bluuebunny/crossref_metadata_2025 huggingface dataset:
Calorimetric investigation of the formation of metastable silicides in Au/a-Si thin film multilayers
Authors: Chromik R. R. , Zavalij L. , Johnson M. D. , Cotts E. J.
Published: July 2002

Metastable phase formations were studied in Au/a-Si thin film multilayers using differential scanning calorimetry and x-ray diffraction. Two nonequilibrium crystalline silicides were found to form below 500 K. Each phase formed by a different mechanism, and the competing growth of the two phases over the temperature range of 375 to 500 K, was found to depend greatly on thickness and grain size in the Au layers. At higher temperatures (500–600 K), these metastable phases decomposed and the a-Si crystallized by metal-induced crystallization to yield a phase mixture of Au and x-Si. The enthalpy of crystallization of a-Si was measured from the calorimetry data to be −12.1±1.2 kJ/mol.
The size of interstellar nanodiamonds revealed by infrared spectra of CH on synthetic diamond nanocrystal surfaces
Authors: Chen C.-F. , Wu C.-C. , Cheng C.-L. , Sheu S.-Y. , Chang H.-C.
Published: July 2002

Infrared spectra of CH stretches have been investigated on the surfaces of synthetic diamond crystallites (5, 100, and 700 nm in diameter) to provide insights into the mid-infrared emission bands of stardusts in interstellar media. While the spectrum of 5-nm diamonds fails to display the distinct 2835 cm−1 (or 3.53 μm) band, a remarkably good match of the absorption bands of larger diamond grains with the emission observed for the stars Elias 1 and HD 97048 is obtained. Suggested by this study, the nanodiamonds that can display such characteristic emission bands at 3.53 μm in these two interstellar systems have a size significantly larger than 5 nm.
Trajectory and molecular binding effects in stopping cross section for hydrogen beams on H2
Authors: Cabrera-Trujillo R. , Öhrn Y. , Deumens E. , Sabin John R.
Published: July 2002

The complex interaction of an atomic projectile with a molecular target is studied by considering the time-dependent electron-nuclear dynamics of the collision. We calculate the energy loss, charge exchange, and differential cross section for a hydrogen beam colliding with molecular hydrogen targets for projectiles energies from 10 eV/amu up to 25 keV/amu. We obtain the total, electronic, nuclear, and rovibrational contribution for the orientationally averaged stopping cross section of the molecular target when scattering over all the angles is considered. We emphasize the violation of Bragg’s rule (additivity of the atomic energy loss for the compound target) and the acceptance angle dependence of the experimental stopping cross section.
Thermal stability of stacked self-assembled InP quantum dots in GaInP
Authors: Jin-Phillipp N. Y. , Du K. , Phillipp F. , Zundel M. , Eberl K.
Published: July 2002

Thermal stability of stacked self-assembled InP quantum dots (QDs) embedded in Ga0.51In0.49P (GaInP) under ex situ rapid thermal annealing (RTA) is studied by photoluminescence spectroscopy and quantitative high-resolution electron microscopy. It is found that InP QDs intermix with surrounding GaInP, and that this is enhanced with increasing temperature and duration of RTA. The preferential direction of the intermixing and reshaping of the QDs changes at different stages of RTA. This anisotropy is attributed to strain-assisted interdiffusion, and is expected in stacked QDs of other material systems.
Atomic-layer epitaxy of GaN quantum wells and quantum dots on (0001) AlN
Authors: Adelmann C. , Brault J. , Rouvière J.-L. , Mariette H. , Mula Guido , Daudin B.
Published: July 2002

We show that a dynamically stable Ga film is formed on (0001) AlN in a large range of Ga fluxes at a substrate temperature of 740 °C. This feature allows for atomic layer epitaxy (ALE) of GaN on AlN by alternate exposure to Ga and N flux. We show that, at a growth temperature of 740 °C, one ALE cycle leads to the formation of a two-dimensional GaN layer, whereas further cycles lead to the formation of GaN quantum dots following a Stranski–Krastanov growth mode. This behavior is confirmed by atomic force microscopy, transmission electron microscopy, and cathodoluminescence.
Simultaneous measurements of Seebeck coefficient and thermal conductivity across superlattice
Authors: Yang B. , Liu J. L. , Wang K. L. , Chen G.
Published: July 2002

A method is developed to simultaneously measure the Seebeck coefficient and thermal conductivity in the cross-plane direction of thin films and applied to an n-type Si/Ge quantum-dot superlattice. In this method, an Au/Cr pattern serves as both a heater and a thermometer, and a microprobe is prepared between the heater and the thin film to extract the Seebeck voltage. Using a differential measurement between the thin films with different thickness, the temperature and voltage drops across the thin film are determined to deduce its cross-plane thermal conductivity and Seebeck coefficient. At room temperature, the cross-plane Seebeck coefficient and thermal conductivity are 312 μV/K and 2.92 W/mK, respectively, for the n-type Si(75 Å)/Ge(15 Å) quantum-dot superlattice doped to 8.7×1019 cm−3.
Time distributions for classically unallowed processes of a two-level system: Nonadiabatic tunneling and above-barrier reflection
Authors: Dumont Randall S.
Published: July 2002

Channel-specific transit time distributions for a two-level model system exhibiting simple avoided crossing are computed and interpreted in terms of a general semiclassical analysis which expresses wave packet evolution in terms of sums over complex trajectories. Novel effects observed include “quantum speedup” (the Hartman effect), for tunneling, and “quantum slowdown” for above-barrier reflection. Nonadiabatic effects are found to appear only at times for which their exponentially small amplitude is comparable or greater than the amplitude of adiabatic contributions. This is in accord with the semiclassical treatment provided. Nonadiabatic effects include, most notably, the appearance of slowly leaking resonances in the excited level potential energy well.
Use of the drift kinetic equation to describe electrostatic turbulent transport
Authors: Baker D. R.
Published: July 2002

It is shown that an appropriate expansion of the drift kinetic equation with an accompanying set of approximations allows the derivation of an expression for the particle flux which is in qualitative agreement with the experimental result that the density profile has the shape of (qH)−0.8, where q the usual tokamak safety factor and ℋ is a slowly varying geometric factor. This procedure reproduces a result which was previously obtained by expressing the Vlasov equation in Lagrangian coordinates, which are the actions of the motion. These actions are the magnetic moment, the parallel momentum invariant and the poloidal flux.
HFCN open-shell isomers in solid argon. I. Spectroscopy of the ground and excited states of HFC=N radical
Authors: Misochko Eugenii Ya. , Akimov Alexander V. , Goldschleger Ilya U. , Wight Charles A.
Published: July 2002

Fluoromethylene amidogen radicals, HFC=N•, were generated in solid argon by solid-state chemical reactions of mobile F atoms with hydrogen cyanide. Highly resolved infrared and electroparamagnetic resonance spectra of HFC=N• were obtained in the temperature range 15–30 K. All six vibrational frequencies and the complete set of isotropic hyperfine coupling constants on magnetic nuclei H1, F19, and N14 were determined experimentally. Calculated spectroscopic characteristics are in excellent agreement with experiments, showing that HFC=N• radical has a planar structure in the ground state. Two electronic absorption transitions were observed in the near-ultraviolet and visible spectral region. The first excited A″2 state of HFC=N• radical is calculated to have a planar structure very similar to the ground state, and lies 20 726 cm−1 above the ground state [at the CCSD(T)/cc-pVTZ level of theory], in good agreement with the experimental value, 20 430 cm−1. The observed Franck–Condon envelope in the laser-induced fluorescence spectra provides strong evidence for the similarity of the structures of the ground and first excited states. An intense absorption band centered at 355 nm is attributed to electronic transition between the ground and second excited states.
Resistive–ideal transition of pressure-driven instabilities in current-carrying plasmas beyond the Suydam criterion
Authors: Ebrahimi F. , Prager S. C. , Sovinec C. R.
Published: July 2002

The linear magnetohydrodynamics stability of local and global resistive pressure-driven instabilities is examined computationally in a cylinder. Both instabilities are resistive from beta values of zero up to several times the Suydam limit. Both transition to ideal modes at higher beta values. No sudden change in growth rate occurs at the Suydam limit. The global pressure-driven modes, of tearing parity, will likely be important in high beta plasmas, such as obtained in the reversed field pinch.
Dimension estimates and physiological data
Authors: Kantz Holger , Schreiber Thomas
Published: July 2002

Dimension estimates for data from physiological systems are notoriously difficult since the data are far from ideal in the sense of deterministic dynamical systems. Possible pitfalls and necessary precautions are pointed out and a recipe is given which is viable for those researchers who want to use the Grassberger–Procaccia algorithm but who are not familiar with the vast existing literature on dimension estimates. The relevance of dimension estimates for the characterization of physiological data is discussed, where both the cases of finding and not finding a low dimension are considered.
Targeting in Hamiltonian systems that have mixed regular/chaotic phase spaces
Authors: Schroer Christian G. , Ott Edward
Published: July 2002

The problem of directing a trajectory of a chaotic dynamical system to a target has been previously considered, and it has been shown that chaos allows targeting using only small controls. In this paper we consider targeting in a Hamiltonian system, whose phase space contains a mixture of regular quasi-periodic and chaotic regions. A multistep forward–backward method targeting strategic intermediate points is found to be efficient and robust. It takes full advantage of the phase space structure and is believed to yield optimal transport times. It is robust under the influence of small noise and small modeling errors and recovers from temporary loss of control. Two illustrative examples, the standard map and the restricted circular three body problem, are presented. (The latter corresponds to motion of a space probe in the presence of the earth and the moon.) Comparisons are made of our method to other targeting strategies.
Quantification of the effects of generation volume, surface recombination velocity, and diffusion length on the electron-beam-induced current and its derivative: Determination of diffusion lengths in the low micron and submicron ranges
Authors: Luke Keung L. , von Roos Oldwig , Cheng Li-jen
Published: July 2002

A systematic and quantitative analysis is carried out to investigate the effects of the shape (point, cube, Gaussian) and size of the generation volume, the surface recombination velocity, and the diffusion length on the electron-beam-induced current (EBIC) and its derivative (DEBIC). Thick homogeneously doped samples exhibiting diffusion lengths in the low micron and submicron range are considered. The results are presented in computed EBIC curves as a function of scanning distance and of the ratio true diffusion length/effective diffusion length. Using these curves, we show (1) a simple and yet rigorous method for the determination of the true diffusion length, taking into consideration all of the factors cited above, (2) a method for the rapid determination of the surface recombination velocity, (3) the condition under which the source shape becomes insignificant, and (4) a new value for the lower limit of the diffusion length which can be determined by the EBIC technique.
Dislocation dynamics during the growth of silicon ribbon
Authors: Dillon O. W. , Tsai C. T. , De Angelis R. J.
Published: July 2002

The thermal viscoplastic stresses and the dislocation densities in silicon ribbon are computed for an axially changing thermal profile by using an iterative finite difference method. A material constitutive equation (Haasen–Sumino model) which involves an internal variable (mobile dislocation density) is used. The results are interpreted as showing that there is a maximum width of silicon ribbon that can be grown when viscoplasticity and dislocations are considered. This maximum width limitation does not exist if the material behavior is elastic.
Plasma density measurement in an imperfect microwave cavity
Authors: Souw Eng-Kie
Published: July 2002

The density of a plasma generated in a TM010/2.45 GHz microwave cavity has been measured from the shift of the TM110 mode excited at a higher frequency. Since the design was aimed at obtaining optimum power coupling, the cavity has several openings and is imperfect from the viewpoint of the density measurement. A technique based on the dipole resonance behavior of cavity modes with nonzero axial mode number has been utilized to calibrate the observed TM110 mode frequency shift, which is found to differ considerably from the ideal cavity formula. A new theoretical formula which accounts for the particular form of cavity imperfection used in the experiment, has been derived and verified. It is also shown that the cavity shift method is conveniently applicable to pulsed microwave discharges.
Linear relaxation: Distributions, thermal activation, structure, and ambiguity
Authors: Macdonald J.Ross
Published: July 2002

The equations governing the small-signal response of relaxing, nonresonant systems which may be described by a distribution of relaxation times (DRT) and/or a distribution of activation energies (DAE) are summarized and generalized and their implications discussed for several popular distributions. Much past work, both experimental and theoretical, associated with these distributions is discussed. A distinction is made between physically realistic distributions, which involve finite shortest and longest relaxation times, and the usual mathematical approaches which involve limiting zero and infinite relaxation times. The Lévy DRT, which is of the latter character and which leads to the popular stretched exponential (SE) time and Williams–Watts (WW) frequency responses, is inconsistent with a temperature-independent DAE, reducing its range of applicability for a thermally activated situation. The SE-WW response has been termed universal; it is not, both because of the above facts and also because it does not lead to the often found symmetrical log-frequency response. Both Gaussian and exponential DAEs can lead to both symmetrical and skewed results, and can involve either temperature-dependent or temperature-independent DAEs. However, the Gaussian DAE does not yield fractional power-law time or frequency response over a finite, nonzero range, behavior found in nearly all distributed data. However, all DAEs involving exponential probability densities do lead to such behavior and provide, as well, an explanation of the temperature dependence of power-law exponents. In addition, it appears that the response of systems involving an exponential DAE can fit that of virtually all previous models, including the SE-WW, and thus can fit all data for thermally activated systems which have been fitted by these models. Problems in data fitting and many sources and types of ambiguity and their resolution are discussed. Special attention is devoted to the distinction between parallel, sequential, and hierarchical microscopic-model structure and response, and the various different, but, surprisingly, equivalent ways the overall response can be represented mathematically or by means of equivalent circuits of different connectivity.
Orientation of acicular particles in magnetic tape
Authors: Paul D. I. , Finkelstein B.I.
Published: July 2002

We present a simple method to determine the orientation distribution of acicular particles in magnetic tape. This result can be used to determine the tape magnetization and the transition width in the presence of a Karlquist field generated by the write head. The particle orientation distribution is determined by a series of measurements on a vibrating sample magnetometer consisting of saturating the tape at successive incremental polar angles, A, away from the ribbon axis, and measuring the remanent magnetization Mr along the ribbon axis after each saturation. Conceptually, particles having their easy axes in the upper half space at an angle greater than 90° from A have their magnetization reversed upon relaxing from saturation. This reduces the tape’s original remanent magnetization along the easy axis. Successive reductions in Mr are entered into an APL program, which computes the corresponding orientation distribution.
Formation of metastable compounds and magnetic properties in rapidly quenched (Fe1−x Cox)5Sm and (Fe1−xCox)7Sm2 alloy systems
Authors: Miyazaki T. , Takahashi Mikio , Yang Xingbo , Saito H. , Takahashi Migaku
Published: July 2002

The formation of metastable (TM)5Sm and (TM)7Sm2 (TM=Fe-Co) compounds and their magnetic properties have been investigatyed by the x-ray diffraction method and analysis of thermomagnetization curves for rapidly quenched (Fe1−x Cox)5Sm and (Fe1−x Cox)7Sm2 (0≤x≤1.0) alloys. The (Fe1−x Cox)5Sm compound is formed with a single phase in the range 0.7≤x≤1.0 and a mixture with other stable compounds in other composition range. On the other hand, the metastable (TM)7Sm2 compound exists in the mixed state with a large amount of (TM)3Sm stable compound in the range 0.7≤x≤1.0. The single phase (RM)5Sm compound exhibits a huge coercivity of 5–14 kOe.
Dynamics of laser-driven shock waves in water
Authors: Harith M. A. , Palleschi V. , Salvetti A. , Singh D. P. , Vaselli M. , Dreiden G. V. , Ostrovsky Yu. I. , Semenova I. V.
Published: July 2002

Shock waves were produced in water by directing unfocused 0.4-J, 20-ns ruby (λ=0.693 μm) or 3-J, 8-ns Nd-glass (λ=1.06 μm) laser light onto the metalized surface of a thin plastic foil. The illuminated areas were 0.35 and 2.3 cm2, respectively, corresponding to laser irradiances of 52.6 and 68.4 MW cm−2. The radial propagation velocity and the profile of the generated waves have been measured via double-exposure interferometric holography and shadowgraphy. Using the obtained values of the shock velocities and the fringe shift in the interferograms, the pressure on the shock wave front, the thickness of the compressed water layer, the laser energy consumed in producing this layer, and the time required for its formation have been calculated.
Crystal growth and characterization of heavily MgO-doped LiNbO3
Authors: Hu L. J. , Chang Y. H. , Yen F. S. , Lin S. P. , Lin I-Nan , Lin W. Y.
Published: July 2002

Single crystals of LiNbO3 doped with up to 25 mol % of MgO were successfully grown. No precipitates were discovered. In studies of the change of the Curie temperatures and Raman spectra with composition, an abrupt change was observed near the composition of 10 mol % of MgO. From the model of Gallagher relating Tc and cation vacancies and the cation occupation calculated by x-ray data, it is proposed that the Mg atoms replace the Nb atoms at Li sites. When the Nb atoms are exhausted, the Mg atoms replace the Li sites and Li vacancies are created. The amount of Mg in the LiNbO3 structure could be predicted better from the 151 cm−1 peak shift in Raman spectra than from Tc.
Thermal annealing study of exchange-biased NiFe-FeMn films
Authors: Toney Michael F. , Tsang Ching , Kent Howard J.
Published: July 2002

We have studied the effects of thermal annealing (between 240 and 270 °C) on the magnetic and interface properties of exchange-biased NiFe/FeMn thin film couples. With moderate annealing, there is a change in the temperature-dependence of the exchange-bias from an initial, linear shape to a convex shape, which better approximates the asymptotic behavior set by the temperature dependence of the sublattice magnetization in bulk FeMn. With extended annealing, the magnitude of the exchange-bias at room temperature increases significantly, and the blocking temperature (where the exchange-bias vanishes) increases beyond the Neel temperature of FeMn. Auger composition analysis shows that after extended annealing, there has been substantial interdiffusion of Ni into the FeMn, Mn into the NiFe, and Fe away from the interface; the original NiFe/FeMn interface becomes preferentially enriched in Mn and Ni, and deficient in Fe. Extended annealing apparently leads to the formation of a NiMnFe alloy that exhibits strong antiferromagnetic order and couples to the NiFe to produce a much stronger exchange bias than the original FeMn alloy.
Influence of the number of wells in the performance of multiple quantum well intersubband infrared detectors
Authors: Steele A. G. , Liu H. C. , Buchanan M. , Wasilewski Z. R.
Published: July 2002

We present the results of a systematic study of a series of intersubband multiple quantum well detector samples having 4, 8, 16, and 32 wells. We find that while the absorption increases with the number of wells in the device, the detector current responsivity is insensitive to the number of wells.
Deposition of copper oxide thin films on different substrates by pulsed excimer laser ablation
Authors: Ogale S. B. , Bilurkar P. G. , Mate Nitant , Kanetkar S. M. , Parikh Nalin , Patnaik Bijoy
Published: July 2002

Stoichiometric films of cupric and cuprous oxide are deposited on Si, MgO, and Y-ZrO2 substrates by pulsed excimer laser ablation technique. It is found that the equilibrium phase diagram based considerations dictate the phase formation. The films are characterized by small-angle x-ray diffraction, infrared, and UV-visible spectroscopies and Rutherford backscattering spectroscopy. It is shown that epitaxy of Cu2O films can be realized on single crystal MgO (100) substrates and the corresponding film resistivity is of the order of 40–60 Ω cm.
Effective medium theory and the attenuation of graphite fiber composites
Authors: Überall Herbert , Howell Barbara F. , Diamond Earl L.
Published: July 2002

Measured microwave attentuation values of 20–65 dB have been reported for composites and mats, containing conductive graphite fibers (‘‘fibrils’’) of 0.01 μm diameter and up to several hundredths of a millimeter length, in weight concentrations of up to 44%. The measured attenuation is interpreted here in terms of effective medium theory, according to which the composites may either behave as a ‘‘transparent’’ conductor (with essentially no conductivity) if the fibers don’t touch each other, or else as an actual, but less efficient conductor for the case where electrical contacts between the fibers have been established. Under this latter assumption, the attenuation could be well explained both in the lower frequency ranges, as well as at frequencies up to the measurement limit of 3 GHz. Quantitative fits were achieved by assuming a reduced ‘‘effective’’ conductivity of the composite due to imperfect contacts between the fibers.
PtGe ohmic contact to n-type InP
Authors: Huang Wen-Chang , Lei Tan-Fu , Lee Chung-Len
Published: July 2002

A new ohmic contact system, Ge/Pt/Ge/Pt/n-InP, was studied systematically by measuring its specific contact resistance and by using Auger electron spectroscopy, x-ray-diffraction analysis, Rutherford backscattering spectroscopy, and scanning electron microscopy. It was found that the system has a wide temperature range for annealing, i.e., 450–550 °C, to achieve the specific contact resistance of the order of 1.0×10−5 Ω cm2. It can achieve a low specific contact resistance of 7.71×10−6 Ω cm2 when it is subjected to rapid thermal annealing at 500 °C for 30 s. The whole process is a solid phase reaction so that a smooth surface morphology is obtained. The ohmicity is due to the heavy doping of Ge in the regrown InP film. The contact system exhibits good thermal stability, being able to maintain a low specific contact resistance of 9.15×10−6 Ω cm2 for 20 h, 400 °C aging, and 2.77×10−5 Ω cm2 for 80 h aging.
Magnetization and coercivity of Mn3−δGa alloys with a D022-type structure
Authors: Niida Hiroshi , Hori Tomiei , Onodera Hideya , Yamaguchi Yasuo , Nakagawa Yasuaki
Published: July 2002

The D022-type (Al3Ti-type) phase in the Mn–Ga system is ferrimagnetic; the spin direction is parallel to the tetragonal c axis. The Curie temperature is about 765 K (for δ=0.67), near the decomposition temperature. We have prepared the D022-type alloys by annealing the quenched ingots of the high-temperature phase (γMn phase) at 400 or 300 °C for a long time. The preparation becomes much easier if the ingots are powdered by filing before the annealing. The single-phase specimens of Mn3−δGa were obtained in the composition range 0.15≤δ≤1.06. The coercivity of some specimens was so large that we measured the magnetization curve in high magnetic fields up to 150 kOe. The room-temperature values of saturation magnetization, remanent magnetization, and coercivity of the powder sample of the alloy with δ=0.67 are 50 emu/g, 25 emu/g, and 13.5 kOe, respectively. We have also studied the effect of magnetic annealing during the transition from the γMn phase to the D022-type phase and the effect of the addition of Fe to Mn3−δGa of the D022-type phase.
Optical properties of degenerately doped silicon films for applications in thermophotovoltaic systems
Authors: Ehsani H. , Bhat I. , Borrego J. , Gutmann R. , Brown E. , Dziendziel R. , Freeman M. , Choudhury N.
Published: July 2002

A detailed study of the dependence of the plasma wavelength and free-carrier absorption on the doping concentration in silicon has been made. Two approaches have been used for introducing impurities into Si to achieve high doping concentration. One was the diffusion technique, using spin-on dopants. The plasma wavelength (λp) of these doped films could be adjusted by controlling the diffusion conditions. The minimum plasma wavelength achieved was 4.8 μm. In addition, a significant amount of absorption was observed for the wavelength 2 μm and below. The second approach was doping by ion implantation followed by thermal annealing with a capped layer of doped glass. Implantation with high dosages of B and As followed by high temperature annealing (>1000 °C) resulted in a plasma wavelength that could be controlled between 3.5 and 6 μm. The high temperature annealing (>1000 °C) that was necessary to activate the dopant atoms and to heal the implantation damage also caused significant redistribution of the dopants. For phosphorous implanted Si, a moderate temperature (800–900 °C) was sufficient to activate most of the phosphorous and to heal the implantation damage. The position of the plasma turn-on wavelength for an implantation dose of 2×1016 cm−2 of P was at 2.9 μm. The absorption at 2 μm was less than 25% and the reflection at 10 μm was about 85%.
Self-consistent modeling of the current–voltage characteristics of resonant tunneling structures with type II heterojunctions
Authors: Lapushkin I. , Zakharova A. , Gergel V. , Goronkin H. , Tehrani S.
Published: July 2002

We present a model and results of self-consistent calculation of current–voltage (I–V) characteristics of the InAs/AlSb/InAs, InAs/AlSb/GaSb resonant tunneling structures with type II heterojunctions. The different current components, charge accumulation in the quantum well and quasi-Fermi level variation in contacts and spacers due to the drift-diffusion processes, are taken into account. The kp band model is used to describe the interband and intraband tunneling processes. The transfer Hamiltonian approach is employed to obtain the resonant tunneling current density and charge density in the quantum well. A good quantitative agreement with the experiment is obtained for both structures including the agreement for the values of peak-to-valley (P/V) current ratio.
Electroluminescence from CaS:TmF3 film prepared by radio frequency magnetron sputtering
Authors: Sun J. M. , Zhong G. Z. , Fan X. W. , Zheng C. W. , Mueller Gerd O. , Mueller-Mach Regina
Published: July 2002

Blue electroluminescence is reported from CaS:TmF3 thin films prepared by radio frequency magnetron sputtering method. The dependences of brightness on substrate temperature, concentration of TmF3, and the applied voltage were investigated on alternating current thin-film electroluminescent devices. The obtained maximum brightness was above the level of ZnS:TmF3 devices. The equivalent average energy of the excited electrons contributed to the electroluminescence in CaS:TmF3 alternating current thin-film electroluminescent devices and was estimated at around 4.23 eV by comparing the ratio of infrared to blue peak in electroluminescent spectra with that in photoluminescent spectra excited by different photon energy. The excitation processes of electroluminescence in CaS:TmF3 thin films were assumed to be energy transfers from the conduction band edge to Tm3+ centers.
Ferroelectric and glassy states in La-modified lead zirconate titanate ceramics: A general picture
Authors: El Marssi M. , Farhi R. , Dellis J.-L. , Glinchuk M. D. , Seguin L. , Viehland D.
Published: July 2002

Raman scattering, pyroelectric current, and dielectric susceptibility measurements have been carried out on a series of tetragonal and rhombohedral-structured Pb1−xLax(ZryTi1−y)1−x/4O3 (PLZT) ceramics, under zero-field heating after zero-field cooling (ZFH/ZFC) and zero-field heating after field-cooling (ZFH/FC) conditions. The results have been shown to be in good agreement with previous transmission electron microscopy observations. A dependence of the Raman spectra on light polarization (i.e., Raman selection rules or RSR), which are known to be a characteristic property of single crystals, has been observed in the PLZT compositions 9/65/35, 12/65/35, 6/40/60, and 12/40/60 illuminated with a 2 μm wide laser spot (i.e., micro-Raman). The texture which has been observed by x-ray diffraction on 6/40/60 is discussed in conjunction with the Raman results. The combined data of Raman scattering experiments and electrical measurements, together with electromechanical and ferroelectric properties, are explained in terms of competing random fields. Furthermore, the disappearance of RSR in the ZFH/FC conditions for the ferroelectric relaxor PLZT 9/65/35 has given evidence for the existence of domains and grain boundaries in the field cooled conditions. Similar Raman spectra were found for the ferroelectric PLZT composition 5/65/35 in zero-field conditions. The occurrence of RSR in PLZT x/65/35 for x>8 (i.e., for only relaxor states with average cubic symmetry) are interpreted as resulting from second-order scattering effects. This supports a model of a cluster glass with short-range-ordered polar clusters rather than a multidomain state (i.e., a multitude of small ferroelectric domains) in these relaxor systems.
The structural and optical properties of high quality ZnTe grown on GaAs using ZnSe/ZnTe strained superlattices buffer layer
Authors: Tu R. C. , Su Y. K. , Chen H. J. , Huang Y. S. , Chou S. T. , Lan W. H. , Tu S. L.
Published: July 2002

This work studied the structural and optical properties of ZnTe epilayers grown on GaAs substrates with ZnSe/ZnTe strained superlattices buffer layers. Material properties were characterized using cross-sectional transmission electron microscopy, photoluminescence (PL), contactless electroreflectance (CER), and piezoreflectance (PzR). PL spectra clearly distinguished the strong free exciton peaks, weak donor-acceptor pair, Y lines, and oxygen-bound exciton peaks, indicating the high quality of the films. In addition, the CER and PzR spectra were compared to ascertain that ZnTe epilayers of 1.5 μm in thickness grown on GaAs substrates are under a biaxial tensile strain. An attempt was also made to identify the origins of the near-band-edge transitions of ZnTe epilayer in the CER and PzR spectra by comparing these spectra with PL spectra and the second harmonic frequency CER. By doing so, the interference below the band gap of ZnTe could be effectively eliminated. Moreover, the energy splitting between heavy- and light-hole valence bands at 15 K was calculated by utilizing the temperature-dependent elastic constants for ZnTe and the thermal-expansion coefficients for ZnTe and GaAs. The discrepancy between experiment and calculation indicates that the residual mismatch-induced strain and the thermally induced strain in ZnTe epilayers grown on GaAs during cooling must be simultaneously considered.
Depth analysis of phase formation in α-Fe after high-dose Al ion implantation
Authors: Kruijer S. , Nikolov O. , Keune W. , Reuther H. , Weber S. , Scherrer S.
Published: July 2002

α-Fe surfaces were implanted at room temperature (RT) and at 300 °C with 50 keV Al+ ions with a nominal dose of 5×1017 cm−2. The samples were studied in the as-implanted state and after annealing at 300 °C. The depth distribution of the Fe–Al phases formed was investigated nondestructively at RT and at 40 K by depth-selective Fe57-conversion–electron Mössbauer spectroscopy (DCEMS) in the energy range of K- as well as L-conversion electrons. Integral conversion–electron Mössbauer spectroscopy (CEMS) was performed between 30 K and RT. In addition, secondary neutral mass spectroscopy and depth-profiling Auger-electron spectroscopy were employed for investigating the element–concentration depth profiles which were observed to extend deeper than Monte Carlo simulations predict. We found an atomically disordered magnetic and an atomically disordered nonmagnetic bcc phase in the as-implanted state the composition of which can be explained by the Al-concentration dependence of bulk disordered Fe–Al alloys. After annealing the nonmagnetic phase becomes atomically ordered (B2 structure) whereas the magnetic phase stays in the atomically disordered state. The layer-like structure of the two phases observed after annealing can be described by the magnetic phase diagram of ordered Fe–Al alloys.
Resonant nonradiative energy transfer to erbium ions in amorphous hydrogenated silicon
Authors: Kühne H. , Weiser G. , Terukov E. I. , Kusnetsov A. N. , Kudoyarova V. Kh.
Published: July 2002

The simultaneous study of absorption, luminescence, and ODMR spectra of erbium doped a-Si:H and a SiCx:H alloy reveals that Er3+ ions are pumped by a resonant but nonradiative energy transfer from electron–hole pairs excited in the host. Direct optical pumping into absorption lines of Er3+ is not observed. The emission of the Er3+ ions is strong and decreases only moderately from 77 K to room temperature. We propose an energy transfer by a Förster mechanism, based on resonant dipole coupling, which quenches efficiently the luminescence of the host in the case of large erbium concentration. Resonance of electron–hole pairs to the excited state of the rare earth ion is achieved as electrons thermalize in tail states.
Effects of physical aging on the relaxation of poled nonlinear optical polymers
Authors: Wong K. Y. , Shen Qishun
Published: July 2002

Isothermal decay of poling induced optical nonlinearity of guest-host polymer thin films was measured by the technique of in situ optical second-harmonic generation. The effect of physical aging on the decay rate was studied over a series of aging temperatures. It was found that aging effects were localized in temperature, and diminished with increasing difference between the aging and decay temperatures. The temperature localization of the aging effects can be understood in terms of the dispersive nature of the relaxation lifetimes of polymers.
Point-source representation for laser-generated ultrasound in an elastic, transversely isotropic half-space
Authors: Hurley David H. , Spicer James B.
Published: July 2002

A point-source representation for laser-generated ultrasound in an elastic, transversely isotropic half-space is developed. This representation is comprised of a set of boundary conditions that approximates a thermoelastic point source located on the bounding surface of the half-space. An analytical expression representing the displacements for wave propagation along the symmetry axis is given for zinc where the bounding surface is the plane of transverse isotropy. The displacements obtained from theory are compared to experimental wave forms generated in a sample of single crystal zinc.
Growth behavior and thermal stability of epitaxial CoSi2 layer from cobalt–carbon films on (100) Si substrate
Authors: Rhee Hwa Sung , Ahn Byung Tae , Sohn Dong Kyun
Published: July 2002

A uniform epitaxial CoSi2 layer was grown on (100) Si substrate by rapid thermal annealing at 800 °C in N2 ambient without capping layers from an amorphous cobalt-carbon film. The amorphous cobalt-carbon film was deposited on Si substrate by the pyrolysis of cyclopentadienyl dicarbonyl cobalt, Co(η5–C5H5)(CO)2, at 350 °C. The discrete epitaxial CoSi2 layers with {111} and (100) faceted interfaces were formed on (100) Si substrate at the initial stage of reaction between Co and Si. Annealing at elevated temperatures lowered the roughness of the CoSi2/Si interface. The leakage current measured on the junction, fabricated with the epitaxial CoSi2 layer and annealed at 1000 °C for 30 s, was as low as that of the as-fabricated junction without silicide. The result indicates that epitaxial (100) CoSi2 is thermally stable at temperatures even above 1000 °C and has potential application to the salicide process in subhalf micron devices.
Changing TiN film morphology by “plasma biasing”
Authors: Geng J. , Schüler A. , Oelhafen P. , Gantenbein P. , Düggelin M. , Mathys D. , Guggenheim R.
Published: July 2002

The influence of the substrate potential with respect to the plasma on the morphology of reactively sputtered TiN thin films on Si(100) has been investigated. It is well known that the film quality with respect to grain size and distribution can be improved by applying a negative substrate bias to increase energetic ion bombardment. For large-area applications, however, a grounded substrate is very much desirable. Therefore, a technique has been developed to deposit films with comparably improved morphology on grounded substrates by means of a so-called “plasma electrode.” Grain size and distribution have been analyzed by top- and side-view scanning electron microscopy. To adjust the parameters for the TiN deposition we have used in situ photoelectron spectroscopy as the process control.
Measuring hot electron temperatures in semiconductors under high injection levels
Authors: Poles E. , Huppert D. , Hanna M. C. , Rosenwaks Y.
Published: July 2002

One of the most widely used methods to measure the excess carrier temperature in semiconductors is super band gap time-resolved photoluminescence. We find that under high carrier injection levels the formalism commonly used to extract the electron temperature using this method is erroneous due to the neglect of the role of the quasi-Fermi levels. A method that can be used to obtain accurate carrier temperatures while taking into account the important factors affecting the excess carrier concentration is proposed. The method is used to analyze time-resolved photoluminescence measurements performed on thin GaAs epilayers. It is found that the proposed method accurately corrects the apparent slow electron cooling frequently found in bulk semiconductors.
Capacitive ultrasound transducer, based on the electrical double layer in electrolytes
Authors: Tankovsky N.
Published: July 2002

An electroacoustic effect in electrolytes is analyzed on the basis of a distributed ionic capacitor, modulated in a wide range by transient electric field. The model explains well the experimental ionic concentration dependence and time dependence of the electroacoustic signal.
Novel magnetostrictive memory device
Authors: Novosad V. , Otani Y. , Ohsawa A. , Kim S. G. , Fukamichi K. , Koike J. , Maruyama K. , Kitakami O. , Shimada Y.
Published: July 2002

A stress-operated memory device consisting of an ellipsoidal magnetic particle array and an electrostrictive grid is proposed. In the device, the magnetic state of the particle can be controlled only by the magnetostriction effect. Each particle is located at the intersection of the grid and has an in-plane uniaxial anisotropy. A pair of electric contacts is connected to the end of each wire. In the writing process, the driving voltages are simultaneously applied to two pairs of the selected contacts. This allows to apply a local electric field whose direction and amplitude can be regulated by varying the voltage intensity and polarity. The exerting stress on the magnetic particle results in the linear magnetostriction and hence an additional anisotropy energy in the particle. The in-plane total energy minimum, corresponding to the magnetization direction, follows the local electric field. Consequently the magnetization of the single magnetic particle located at the intersection can therefore be selectively switched.
Crystal field excitations in YbT2Si2 (T = Fe, Co, Ni)
Authors: Goremychkin Eugene A. , Osborn Raymond
Published: July 2002

The results of an inelastic neutron scattering investigation of the crystal field potential of YbT2Si2 (T = Ni, Co, Fe) are reported. We have observed well-defined inelastic peaks due to crystal field transitions for all three compounds. By performing a profile refinement of the measured spectra, we have determined the parameters of the crystal field Hamiltonian for YbCo2Si2 and YbNi2Si2.
Structure and magnetic properties of N-containing Nd–Fe–B alloys prepared by mechanical alloying
Authors: Liu W. , Zhang Z. D. , Liu J. P. , Sun X. K. , Sellmyer D. J.
Published: July 2002

The structure and magnetic properties of Nd2Fe14BNδ-based alloys prepared by mechanical alloying have been investigated. For the Nd14Fe78B8Ny series, a large amount of NdN, α-Fe, and a small Nd2Fe14B phase are observed in x-ray diffraction patterns. With increasing Nd content, the amount of the Nd2Fe14BNδ phase increases and α-Fe decreases gradually. When x⩾25 in the NdxFe92−xB8Ny systems, some amount of the N-containing, Nd-rich phase appears. Meanwhile, the content of the nitrogen in the Nd2Fe14BNδ phase decreases and, correspondingly, the Curie temperature decreases. The coercivity and the maximum magnetic energy product of this series attain maxima at x=25. A coercivity as high as 20 kOe has been achieved. The effect of nitrogen on the formation, composition, and magnetic properties of the Nd2Fe14BNδ compound is discussed.
Dynamic radio-frequency transverse susceptibility in magnetic nanoparticle systems
Authors: Spinu L. , Srikanth H. , Carpenter E. E. , O’Connor C. J.
Published: July 2002

A novel resonant method based on a tunnel-diode oscillator is used to study the dynamic transverse susceptibility in a Fe nanoparticle system. The magnetic system consists of an aggregate of nanometer-size core (Au)-shell (Fe) structure, synthesized by reverse micelle methods. Static and dynamic magnetization measurements carried out in order to characterize the system reveal a superparamagnetic behavior at high temperature. The field-dependent transverse susceptibility at radio-frequencies for different temperatures reveals distinct peak structure at characteristics fields (±HK,HC) which changes with temperature. It is proposed that relaxation processes could explain the influence of the temperature on the field dependence of the transverse susceptibility.
FTIR observation of N≡N stretching fundamentals in hydrogen-bonded complexes in solid argon
Authors: Andrews Lester , Davis Steven R.
Published: July 2002

Codeposition of Ar/HF and Ar/N2 samples at 10 K produced the N2--H–F complex as evidenced by a very strong H–F stretching fundamental at 3881.4 cm−1 and a weak new 2332.1 cm−1 band assigned here to the N 3/4 N stretching fundamental based on 14,15N2 and 15N2 substitution. A blue DF shift to 2332. 8 cm−1 indicates a slightly stronger deuterium bond in these floppy complexes relative to the 2327.5 cm−1 Raman fundamental for N2 isolated in solid argon. Similar results were obtained for the weaker HCl and H2O complexes. Three bond stretching modes were observed for the N2--H–F--H–F complex.
Freezing of polydisperse hard spheres
Authors: McRae Robin , Haymet A. D. J.
Published: July 2002

Modern density functional theory is used to study the freezing of a polydisperse liquid of hard spheres into both face centered cubic (fcc) and hexagonally close packed (hcp) crystals. Two physically relevant, continuous distributions of particle size are studied: the gamma (or Schulz) distribution and the Gaussian distribution. The structure of a liquid of polydisperse hard spheres can be calculated analytically—and quite accurately—from the approximate Percus–Yevick integral equation. For both distributions we find that when the standard deviation of the particle size distribution exceeds approximately 5% of the mean size, the liquid no longer freezes into a crystalline array. Despite the approximations involved in the interactions between the particles in our model, this result is in agreement with experiments on real colloidal suspensions.
Liquid structuring at solid interfaces as probed by direct force measurements: The transition from simple to complex liquids and polymer fluids
Authors: Israelachvili Jacob N. , Kott Stephen J.
Published: July 2002

Results are presented of the equilibrium forces between two molecularly smooth surfaces immersed in pure monodisperse polymer liquids or ‘‘melts’’: liquid polybutadienes of molecular weights ∼1000 and ∼3500, corresponding to n≈20 and n≈65 segments (monomer units) per molecule, respectively. The aims were (i) to establish the minimum segment number when a flexible chain molecule begins to interact like ‘‘a polymer,’’ (ii) to investigate the nature of the transition from simple liquid-like behavior (as occurs with short-chained alkanes) to that characteristic of long-chained polymers, and (iii) to compare the forces in polymer melts with those where dissolved polymers are adsorbed from solution, and thereby ascertain whether there are any fundamental differences between these two types of interactions. The results show that, unlike liquid alkanes (up to hexadecane, n=16), liquid polybutadienes with n>20 already exhibit ‘‘polymer-like’’ interactions. There is also a striking similarity between the forces measured in pure polymer melts with those previously measured when polymers are adsorbed to saturation onto surfaces from good solvents; viz., a purely monotonic repulsive force law extending out to a few radii of gyration between the surfaces. It is concluded that the transition from the simple liquid-like ordering of spherical or short-chained molecules at surfaces to that characteristic of long-chained polymers is essentially a transition from a quasiordered layering of segments parallel to surfaces, extending a few segment diameters, to a more random disordered state farther out describable by mean field and scaling theories.