bluuebunny/crossref_metadata_2025 huggingface dataset:
Boron implantation in silicon: Isotope effects studied by secondary ion mass spectrometry
Authors: Svensson B. G. , Linnros J. T. , Holmén G.
Published: July 2002

The range distributions of 10B+ and 11B+ ions implanted into silicon have been studied by secondary ion mass spectrometry. Implantation energies in the range of 50 to 250 keV were used. An isotope effect on the boron profiles is clearly resolved for the energies above ∼100 keV. Projected range and parallel straggling values extracted from the measured profiles are compared with calculated values obtained from Monte Carlo simulations and from numerical computations applying Boltzmann’s transport equation. A good agreement is found, and also the computed values reveal a clear isotope dependence. This effect is attributed to a larger electronic stopping cross section Se for the 10B+ ions than for the 11B+ ions at a given energy in the range where Se is roughly proportional to the ion velocity. 5
Improvement of photoconductivity of a-SiC:H films by introducing nitrogen during magnetron sputtering process
Authors: Saito N. , Goto T. , Tomioka Y. , Yamaguchi T. , Shibayama M.
Published: July 2002

Nitrogen has been introduced in the preparation of hydrogenated amorphous silicon carbon alloy films by magnetron sputtering of silicon in a methane-argon gas mixture. The partial pressure of nitrogen was changed, and its effects on the structural, optical, electrical, and optoelectronic properties of the deposited films were investigated. The SiN bond becomes predominant with increasing partial pressure of nitrogen pN above 10%, at which the optical band gap is rapidly increased. The concentration of silicon and carbon in the films remained almost unchanged by changing pN up to 20%. It was found that the photoconductivity is also almost unchanged up to 20%, while the optical band gap is increased. This indicates that the photoconductivity is improved over a wider optical band gap, as compared with previous studies when the optical band gap was changed by changing deposition conditions but without introducing nitrogen.
Micromagnetic calculations of 180° surface domain walls
Authors: Scheinfein M. R. , Blue James L.
Published: July 2002

We compare quantitative results from two methods of solving the micromagnetics equations for 180° domain walls. The first method solves the Landau–Lifshitz–Gilbert equation by time integration of the stiff differential equations. The second method uses a relaxation scheme to determine the equilibrium domain wall magnetization configuration. We compare results from these calculations for various damping parameters and discretization mesh densities, and also compare our results with domain wall measurements made using scanning electron microscopy with polarization analysis. We conclude that equilibrium domain wall microstructure is insensitive to the damping parameter used in the time method of solution for domain walls in magnetically soft iron and Permalloy. For large damping parameters, the approach to equilibrium is the same for the time method and the relaxation method. Further, we can obtain metastable equilibrium states with both methods.
High power ultrafast excimer lasers
Authors: McIntyre Iain A. , Rhodes Charles K.
Published: July 2002

Rare-gas–halide excimer lasers are suitable for the production of extremely high peak brightness output in the ultraviolet. The basic properties of these systems are examined and the various techniques employed to produce high power (multiterawatt) operation are described. A specific system using short-pulse injection is examined and its ability to focus to intensities above ∼1019 W cm−2 is discussed. Various applications of such high intensity subpicosecond light sources are also considered.
Theoretical operational life study of the closed-cycle transversely excited atmospheric CO2 laser
Authors: Hokazono Hirokazu , Obara Minoru , Midorikawa Katsumi , Tashiro Hideo
Published: July 2002

By using a comprehensive theoretical model that assumes a stable excitation discharge and homogeneous plasma chemical reactions in the discharge plasma, the laser output performance and the variations of the laser gas components during the sealed-off operation of the high-power, closed-cycle transversely excited atmospheric CO2 laser have been investigated. The fractional CO2/N2, molecules decomposition, and the concentration of the various minor impurities accumulated in the laser gas mixture have been theoretically calculated as a function of shots and number of repetitive discharge pulses. According to the results, the gradual reduction of the laser output energy with the successive excitation pulses was mainly due to the depletion of the CO2 molecules and the reduction of the excitation efficiency; the excitation efficiency was decreased in consequence of the increased operational E/N (E is the discharge field strength, N is the total laser gas number density) caused by the accumulation of highly electronegative impurities such as O2 and O3. The nitrogen oxides were found to show little effect on the operational E/N in spite of their large electron attachment cross sections, because these molecules were much less accumulated in the laser gas mixture than O2 or O3. The theoretical model has clarified for the first time that a trace of water (H2O) vapor in the laser chamber effectively acts as a gaseous catalyst to enhance the CO2 reforming reaction in the discharge plasma. Furthermore, this CO2 reforming reaction by H2O, rather than the other backward reactions, predominantly determines the equilibrium CO2 decomposition level in the actual laser chamber. Finally, with regard to the ultraviolet (UV) preionization, it was theoretically shown that the UV absorption depth of the laser gas mixture steeply decreased as the CO2 decomposition increased owing to the contamination of strong UV absorbing species such as O2 and O3.
Modeling plasma flow in straight and curved solenoids
Authors: Boercker David B. , Sanders David M. , Storer Jonathan , Falabella Steven
Published: July 2002

The ‘‘flux-tube’’ model originated by Morozov is a very simple and numerically efficient method for simulating ion motion in plasma filters. In order to test its utility as a design tool, we compare the predictions of the model to recent experimental measurements of plasma flow in both straight and curved solenoids.
The production of high coercivity cast magnets using the HDD process
Authors: Zhang X. J. , McGuiness P. J. , Harris I. R.
Published: July 2002

Nd-Fe-B as-cast alloys in the form of small ingots 4–5 mm thick and with composition Nd2.1Fe14B were transformed into high coercivity magnets using the hydrogenation disproportionation desorption (HDD) process without decrepitation of the blocks taking place. Magnetic measurements revealed that it was possible to produce blocks with coercivity of ∼780 kA m−1, remanence of ∼680 mT and a BHmax of ∼68 kJ m−3 (8.6 MGOe). The high coercivities in these samples were attributed to the disproportionation and subsequent recrystallization process. SEM metallographs of the samples processed at optimum temperature revealed an absence of the original large (∼200 μm) grains of Nd2Fe14B containing areas of free iron; instead an extremely fine grained, submicron, microstructure of single phase Nd2Fe14B grains was observed. In samples processed at temperatures higher than that found to be the optimum, larger Nd2Fe14B grains with extremely rectangular morphologies could be observed growing from surrounding submicron matrix grains. Domain patterns observed on these larger grains, using the Kerr effect, indicated that Nd2Fe14B matrix grains appear to grow predominantly with rectangular morphologies from the disproportionated material with the a and c crystal axis running parallel to the edges of the rectangle.
Crystallography and magnetic properties of CoCrTa films prepared on Cr underlayers with different substrate bias conditions
Authors: Pressesky Jason , Lee Sang Y. , Duan Shanlin , Williams Deborah
Published: July 2002

Thin CoCrTa alloy films on Cr underlayers were prepared on disk substrates by dc magnetron sputtering. The Cr thickness was varied and three bias conditions were used, viz., no bias, dc bias, and rf bias. Film morphology, crystallography, and magnetic properties were studied. Bias was observed to promote Cr 〈100〉 and Co 〈112̄0〉 preferred orientation in the films. The coercivity was observed to increase with increasing Cr thickness. The application of bias was observed to increase the coercivity and coercive squareness, and to reduce the grain size.
Effects of photochemical vapor deposition phosphorus-nitride interfacial layer on electrical characteristics of Au-InP Schottky diodes
Authors: Jeong Yoon-Ha , Kim Geon-Tae , Kim Seok-Tae , Kim Kwang-Il , Chung Wook-Jin
Published: July 2002

The enhancement of Schottky barrier heights on InP substrate using a thin layer of phosphorus-nitride(P3N5), grown by a direct photochemical vapor deposition process with a gaseous mixture of PCl3 and NH3, has been studied. A Au-Schottky diode formed using the in situ processes has a 50-Å P3N5 intermediate layer showed a barrier height of 0.81 eV, an ideality factor of 1.08, and a reverse leakage current of 5.0 × 10−8 Å/cm2 at 1 V. The breakdown voltage was larger than 30 V. The thin-film/InP interface was investigated using x-ray photoelectron spectroscopy.
Optimized tight-binding valence bands and heterojunction offsets in strained III-V semiconductors
Authors: Anderson Neal G. , Jones Sean D.
Published: July 2002

An optimized nearest-neighbor tight-binding description of valence bands in strained-layer III-V semiconductors is developed and applied to the calculation of valence-band offsets at strained heterojunctions. It is first shown that a single set of universal interatomic matrix elements can be found which, when appropriately scaled for bond length, simultaneously provide near-optimum tight-binding predictions of valence-band uniaxial deformation potentials, trends in photoelectric thresholds, and valence bandwidths for the common III-V compounds. Application of the optimized tight-binding model to the calculation of valence-band offsets at strained heterojunctions is then discussed, and one simple approach is described which combines a fully strain-dependent version of the optimized tight-binding model with Tersoff’s quantum-dipole heterojunction model. Offsets calculated using this combined approach are shown to agree with experimental data better than either strain-dependent natural tight-binding offsets or offsets calculated directly from Tersoff’s model. Finally, convenient quadratic expressions for the composition dependence of light-and heavy-hole valence-band offsets, as calculated using the combined approach, are tabulated for several strained and unstrained ternary-on-binary III-V heterojunctions. The balance between accuracy and simplicity offered by our approach should render it useful for exploratory heterojunction device modeling.
The effects of deposition rate on the structural and electrical properties of ZnO:Al films deposited on (112̄0) oriented sapphire substrates
Authors: Igasaki Yasuhiro , Saito Hiromi
Published: July 2002

Aluminum doped zinc oxide (ZnO:Al) films were deposited on (112̄0) oriented sapphire substrates heated to 200 °C with a radio-frequency (rf) power ranging from 25 to 170 W for a deposition rate in the range 0.7–27.4 nm min−1 by rf-magnetron sputtering from a ZnO target mixed with Al2O3 of 2 wt. %. All of the films deposited were (0001) oriented single-crystalline films with an internal stress. The stress was increased and degraded the crystallinity of the epitaxial film as a deposition rate was increased, and thus the Hall mobility and the resistivity of the film were, respectively, decreased and increased. However, the resistivities obtained were in the range about 1.4–3.0×10−4 Ω cm, the values comparable to those for indium tin oxide film presently used as a transparent electrode.
Potential formation near powered electrode in radio-frequency-driven discharge
Authors: Okuno Yoshihiro , Fujita Hiroharu
Published: July 2002

Potential formation at the crest and trough phases in a radio-frequency (rf) oscillation (13.56 MHz) near a powered electrode has been observed for the first time in a rf-driven discharge plasma on the basis of a novel method of an emissive probe. The spatial profile at the crest phase was found to form an electron sheath extremely different from the conventional one, whereas the profile at the trough phase forms apparently a conventional ion sheath. The sheath thickness estimated from the time-averaged potential profile was also discussed by comparing with the spatial profiles of plasma density and optical emission.
All-optical switching of solitons in two- and three-core nonlinear fiber couplers
Authors: Soto-Crespo Jose M. , Wright E. M.
Published: July 2002

We present a numerical investigation of all-optical switching of solitons in two- and three-core nonlinear fiber couplers. Both self-switching of solitons and phase-sensitive switching are considered. The extension of the results to multicore couplers is briefly discussed.
A surface recombination model applied to large features in inorganic phosphor efficiency measurements in the soft x-ray region
Authors: Benitez E. L. , Husk D. E. , Schnatterly S. E. , Tarrio C.
Published: July 2002

The photoluminescent quantum efficiencies of the inorganic phosphors Y2O2S:Eu, Y2O3:Eu, La2O2S:Tm, Gd2O2S:Tb, and Sr5Cl(PO4)3:Eu have been measured in the range 17 to 450 eV. The optical properties of these phosphors from 2 to 160 eV have been determined from inelastic electron scattering measurements. Using a model which involves nonradiative recombination at the surface of the material, we relate photoluminescent efficiency to optical absorption properties, and find that surface recombination is the predominant source of efficiency loss for these materials in the soft x-ray range. From the model, we obtain values for the diffusion length, surface recombination velocity, and bulk quantum efficiency of these materials.
Temperature dependent photoluminescence of the Sn-implanted InSe
Authors: Gridin Vladimir V. , Kasl C. , Comins J. D. , Beserman R.
Published: July 2002

Measurements of temperature dependent photoluminescence (PL) of Sn-implanted InSe were carried between 5 K and room temperature (RT). We observe two broad structures in the PL spectra: one in the vicinity of E1≊2.2 eV and another at E2≊1.8 eV. With a decrease of temperature the energy E1 (E2) increases (decreases) by about 35 meV with respect to the RT values. The relative intensity of the PL structures, R12, also shows considerable temperature dependence. We present empirical relations for the temperature dependence of E1 and E2. Except for the low temperature data the temperature dependence of the absolute changes in E1 and E2 was found to be in reasonable agreement with the usual band gap behavior: αT2/(T+β). This was compared with the available PL data of virgin InSe. The results are interpreted in terms of the appearance of deep impurity levels approximately 0.6 eV above the valence band edge.
Atomic disorder induced by mechanical milling in the Nb3Au intermetallic compound
Authors: Di L. M. , Bakker H.
Published: July 2002

The behavior of the intermetallic A15 compound Nb3Au in the early stage of mechanical milling was studied. By following the degradation of the superconducting transition temperature and the increase of the lattice parameter as a function of milling time, it was found that Nb3Au exhibits atomic (chemical) disorder due to milling. A value for the ballistic jump frequency of the atoms was assessed. The behavior turns out to be very similar to that of Nb3Sn, although for long milling time Nb3Sn amorphizes, whereas Nb3Au transforms from the A15 structure to the bcc structure.
Uniaxial stress dependence of the properties of the DX center in n-Al0.32Ga0.68As
Authors: Wang Zhiguo , Chung Ki-woong , Miller T. , Williamson F. , Nathan M. I.
Published: July 2002

We have measured the emission energy, Ee, and the capture energy, Ec, and derived the thermal activation energy of the DX center, EDX, in n-Al0.32Ga0.68As as a function of uniaxial stresses along the 〈100〉 and the 〈111〉 directions. We found that the stress coefficients of EDX change sign when the band gap of Al0.32Ga0.68As changes from direct to indirect for both stress directions, and the stress coefficients of EDX in the indirect region are about same for both stress directions. These results agree with the model proposed by Chadi and Chang which assumes that the DX center is a highly localized center and disagree with the model which takes the DX center to be the effective-mass state associated with the L minima.
Faraday rotation of spontaneous magnetization in a laser-produced plasma from solid target
Authors: Khan Manoranjan , Sarkar Susmita , Bhattacharyya B. , Chakraborty B. , Desai T. , Pant H. C.
Published: July 2002

Interaction of elliptically polarized laser radiation with plasma produced from a solid target generates magnetic fields simultaneously along axial and transverse directions. The axial magnetic field induces the plane of polarization of the incident laser beam to rotate due to the Faraday effect. Numerically the Faraday rotation has been calculated for interaction of Nd glass laser (λ=1.06 μm) in the subdense plasma in which the plasma density, temperature, and self-generated magnetic field vary in the axial direction.
Analysis of magnetic relaxation in YBa2Cu3O7−δ leading to temperature-independent activation energies
Authors: Theuss H. , Reininger T. , Kronmüller H.
Published: July 2002

Within the framework of first-order kinetics the activation energy spectrum p(U) for thermal activated flux motion in YBa2Cu3O7−δ single crystals is derived from the time decay of the magnetic polarization μ0M(t). The distribution function p(U) turns out to be temperature independent, so that the time dependence of μ0M(t,T) can be described by one unique spectrum p(U) in the whole temperature range 0 < T < Tc. Concerning the crystal directions H∥ĉ and H⊥ĉ, respectively, a threshold energy in the case H⊥ĉ is noticed which may be related to the Peierls potential, formed by the Cu-O planes.
Electrical characterization of a GaAs quantum well confined by GaAlAs layers or by two superlattices
Authors: Ababou S. , Guillot G. , Regreny A.
Published: July 2002

Capacitance-voltage, deep level transient spectroscopy (DLTS), and admittance spectroscopy measurements have been performed to characterize a GaAs quantum well confined either by Ga0.57Al0.43As layers or by two GaAs-Ga0.54Al0.46As superlattices. Due to a large capacitance decrease at low temperature, the well response cannot be obtained from DLTS measurements. This capacitance step is related to the thermionic emission from the well to the barrier regions. For the case of the enlarged well in the superlattice, the electron emission takes place towards the conduction miniband. In order to deduce the conduction-band discontinuity, admittance spectroscopy measurements have been applied. The bottom of the superlattice miniband is at 107±10 meV from the GaAs conduction-band minimum. The corresponding band offset deduced from a theoretical calculation is ΔEc=410±10 meV=(0.65±0.02) ΔEg. For the GaAs quantum well in GaAlAs, we measure a conduction-band discontinuity of 0.35 eV between GaAs and GaAlAs that has its minimum at the X point. At the Γ point, the band offset ratio ΔEc/ΔEg is once more confirmed.
Surface microstructures of diamond crystals synthesized in the H2-CH4-O2 system
Authors: Hirabayashi Keiji
Published: July 2002

The dependence of the microstructures of the {111} surfaces of diamond crystals synthesized in the H2-CH4-O2 system on the oxygen-carbon ratio (O/C) has been studied to elucidate the mechanism of the crystal growth. On the {111} surfaces of diamond crystals synthesized in high O/C (≥0.8, in our synthesized conditions), the two-dimensional nucleation rate is suppressed and the two-dimensional crystal growth rate is promoted. The Raman spectrum of the diamond crystals synthesized in high O/C shows that the half width at half maximum and the peak position are equivalent to those of natural diamond crystals.
Arsenic cluster dynamics in doped GaAs
Authors: Melloch M. R. , Otsuka N. , Mahalingam K. , Chang C. L. , Woodall J. M. , Pettit G. D. , Kirchner P. D. , Cardone F. , Warren A. C. , Nolte D. D.
Published: July 2002

We have studied the formation of As precipitates in doped GaAs structures that were grown by molecular beam epitaxy at low substrate temperatures and subsequently annealed. We find that the As precipitates form preferentially on the n side of such fabricated GaAs pn junctions. As the coarsening process proceeds, there is a gradual increase in the amount of As in precipitates in the n-GaAs region and a decrease in the p-GaAs region; the depletion region between the pn junction becomes free of As precipitates. These observations can be understood qualitatively based on the charge states of the As interstitial and using thermodynamic arguments in which the crystal attempts to minimize the chemical potential during the anneal. The presence of the excess As results in a stable Be profile even to anneals of 950 °C. Finally, a temperature cycling technique to grow arbitrarily thick GaAs epilayers containing As precipitates was demonstrated.
Method for investigating the reversal properties of isolated barium ferrite fine particles utilizing magnetic force microscopy (MFM)
Authors: Chang Thomas , Zhu Jian-Gang , Judy Jack H.
Published: July 2002

The reversal properties of isolated barium ferrite recording particles with Co-Ti substitution (BaFe12−2xCoxTixO19) are studied by a newly developed magnetic force microscopy (MFM)-based technique. The size dependence of the switching field on particles with diameters ranging from 30 to 80 nm were investigated. A decrease in the switching field was observed, which corresponds to a decrease in the particle diameter below 50 nm. This indicates thermally assisted switching in small size particles. As the diameter increases, the switching field reaches a maximum at 55 nm then sharply decreases. An atomic force microscopy (AFM) profile of the particle’s thickness shows that the aspect ratio variation among these particles cannot account for the observed difference in the switching field. Thus, it is concluded that incoherent reversal is occurring in particles with diameter greater than 60 nm.
Predicting the dynamic behavior of moving magnet actuators
Authors: Furlani E. P. , Lee J. K. , Dowe D.
Published: July 2002

Electromagnetic actuators are widely used for applications involving controlled motion. An important class of these devices is the moving magnet actuator. This type of actuator is gaining widespread use due to the concurrent development of high-strength NdFeB magnetic materials. In this article, a method is presented for computing the dynamic behavior of moving magnet actuators. This method comprises a hybrid mathematical approach that includes magnetic circuit analysis, finite element analysis, and the direct computation of force and torque using integral expressions. The method is demonstrated herein using a specific actuator.
The power sensitivity of magnetic resonance experiments
Authors: Iriguchi Norio
Published: July 2002

On the basis of formulae given by Hoult and Lauterbur [J. Magn. Res. 34, 425 (1979)] for magnetic resonance in conductive samples, formulae for the signal-to-noise ratio (SNR) and the SNR per unit input power are presented.
Effects of melt temperature on the magnetic properties of FeCuNbSiB alloy
Authors: Lim S. H. , Pi W. K. , Noh T. H. , Kim H. J. , Kang I. K.
Published: July 2002

The magnetic properties of an Fe73.5Cu1Nb3Si13.5B9 alloy with nanoscale crystalline structure have been investigated as a function of the melt temperature. In the temperature range of 1240 to 1380 °C investigated in the present study, the permeability increases with the melt temperature. This result may be explained in terms of the reduction in the magnetic anisotropy as a function of the melt temperature. This is supported by the results for the remanence ratio, which also increases with melt temperature in a similar manner to the permeability. In the latter part of the work described in this paper, the relationships between the permeability and the coercivity, and the permeability and the remanence ratio have been investigated for the nanocrystalline alloy. It was found that the permeability is correlated more closely to the remanence ratio than the coercivity.
Optical properties of μc-Si:H/α-Si:H layered structures: Influence of the hydrogen bonds, crystallite size, and thickness
Authors: Boultadakis S. , Logothetidis S. , Ves S. , Kircher J.
Published: July 2002

Thin films of layered μc-Si:H/α-Si:H structures grown with rf magnetron sputtering have been studied with spectroscopic ellipsometry (SE) and Raman spectroscopy (RS). Analysis of the dielectric function spectra with the Si-centered tetrahedron model and deconvolution of the Raman spectra suggest that the silicon-hydrogen bonds are correlated with the microscopic voids in α-Si:H while the good quality α-Si:H films exhibit SiH3 bonds and small amount of SiH2 bonds. The quality of the surface and interface of the μc-Si:H/α-Si:H was analyzed by SE and transmission electron microscopy and the dependence of an interfacial amorphous layer at the initial stages of the μc-Si:H growth on the argon and hydrogen pressure was found. Furthermore, a correlation between the SE and RS results on μc-Si:H layers is shown and commented, while a dependence of the energy shift and the broadening of the interband transitions and the Raman transverse optical peak on the mean crystallite size has been found and discussed in view of the finite size and stress effects on the electronic structure of these materials. This analysis suggests that strain is one of the most important factors during the growth of μc-Si:H.
Crossover between equilibrium and nonequilibrium dynamics in a re-entrant CrFe ferromagnet
Authors: Mitchler P. , Roshko R. M. , Ruan W.
Published: July 2002

Measurements of the low field static magnetization and thermoremanent decay of a Cr-17 at. % Fe spin glass and a re-entrant Cr-22 at. % Fe ferromagnet are presented. The thermoremanent relaxation within the pure spin glass phase exhibits nonequilibrium effects and is describable by the product of a power law and a stretched exponential superposed on a constant term. In the ferromagnet, a crossover is observed between two distinct dynamic regimes: a high temperature, power law regime, with negligible aging effects, which is essentially coincident with the ferromagnetic phase, and a low temperature, stretched exponential regime, with nonequilibrium, age-dependent behavior, which is correlated with the re-entrant phase.
X-ray measurements of mercury density in arc discharge lamps during warm-up and following extinction
Authors: Fohl Timothy , Kramer Jerry M. , Lester Joseph E.
Published: July 2002

A method for measuring the density of mercury in high intensity discharge (HID) lamps using x-ray absorption is described. The method is suitable for use with lamps with any outer envelopes. Time dependent values of absolute density and cold spot temperature were obtained for a variety of HID lamp types during the period following the extinguishing of the arc. Values for the time dependent mercury mass per unit area at various locations in the lamp were obtained during the warming up of the lamps.
Properties of epitaxial Ba-hexaferrite thin films on A-, R-, and C-plane oriented sapphire substrates
Authors: Hylton T. L. , Parker M. A. , Coffey K. R. , Howard J. K.
Published: July 2002

Thin films of Ba-hexaferrite of the magnetoplumbite structure have been prepared on sapphire substrates of orientation (112̄0), (11̄02), and (0001). Torque magnetometry indicates easy axes parallel, tilted off parallel, and perpendicular to the substrate for the A-, R-, and C-plane substrates. X-ray diffraction shows that this is due to epitaxy between the Ba-ferrite and sapphire. For each substrate orientation, the c axes of the Ba-hexaferrite and sapphire are parallel and the directions of close packing in the close-packed oxygen planes perpendicular to the c axes are parallel. Torque measurements are consistent with the combined effects of crystal and shape anisotropy. These results suggest that Ba-hexaferrite thin films with an easy axis at any angle with respect to the substrate can be prepared by appropriate choice of the sapphire substrate orientation, which may be of interest for microwave and other applications.
Higher order nonlinear response in dilute random composites
Authors: Hui P. M.
Published: July 2002

The effective response of a random nonlinear composite consisting of a nonlinear material with a displacement-electric field relation of the form D=εB0E+χB‖E‖βE randomly embedded in a host with a linear D-E relation is studied. Using the Maxwell–Garnett approximation and taking into account the correction to the local field in the presence of nonlinear inclusions, expressions are derived for the effective response to the order of χB and χB2, which are valid in the dilute limit of the nonlinear component in the composite.
Electron mobility in single- and polycrystalline Ga2O3
Authors: Fleischer M. , Meixner H.
Published: July 2002

Ga2O3 is a purely n-conducting metal oxide at high temperatures. Increasing interest is being shown in this material as the basic material for gas sensors since its electrical conductivity is gas dependent. To investigate its electrical conductivity mechanism in the temperature range between 800 and 1000 °C, conductivity and Hall measurements were performed on single crystals and on polycrystalline ceramics. In the conductivity measurements, identical results were obtained with dc and low-frequency ac, thus confirming the notion of purely electron current transport. A thermally activated specific conductivity is observed in both cases, that of the single crystals lying around three and a half orders of magnitude above that of the ceramic. The carrier densities are determined from the Hall measurements, a thermally activated behavior always being observed. The carrier density of the ceramic is only 2×1013 cm−3 at 1000 °C, that of the single crystals lying somewhat more than three orders of magnitude above this. The Hall mobilities are determined by combining the conductivity and Hall measurements. Within the limits of measurement accuracy, the Hall mobilities in the ceramic and the single crystals are shown to be identical. The Hall mobility is about 10 cm2/V s at 1000 °C with a thermal activation energy of about 0.6 eV. The results suggest that conduction in Ga2O3 at high temperatures is well described by the small polaron model.
Detection of local conductivity by laser-frequency mixing in a scanning force microscope
Authors: Völcker M. , Krieger W. , Walther H.
Published: July 2002

Coupling of laser radiation into a conducting tip of a scanning force microscope allows one to distinguish between conducting and nonconducting parts of a sample. This is demonstrated for a pattern of small metal islands on a nonconducting BaF2 substrate. In the experiment two infrared laser beams are coupled into the tip. The difference frequency is generated in the tip-sample junction, emitted, and detected by means of an open waveguide. Images with this signal are recorded simultaneously with the topography. Difference-frequency generation is observed only on conducting parts of the surface and at islands larger than about 1 μm in diameter. The size of the conducting island as well as the tunneling distance between the tip and conducting surface determine the magnitude of the difference-frequency signal. Frequency mixing of visible laser light, using the excitation of localized plasmons for field enhancement, may lead to the detection of local conductivity on much smaller structures.
Ion-matrix sheath in a cylindrical bore
Authors: Sheridan T. E.
Published: July 2002

The potential structure, electric field, and sheath width for the ion-matrix sheath in a cylindrical bore are calculated. The appropriate scaling length for this problem is found to be √2 times larger than the planar ion matrix sheath width. If the radius of the bore is less than this scaling length, then the sheaths from ‘‘opposite’’ sides of the bore overlap, and the potential drop from the axis to the sidewalls decreases with the square of the bore radius. This limits the maximum energy of ions impacting the target. For bores larger than this scaling length, the asymptotic planar solutions are rapidly approached.
Excimer laser ablation of polyimide-doped poly(tetrafluoroethylene) at 248 and 308 nm
Authors: D’Couto G. C. , Babu S. V. , Egitto F. D. , Davis C. R.
Published: July 2002

Experimental data on the 248 and 308 nm wavelength excimer laser ablation at poly(tetrafluoroethylene) doped with polymide (PI) are reported for a range of fluences and dopant concentrations. Threshold fluences were determined and a correlation was obtained between the dopant concentration and the threshold fluence. The threshold fluences and the limiting etch rates at high fluences decreased with increasing dopant concentration, and there is a minimum dopant concentration below which there is no ablation at both of the wavelengths. The side wall taper of the ablated holes increased with increasing dopant concentration. At subthreshold fluences, the polymer surface was modified with selective removal of PI from the polymer blend. The etch rates have been modeled using a two parameter thermal model to describe the etching process. The parameters obtained by fitting the data are qualitatively correlated to the dopant concentration and the measured limiting etch rates.
Conductance technique measurements of the density of states between Si and ZnS grown by molecular beam epitaxy
Authors: McClean I. P. , Konofaos N. , Thomas C. B.
Published: July 2002

Electrical measurements have been performed on Al/ZnS/p−Si diodes, where the ZnS film was grown using the molecular beam epitaxy process. Capacitance-voltage data confirm that the ZnS layer is conducting as shown by the fall of capacitance in accumulation and the associated feature of deep depletion that is frequency independent. A wide dispersion of capacitance at different ac signal frequencies is observed in depletion. Also, admittance measurements are taken, using the conductance technique. The discrete energy-state model provided the best fit to the admittance data, inherently implying a low defect interface has been created. The density of interface states varies between 8.2×1010 cm−2 eV−1 at the Si Fermi-level and 2.7×1010 cm−2 eV−1 at the Si midgap.
Interactions between single domain particles
Authors: Proksch Roger , Moskowitz Bruce
Published: July 2002

We present a variation of the Wohlfarth–Henkel technique for studying interactions in single domain particles (SDPs) in which samples are prepared in different remanent states before the remanent magnetization curves are measured. By analyzing the resulting series of switching field distributions (SFDs), it is possible to separate the effects of positive (magnetizing) and negative (demagnetizing) interactions, even when one type dominates the other. The method is applied to two types of samples consisting of uniform SPDs of magnetite produced by magnetotactic bacteria: (1) whole bacterial cells containing single, linear chains of SDPs; and (2) SDPs extracted from the cells and allowed to aggregate into clumps.
Micromagnetic modeling and experimental study of transition noise correlation in thin-film media
Authors: Zhu Jian-Gang , Wang Haiyun , Arnoldussen Thomas C.
Published: July 2002

Micromagnetic modeling and time-domain experimental measurements are combined to study spatial noise correlations between two closely recorded transitions in oriented longitudinal thin-film media. The Karhunen–Loeve expansion method is utilized to characterize the noise correlations. The analysis shows that the noise correlation of the dipulses exhibits three principle modes: amplitude variation, shift in unison, and breathing. The supralinear increase of noise at small bit intervals in well-oriented longitudinal film media is dominated by the amplitude variation, which arises from large variation of the degree of percolation between adjacent transition boundaries. The results from the micromagnetic modeling shows excellent agreement with the results obtained from the experimental time-domain noise measurements.
Noise correlations in dibit recording
Authors: Lin Gang Herbert , Bertram H. Neal , Simmons Ralph
Published: July 2002

Medium noise correlations in dibit data patterns have been studied experimentally in thin film disk/head recording systems. An empirical eigenfunction expansion was utilized to identify the dominant noise modes and possible associated physical mechanisms. At large bit separation where noise power adds linearly, dibit in-phase jitter and out-of-phase jitter were found to be the leading noise modes. The noise waveform shape associated with out-of-phase jitter approaches that of the dibit signals (i.e., this mode of noise becomes amplitude-like) as the bit separation is reduced. In addition, dibit amplitude variation due to the interaction between the two pulses generates the same amplitude-like noise for a small dibit separation. Amplitude mode noise increases as the bit separation is reduced, causing the supralinear noise increase in high recording density. Results from nonlinear amplitude reduction measurement in dibit recording indicate a close correlation between the nonlinearities and the supralinear noise enhancement at high recording density.
Multitarget sputtering of high magnetostrictive Tb-Dy-Fe films
Authors: Quandt E.
Published: July 2002

Giant magnetostrictive thin films of both the binary compound TbxFe1−x and the ternary compound (TbyDy1−y)xFe1−x have been prepared by multitarget magnetron sputtering in wide compositional ranges. In view of their potential use as actuators in microsystem technology the research was focused on low-field magnetostrictive properties in connection with low coercivity. Giant magnetostriction was found for a range of compositions with Tb contents higher than those of the crystalline Tb-Fe compounds. Depending on the composition, amorphous films with a magnetostriction as large as λ≊450×10−6 at an applied field of 6 kOe and a coercive field Hc<10 Oe could be prepared. The improvement of the low-field magnetostriction of the ternary alloy Tb-Dy-Fe in comparison to Tb-Fe was found to be less distinct than for crystalline materials, probably because all reported films showed a perpendicular easy axis. The amorphous state of the films are stable at least up to 300 °C.
Non-Rutherford cross sections in heavy ion elastic recoil spectrometry: 40–70 MeV 32S ions on carbon, nitrogen, and oxygen
Authors: Räisänen J. , Rauhala E. , Knox J. M. , Harmon J. F.
Published: July 2002

Elastic scattering cross sections for 12C, 14N, and 16O recoils by 32S ion bombardment below 70 MeV at recoil angles of 20°, 25°, 30°, and 40° have been investigated. The experiments are performed by measuring the elastic scattering yields near the Coulomb barrier energies for the kinematically reversed 32S (12C, 12C) 32S, 32S(14N, 14N)32S and 32S(16O, 16O)32S reactions. At a threshold energy, above which the Rutherford model becomes invalid, the cross sections are found to fall off rapidly. No prominent resonancelike structures are observed in the present energy region. The non-Rutherford threshold energies, and the elastic recoil excitation functions are presented. Based on fits to the present heavy ion recoil data and earlier heavy ion scattering data from the literature, a classical semiempirical formula for predicting the projectile threshold energies for 12C, 14N, and 16O recoils is introduced. Above the threshold, the rapid decrease of the cross sections renders useful elastic recoil detection analysis impossible. These energies thus constitute a practical high energy limit for elastic recoil spectrometry.
Analysis of conventional and three-dimensional atom probe data for multiphase materials
Authors: Camus Eric , Abromeit Christian
Published: July 2002

A statistical analysis of depth profiles obtained by field-ion microscopy with atom probe is developed for the investigation of phase separation in concentrated alloys. Theoretical correlation coefficients, contingency tables, and contingency coefficients are calculated and are compared with those values obtained from experimentally observed depth profiles in ion irradiated alloy Nimonic PE16. This paper shows that a refined contingency analysis is the appropriate concept for the evaluation of multiphase alloy data. The analysis is applied to the diffusion-controlled dissolution of randomly dispersed precipitates.
Influences of chain rigidity, in-plane orientation, and thickness on residual stress of polymer films
Authors: Ree Moonhor , Chu Cheng-Wu , Goldberg Martin J.
Published: July 2002

Films of several polyimides having various chain rigidities were prepared on silicon substrates from the poly(amic acid) precursors by a conventional spin-coat/dry/thermal imidization process: poly(p-phenylene pyromellitimide) (PMDA-PDA), poly(4,4′-oxydiphenylene pyromellitimide) (PMDA-ODA), poly(p-phenylene biphenyltetracarboximide) (BPDA-PDA), and poly(4,4′-oxydiphenylene benzophenonetetracarboximide) (BTDA-ODA). The film properties, such as residual stress, refractive indices, and birefringence were investigated using wafer bending and prism coupling techniques. The stress was relatively low for both fully rodlike PMDA-PDA and semirigid BPDA-PDA, intermediate for semiflexible PMDA-ODA, and high for flexible BTDA-ODA. However, even for the rigid polyimides such low stress is achievable only in thin films (≤10 μm thick for PMDA-PDA and ≤20 μm thick for BPDA-PDA). Otherwise, they are no longer low stress materials. Overall, the film stress was very sensitive to the film thickness in the rigid polyimides but less sensitive to it in the flexible polyimides. The lower stress polyimide film exhibited the higher anisotropy of refractive indices (i.e., higher birefringence, which is a measure of molecular in-plane orientation). The molecular in-plane orientation, which is a critical factor to the film stress, becomes higher as the polymer chain rigidity is greater and the film is thinner. In the polyimide films prepared from the precursors in N-methyl-2-pyrrolidone, which has a high boiling point and low vapor pressure, the molecular in-plane orientation is mainly induced by the surface of substrates rather than other film process factors including spinning in spin coating.
Contribution of defects to electronic, structural, and thermodynamic properties of amorphous silicon
Authors: Stolk P. A. , Saris F. W. , Berntsen A. J. M. , Weg W. F. van der , Sealy L. T. , Barklie R. C. , Krötz G. , Müller G.
Published: July 2002

The structure of pure, nonhydrogenated amorphous silicon (a-Si) was modified by means of ion implantation, furnace annealing, and pulsed laser annealing. Defects in a-Si were probed by measuring the photocarrier lifetime τ at low carrier densities (1018/cm3) with subpicosecond resolution using pump-probe reflectivity measurements. The average cross section of defect-related midgap states for free-carrier capture is found to be 6×10−16 cm2. In addition, the average bond-angle distortion Δθ in a-Si was derived from Raman spectroscopy. Annealing as-implanted a-Si for 1 h at T≤500 °C induces defect annihilation as well as network relaxation. In contrast, 32 ns pulsed laser heating of a-Si just below the melting threshold leads to relaxation of Δθ without significant defect annihilation. This annealing behavior can be understood on the basis of defect diffusion kinetics. Implanting fully relaxed a-Si with 1 MeV B+, Si+, and Xe+ up to damage levels of 0.004 displacements per atom raises the defect density without affecting Δθ. Only after the defect density has saturated at higher damage levels is Δθ returned to the as-implanted level. The electronic density of states of a-Si is determined using optical-absorption spectroscopy, yielding Nsat≊0.5 at. % for the saturation defect density in a-Si at room temperature. Electron paramagnetic resonance shows that a minor fraction (0.02 at. %) of these defects is spin active. The response of c-Si and relaxed a-Si to implantation damage is comparable, suggesting that the defect populations in both materials are similar. Comparing carrier lifetime measurements and Raman spectroscopy for the various experimental treatments demonstrates that there is no unique correlation between the defect density and Δθ in a-Si. Assuming that defects and Δθ have independent enthalpic contributions, the Gibbs free energy of various structural states of a-Si is calculated. These calculations indicate that the melting temperature of a-Si may vary from 1010 to 1490 K.
Magnetic and crystallographic properties of molecular beam epitaxially grown Fe3O4/NiO superlattices and Fe3O4 films
Authors: Krebs J. J. , Lind D. M. , Lochner E. , Shaw K. A. , Portwine W. , Berry S. D.
Published: July 2002

Ferromagnetic resonance, SQUID magnetometry, and x-ray diffraction have been used to characterize a set of [Fe3O4(68 Å)/NiO(17 Å)]N superlattices (SL) with N=3, 10, 30, and 100, as well as a 1.5-μm-thick Fe3O4 film. For this NiO thickness, Fe3O4 layers are strongly coupled and the in-plane anisotropy is much less than the 330-Oe ferromagnetic resonance (FMR) linewidth at 35 GHz. Both in-plane and perpendicular FMR at 9.5 and 35 GHz have been used, with the 9.5-GHz data showing significant hysteresis associated with the sample magnetization. X-ray diffraction indicates that both the film and SL’s are nearly cubic single-crystalline structures with long-range coherence. The 300 K magnetization data indicate the presence of small cubic anisotropy in the SL’s, although bulklike Fe3O4 magnetic ordering in the thick single film. When the Fe3O4 film is cooled below the Verwey transition in a 10 kOe field (aligned along 〈100〉), the FMR shows that the sample develops a large uniaxial (Ku=1.8 kOe) in-plane anisotropy with its easy axis along the field direction. Decreased microwave loss also occurs abruptly at the film TV.
Giant magnetoresistance in electrodeposited Co–Ni–Cu/Cu superlattices
Authors: Alper M. , Attenborough K. , Baryshev V. , Hart R. , Lashmore D. S. , Schwarzacher W.
Published: July 2002

We have electrodeposited a series of Co–Ni–Cu/Cu superlattices in which the Cu layer thickness was varied between 7 and 35 Å and the Co–Ni–Cu alloy layer thickness held constant. ‘‘Giant magnetoresistance’’ was observed for all films, with the magnitude of the effect decreasing with increasing Cu spacer layer thickness.
Comparison of the lateral carrier transport between a GaAs single quantum well and the AlGaAs barrier during cathodoluminescence excitation
Authors: Araújo D. , Oelgart G. , Ganière J.-D. , Reinhart F. K.
Published: July 2002

In a recent paper we measured the lateral hole diffusion in a GaAs/AlGaAs single quantum well (SQW) by a novel method. At helium temperature, we estimate a lateral hole diffusion length in the QW of 1.5 μm. However, the assumption that diffusion takes place mainly in the SQW needs to be checked, as the measured diffusion length is the result of two competing processes: (i) hole diffusion in the SQW plane itself and (ii) hole diffusion in the barrier followed by recombination in the SQW. We present here a comparison between the lateral hole distribution in the SQW and in the AlGaAs barrier. First, we estimate the hole diffusion length in the barrier fitting experimental cathodoluminescence linescans on simulated ones. Second, using the measured diffusion lengths in the QW plane and in the bulk barrier and modeling the carrier transport, we deduce the lateral hole distribution in both layers. It is found that even for very large barriers (1.2 μm), the hole diffusion in the barrier contributes less than 0.1% of the total lateral hole diffusion. The lateral transport is mainly carried by holes in the QW (2D diffusion) due to their confinement in the well.