaporation with high-concentration ozone. PZT thin films that demonstrate the highest charge storage density (280 fC/μm2 at 1.5 V for 75-nm-thick film) yet reported have been fabricated. No fatigue was observed after 1011 polarization switching cycles even though a Pt electrode is used. A low leakage current of <10−7 A/cm2 at 1.5 V was attained. These PZT films are promising candidates of an alternative capacitor dielectric for dynamic random access memory (DRAM) and ferroelectric nonvolatile memories.
Interaction of wide band gap single crystals with 248 nm excimer laser radiation. V. The role of photoelectronic processes in the formation of a fluorescent plume from MgO
Authors: Ermer D. R. , Langford S. C. , Dickinson J. T.
Published: July 2002

We examine the formation of a fluorescent plume during pulsed laser irradiation of wide band gap materials at sub-band gap photon energies. We show that fluences near the threshold for plume fluorescence in MgO do not produce adequate electron densities for significant laser-plume interactions via inverse bremsstrahlung processes. At a wavelength of 248 nm, 30 ns pulse width, and fluences well below the onset of detected plume fluorescence, we observe intense Mg+ emissions and electron temperatures in excess of 1 eV. The onset and growth of plume fluorescence closely parallels the neutral emission intensities, suggesting that the lack of plume fluorescence below the fluence threshold is due to the lack of gas phase neutral Mg. Time-resolved measurements of the atomic line emissions show peaks well after the peak laser intensity, arguing against direct laser-plume interactions. We propose that electron acceleration involves electrostatic interactions between photoelectrons and photoelectronically emitted ions. The unusual temporal evolution of the atomic line emissions can be attributed to the time dependence of the spatial overlap between neutral particles and the accelerated electrons that collisionally excite the atomic lines.
Thickness dependence of room temperature permittivity of polycrystalline BaTiO3 thin films by radio-frequency magnetron sputtering
Authors: Jang Jin Wook , Chung Su Jin , Cho Woon Jo , Hahn Taek Sang , Choi Sang Sam
Published: July 2002

Polycrystalline BaTiO3 thin films with thickness ranging from 2100 to 20 000 Å were prepared on platinum substrates using off-axis radio-frequency magnetron sputtering. The variation in room temperature permittivity of the films was investigated with respect to thickness using x-ray diffraction and transmission electron microscopy. All films were ferroelectric and their room temperature permittivity, which was significantly higher than previously reported values, showed a strong dependence on film thickness. Higher permittivity was attributed primarily to the presence of ferroelectric domains. The room temperature permittivity of the thin films showed large variations with grain size, as in the case of BaTiO3 ceramics. The increase in permittivity with increasing film thickness was attributed to the decrease in defect concentration with grain growth. The 20 000 Å film showed an abrupt decrease in permittivity and the presence of an intergranular phase having titanium-excess composition; these phenomena are discussed in terms of domain boundary pinning and recrystallization.
High-lying metastable states of Si donors in GaAs under magnetic field
Authors: Chen Zhanghai , Chen Zhonghui , Liu P. L. , Shi G. L. , Hu C. M. , Shi X. H. , Shen S. C.
Published: July 2002

Fourier transform infrared photoconductivity spectra of Si donors in GaAs under magnetic field are presented in this paper. Four new spectral lines corresponding to the transitions from hydrogen-like ground state to the metastable states (510), (610), (710), and (810) under nonzero magnetic field were observed. The variational calculation model proposed by Barmby et al. was applied to the high-lying metastable states. The experimental data of the transition energies agree well with the variational results under high magnetic field and below the resonant polaron region. Deviations due to omitting the bound Landau continuum coupling in the trial wave function were found in the comparison at the low field region for the metastable states with higher Landau index. The ionization energies of the high-lying metastable states were also determined.
The influence of the Pt buffer layer on the perpendicular magnetic anisotropy in epitaxial FePd(001) ordered alloys grown by sputtering
Authors: Caro P. , Cebollada A. , Ravelosona D. , Briones F. , Garcı́a D. , Vázquez M. , Hernando A.
Published: July 2002

Single-crystalline FePd(001) 1000-Å-thick films were grown at 500 °C by codeposition in an UHV sputtering system. It is found that both the crystalline structure and therefore the magnetic properties strongly depend on the Pt buffer layer thickness. The films grown on Pt(100) layers 50 Å thick and thinner do not exhibit clear chemical order and show in-plane magnetic anisotropy. FePd grown on thicker Pt buffers adopts a fct structure accompanied by a strong chemical ordering, leading to a perpendicular magnetic anisotropy.
Extraordinary Hall effect in (111) and (100)-orientated Co/Pt superlattices
Authors: Canedy C. L. , Li X. W. , Xiao Gang
Published: July 2002

We have fabricated a series of (111)- and (100)-orientated Co/Pt superlattices of high quality. (111)-orientated superlattices acquire a perpendicular magnetic anisotropy for samples with thin Co layers whereas for the (100)-orientated superlattices the easy magnetization direction remains in plane. Analysis of the extraordinary Hall effect (EHE) verifies the applicability of simple scaling laws between the resistivity and the EHE coefficient for these layered materials. In addition, we have found a marked dependence of the skew coefficient on Co layer thickness. The contribution to EHE from quantum side-jump remains relatively constant.
The effects of particle size and distribution on the magnetic properties of coercive Sm(Co,Fe,Cu,Zr)z alloy powders for bonded magnet applications
Authors: Gong W. , Ma B. M. , Bounds C. O.
Published: July 2002

The microstructure of Sm(CobalFexCu0.08Zr0.03)8.2, where x=0.23, 0.26, and 0.28, in the as-cast state and after various processing stages has been examined by optical microscopy. The size of the 2:17 matrix phase was found to be approximately 100 μm in the as-cast state. A slight increase in the size of the 2:17 matrix was observed after thermal processing. Subgrains of 10 to 20 μm are present in the 2:17 matrix of the fully processed ingots. Powders with a mean particle size range of 3–300 μm were found to exhibit a Gaussian distribution. A slight increase in intrinsic coercivity (Hci) was observed when the mean particle size was decreased from 300 to 200 μm and remained nearly constant for sizes ranging between 10 and 200 μm. A significant decrease in Hci was observed when powders were further reduced below 10 μm. Similar trends were also observed for remanence (Br), maximum energy product (BHmax), and squareness of the second quadrant demagnetization curve. The size of the subgrains was found to be critical to these properties. The Hci of alloy powders with a high Fe content appeared to degrade more severely when reduced below 10 μm. For a fixed mean particle size, alloy powders with a high Fe content also exhibited a less-square second quadrant demagnetization curve. A Br of 9.2 kG, Hci of 18 kOe, Hcb of 7.4 kOe, BHmax of 19 MGOe, and a squareness ratio of 0.91 have been obtained on Sm(CobalFe0.23Cu0.08Zr0.03)8.2. As expected for alloys with a higher Fe content, Br of 11.3 kG, Hci of 20 kOe, Hcb of 8.1 kOe, BHmax of 25 MGOe, and a square ratio of 0.83 have been obtained on Sm(CobalFe0.28Cu0.08Zr0.03)8.2.
Magnetic domains in epitaxial Fe/GaAs micro-patterned wires
Authors: Ebels U. , Adeyeye A. O. , Gester M. , Daboo C. , Cowburn R. P. , Bland J. A. C.
Published: July 2002

The magnetic domain evolution during the magnetization reversal in in-plane magnetized epitaxial Fe/GaAs(001) wire elements with dimensions of 15 μm width × 500 μm length × 300 Å thickness has been studied using a scanning Kerr microscope. The two jump switching processes, characteristic of the magnetization reversal in continuous epitaxial Fe(001) films with four-fold in-plane anisotropy, are observed. However, the domain nucleation and growth processes which mediate the discontinuous and irreversible magnetization switching at the two critical reverse fields Hc1 and Hc2 are different from those in the continuous film. They are found to be determined by the orientation of the applied field with respect to the short and the long wire axis and hence are “shape”-anisotropic. This shape-anisotropy is a combined local dipolar field and anisotropic finite size effect, allowing the growth of domains only from the long/short wire axis and the propagation of domains only in direction of the short/long wire axis respectively according to the applied field direction.
Current dependence of the magnetization rise time in thin film heads
Authors: Freeman M. R. , Elezzabi A. Y. , Stotz J. A. H.
Published: July 2002

Time-domain optical measurements of magnetization dynamics can now be performed with sufficient resolution to reveal the intrinsic speed of many structures relevant to magnetic recording. Here we describe the behavior of the magnetization rise time at the air-bearing surface of a thin film recording head, as a function of the amplitude of current pulses in the write coil. The spatial profile of the magnetization on both sides of the gap is also examined through time-resolved, current dependent measurements. Spatial resolution enhancement via a solid immersion lens allows domain features to be discerned in the data.
Itinerant-to-localized electron transition in perovskite CaRu1−xRhxO3
Authors: Cao G. , Freibert F. , Crow J. E.
Published: July 2002

Magnetic susceptibility χ(T) (2⩽T⩽400 K), electrical resistivity ρ(T) (2⩽T⩽300 K), and Hall effect of the distorted perovskite CaRu1−xRhxO3 with 0⩽x⩽0.35 were measured. This study indicates that the magnetic and transport properties of CaRuO3, a paramagnetic conductor with a narrow 4d-band, depend in an extremely sensitive way on the degree of band filling and bandwidth. Slight Rh doping, while retaining the crystals structure, effectively narrows the 4d-band, and enhances the spin correlation. The occurrence of the enhanced spin correlation is accompanied by an itinerant-to-localized electron transition and a change in crystallographic constants. The results presented together with others strongly suggests that CaRuO3, is on the verge of the metal-insulator transition and magnetic ordering.
A new technique for measuring magnetic anisotropies in thin and ultrathin films by magneto-optics
Authors: Cowburn R. P. , Ercole A. , Gray S. J. , Bland J. A. C.
Published: July 2002

A new technique for high precision measurement of magnetic anisotropy fields in thin and ultrathin films called modulated field magneto-optical anisometry (MFMA) is described. MFMA can be performed by a simple extension to a conventional magneto-optical Kerr effect magnetometer, and is therefore experimentally simple. It can resolve an arbitrary combination of anisotropies of different symmetries with a very high precision, even when there is little magneto-optical signal (e.g., ultrathin in-plane magnetized films) and high optical noise. It is spatially resolving and readily suited to the ultrahigh vacuum environment. MFMA thus offers many advantages over existing anisotropy measurement methods. A quantitative comparison is made between anisotropy field measurements made by MFMA and by Brillouin light scattering on an ultrathin Fe(001) epitaxial film. Agreement is found to within a high precision.
Theory of acoustic scattering by a supported wire
Authors: Khelif A. , Djafari Rouhani B.
Published: July 2002

We present the application of a general Green function formalism to the study of near- and far-field scattering of an incident acoustic plane wave by a perturbation (an inhomogeneity) existing at the planar surface of a substrate. The perturbing element will be a supported wire which, in principle, can have an arbitrary shape and composition. Considering the case of shear horizontal vibrations, we discuss for a wire of parabolic section the behavior of the scattered field as a function of frequencyω, of material parameters, and of incidence angle. At normal incidence, typical behaviors depend on the relative impedance and sound velocity of both materials. For an oblique incidence, there are a large variety of cases depending on ω and on material parameters; one typical behavior will be the predominance of a scattered wave towards the specular reflection direction.
Intense ion beam optimization and characterization with infrared imaging
Authors: Davis H. A. , Bartsch R. R. , Olson J. C. , Rej D. J. , Waganaar W. J.
Published: July 2002

We have developed two-dimensional calorimetry with infrared imaging of beam targets to optimize and measure the energy-density distribution of intense ion beams. The technique, which measures a complete energy-density distribution on each machine firing, has been used to rapidly develop and characterize two very different beams—a 400 keV beam used to study materials processing and an 80 keV beam used for magnetic fusion diagnostics. Results of measurements, using this technique, varying the diode applied magnetic field strength and geometry, anode material type and configuration, and anode-cathode gap spacing are presented and correlated with other observations. An assessment of calorimeter errors due to target ablation is made by comparison with Faraday cup measurements and computer modeling of beam-target interactions.
Saturated electric field effect at semi-insulating GaAs-metal junctions studied with a low energy positron beam
Authors: Hu Y. F. , Ling C. C. , Beling C. D. , Fung S.
Published: July 2002

The interfacial electric field established under different reverse bias conditions in Au and Ni on semi-insulating GaAs junctions has been studied by means of a low energy positron beam. The technique used is that of monitoring the positron drift to the interface through changes in the annihilation radiation lineshape as a function of incident positron beam energy at different reverse biases. The data show a small but clear electric field drift of positrons towards the interface that increases more rapidly at low voltages (less than 50 V) which at higher biases tends towards saturation. This confirmation of electric field saturation adds further weight to the picture of an electric field enhanced electron capture cross section for the ionized EL2 defect. Electric field values extracted from the data are compared with results from other techniques and suggest that enhanced electron capture is already occurring at the relatively low built-in fields (∼1 kV cm−1) found at the unbiased junction, with a rapid increase of EL2+ neutralization occurring for biases above 10 V. At still higher fields ∼10 kV cm−1 (biases>50 V), there appears to be an additional threshold for more complete EL2+ neutralization adjacent to the contact. The present study clearly demonstrates the often overlooked necessity of catering for built-in electric fields in positron diffusivity studies of III–V semiconductors where surface midgap Fermi-level pinning is common.
Photoluminescence and the thermal stability of color centers in γ-irradiated LiF and LiF(Mg)
Authors: Davidson A. T. , Kozakiewicz A. G. , Comins J. D.
Published: July 2002

Pure and magnesium doped (2000 ppm) LiF crystals have been irradiated with γ rays from Am241 and Co60 sources and the thermal stability of the resulting defects has been studied using photoluminescence (PL) and optical absorption (OA). The composite nature of the M band at 440 nm is highlighted by PL data which reveal emission attributed to F3+ defects otherwise hidden in OA measurements. The following bands appear in excitation spectra; Z3 (222 nm), Z2 (285 nm), R1 (329 nm), R2 (375 nm), F3+ (440 nm), and F2 (440 nm). Notable differences are observed between pure and doped samples during thermal annealing relating to the F2 and F3+ bands in particular. Results support the idea of Z centers as being F and F− centers trapped at impurity ions in the crystal.
Anomalous temperature dependence of the Hall mobility in undoped bulk GaAs
Authors: Siegel W. , Schulte S. , Reichel C. , Kühnel G. , Monecke J.
Published: July 2002

Undoped liquid encapsulated Czochralski grown GaAs crystals with a transition from semi-insulating to medium-resistivity behavior show unusual low values of the Hall mobility at 300 K in this transition region. Moreover, in samples of this region an anomalous temperature dependence of μH characterized by an increase of μH with increasing temperature for T<400 K is observed. By model calculations using a standard effective medium theory it is shown that this anomalous behavior of the Hall mobility is due to the existence of mesoscopic electrical nonuniformities connected with the cellular structure of dislocations.
Growth mechanism of nanoparticles prepared by radio frequency sputtering
Authors: Hirasawa M. , Shirakawa H. , Hamamura H. , Egashira Y. , Komiyama H.
Published: July 2002

Growth mechanisms of nanometer-sized particles prepared by rf sputtering on silica glass layers were examined. Gold and gallium arsenide (GaAs) particles synthesized with varying sputtering times on a SiO2 sputtered layer were subsequently buried in a SiO2 film by sputtering a SiO2 target. Transmission electron microscopy showed that in both cases, with increasing sputtering time, the number density decreased and the distance between neighboring particles increased in the initial stage of the growth, which suggests that the particles migrate on the SiO2 surface and coalesce with each other as they grow. Differences of GaAs and gold particle formation suggest that the mobility of the GaAs particles is much larger than that of gold. The results suggest that the migration of nanoparticles is activated by the bond-formation energy released during the incorporation of precursors into growing particles.
Polarity effect on failure of Ni and Ni2Si contacts on Si
Authors: Huang J. S. , Tu K. N. , Bedell S. W. , Lanford W. A. , Cheng S. L. , Lai J. B. , Chen L. J.
Published: July 2002

Stability of contacts in shallow junction devices against high current density is a reliability issue for very large scale integration technology. We have observed a strong polarity effect on failure at nickel and nickel silicide contacts on both n- and p-type Si under high stress conditions. In a pair of cathode and anode contacts the Ni/n+-Si contact pair fails at the anode, while the Ni/p+-Si pair fails at the cathode. The Ni/Ni2Si/n+-Si and Ni/Ni2Si/p+-Si were found to fail preferentially at the cathode. Microbeam Rutherford backscattering spectrometry and Auger electron spectroscopy depth profiles show that a silicide reaction occurs between Ni and Si during current stressing, especially at the failed contacts. In situ resistance data indicate that the resistance of the failed contact increases with time while that of the other contact in the pair remains constant. Transmission electron microscopy shows that the silicide formation is not uniform at the damaged contacts. A mixture of dominant epitaxial NiSi2 and a minor amount of polycrystalline NiSi2 phases was identified. We have proposed mechanisms to explain the polarity effect on failure: wear-out mechanism for the damaged positive contacts of Ni/n+-Si, electromigration enhanced silicide formation for the damaged negative contacts of Ni/Ni2Si/n+-Si and electron-hole recombination mechanism for the damaged negative contacts of Ni/p+-Si and Ni/Ni2Si/p+-Si.
Dynamic analysis of the response of lateral piezoresistance gauges in shocked ceramics
Authors: Feng R. , Gupta Y. M. , Wong M. K. W.
Published: July 2002

The ability to quantify the complete stress state in solids subjected to shock wave, uniaxial strain loading, is an important need. Toward this end, the dynamic behavior of lateral piezoresistance gauges embedded in shocked ceramics was examined using two-dimensional, numerical calculations to understand the relationship between the mechanical and piezoresistance response of manganin foils and the sample lateral stresses. The results show that the dynamic mechanical states within and near a lateral gauge deviate significantly from uniaxial strain due to perturbations caused by gauge emplacement including the presence of epoxy bonds. The dynamic response of the gauge represents a coupling of the sample material response and gauge emplacement details. In general, direct time-resolved inference of the sample lateral stress from the lateral gauge data, using simple assumptions about the gauge mechanical state, is not meaningful. A rigorous interpretation of the lateral gauge data requires the use of two- or three-dimensional computations. However, the present results show that for an assumed time-independent sample response, the equilibrium gauge response (corresponding to a constant state) is a good measure of the far-field, lateral stress in the shocked sample. Hence, lateral gauge data can be analyzed to provide sample lateral stresses with reasonable accuracy in particular situations. Given the importance of determining lateral stresses in shocked solids, various issues related to the use of lateral piezoresistance gauges including the limitations in extrapolating the present results are discussed.