bluuebunny/crossref_metadata_2025 huggingface dataset:
Enhanced photogeneration of carriers in a semiconductor via coupling across a nonisothermal nanoscale vacuum gap
Authors: DiMatteo R. S. , Greiff P. , Finberg S. L. , Young-Waithe K. A. , Choy H. K. H. , Masaki M. M. , Fonstad C. G.
Published: July 2002

Enhanced generation of carriers when a thermophotovoltaic cell is placed in submicron proximity to a heated surface is demonstrated using custom-designed InAs photodiodes and special silicon-based heater chips produced using microelectromechanical system techniques. The short-circuit current of the photocells is shown to increase sharply (up to fivefold) when the spacing between the heater and photodiode surfaces is reduced, while at the same time, the heater temperature decreases, consistent with increased radiative transfer between the two surfaces. By varying the spacing sinusoidally (at up to 1 kHz), it is demonstrated that the increase in the short-circuit current occurs in phase with the decrease in separation, thereby ruling out thermal effects. It is argued that the increase in short-circuit current is due to increased evanescent coupling of blackbody radiation from the hot surface to the cold photocell, consistent with recent theoretical predictions. The demonstration of this effect is the initial step in the development of a class of energy conversion devices.
Wave packet calculations for H2(v1=10–14)+H2(v2=0–2): Reaction and dissociation mechanisms
Authors: di Domenico Daniela , Hernández Marta I. , Campos Martı́nez José
Published: July 2002

Collision induced dissociation and four center exchange reaction in H2(v1=high)+H2(v2=low) are studied by means of time-dependent wave packet calculations and within a three-degree-of-freedom reduced dimensionality model. The role of both—vibrationally excited and vibrationally cold—collision partners is examined varying v1 between 10 and 14 and v2 between 0 and 2, respectively. From the analysis of the results, a clear picture of the main mechanisms of dissociation and reaction has been obtained, and the regions of the potential energy surface most sensitive to the dynamics have been identified. In this way, reaction bottlenecks are found to significantly depend on the initial v1 state, owing to the anharmonicity of these v1 states near dissociation and the different regions of the potential explored by the associated wave packets. The topography of such bottlenecks provide a basis for the existence of tunneling in (v1=10,12, v2=0–2) reactions. Regarding the dissociation process, we find that there are two main mechanisms for dissociation; one where the unbroken diatom recoils with respect to dissociated fragments, and the second, where the diatom passes through the dissociated fragments. These mechanisms are responsible of a double peak observed in some dissociation probabilities. For (v1=14, v2=0–2) reagents, new processes appear with non-negligible probabilities: (i) inelastic collision by insertion of the cold diatom into the vibrationally excited one and (ii) dissociation of the initially cold diatom. These features, together with the observation of structures in all channel probabilities, suggest that four-atom complexes are formed during collision.
Optimized energy separation for phonon scattering in three-level terahertz intersubband lasers
Authors: Williams Benjamin S. , Hu Qing
Published: July 2002

We examine the impact of the complex phonon spectrum on the design rules for three-level THz intersubband emitters that utilize resonant optical phonon scattering to obtain intersubband population inversion. In GaAs/AlxGa1−xAs multiple quantum wells, electron–optical phonon scattering occurs due to interaction with “GaAs-like” modes at ℏωLO∼36 meV and “AlAs-like” modes at ℏωLO∼47 meV. Scattering rates are calculated for interface and confined phonon modes to determine the optimal subband energy separations for enhancement of fast depopulation scattering and reduction of parasitic nonradiative scattering. While the results depend sensitively on details of the structure, we find that, for electrically pumped THz lasers, there is not an overwhelming advantage in increasing the subband separation to allow scattering by the higher energy modes. © 2001 American Institute of Physics.
The entropy loss on supercooling a liquid and anharmonic contributions
Authors: Johari G. P.
Published: July 2002

Interdependence of the configurational, vibrational, and anharmonic-force contributions to the entropy of a liquid during its supercooling is investigated, and the effect of the anharmonic-force contribution to the entropy at constant pressure and constant volume conditions is analyzed. For realistic values of the Grüneisen parameter and expansion coefficient of a liquid, this contribution is found to be 30–40% of the heat capacity, which is high enough to affect the relation between the viscosity and excess entropy. An analysis of the resolved contributions to entropy of liquid selenium, an N-type liquid, shows that its configurational entropy is not proportional to its excess entropy.
Linear response properties for solvated molecules described by a combined multiconfigurational self-consistent-field/molecular mechanics model
Authors: Poulsen Tina D. , Ogilby Peter R. , Mikkelsen Kurt V.
Published: July 2002

A multiconfigurational self-consistent-field/molecular mechanics (MC-SCF/MM) linear response method that includes polarization effects is presented for calculating molecular properties of solvated systems. Accessible properties include frequency-dependent molecular polarizabilities, electronic excitation energies, and transition moments. Sample calculations are presented for H2O in aqueous solution.
Structure and dynamics of the keto and enol forms of acetylacetone in the solid state
Authors: Johnson M. R. , Jones N. H. , Geis A. , Horsewill A. J. , Trommsdorff H. P.
Published: July 2002

The tunneling and librational dynamics of the methyl groups of acetyl-acetone were investigated by inelastic and quasielastic neutron scattering at ambient and high pressure (4 kbar) for a variety of isotopic compounds. Samples, prepared by quenching the liquid, are shown to consist of a mixture of keto and enol forms of the molecule. This fact explains difficulties in the data analysis of previous studies. In the present work the contributions of the two forms could be separated, by preparing pure enol samples as well as keto-enriched samples. Two inequivalent methyl groups are identified for the enol form with barrier heights of the hindering potential in the range of 220–800 K. These potential barriers are fairly sensitive to deuteration of the nonmethyl protons and to disorder in the crystal. In contrast, for the keto form the potential is insensitive to these factors. These differences reflect the influence of the hydrogen-bonded proton on the methyl group dynamics in the enol molecule.
Asymmetry of relaxation processes and the creation of high-energy phonons in the anisotropic phonon systems of He II
Authors: Adamenko I. N. , Nemchenko K. É. , Wyatt A. F. G.
Published: July 2002

A study is made of relaxation processes in anisotropic phonon systems in superfluid helium. The rates of creation and annihilation of high-energy (h) phonons in all possible processes in an anisotropic phonon system are obtained as functions of momentum, temperature, and the anisotropy parameter. The physical causes of the asymmetry in the creation and annihilation of h phonons are elucidated. All of the processes of interaction of h phonons with low-energy phonons (l phonons) and h phonons with each other are investigated, and the role of each of these processes in the formation of the distribution function of h phonons is determined. All of the processes that do not conserve the total number of h phonons are taken into account in constructing an equation describing the change in energy density of the h phonons in the main beam, and a quasistationary solution of this equation is found. The solution of this equation can be used to find the energy density of the created h phonons at any point in space at an arbitrary time and the energy density of l phonons in the main beam.
Wave driven N2–Ar discharge. I. Self-consistent theoretical model
Authors: Henriques J. , Tatarova E. , Guerra V. , Ferreira C. M.
Published: July 2002

A theoretical model for a low pressure surface wave driven N2–Ar discharge accounting in a self-consistent way for electron and heavy particle kinetics, gas thermal balance, and wave electrodynamics is developed. The inhomogeneous wave power transfer along the discharge and the concentrations of active species as a function of the spatial position and mixture composition are investigated. N2+ are the dominant ions over a wide range of Ar percentages in the mixture due to the contributions of charge transfer processes between Ar+ and N2 and an effective associative ionization from N2(A 3Σu+). Dissociative recombination of N2+ with electrons provides an increase in the dissociation degree of nitrogen molecules at high Ar fractional concentration.
Tapping mode and elasticity imaging in liquids using an atomic force microscope actuated by acoustic radiation pressure
Authors: Onaran A. Guclu , Degertekin F. Levent , Hadimioglu Babur
Published: July 2002

We report the use of the radiation pressure generated by a focused acoustic beam near 170 MHz to implement tapping mode and elasticity imaging using the atomic force microscope (AFM) in liquids. Since the acoustic radiation force can be localized on an area of a few microns in diameter, this method enables efficient excitation and spatial mapping of both higher-order flexural and torsional modes of AFM cantilevers in liquids. We exploit the sensitivity of the higher-order cantilever mode shapes to the tip–sample contact stiffness for elasticity imaging. We present higher-order flexural and torsional AFM cantilever mode shape measurements in the 1–250 kHz range, and initial results on elasticity imaging on a sample with 1.3 μm thick patterned photoresist layer on silicon obtained at 50 kHz.
Rescaled range analysis of resting respiration
Authors: Hoop Bernard , Kazemi Homayoun , Liebovitch Larry
Published: July 2002

Fluctuations in resting depth of breathing (tidal volume) at constant breathing rate in the anesthetized adult rat exhibit fractal properties when analyzed by a rescaled range method characterized by a mean (±SD) exponent H=0.83±0.02 and 0.92±0.03 with and without sighs, respectively, for up to 400 breaths. Values of H determined from shuffled tidal volumes and simulated tidal volumes taken randomly from a Gaussian distribution of mean and variance approximating that of the actual data are consistent with the expected value of H=0.5 for an independent random process with finite variances. An empirical description is proposed to predict the change in H with length of time record.
Magnetic scattering of x rays (invited)
Authors: Blume M.
Published: July 2002

The scattering of x rays is used to determine the electric charge distribution in matter. Since x rays are electromagnetic radiation, we should expect that they will be sensitive not only to the charge distribution, but also to the magnetization density. That this is indeed the case has been pointed out and studied experimentally. In this paper the magnetic scattering is discussed in a way which allows consideration of the effects of electron binding. The cross section, compared with that for neutron scattering from magnetically ordered materials, is reduced by (ℏω/mc2)2 (about 5×10−4). With a synchrotron radiation source, however, this factor can be made up, and magnetic x-ray Bragg peaks can be collected in the same time as neutron peaks. Special effects of interest include high momentum resolution, polarization phenomena which separate spin and orbital densities, and resonance effects which give a large enhancement of the x-ray cross section and which may make the study of surface magnetism possible.
Fabrication of NbN/AlN superconducting multilayers
Authors: Murduck J. M. , Vicent J. , Schuller Ivan K. , Ketterson J. B.
Published: July 2002

Multilayers consisting of superconducting niobium nitride alternated with layers of insulating aluminum nitride have been fabricated using dc magnetron reactive sputtering. Under the proper conditions NbN can be produced in a (111) orientation and AlN in a (0001) orientation with an in-plane lattice mismatch of only 0.27%. Proper preparation of the aluminum target was necessary to ensure uniform deposition. X-ray diffraction and transmission electron microscopy verify the layered nature of these samples. Transport properties are studied as they relate to the structure of the system.
An investigation of the Nd-rich phases in the Nd-Fe-B system
Authors: Tang Weizhong , Zhou Shouzeng , Wang Run , Graham C. D.
Published: July 2002

The crystal structures and the compositions of the Nd-rich phases in the Nd-Fe-B system have been investigated by means of transmission electron microscopy, x-ray diffraction, and Auger spectroscopy techniques. It has been observed that there are two kinds of Nd-rich phases with different structures and compositions. Most of the Nd-rich phases will undergo a phase transformation when the powder is sintered at high temperatures. This phase transformation is accompanied by the introduction of oxygen into the alloy which will change from a ternary system into a quaternary one. Both of the two Nd-rich phases are stable phases in this later system. With the aid of the EDX and the Auger spectroscope, the compositions of the dhcp and the fcc Nd-rich phases have been determined. The Nd contents of the two phases are about 90 and 70 at. %, respectively, with the later phase containing about 15 at. % of oxygen.
The structures and magnetic properties of FeN films prepared by the facing targets sputtering method
Authors: Jiang En-yong , Sun Chang-qing , Li Jin-e , Liu Yu-guang
Published: July 2002

The Fe nitride films were easily produced by facing targets sputtering (FTS). The structures and magnetic properties of the films depend on the reactive pressure (PN) and dc substrate biasing voltage (Vb). α-Fe, α″-Fe16N2, γ′-Fe4N, ε-Fe3N, and ζ-Fe2N as well as the amorphous FeN phase have been produced at the PN ranges of 0–8×10−3 Torr. The magnetic moment per iron atom increases with PN until PN=1.0 mTorr at which the α′-Fe16N2 phase appears with the coercive force Hc=103 A/m, susceptibility χm=161, and μS=2.85 μB. The relation between conductivity and temperature σ-T indicates that Fe nitride films resemble a semiconductor. The Curie temperature of these films is reduced with the increase of PN. Magnetic relaxation phenomena were observed in the vicinity of 540 °C.
Quenching of flames by magnetic fields
Authors: Ueno S.
Published: July 2002

An experiment has been demonstrated to show a phenomenon involving quenching of candle flames using magnetic fields. An electromagnet with a pair of columnar magnetic poles in which inner sidepieces were hollowed out was used. Magnetic fields of 1.5 T at the brim gave a gradient of 50–300 T/m in the direction perpendicular to the pole axis when the distance of the airgap was in a range of 5–10 mm. A candle was burned in the hollowed space between magnetic poles, and candle flames were exposed to magnetic fields. The flames were quenched a few seconds after the onset of field exposures. A model, called a ‘‘magnetic curtain,’’ was introduced to explain this phenomenon. It is assumed that the magnetic curtain is a wall of air which is formed by magnetic fields. The magnetic curtain, which is also called an ‘‘air curtain,’’ blocks air flow in and out of the hollowed space. The interception of oxygen by the magnetic curtain quenches flames. The magnetic curtain also presses back flames and other gases.
Epitaxial layer thickness measurement of double heterostructures using reflectance spectroscopy
Authors: Tarof L. E. , Miner C. J. , Blaauw C.
Published: July 2002

Reflectance spectroscopy has been applied to the study of A-B-A double-heterostructure (DH) layer sequences in the AlGaAs and InGaAsP materials systems, consisting of an A epitaxial layer on a B epitaxial layer on a substrate of material A. In the spectral regime where the materials are transparent, the reflectance spectrum exhibits periodic oscillations due to thin-film interference. Both epitaxial layer thicknesses may always be determined by performing computer simulations of the DH reflectance spectrum, obtaining a best fit to spectral data. Simple algorithms far less computation intensive than curve fitting the spectrum are developed here, which can be used to deduce both epitaxial layer thicknesses in many cases. The choice of algorithm depends primarily on the thickness of the B layer. Fourier transform infrared measurements have been used to apply this technique to a variety of A-B-A DH systems for B layers ranging from 0.01 to 3.8 μm thick. The simulated and measured reflectance spectra are in good agreement. The optical results show a systematic offset of 2% from transmission electron microscopy measurements with a standard deviation of 1%. An analytical expression for the modulation amplitude of the reflectance spectrum oscillations, which depends on the thickness and composition of the epitaxial layers, is developed. This theoretical expression justifies the thickness algorithms and predicts a nearly linear relationship between the modulation amplitude of the reflectance spectrum and the refractive index (composition) of the B layer.
Investigation of the amorphization reaction induced by amorphous phase through mechanical milling
Authors: Wang K. Y. , He A. Q. , Shen T. D. , Quan M. X. , Wang J. T.
Published: July 2002

We report the study of the amorphization reaction induced by the amorphous phase through mechanical milling (MM). The amorphous iron-based powders (Fe78Si12B10) and elemental Al powders were mechanically milled in a planetary ball mill. X-ray diffraction, differential scanning calorimetry, and transmission electron microscopy were used to investigate the MM processes. Results showed that the Al atoms dissolved into the amorphous iron-based phase (Fe78Si12B10) and an Al-rich amorphous phase was produced.
Population inversions behind normal shock waves in CO2-N2-He mixtures seeded with solid particles: Further results
Authors: Reddy K. P. J.
Published: July 2002

The analysis of propagation of a normal shock wave in CO2-N2-He or H2 or H2O system seeded with solid particles is presented. The variation of translational and vibrational temperatures of gas phase and the particle temperatures in the relaxation zone behind the shock front are given in graphical form. These results show that the peak value of population inversion and the width of the inversion zone are highest for He catalyst and lowest for H2O catalyst.
Dielectric constant for binary piezoelectric 0-3 composites
Authors: Jayasundere N. , Smith B. V.
Published: July 2002

In this article an analytic expression is presented for the dielectric constant of a binary piezoelectric 0-3 composite by modifying the well known Kerner expression [Proc. Phys. Soc. London Sec. B 69, 802 (1956)] to include interactions. The developed expression is a function of the constituent dielectric constants and their volume fractions only and compares very favorably with available experimental data for piezoelectric ceramic inclusions in a dielectric continuum having widely differing dielectric constants.
Electron irradiation of GaAs: Improvement of transport properties and observation of DX-like centers at ambient pressure
Authors: Ghamlouch H. , Aubin M. , Carlone C. , Khanna S. M.
Published: July 2002

Epitaxial n-type GaAs samples were studied before and after irradiation with 7 MeV electrons at fluences varying from 1×1013 to 5×1015/cm2. The measurements involved the Hall effect and resistivity from 15 to 300 K. Fitting of the data revealed the evolution of the density and activation energy of impurity levels as a function of fluence. At the higher fluences, the carrier concentration and mobility decrease as expected but at the lower fluences, both of these quantities increase in a manner that depends on doping and fluence levels, and on temperature. The results are consistent with the radiation-stimulated gettering effect. It is shown that the maximum observed in the Hall coefficient as a function of temperature is possibly due to a manifestation of DX centers in the absence of applied pressure.
Comparison of neutron and electron irradiation on the EL2 defect in GaAs
Authors: Lai S. T. , Nener B. D. , Alexiev D. , Faraone L. , Ku T. C. , Dytlewski N.
Published: July 2002

The deep level transient spectroscopy technique has been used to study the EL2 defect in n-type semiconducting GaAs subjected to 1 MeV fast neutrons at room temperature. After neutron irradiation, the EL3 defect which is usually detected between 180 and 210 K disapperared and the EL2 defect measured between 280 and 320 K was found to remain single exponential (Ec−0.820 eV) despite the creation of a broad U band measured between 100 and 270 K. From this result, together with our earlier reports on the double exponential capacitance transient of the EL2 defect after 1 MeV electron irradiation [Lai, Nener, Faraone, Nassibian, and Hotchkis, J. Appl. Phys. 73, 640 (1993)] and the behavior of the electron irradiated EL2 defect upon isochronal annealing [Lai and Nener, J. Appl. Phys. 75, 2354 (1994)], we observe a difference in the behavior of the EL2 defect after neutron and electron irradiation. The results of the present study indicate that the EL2-B level reported in an earlier work is not due to any interaction of the stable EL2 (or EL2-A) level with either the U-band or EL6 defect. The EL2 defect is likely to be a complex defect which can manifest itself as a number of different defect levels depending on the particular details of the irradiation used. The U band is likely to be a cluster defect caused by the large number of atoms displaced from lattice sites by the fast neutrons, and is not likely to be due to any interaction mechanism between the EL2 and EL6 defects.
Ga2O3 films for electronic and optoelectronic applications
Authors: Passlack M. , Schubert E. F. , Hobson W. S. , Hong M. , Moriya N. , Chu S. N. G. , Konstadinidis K. , Mannaerts J. P. , Schnoes M. L. , Zydzik G. J.
Published: July 2002

Properties of Ga2O3 thin films deposited by electron-beam evaporation from a high-purity single-crystal Gd3Ga5O12 source are reported. As-deposited Ga2O3 films are amorphous, stoichiometric, and homogeneous. Excellent uniformity in thickness and refractive index was obtained over a 2 in. wafer. The films maintain their integrity during annealing up to 800 and 1200 °C on GaAs and Si substrates, respectively. Optical properties including refractive index (n=1.84–1.88 at 980 nm wavelength) and band gap (4.4 eV) are close or identical, respectively, to Ga2O3 bulk properties. Reflectivities as low as 10−5 for Ga2O3/GaAs structures and a small absorption coefficient (≊100 cm−1 at 980 nm) were measured. Dielectric properties include a static dielectric constant between 9.9 and 10.2, which is identical to bulk Ga2O3, and electric breakdown fields up to 3.6 MV/cm. The Ga2O3/GaAs interface demonstrated a significantly higher photoluminescence intensity and thus a lower surface recombination velocity as compared to Al2O3/GaAs structures.
Observation of phase transitions in an antiferroelectric liquid crystal investigated by scanning tunneling microscopy
Authors: Jandt K. D. , Tixier S. , Towler M. J. , Blackmore J. M. , Miles M. J.
Published: July 2002

Here, we present scanning tunneling microscopy images of antiferroelectric liquid-crystalline (AFELC) molecules observed at different elevated temperatures corresponding to different bulk mesophases of the material. These images show evidence for a commensurate changing of the molecular arrangement on the graphite substrate as a function of temperature. Furthermore, a nontrivial angular relationship between the graphite substrate and the AFELC domains was obtained, indicating that there may be a stronger, polar (electrostatic) interaction between the AFELC molecules than between the adsorbate and the substrate.
Donor-acceptor pair luminescence involving the iodine A center in CdTe
Authors: Lee Jaesun , Giles N. C. , Rajavel D. , Summers C. J.
Published: July 2002

Photoluminescence studies from 5 to 296 K have been performed on a series of iodine-doped CdTe epilayers grown by molecular beam epitaxy. The samples exhibit excess electron concentrations in the range from 8×1016 to 3×1018 cm−3. Bright edge emission is observed at 296 K from all samples. A deep-level band centered near 1.45 eV is observed at T<210 K and increases in intensity with doping level. A correlation of growth parameters with photoluminescence data fits a model of the deep-level band being predominantly donor-acceptor pair recombination involving the shallow iodine donor (ITe) and the iodine A-center acceptor complex (VCd-ITe). Zero-phonon emission related to this pair recombination occurs at 1.470 eV at 5 K. Thermal quenching of the integrated intensity of this donor-acceptor band is described by activation energies of 15 and 125 meV corresponding to thermalization of electrons from shallow ITe donors to the conduction band and complete thermalization from the valence band to iodine A centers, respectively.
Magnetization reversal mechanisms in NiFe/Cu/NiFe/FeMn spin-valve structures
Authors: Gillies M. F. , Chapman J. N. , Kools J. C. S.
Published: July 2002

Transmission electron microscopy is used to provide a detailed description of how magnetization reversals take place in NiFe/Cu/NiFe/FeMn spin-valves. Direct observation is made of how both NiFe layers respond to an applied field. Marked differences from the behavior observed in single Permalloy layers of the same thickness are identified. Complex 360° wall structures frequently form and are studied in some detail. A description of their structure is given and a theory involving the compensation of charges from the biased layer is suggested to explain stability. The work has a direct bearing on the performance of spin-valves as sensors as the way the magnetization changes under the influence of an applied field affects the noise characteristics.
Characteristics of photoacoustic displacement for silicon damaged by ion implantation
Authors: Takamatsu Hiroyuki , Sumie Shingo , Morimoto Tsutomu , Kawata Yutaka , Muraki Takeshi , Hara Tohru
Published: July 2002

A decrease in the thermal conductivity and the optical absorption length of a silicon wafer damaged by ion implantation causes an increase in the amplitude of photoacoustic displacement (PAD) on the sample surface. The behaviors of the PAD were investigated by thermoelastic analysis. The theoretical results indicate that the amplitude of PAD is approximately proportional to the square of the thickness of the damaged layer and is characterized by the ratio of the optical-absorption coefficient to the thermal conductivity of the damaged layer. Experimental results, which were quantitatively measured by a highly sensitive PAD interferometer, can be explained well by the theoretical predictions. The thermal conductivity of the damaged layer by implantation above critical amorphization dose can be estimated to be 1–3 W/mK by comparison of the experimental data with the theoretical results.
Neutron-diffraction study of the insertion scheme of hydrogen in Nd2Fe14B
Authors: Isnard O. , Yelon W. B. , Miraglia S. , Fruchart D.
Published: July 2002

Results are presented of a neutron powder diffraction experiment performed at room temperature on the Nd2Fe14BHx series. The location of the hydrogen (or deuterium) atoms within the crystal lattice is described. After an analysis of the crystal structure of Nd2Fe14B, Nd2Fe14BH, Nd2Fe14BH2, Nd2Fe14BH3, and Nd2Fe14BH4, a filling scheme of the interstitial sites by hydrogen atoms is proposed. This filling scheme is compared and discussed with that previously obtained on other R2Fe14B alloys in particular with R=Ce, Y, Ho, Er. The use of all these compositions provides a better understanding of the way the Nd2Fe14B structure is modified by insertion of hydrogen. The evolution of the shortest iron-iron interatomic distances is also analyzed and related to the change in magnetism.
Thermal stability of α-titanium in contact with titanium nitride
Authors: Wang Shi-Qing , Allen Leslie H.
Published: July 2002

The thermal stability of an α-Ti film in contact with a δ-TiN film in the structure of a TiN/Ti/TiN film stack on SiO2 substrates was studied by in situ sheet resistance (Rs) measurement, Auger electron spectroscopy, glancing angle x-ray diffractometry, cross-sectional transmission electron microscopy, scanning electron microscopy, and atomic force microscopy. It was found that nitrogen dissolves from TiN into Ti between 405 and 474 °C and a significant reaction between Ti and TiN results in the formation of ε-Ti2N in the temperature range of 505–548 °C.
Equilibrium states in multijunction superconducting quantum interferometers
Authors: Konopka A. , Lewandowski S. J. , Mikheenko P. N. , Monaco Roberto
Published: July 2002

Multijunction superconducting quantum interferometers can be created spontaneously by intrinsic junctions in granular high-Tc materials, and as a by-product of step-edge technology used to fabricate high-Tc thin film dc superconducting quantum interference devices. The properties of such systems are relatively little known. The theory predicts multivalued dependence of critical current Ic on applied magnetic flux Φe and the appearance of different stationary ‘‘phase states’’ related to different possible relationships between the phases of superconducting order parameter in individual junctions. We report on the measurements performed on a (2×2) quantum interferometer, comprising a series array of two underdamped all-niobium tunnel junctions in each parallel arm. Experimental results closely follow the theoretical predictions and reveal several instability regions, in which noise generation takes place. These findings can possibly be related to the random telegraph noise observed in high-Tc materials.
Hydrodynamic instabilities in laser pulse-produced melts of metal films
Authors: Bostanjoglo O. , Nink T.
Published: July 2002

The dynamics of melts, as induced by 7 ns laser pulses in Al and Au films, were investigated by in situ time-resolved transmission electron microscopy. Melting, motion of the liquid, and crystallization were observed by tracing the image intensity with a photomultiplier (space/time resolution 100 nm/3 ns) and by streak imaging (streak times 15 ns–4 μs). Films with native oxides/adsorbed atmospheric contaminations and films purified by pulse melting were found to show a completely different behavior of their melts. The melts of purified films either remained almost flat (Al) or revealed a gradual pileup of liquid in cold regions within 500 ns (Au), caused by thermocapillarity with the negative thermal coefficient of the surface tension of pure metals. In contrast, contaminated films showed three distinctly different types of perturbations: (1) a fast expulsion of the melt from the center of the laser spot within 20 ns after the laser pulse; (2) a gradual contraction of liquid at the center within 0.5–1 μs; (3) thickness oscillations with frequencies of 5–10 MHz and time constants of 500 ns. These effects are explained by recoil from evaporating contaminations, by thermocapillary flow with a positive thermal coefficient of the surface tension, caused by surface active impurity atoms, and by thermocapillary waves.
Preparation and properties of ferromagnetic carbon-coated Fe, Co, and Ni nanoparticles
Authors: Jiao Jun , Seraphin Supapan , Wang Xikun , Withers James C.
Published: July 2002

Carbon-coated iron, cobalt, and nickel particles were produced by an arc discharge process modified in the geometry of the anode and the flow pattern of helium gas. Field emission scanning electron microscopy shows that the resulting material consists of only carbon-coated metal particles without any nanotubes or other unwanted carbon formations present. The diameters of iron, cobalt, and nickel particles range predominantly from 32 to 81 nm, 22 to 64 nm, and 16 to 51 nm, respectively. X-ray diffraction analysis confirmed that the as-made particles are carbon-coated elements rather than metal carbides. High resolution transmission electron microscopy reveals that the as-made cobalt and nickel particles are covered by 1–2 graphitic layers, while iron particles are surrounded by amorphous carbon. When the samples were treated by annealing or immersion into nitric acid, particles completely coated by carbon resisted both postdeposition treatments. However, further graphitization of the carbon coating by either of the two treatments was observed. Particles only partially coated by carbon were not protected, but sintered by annealing or dissolved in the acid. The magnetic properties of the as-made particles were measured by a vibrating sample magnetometer. The values of the saturation magnetic moment per gram of each type of metal particle are 56.21, 114.13, and 34.9 emu/g representing 26%, 71%, and 64% of the saturation moments of the bulk ferromagnetic elements iron, cobalt, and nickel, respectively. All the metal particles were shown to be ferromagnetic with a ratio of remnant to saturation magnetization MR/MS∼0.3 at room temperature (25 °C). In this article the detailed preparation and the properties of these carbon-coated metal particles will be discussed.
Nitrogen in germanium
Authors: Chambouleyron I. , Zanatta A. R.
Published: July 2002

The known properties of nitrogen as an impurity in, and as an alloy element of, the germanium network are reviewed in this article. Amorphous and crystalline germanium–nitrogen alloys are interesting materials with potential applications for protective coatings and window layers for solar conversion devices. They may also act as effective diffusion masks for III-V electronic devices. The existing data are compared with similar properties of other group IV nitrides, in particular with silicon nitride. To a certain extent, the general picture mirrors the one found in Si–N systems, as expected from the similar valence structure of both elemental semiconductors. However, important differences appear in the deposition methods and alloy composition, the optical properties of as grown films, and the electrical behavior of nitrogen-doped amorphous layers. Structural studies are reviewed, including band structure calculations and the energies of nitrogen-related defects, which are compared with experimental data. Many important aspects of the electronic structure of Ge–N alloys are not yet completely understood and deserve a more careful investigation, in particular the structure of defects associated with N inclusion. The N doping of the a-Ge:H network appears to be very effective, the activation energy of the most effectively doped samples becoming around 120 meV. This is not the case with N-doped a-Si:H, the reasons for the difference remaining an open question. The lack of data on stoichiometric β-Ge3N4 prevents any reasonable assessment on the possible uses of the alloy in electronic and ceramic applications.
Colossal magnetoresistance in Ce doped manganese oxides
Authors: Gebhardt J. R. , Roy S. , Ali N.
Published: July 2002

In this study we investigate the effects of Ce doping in R1−xAxMnO3 (R=La, Ce, and A=Sr, Ce) on the magnetic and transport properties of this system. For La1−xCexMnO3 (LCMO), an increase in Ce concentration is accompanied by an increase in TC from 225 to 236 K, as well as an increase in the electrical resistivity. An extremely high resistivity is observed in the new system Ce1−xSrxMnO3 (CSMO) which becomes insulating below its Curie temperature of 43 K. A maximum magnetoresistance (MR) ratio of 40% for CSMO and 53% for LCMO is observed. A larger change in resistivity is seen to correspond to an increase in the Ce concentration, however this is offset by an overall resistivity increase which keeps the MR ratio low. The high resistivity may be due to unreacted oxides in the samples. If true, the amount of impurity appears to be proportional to the Ce doping. If this impurity level can be reduced, a significant colossal magnetoresistance effect could be exhibited by these systems.
Characterization and aging response of the d31 piezoelectric coefficient of lead zirconate titanate thin films
Authors: Shepard Joseph F. , Chu Fan , Kanno Isaku , Trolier-McKinstry Susan
Published: July 2002

The wafer flexure technique was used to characterize the d31 coefficient of a number of sol–gel and radio frequency (rf) sputtered lead zirconate titanate (PZT) thin films with thicknesses between 0.6 and 3 μm. Typical d31 values for well-poled 52/48 sol–gel films were found to be between −50 and −60 pC/N. The rf sputtered films possessed large as-deposited polarizations which produced d31 coefficients on the order of −70 pC/N in some unpoled films. The subsequent poling of the material, in a direction parallel to the preferred direction increased the d31 coefficient to values of about −85 pC/N. The aging behavior of the d31 coefficient was also investigated. For sol–gel films the aging rate was found to be independent of poling direction and to range from 4% per decade for a 2.5 μm film to 8% per decade for a 0.6 μm film. In contrast, the aging rate of sputtered films was strongly dependent on poling direction, with maximum and minimum rates of 26% and 2% per decade recorded. These aging rates are very high in light of the limited twin wall motion in PZT films, and are believed to result from the depolarizing effects of internal electric fields in the rf sputtered films and interfacial defects in the sol–gel films.
Nanoscale friction and wear mechanisms at the interface between a boron carbide film and an atomic force microscope tip
Authors: Prioli R. , Reigada D. C. , Freire F. L.
Published: July 2002

The investigation of the modifications of the nanoscale tribological properties of boron carbide films induced by the energy dissipation at the interface between the atomic force microscope tip and the film surface is presented. It is shown that the microscope tip induces a modification at the surface that results in a decrease on the friction forces between the tip and the film surface. The influences of the friction coefficient, the scanning speed, and the applied normal force on the film wear are investigated. Using a microscopic model, the dissipated energy at the tip–surface interface during scanning was estimated. The influence of the dissipated energy on the nanoscale wear is presented and a strong correlation between friction and wear, in nanoscale, is shown.
Strain relaxation in In0.2Ga0.8As/GaAs quantum-well structures by x-ray diffraction and photoluminescence
Authors: Chen J. F. , Wang P. Y. , Wang J. S. , Chen N. C. , Guo X. J. , Chen Y. F.
Published: July 2002

The onset of strain relaxation in In0.2Ga0.8As/GaAs quantum-well structures is investigated. X-ray diffraction shows that when the InGaAs thickness increases beyond its critical thickness, another peak on the right shoulder of the GaAs peak appears, indicating that the top GaAs layer is being compressed in the growth direction by the relaxation of the InGaAs layer. Energy shifts of 44 and 49 meV are observed, respectively, from the strains of the InGaAs and GaAs top layers when increasing the InGaAs thickness from 300 and 1000 Å. These energy shifts are in agreement with theory calculated based on the relaxation process observed in x-ray diffraction, providing evidence that the relaxation occurs from near the bottom InGaAs/GaAs interface while the top interface still remains strained. This result is further corroborated by the images of cross-sectional transmission electron micrographs which show that most of the misfit dislocations are confined near the bottom interface.
Modeling a metal–vapor buffer-gas hollow cathode discharge
Authors: Lichtenberg A. J. , Lieberman M. A.
Published: July 2002

The operation of a metal–vapor buffer-gas hollow cathode discharge is re-examined using modeling techniques that have been developed for electronegative plasma discharges. It is shown that a previously developed global model can be extended to give spatially resolved densities as well as more accurate values for the plasma parameters. A numerical calculation for a neon–copper (Ne–Cu) discharge is used to illustrate the results of the modeling technique. The density profiles of the neutral and ion components, and their variation with the buffer gas pressure, are similar to those found by numerical solution of the complete equations.
Charge distribution and conduction characteristics of 2-vinylpyridine-grafted polyethylenes
Authors: Suh Kwang S. , Kim Jong Eun , Oh Woo Jung , Yoon Ho Gyu , Takada Tatsuo
Published: July 2002

This article describes electrical properties such as space charge distribution and electrical conduction characteristics of 2-vinylpyridine (VP) grafted low density polyethylenes (PE). The VP incorporated onto PE chains changes electrical properties of PE. For example, the homocharge found in virgin PE decreases with the increase of graft ratio. As a result of the nitrogen acting as an electron donor, no space charge develops at high graft ratios. It has been found that VP reduces the current density in PE. Grafted PE with VP content of 0.754 wt % has also been found to obey a space charge limited conduction mechanism: effective charge mobility at 70 °C was calculated to be 4.6×10−16 m2/V s: an activation energy for the conduction was estimated to be 0.81 eV at 70 °C.
Magnetic characterization of λ-MnO2 and Li2Mn2O4 prepared by electrochemical cycling of LiMn2O4
Authors: Jang Young-Il , Huang Biying , Chou F. C. , Sadoway Donald R. , Chiang Yet-Ming
Published: July 2002

Magnetic characterization has been performed on the cubic phases λ-Li0.07Mn2O4, Li0.98Mn2O4, and the tetragonal phase Li1.82Mn2O4, prepared by electrochemical cycling of a single starting LiMn2O4 spinel. The magnitude of the negative Weiss constant increases with x in LixMn2O4, indicating that antiferromagnetic interactions increase in strength as the Mn4+ concentration decreases. Spin-glass behavior is observed in all three materials, as previously seen in nearly stoichiometric LiMn2O4 [Jang et al., Appl. Phys. Lett. 74, 2504 (1999)]. A short-range antiferromagnetic correlation is observed in Li2Mn2O4 at temperatures below the paramagnetic temperature regime, which is ascribed to collective Jahn–Teller distortion causing a two-dimensional nearest-Mn-neighbors interaction.
Study of concentration-dependent Be diffusion in GaInP layers grown by gas source molecular beam epitaxy
Authors: Li Wei , Likonen Jari , Haapamaa Jouko , Pessa Markus
Published: July 2002

Concentration-dependent Be diffusion in GaInP layers grown by gas source molecular beam epitaxy has been studied using secondary ion mass spectrometry. At a growth temperature of 500 °C, apparent Be diffusion occurs at a doping level over 4×1019 cm−3. At lower temperature, the Be profile exhibits a significantly reduced diffusion. In contrast to Zn diffusion in metalorganic vapor phase epitaxy, no enhancement of Be redistribution in GaInP by adjacent n-type layers occurs. These results are explained based on the interstitial–substitutional diffusion model. A p+-n+ GaInP tunnel diode with a high reverse-biased conductance of 15 mA/cm2 at 1.7 mV has been achieved.
Comment on “Electron microprobe quantification: A new model based on electrons rather than on mass” [J. Appl. Phys. 86, 2790 (1999)]
Authors: Reed S. J. B.
Published: July 2002

The “electron fraction” model for electron microprobe quantification described by Pingitore et al. [J. Appl. Phys. 86, 2790 (1999)] does not, as claimed, rest upon more rigorous physical foundations than the models used in this field for several decades, but actually is less rigorous. Comparisons with the “first approximation” of Castaing [R. Castaing, Ph.D. thesis, University of Paris (1951)] are irrelevant since this is never used in practice. Also, the observation that characteristic x-ray intensities are constant for different isotopes of the same element is not, as suggested, evidence that the physical basis of the orthodox approach is unsound.
Silicon wafer surface passivation by low-temperature oxidation prior to ultraviolet radiation for photoconductive decay measurement
Authors: Lee W. P. , Khong Y. L.
Published: July 2002

Conventionally, wafers are passivated by thermal oxidation at temperatures exceeding 900 °C for about an hour prior to effective minority carrier recombination lifetime measurement. This article discusses a method where a reduced duration and low-temperature (650 °C) oxidation followed by ultraviolet radiation was found to be efficacious in passivating silicon wafers for photoconductive decay measurements. The effective minority carrier recombination lifetimes of wafers deliberately contaminated with iron, measured by the conventional and the present methods were well correlated.
High field transport in strained Si/GeSi double heterostructure: A Fokker–Planck approach
Authors: Comas F. , Studart Nelson
Published: July 2002

We report calculations of high electric field transport for the case of a strained Si/GeSi double heterostructure (DHS) considering transport along the Si channel and by applying the analytical Fokker-Planck approach (FPA), where the process is modeled as drift diffusion in energy space. We limit ourselves to electronic transport in the conduction band of the strained Si, where an energy shift between the otherwise degenerate six energy valleys characterizes the band alignment in the DHS. Inter-valley phonon scatterings are considered while intra-valley acoustic phonon scattering is ignored, leading to results valid for high enough temperatures. Our results are compared to previous theoretical work where Monte Carlo simulations were applied. A reasonable agreement between the two approaches is obtained in the high electric field regime.
Asymmetric doping effects on electronic properties of coupled quantum wells in an in-plane magnetic field
Authors: Aceituno P. , Hernández-Cabrera A. , Ramos A. , Vasko F. T.
Published: July 2002

The purpose of this work is to study the influence of asymmetric doping positions on the electronic properties of tunnel-coupled double quantum wells (DQWs) in an in-plane magnetic field. The doping asymmetry introduces peculiarities in the gap due to the anticrossing of the two branches of the electronic dispersion relations. The built-in potential caused by the doping dramatically affects the self-consistent Fermi energy renormalization and strongly opposes an applied bias. As a result, the magnetization shows an abrupt jump when the transition from vertical to horizontal anticrossing occurs. The absolute value of the magnetoinduced voltage of two dimensional electrons in DQWs increases if the Fermi level is localized near such peculiarities and shows additional features in comparison with the symmetric-doping case. We suggest that the transverse voltage induced by a magnetic field and the magnetization are powerful tools for the experimental study of doping characteristics and equilibrium magnetic properties of quantum heterostructures.