bluuebunny/crossref_metadata_2025 huggingface dataset:
Tunneling giant magnetoresistance in coevaporated Fex(SiO)1−x thin films
Authors: Anas M. , Bellouard C. , Vergnat M.
Published: July 2002

Fe x ( SiO ) 1−x thin films, with volume fraction x ranging from 0.04 to 0.6, have been prepared by coevaporation. Magnetization, resistivity, and magnetoresistance measurements show that the behavior of this composite is metallic like for x⩾0.35. For 0.2⩽x⩽0.27, the confrontation of all measurements prove the existence of a tunneling giant magnetoresistance between ferromagnetic grains at T<300 K, where the mean radius of the grains is estimated to be about 1 nm.
Absolute measurements and modeling of radio frequency electric fields using a retarding field energy analyzer
Authors: Charles C. , Degeling A. W. , Sheridan T. E. , Harris J. H. , Lieberman M. A. , Boswell R. W.
Published: July 2002

Measurements of the rf electric field have been made along the z axis of a helicon reactor using a retarding field energy analyzer. A fluid code and a simple analytical model have been developed to analyze the ion energy distribution functions, especially in the case of bimodal distributions where the ion transit time through the sheath in front of the analyzer is comparable to the rf period. A generalized curve (and an analytical approximation to that curve) has been developed from the analytical model and confirmed by the self-consistent fluid model for high, low, and intermediate ion transit time, which can be used by experimenters to quickly convert the experimental results (energy peak separation, plasma potential and density, electron temperature), which are related to rf sheath oscillations, to absolute values of the rf electric field. An analysis of the errors involved in the derivation of the field is given. The results agree qualitatively with rf pickup measured with a floating Langmuir probe.
Role of nitrogen in the reduced temperature dependence of band-gap energy in GaNAs
Authors: Suemune I. , Uesugi K. , Walukiewicz W.
Published: July 2002

We have observed a significant reduction in the temperature dependence of the absorption-edge energy in GaNxAs1−x alloys with x<0.04. The effect has been analyzed in terms of the recently introduced band anticrossing model that considers a coupling of the temperature-independent localized states of substitutional nitrogen atoms and the temperature-dependent extended states of GaAs. The model explains very well the alloy composition and the temperature dependence of the absorption-edge energy. We also compare the parameters that determine the temperature dependence of the band-gap energies in GaNAs and GaInNAs alloys.
Strong anisotropy in thin magnetic films deposited on obliquely sputtered Ta underlayers
Authors: McMichael R. D. , Lee C. G. , Bonevich J. E. , Chen P. J. , Miller W. , Egelhoff W. F.
Published: July 2002

Anisotropy fields in excess of 120 kA/m (1500 Oe) have been produced in 3–5-nm-thick polycrystalline films of Co by oblique sputtering of Ta underlayers. The unusually high anisotropy is magnetostatic in origin and is induced by corrugations on the surface of an obliquely sputtered Ta underlayer. Cross-sectional transmission electron microscopy reveals 4 nm columnar grains of Ta tilted toward the Ta source and elongated perpendicular to the Ta flux in the film plane. The anisotropy field of the Co film increases with both the underlayer thickness and the angle between the Ta source and the film normal. In spin valve samples, the anisotropy is attenuated by more than an order of magnitude across a 4-nm-thick Cu spacer. Magnetoresistance measurements on a spin valve indicate less than 2° dispersion in hard axis directions, and despite the nanometer-scale roughness of the underlayer there is weak broadening of the ferromagnetic resonance line.
Consistency conditions and equation of state for additive hard-sphere fluid mixtures
Authors: Barrio C. , Solana J. R.
Published: July 2002

A number of consistency conditions for the contact values gij(σij) of the pair correlation function of species i and j in an additive hard-sphere fluid mixture are discussed. It is shown that most of the theoretically-based expressions, as well as other more empirical in character, existing for these functions, fail to satisfy at least one of the conditions. It is suggested that one could improve the performance of the expressions for gij(σij) and the equation of state by using the consistency conditions. This is illustrated by modifying the Boublı́k–Mansoori–Carnahan–Starling–Leland expressions for gij(σij), which results in better predictions for these functions as well as for the compressibility factor and the fourth and fifth virial coefficients.
Short circuits in the Corrsin–Obukhov cascade
Authors: Villermaux Emmanuel , Innocenti Claudia , Duplat Jérôme
Published: July 2002

Experiments show that a blob of scalar released in the inertial range of scales of a turbulent medium is rapidly converted into a set of disjointed sheets whose spectrum exhibits a k−1 shape for wave numbers larger than the injection wave number 1/d. The sheets diffusive uniformization onsets at a time ts=(d/u)f(Sc) with f(Sc)∼ln(Sc) function of the injection time of the blob in the medium d/u, of the Schmidt number Sc, independently of the Reynolds number. This time is appreciably smaller than the time needed to cross the Kolmogorov cascade, which is “bypassed” by a strong and constant stretching rate acting at the injection scale.
A probe for investigating the effects of temperature, strain, and magnetic field on transport critical currents in superconducting wires and tapes
Authors: Cheggour Najib , Hampshire Damian P.
Published: July 2002

A variable-temperature probe has been developed to study the effect of strain on the transport properties of superconducting wires and tapes in high magnetic fields. The strain is applied to the wire by soldering it to a thick coiled spring and twisting one end of the spring with respect to the other. Strain can be applied reversibly from −0.7% to +0.7%. Measurements can be performed either in (pumped) cryogen or under vacuum. When immersed in liquid helium at 4.2 K, the probe can carry at least 200 A. From 6 to 16 K, with thin (low-loss) current leads the temperature of the sample is stable to ±45 mK for currents around 15 A, and to ±100 mK for currents around 25 A. With thick current leads, ±10 mK stability is achieved for currents up to 85 A. Full details of the probe design are described. Results obtained for a bronze processed niobium–tin multifilamentary wire are presented.
Growth and characterization of non-c-oriented epitaxial ferroelectric SrBi2Ta2O9 films on buffered Si(100)
Authors: Lee Ho Nyung , Senz Stephan , Zakharov Nikolai D. , Harnagea Catalin , Pignolet Alain , Hesse Dietrich , Gösele Ulrich
Published: July 2002

We have grown non-c-oriented SrBi2Ta2O9 (SBT) epitaxial thin films with well-defined (116) orientation by pulsed laser deposition on yttria-stabilized zirconia-buffered (YSZ-buffered) Si(100) substrates covered with electrically conductive (110)-oriented SrRuO3 (SRO) bottom electrodes. The epitaxial growth of (110)-oriented SRO films on (100)-oriented YSZ on Si(100) was confirmed both by x-ray pole figures and transmission electron microscopy (TEM) analyses showing a diagonal-type rectangle-on-cube epitaxy of SRO on YSZ with respect to the substrate and yielding specific multiple twins which originate from the particular in-plane positioning of SRO on YSZ. Cross-sectional TEM analyses revealed a roof-like morphology at the SBT/SRO interface while the other interfaces are sharp. The ferroelectric measurements of the (116)-oriented SBT films show a remanent polarization (2Pr) of 6.8 μC/cm2 and a coercive field (2Ec) of 142 kV/cm for a maximum applied electric field of 283 kV/cm. A comparable hysteresis loop recorded from local piezoresponse by an atomic force microscope working in a piezoelectric mode has also been obtained.
Electronic states of intrinsic layers in n-i-p solar cells near amorphous to microcrystalline silicon transition studied by photoluminescence spectroscopy
Authors: Yue Guozhen , Han Daxing , Williamson D. L. , Yang Jeffrey , Lord Kenneth , Guha Subhendu
Published: July 2002

Thin film n-i-p solar cells were prepared using decomposition of disilane-hydrogen mixtures by plasma-enhanced chemical vapor deposition. By increasing either the H dilution ratio or the thickness, the i-layer structure showed a transition from amorphous to microcrystalline silicon characterized by x-ray diffraction. The electronic states of the i layer were examined by photoluminescence (PL) spectroscopy, which showed that: (a) below the onset of microcrystallinity, a blueshift of the 1.4 eV PL peak energy along with a decrease of the band width occur as the structural order is improved; (b) above the onset of microcrystallinity, the PL efficiency decreases by a factor of 4–5 and the PL peak energy is redshifted toward 1.2 eV as the μc-Si volume fraction is increased. In addition, the solar cell open circuit voltage shows first an increase and then a decrease, correlating with the PL peak energy position. We conclude that the PL spectroscopy is a sensitive tool for characterizing the gradual amorphous-to-microcrystalline structural transition in thin film solar cells.
Local probing of the polarization state in thin Pb(ZrTi)O3 films during polarization reversal
Authors: Mishina E. D. , Sherstyuk N. E. , Pevtsov E. Ph. , Vorotilov K. A. , Sigov A. S. , Moret M. P. , Rössinger S. A. , Larsen P. K. , Rasing Th.
Published: July 2002

The polarization state of a thin Pb(ZrTi)O3 film is probed by optical second-harmonic generation (SHG) while applying an external voltage (a sine wave). A hysteresis in the SHG intensity is observed that corresponds to the dielectric hysteresis and is analyzed using a phenomenological relation between the SHG intensity and the dielectric polarization. Based on this model, the polarization state of the film during polarization reversal is mapped.
Oscillatory thermocapillary convection in liquid bridges with highly deformed free surfaces: Experiments and energy-stability analysis
Authors: Sumner L. B. S. , Neitzel G. P. , Fontaine J.-P. , Dell’Aversana P.
Published: July 2002

Laboratory experimentation, numerical simulation, and energy-stability theory are used to examine the effect of interface deformation on the onset of oscillatory thermocapillary convection in half zones. Experiments are performed to map the stability boundaries marking the onset of oscillatory flow, modifying the free-surface deformation by adjusting the volume of liquid in the bridge. The stability results presented here along with those of other researchers [Monti et al., Proceedings of the 43rd Cong. Int. Artro. Fed. (1992); Hu et al., J. Cryst. Growth 142, 379 (1994)] show that free-surface curvature can have a pronounced influence on flow stability. Steady, axisymmetric flow simulations are computed using the commercial code FIDAP to model the conditions of the experiments, and reveal that flow structure near the stability boundary is sensitive to several parameters. Energy theory is applied to these simulations to determine sufficient conditions for stability. Comparisons between the theoretical and experimental results show nonconservative energy limits falling above the experimentally determined stability boundaries for bridges of various liquid volumes. While the trend of the experimental data is predicted for zones of large volume ratio (bulging zones), the same cannot be said for those with small volume ratio (necked-down zones). In addition, energy-stability limits for some undeformed-free-surface cases were determined which are above the linear-stability limits determined by other researchers, in clear contradiction of the roles of the respective theories.
Electrical and structural properties of solid phase crystallized polycrystalline silicon and their correlation to single-electron effects
Authors: Tan Y. T. , Durrani Z. A. K. , Ahmed H.
Published: July 2002

Single-electron transistors have been fabricated in solid phase crystallized polycrystalline silicon films deposited on SiO2 layers grown on silicon substrates. The single-electron transistors consist of lateral side-gated nanowires. A Coulomb staircase is observed at 4.2 K, which is fully modulated by the side-gate voltage. Two-period conductance oscillations are observed in nanowires fabricated on 10-nm-thick buried oxide layers, while single-period oscillations are observed in nanowires fabricated on 40-nm-thick buried oxide layers. The two-period oscillations are attributed to the formation of a charge layer in the silicon substrate. The single-electron effects are also studied as a function of the nanowire dimensions and annealing or oxidation treatments. The effects are correlated to the structure of the polysilicon film, characterized using transmission electron microscopy, Raman spectroscopy, and electron spin resonance analysis. These measurements demonstrate the significance of single-electron charging effects on electron transport in nanometer-scale complementary metal-oxide semiconductor systems.
Ordered anodic alumina nanochannels on focused-ion-beam-prepatterned aluminum surfaces
Authors: Liu C. Y. , Datta A. , Wang Y. L.
Published: July 2002

A combined process of electropolishing, focused-ion-beam lithography, and controlled anodization is used to fabricate anodic alumina films with ordered nanochannels. The ion beam is used to create a hexagonally close-packed lattice of concaves on a polished aluminum surface and the concaves act as pinning points for guiding the growth of nanochannels in the following anodization step. By carefully matching the lattice constant (100 nm) with the anodization voltage, ordered nanochannels with aspect ratio of ∼100 are fabricated. The effects of the ion dose and its corresponding depth of the concaves on the ordering of the nanochannel array are investigated and a minimum depth of 3 nm is found to be necessary for effective guidance of the growth of ordered nanochannels.
Frequency variation under constant power conditions in hydrogen radio frequency discharges
Authors: Amanatides E. , Mataras D.
Published: July 2002

The effect of driving frequency (13.56–50 MHz) on the electrical characteristics and the optical properties of hydrogen discharges has been studied, under constant power conditions. The determination of the discharge power and impedance was based on current and voltage wave form measurements, while at the same time spatially resolved Hα emission profiles were recorded. As frequency is increased, the rf voltage required for maintaining a constant power level is reduced, while the discharge current increases and the impedance decreases. Concurrently the overall Hα emission intensity decreases and its spatial distribution becomes more uniform. Further analysis of these measurements through a theoretical model reveals that frequency influences the motion of charged species as well as the electron energy and the electric field, resulting in a modification of their spatial distribution. Moreover, the loss rate of charged species is reduced, leading to an increase of the plasma density and to a decrease of the electric field. Under these conditions, the total power spend for electron acceleration increases with frequency, but combined to the higher electron density, leads to a drop of the average energy gained per electron, a drop of the mean electron energy, and an enhancement of the low-energy electron-molecule collision processes against high energy ones.
Third-generation plasma immersion ion implanter for biomedical materials and research
Authors: Chu P. K. , Tang B. Y. , Wang L. P. , Wang X. F. , Wang S. Y. , Huang N.
Published: July 2002

A third generation plasma immersion ion implanter dedicated to biomedical materials and research has been designed and constructed. The distinct improvement over first and second generation multipurpose plasma immersion ion implantation equipment is that hybrid and combination techniques utilizing metal and gas plasmas, sputter deposition, and ion beam enhanced deposition can be effectively conducted in the same machine. The machine consists of four sets of high-efficiency metal arc plasma sources with magnetic filters, a custom designed high voltage modulator for operation up to 60 kV, a separate high-frequency, low-voltage power supply for hybrid treatment processes, special rotating sample stage for samples with an irregular shape, and other advanced features. The machine has been installed at Southwest Jiaotong University and operated reliably for 6 months. This article describes the design principles and performances of the machine as well as pertinent biomedical applications.
Atom scattering from atomic surfactants: Collisions of argon with a dilute Bi:Ga solution
Authors: Morgan Jason A. , Nathanson Gilbert M.
Published: July 2002

Gas–liquid scattering experiments are used to determine the ways in which surfactant bismuth atoms alter energy flow in collisions of argon atoms with a liquid solution of 0.02% bismuth: 99.98% gallium. We find that argon–alloy energy exchange and argon scattering intensities are governed by the surface coverage of Bi atoms. A single Bi monolayer masks the underlying Ga atoms, causing the Ga-rich solution to behave like pure liquid Bi.
Development of Pt-based ohmic contact materials for p-type GaN
Authors: Arai T. , Sueyoshi H. , Koide Yasuo , Moriyama M. , Murakami Masanori
Published: July 2002

The stabilities of the electrical properties and microstructures of Pt, PtAu, NiAu, and TaTi ohmic contacts after contact formation were studied. The Pt and PtAu contacts annealed in an O2 and N2 mixed gas ambient had the specific contact resistance (ρc) of high-10−3 Ω cm2 and strong adhesion to the GaN substrates. The NiAu contact annealed in the partial O2 ambient had poor adhesion to the GaN, although the ρc value of low-10−3 Ω cm2 was obtained. The TaTi contact had the lowest ρc values of less than 10−4 Ω cm2. After contact formation, the Pt and NiAu ohmic contacts prepared by annealing in the partial O2 ambient showed the excellent electrical and microstructural stabilities during room temperature storage and current injection. However, the contact resistance of the TaTi contact prepared by annealing at 800 °C increased during room temperature storage, and the mechanical failure of the contact was also observed after injecting current as low as 4 kA/cm2. From the present experiments, it was concluded that the Pt and PtAu contacts were the most reliable ohmic contact materials for p-GaN among four contact metals.
Normal stress and diffusion in a dilute suspension of hard spheres undergoing simple shear
Authors: Zarraga Isidro E. , Leighton David T.
Published: July 2002

The complete set of normal stresses in a dilute suspension of hard spheres undergoing simple shear at low Reynolds number is calculated using a path integration approach for the cases where the concentration is uniform and where a small gradient in concentration is present. As expected, the normal stresses are seen to be a strong function of εs=2(b−a)/a, where b is the hard sphere radius and a is the particle radius. The normal stress differences N1 and N2, are negative while the osmotic pressure is large and positive, with Π>|N2| and N1→0 as εs→∞. For εs≪1, the asymmetry in the pair distribution function due to a depletion of particles in the extensional side of a pair interaction leads to |N1|>|N2|. On the other hand, for εs→∞, the additional stresslet induced when hard sphere radii touch dominates the stress generated in the suspension, and N2 becomes the prevailing normal stress difference. The self and gradient diffusivities are calculated using da Cunha and Hinch’s [J. Fluid Mech. 309, 211 (1996)] trajectory method. Numerical results for the diffusivities are in agreement with those obtained by da Cunha and Hinch for εs⩽0.08 while matching the analytically obtained diffusivities for large εs. Finally, we calculate the normal stress in the presence of a small concentration gradient and compare two models of migration for this case, namely the suspension balance model of Nott and Brady [J. Fluid Mech. 275, 157 (1994)] and the diffusive flux model first introduced by Leighton and Acrivos [J. Fluid Mech. 181, 415 (1987)]. The results show that although the two models equally describe migration in the presence of a concentration gradient for the case where b≫a (or εs→∞), the two models are shown to be quantitatively different when near-field hydrodynamics are relevant.
Observation of intersubband real-space transfer in GaAs/AlAs quantum-well structures due to Γ–X mixing
Authors: Schmidt S. R. , Zibik E. A. , Seilmeier A. , Vorobjev L. E. , Zhukov A. E. , Ustinov U. M.
Published: July 2002

The Γ2–Xz1 intersubband dynamics in GaAs/AlAs quantum-well structures is investigated by time-resolved infrared pump and probe experiments. In the studied structure, the second Γ level in GaAs is nearly resonant to the first Xz level in AlAs, resulting in elastic Γ–X scattering. A biexponential behavior of ground-state recovery has been found with a time constant in the order of 1 ps for the Γ2–Γ1 relaxation and a time constant of 7 ps (at 10 K), which represents the Xz–Γ2 transfer.
Initial reactions in Ti–Si bilayers: New indications from in situ measurements
Authors: Cocchi R. , Giubertoni D. , Ottaviani G. , Marangon T. , Mastracchio G. , Queirolo G. , Sabbadini A.
Published: July 2002

The interactions occurring in a Ti–Si bilayer have been investigated by in situ resistance measurements and the reaction products have been analyzed by complementary techniques, such as x-ray diffraction, Auger electron spectroscopy, and MeV He+4 Rutherford backscattering. The measurements were performed by heating the samples at a constant rate in the 5–425 °C/min range. The samples are 60 nm of titanium sputter deposited on 250 nm of a silicon film deposited by chemical vapor technique; the bilayers were placed on 650 nm of SiO2, thermally grown on p-type 〈100〉 silicon wafers. Samples were also prepared by reactively depositing 25 nm of TiN on the Ti film. The TiN cap, deposited without breaking the vacuum, protects the metal films from contaminants, namely oxygen, during handling and heat treatments. It has been shown that, at temperatures around 400 °C and before any massive Ti–Si interaction, the resistance increases. This effect, attributed in the literature to the silicon diffusion in the metal film, is due to oxygen, generally contained in the annealing ambient, which diffuses in the Ti film. The Ti–Si reaction produces a decrease in the value of resistance; the first decrease, generally attributed to the formation of a C49–TiSi2 phase, is due to the growth of an amorphous compound having a graded in-depth composition between Ti5Si3 and TiSi. The activation energy for such formation is 2.8±0.1 eV. At higher temperatures C49–TiSi2 starts to form, more likely between the silicon and the amorphous layer. The formation of silicide induces a segregation of oxygen in the unreacted Ti film, thus slowing down the growth of silicide.
Comparison of optical transitions in InGaN quantum well structures and microdisks
Authors: Dai Lun , Zhang Bei , Lin J. Y. , Jiang H. X.
Published: July 2002

In 0.22 Ga 0.78 N/In 0.06 Ga 0.94 N multiple quantum well (MQW) microdisks 6.0 μm in diameter have been fabricated by photolithography and ion beam etching. Photoluminescence (PL) spectroscopy has been employed to study the optical transitions in these microdisks as well as in the original MQW structures prior to microdisk formation. With respect to the original MQWs, a blueshift in the PL peak position, enhancement of the PL intensity, and narrowing of the PL linewidth were observed at 10 K in the microdisks. These observations can be understood mainly in terms of a reduction of piezoelectric field strength due to partial strain relief in the microdisks. The magnitude the piezoelectric field reduction was estimated to be around 0.27 MV/cm, which is of the same order as the previously reported value of the piezoelectric field in similar MQW structures.
Magnetic phase transitions in CePtSn
Authors: Khmelevska T. , Svoboda P. , Janoušová B. , Sechovský V. , Chang S. , Nakotte H. , Torikachvili M. S.
Published: July 2002

Magnetization (M) and specific-heat measurements of a CePtSn single crystal in magnetic fields up to 14 T applied along principal crystallographic axes are reported. CePtSn becomes antiferromagnetic at TN=7.5 K and undergoes an order–order transition at 5 K (in zero field). In a field applied along the a axis (B//a) a metamagnetic transition (MT) is induced that is manifest by a cusp on the M(B) curve at a critical field Bc, an exponential increase of the low-field magnetization, and a linear increase for B>Bc(=12.5 T) at 2 K; Bc decreases with increasing temperature. For B//b, qualitatively different M(B) curves are observed in two temperature regimes. For T<3 K, two MTs are found at 4 and 11 T, respectively, the former one irreversible. For T⩾3 K only the “11 T” transition remains with modified character and Bc is rapidly decreasing with increasing temperature. These MTs are associated with previously reported GMR effects. Microscopic aspects of these phenomena are discussed and updated magnetic-phase diagrams for B//a and B//b, respectively, are presented.
Interface composition in magnetic tunnel junctions
Authors: Schad R. , Mayen K. , McCord J. , Allen D. , Yang D. , Tondra M. , Wang D.
Published: July 2002

The magnetoresistance of magnetic tunnel junctions critically depends on the exact composition at the interfaces. As such the completeness of the oxidation process of the Al layer (used to produce Al2O3-based tunnel junctions) plays an essential role in the magnetoresistance. We studied the chemical properties of ferromagnet/Al2O3 interfaces as a function of original Al layer thickness. We have studied the concentrations of elementary and oxidized Al, Co, Ni, and Fe for varying roughness of the ferromagnetic layer. The oxidation process critically depends on the roughness of the underlying ferromagnetic (FM) layer. Al layers grown onto smooth FM layers oxidize homogeneously whereas Al layers grown on rough FM layers show a complicated oxidation behavior. Within the sensitivity of the analysis technique, we did not observe oxidation of the ferromagnetic layers, even for the overoxidized part of the samples.
Nonlinear amplification and compression of envelope solitons by localized nonstationary parametric pumping
Authors: Melkov G. A. , Kobljanskyj Yu. V. , Serga A. A. , Tiberkevich V. S. , Slavin A. N.
Published: July 2002

Amplification of envelope solitons by localized parametric pumping has been investigated. The possibility of extremely large single-soliton amplification (above the theoretical limit for an ideal linear amplifier) due to signal compression is predicted theoretically and demonstrated experimentally for backward volume magnetostatic waves in yttrium–iron–garnet films. In the case of a strongly localized nonstationary pumping signal, compression occurs due to the amplification of a part of the signal. In the case of a quasiuniform parametric pumping signal compression results from the development of collective oscillations of parametrically coupled spin waves. A single-soliton amplification gain of 17 dB has been obtained experimentally.
The continuous spectrum for a boundary layer in a streamwise pressure gradient
Authors: Maslowe S. A. , Spiteri R. J.
Published: July 2002

Solutions of the Orr–Sommerfeld equation belonging to the continuous spectrum are presented for boundary layers developing in the presence of a streamwise pressure gradient. Although the continuous spectrum has received considerable attention in the Blasius case, in most engineering applications transition to turbulence occurs in a region where there is a pressure gradient. This investigation, so far as we know, is the first to examine what effect this has on the eigenfunctions. Our results show that when there is a pressure gradient the magnitude of the eigenfunctions near the edge of the boundary layer can be much larger than it is for Blasius flow. This is particularly true when the pressure gradient is adverse, but such is the case even when it is favorable. We also investigate the effect of Reynolds number and frequency on the penetration depth; the latter term refers to one of the properties of these modes that distinguishes them from Tollmien–Schlichting waves, namely, that their magnitude is largest near the edge of the boundary layer, but much smaller inside.
Magnetic properties of Nd1/3Sr2/3FeO3 by neutron and Mössbauer spectroscopy
Authors: Uhm Young Rang , Kim Chul Sung
Published: July 2002

Charge transfer type Nd1/3Sr2/3FeO3 was synthesized and its magnetic properties were investigated by neutron diffraction and Mössbauer spectroscopy at various temperatures ranging from 4.2 K to room temperature. The powder neutron diffraction shows magnetic ordering structures. The Nd1/3Sr2/3FeO3 with small lattice distortion underwent a charge ordering (CO) phase transition at and below TCO=163 K corresponding to a sequence of Fe+3Fe+3Fe+5Fe+3Fe+3Fe+5 aligned along the z direction of rhombohedral, as well as canted antiferromagnetic spin ordering. The spin rotation resulting from helical spin of Nd was observed at and below 25 K. At 4.2 K, the electric quadrupole splittings for Fe3+ and Fe5+ are 0.06 and 0.15 (mm/s) which were changed to −0.06 and −0.02 (mm/s) at and above 20 K. It is evidence of spin rotation in Nd1/3Sr2/3FeO3.
Lithium ion attachment mass spectrometry: Instrumentation and features
Authors: Selvin P. Christopher , Fujii Toshihiro
Published: July 2002

The design and construction of a lithium ion attachment mass spectrometer that is used to study the occurrence of free radical intermediates and molecular and atomic species (which are not found under ordinary conditions) on a real-time basis in the gas phase are described. The newly developed device exhibits several advantages over conventional mass spectrometers. The performance of this equipment was extensively tested and optimized, and experimental tests show that the efficiency and sensitivity of the instrument are moderately high. The main applications and possible future perspectives of this approach are briefly reviewed.
Irradiation enhanced diffusion of boron in delta-doped silicon
Authors: Lévêque P. , Kuznetsov A. Yu. , Christensen J. S. , Svensson B. G. , Larsen A. Nylandsted
Published: July 2002

Two kinds of silicon samples have been used in this work: one containing a sequence of boron spikes and one with a sequence of alternating boron and antimony spikes, both grown by molecular beam epitaxy. These samples were irradiated with 2.5 MeV protons at an elevated temperature ranging from 500 to 830 °C and characterized by secondary-ion-mass spectrometry. The energy of the proton beam was chosen such that the generation rate of point defects can be considered as uniform throughout the delta-doped Si samples. The influence of the sample surface and of the boron concentration (ranging from 5×1015 to 3.2×1018 B/cm3 in the different samples) on the diffusion of boron have been studied in detail. The effect of antimony on boron diffusion has also been analyzed. For each sample, the B diffusion coefficient is increased under irradiation as compared to the B diffusion coefficient in unirradiated areas. This enhancement is dependent on the irradiation temperature, on the position of the boron spike and on the boron concentration. Further, the presence of antimony enhances the boron diffusion during low-temperature proton irradiation.
Growth kinetics of amorphous interlayers in Ti thin films on epitaxial Si–Ge layers on silicon and germanium
Authors: Lai J. B. , Chen L. J.
Published: July 2002

The growth kinetics of amorphous interlayers (a interlayers) formed by solid-state diffusion in ultrahigh vacuum deposited polycrystalline Ti thin films on germanium and epitaxial Si1−xGex (x=0.3, 0.4 and 0.7) alloys grown on (001) Si and (111) Ge has been investigated by transmission electron microscopy and Auger electron spectroscopy. The growth of a interlayers in all systems was found to follow a linear growth behavior initially. The activation energies for the linear growth of a interlayers were found to decrease with the Ge content and are 1.0±0.2, 0.95±0.2, 0.85±0.2, and 0.7±0.2 eV for Ti/Si0.7Ge0.3, Ti/Si0.6Ge0.4, Ti/Si0.3Ge0.7, and Ti/Ge systems, respectively. The maximum thickness of a interlayer was found to increase with the Ge content with x⩽0.4. On the other hand, the formation temperature of crystalline phase was observed to decrease with the Ge content. Essential factors for the formation and growth of a interlayer are discussed. The results are compared with the Ti/Si system.
Simulation of stress-induced leakage current in silicon dioxides: A modified trap-assisted tunneling model considering Gaussian-distributed traps and electron energy loss
Authors: Chang Wai Jyh , Houng Mau Phon , Wang Yeong Her
Published: July 2002

In this article, a modified generalized trap-assisted tunneling model (GTAT) is proposed to explain the excessive currents occurring at low electric fields during stressing (stress-induced leakage current, SILC). Parameters such as trap energy level, Gaussian-distributed traps, and energy loss (when electrons tunnel through an oxide) are all included in this model. The trap energy levels relative to the effective Fowler–Nordheim tunneling barriers (ΦB) are classified into either shallow traps or deep traps. Quantitative analyses of the effects of oxide thickness, trap energy levels, trap concentrations, and energy losses on SILC are performed. Examples relating to the SILC of thermal oxides are shown to validate the suitability of our GTAT model. Good agreement between experimental data and the simulated current–voltage curves using this model is obtained for various SILC phenomena. The extracted trap energy levels exist between 1.5 and 2.0 eV for shallow traps and at 3.2 eV for deep traps, while trap concentrations are in the range of 1018–1020 cm−3 depending on various stress conditions. The energy level of induced traps and trap concentration can be easily derived from this model without the need for other complicated measurements. This model is demonstrated to be an accurate and reliable SILC model for investigating ultrathin gate oxide devices in integrated circuits of future generations.
Dielectric response in narrow-gap semiconductor quantum wells in a magnetic field
Authors: López-Richard V. , Marques G. E. , Trallero-Giner C.
Published: July 2002

A multiband theory of the dielectric response in a quasi-two dimensional (Q2D) electron plasma of narrow-gap semiconductor structures in an external magnetic field has been developed. The strong interband mixing plays an essential role in the determination of the shape and behavior of the dielectric response in narrow-band Q2D systems. We have used a full 8×8 k⋅p Hamiltonian model to describe the Landau levels. A rigorous theoretical analysis shows, among other interesting features, that spin–flip-like intrasubband transitions are allowed and its effects on the related optical properties of Q2D systems are investigated. The influence of the Landau level filling factor on the dielectric response and the limit of integer and fractional filling factors cases are discussed. We found that the occupation of Landau levels modulates the polarizability strength of the plasma.
Gap states of hydrogenated amorphous silicon near and above the threshold of microcrystallinity with subtle boron compensation
Authors: Sheng Shuran , Liao Xianbo , Kong Guanglin
Published: July 2002

The effects of hydrogen dilution, subtle boron compensation, and light-soaking on the gap states of hydrogenated amorphous silicon films (a-Si:H) near and above the threshold of microcrystallinity have been investigated in detail by the constant photocurrent method and the improved phase-shift analysis of modulated photocurrent technique. It is shown that high hydrogen dilution near the threshold of microcrystallinity leads to a more ordered network structure and to the redistribution of gap states; it gives rise to a small peak at about 0.55 eV and a shoulder at about 1.2 eV below the conduction band edge, which are associated with the formation of microcrystallites embedded in the amorphous silicon host matrix. A concurrent subtle boron compensation is demonstrated to prevent excessive formation of microcrystallinity, and to help promote the growth of the ordered regions and reduce the density of gap defect states, particularly those associated with microcrystallites. Hydrogen-diluted and appropriately boron-compensated a-Si:H films deposited near the threshold of microcrystallinity show the lowest density of the defects in both the annealed and light-soaked states, and hence, the highest performance and stability.
Thermopower behavior for the shape memory alloy NiTi
Authors: Lee J. Y. , McIntosh G. C. , Kaiser A. B. , Park Y. W. , Kaack M. , Pelzl J. , Kim Chul Koo , Nahm Kyun
Published: July 2002

We report thermopower measurements for the nickel titanium shape memory alloy Ni0.507Ti0.493. Our measurements reveal abrupt changes in the temperature dependence of thermopower, which correlate well with the structural phase transition between the austenitic and martensitic phases. These transition effects in thermopower are more clearly defined than in the resistivity, which is also reported. In the martensitic phase, thermopower exhibits standard metallic diffusion behavior with a nonlinearity, which is consistent with either a small peak in the density of states just below the Fermi level, as calculated by Kulkova, Egorushkin, and Kalchikhin [Solid State Commun. 77, 667 (1991)], or else electron–phonon mass enhancement. For thermally or mechanically treated samples, the magnitude of the transition effects in thermopower are reduced.
Thermal stability of the strained-Si/Si0.7Ge0.3 heterostructure
Authors: Sugii Nobuyuki
Published: July 2002

Thermal stability of the strained-Si/Si0.7Ge0.3 heterostructure was investigated by secondary-ion mass spectroscopy, Raman spectroscopy, and atomic force microscopy. Ge atoms diffused out through the strained-Si layer during heat treatment of 1000 °C for 1 h. The activation energy of Ge diffusion in strained Si was 3.3 eV, which was lower than the value in unstrained Si (4.7–5.3 eV). Strain in the strained-Si layer did not change after thermal treatment at 950 °C or less for 1 h. Slip lines due to strain relaxation formed at the surface of the strained-Si layer for the samples treated at 950–1000 °C for 1 h. For practical application of the strained-Si/Si0.7Ge0.3 heterostructure to electron devices, the maximum thermal budget should be made less than that equivalent to 900 °C annealing for 1 h.
Simultaneously efficient blue and red light generations in a periodically poled LiTaO3
Authors: Luo G. Z. , Zhu S. N. , He J. L. , Zhu Y. Y. , Wang H. T. , Liu Z. W. , Zhang C. , Ming N. B.
Published: July 2002

Generations of efficient blue light at 447 nm and red light at 671 nm were achieved by frequency doubling and tripling of a diode-pumped, Q-switched 1342 nm Nd:YVO4 laser with a periodically poled LiTaO3 (PPLT). The blue light at 447 nm was generated by sum-frequency mixing of the fundamental at 1342 nm with the generated second harmonic at 671 nm. The first-order and third-order reciprocals of the PPLT compensated the phase mismatches of second-harmonic and sum-frequency processes, respectively, making them quasiphase matched. The resulting averaged blue light power of 51 mW and red light power of 207 mW under the averaged fundamental power of 500 mW indicate that the PPLT may be used to construct an all-solid-state blue and red dual wavelength laser.
Growth of epitaxial germanium films on silicon using hot-wire chemical vapor deposition
Authors: Mukherjee C. , Seitz H. , Schröder B.
Published: July 2002

We report the epitaxial growth of germanium (Ge) films from germane gas at low substrate temperature of 350 °C and deposition rates up to 3 Å/s on crystalline silicon (Si) using hot-wire chemical vapor deposition. In situ kinetic ellipsometry measurements reveal that deposition rate is very critical to obtain thick epitaxial films. Cross-sectional transmission electron microscopy and spectroscopic ellipsometry measurements yield growth of epitaxial Ge layer with 98% crystalline fraction. Epitaxy was also confirmed by x-ray diffraction measurements and lattice stress in the epilayer is estimated. Raman measurements also reflect growth of crystalline Ge films without Si impurities.
Temperature and photopolymerization effects in microhardness of C70 crystals
Authors: Tachibana M. , Yamaguchi T. , Kojima K. , Sakuma H.
Published: July 2002

The microhardness of C70 crystals was investigated in the temperature range of 295–425 K. The hardness gradually decreased with increasing temperature. The photoinduced hardening was observed after the long-time illumination of light. The hardening reached the maximum near the phase transition temperature of 348 K. These results are discussed and compared with those of C60 crystals.
Nonlinear effects resulting from the interaction of a large-scale Alfvén wave with a density filament
Authors: Drozdenko T. , Morales G. J.
Published: July 2002

This analytic study investigates the nonlinear plasma response when a shear Alfvén wave of large transverse scale interacts with a field-aligned density perturbation whose transverse scale is comparable to the electron skin depth. The interaction between the large-scale wave and the density perturbation produces a small-scale shear mode [T. Drozdenko and G. J. Morales, Phys. Plasmas 7, 823 (2000)], which facilitates the transfer of energy to the plasma particles. The beat of the large-scale wave with the small-scale wave can lead to ponderomotive forces and flows in the plasma, or, if the field amplitudes of the small-scale wave are large enough to produce oscillatory velocities comparable to the electron thermal velocity, a streaming instability may develop. In this study, it is demonstrated that nonlinear effects arise in regions remote from the seed perturbation, and estimates of the amplitude of the large-scale wave necessary to produce significant ponderomotive density changes or trigger significant streaming instabilities are presented.
Interface state generation after hole injection
Authors: Zhao C. Z. , Zhang J. F. , Groeseneken G. , Degraeve R. , Ellis J. N. , Beech C. D.
Published: July 2002

After terminating electrical stresses, the generation of interface states can continue. Our previous work in this area indicates that the interface state generation following hole injection originates from a defect. These defects are inactive in a fresh device, but can be excited by hole injection and then converted into interface states under a positive gate bias after hole injection. There is little information available on these defects. This article investigates how they are formed and attempts to explain why they are sensitive to processing conditions. Roles played by hydrogen and trapped holes will be clarified. A detailed comparison between the interface state generation after hole injection in air and that in forming gas is carried out. Our results show that there are two independent processes for the generation: one is caused by H2 cracking and the other is not. The rate limiting process for the interface state generation after hole injection is discussed and the relation between the defects responsible for this generation and hole traps is explored.
Quantification of resputtering during pulsed laser deposition
Authors: Sturm Kai , Krebs Hans-Ulrich
Published: July 2002

During pulsed laser deposition in ultrahigh vacuum, the deposited material consists of a large fraction of ions with kinetic energies in the range of 100 eV. In many cases, these energetic particles induce resputtering at the film surface and lead to composition deviations. For Fe–Ag the resputtering effects are quantified by monitoring the deposition rate during growth of elementary and multilayer films. It is shown that preferential resputtering and, therefore, composition deviations of alloy films can be reduced at laser fluences close to the ablation threshold or even better under Ar gas pressure of about 0.05 mbar. The experimental results are described by a model, which includes atom deposition on the film surface, implantation of energetic ions below the surface and resputtering of atoms from the top monolayer.
Transient analysis in Al-doped barium strontium titanate thin films grown by pulsed laser deposition
Authors: Saha S. , Krupanidhi S. B.
Published: July 2002

Thin films of (Ba0.5Sr0.5)TiO3 (BST) with different concentrations of Al doping were grown using a pulsed laser deposition technique. dc leakage properties were studied as a function of Al doping level and compared to that of undoped BST films. With an initial Al doping level of 0.1 at. % which substitutes Ti in the lattice site, the films showed a decrease in the leakage current, however, for 1 at. % Al doping level the leakage current was found to be relatively higher. Current time measurements at elevated temperatures on 1 at. % Al doped BST films revealed space-charge transient type characteristics. A complete analysis of the transient characteristics was carried out to identify the charge transport process through variation of applied electric field and ambient temperature. The result revealed a very low mobility process comparable to ionic motion, and was found responsible for the observed feature. Calculation from ionic diffusivity and charge transport revealed a conduction process associated with an activation energy of around 1 eV. The low mobility charge carriers were identified as oxygen vacancies in motion under the application of electric field. Thus a comprehensive understanding of the charge transport process in highly acceptor doped BST was developed and it was conclusive that the excess of oxygen vacancies created by intentional Al doping give rise to space-charge transient type characteristics.
Electrical characteristics of ZrOxNy prepared by NH3 annealing of ZrO2
Authors: Jeon Sanghun , Choi Chel-Jong , Seong Tae-Yeon , Hwang Hyunsang
Published: July 2002

The electrical characteristics of ZrOxNy prepared by NH3 annealing of ZrO2 were investigated for use in metal–oxide–semiconductor gate dielectric applications. Compared with conventional ZrO2, ZrOxNy exhibits excellent electrical characteristics such as high accumulation capacitance, low leakage current density, and superior thermal stability. Based on high resolution transmission electron microscope analysis of both ZrO2 and ZrOxNy samples which had been annealed at 800 °C for 5 min, the ZrO2 exhibited a polycrystalline state but the ZrOxNy was amorphous in structure. In addition, the thickness of ZrOxNy was thinner than that of ZrO2. The improvement in electrical characteristics can be explained by the better thermal stability and lower rate of oxidation of ZrOxNy.
Ring opening dissociation of d6-benzene
Authors: Tsai Shang-Ting , Huang Cheng-Liang , Lee Yuan T. , Ni Chi-Kung
Published: July 2002

The photodissociation of jet-cooled d6-benzene at 193 nm was studied. Photofragment translational energy distributions were measured by multimass ion imaging techniques. In addition to the major channel of D atom elimination from one-photon excitation, elimination of two D atoms and two ring opening dissociation channels, C6D6→CD3+C5D3 and C6D6→C2D3+C4D3, resulting from two-photon dissociation was observed. These ring opening channels were confirmed by the momentum match between the two fragments in each channel.
Solution of the plasma-sheath equation with a cool Maxwellian ion source
Authors: Sheridan T. E.
Published: July 2002

The plasma-sheath equation with a cool Maxwellian ion source is solved using a hybrid simulation with kinetic ions and Boltzmann electrons. Results are shown for ion source temperatures TS of 0.003 to 1.0 times the electron temperature Te. When TS≲0.1Te, the potential and density profiles in the plasma are well approximated by cold Tonks–Langmuir theory. However, the shape and width of the ion velocity distribution function depend sensitively on the source temperature. The calculated distribution functions agree qualitatively with the experimental measurements of Bachet et al. [Phys. Plasmas 2, 1782 (1995)].