bluuebunny/crossref_metadata_2025 huggingface dataset:
Dislocation scattering effects on electron mobility in InAsSb
Authors: Egan R. J. , Chin V. W. L. , Tansley T. L.
Published: July 2002

Heteroepitaxial InAsSb is routinely prepared on InAs, InSb, or GaAs substrates under conditions favorable to dislocation formation, since the binary components are lattice mismatched by about 7.4%, and the ternary are mismatched to GaAs by between 7.2% and 14.5%, depending on composition. We here extend the description of electron scattering in InAsSb to include the effects of grain boundaries and dislocations. Comparison with experiment confirms that dislocation scattering has a strong effect on transport, while alloy scattering limits mobility in ternary samples grown with a minimum of defects.
Buckyball microwave plasmas: Fragmentation and diamond-film growth
Authors: Gruen Dieter M. , Liu Shengzhong , Krauss Alan R. , Pan Xianzheng
Published: July 2002

Microwave discharges (2.45 GHz) have been generated in C60-containing Ar. The gas mixtures were produced by flowing Ar over fullerene-containing soot at a variety of temperatures. Optical spectroscopy shows that the spectrum is dominated by the d 3Πg–a  3Πu Swan bands of C2 and particularly the Δv=−2, −1, 0, +1, and +2 sequences. These results give direct evidence that C2 is in fact one of the products of C60 fragmentation brought about, at least in part, by collisionally induced dissociation. C60 has been used as a precursor in a plasma-enhanced chemical vapor deposition experiment to grow diamond-thin films. The films, grown in an Ar/H2 gas mixture (0.14% carbon content, 100 Torr, 20 sccm Ar, 4 sccm H2, 1500 W, 850 °C substrate temperature) were characterized with scanning electron microscopy, x-ray diffraction, and Raman spectroscopy. The growth rate was found to be ∼0.6 μm/h. Assuming a linear dependence on carbon concentration, a growth rate at least six times higher than commonly observed using methane as a precursor would be predicted at a carbon content of 1% based on C60. Energetic and mechanistic arguments are advanced to rationalize this result based on C2 as the growth species.
Stable levitation of steel rotors using permanent magnets and high-temperature superconductors
Authors: Hull J. R. , Passmore J. L. , Mulcahy T. M. , Rossing T. D.
Published: July 2002

Individual freely spinning magnetic steel rotors were levitated by combining the attractive force between permanent magnets and the rotor with the repulsive force between high-temperature superconductors and the steel. The levitation force and stiffness of several configurations are presented, and the application of this levitation method to high-speed bearings is discussed.
Hot-filament-activated chemical-vapor deposition of carbon: Film growth and filament reactions
Authors: Rye Robert R.
Published: July 2002

Pure glassy carbon films [no x-ray photoelectron spectroscopy (XPS) detectable impurities above the 0.5% level] as thick as 25 000 Å have been grown on nearby silicon substrates (T≳100 °C) as a result of reactions between a hot tungsten filament and cyclopentane. Above ∼2500 °C, cyclopentane-tungsten reactions yield a liquid W/C eutectic which limits filament operation. Below ∼2500 °C, resistance changes of the filament and XPS spectra show such reactions form carbides and graphite. It is shown that the temperature dependence of the carbon deposition rate is similar to the sublimation rate of carbon from graphite and tungsten carbide. Moreover, it is also shown that C1, C2, and C3 (carbon monomers, dimers, and trimers) are evaporated from carbarized tungsten and also from graphite. These results suggest that carbon film growth is a consequence of evaporation of carbon from the carbarized tungsten filament, with steady-state film deposition occurring as a result of a quasisteady state in the formation and decomposition of the carbarized tungsten. Carbarization of the tungsten filament is expected for a wide variety of hydrocarbon gases, but evaporation of C species from this carbarized filament should be independent of the way it is produced. These same processes should occur at the tungsten filament during diamond film deposition using the hot-filament chemical-vapor deposition method.
Interaction effects and magnetic ordering in GMR alloys
Authors: Greaves S. J. , El-Hilo M. , O’Grady K. , Watson M.
Published: July 2002

In this paper AgNiFe alloy films were examined both before and after annealing. Characterization of the samples and examination of interaction effects was carried out by measuring the temperature decay of remanence, initial susceptibility, and magnetoresistance. The temperature decay of remanence reveals that annealing widens the distribution of energy barriers, which is indicative of grain growth. The behavior of the initial susceptibility as a function of temperature is analogous to that found in spin glasses. From these measurements, it is believed that interaction effects in these systems are small.
Ablation plume particle dynamics during excimer laser ablation of polyimide
Authors: Hahn D. W. , Ediger M. N. , Pettit G. H.
Published: July 2002

The dynamics of polyimide ablation plume particles generated during ArF excimer laser (193 nm) ablation were investigated using time-resolved, laser light scattering and Raman spectroscopy. The evolution of plume particle size is determined over a range of 300–1150 μm above the polymer target surface. Raman-scattering measurements of the ablation plume allow in situ identification of the particle composition, which is equivalent to flame generated soot. The total mass of particulate carbon within the plume accounts for approximately 1% of the total carbon mass within the ablated polyimide. Scanning electron microscopy was used to examine the size and morphology of collected plume particles, which were found to correlate remarkably with the light-scattering results. The formation of particulate carbon within the ablation plume is apparently complete within 500 μm above the polymer surface, corresponding to a time scale of 30 μs, and then gives way to carbon oxidation at greater heights. It is concluded that particulate carbon is formed within the resulting ablation plume, with subsequent particle growth and oxidation processes being important in the evolution of the particles.
Observation of a near-static condensation of polarization fluctuations in strontium barium niobate
Authors: Huang Weng-Hsing , Xu Z. , Viehland Dwight , Neurgaonkar R. R.
Published: July 2002

The dielectric response characteristics of SrxBa1−xNb2O6 for 0.50<x<0.75 have been investigated as a function of temperature. Cold-stage transmission electron microscopy studies were performed for comparative purposes. An anomalous relaxation of tan δ was observed in the temperature range of −100 to −50 °C, which was previously unreported. Near this temperature range the dielectric constant was also found to become frequency independent. These results are interpreted in terms of a near-static condensation of polarization fluctuations, reflecting the trailing edge of the relaxation time distribution approaching long time periods. Selected-area electron-diffraction investigations revealed an incommensurate reflection along the 〈110〉 which exhibits diffuse scattering. Near the temperature of the static condensation of polarization fluctuations, the degree of diffuse scattering was found to decrease significantly. In addition, bright-field imaging revealed a significant increase in the size of needlelike nanosized ferroelectric regions in this temperature range. It is proposed that the near-static condensation of polarization fluctuations is driven by small structural changes in the channels of the tungsten bronze structure.
Current-voltage characteristics of strained piezoelectric structures
Authors: Bykhovski A. , Gelmont B. , Shur M. , Khan A.
Published: July 2002

Experimental and theoretical studies are presented of the current-voltage characteristics of symmetrically doped n-type GaN-AlN-GaN semiconductor-insulator-semiconductor (SIS) structures. The asymmetry caused by the strain-induced electric field leads to the depletion layer barrier in addition to the barrier presented by a thin insulating layer of AlN. It is shown that the tunnel current depends on the degree of the elastic strain relaxation which, in turn, is related to the AlN film thickness. This dependence provides quantitative information about the film relaxation. This characterization technique is compared with the capacitance-voltage characterization of the SIS structures. The data indicate that the low bound of the conduction-band offset for the AlN/GaN heterointerface is close to 1 eV.
Near-threshold laser sputtering of gold
Authors: Bennett Ted D. , Grigoropoulos Costas P. , Krajnovich Douglas J.
Published: July 2002

This work characterizes the laser sputtering of gold by 248 nm laser pulses at near-threshold fluences (material removal rates ≤10 Å/pulse) using time-of-flight plume diagnostics, scanning electron microscope analysis of the surface topography, and thermal analysis of the transient near surface conditions. Pulsed laser irradiation leads to development of surface topography characterized by droplet and ridge formations, and to the liberation of micrometer-sized droplets into the plume. The development of surface topography has been identified with a hydrodynamic response to phase change occurring at the surface of the target. Drawing upon a Rayleigh–Taylor instability description of the melt surface, the readily observable ∼5 μm periodicity in topography formation can be theoretically predicted. Additionally, the preferential formation and liberation of ∼1 μm diameter droplets at the target surface is observed. Nevertheless, the majority of sputtered mass flux is not comprised of droplets, but of neutral gold atoms with almost perfect Boltzmann translational energy distribution. The mean translational energy of the gold atoms, however, is much too high to reconcile with a simple thermal vaporization model. The yield, translational energy, and angular characteristics of the plume are strongly influenced by the surface topography. Local variations in the light absorption and heat transfer explain the qualitative trends in the experimental results.
Scanning tunneling microscope with gallium arsenide microtip fabricated by selective epitaxial growth
Authors: Yamaguchi Koichi , Okamoto Kotaro , Yugo Shigemi
Published: July 2002

GaAs microtip for scanning tunneling microscope was fabricated by selective metalorganic chemical vapor deposition. The GaAs tip was constructed by two {111}B growth facets and a cleaved (110) surface. The surface of the GaAs tip was treated by ammonium sulfide solution, and highly oriented pyrolytic graphite surfaces were observed by using the GaAs tip in air. As the result, imagings of the atomic arrangement could be successfully obtained.
Radio-frequency plasmas in CF4: Self-consistent modeling of the plasma physics and chemistry
Authors: Mantzaris Nikolaos V. , Boudouvis Andreas , Gogolides Evangelos
Published: July 2002

A self-consistent, one-dimensional simulator for the physics and chemistry of radio frequency plasmas is developed and applied for CF4. The simulator consists of a fluid model for the discharge, a commercial Boltzmann code for calculations of electron energy distribution function (EEDF), a generalized plasma chemistry code, and an interface among the three models. Chemistry calculations are fed back into the physics model and the procedure is repeated until a self-consistent solution is obtained. The CF4 discharge shows an electronegative behavior with ten times more negative ions than electrons even at low pressures of 100 mTorr. The EEDF high energy tail lies between the Maxwell and Druyvensteyn distribution. The chemistry model predicts densities of 3.5×1012 cm−3 for CF3, 3×1012 cm−3 for CF2, 2.5×1013 cm−3 for F, and 0.7×1012 cm−3 for CF, in agreement with experimental data from a Japanese group. CF and to a lesser extent CF2, are consumed at the surface, and CF, CF2, and F densities and profiles are sensitive to the sticking coefficient and residence time. CF2 and CF are produced mainly from the parent gas CF4 and not its fragments. Finally, the chemistry results are fed back into the physics model and influence the discharge structure, mainly by changing electron densities and the width of the inner core of the positive-negative ion plasma. Thus, the importance of self-consistent plasma calculations is demonstrated and justified.
Improved Si3N4/Si/GaAs metal-insulator-semiconductor interfaces by in situ anneal of the as-deposited Si
Authors: Tao Meng , Botchkarev Andrei E. , Park Daegyu , Reed John , Chey S. Jay , Van Nostrand Joseph E. , Cahill David G. , Morkoç Hadis
Published: July 2002

Si interlayers in GaAs metal-insulator-semiconductor structures are essential for interfaces with device quality. The incompatible growth temperature of Si on GaAs, however, presents a dilemma between the crystallinity of Si and the stoichiometry of GaAs. We circumvented this dilemma by a new approach: a high-temperature in situ anneal following the low-temperature Si deposition. The idea is that the GaAs surface covered with a few monolayers of Si can stand a much higher temperature, and the crystal quality of the Si is resumed during the high-temperature anneal. The surface morphology of the as-deposited and the in situ annealed Si was examined with a scanning tunneling microscope, the results of which confirmed high crystal quality of the Si layer and full coverage of the GaAs surface. With in situ anneal, interface trap densities of high 1010 eV−1 cm−2 were routinely obtained in Si3N4/Si/GaAs metal-insulator-semiconductor capacitors, as determined with conductance measurements.
Hall factor for ionized impurity scattering
Authors: Gelmont B. , Shur M. S.
Published: July 2002

We propose an interpolation formula which describes the dependence of the Hall factor for impurity scattering in semiconductors on the carrier concentration and temperature with sufficient accuracy for practical calculations. This formula applies to semiconductors with an arbitrary degree of degeneracy which makes it especially useful for the interpretation of the measured temperature dependencies of a Hall mobility.
Radiation resistance of compound semiconductor solar cells
Authors: Yamaguchi Masafumi
Published: July 2002

1-MeV electron irradiation effects on molecular beam epitaxy-grown AlGaAs and InGaAs solar cells have been determined and compared with our previous results for radiation damage of InP and GaAs solar cells in order to clarify radiation resistance of compound semiconductor solar cells. Differences of radiation resistance of AlGaAs, GaAs, and InGaAs cells are found to be explained by band-gap energy effects on solar cell degradation. Moreover, 1-MeV electron irradiation results of several solar cells such as InP, InGaP, InGaAsP, GaAs, AlGaAs, InGaAs, Si, Ge, and CuInSe2 have also been analyzed by considering their damage constants, band-gap energies, and optical absorption coefficients. It is found that superior radiation-resistance of CuInSe2 and InP-based solar cells is explained by the higher optical absorption coefficient of CuInSe2 and lower defect introduction rates (damage constants) of InP-based materials compared to other compound semiconductor materials.
The influence of space charge and electric field on the excitation efficiency in thin film electroluminescent devices
Authors: Corlatan D. , Neyts K. A. , De Visschere P.
Published: July 2002

In electroluminescent devices electrons move every half period from the cathodic to the anodic interface of the phosphor layer and excite atoms which can lose their energy by the emission of a photon. It has been found experimentally by others that the efficiency for excitation of the luminescent atoms is largest close to the cathodic interface. By comparing transient measurements with the steady state situation we show that in steady state the efficiency is reduced and the difference between cathodic and anodic efficiency is larger. Both phenomena could be ascribed to creation of positive space charge during the first current pulses.
Optical mixing of CO2-laser radiation in a scanning tunneling microscope
Authors: Sammet C. , Völcker M. , Krieger W. , Walther H.
Published: July 2002

Two infrared laser beams coupled into the tunneling junction of a scanning tunneling microscope lead to the generation of a signal at the difference frequency. In this article it is described that two different frequency mixing mechanisms are responsible for this process. One part of the signal is generated through a mixing process owing to the nonlinearity in the static current-voltage characteristic. Another part has its origin in a nonlinear susceptibility at the surface; it therefore corresponds to frequency mixing in nonlinear optics. It will be shown that the difference-frequency signals generated by the two processes can be separated owing to their different dependence on the tip-sample distance.
High power operation of first and second harmonic gyrotwystrons
Authors: Lawson W. , Latham P. E. , Calame J. P. , Cheng J. , Hogan B. , Nusinovich G. S. , Irwin V. , Granatstein V. L. , Reiser M.
Published: July 2002

We report the first experimental operation of overmoded first and second harmonic gyrotwystron amplifier configurations. Both devices utilize a single cavity which is driven near 9.87 GHz in the TE011 mode, heavily attenuated drift tubes, and long tapered output waveguide sections. A magnetron injection gun produces a 460 kV, 245 A beam with a maximum average perpendicular-to-parallel velocity ratio approximately equal to one. The axial magnetic field profile is sharply tapered in the output section. Peak powers above 21 MW are achieved in 1 μs pulses with an efficiency exceeding 22% and a large signal gain near 24 dB in the first harmonic tube. The second harmonic tube achieves nearly 12 MW of the peak power with an efficiency of 11% and a gain above 21 dB. First harmonic amplifier performance is limited principally by competition from a fundamental mode output waveguide interaction; the second harmonic tube is limited by both travelling wave output modes and by a down-taper oscillation.
Spatial resolved emission spectroscopic study on laser plume of Y1Ba2Cu3Ox at different oxygen pressures
Authors: Fukushima K. , Badaye M. , Morishita T.
Published: July 2002

To investigate the characteristics of the plume generated by the laser ablation of Y1Ba2Cu3Ox, emission spectra of the ejected species were observed at different distances from the target in an oxygen atmosphere. It was found that velocities of ejected species increased with the distance from the target. At 35 mm from the target the emission intensities of Y, Ba, O atoms, and YO molecules drastically increased to an order of magnitude larger beyond several tens mTorr of oxygen pressure. This increase of emission intensity was due to the increment of the plasma density by a shock front formed at ∼35 mm. Above 0.5 Torr of oxygen pressure, molecular YO was produced by the oxidation of Y atoms. The plasma temperature was also elevated by shock heating.
Density-functional studies of the electronic structure of the perovskite oxides: La1−xCaxMnO3
Authors: Satpathy S. , Popović Zoran S. , Vukajlović Filip R.
Published: July 2002

Using density-functional methods, we study the electronic structures of the lanthanum-based ‘‘double-exchange’’ perovskite magnets. Antiferromagnetic insulating solutions are obtained for both the end members, LaMnO3 and CaMnO3, within the local density approximation (LDA), with the Jahn-Teller (JT) distortion of the oxygen octrahedron taken into account. The JT distortion splits off the Mn(3d)eg bands producing an energy gap within the LDA, with the bands derived from the (z2−1) orbital, pointed along the long basal-plane Mn—O bond, occupied and the (x2−y2) bands empty. The on-site Coulomb repulsion and the intra-site exchange terms are found to be, respectively, U≂8–10 eV and J≂0.9 eV, from the ‘‘constrained’’ density-functional theory. The large value of U as compared to the bandwidth indicates that the manganese perovskite oxides are strongly correlated systems.
Real time spectroellipsometry characterization of optical gap profiles in compositionally-graded semiconductor structures: Applications to bandgap engineering in amorphous silicon-carbon alloy solar cells
Authors: Kim Sangbo , Burnham J. S. , Koh Joohyun , Jiao Lihong , Wronski C. R. , Collins R. W.
Published: July 2002

We have applied a real time spectroellipsometry data analysis procedure developed previously [S. Kim and R. W. Collins, Appl. Phys. Lett. 67, 3010 (1995)] to characterize depth profiles in the optical gap for compositionally-graded semiconductor alloy thin films prepared by plasma enhanced chemical vapor deposition. The analysis procedure employs a two-layer (four-medium) optical model consisting of the ambient, a thin surface roughness layer and outer-layer (5–15 Å) whose properties are to be determined, and a pseudo-substrate that contains the past history of the graded-layer deposition. The ellipsometric spectra (2.3–4.0 eV) are analyzed to provide, not only the depth-profile of the optical gap and alloy composition for the graded layer, but also the instantaneous deposition rate and the surface roughness layer thickness versus time or accumulated layer thickness. To apply the previous analysis approach, it was necessary to (i) parameterize the dielectric function of the alloys as a continuous function of composition over the desired alloy range and (ii) express the optical gap as an accurate function of alloy composition. As an example, we have applied the extended analysis to obtain the depth-profile of the optical gap and alloy composition with <15 Å resolution for a hydrogenated amorphous silicon-carbon alloy (a-Si1−xCx:H) film prepared by continuously varying the gas flow ratio z=[CH4]/{[CH4]+[SiH4]}. In order to demonstrate the technological importance of such structures, the graded layer has been incorporated at the p/i interface of widegap a-Si1−xCx:H p-i-n solar cells, and improvements in open-circuit voltage have been observed.
Dependence of InGaP/InGaAlP heterointerface smoothness on substrate misorientation
Authors: Watanabe Minoru , Okajima Masaki
Published: July 2002

InGaP/InGaAlP single quantum wells (SQWs) were fabricated on (100)GaAs substrates misoriented towards the [011] direction at different growth rates. Photoluminescence (PL) measurements were carried out at 4.2 K to investigate dependence of the interface smoothness on substrate misorientation and growth rate for InGaP/InGaAlP SQWs. It has been found that the PL linewidth of the SQWs becomes a minimum value in the 10°–40° substrate misorientation range. This means the smoothest InGaP/InGaAlP heterointerface can be obtained on these misoriented substrates. It has also been found that the heterointerface can be improved with reduced growth rate.
Transient processes in an Ar+I2 dc discharge
Authors: Fernández Palop J. I. , Ballesteros J. , Colomer V. , Hernández M. A.
Published: July 2002

The experimental results obtained in an Ar+I2 dc discharge during a transient process in which the iodine concentration decreases with time are presented. The following quantities have been measured during the process: electron energy distribution function, electron density and temperature, plasma potential and floating potentials. The temporal evolution of such quantities is analyzed. A transition from electronegative to electropositive plasma is observed. The floating potential values measured during the process are compared with those obtained from a theoretical model, obtaining a quite good consistency between both values. The floating potential is shown to be a powerful tool in the analysis of electronegative plasmas.
Ion beam induced recrystallization of amorphous silicon: A molecular dynamics study
Authors: Marqués Luis A. , Caturla María-J. , Díaz de la Rubia Tomás , Gilmer George H.
Published: July 2002

We use molecular dynamics techniques to study the ion beam induced enhancement in the growth rate of microcrystals embedded in an amorphous silicon matrix. The influence of the ion beam on the amorphous-to-crystal transformation was separated into thermal annealing effects and defect production effects. Thermal effects were simulated by heating the sample above the amorphous melting point, and damage induced effects by introducing several low energy recoils in the amorphous matrix directed at the crystalline grain. In both cases, the growth rate of the microcrystals is enhanced several orders of magnitude with respect to the pure thermal process, in agreement with experimental results. The dynamics of the crystallization process and the defect structures generated during the growth were analyzed and will be discussed.
Au/GaAs(100) interface Schottky barrier modification by a silicon nitride intralayer
Authors: Almeida J. , Coluzza C. , dell’Orto T. , Margaritondo G. , Terrasi A. , Ivanco J.
Published: July 2002

We report a modification by thin silicon nitride intralayers of the Au/n-GaAs(100) Schottky barrier height. Thin intralayers were obtained by nitridation of evaporated Si films on decapped GaAs substrates in an argon–nitrogen mixture plasma. The nitridation was performed at a beam energy <40 eV, with 573 K sample temperature. Gold was deposited to study in situ the Schottky barrier formation process with x-ray photoelectron spectroscopy. Internal photoemission spectroscopy and current–voltage measurements were used to evaluate the barrier modification on fully formed interfaces. Such a modification was analyzed in terms of theoretical calculations of the dipole created by the substrate–intralayer bonds.
Phase transformation of SmCo5 due to hydrogenation and nitridation
Authors: Li X. G. , Chiba A. , Takahashi S.
Published: July 2002

Structural changes of the intermetallic compound SmCo5 after hydrogenation and nitrogenation were investigated by x-ray diffraction, transmission electron microscopy, and differential scanning calorimetry. Amorphization was not observed by hydrogenation of SmCo5. On the contrary, an amorphous a-SmCo5N1.2 alloy was synthesized by nitrogenation.
Magnetic field induced properties of manganite perovskites with colossal magnetoresistance (invited)
Authors: Xiao Gang , Gong G. Q. , Canedy C. L. , McNiff E. J. , Gupta A.
Published: July 2002

We present a systematic study of the magnetotransport and magnetic properties of the half-doped La0.5Ca0.5MnO3+δ system. The solid is a metamagnet which undergoes a first-order antiferromagnet (AFM) to ferromagnet (FM) phase transition under a field or by changing temperature. Associated with the AFM–FM transition is an insulator to metal transition. A maximum 109-fold magnetoresistance ratio has been observed at 4.2 K between the least and the most conductive states. At low T (⩽50 K), we have also observed two additional metastable electronic states in the canted AFM state at certain fields. The resistivity of each state differs from one another by at least one order of magnitude. The existence of these multiple states may be related to the unique charge- and spin-ordered state of the half-doped manganite.
SmCo based sputtered films with CaCu5 and TbCu7 structures
Authors: Rani R. , Cadieu F. J. , Qian X. R. , Mendoza W. A. , Shaheen S. A.
Published: July 2002

SmCo based films containing 20 at. % Fe and with Sm varying from 15.5 to 17 at. % have been synthesized to show the transition from ordered CaCu5 to disordered TbCu7 phase. For either phase, the demagnetization energy present during deposition can result in films with the crystallite c axes randomly splayed onto the film plane. It has been possible to synthesize the high anisotropy CaCu5 phase with either a (200) or (110) texture by adjusting the sputtering parameters to correspond to either a relatively high or relatively low surface mobility during the film deposition. It has been possible to synthesize ordered CaCu5 type films with very low values of BR(⊥)/BR(∥)=0.042. Previously, SmCo based single phase TbCu7 structure films have been synthesized with close to the usual high energy product 2–17 compositions of nominally 7 at. % Cu and 2 at. % Zr. The films have been characterized by vibrating-sample magnetometer and superconducting quantum interference device magnetometry, x-ray diffraction, and scanning electron microscope measurements for composition and film morphology. The films have been directly crystallized onto heated substrates so that the crystallite c axes are predominantly randomly aligned in the film plane. The crystal structures observed could be switched from the ordered CaCu5 to the disordered TbCu7 phase for small variations in the Sm concentrations.
Giant Hall effect in granular Fe–SiO2 film
Authors: Zhao B. , Yan X.
Published: July 2002

We have studied the resistivity, magnetoresistivity, and Hall resistivity for a few Fex(SiO2)1−x granular films with Fe volume fraction x near the metal insulator transition threshold xc. Both the ordinary Hall coefficient R0 and the saturated extraordinary Hall resistivity ρxys were found to be enhanced by a factor of 104 when x approaches xc. The largest measurable R0 and ρxys at room temperature are about 10−3 μΩ cm/G and 250 μΩ cm, respectively. By comparing with the Ni-rich NiFe–SiO2 system [A. B. Pakhomov, X. Yan, and B. Zhao, Appl. Phys. Lett. 67, 3497 (1995)], we found that the correlation between giant Hall effect and the resistivity with a −log T-like dependence is independent of the sign of the carrier. The feasibility of using such a system as a magnetic-field sensor is discussed.
Statistical thermodynamic foundation for photovoltaic and photothermal conversion III: Application to hybrid solar converters
Authors: Badescu Viorel , Landsberg Peter T. , De Vos Alexis
Published: July 2002

This article considers different optimized applications of hybrid (i.e., combined) solar converters (which are devices to generate simultaneously electrical power, mechanical power, and, possibly, thermal energy). A theoretical model proposed recently by the authors is used. An improved model to evaluate upper bounds for the efficiency of photovoltaic converters is elaborated. Its predictions are used to discuss the performance of hybrid converters. It follows that band gap materials are appropriate for these combined converters.
Effect of geometrical shape in the cross section of quantum wires on exciton binding energy
Authors: Thilagam A.
Published: July 2002

Quasi-one-dimensional excitons in quantum wires are investigated using an isotropic “hard” Coulomb potential of the type: V(r)=−(1/ar), where a is a specified length parameter of the confined region. Using the virial theorem, this interaction potential gives a constant ratio of 34 for the exciton binding energy to its potential energy, a result that has been recently verified for various configurations of GaAs/AlxGa1−xAs quantum wires. We utilize this simple model to show a semiempirical approach to determine the exciton binding energy in quantum wires with transverse cross sections of widely used geometrical shapes. The significance of channel points and strong curvatures in the cross section of quantum wires is highlighted.
Current transport properties of SiO2 films containing Ge nanocrystals
Authors: Fujii Minoru , Mamezaki Osamu , Hayashi Shinji , Yamamoto Keiichi
Published: July 2002

The electrical transport properties of SiO2 films (⩾3 μm in thickness) containing Ge nanocrystals have been studied. We found that the films exhibit T−1/2 dependence of ln(σ) under relatively low electric fields independent of the volume fraction of Ge in the films, where T and σ are the temperature and the conductivity, respectively. The observed electrical properties could be well explained by the theory developed by Šimánek [Solid State Commun. 40, 1021 (1981)] which considers the tunneling of thermally activated carriers between neighboring nanocrystals.
Monte Carlo calculations of the hydrostatic compression of hexahydro-1,3,5-trinitro-1,3,5-triazine and β-octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine
Authors: Sewell Thomas D.
Published: July 2002

Rigid molecule Monte Carlo simulations are used in a computational study of the isothermal, hydrostatic compression of crystalline hexahydro-1,3,5-trinitro-1,3,5-triazine and β-octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine for pressures in the range 0 GPa ⩽p⩽ 7.5 GPa. The purpose of the investigation is to assess the utility of simple intermolecular potential-energy functions, generally parameterized for ambient conditions of temperature and pressure, for studies of polyatomic molecular crystals under the extremes of pressure that are important in accident scenarios involving high explosives. The calculated results are found to be in good agreement with published x-ray diffraction data.
Electron transport characteristics of GaN for high temperature device modeling
Authors: Albrecht J. D. , Wang R. P. , Ruden P. P. , Farahmand M. , Brennan K. F.
Published: July 2002

Monte Carlo simulations of electron transport based upon an analytical representation of the lowest conduction bands of bulk, wurtzite phase GaN are used to develop a set of transport parameters for devices with electron conduction in GaN. Analytic expressions for spherical, nonparabolic conduction band valleys at the Γ, U, M, and K symmetry points of the Brillouin zone are matched to experimental effective mass data and to a pseudopotential band structure. The low-field electron drift mobility is calculated for temperatures in the range of 300–600 K and for ionized impurity concentrations between 1016 and 1018 cm−3. Compensation effects on the mobility are also examined. Electron drift velocities for fields up to 500 kV/cm are calculated for the above temperature range. To aid GaN device modeling, the drift mobility dependences on ambient temperature, donor concentration, and compensation ratio are expressed in analytic form with parameters determined from the Monte Carlo results. Analytic forms are also given for the peak drift velocity and for the field at which the velocity peak is reached as functions of temperature.
298 K operation of Nb/Nb oxide-based single-electron transistors with reduced size of tunnel junctions by thermal oxidation
Authors: Shirakashi Jun-ichi , Matsumoto Kazuhiko , Miura Naruhisa , Konagai Makoto
Published: July 2002

We present the successful operation of Nb/Nb oxide-based single-electron transistors at room temperature. At first, devices were fabricated by scanning probe microscope based anodic oxidation technique. Then, the effective area of tunnel junctions was further shrunken by thermal oxidation. Ultrasmall tunnel junctions were easily obtained utilizing additional thermal oxidation process, and single-electron charging effects were observed by means of the modulation of Coulomb blockade voltages at room temperature.
Study on growth processes of particles in germane radio frequency discharges using laser light scattering and scanning electron microscopic methods
Authors: Kawasaki Hiroharu , Kida Junichirou , Sakamoto Kazutaka , Fukuzawa Tsuyoshi , Shiratani Masaharu , Watanabe Yukio
Published: July 2002

Growth processes of particles formed in germane (GeH4) rf parallel plate discharges are studied using a laser light scattering and scanning electron microscopic methods. For GeH4(5%)+He, 30 sccm, 80 Pa, and a relatively high power of 40 W(0.51 W/cm2), particles begin to be observed from a very early time of about 0.13 s after the discharge initiation around the plasma/sheath boundary near the powered electrode, where emission intensity of Ge atoms is high. This appearance time of particles is extremely early compared to that (about 0.5 s) in silane (SiH4) rf discharges. The localized existence of particles suggests that short-lifetime radicals being generated at a high rate may contribute to the particle nucleation, while little information about reaction rates for GeHx (x=0−3) radicals is available. After nucleation and subsequent initial growth of particles, they coagulate quickly with one another, which brings about a growth rate considerably high compared to that for SiH4. Some Ge particles become submicron in size at an early time of 0.3 s and fall to the plasma/sheath boundary near the lower grounded electrode. For such a high coagulation rate (growth rate is about 800 nm/s), particles have irregular nonspherical shapes and most of them are agglomerates composed of chains, while they are almost spherical with a roughness of primary particle size (about 10 nm) for a low coagulation rate (growth rate is about 100 nm/s). Fast appearance of particles for GeH4 discharges also brings about rapid decrease in discharge voltage and absolute value of self-bias voltage.
Characterization of photovoltaic cells using poly(phenylenevinylene) doped with perylenediimide electron acceptors
Authors: Angadi Maki A. , Gosztola David , Wasielewski Michael R.
Published: July 2002

The performance of a sandwich junction device based on doped 1,4-bis(2-ethylhexyloxy)phenylenevinylene (BEH-PPV) is reported. The power conversion efficiency is dependent on the doping level, external load, and work function of the cathode. The increase in power conversion efficiency (ηe=0.8%) is due to efficient charge separation resulting from photoinduced electron transfer from the BEH-PPV donor to the perylene dicarboximide acceptor.
Further analysis of space-charge-limited currents in polybenzo[c]thiophene films
Authors: Musa I. , Eccleston W. , Higgins S. J.
Published: July 2002

The temperature dependence of space-charge-limited currents in polybenzo[c]thiophene (PBCT) films in a sandwich structure of aluminum/PBCT/indium–tin–oxide is presented. Free carrier space-charge-limited currents are identified and the free carrier mobility has been estimated to be ∼3×10−5 cm2 V−1 s−1. A trap of concentration ∼1.8×1017 cm−3 and energy ∼0.67 eV from the highest occupied molecular orbital level is estimated. The thickness dependence of the current, at constant voltage, is very close to that expected for a space-charge-limited mechanism. The Tauc band gap determined by optical spectrophotometry was found to be ∼1.34 eV.
Magnetomechanical coupling and elastic moduli of polymer-bonded Terfenol composites
Authors: Hudson J. , Busbridge S. C. , Piercy A. R.
Published: July 2002

The dependence of coupling coefficient (k) and elastic moduli (EH and EB) on particle size and volume fraction of Terfenol powder in polymer-bonded composites has been investigated. Materials were prepared with powder in five size ranges between 106 and 710 μm and in three volume fractions (VF). The moduli show a ΔE effect, which is negative for small bias fields and positive for larger fields. EH is found to be independent of particle size and to vary with VF in agreement with model predictions. The maximum value of k, for each sample, is found to be independent of both particle size and VF and a simple model is presented which predicts this behavior and indicates that the low values of kmax arise mainly from the low modulus of the epoxy binder.
Magnetic structures of the triangular lattice magnets AFe(SO4)2 (A=K, Rb, Cs)
Authors: Serrano-González H. , Bramwell S. T. , Harris K. D. M. , Kariuki B. M. , Nixon L. , Parkin I. P. , Ritter C.
Published: July 2002

In the crystal structures of CsFe(SO4)2, RbFe(SO4)2, and KFe(SO4)2, the magnetic Fe3+ ions form a triangular array in well separated layers. CsFe(SO4)2 and RbFe(SO4)2 may be regarded as realizations of the highly frustrated triangular lattice antiferromagnet, whereas KFe(SO4)2 is a suspected realization of the row model. The latter model is characterized by two couplings J′ and J, and for J′/J>0.5 forms a helical spin structure with an incommensurate repeat distance. The regular triangular lattice magnet may be described by the row model with J′=J, and its “120°” spin structure may be regarded as a special case of this helical structure. We have determined the low temperature (1.3 K) magnetic structures adopted by CsFe(SO4)2, RbFe(SO4)2, and KFe(SO4)2 by powder neutron diffraction. CsFe(SO4)2 and RbFe(SO4)2 adopt the expected 120° helical spin structure of the triangular lattice magnet, but KFe(SO4)2 does not adopt the expected incommensurate helical structure of the row model. Rather, it adopts a sine wave modulated structure. Possible reasons for this behavior are discussed.
Measurements and comparison of low frequency noise in npn and pnp polysilicon emitter bipolar junction transistors
Authors: Jamal Deen M. , Rumyantsev S. , Bashir R. , Taylor R.
Published: July 2002

Low frequency noise characteristics of new high voltage, high performance complementary polysilicon emitter bipolar transistors have been studied. The influence of the base biasing resistance, emitter geometry, and temperature on the noise spectra are discussed. The npn transistors studied exhibited 1/f and shot noise. The pnp transistors, on the other hand, are characterized by significant generation-recombination noise contributions to the total noise. For both types of transistors, the measured output noise is determined primarily by the noise sources in the polysilicon–monosilicon interface. The level of the 1/f noise is proportional to the square of the base current (IB2) for both npn and pnp transistors. The contribution of the 1/f noise in the collector current is also estimated. The magnitude of the 1/f noise normalized to the square of the base current for devices with different emitter areas was found to be inversely proportional to the emitter area, but for the transistors with a large ratio of emitter perimeter to emitter area, the contribution of noise sources located in the emitter perimeter may be significant. For both pnp and npn transistors, 1/f noise was found to be independent of temperature, and for pnp transistors, generation-recombination noise decreases with increasing temperature.
How to grow cubic GaN with low hexagonal phase content on (001) SiC by molecular beam epitaxy
Authors: Daudin B. , Feuillet G. , Hübner J. , Samson Y. , Widmann F. , Philippe A. , Bru-Chevallier C. , Guillot G. , Bustarret E. , Bentoumi G. , Deneuville A.
Published: July 2002

Molecular beam epitaxy (MBE) of cubic GaN on SiC films deposited by chemical vapor deposition on Si has been investigated by reflection high-energy electron diffraction, x-ray diffraction, photoluminescence, and micro-Raman spectroscopy. The wurtzite/zinc-blende ratio, indicative of the material quality, has been found to depend on both the initial substrate roughness and the N/metal ratio impinging on the surface. The results were consistently analyzed by assuming that the MBE growth of cubic GaN is mainly governed by the impinging active N flux, which directly determines the mean-free path of Ga adatoms.
Determination of molecular orientational distribution using surface second-harmonic generation analyzed by five-layer model and maximum-entropy method
Authors: Yoo Jeong-Geun , Hoshi Hajime , Sakai Takahiro , Park Byoungchoo , Ishikawa Ken , Takezoe Hideo , Lee Yoon Sup
Published: July 2002

A novel analytical method to determine molecular orientational distribution functions (ODFs) at interfaces has been developed to apply to polymer film possessing second-harmonic generation (SHG) active side chains with C∞v symmetry. This method treats SHG in five layers, i.e., air, an upper SHG-active layer, an SHG-inactive polymer bulk, a lower SHG-active layer and a substrate (five-layer model). We first obtain tentative ODFs of the side chains at two SHG-active layers by the assumption of two undetermined multipliers in the maximum-entropy method. These ODFs give the order parameters of the side chains and in turn the dielectric constants of the SHG-active layers. Achieving the best fit of the theoretical SHG intensity to the experimental results obtained in four different geometries, we succeeded in obtaining unbiased ODFs together with the dielectric constants of the SHG-active layers.
The correlation of the magnetic properties and the magnetocaloric effect in (Gd1−xErx)NiAl alloys
Authors: Korte B. J. , Pecharsky V. K. , Gschneidner K. A.
Published: July 2002

A study of the magnetic properties of several (Gd1−xErx)NiAl alloys (where x=0, 0.30, 0.40, 0.46, 0.50, 0.55, 0.60, 0.80, and 1.00) was undertaken using both ac and dc magnetic and heat capacity measurements in an attempt to understand the table-like magnetocaloric effect previously observed in (Gd0.54Er0.46)NiAl. Results indicate the presence of both antiferromagnetic and ferromagnetic ordering processes in all alloys containing Gd. For ErNiAl, a metamagnetic transition from an antiferromagnetic ground state was observed. Within each alloy, several magnetic transitions occur over a temperature range from 10 K [in (Gd0.20Er0.80)NiAl] up to 35 K (in GdNiAl), with all but the lowest temperature transition shifting to higher temperatures with increasing Gd content. The change in magnetic entropy (ΔSmag) induced by a change in field is observed to peak around the Néel temperature for ErNiAl while gradually broadening and shifting toward the Curie temperature as the Gd content is increased. For Gd-rich alloys, a significant contribution to ΔSmag is observed at both the low and high temperature transitions, resulting in a rounded, skewed caret-like temperature profile of the magnetocaloric effect. Factors, which are believed to contribute to this effect, include the presence and temperature spacing of multiple zero-field transitions, which most likely result from competing anisotropy and exchange interactions within a frustrated hexagonal spin lattice. This leads to broad peaks in the magnetic heat capacity that span several transition temperatures, providing for a substantial ΔSmag over an extended temperature range. This characteristic is desired for application to magnetic refrigeration, where certain thermodynamic cycles (e.g., Ericsson cycle) require specific temperature profiles of the magnetocaloric effect in refrigerant materials (e.g., a constant change in magnetic entropy as a function of temperature within the region of cooling). In general, the best materials are those which supply the maximum amount of cooling over the widest temperature range.
Nonlinear TE surface waves on an antiferromagnetic crystal
Authors: Youfa Wang , Qi Wang , Jiashan Bao
Published: July 2002

A study of nonlinear magnetodynamic waves in antiferromagnetic materials is presented. Attention is restricted to an exact theory of electromagnetic waves along the single interface between a linear dielectric material and an antiferromagnetic crystal. The nonlinear motion equation for the TE waves is converted to the Bernoulli differential equation and its exact solution is found in a form of inverse function, and the exact dispersion relation is obtained. The necessary condition for the existence of the nonlinear TE surface wave is μxL>0. The dispersion equation and the frequency regime are analyzed. The theoretical results show that the peak position of the magnetic field is not a function of the effective index and is located steadily at the surface of the crystal, and in some cases one guided power corresponds to two different effective refraction indexes showing the bistable property of the waves.
Study of the hot spot of an in-plane gate transistor by scanning Joule expansion microscopy
Authors: Bolte J. , Niebisch F. , Pelzl J. , Stelmaszyk P. , Wieck A. D.
Published: July 2002

The local heat dissipation of an in-plane gate (IPG) transistor was investigated by means of a thermally modulated scanning Joule expansion microscope. The nanostructured sample was prepared by focused ion beam techniques. The temperature induced thermal expansion and the topographic information are measured simultaneously. The spatial resolution of the constructed microscope is below 50 nm. Heat spots of the semiconducting devices are visualized by heating them with modulated drain voltage. The heat spot position and the temperature amplitude of the heat spot on the IPG transistor depend on the gate voltage VG. With an increasing negative magnitude of VG the heat spot displaces in the direction from drain towards source. Simultaneously the temperature amplitude of the maximum of the heat spot and the thermal expansion integrated over the heated area decrease linearly.