bluuebunny/crossref_metadata_2025 huggingface dataset:
Facet temperature reduction by a current blocking layer at the front facets of high-power InGaAs/AlGaAs lasers
Authors: Rinner F. , Rogg J. , Kelemen M. T. , Mikulla M. , Weimann G. , Tomm J. W. , Thamm E. , Poprawe R.
Published: February 2003

The facet heating of a single-quantum well InGaAs/AlGaAs broad-area high-power laser-diodes emitting at 940 nm was reduced by the introduction of a 30 μm long current blocking region located at the front facet of the laser, also increasing the level of catastrophical optical mirror damage. The blocking of the pump current close to the facet reduces the carrier density and then the surface recombination current. The temperature rise of 2 mm long and 200 μm wide lasers is reduced by a factor of 3–4.
Preparation of different BeTe surface reconstructions by decapping and thermal treatment
Authors: Wagner J. W. , Wagner V. , Hansen L. , Schmidt G. , Geurts J. , Vogt P. , Esser N. , Richter W.
Published: February 2003

We report on the application of protective capping layers for nonultrahigh vacuum (UHV) transport of BeTe layers. It allows the subsequent recovery of the clean BeTe(100) surface by thermal decapping in UHV. For capping tellurium/selenium double layers with a thickness of about 0.2 μm are employed. After decapping by annealing which results in the desorption of Se at 150 °C and of Te at 260 °C, the low-energy electron diffraction pattern confirms the reestablishment of (2×1) surface reconstruction of BeTe(100). Subsequent annealing to 520 °C results in (3×1) reconstruction. In situ monitoring by Raman spectroscopy and reflection anisotropy spectroscopy is used to control the decapping process and the subsequent annealing-induced change in surface reconstruction.
Dielectric anisotropy and photoinduced voltage in Langmuir–Blodgett films of retinal derivatives
Authors: de Oliveira Helinando P. , Tenorio Alexandro C. , de Melo Celso P.
Published: February 2003

We have investigated the photovoltaic response and measured the impedance of mixed Langmuir–Blodgett films of retinal derivatives and behenic acid along three perpendicular directions. The results indicate that the molecular dipoles in these samples are preferentially oriented in the direction perpendicular to the substrate. This microscopic arrangement is responsible for the observed anisotropy of both the dielectrical and photovoltaic properties of these thin organic films.
Dielectric permittivity of SiO2 thin films in dependence on the ambient hydrogen pressure
Authors: Holten Stephan , Kliem Herbert
Published: February 2003

Metal–silicondioxide–silicon structures with palladium electrodes, instead of standard gold or aluminum electrodes, show an increasing dielectric permittivity of the SiO2 films with increasing ambient hydrogen pressure. Measurements of the complex dielectric permittivity ε_SiO2(f ) in dependence on the hydrogen pressure are carried out at room temperature. The broad relaxational response spectra of the thin films result from a volume polarization effect. Correspondingly, the contribution of absorbed hydrogen to the dielectric permittivity is modeled by proton fluctuations in coulombic double-well potentials between pairs of oxygen ions. The broad distribution of relaxation times can be attributed to the distribution of interatomic distances between the oxygens within the amorphous SiO2. From the experimental dielectric response spectra, a pair distribution function (PDF) of oxygens is derived. This PDF is similar to PDFs from literature, which originate from theoretical lattice simulations of amorphous SiO2. As a quantitative result, we estimate the number of relaxation centers, i.e., oxygen pairs occupied by a proton, which contribute to the dielectric permittivity of the SiO2 thin films in dependence on the hydrogen pressure.
Relationship between interfacial adhesion and electromigration in Cu metallization
Authors: Lane M. W. , Liniger E. G. , Lloyd J. R.
Published: February 2003

A relationship between the adhesion of a Cu conductor to its surrounding medium, the electromigration drift velocity, and lifetime in a conventional electromigration test has been demonstrated. Lifetime measurements demonstrate that the drift velocity and the activation energy for mass transport along an interface correlate with the measured adhesion energy of the interface as determined by a four-point bending test. The results indicate that a linear relationship is expected between the electromigration activation energy and the intrinsic work of adhesion, which is consistent with a simple model relating the two. The data indicate that interfacial cleanliness and bond character at the interface (i.e., metal/dielectric versus metal/metal bonding) have a significant impact on all measured parameters.
Dielectric properties of Zn1−xMnxTe epilayers
Authors: Fu S. P. , Chen Y. F. , Wang J. C. , Shen J. L. , Chou W. C.
Published: February 2003

We report on the dielectric properties of Zn1−xMnxTe (0≦x≦0.268) epilayers studied by capacitance and dissipation factor measurements at a temperature of 200 K<T<460 K and a frequency of 20 Hz<f<1 MHz. A Debye-like relaxation in the dielectric response has been observed, which is explained in terms of the presence of charge redistribution. The relaxation is found to be a thermally activated process, and the activation energies obtained from both dissipation factor and capacitance are in good agreement. It is also found that the activation energy decreases with increasing Mn content and this behavior is interpreted in terms of the four-center model, in which the number of Mn atoms appearing in the nearest-neighbor sites of a defect can have four possible configurations. In addition, we demonstrate that the mechanism responsible for the conduction of carrier hopping among structural defects can be attributed to the correlated barrier hopping model.
Effect of steady-state hydrogen coverage on the evolution of crosshatch morphology during Si1−xGex/Si(001) growth from hydride precursors
Authors: Spila T. , Desjardins P. , D’Arcy-Gall J. , Twesten R. D. , Greene J. E.
Published: February 2003

Compressively strained Si0.7Ge0.3 layers were grown on Si(001) by gas-source molecular beam epitaxy from Ge2H6/Si2H6 mixtures at 450 °C. The combination of the relatively low growth temperature and high steady-state hydrogen surface coverage, θH=0.52 monolayer, completely suppresses strain-induced roughening and provides extremely flat surfaces with root mean square widths w<1.5 Å for fully coherent layers. These samples were used as the starting point to probe mechanisms that control misfit-dislocation-induced surface roughening (i.e., crosshatch) along 90°-rotated 〈110〉 directions. For film thicknesses t just larger than the critical value for misfit dislocation formation, tc≃1000 Å, surface roughness is dominated by single- and multiple-atomic-height steps generated by the motion of threading dislocations associated with interfacial misfits. The surface steps are preferential H desorption sites and the increase in total step length results in a decrease in θH on terraces as well as at step edges. The latter effect allows a higher adatom crossing probability at ascending steps, leading to the formation of periodic ridges in response to local strain fields associated with misfit dislocation clusters; w increases from 3.1 Å at t=1350 Å (corresponding to strain relaxation R of 1%) to 27 Å at t=4400 Å (R=78%). Simultaneously, the decrease in θH on terraces strongly affects film growth kinetics as the deposition rates increase from 10 Å min−1 with t<tc to ≃60 Å min−1 with t≃1400–4400 Å. Overall, in films with t≲1440 Å (R≲5%), crosshatch is due to surface steps that result from multiple misfit dislocations on single glide planes. At higher film thicknesses (R=22–78%), crosshatch becomes dominated by local strain-induced roughening and leads to periodic ridge formation.
Basis set and correlation dependent extrapolation of correlation energy
Authors: Huh Soon Bum , Lee Jae Shin
Published: February 2003

A simple extrapolation formula of (X+γ)−3 which fits correlation energies with correlation consistent (aug-)cc-pVXZ and (aug-)cc-pV(X+1)Z[X=D(2),T(3),Q(4)] basis sets to estimate the basis set limit was devised by varying the parameter γ according to basis set quality and correlation level. The explicit extrapolation formulas suitable for calculations at the second order Møller–Plesset perturbation theory and single and double excitation coupled cluster theory with perturbative triples correction level are presented and applications are made to estimate the basis set limit binding energies of various hydrogen-bonded and van der Waals clusters. A comparison of the results by this formula with the reference basis set limit results and the results by other extrapolation methods reveals that the extrapolation formulas proposed here can yield the reliable basis set limit estimates even with the small basis sets and could be used effectively for investigating large weakly bound complexes.
Rotational spectra of C4N, C6N, and the isotopic species of C3N
Authors: McCarthy M. C. , Fuchs G. W. , Kucera J. , Winnewisser G. , Thaddeus P.
Published: February 2003

Two new carbon chain radicals terminated with a nitrile group, C4N and C6N, have been detected in a supersonic molecular beam by Fourier transform microwave spectroscopy. In addition, at least three hyperfine-split rotational transitions of the singly-substituted isotopic species of C3N have also been observed. Both C4N and C6N are linear chains with Π2 electronic ground states, and both radicals have resolvable hyperfine structure and lambda-type doubling in their lowest rotational levels. At least four transitions in the lowest-energy fine structure component (2Π1/2) were measured between 7 and 22 GHz for both molecules, and at most nine spectroscopic constants were required to reproduce the measured spectra to a few parts in 107. Precise sets of rotational, centrifugal distortion, spin-rotation, and hyperfine coupling constants were also determined for the isotopic species of C3N by combining the centimeter-wave measurements here with previous millimeter-wave data. The C13 hyperfine coupling constants of isotopic C3N differ from those of the isoelectronic chain C4H, but are fairly close to those of isovalent C2H, indicating a nearly pure Σ2 electronic ground state for C3N. Although the strongest lines of C6N are more than five times less intense than those of C5N, owing to large differences in the ground state dipole moments, both new chains are more abundant than C5N. Searches for C7N have so far been unsuccessful. The absence of lines at the predicted frequencies implies that the product of the dipole moment times the abundance (μ⋅Na) is more than 60 times smaller for C7N than for C5N, suggesting that the ground state of C7N may be Π2, for which the dipole moment is calculated to be small.
Recalescence after bulk solidification in germanium films melted by ns laser pulses
Authors: Armengol J. , Vega F. , Chaoui N. , Solis J. , Afonso C. N.
Published: February 2003

Rapid solidification dynamics in amorphous germanium films melted by nanosecond laser pulses has been analyzed by means of single-shot subnanosecond time resolved reflectivity measurements using a streak camera based setup. The results show that once a minimum melt depth is induced, a bulk solidification process followed by the release of the solidification enthalpy dominates the solidification scenario. Moreover, the laser-melted material solidifies completely before being remelted as a consequence of the solidification enthalpy release, something only observed, up to date, upon irradiation with picosecond laser pulses.
Cubic polytype inclusions in 4H–SiC
Authors: Iwata Hisaomi , Lindefelt Ulf , Öberg Sven , Briddon Patrick R.
Published: February 2003

Multiple stacking faults in 4H–SiC, leading to narrow 3C polytype inclusions along the hexagonal c direction, have been studied using an ab initio supercell approach with 96 atoms per supercell. The number of neighboring stacking faults considered is two, three, and four. The wave functions and the two-dimensional energy bands, located in the band gap and associated with the narrow inclusions, can be reconciled with a planar quantum-well model with quantum-well depth equal to the conduction band offset between 3C– and 4H–SiC. We show that the existence of the electronic dipole moment due to the spontaneous polarization leads to a clear asymmetry of the bound wave functions inside the quantum well, and that the perturbation associated with the change in the dipole moment caused by the 3C–like inclusion accounts for the appearance of very shallow localized states at the valence band edge. We have also calculated the stacking fault energies for successive stacking faults. It is found that the stacking fault energy for two stacking faults in adjacent basal planes is reduced by approximately a factor of 4 relative to that of one isolated stacking fault, indicating that double stacking faults in 4H–SiC could be quite common.
Morphology and crystallization kinetics in HfO2 thin films grown by atomic layer deposition
Authors: Ho M.-Y. , Gong H. , Wilk G. D. , Busch B. W. , Green M. L. , Voyles P. M. , Muller D. A. , Bude M. , Lin W. H. , See A. , Loomans M. E. , Lahiri S. K. , Räisänen Petri I.
Published: February 2003

We report the effects of annealing on the morphology and crystallization kinetics for the high-κ gate dielectric replacement candidate hafnium oxide (HfO2). HfO2 films were grown by atomic layer deposition (ALD) on thermal and chemical SiO2 underlayers. High-sensitivity x-ray diffractometry shows that the as-deposited ALD HfO2 films on thermal oxide are polycrystalline, containing both monoclinic and either tetragonal or orthorhombic phases with an average grain size of ∼8.0 nm. Transmission electron microscopy shows a columnar grain structure. The monoclinic phase predominates as the annealing temperature and time increase, with the grain size reaching ∼11.0 nm after annealing at 900 °C for 24 h. The crystallized fraction of the film has a strong dependence on annealing temperature but not annealing time, indicating thermally activated grain growth. As-deposited ALD HfO2 films on chemical oxide underlayers are amorphous, but show strong signatures of ordering at a subnanometer level in Z-contrast scanning transmission electron microscopy and fluctuation electron microscopy. These films show the same crystallization kinetics as the films on thermal oxide upon annealing.
Hardness and Young’s modulus of high-quality cubic boron nitride films grown by chemical vapor deposition
Authors: Jiang X. , Philip J. , Zhang W. J. , Hess P. , Matsumoto S.
Published: February 2003

The elastic and mechanical properties of high-quality cubic boron nitride (cBN) films with a few microns thickness and submicron grain size grown on silicon substrates by chemical vapor deposition were determined by measuring the dispersion of surface acoustic waves propagating along the surface of the layered system. The values are compared with those obtained with an ultralow load indenter (Triboscope). Specifically, the hardness, Young’s modulus and density of the film were measured.
Impact ionization and field-enhanced trapping: Fitting current density curves for semi-insulating GaAs
Authors: Albuquerque H. A. , de Oliveira A. G. , Ribeiro G. M. , da Silva R. L. , Rodrigues W. N. , Moreira M. V. B. , Rubinger R. M.
Published: February 2003

Semi-insulating GaAs samples present N-shaped negative differential conductivity under high-electric fields. This behavior can be associated with two physical processes: Impact ionization (generation) and field-enhanced trapping (recombination), both of which involve trapped and free electrons. We have analyzed the j(E) characteristic curves of a GaAs sample rich in As antisite defects at different conditions of temperature and illumination. The fitting was carried out using an analytical expression for j(E) based on the competition between the above-mentioned processes. Our analysis permits us to identify the temperature and illumination ranges in which those processes are relevant. The best fittings were obtained for measurements between 150 and 200 K and using an infrared photon flux of the order of 1011 photons/cm2 s.
Orientation of single-walled carbon nanotubes by uniaxial pressure
Authors: Bendiab N. , Almairac R. , Sauvajol J.-L. , Rols S. , Elkaim E.
Published: February 2003

This article presents a very simple process to align macroscopic quantities of single-wall carbon nanotubes (SWNT), the alignment being controlled by x-ray diffraction. A pellet of SWNT is obtained by pressing a raw nanotube powder up to a pressure of 10 kbars. A detailed study of the most characteristic lines in the diffraction pattern reveals that the tubes have their axis aligned in the plane of the pellet within an out-of-plane spread angle of ∼4°. We also show that the intense small-angle signal—which is observed in all SWNT diffraction diagrams—can be assigned to both the form factor of the isolated nanotubes in the sample and the form factor of the bundles with various size and shape.
Spectral method for the electrostatic modeling of semiconductor γ-ray spectrometer arrays
Authors: Dolgin M. , Einziger P. D. , Nemirovsky Y.
Published: February 2003

Semiconductor γ-ray spectrometer devices, which possess attractive properties over alternative devices (e.g., scintillators), have received due attention recently. Evidently, efficient and accurate modeling of these devices, in particular, of the induced charges and internal fields, is crucially important for further development and engineering implementation. Previous attempts to analyze the spectrometers were either oversimplified, i.e., Image Charges (IC) in an idealized infinite parallel plate capacitor, or time consuming, utilizing a Point Charge Moment Method (PCMM) scheme. While the first attempt results in poor accuracy, the second leads to an enhanced accuracy but with the cost of large computation time. Thus, it is desirable to develop a modeling technique that is capable of generating both accurate and efficient solutions. An attempt in this direction is outlined in our work, which introduces the Fourier Series Expansion (FSE) scheme. The model utilizes the eigenfunctions of an idealized infinite two-dimensional array of anodes, namely, Fourier harmonics. These base functions exhibit an accurate and fast converging solution. Thus, it can be effectively applied for practical (finite) devices as long as edge effects can be neglected. This constraint is generally satisfied in practical devices, since the device contains a large number of anodes and the detectable photogenerated carriers are located sufficiently far from the device edges. Numerical comparison of the FSE scheme with the PCMM and IC techniques reveals the effectiveness and usefulness of the proposed scheme. While the FSE and PCMM techniques agree within 2%, the agreement with the IC method is within 7%, as expected.
Inductively coupled plasma etching of SiC in SF6/O2 and etch-induced surface chemical bonding modifications
Authors: Jiang Liudi , Cheung R. , Brown R. , Mount A.
Published: February 2003

4H silicon carbide (SiC) substrates were dry etched in an inductively coupled plasma (ICP) system, using SF6/O2 gas mixtures. Etch rate and etch mechanisms have been investigated as a function of oxygen concentration in the gas mixture, ICP chuck power, work pressure, and flow rate. Corresponding to these etch conditions, surface information of the etched SiC has been obtained by x-ray photoelectron spectroscopy measurements. The fact that no obvious Si–Si and Si–F bonds were detected on the etched surface of SiC in all our etch experiments suggests efficient removal of Si atoms as volatile products during the processes. However, various kinds of C–F bonds have been detected on the etched SiC surface and the relative intensities of these bonds vary with the etch conditions. In addition, the nature of the incorporated F atoms on the etched surface also depends strongly on etch conditions, which was identified by the change of the relative ratio between semi-ionic and covalent carbon fluorine bonds. The electrical behavior for different bond structures on the etched SiC surface can be one of the basic reasons affecting related devices.
Interface stability during the growth of Al2O3 films on Si(001)
Authors: Kundu Manisha , Miyata Noriyuki , Ichikawa Masakazu
Published: February 2003

We grew thin Al2O3 films on Si(001)-2×1 surfaces using three different growth procedures and investigated the Al2O3/Si(001) interface structure and stability for each case. We observed that stacked Al2O3 film grew with an atomically abrupt interface on Si(001). However, depositing a relatively thick initial Al film on Si(001) followed by oxidation, resulted in Al2O3 films being formed having a significantly roughened interface with the Si(001). The interfacial roughness was attributed to the Si–Al interdiffusion near the interfacial region, which with increasing oxidation time, resulted in a nonuniform interfacial region being formed with Al–O–Si compounds. In the growth of Al2O3 film on an Al2O3 prelayer/Si(001) system by depositing Al in an oxygen ambient, about one layer of roughening of the Si substrate occurred at the interface, which was attributed to nonuniform oxidation of the Si substrate. Furthermore, the Al2O3 film growth rate was very slow in this case. These results indicate that the growth procedure adopted to form Al2O3 films plays an important role in controlling the composition and structure of the Al2O3/Si(001) interface.
Loss rate of a plasticizer in a nylon matrix calculated using macroscopic reaction–diffusion kinetics
Authors: Zhang M. L. , March N. H. , Peeters A. , Van Alsenoy C. , Howard I. , Lamoen D. , Leys F.
Published: February 2003

When a nylon–plasticizer complex is unstable or if the initial concentration of a plasticizer is higher than the equilibrium concentration at the corresponding temperature and pressure, the plasticizer molecules will dissociate with the amides of nylon and diffuse in the nylon matrix. In the process of diffusion, a plasticizer molecule may recombine with other amides and dissociate after some time. Eventually the excess plasticizer molecules will escape from the nylon–plasticizer system. The loss rate of a plasticizer as a function of time is calculated from macroscopic reaction–diffusion kinetics.
High open-circuit voltage photovoltaic devices from carbon-nanotube-polymer composites
Authors: Kymakis E. , Alexandrou I. , Amaratunga G. A. J.
Published: February 2003

Organic photovoltaic devices based on the bulk heterojunction concept, containing a blend of single-wall carbon nanotubes (SWNTs) and soluble polythiophenes (P3OT) were studied. The open circuit voltage Voc of the devices was found to be 0.75 V, which is larger than the theoretical limit calculated by the metal–insulator–metal (MIM) model. In order to investigate the origin of this unusually high Voc, we have prepared P3OT–SWNT based devices with different metal negative electrodes. The Voc measured is only very weakly dependent on the work function of the metal, suggesting that the MIM model does not apply in this case. From the analysis of the current–voltage characteristics and electron microscopy imaging of the composite structure, it is proposed that the photovoltaic response of these devices is based on the introduction of internal polymer/nanotube junctions within the polymer matrix, which due to a photoinduced electron transfer from the polymer to the nanotube contribute to enhanced charge separation and collection. The data suggest that the negligible influence of the negative electrode work function on Voc can be explained by the metal negative electrode forming ohmic contacts to the nanotube percolation paths.
AlN/AlGaInN superlattice light-emitting diodes at 280 nm
Authors: Kipshidze G. , Kuryatkov V. , Zhu K. , Borisov B. , Holtz M. , Nikishin S. , Temkin H.
Published: February 2003

Ultraviolet light-emitting diodes operating at 280 nm, grown by gas source molecular-beam epitaxy with ammonia, are described. The device is composed of n- and p-type superlattices of AlN(1.2 nm thick)/AlGaInN(0.5 nm thick) doped with Si and Mg, respectively. With these superlattices, and despite the high average Al content, we obtain hole concentrations of (0.7–1.1)×1018 cm−3, with the mobility of 3–4 cm2/V s and electron concentrations of 3×1019 cm−3, with the mobility of 10–20 cm2/V s, at room temperature. These carrier concentrations are sufficient to form effective p–n junctions needed in UV light sources.
Superhyperfine interactions in inhomogeneously broadened paramagnetic centers observed via a hole-burned free induction decay
Authors: Drew S. C. , Pilbrow J. R. , Newman P. J. , MacFarlane D. R.
Published: February 2003

Superhyperfine interactions in inhomogeneously broadened paramagnetic centers are observed using a single high-turn-angle microwave pulse. The free induction signal that follows the hole-burning pulse exhibits oscillations that are distinct from the oscillatory free induction decay observable in some inhomogeneously broadened systems. It contains frequencies characteristic of the superhyperfine splittings, together with a zero frequency component. Experimental examples of the effect in both orientationally disordered (powdered) and structurally disordered (glassy) systems are presented and compared with the conceptually similar Fourier transform electron paramagnetic resonance detected nuclear magnetic resonance experiment, together with numerical simulations.
Effect of elastic anisotropy on the elastic fields and vertical alignment of quantum dots
Authors: Pei Q. X. , Lu C. , Wang Y. Y.
Published: February 2003

The elastic fields in the self-organized quantum dot (QD) structures are investigated in details by three-dimensional finite element analysis for an array of lens shaped QDs. Emphasis is placed on the effect of elastic anisotropy of the materials with the anisotropy ratio A ranging from 0.25 to 4.0 for both the QDs and the matrix. It is found that the elastic anisotropy strongly influences the distributions of strain, stress, and strain energy density in the QD structures. It is shown that the elastic interactions among the buried QDs play crucial role in the formation of the satellite energy minima at the cap layer surface, while the materials anisotropy and the cap layer thickness also play important roles. By changing the elastic anisotropy ratio and the cap layer thickness, substantially different distributions of strain energy minima on the cap layer surface are obtained, which may result in various QD ordering phenomena such as vertical alignment, partial alignment, or complete misalignment. Based on the calculation results, a phase diagram is constructed to show the effect of material anisotropy and cap layer thickness on the vertical correlation of QDs.
Constitutive response of passivated copper films to thermal cycling
Authors: Shen Y.-L. , Ramamurty U.
Published: February 2003

The thermomechanical behavior of passivated thin copper films is studied. Stresses in copper films of thickness ranging from 125 to 1000 nm, deposited on quartz or silicon substrates and passivated with silicon oxide, were measured using the curvature method. The thermal cycling spans a temperature range from −196 to 600 °C. The measured mechanical behavior was found to be rate insensitive within the heating/cooling rate range of 5–25 °C/min. It was observed that the passivated films do not exhibit a significant stress relaxation at elevated temperatures that is normally found in unpassivated films. Furthermore, a significant strain hardening during the course of thermal loading was noted. Simple continuum plasticity analyses show that the experimentally measured stress–temperature response can only be rationalized with a kinematic hardening model. Analytical procedures for extracting the constitutive properties of the films that were developed on the basis of such a model are presented. The initial yield strength is higher and tends to be less temperature dependent in thinner films. The strain hardening rate is found to increase with decreasing film thickness.
Reversible and irreversible polarization processes in ferroelectric ceramics and thin films
Authors: Bolten D. , Böttger U. , Waser R.
Published: February 2003

In this article, the separation between reversible and irreversible polarization where the reversible polarization component is determined by capacitance–voltage curve measurements, is used to characterize ferroelectric materials. After giving a thorough foundation of the method, it is used to investigate the influence of the composition on the reversible and irreversible polarization contributions in ferroelectric thin films and/or bulk ceramics. The reversible polarization is also monitored during fatigue. A comparison to bulk ceramics suggests that the domain wall motion in ferroelectric thin films is reduced compared to bulk ceramics.
Radio frequency magnetron sputtering deposition of hetero-epitaxial strontium barium niobate thin films (SrxBa1−xNb2O6)
Authors: Cuniot-Ponsard M. , Desvignes J. M. , Ea-Kim B. , Leroy E.
Published: February 2003

Excellent electro-optic properties of SrxBa1−xNb2O6 crystals (SBN:x) motivate the attempts to control the deposition of high ordered SBN thin films with the aim of optical waveguiding and processing integration. We have examined the ability of sputtering techniques to deposit stoichiometric SBN thin films. Composition analysis has enabled us to probe the mechanisms which control the target-film composition transfer and to define an experimental strategy for stoichiometry control. Epitaxial (001) SBN thin films have been obtained on MgO (100) substrates, which exhibit two in-plane orientations (±31°) mirror symmetric to the MgO cell axis.
Thickness dependence of leakage current behavior in epitaxial (Ba,Sr)TiO3 film capacitors
Authors: Ahn Kun Ho , Kim Sang Sub , Baik Sunggi
Published: February 2003

Thickness dependence of leakage current behavior in Pt/(Ba,Sr)TiO3/Pt film capacitors was investigated by fabricating epitaxial (Ba,Sr)TiO3 thin films having different thicknesses of 55–225 nm but with an identical interface state on Pt(001)/MgO(001) substrates by a radio-frequency magnetron sputtering technique. In the 55-nm-thick film, the Schottky emission is found to be a main leakage mechanism over a wide electric field range. In sharp contrast, instead of the Schottky emission, the Fowler–Nordheim tunneling dominates particularly in the thicker films (⩾90 nm) at high electric fields. According to the energy band model proposed to explain these results, the major leakage conduction mechanism is likely to be associated with overlapping of the depletion layers formed at the top and bottom interfaces.
Structures and stabilities of small silicon clusters: Ab initio molecular-orbital calculations of Si7–Si11
Authors: Zhu Xiaolei , Zeng X. C.
Published: February 2003

Ab initio all-electron molecular-orbital calculations have been carried out to study the structure and relative stability of small silicon clusters (Sin, n=7–11). A number of low-energy geometric isomers are optimized at the second-order Møller–Plesset (MP2) MP2/6-31G(d) level. Harmonic vibrational analysis has been performed to assure that the optimized geometries are stable. The total energies of stable isomers are computed at the coupled-cluster single and double substitutions (including triple excitations) [CCSD(T)] CCSD(T)/6-31G(d) level. The calculated binding energies per atom at both the MP2/6-31G(d) and CCSD(T)/6-31G(d) levels agree with the experiments. For Si7, Si8, and Si10, the lowest-energy structures are the same as those predicted previously from the all-electron optimization at the Hartree–Fock (HF) HF/6-31G(d) level [Raghavachari and Rohlfing, J. Chem. Phys. 89, 2219 (1988)]. For Si9, the lowest-energy isomer is same as that predicted based on density-functional plane-wave pseudopotential method [Vasiliev, Ogut, and Chelikowsky, Phys. Rev. Lett. 78, 4805 (1997)]. Particular attention has been given to Si11 because several low-energy geometric isomers were found nearly isoenergetic. On the basis of MP2/6-311G(2d)//CCSD(T)/6-311G(2d) calculation, we identified that the C2v isomer, a tricapped trigonal prism with two additional caps on side trigonal faces, is most likely the global-minimum structure. However, another competitive geometric isomer for the global minimum is also found on basis of the MP2/6-311G(2d)//CCSD(T)/6-311G(2d) calculation. Additionally, calculations of the binding energy and the cluster polarizability offer more insights into relatively strong stability of two magic-number clusters Si6 and Si10.
Ferroelectric properties of lanthanide-substituted Bi4Ti3O12 epitaxial thin films grown by metalorganic chemical vapor deposition
Authors: Kojima Takashi , Watanabe Takayuki , Funakubo Hiroshi , Saito Keisuke , Osada Minoru , Kakihana Masato
Published: February 2003

The ferroelectricity of epitaxially grown lanthanide-substituted Bi4Ti3O12, (Bi4−xLnx)Ti3O12 (Ln=La, Nd, and Sm) thin films with various lanthanide contents (x) was systematically investigated; (104)-oriented epitaxial (Bi4−xLnx)Ti3O12 thin films were grown on (111)SrRuO3∥(111)SrTiO3 substrates by metalorganic chemical vapor deposition. The remanent polarization (Pr) increased with x irrespective of the kind of lanthanide element. The maximum Pr values were 17, 25, and 20 μC/cm2 for (Bi3.44La0.56)Ti3O12, (Bi3.54Nd0.46)Ti3O12, and (Bi3.87Sm0.13)Ti3O12 thin films, where the coercive field (Ec) values were 145, 135, and 135 kV/cm, respectively. Reduction in the Pr values due to the excess substitution of the lanthanide is considered to originate from the solubility limit of the lanthanide in the Bi site in the pseudoperovskite layer. The Nd-substituted film with the largest polarization is comparable to the commercially used Pb(Zr,Ti)O3 films and is a useful candidate for lead-free ferroelectric applications.
Triboelectric charging of a perfluoropolyether lubricant
Authors: Wasem J. V. , LaMarche B. L. , Langford S. C. , Dickinson J. T.
Published: February 2003

Perfluoropolyethers (PFPE) are used extensively to lubricate hard disks for high density information storage. Some PFPE lubricants have been shown to be vulnerable to dissociative electron attachment as well as electron bombardment. Triboelectric charges generated during sliding contact may generate quasi-free electrons capable of degrading the lubricant. We describe measurements of triboelectric charging in a model polymer-metal system with and without a PFPE lubricant and show that negative charge accumulates on the substrate polymer surface, both with and without the lubricant. In the presence of the lubricant, some of this charge moves through and becomes bound to the lubricant. Electron and positive ion emission from PTFE-lubricated surfaces during contact sliding again indicate motion of charge through the lubricant. Ion emission in particular suggests possible degradation of the lubricant.
Molecular dynamics simulations of CH3 sticking on carbon surfaces
Authors: Träskelin P. , Salonen E. , Nordlund K. , Krasheninnikov A. V. , Keinonen J. , Wu C. H.
Published: February 2003

Employing both quantum mechanical and empirical force models, we use molecular dynamics simulations to obtain sticking cross sections for CH3 radical chemisorption on unsaturated sites of carbon surfaces. Effects of the local atomic neighborhood on the chemisorption are examined for the comparison of the results with experiments. Our results show that the chemisorption of a CH3 radical onto a dangling bond is highly affected by the neighborhood of the unsaturated carbon atom sites. Notably, sticking cross sections of totally bare dangling bond sites at the surface and sites partly shielded by neighboring methyl groups are observed to differ by two orders of magnitude, (15.3±1.7) Å2 and (0.2±0.1) Å2, respectively. We describe a steering effect which explains the recent experimental observation that the sticking cross section can be larger than the average area per surface site.
Low-threshold lasing in a microcavity of fluorene-based liquid-crystalline polymer blends
Authors: Lee Tae-Woo , Park O. Ok , Cho Hyun Nam , Kim Dong Young , Kim Young Chul
Published: February 2003

We report the lasing characteristics of a microcavity device made of fluorene-based luminescent polymer blends with liquid crystallinity. Poly(2,7-bis(p-stiryl)-9,9′-di-n-hexylfluorene sebacate) (PBSDHFS) and poly(9,9′-di-n-hexyl fluorenediylvinylene-alt-1,4-phenylenevinylene) (PDHFPPV) were used to comprise a binary blend system. We employed the Förster-type energy transfer from a liquid crystalline donor to a non-liquid-crystalline acceptor to obtain a low lasing threshold. The binary blend film of PBSDHFS/PDHFPPV (98/2 by wt) demonstrated a very low-threshold energy (∼3 nJ/cm2/pulse) for microcavity lasing, which is lower than any other values previously reported on the organic or polymeric microcavity devices with metal or dielectric mirrors. This result implies that the liquid crystalline polymer blends could be a good candidate for the gain material of photo and electrically pumped lasing devices.
X-ray photoelectron spectroscopic study of rare-earth-doped amorphous silicon–nitrogen films
Authors: Zanatta A. R. , Ribeiro C. T. M. , Alvarez F.
Published: February 2003

Amorphous silicon–nitrogen (a-SiN) films independently doped with different rare-earth (RE) elements (Y, La, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu) have been prepared by cosputtering. The films were investigated in detail by x-ray photoelectron spectroscopy employing 1486.6 eV photons. Additional information was also achieved by optical techniques and ion beam analyses. As a result of the deposition method and conditions, the films present similar contents of Si and N, and rare-earth concentrations below 1.0 at. %. In spite of this relatively low concentration, and taking advantage of the high photoionization cross section of the rare-earth elements at 1486.6 eV, the signal of several different core-levels and Auger transitions could be detected and analyzed. The electronic states at the top of the valence band of the RE-doped a-SiN films were also investigated with 1486.6 eV photons. Compared to the spectroscopic data of pure metals, the RE-related core levels of the present a-SiN films exhibit an energy shift typically in the 0.8–2.5 eV range, which is attributed to the presence of nitrogen atoms. According to the experimental data, most of the RE ions remain in the 3+ state. The only clear exception occurs in the Yb-doped a-SiN film, where a large fraction of Yb2+ coexisting with Yb3+ ions is evident.
Ideal magnetocaloric effect for active magnetic regenerators
Authors: Rowe A. M. , Barclay J. A.
Published: February 2003

The active magnetic regenerator (AMR) uses a magnetic solid as a thermal storage medium and as a working material in a refrigeration cycle. Thermodynamically coupled to a heat transfer fluid, the regenerator produces a cooling effect and generates a temperature gradient across the AMR. The coupling between the heat transfer fluid and the magnetic refrigerant is a key aspect governing the operating characteristics of an AMR. To increase our understanding of AMR thermodynamics, we examine the entropy balance in an idealized active magnetic regenerator. A relation for the entropy generation in an AMR with varying fluid capacity ratios is derived. Subsequently, an expression describing the ideal magnetocaloric effect (MCE) as a function of temperature is developed for the case of zero entropy generation. Finally, the link between ideal MCE and refrigerant symmetry is discussed showing that an ideal reverse Brayton-type magnetic cycle cannot be achieved using materials undergoing a second-order magnetic phase transition.
Use of harmonics for femtosecond micromachining in pure dielectrics
Authors: Bityurin N. , Kuznetsov A.
Published: February 2003

A controllable reproducible modification of dielectrics within the bulk of material by focused ultrashort laser pulses provides the opportunity for writing three-dimensional (3D) photonic structures including bit-wise 3D information recording. Here a nonlinear response of material is employed, the multi-photon absorption being the first step. The present article concerns the expedience of the use of harmonics, first of all, second harmonic, for 3D micromodification of transparent dielectrics. If modification is provided by multi-photon transition to the first electronic excited level either due to subsequent photochemical reaction or material heating, then the reduction of the order of multi-photon transition provided by the second harmonic appears to be effective. If modification is connected with ionization of material and creation of free electrons, i.e., by breakdown, then an advantage of second harmonic is less obvious because impact ionization is more effectively provided by the fundamental frequency. Here a combined effect of both the second harmonic and fundamental frequency beams can be effective. Impact ionization is described by the Fokker–Planck equation. A relation between exponent of avalanche development and impact ionization delay time is discussed.
Dependence of the electron minority carrier lifetime on interstitial oxygen and substitutional carbon in pseudomorphically strained SiGeC heterostructures
Authors: Carroll M. S. , King C. A.
Published: February 2003

The electron minority carrier lifetimes in 13.5 nm Si0.7−xGe0.3Cx layers pseudomorphically grown by rapid thermal chemical vapor chemical deposition on low oxygen (∼5×1017 cm−3), 〈100〉 silicon substrates, capped with a 100 nm of silicon are extracted from the transient time response of the combined Si/Si0.7−xGe0.3Cx/Si layers' photoconductance for substitutional carbon concentrations up to 0.76 at. %. To this end, an analytic expression relating the transient response of the photoconductance to the minority carrier lifetime in the Si0.7−xGe0.3Cx layer is developed. The average electron minority carrier lifetime for all Si0.7Ge0.3 layers grown is 2.2×10−6 s, which is found to decrease weakly on the substitutional carbon incorporation, ∼ [Cs]−1/2, shortening to 1.2×10−7 s in the Si0.6934Ge0.3C0.0076 layer. Finally, the electron minority lifetime in the Si0.7Ge0.3 layer is found to be much more sensitive to oxygen concentrations becoming shorter than 10−7 s for concentrations as small as 3×1019 cm−3.
Energy dependence of the Penning ionization electron spectrum of Ne*(3s 3P2,0)+Ar
Authors: Jacobs Brian A. , Rice Wesley A. , Siska P. E.
Published: February 2003

Crossed supersonic beam measurements of the Penning ionization electron spectrum for the title system are reported for a collision energy range E=1.6 to 7.4 kcal/mol. The spectra are deconvoluted to obtain separate line shapes for the four possible combinations of J(Ne) and J′(Ar+). The reagent fine structure ionization cross-section ratio QJ=0/QJ=2 is found to increase from 1.1 to 1.5 over this energy range, in good agreement with other studies. The anomalous product branching ratios QJ,J′=3/2/QJ,J′=1/2 found by Hotop et al. [J. Electron Spectrosc. Relat. Phenom. 23, 347 (1981)] are reproduced; these also depend weakly on E. The width and blueshift of all lines increase with E, while line shape asymmetry increases rapidly at low E and levels off at high. Comparison is made with quantum-mechanical calculations based on previously proposed potentials; the calculations predict oscillatory E dependence of the linewidth and asymmetry.
Effect of anisotropy on compensation point in Fe3O4/Mn3O4 grown on MgO(011)
Authors: Wang Y. C. , Horng Lance , Chern G.
Published: February 2003

[ Fe 3 O 4 (40 Å)/ Mn 3 O 4 (160 Å)] ×10 superlattice is grown on MgO(110), and its in-plane angular-dependent magnetic response is studied. Magnetic compensation and spin–flop state are observed at ∼42 K and external field >15 kOe when the magnetic measurement is along the [1-10] direction. These features disappear when the measurement is along the [001] direction. This strong anisotropic effect may originate from the tetragonal structure of Mn3O4 whose magnetic easy axis is in the a-b plane and hard axis is perpendicular to the a-b plane. This study demonstrates that the compensation point and spin–flop phase transition both are orientationally dependent in an antiparallel-coupled magnetic layer system if there is at least one constituent component with strong magnetic anisotropy.
Shearing x-ray interferometer with an x-ray prism
Authors: Kohmura Yoshiki , Ishikawa Tetsuya , Takano Hidekazu , Suzuki Yoshio
Published: February 2003

A wave front dividing interferometer was devised in which half of a highly spatially coherent beam deflected by an x-ray prism is overlapped with another half. By placing the samples in the two beams and the x-ray imaging detector closely downstream of the sample, the differential phase of the sample is recorded on the imaging detector. The separation of the two paths at the sample plane, the amount of shear, was 2.2 μm in the present experiment. This is easily controllable by changing the distance between the sample and the imaging detector. Phase retrieval using the fringe scanning method was successfully demonstrated for various kinds of weakly absorbing samples.
Giant Hall effect in FexSn100−x granular alloy films
Authors: Gao Jun , Wang Fei , Jiang Xiaolong , Ni Gang , Zhang Fengming , Du Youwei
Published: February 2003

The Hall resistivity of FexSn100−x granular alloy films fabricated by ion-beam sputtering has been studied. A saturated Hall resistivity up to 8.68 μΩ cm, the highest value ever observed for ferromagnetic metal/nonmagnetic metal system, has been obtained for the sample with Fe concentration x=68 at. %. This value is almost 60 times higher than that of pure Fe and eight orders higher than that of pure Sn, respectively. The scale relation between ρxys and ρxx was also investigated. The effect of both the concentration and average grain size of the films on the Hall resistivity has been discussed. Magnetoresistance measurement indicated no giant magnetoresistance effect has been found in this system.
Measurement of expanding plasma sheath from a target biased by a negative pulse with a fast rise time
Authors: Kim Gon-Ho , Kim Young-Woo , Han Seunghee , Uhm Han-Sup , Cho Jeonghee , Jung Min Joong , Hong Munpyo
Published: February 2003

The pulse rise time effect on sheath size is investigated. Experiments are carried out on a planar target biased by a negative pulse with a fast rise time in a plasma source ion implantation (PSII) system. The fast rise time of the pulse provides that the speed of sheath expansion is larger than the Bohm speed during the pulse rise time. Results show that the sheath size increases with increasing pulse voltage and pulse rise rate (the ratio of the pulse voltage and the rise time), which is a very important factor for characterizing the expanding sheath in PSII. During the rise time, sheath size is proportional to the square root of the pulse rise rate over plasma density but it is insensitive to ion mass. Experimental results are in good agreement with the developed model based on the expanding ion matrix sheath model. After the full pulse has been attained, the sheath increases linearly with a constant speed as the Bohm speed.
Interlayer exchange coupling in spin valves with specularly reflective oxide layers
Authors: Hong Jongill , Kanai Hitoshi
Published: February 2003

We found that capping a spin valve with thin oxide can make the interlayer coupling oscillatory as a function of thickness of the Cu spacer. The coupling was antiferromagnetic and it allowed us to reduce the thickness of the Cu spacer down to 20 Å without sacrificing the good properties of the spin valve. In addition, the oxide layer enhanced specular reflection of conduction electrons at the interface. As a result, we could greatly increase the giant magnetoresistance (GMR) value of the spin valve. The fact that less current shunted through the thinned Cu spacer and the enhancement of specular reflection contributed to the large GMR value. We also report the observation of the oscillation of interlayer coupling as a function of thickness of the magnetic layer in a spin valve.