bluuebunny/crossref_metadata_2025 huggingface dataset:
Elevated-temperature electrical characteristics of mechanically strained Si devices
Authors: Haugerud B. M. , Bosworth L. A. , Belford R. E.
Published: February 2004

Device characteristics and analyses are reported for strained silicon n- and p-channel, metal-oxide-semiconductor field-effect transistors at five temperatures ranging from 296 to 367 K. Both partially depleted and bulk architectures were investigated. The devices were fabricated commercially on homogeneous silicon-based substrates and strain was applied mechanically after fabrication. Tensile uniaxial strain was applied within the elastic region using a back-end process. It was applied either parallel to or perpendicular to the carrier transport direction. Tensile biaxial strain was also induced in selected samples by using high thermal expansion Al substrates. Samples mounted on Al substrates experienced increasing strain as the temperature was raised. The structures were relaxed and characterized under steady-state conditions at each temperature level. No degradation of strain-induced mobility enhancement was observed due to increased temperature. We conclude that a reduction in average effective mass may be the greatest contribution to strain-enhanced mobility.
Structural and magnetic properties of transition metal substituted ZnO
Authors: Kolesnik S. , Dabrowski B. , Mais J.
Published: February 2004

Structural and magnetic properties have been studied for polycrystalline Zn1−xTMxO, where TM (transition metal ions)=Mn, Fe, and Co. No bulk ferromagnetism was observed for single-phase materials, contrary to the existing theories. Single-phase samples demonstrate paramagnetic Curie–Weiss behavior with antiferromagnetic interactions, similar to other diluted magnetic semiconductors. Nonoptimal synthesis conditions lead to formation of second phases that are responsible for spin-glass behavior {ZnMnO3 impurity for Zn1−xMnxO [S. Kolesnik et al., J. Supercond. 15, 251 (2002)]} or high-temperature ferromagnetic ordering [Co metal for Zn1−xCoxO with the Curie temperature TC>800 K or (Zn,Fe)3O4 for Zn1−xFexO with TC=440 K].
Fast reconnection in relativistic pair plasmas: Analysis of particle acceleration in self-consistent full particle simulations
Authors: Jaroschek C. H. , Treumann R. A. , Lesch H. , Scholer M.
Published: February 2004

Particle acceleration in collisionless magnetic reconnection is studied in the relativistic regime of an electron-positron plasma. For the first time, the highly dynamic late-time evolution of reconnection is simulated in two dimensions (2D) and the finite size of the acceleration region is resolved in 3D applying a fully electromagnetic relativistic particle-in-cell (PIC) code. The late-time evolution is extremely important with respect to particle acceleration, because thin current sheets show a highly dynamic late-time phase with instabilities evolving in the Alfvén velocity vA0 regime. Consequently, since c∼vA0 is valid as a peculiarity of pair plasmas, v×B-contributions become dominant in the accelerating electric field. Most remarkable: Though acceleration regions are highly variable at late times, the power-law shape of the particle energy distribution is smoothed compared to quasi-static reconnection configurations at early times [S. Zenitani and M. Hoshino, Astrophys. J. 562, L63 (2001)]. Spectral power indices of s∼−3 for the complete simulation box, s∼−1 within the X-zone, are preserved at late times and appear as a characteristic of pair plasma reconnection of thin current sheets! The spectral high-energy cut-off depends on the sheet width at late times and is most efficiently tuned by the ratio c/vA0. In 3D, sheet instabilities limit the acceleration potential of a single X-zone, but current driven instabilities like the relativistic drift kink mode can also significantly contribute to particle acceleration. Via the analysis of particle trajectories, the consequences of a finite 3D acceleration zone are resolved and efficient acceleration mechanisms in the context of dynamic X-points are identified.
Damage evolution on Sm and O sublattices in Au-implanted samarium titanate pyrochlore
Authors: Zhang Y. , Weber W. J. , Shutthanandan V. , Devanathan R. , Thevuthasan S. , Balakrishnan G. , Paul D. M.
Published: February 2004

Damage evolution on the Sm and O sublattices in Sm2Ti2O7 single crystals irradiated with 1 MeV Au2+ ions at 170, 300, and 700 K was studied by Rutherford backscattering spectroscopy and O16(d,p)17O nuclear reaction analysis along the 〈001〉 direction. The damage accumulation behavior at each irradiation temperature indicates that the relative disorder on the O sublattice is higher than that on the Sm sublattice, and the relative disorder, determined by ion channeling, on each sublattice follows a nonlinear dependence on dose that is well described by a disorder accumulation model. While there is little difference in damage accumulation behavior on the Sm sublattice at 170 and 300 K irradiation, the rate of damage accumulation decreases dramatically at 700 K due to dynamic recovery processes. The critical dose for amorphization at 170 and 300 K is 0.14 displacements per atom (dpa), and a higher dose of 0.22 dpa is observed under irradiation at 700 K. During thermal annealing in an O18 environment, a significant increase in the O18 exchange was observed between 800 and 900 K, which is just below the previously determined critical temperature, 950 K, for amorphization in Sm2Ti2O7, suggesting that the mobility of O vacancies may be important in defining the critical temperature.
Low-power all-optical switch by superposition of red and green laser irradiation in thin-film cadmium sulfide on glass
Authors: Erlacher A. , Miller H. , Ullrich B.
Published: February 2004

All-optical transmission switching is observed by superposition of continuous red (623.8 nm) and green (532 nm) laser light in a thin-film CdS on glass. The results demonstrate that all-optical digitizing can be realized by fairly simple means at modest laser powers not exceeding 50 mW. The transmittance alterations are induced by occupancy variations of the ground state and impurities.
A model of ferroelectric behavior based on a complete switching density
Authors: Cima Lionel , Labouré Eric
Published: February 2004

In this article, a ferroelectric capacitor is characterized by an experimental Preisach density (PD), determined by first order reversal curves (FORC) method. It is experimentally shown that this density, defined under the usual Preisach assumptions, does not allow a complete model of the ferroelectric behavior. Considering the domain walls phenomenon, an extension of the Preisach assumptions is suggested, leading to the complete switching density (CSD) defined on the whole Preisach plane. It is theoretically shown that the experimental PD determined by the FORC method is an exact image of the CSD truncated outside of the so-called Preisach triangle. It is finally verified that the CSD improves the ferroelectric modeling on a wide range of electric field strength, including the Rayleigh zone.
Growth kinetics of highly undercooled Al2O3 melts
Authors: Li Mingjun , Kuribayashi Kazuhiko
Published: February 2004

An aeroacoustic levitator was employed to solidify the Al2O3 melt in a containerless condition at different melt undercoolings when a laser beam heating system was utilized. The sample was simulated to crystallize at well-defined temperatures and the recalescence front was imaged using a high-speed video. Both the observation of solidified microstructures and the theoretical calculation of the hypercooling limit of the undercooled melt indicated that the final microstructure should consist of the primary dendrite formed during rapid recalescence and the subsequent product yielded after recalescence near the melting temperature. Microstructure analysis showed that the advancement rate of the recalescence front should be viewed as the growth velocity of the undercooled melt. The accurate relationship of growth velocity versus melt undercooling was acquired, which was essential in characterizing the growth kinetics of the undercooled melts. Further analysis indicated that the linear kinetic coefficient for the free growth of Al2O3 was about 0.05 m/sK, which was much higher than that of some other compounds with complicated crystalline structures. The growth kinetics of the undercooled melt can be well clarified when considering the complexity level of the crystalline phase and the structure of coordination in the undercooled melt in comparison with some other oxides.
Silicon implantation in epitaxial GaN layers: Encapsulant annealing and electrical properties
Authors: Matsunaga S. , Yoshida S. , Kawaji T. , Inada T.
Published: February 2004

Silicon ions have been implanted into GaN layers epitaxially grown on an AlN/(0001)-Al2O3 substrate to a dose of 1×1015/cm2. The Si-implanted GaN has been rapid thermal annealed at temperatures between 800 and 1250 °C. Thermal pits are formed in the surface during annealing at 1200 °C, resulting in deterioration of the morphology. The surface deterioration is effectively suppressed by using a 140-nm-thick Si3N4 film as an encapsulant during annealing up to 1250 °C. The electrical activation process for Si atoms has an activation energy of 3.1 eV in the range of 1000 to 1250 °C. Carrier concentration and mobility profiles for n-type layers formed by Si implantation have been examined by differential Hall-effect measurements. A very high electrical activity for implanted Si atoms of 86% can be achieved, and a highly doped n-type layer with a peak carrier concentration of 6×1019/cm3 is formed after annealing at 1250 °C. Electrons generated from Si atoms located near the end of range are trapped by defects remaining after annealing at 1250 °C.
High-resolution transmission electron microscopy of as-deposited boron nitride on the edge of ultrathin Si flake
Authors: Yang Hangsheng , Iwamoto Chihiro , Yoshida Toyonobu
Published: February 2004

Boron nitride was deposited on the edge of ultrathin Si flake substrates by low-pressure inductively coupled plasma-enhanced chemical vapor deposition. Since the as-deposited boron nitride was sufficiently thin for cross-sectional high-resolution transmission electron microscopy, the postdeposition thinning process was omitted. The morphology of the boron nitride was revealed to vary from an amorphous structure to a three-layer structure, that is, amorphous/turbostratic/cubic boron nitride, upon increasing the thickness of the edge of Si flake substrates from a few to 50 nm. The critical thickness of the edge of Si flakes required for the nucleation of cubic boron nitride was found and estimated to be approximately 40 nm. The surface of cubic boron nitride nuclei was composed of {111} nanofacets, and no evidence of a thin sp2-bonded BN structure was observed at the top surface. This clearly reveals that cubic boron nitride is nucleated and grows on the top surface, not in the subsurface of the turbostratic boron nitride layer, during chemical vapor deposition.
Collapse of microchannels during anodic bonding: Theory and experiments
Authors: Shih Wen-Pin , Hui Chung-Yuen , Tien Norman C.
Published: February 2004

By fabricating a large number of microchannels with different aspect ratios using anodic bonding, we demonstrate that a channel collapse can occur during anodic bonding as a result of the high electric field inside the channels. A theory is developed to establish a criterion for the collapse. This theory shows that the collapse can be avoided as long as the condition εaV2a/Eeffd3<1 is satisfied, where εa is the permittivity of air, V is the applied voltage for anodic bonding, Eeff is a material constant which characterizes the elastic stiffness of the materials, a is the half width of the channel, and d is the channel depth. The validity of this theory is checked by comparing with experimental results.
Local characterization and transformation of phase-change media by scanning thermal probes
Authors: Bichet O. , Wright C. D. , Samson Y. , Gidon S.
Published: February 2004

Scanning thermal microscopy is applied to investigate the local thermal properties of thin phase-change (Ge2Sb2Te5) layers. The thermal contrast allows crystalline and amorphous areas to be detected unambiguously, and can be quantitatively linked to the crystalline ratio of the phase change layer. It is demonstrated that the heated tip can be used to locally crystallize parts of an amorphous Ge2Sb2Te5 layer, thereby suggesting thermal probes may be used for data storage applications on phase-change media (write once).
Modeling negative bias temperature instabilities in hole channel metal–oxide–semiconductor field effect transistors with ultrathin gate oxide layers
Authors: Houssa M. , Aoulaiche M. , Autran J. L. , Parthasarathy C. , Revil N. , Vincent E.
Published: February 2004

The decrease of the threshold voltage Vth of hole channel metal–oxide–semiconductor field effect transistors with ultrathin gate dielectric layers under negative bias temperature stress is studied. A degradation model is developed that accounts for the generation of Si3≡Si• (Pb0) centers and bulk oxide defects, induced by the tunneling of electrons or holes through the gate dielectric layer during electrical stress. The model predicts that Vth shifts are mainly due to the tunneling of holes at low gate bias |VG|, typically below 1.5 V, while electrons are mainly responsible for these shifts at higher |VG|. Consequently, device lifetime at operating voltage, based on Vth shifts, should not be extrapolated from measurements performed at high gate bias. The impact of nitrogen incorporated at the Si/dielectric interface on Vth shifts is investigated next. The acceleration of device degradation when the amount of nitrogen increases is attributed to the increase in local interfacial strain, induced by the increase in bonding constraints, as well as to the increase in the density of Si–N–Si strained bonds that act as trapping centers of hydrogen species released during the electrical stress.
High injection and carrier pile-up in lattice matched InGaAs/InP PN diodes for thermophotovoltaic applications
Authors: Ginige R. , Cherkaoui K. , Wong Kwan V. , Kelleher C. , Corbett B.
Published: February 2004

This article analyzes and explains the observed temperature dependence of the forward dark current of lattice matched In0.53Ga0.47As on InP diodes as a function of voltage. The experimental results show, at high temperatures, the characteristic current-voltage (I–V) curve corresponding to leakage, recombination, and diffusion currents, but at low temperatures an additional region is seen at high fields. We show that the onset of this region commences with high injection into the lower-doped base region. The high injection is shown by using simulation software to substantially alter the minority carrier concentration profile in the base, emitter and consequently the quasi-Fermi levels (QFL) at the base/window and the window/cap heterojunctions. We show that this QFL splitting and the associated electron “pile-up” (accumulation) at the window/emitter heterojunction leads to the observed pseudo-n=2 region of the current-voltage curve. We confirm this phenomenon by investigating the I–V–T characteristics of diodes with an InGaAsP quaternary layer (Eg=1 eV) inserted between the InP window (Eg=1.35 eV) and the InGaAs emitter (Eg=0.72 eV) where it serves to reduce the barrier to injected electrons, thereby reducing the “pile-up.” We show, in this case that the high injection occurs at a higher voltage and lower temperature than for the ternary device, thereby confirming the role of the “accumulation” in the change of the I–V characteristics from n=1 to pseudo-n=2 in the ternary latticed matched device. This is an important phenomenon for consideration in thermophotovoltaic applications. We, also show that the activation energy at medium and high voltages corresponds to the InP/InGaAs conduction band offset at the window/emitter heterointerface.
Electrical transport in metallic films
Authors: Fan Ping , Yi Kui , Shao Jian-Da , Fan Zheng-Xiu
Published: February 2004

On the basis of the Boltzmann equation, the authors propose a model that includes scattering from both film surfaces and grain boundaries, and have studied the quasiclassical electrical transport in metallic films. The in-plane electric conductivity of metallic films is obtained, and the theoretical results are shown to be in good agreement with experimental data. We also give the relation between temperature coefficient of resistivity and thickness of metallic films and make a comparison with experiment.
Porosity in low dielectric constant SiOCH films depth profiled by positron annihilation spectroscopy
Authors: Brusa R. S. , Spagolla M. , Karwasz G. P. , Zecca A. , Ottaviani G. , Corni F. , Bacchetta M. , Carollo E.
Published: February 2004

The 3γ annihilation of orthopositronium and the Doppler broadening of the positron annihilation line have been measured by implanting low energy positrons in low dielectric constant (low-k) SiOCH films. The evolution and stability of film porosity with thermal treatments in the 400–900 °C temperature range has been studied. The films have been produced by plasma enhanced chemical vapor deposition and after annealing in N2 atmospheres at 480 °C have been treated in N2+He plasma. The minimum free volume of the pores in the as-produced samples has been estimated to correspond to that of a sphere with radius r=0.6 nm. The treatment in the N2 plasma was found to seal the pores up to 45 nm depth. Both the composition of the films (as obtained by Rutherford backscattering spectroscopy and elastic recoil detection analysis) and the chemical environment of the pores probed by positrons were found to be very stable up to 600 °C thermal treatment. Above such a temperature a reduction of the hydrogen content accompanied by a change in the structure and in the chemical environment of the pores has been observed. In the samples thermal treated at 800–900 °C, the positronium formation is reduced by one-third respect with the as produced sample. In the annealed and as-produced films, a natural aging of 30 days in air was enough to contaminate the porosity, as pointed out by a strong reduction of the 3γ annihilations. The effect of contamination and the distribution of the pores were completely recovered after a thermal treatment at 400 °C. Artificial aging of SiOCH films in controlled atmospheres of H2, O2, H2O has shown that H2O is the more efficient contaminant in reducing the effective volume of the pores.
Optical and mechanical properties of MgO crystals implanted with lithium ions
Authors: Savoini B. , Cáceres D. , Vergara I. , González R. , da Silva R. C. , Alves E. , Chen Y.
Published: February 2004

Defect profile induced by implantation of Li+ ions with an energy of 175 keV and a fluence of 1×1017 ions/cm2 in MgO single crystals was characterized by Rutherford backscattering and optical absorption measurements. Several absorption bands at 5.0, 3.49, 2.16, and 1.27 eV, identical to those found in neutron irradiated crystals, were observed and have been previously associated with oxygen vacancies and higher-order point defects involving oxygen vacancies. Despite the high fluence of Li+ ions, no evidence was found for the formation of Li nanocolloids during implantation. Nanoindentation experiments demonstrated that both the hardness and Young’s modulus were higher in the implanted layer than in the sample before implantation. The maximum values were H=(17.4±0.4) and E=(358±9) GPa, respectively, at a contact depth of ≈165 nm. Thermal annealings in flowing argon at increasing temperatures improved the crystalline quality of the implanted layer. After annealing at 500 K, two extinction bands at ≈2.75 and 3.80 eV emerged. These bands are attributed to Mie scattering from metallic lithium nanocolloids with either a face-centered- or a body-centered-cubic structure. The latter band was almost absent by 950 K. The former reached a maximum intensity after the thermal treatment at 1050 K and disappeared by 1250 K. The behavior of these bands can be satisfactorily explained by the Maxwell–Garnett theory. The decrease in hardening cannot be correlated with the thermal destruction of the absorption bands at 5.0, 3.49, 2.16, and 1.27 eV, but rather with the annihilation of both lithium and oxygen interstitials. Lithium outdiffusion from the implanted region takes place at temperatures of ≈1100 K. It is concluded that the hardening observed in the implanted region was primarily due to the extraordinarily large concentration of both lithium and oxygen interstitials.
Characteristics of gravitationally affected colloidal plasma sheath
Authors: Rajkhowa Kavita Rani , Ramachandran H.
Published: February 2004

The presence of massive dust grains modifies the flow structures of plasma species in presheath and sheath regions as well as the nature of boundary conditions at the sheath edge. The work presents a one-dimensional approach to study the plasma sheath profiles in the presheath and sheath regions of a colloidal plasma. Electrons are assumed to be Boltzmannian and ions and dusts are assumed to be adiabatic fluid with finite temperature in the absence of a source. The set of equations is solved numerically for the presheath and the sheath regions to explore the possible physical solutions. Solutions for presheath and sheath regions in continuity are obtained. Combined effects of electrostatic, neutral drag, and external gravity forces are included to analyze the solutions for three distinct cases for both interacting and noninteracting dust. The likely existence of levitational equilibrium is explored.
Renormalization-group analysis on the stability of large-scale electrostatic fluctuations of two-dimensional plasmas
Authors: Kim Chang-Bae
Published: February 2004

Stability of large-scale electrostatic fluctuations of forced two-dimensional plasma is studied by the use of the renormalization-group method. Large-scale flows have been rendered important in regulating turbulent plasma states and many recent works focus on their generation out of turbulence. This work is motivated by the recent result that the long-wavelength fluctuations can either grow or decay as a result of the interactions between the short-scale fluctuations depending anisotropically on the spectra of the small-scale turbulence. The Hasegawa–Mima model that is forced by an anisotropic Gaussian power-law noise is used and the renormalized viscosity up to the lowest-order coupling is computed. The result shows that the anisotropic portion of the viscosity may have opposite sign compared to the isotropic part, depending on the strength of the anisotropy.
Anisotropic magnetoresistance in CoO/Co and CoO/Fe bilayers in the biased and unbiased state
Authors: Gruyters M.
Published: February 2004

Magnetoresistance and magnetic hysteresis loop measurements have been used to investigate the strong interfacial coupling in cobalt oxide/ferromagnet bilayers at low temperatures. For the biased state, an asymmetry in the magnetization reversal processes has been found. It is characterized by domain wall motion as the dominant mechanism for the decreasing field branch and an appreciable amount of magnetization rotation for the increasing field branch. The observed behavior is different from other exchange bias systems.
Observation of negative differential resistance in GaAlAs single-barrier heterostructure at room temperature
Authors: Emelett Stephen J. , Goodhue William D. , Karakashian Aram S. , Vaccaro Kenneth
Published: February 2004

Observation of experimental negative differential resistance at room temperature due to electron tunneling in a multiple-step single-barrier GaAlAs heterostructure is reported. Theoretical investigations of a three-step single-barrier heterostructure were conducted using the transfer matrix method and the Tsu–Esaki approach to obtain the transmission coefficients and current–voltage characteristic, respectively. The system was designed based on these calculations, and grown with molecular beam epitaxy. The diode exhibited negative differential resistance at 300 K and had a peak-to-valley current ratio above unity, which was in agreement with the predicted values.
Transient and alternating current conductivity of nanocrystalline porous alumina thin films on silicon, with embedded silicon nanocrystals
Authors: Karahaliou P. K. , Theodoropoulou M. , Krontiras C. A. , Xanthopoulos N. , Georga S. N. , Pisanias M. N. , Kokonou M. , Nassiopoulou A. G. , Travlos A.
Published: February 2004

Structural characterization and surface topography of porous alumina thin films on silicon with embedded silicon nanocrystals were performed using scanning and transmission electron microscopy. The nature of porous alumina thin films is nanocrystalline with a high density of uniformly distributed silicon nanocrystals. The pores were randomly distributed with an average size of 35 nm. ac impedance spectroscopy measurements were performed at room temperature, from 0.05 up to 3.0 V in the range of 1–105 Hz for both porous alumina thin films with and without embedded silicon nanocrystals. Transient current measurements were also performed from 0.5 up to 50.0 V in the time interval 1–100 s both in forward and reverse bias conditions. The electrical conduction is dominated by the porous alumina matrix and there is no evidence of participation of the contacts to the electrical properties of the thin films. ac conductivity results follow the dielectric universal response through the whole frequency range of investigation. The real part of the specific electrical conductivity σ′ is voltage independent, in the samples studied, implying the presence of a conduction mechanism. The analysis of the experimental data reveals that the conductivity is governed by two different conduction mechanisms regardless of bias conditions, forward or reverse. In the low applied voltage region the conduction is due to thermally excited electrons, hopping from one state to another. This conduction mechanism is ohmic. For higher voltages the electrical conduction is space charge limited.
Density and concentration fluctuations in F-doped SiO2 glass
Authors: Watanabe T. , Saito K. , Ikushima A. J.
Published: February 2004

The fictive temperature, Tf, and fluorine concentration dependences of structure fluctuation in SiO2 glass was investigated by small-angle x-ray scattering measurement. Regardless of F concentration, density fluctuation in SiO2 glass is proportional to Tf and depends on structural relaxation. Concentration fluctuation does not depend on Tf and is almost proportional to F concentration. And it implies that F is incorporated into the glass structure at temperatures considerably higher than the Tf on which glass structure freezes. Contributions of the density and concentration fluctuations to the structure fluctuation were estimated.
Modeling extended defect ({311} and dislocation) nucleation and evolution in silicon
Authors: Avci Ibrahim , Law Mark E. , Kuryliw Erik , Saavedra Antonio F. , Jones Kevin S.
Published: February 2004

End of range (EOR) defects are the most commonly observed defects in ultrashallow junction devices. They nucleate at the amorphous-crystalline interface upon annealing after amorphization due to ion implantation. EOR defects range from small interstitial clusters of a few atoms to {311} defects and dislocation loops. They are extrinsic defects and evolve during annealing. Li and Jones [Appl. Phys. Lett., 73, 3748 (1998)] showed that {311} defects are the source of the projected range dislocation loops. In this article, the same theory is applied to EOR dislocation loops to model the nucleation and evolution of the loops. The model is verified with experimental data and accurately represents the nucleation, growth, and Ostwald ripening stages of dislocation loop evolution. The density and the number of interstitials trapped by dislocation loops are compared with the experimental results for several annealing times and temperatures.
Thickness dependent and annealing effects of underdoped lanthanum manganite thin films grown on Si substrates
Authors: Murugavel P. , Noh T. W. , Yoon Jong-Gul
Published: February 2004

We have investigated the thickness dependent transport properties of La0.92Ba0.08MnO3 and La0.9Ca0.1MnO3 polycrystalline films to clarify their origin in the underdoped region. For comparison, we have compared the results with properties of epitaxial films. All the underdoped films showed metal–insulator transitions after postannealing, whose transition temperature TMI increased with decrease in thickness, irrespective of their microstructures and strained states. We suggest the increase in effective hole doping, induced by cationic vacancies due to the excess oxygen, as a possible reason for the observed trend in TMI with respect to the thickness.
Comparison of interfaces for (Ba,Sr)TiO3 films deposited on Si and SiO2/Si substrates
Authors: Suvorova N. A. , Lopez C. M. , Irene E. A. , Suvorova A. A. , Saunders M.
Published: February 2004

( Ba,Sr ) TiO 3 ( BST ) thin films were deposited by ion sputtering on both bare and oxidized Si. Spectroscopic ellipsometry results have shown that a SiO2 underlayer of nearly the same thickness (2.6 nm in average) is found at the Si interface for BST sputter depositions onto nominally bare Si, 1 nm SiO2 on Si or 3.5 nm SiO2 on Si. This result was confirmed by high-resolution electron microscopy analysis of the films, and it is believed to be due to simultaneous subcutaneous oxidation of Si and reaction of the BST layer with SiO2. Using the conductance method, capacitance–voltage measurements show a decrease in the interface trap density Dit of an order of magnitude for oxidized Si substrates with a thicker SiO2 underlayer. Further reduction of Dit was achieved for the capacitors grown on oxidized Si and annealed in forming gas after metallization.
Dependence of giant magnetoelectric effect on interfacial bonding for multiferroic laminated composites of rare-earth-iron alloys and lead–zirconate–titanate
Authors: Liu Gang , Nan Ce-Wen , Cai Ning , Lin Yuanhua
Published: February 2004

The giant magnetoelectric effect of laminated composites of rare-earth-iron alloys (Terfenol-D) and lead–zirconate–titanate (PZT) is calculated by using the finite element method. Our simulations show that the magnetoelectric response of the laminated Terfenol-D/PZT composites is strongly dependent on interfacial bonding between Terfenol-D and PZT layers. The giant magnetoelectric effect and its dependence on the interfacial bonding predicted by the finite element method for the composites are in good agreement with predictions by a recent analytical method and recent experimental observations available.
Modeling the transient flow of undercooled glass-forming liquids
Authors: Demetriou Marios D. , Johnson William L.
Published: February 2004

In a recent experimental study on flow behavior of Vitreloy-1 (Zr41.25Ti13.75Cu12.5Ni10Be22.5), three distinct modes of flow are suggested: Newtonian, non-Newtonian, and localized flow. In a subsequent study, the experimental flow data is utilized in a self-consistent manner to develop a rate equation to govern local free volume production. In the present study the production-rate equation is transformed into a transport equation that can be coupled with momentum and energy transport via viscosity to formulate a model capable to govern the flow of undercooled glass forming liquids. The model is implemented to study the flow behavior of undercooled Vitreloy-1 melt. For a temperature of 700 K and shear loading of 1.0 MPa, the model predicts that the flow profile gradually stabilizes to its Newtonian limit while the liquid is maintained in structural and thermal equilibrium. For the conditions of 675 K and 100 MPa, the model predicts that the flow profile departs from its Newtonian limit and gradually stabilizes to a non-Newtonian limit. The non-Newtonian profile is evaluated independently by considering structurally quasistatic conditions, which yield the shear-rate dependency of flow. For the conditions of 650 K and 2.0 GPa, the model predicts that the flow continuously localizes and ultimately accelerates unconstrained, while the system is driven out of structural and thermal equilibration towards an unstable state associated with free volume generation, viscosity degradation, and temperature rise. The computed temperature and shear rate evolutions for the three distinct flow modes are superimposed on a temperature-shear rate diagram and appear to computationally reproduce the experimental flow map. The system’s structural state that appears to dictate flow behavior is quantified by a dimensionless number, which results from a time scale analysis of the free volume production equation.
Energy distribution of electrons ejected from a copper target in a femtosecond laser field of 1017 W/cm2
Authors: Okano Yasuaki , Hironaka Yoichiro , Nakamura Kazutaka G. , Kondo Ken-ichi , Oishi Yuji , Nayuki Takuya , Nemoto Koshichi
Published: February 2004

The energy distribution of electrons ejected from laser-induced plasma was measured using a magnetic spectrometer. The spectrometer was developed for quantitative analysis using an imaging plate. The efficiency of the imaging plate for detecting energetic electrons was calibrated using a transmission electron microscope, which accelerates electrons into energies between 80 and 200 keV. The kinetic energy distribution of electrons, which are ejected towards the backward direction, was measured on a copper bulk target irradiated with an infrared 60 fs laser. The obtained effective electron temperature in the energy range between 60 and 200 keV corresponded to approximately 130 keV at an intensity of 3.3×1017 W/cm2. This temperature was consistent with a scaling of 100[Iλ2/1017 (W μm2/cm2)]1/3 keV by Beg et al. [Phys. Plasma 4, 447 (1997)] derived from resonance absorption.
Control over a phase state of the laser plume ablated by femtosecond laser: Spatial pulse shaping
Authors: Gamaly E. G. , Rode A. V. , Uteza O. , Kolev V. , Luther-Davies B. , Bauer T. , Koch J. , Korte F. , Chichkov B. N.
Published: February 2004

The conditions for the formation of a fully atomized laser-ablated plume using subpicosecond laser pulses have been studied theoretically and implemented experimentally. It is shown that the low-intensity wings, which generally exist in the spatial distribution of laser intensity in the focal plane and can contain a substantial part of the incident laser energy, are responsible for low-threshold phase transformations of the target and this is the major source of particulates in the ablated plume. By truncating these wings with a simple aperture and positioning the target in the image plane of that aperture, particulates can be eliminated from the plume. This is demonstrated in experiments on deposition of Si films using the truncated beam in combination with the proper choice of laser fluence. This results in an almost totally atomized plume and consequently in droplet-free deposition of thin films.
Determination of the capacitance of nm scale Josephson junctions
Authors: Deppe Frank , Saito Shiro , Tanaka Hirotaka , Takayanagi Hideaki
Published: February 2004

Superconducting three-Josephson-junction flux qubits (3JFQB) are possible candidates for the basic elements of a (future) scalable quantum computer. An important design parameter is the capacitance of their Josephson junctions. We estimate the capacitance per junction area (specific capacitance) of junctions typically used in 3JFQBs, i.e., of Al/AlOx/Al junctions with 5–10 Å thick oxide layers. The capacitance is obtained by analyzing resonant voltage steps in the current–voltage characteristics of specifically designed dc superconducting quantum interference devices (SQUIDs). The junction area is deduced from scanning electron microscope images. We find that the specific capacitance of our junctions is Cs=100±25 fF/μm2. Finally we compare this result to capacitance estimates obtained with 3JFQB microwave spectroscopy and find that the SQUID resonance method provides a much higher accuracy.
Safety factor scaling of energy transport in L-mode plasmas on the DIII-D tokamak
Authors: Petty C. C. , Kinsey J. E. , Luce T. C.
Published: February 2004

The scaling of energy transport with safety factor (q) at fixed magnetic shear has been measured on the DIII-D tokamak [Luxon, Nucl. Fusion 42, 614 (2002)] for low confinement (L) mode discharges. At constant density, temperature, and toroidal magnetic field strength, such that the toroidal dimensionless parameters other than q are held fixed, the one-fluid thermal diffusivity is found to scale like χ∝q0.84±0.15, with the ion channel having a stronger q dependence than the electron channel in the outer half of the plasma. The measured q scaling is in good agreement with the predicted scaling by the GLF23 transport model for the ion temperature gradient and trapped electron modes, but it is significantly weaker than the inferred scaling from empirically derived confinement scaling relations.
Linear and nonlinear properties of Rao-dust-Alfvén waves in magnetized plasmas
Authors: Kourakis I. , Shukla P. K.
Published: February 2004

The linear and nonlinear properties of the Rao-dust-magnetohydrodynamic (R-D-MHD) waves in a dusty magnetoplasma are studied. By employing the inertialess electron equation of motion, inertial ion equation of motion, Ampère’s law, Faraday’s law, and the continuity equation in a plasma with immobile charged dust grains, the linear and nonlinear propagation of two-dimensional R-D-MHD waves are investigated. In the linear regime, the existence of immobile dust grains produces the Rao cutoff frequency, which is proportional to the dust charge density and the ion gyrofrequency. On the other hand, the dynamics of amplitude modulated R-D-MHD waves is governed by the cubic nonlinear Schrödinger equation. The latter has been derived by using the reductive perturbation technique and the two-timescale analysis which accounts for the harmonic generation nonlinearity in plasmas. The stability of the modulated wave envelope against non-resonant perturbations is studied. Finally, the possibility of localized envelope excitations is discussed.
Influence of strong laser fields on Coulomb explosions energy losses of correlated-ion clusters in plasmas
Authors: Wang Gui-Qiu , Wang You-Nian
Published: February 2004

Interactions of correlated-ion clusters with classical plasma targets are studied in the presence of a strong laser field. General expressions of the dynamic interaction potential are derived within the framework of the linearized Vlasov–Poisson theory. Coulomb explosions and the energy losses of the cluster moving in the plasma target are simulated by solving the equations of motion for the constituent ions. Numerical results show that the laser field affects the correlation between the two ions and contributes to decrease of the plasma stopping power, as compared to the laser-free case. In addition, the Coulomb explosions and the energy losses are discussed for a range of laser parameters.
Electrical resistivity of acceptor carbon in GaAs
Authors: Ferreira da Silva A. , Pepe I. , Sernelius Bo E. , Persson C. , Ahuja R. , de Souza J. P. , Suzuki Yoko , Yang Y.
Published: February 2004

The electrical resistivity was investigated from room temperature down to 1.7 K for the shallow acceptor carbon in GaAs prepared by ion implantation with impurity concentrations between 1017 and 1019 cm−3. Good agreement was obtained between the measured resistivities and resistivities calculated by a generalized Drude approach at similar temperatures and doping concentrations. The critical impurity concentration for the metal–nonmetal transition was found to be about 1018 cm−3.
Optical waveguide characterization of a tristable antiferroelectric liquid crystal cell
Authors: Jewell S. A. , Sambles J. R. , Goodby J. W. , Hall A. W. , Cowling S. J.
Published: February 2004

The optical convergent-beam waveguide technique has been used to characterize a homogeneously aligned 3 μm cell containing a liquid crystal in the antiferroelectric phase. The director structure has been quantified with the cell at 0 V and at ±50 V dc, and three distinct states have been observed. From the optical data collected, it is found that the material forms a tilted-bookshelf ferroelectric structure in the presence of a suitable voltage, and the characteristic alternating (anticlinic) structure of the antiferroelectric phase when the cell is short-circuited. The biaxiality of the antiferroelectric state has been measured, and (approximately) uniaxial refractive indices, the cone angle, and layer tilt have been determined for the ferroelectric state.
Magnetic interactions in dot arrays with perpendicular anisotropy
Authors: Repain V. , Jamet J.-P. , Vernier N. , Bauer M. , Ferré J. , Chappert C. , Gierak J. , Mailly D.
Published: February 2004

The ΔM method is based on the difference of field-induced magnetization M(H) curves starting from different initial states. It is widely used to determine the magnetic interactions in recording media made of granular or self-organized nanoparticles. It is applied here to a two-dimensional Ising model system of interacting magnetic dots fabricated by focused ion-beam patterning. This allowed us to vary the sign and the strength of the interparticle coupling in a controlled way. By high-resolution magneto-optical microscopy, the local switching processes have been investigated for these dot arrays, giving insight into the understanding of the ΔM method. Numerical simulations, using realistic values of the interactions, allowed us to reproduce well the observed field-induced magnetic domain patterns and to fit the M(H) and ΔM(H) curves.
Modeling of particle size dependence of x-ray line width of rhombohedral phase in Pb(Zn1/3Nb2/3)O3-(6,7)%PbTiO3
Authors: Rajan K. K. , Lim L. C.
Published: February 2004

A geometric model has been developed to explain the particle-size-dependent full width at half-maximum (FWHM) of rhombohedral x-ray diffraction lines in powder samples of Pb(Zn1/3Nb2/3)O3-(6,7)%PbTiO3 [PZN-(6,7)%PT] solid solutions. The model, fitted to experimental data, shows that transformation stresses are responsible for the line broadening of rhombohedral peaks of powder samples of PZN-(6,7)%PT when the particles are only a few domains in dimensions. The results also show that trapped metastable phases are an effective means of stress relaxation in both relaxor single-crystal plate and coarse powder samples. With the aid of the model, the ferroelastic domain size of the rhombohedral phase in PZN-(6,7)%PT was determined to be about 55–70 μm, which agrees reasonably with the reported values from optical, scanning electron, and atomic force microscopy.
90° domain wall relaxation and frequency dependence of the coercive field in the ferroelectric switching process
Authors: Lente M. H. , Picinin A. , Rino J. P. , Eiras J. A.
Published: February 2004

The mechanisms involved in the polarization switching process in soft and hard Pb(Zr53,Ti47)O3 (PZT) bulk ceramics were investigated through the dependency of the hysteresis loop on the frequency. In order to determine the influence of the defects on the domain switching dynamics, the samples were characterized in the virgin state and after a fatigue or a depinning process. The frequency dependence of the polarization revealed a strong relaxation of the 90° domain walls at ∼100 Hz. The results also revealed a strong influence of the kind of defect and their distribution in the ferroelectric matrix on the domain switching dynamics, which were reflected in the frequency dependence of the coercive field and the percentage of the backswitching. Initially, it was observed that the frequency dependence of the coercive field for the soft and the hard PZT in the virgin state had just one rate of change per decade in the entire frequency range investigated, which is the standard behavior found in the literature. However, after the fatigue or the depinning process, two rates of changes were noticed. Consequently, evidence of an upper-frequency limit for the coercive field changes was found. The percentage of the backswitching and its behavior for the soft PZT was almost independent of the fatigue state in the entire frequency range investigated. Nevertheless, for the hard PZT, an opposite behavior was verified. The reorientation of the domains was modeled as occurring in a viscous medium where several forces, such as viscous and restoring forces, act on them.
The effect of carbon and germanium on phase transformation of nickel on Si1−x−yGexCy epitaxial layers
Authors: Hållstedt J. , Blomqvist M. , Persson P. O. Å. , Hultman L. , Radamson H. H.
Published: February 2004

The influence of carbon and germanium on phase transformation and sheet resistance of Ni on epitaxially grown Si1−x−yGexCy (0⩽x⩽0.24 and 0⩽y⩽0.01) layers annealed in a temperature range of 360 to 900 °C has been investigated. The role of strain relaxation or compensation in the reaction of Ni on Si1−x−yGexCy layers due to Ge or C out-diffusion to the underlying layer during the phase transformation has also been investigated. The formed NiSiGe layers were crystalline, with strong (020)/(013) growth orientation in the direction, but the thermal stability decreased rapidly with increasing Ge amount due to agglomeration. However, this thermal behavior was shifted to higher annealing temperatures when carbon was incorporated in the SiGe layers. A carbon accumulation at the interface of NiSiGeC/SiGeC has been observed even at low-temperature annealing, which is suggested to retard the phase transformation and agglomeration of Ni/SiGeC system.
Heat-induced nanocluster formation in codeposited Ag1−xCox thin films: Nuclear magnetic resonance study
Authors: Jȩdryka E. , Wójcik M. , Nadolski S. , Pattyn H. , Verheyden J. , Dekoster J. , Vantomme A.
Published: February 2004

Co 59 nuclear magnetic resonance has been used to study the formation of Co nanoclusters in several series of thin-film Ag1−xCox alloys (0.08⩽x⩽0.4) prepared by coevaporation and subjected to different heat treatments. Co particles originally formed during the deposition process were found to be single-domain fcc Co clusters, having the shape of truncated octahedra of about 4.5 nm in diameter. Annealing at 200 °C resulted in a size increase to about 7 nm, but after annealing at 350 °C, the particles grew to about 26 nm while remaining single magnetic domain. Upon further annealing, the onset of large, multidomain Co clusters is evidenced. The Co concentration in the original alloy influenced the density of Co particles, but not their size, which was essentially determined by the annealing conditions.
Energy loss rate for guiding-center antihydrogen atoms
Authors: Bass Eric M. , Dubin Daniel H. E.
Published: February 2004

Collisional drag between a bound positron and a background positron plasma is considered as a mechanism for guiding-center antihydrogen atoms to relax to deeply bound states. Contrary to previous assessment, an adiabatic cutoff to the drag is predicted at deep binding, when the bound positron’s E×B drift speed vd exceeds the plasma positron thermal speed. In this regime, small-impact parameter collisions neglected in the drag calculation become the dominant 3-body recombination mechanism. At shallow binding, when ξ=vd/v̄≪1, the atom’s energy loss rate due to drag scales like ξ3/2 log2 ξ. When ξ≫1 the adiabatic cutoff takes over and the rate scales as ξ7/6 exp(−32(2ξ)2/3). The adiabatic cutoff implies that collisional drag can only assist positron–antiproton recombination up to a finite binding energy.