bluuebunny/crossref_metadata_2025 huggingface dataset:
Wall-induced effects in gas transport through micromachined channels in silicon
Authors: Lundström I. , Norberg P. , Petersson L.-G.
Published: July 2002

It has recently been shown that micromachined channels can be used to study gas transport at Knudsen-like conditions also close to atmospheric pressures. The channels are etched into a silicon surface and covered by a glass lid through anodic bonding. An unexpected pressure dependence of diffusion lag times through channels around 100 nm deep and a few centimeters long is concentrated on. A simple tentative model, based on the adsorption of gas molecules onto the channel walls, is presented which can explain an observed decrease in lag time with pressure and an influence of one species on the diffusion lag time of another species in a gas mixture. Some observations on chemical reactions on modified channel walls are also discussed.
Ultraviolet excited Cl-radical etching of Si through native oxides
Authors: Sugino Rinji , Nara Yasuo , Horie Hiroshi , Ito Takashi
Published: July 2002

Si etching at over 200 °C by ultraviolet (UV) irradiation-generated Cl radicals was found to occur by diffusion through native oxides present on Si surfaces. The Si etch rate and surface morphologies formed by the Cl radicals depended on the various native oxides formed by different wet chemical treatments before etching. The ‘‘etching through the native oxide film’’ phenomenon was confirmed by scanning electron microscopy observation of caves on the etched Si surfaces when poly-Si deposition formed lids on the existing native oxide layer. The native oxide layer was supported by the thermally grown thick-oxide (SiO2) etching mask and did not collapse during Cl-radical etching.
Calculations of electronic and optical properties of Si/Ge alloys and superlattices: Application to planar waveguides
Authors: Theodorou G. , Vlachos N. D. , Tserbak C.
Published: July 2002

A systematic comparative study between the electronic and optical properties of the strain layer superlattices (Si)10−n/(Ge)n coherently grown on a Si1−n/10Gen/10(001) alloy surface and those of the corresponding bulk alloys Si1−n/10Gen/10 is presented. We find that the superlattices have a smaller fundamental gap than the corresponding alloys; also for energies smaller than 1.5 eV and polarization along the growth plane, the real part of the dielectric function, ε1, for the SLS is larger than that of the corresponding alloy, while for perpendicular polarization, the two dielectric functions practically coincide. The utilization of this property for the construction of planar waveguides is investigated. In particular, the transverse electric modes of a waveguide consisting of a finite thickness SLS (Si)6/(Ge)4, sandwiched between two layers of the alloy Si0.6Ge0.4, are studied. No transverse magnetic modes exist in this structure.
Photoreflectance and photoluminescence of partially intermixed GaAs/AlGaAs double quantum wells
Authors: Gontijo I. , Tang Y. S. , De La Rue R. M. , Sotomayor Torres C. M. , Roberts J. S. , Marsh J. H.
Published: July 2002

Photoreflectance and photoluminescence were used to study GaAs/AlGaAs quantum wells (QWs) as they were partially intermixed using SiO2 capped rapid thermal annealing. As the annealing temperature was increased, the experimental photoreflectance results showed spectral features moving to higher energies and merging to form broad peaks. This is explained by changes in the shapes of the originally square wells, which result in a convergence of their subbands around certain energies. The interpretation of these changes showed that the partially intermixed QWs were well described by an error-function profile.
Low-temperature homoepitaxial growth on nonplanar Si substrates
Authors: Adams D. P. , Yalisove S. M.
Published: July 2002

The kinetics associated with the breakdown of epitaxy at low temperatures are studied for growth onto a number of Si surfaces, including (001), (117), (115), and (113). These surfaces are all initially generated at trench edges on a single patterned substrate. Growth on each of these surfaces at low temperatures is shown to result in a well-defined crystalline-to-amorphous transition. The epitaxial thicknesses hepi have been measured over a range of substrate temperatures below 280 °C, and activation energies characteristic of this transition were determined. In general, the breakdown in epitaxy occurs such that hepi(001)≳hepi(117)≳hepi(115)≳hepi(113). Growth at slightly higher temperatures, Tsubstrate≳300 °C, shows a different microstructure than that at lower temperatures. Epitaxial growth continues for longer times on (113) facets, as compared with (001). These results are discussed in terms of a recently proposed model explaining the breakdown of epitaxy at lower temperatures and an epitaxial temperature for Si.
Transresistance and current gain of high-T
c flux flow transistors
Authors: Miyahara Kazunori , Kubo Shugo , Suzuki Minoru
Published: July 2002

The transresistance and current gain of high-Tc flux flow transistors were investigated phenomenologically and experimentally in relation to the flux flow characteristics of the devices. It was shown that the transresistance was in proportion to the vortex flow velocity, the channel width, and the magnetic coupling coefficient between the channel and the control current line. The devices were fabricated from thin films of coevaporated NdCeCuO, and the control current dependence of their transresistance was measured. It was found that the transresistance peak appeared at a high control current level that corresponded to the saturation of the flow voltage. The peak value of the transresistance was 1.25 Ω. The flow voltage saturation mechanism is discussed in relation to the channel etching of the devices.
Kinetics and quenching mechanisms of photoluminescence in Yb-doped InP
Authors: Lozykowski H. J. , Alshawa A. K. , Brown I.
Published: July 2002

The photoluminescence spectra of Yb-implanted InP samples were studied under pulsed and cw excitations using an Ar+-ion laser (above band-gap excitation) at different temperatures and excitation intensities. The photoluminescence spectrum of the 4f transitions 2F5/2–2F7/2 consists of a sharp peak at 1001 nm and broader peaks in the spectral range between 1002.5 and 1010 nm. Time-resolved spectra were recorded at different temperatures. Using a tunable Ti:sapphire laser (above and below band-gap excitation) the photoluminescence excitation spectra were investigated for different emission lines. Study of the rise and decay time under intrinsic excitation (band to band) show that the rise time is a function of excitation intensity, which reflects an indirect excitation process for Yb3+. The experimental data are explained using a kinetics model of energy transfer from the host lattice to the localized core excited states of rare-earth isoelectronic structured traps. The numerically simulated luminescence rise and decay times show a good general quantitative agreement with experimental data, over a wide range of generation rates. The photoluminescence spectra and decay time were also studied as a function of temperature. A quenching mechanism of ytterbium luminescence involving Yb and Fe ions is proposed. Finally, the electric-field quenching of InP:Yb photoluminescence is investigated.
The effect of buffer layer on the structural and electrical properties of (BaSr)TiO3 thin films deposited on indium tin oxide-coated glass substrate by using a rf magnetron sputtering method
Authors: Kim Tae Song , Kim Chong Hee , Oh Myung Hwan
Published: July 2002

Y2O3 and Si3N4 layers of 50–60 nm thickness were employed as a buffer layer to improve the electrical characteristics between (BaSr)TiO3 thin film and ITO-coated glass substrate. Films were deposited by a rf magnetron sputtering method. In order to investigate the influence of the buffer layer on the structural and electrical properties, XRD, SEM, and SIMS analysis were conducted, and dielectric constant (ε′), dissipation factor (tan δ), voltage-current (V-I), and breakdown fields were measured as a function of (BaSr)TiO3 film thickness. From the results of the SEM and SIMS measurements, it was observed that the buffer layers reduced the influences of the substrate on grown films. Even though the dielectric constant of (BaSr)TiO3 film (440 nm in thickness) was lowered from 238–249 (nonbuffered) to 73–81 (Y2O3 buffered) and 37–43 (Si3N4 buffered) by adopting a buffer layer, the dissipation factor and leakage currents were significantly reduced, which could be explained by the decrease of oxygen diffusion due to the buffer layer. It was also shown that the shift of the breakdown field maximum toward thinner film and the increase of breakdown field occurred by adopting a buffer layer.
Spatially resolved measurement of lattice damage in alpha-particle-irradiated type IIa natural diamond by confocal photoluminescence microscopy
Authors: Ager Joel W. , Han Sung , Prussin Stanley G. , Wagner Ron S. , Pan Lawrence S. , Kania D. R. , Lane Stephen M.
Published: July 2002

Confocal fluorescence microscopy has been used to measure the three-dimensional distribution of the H3 color center produced in type IIa natural diamonds by 5 MeV He+ irradiation at a total fluence of 8×1015 cm−2. The peak of the H3 emission occurs 16 μm below the surface of the irradiated side of the diamond, which is in fair agreement with the peak of the vacancy distribution predicted by a Monte Carlo calculation (transport of ions in matter or trim). The H3 distribution is broader in the direction normal to the surface (10 μm full width at half maximum) than the trim calculation. This is attributed to diffusion of vacancies caused by self-annealing during irradiation.
Formation of shallow p+n junctions by BF+2 implantation into thin polycrystalline Si films
Authors: Lin C. T. , Juang M. H. , Jan S. T. , Chou P. F. , Cheng H. C.
Published: July 2002

Excellent shallow p+n junctions have been formed by implanting BF+2 ions into thin polycrystalline Si films and subsequent annealing. The samples implanted with 5×1015 cm−2 at 50 keV show a leakage of 1 nA/cm2 and a junction depth of about 0.05 μm after a 800 °C annealing. Various implant and annealing cases were examined to determine and characterize their effects on the resultant junctions. High energy implantations (125 and 150 keV) exhibit poor characteristics at all annealing temperatures because the Si substrates are severely damaged. However, the specimens implanted below 100 keV result in excellent diode characteristics for all implantation doses after an 800 °C annealing since the implantation defects are confined in the poly-Si layer. The major factors affecting the junction depth were found to be the implantation energy and annealing temperature, while a minor for the implantation dosage. Furthermore, the effects of the subsequent silicidation on the resultant junction characteristics were also investigated.
Effects of overheating in a single-electron transistor
Authors: Korotkov A. N. , Samuelsen M. R. , Vasenko S. A.
Published: July 2002

Heating of a single-electron transistor (SET) caused by the current flowing through it is considered. The current and the temperature increase should be calculated self-consistently taking into account various paths of the heat drain. Even if there is no heat drain from the central electrode of the SET due to transfer of phonons, the temperature of this electrode remains finite because electron tunneling decreases the temperature difference between the central and outer electrodes. Overheating effects can cause hysteresis in the I-V curve of the SET in the vicinity of the Coulomb blockade threshold.
Low-frequency resistance fluctuation measurements on conducting polymer thin-film resistors
Authors: Bruschi P. , Cacialli F. , Nannini A. , Neri B.
Published: July 2002

Low-frequency resistance fluctuations were measured on polypyrrole thin-film resistors. The samples were obtained by chemical-vapor deposition onto copper chloride patterned precursors. The measurements were devoted to characterize the noise spectral density of constant-current-biased samples. The dependence of the noise magnitude on the applied dc voltage was studied revealing that the noise cannot be ascribed only to equilibrium resistance fluctuations. Application of the Hooge formula provided an estimate of carrier density in the material.
Plasma assisted two stage selenization process for the preparation of selenide semiconductor thin films using elemental selenium vapor
Authors: Lakshmikumar S. T. , Rastogi A. C.
Published: July 2002

A plasma assisted two stage selenization process using elemental selenium vapor is demonstrated for the preparation of selenide semiconductor thin films. Vacuum evaporated copper films are selenized using the vapor of elemental selenium which has been ionized by a radio frequency plasma. This enhances the reaction kinetics by modifying the availability and ionization state of Sex species present in the gas phase. In contrast to the H2Se selenization, the availability of the Sex species in the gas phase is not dependent on the kinetics of its reaction with the precursor. Formation of highly crystalline CuSe films has been observed at low temperatures (250 °C) within 2–5 min. CuSe films show large (≊2–5 μm) rodlike crystallites, with a highly oriented microstructure in plasma assisted growth as different from the small (≤1 μm) coalescing grains in the polycrystalline films formed by the conventional selenization scheme. X-ray diffraction shows a threefold enhancement in the intensity of the (006) reflection of the hexagonal CuSe phase indicating the orientation of the c axis of the crystallites, normal to the plane of the film. The observed hexagonal CuSe growth phase is in agreement with the thermodynamics of copper selenium vapor reaction.
Satellite peak generation in the electroluminescence spectrum of ZnS:Sm grown by metalorganic chemical vapor deposition with Cl codoping
Authors: Kato Akira , Katayama Masayuki , Mizutani Atsushi , Hattori Yutaka , Ito Nobuei , Hattori Tadashi
Published: July 2002

ZnS:Sm, ZnS:Sm,Cl, and ZnS:Cl thin films were grown by metalorganic chemical vapor deposition using the thd-chelate (thd=2,2,6,6-tetramethyl-3,5-heptanedione) of Sm and hydrogen chloride. The electroluminescence spectrum of the ZnS:Sm,Cl showed three satellite peaks appearing in each of the 7 nm ranges around the three dominant peaks that the ZnS:Sm has originally. The ZnS:Sm,Cl also showed photoluminescence attributed to the self-activated centers, but its intensity was less than 10−2 of that of the ZnS:Cl. The effect of the Cl codoping on Sm3+ luminescent properties is discussed on the basis of the Sm-VZn (zinc vacancy)-Cl complex formation.
In situ growth of layered, spinel, and rock-salt LiCoO2 by laser ablation deposition
Authors: Antaya M. , Cearns K. , Preston J. S. , Reimers J. N. , Dahn J. R.
Published: July 2002

A study is reported of the in situ growth of three distinct phases of LiCoO2 by laser ablation deposition on heated substrates in an oxygen background. Films were characterized by x-ray diffraction from which crystal structure, crystal orientation, lattice constants, and phase information were obtained. Electron microscopy was used to investigate crystal grain size and overall film morphology. For deposition under 2000 mTorr O2, substrate temperatures of 22–250 °C resulted in a rock-salt structure, 300–450 °C produced a modified spinel structure (low-temperature LiCoO2), and 680 °C gave a hexagonal layered structure (high-temperature LiCoO2). Growth at 500 and 550 °C produced mixed-phase spinel and layered LiCoO2; however, at 550 °C, reducing the O2 pressure to 100 mTorr resulted in single-phase high-temperature LiCoO2. In the case of the rock-salt phase, modified film stoichiometries of composition Li0.5Co1.5O2 and Li0.75Co1.25O2 were also produced by using ablation targets of the same composition and growing at 150 °C under 200 mTorr O2. Average crystal grain size for all films was found to increase with growth temperature. Ordering of the cations in rock-salt films by heating was unsuccessful.
Quasiparticle trapping and the density of states in superconducting proximity structures
Authors: Warburton P. A. , Blamire M. G.
Published: July 2002

An experimental study of quasiparticle trapping in epitaxial and polycrystalline Ta films on epitaxial Nb is presented using three-terminal double tunnel junction devices. It is shown that polycrystalline Ta is a more effective trap than epitaxial Ta. The experimentally measured tunneling density of states is used to calculate the inelastic quasiparticle scattering rates in the two types of Ta using the standard theory of Kaplan et al. [Phys. Rev. B 14, 4854 (1976)]. The agreement of this calculation with the experimental results shows that the tunneling density of states may be used to determine scattering rates in proximitized superconducting films whose thickness is greater than the coherence length. This result is important since no existing theory satisfactorily describes the density of states in such proximity structures, which are currently being developed for use in high-resolution particle spectrometers.
Gas dynamics resulting from laser vaporization of metals in one dimension. II
Authors: Bellantone R. , Hahn Y.
Published: July 2002

A mathematical model for the transient gas dynamics in one dimension resulting from laser vaporization of metals is presented. The model treats the case of inviscid isentropic (the material derivative of the specific entropy vanishes) flow. A general solution is obtained for the flow velocity and thermodynamic parameters as functions of time and position. The flow field is determined in terms of the time dependence of the velocity νK, which is the velocity of the vapor exiting a transition zone between the melt surface and the bulk vapor flow (known as the Knudsen layer). It was found that the stagnation parameters c0, p0, ρ0, and T0 (the sound speed, pressure, density, and temperature for the field when the flow velocity vanishes) were functions only of the specific entropy s and thus their material derivatives also vanish. The general solution is applied to a specific case in which νK is taken to be linear in time. The isentropic results are compared with the homentropic results using a quasisteady approximation. It was found that for reasonably small variations in νK, comparison of isentropic and homentropic results show them to be consistent. Thermodynamics relations derived and used to test the results are not satisfied by the quasisteady homentropic calculations but are satisfied by the isentropic results. It is concluded that the specific entropy varies with νK and the isentropic model extends the range of applicability of the flow model to all subsonic flow situations.
In situ Raman spectroscopy during diamond growth in a microwave plasma reactor
Authors: Fayette L. , Marcus B. , Mermoux M. , Rosman N. , Abello L. , Lucazeau G.
Published: July 2002

An experimental set-up designed for in situ Raman analysis during the growth of diamond films in a microwave plasma reactor is described. A gated multichannel detection synchronized with a pulsed YAG laser is used to discriminate the Raman signals from the plasma emission. The in situ detection of a diamond film during its growth on a single crystal of alumina substrate is presented. The detectivity of the method has been estimated to be about a few tens of μg/cm2 for an acquisition time of 800 s. Peak shifts are interpreted in terms of temperature and stress dependences. It is shown that the diamond in the first stages of deposition is free of stress, then when grains come into contact compressive stresses are observed, when the film thickness reaches about 1 μm stresses are relaxed.
Crystallographic orientation dependence of impurity incorporation into III-V compound semiconductors grown by metalorganic vapor phase epitaxy
Authors: Kondo Makoto , Anayama Chikashi , Okada Naoko , Sekiguchi Hiroshi , Domen Kay , Tanahashi Toshiyuki
Published: July 2002

This article presents a comprehensive study of the dependence of impurity incorporation on the crystallographic orientation during metalorganic vapor phase epitaxy of III-V compound semiconductors. We performed doping experiments for group-II impurities (Zn and Mg), group-VI impurities (Se and O), and a group-IV impurity (Si form SiH4 and Si2H6). The host materials were GaAs, Ga0.5In0.5P, and (Al0.7Ga0.3)0.5In0.5P grown on GaAs substrates. We examined the doping efficiency on the surfaces lying between {100} and {111}A/B. Even though we grew epitaxial layers in a mass-transport-limited regime, the doping efficiency significantly depended on the orientation, indicating that the surface kinetics plays an important role in impurity incorporation. Comparing our results with other reports, we found that acceptor impurities residing on the group-III sublattice and donor impurities residing on the group-V sublattice, respectively, have their own distinctive orientation dependence. Si donors exhibit orientation dependences which are either negligible or are similar to group-VI donors, depending on the growth conditions. We constructed a model for the orientation dependences, considering atomic bonding geometries between impurity adsorbates and adsorption sites.
Effect of spacer layers on capacitance of resonant tunneling diodes
Authors: Wei T. , Stapleton S.
Published: July 2002

The capacitances of double barrier resonant tunneling diodes have been obtained by calculating the charge distributions in the devices self-consistently. Self-consistent calculations were performed based on the solutions of the time-independent Schrödinger equation and the Poisson equation. The capacitance was calculated by considering the displacement current and electron density distribution in the device in detail. GaAs/Al0.3Ga0.7As diodes with different undoped spacer layers but identical quantum well structure have been studied and compared. The capacitance was found to decrease generally with increasing spacer layer. The experimental peak in the capacitance-voltage (C-V) measurements has been observed in the calculations. The result shows that a longer cathode spacer layer can result in a higher peak in the C-V characteristic, while the anode spacer layer has relatively small effect on the peak. The optimum choice for spacer layers for higher cutoff frequency are discussed.
Thin film growth of layered cobalt-oxide Bi2Sr3Co2O9+δ nearly isomorphic to Bi2Sr2CaCu2O8+δ superconductors
Authors: Tsukada I. , Terasaki I. , Hoshi T. , Yura F. , Uchinokura K.
Published: July 2002

Thin films of Bi2Sr3Co2O9+δ, which is nearly isomorphic to Bi2Sr2CaCu2O8+δ, are grown in situ by molecular beam epitaxy with pure ozone. For producing a molecular beam of Co by a Knudsen cell, we used MgO as a crucible material. The c-axis oriented films are successfully grown on Nd:YAlO3 (001) substrates, which are untwinned with their structural modulation along Nd:YAlO3[100]. With cooling the films from room temperature, it was found that they show a metallic behavior first (dρ/dT≳0), and then a semiconducting one at lower temperatures (dρ/dT<0). One of the films, which shows the lowest resistivity, exhibits dρ/dT≳0 down to 86 K.
Scaling behavior of virgin magnetization curve of DyBa2Cu3Oy crystal prepared by melt-textured growth method
Authors: Wei C. D. , Liu Z. X. , Gan Z. Z. , Ren H. T. , Xiao L. , He Q.
Published: July 2002

The magnetization curves of a DyBa2Cu3Oy crystal prepared by the melt-textured growth method were measured from 10 to 85 K with the magnetic field parallel to the c axis. The scaling behavior of the virgin magnetization curves is observed and explained in terms of an extended Bean model.
The maximum overall coefficient of performance of a solar-driven heat pump system
Authors: Yan Zijun , Chen Jincan
Published: July 2002

The optimal performance of the solar-driven heat pump system consisting of a solar collector and a three-heat-source heat pump is studied, based on the linear heat-loss model of solar collectors and the general optimum relation of endoreversible three-heat-source heat pumps. The overall coefficient of performance of the system is adopted to be an objective function. The optimum operating temperature of the solar collector and the maximum overall coefficient of performance and maximum rate of heat pumping of the system are found. The results obtained here have more realistic meaning than those of classical thermodynamics for the optimal design and operation of practical solar-driven heat pump systems. Moreover, it is proven that it is not economic to use a solar-driven heat pump under certain circumstances. Finally, it is expounded that for a solar-driven heat pump system, the product of the efficiency of the solar collector and the coefficient of performance of the three-heat-source heat pump at maximum rate of heat pumping may not be used to calculate the optimal operating temperature of the solar collector and determine the optimal performance of the system.
Photoluminescence measurements of complex defects in Si-doped Al0.3Ga0.7As
Authors: de Paula Ana M. , Medeiros-Ribeiro G. , de Oliveira A. G.
Published: July 2002

The optical properties of a Si-doped Al0.3Ga0.7As alloy are studied as a function of the Si dopant concentration by means of photoluminescence measurements. The photoluminescence spectra show peaks due to electron Si acceptors and Si-related complex-defects transitions, which we tentatively attribute to Si acceptor coupled to an As vacancy (SiAs-VAs) and Si donor coupled to a Ga(Al) vacancy (SiIII-VIII). We show that the importance of each of these defects to the alloy optical properties is strongly dependent on the growing parameters. Spectrum for a planar-doped sample also showing peaks related to Si complex defects is presented.
Hot-electron relaxation dynamics in quantum wires
Authors: Gaška R. , Mickevičius R. , Mitin V. , Stroscio Michael A. , Iafrate Gerald J. , Grubin H. L.
Published: July 2002

Monte Carlo simulations of hot nonequilibrium electron relaxation in rectangular GaAs quantum wires of different cross sections are carried out. The simulations demonstrate that the initial stage of hot-electron cooling dynamics is determined by cascade emission of optical phonons and exhibits strong dependence on the excitation energy. The second (slow) relaxation stage is controlled by strongly inelastic electron interactions with acoustic phonons as well as by nonequilibrium (hot) optical phonons. The relaxation times obtained in our simulations are in good agreement with the results of recent luminescence experiments. At low electron concentrations where hot phonon effects are negligible the cascade emission of optical phonons may lead to the overcooling of the electron system to temperature below the lattice temperature. These electrons then slowly (during tens of picoseconds) relax to equilibrium due to the interaction with acoustic phonons. At certain excitation energies strong intersubband electron scattering by optical phonons leads to electron redistribution among subbands and intersubband population inversions. If the electron concentration exceeds 105 cm−1, hot phonon effects come into play. In contrast to bulk materials and quantum wells, hot phonon effects in quantum wires exhibit strong dependence on the initial broadening of the energy distribution of the electrons. The very initial electron gas relaxation stage in quantum wires is faster in the presence of hot phonons, while for t≳0.5 ps the hot phonon thermalization time defines the characteristic electron cooling time.
The transition from dilute aluminum δ structures to an AlAs monolayer in GaAs and a comparison with Si δ doping
Authors: Ashwin M. J. , Fahy M. R. , Hart L. , Newman R. C. , Wagner J.
Published: July 2002

Superlattices of Al δ layers embedded in GaAs have been grown by molecular beam epitaxy at 400 °C on (001) GaAs. Infrared absorption measurements revealed a shift of the Al localized vibrational mode (LVM) from 361.4 to 358.3 cm−1 as the doping was increased from 0.25 monolayers (ML) to 1.4 ML per δ plane, while Raman scattering demonstrated a clear transition from the TO-like LVM to a longitudinal LO-like mode. X-ray measurements showed that the Al atoms were confined to a thickness of 6 Å for the highest areal concentrations. The LVM behavior is compared with that of Si δ layers where the absorption tends to zero as the areal concentration increases up to 0.5 ML. It is concluded that SiGa atoms move off their lattice sites and/or change their charge states as the coverage approaches 0.5 ML.
Growth of device quality amorphous SiGe:H alloys with high deposition rate under helium dilution
Authors: Middya A. R. , Hazra Sukti , Ray Swati
Published: July 2002

Highly photoconductive amorphous silicon-germanium alloy (a-SiGe:H) films have been developed by plasma-enhanced chemical-vapor deposition using helium dilution (HeD) of the process gases (silane and germane). On comparison with high-quality a-SiGe:H alloys prepared under hydrogen dilution it has been observed that HeD films have higher deposition rates as well as higher mobility lifetime products ημτ throughout the alloy range; however, midgap defect densities and Urbach energy values of the two types of materials are nearly the same. Improvement in ημτ values of HeD films are found to be consistent with the reduction of microstructural defects in the films. Device potential of the helium-diluted a-SiGe:H films has also been investigated.
Influence of nitrogen content on coercivity in remanence-enhanced mechanically alloyed Sm-Fe-N
Authors: O’Donnell K. , Kuhrt C. , Coey J. M. D.
Published: July 2002

In nanostructured Sm2Fe17Nx+α-Fe two-phase permanent magnets, exchange interactions between hard and soft magnetic phases result in remanence enhancement. With a starting composition of Sm7Fe93, mechanical alloying, annealing at 625 °C for 10 min and subsequent nitriding at 330 °C for 45 h results in hard magnetic powder with nanocrystallites of Sm2Fe17N3 and α-Fe some 20 nm in size. There is a coercivity of μ0Hc=0.4 T and a remanence Jr=1.2 T, after saturating in a field of 7.5 T. The deduced value of the remanence depends strongly on the choice of demagnetizing factor.
Penetration of electromagnetic fields into superconductors with gradual resistive transition
Authors: Mayergoyz I. D.
Published: July 2002

The penetration of electromagnetic fields into superconductors is studied for the case of gradual resistive transition described by the ‘‘power law.’’ Simple self-similar solutions are first found, which are then used to further generalize the Bean (critical state) model of superconducting hysteresis.
Study of the frequency and low-field dependence of ac susceptibility in YBaCuO
Authors: Zazo M. , Torres L. , Iñiguez J. , de Francisco C. , Muñoz J. M.
Published: July 2002

The frequency dependence of the ac susceptibility in polycrystalline samples of YBaCuO has been studied at the ac field range 0.06–1.2 Oe. The curve of the real part of the susceptibility χ′ exhibits two drops, the first one close to Tc and the other well below Tc which could correspond to screening behavior of the grains and grain boundaries, respectively. The imaginary part of the susceptibility shows only a peak below Tc which is related to hysteresis losses at the grain boundaries. The peak corresponding to intragrains hysteresis losses is very small and appears masked by the losses of the intergrains. In the range of 1–20 kHz, the onset temperature of χ′ showed no frequency dependence. However, the width of the transition decreases lightly and there is a small shift in the peak of χ″ to higher temperature as the frequency increases. This behavior could be explained in terms of the thermally activated Anderson flux creep. The shift depends on the amplitude of the measuring field. The activation energy for flux creep ranges from 6.4 eV at 0.02 Oe to 3.23 eV at 1.2 Oe in the zero-field-cooled measurements and from 3.41 eV at 0.02 Oe to 1.41 eV at 1.2 Oe in the field-cooled measurements.
Tunable bistability from magnetostriction (abstract)
Authors: Arrott A. S. , Lee J.-G.
Published: July 2002

When a current passes along the axis of a [100] iron whisker in the presence of a field along the same axis, the magnetization in a central domain points along the field direction while the mangetization in four domains surrounding the central domain circulates about the axis, see Fig. 1. The size of the central domain grows with applied field, which overcomes the effect of the field from the current and the magnetostriction that favor the collapse of the central domain to an irreducible core. Depending on the current, the growth of the central domain can be a continuous or discontinuous process. At certain fields the energy is a double-well function of the size of the central domain, which determines the magnetization. The size of the energy barrier can be made arbitrarily small by suitable choice of the current. It is the ability to adjust the size of the barrier in such a clearly defined system that makes this an attractive experimental approach to understanding the role of fluctuations in magnetization processes. This structure has been verified using measurements of ac susceptibility.
Magnetic properties of Sm2(Fe1−xGax)17 (x=0−0.5) compounds and their nitrides
Authors: Li W.-Z. , Tang N. , Wang J.-L. , Yang Fuming , Zeng Y. W. , Zhu J. J. , de Boer F. R.
Published: July 2002

Magnetic properties of Sm2(Fe1−xGax)17 compounds and their nitrides have been studied. Substitution of Ga for Fe leads to an increase in lattice constants. Introduction of nitrogen results in a further increase in lattice constants. Substitution of Ga for Fe causes a dramatic change of the Curie temperature of the Sm2(Fe1−xGax)17 compounds. When x=0.2 the Curie temperature is enhanced by about 200 K. X-ray-diffraction patterns of aligned samples of Sm2(Fe1−xGax)17 compounds show that alloys with x=0.15, 0.20, and 0.25 exhibit uniaxial anisotropy at room temperature. The introduction of nitrogen made the samples with x≤0.4 exhibit uniaxial anisotropy at room temperature. The Curie temperature of the nitrides decreases with the Ga concentration. The anisotropy fields of the nitrides derived from the high-field magnetization. The changes of the magnetic anisotropy, saturation magnetization, and the moment of the Fe atoms in the nitrides and their parent compounds with Ga concentration are discussed.
Formation kinetics of polycrystalline Eu2−xCexCuO4−y obtained from a sol-gel precursor
Authors: Suzuki P. A. , Jardim R. F. , Gama S.
Published: July 2002

Polycrystalline samples of Eu2−xCexCuO4−y (0.0≤x≤0.18) were prepared from a sol-gel precursor and sintered in air at different temperatures. From the x-ray diffraction results, the beginning of Eu2−xCexCuO4−y phase formation is observed by increasing the sintering temperature up to 500 °C, as indicated by the presence of very broad Bragg reflections belonging to the desired phase. At 700 °C, Eu2−xCexCuO4−y coexists with Eu2O3, CeO2, and CuO. Additional sintering at 950 °C in air for 20 h results in single phase materials. The effectiveness of Eu replacement by Ce in the Eu2CuO4 pristine phase has been confirmed by a decrease in the lattice parameter c with increasing x. Also, the solubility limit of Ce in these series was found to be higher than x=0.15. The results of thermal analysis reveal that the eutectic temperature Te≊1020 °C is Ce independent. On the other hand, the peritectic temperature Tp increases significantly with increasing Ce concentration. It is close to Tp≊1180 °C for Eu2CuO4 and Tp≊1195 °C for Eu1.82Ce0.18CuO4−y. Thermogravimetric analysis performed during the heating process in all samples studied revealed a weight loss of ≊1.5% at the peritectic temperature. This weight loss has been attributed to an oxygen removal which is partially recovered during the cooling process.
Influence of size and magnetocrystalline anisotropy on spin canting anomaly in fine ferrimagnetic particles
Authors: Han D. H. , Wang J. P. , Feng Y. B. , Luo H. L.
Published: July 2002

Fine equiaxed γ-CoxFe2−xO3 (x=0, 0.06) particles with a diameter ranging from 200 to 1000 Å were prepared by chemical precipitation. The average crystallite sizes were determined from x-ray line broadening measurements. The saturation magnetization and magnetocrystalline anisotropy of the particles were determined by using the approach to saturation. An empirical linear dependence of the specific saturation magnetization σs on the specific surface area Sa of the fine crystallites was obtained in the form of σs(S)=σs(∞)(1−ASa). The slope A which reflects the surface spin canting anomaly is different for γ-Fe2O3 and γ-Co0.06Fe1.94O3 particles. Under the supposition of the fine crystallite consisting of two parts, i.e., the surface layer, whose magnetic moment cannot be turned entirely along the direction of the applied field, but makes an average canting angle with the field, and the inner part, whose magnetic moment can be aligned along the direction of the applied field, the above formula can be interpreted well. The different slope A for γ-Fe2O3 and γ-Co0.06Fe1.94O3 particles may be caused by the different anisotropies of the two series particles.
Heat treatment to control the coercivity of Pt/Co multilayers
Authors: Miller J. , Pitcher P. G. , Pearson D. P. A.
Published: July 2002

We report on controlled changes in the coercivity of sputter deposited Pt/Co multilayer films by thermal treatment under controlled atmospheres. The as-sputtered coercivities of typically 3.3 kOe can be increased to 10.5 kOe by annealing in air, however for coercivities greater than 6 kOe the rectangular ratio of the multilayer decreases with increasing coercivity. The loop shape can be regained and the coercivity decreased by annealing in a H2/N2 atmosphere.
Changes in resistivity behavior of metallic glass Fe70Ni12B16Si2 due to molybdenum substitution for nickel
Authors: Bhatnagar A. K. , Seshu B. , Rathnayaka K. D. D. , Naugle D. G.
Published: July 2002

The changes in the resistivity behavior of metallic glass Fe70Ni12−xMoxB16Si2 are reported as a function of Mo substitution (x=0, 2, 4, and 6) for Ni at temperatures between 300 and 1.5 K.
Mössbauer study of permanent-magnet materials: Sm2Fe17−xAlx compounds
Authors: Al-Omari I. A. , Jaswal S. S. , Fernando A. S. , Sellmyer D. J.
Published: July 2002

The Fe57 Mössbauer spectra of Sm2Fe17−xAlx, where x=0, 1.0, 2.0, 3.0, and 4.0, have been measured at room temperature and analyzed. The ternary compounds Sm2Fe17−xAlx have the rhombohedral Th2Zn17 structure. Mössbauer measurements showed that all the compounds studied were ferromagnetic. The average hyperfine field was found to decrease with the increasing aluminum concentration, which is in qualitative agreement with magnetic measurements. The decrease in the average hyperfine field was from 224 kOe at x=0 to 174 kOe at x=4. By fitting the spectra we found that the hyperfine fields for the iron sites decrease in the order 6c, 9d, 18f, and 18h. The measured average isomer shift relative to α-iron was found to increase linearly with x. Analysis of the spectra showed that Al atoms occupy the 6c, 18h, and 18f, but not 9d, Fe sites and the fraction of occupancy of Al was found to depend on x.
Film-compensated homeotropic liquid-crystal cell for direct view display
Authors: Wu Shin-Tson
Published: July 2002

The viewing angle, contrast ratio, and response time of a negative birefringent film-compensated homeotropic liquid-crystal cell were evaluated. A spin-coated polyimide film was found to cancel the phase retardation of the liquid-crystal cell over a wide range of incident angle. Such a film-compensated homeotropic liquid-crystal cell exhibits a reasonably wide viewing angle, excellent contrast ratio, and relatively fast response time. This film also makes a significant improvement on the horizontal viewing angle of the twisted nematic cell.
Examination of electrical and optical properties of vanadium in bulk n-type silicon carbide
Authors: Evwaraye A. O. , Smith S. R. , Mitchel W. C.
Published: July 2002

Deep level transient spectroscopy (DLTS) has been used to characterize deep impurity levels in n-type 6H-SiC single crystals. A defect level at Ec−0.71 eV with an electron capture cross section σ=5.63×10−20 cm2 was observed. Defect concentration profiles confirm that the defect is a bulk defect. Infrared absorption measurements in the spectral range of 7000–7700 cm−1 were made using these samples. The infrared absorption spectrum characteristic of vanadium in silicon carbide is composed of a group of three absorption lines in the spectral range of 7000–7700 cm−1. This infrared signature was seen in the specimens in which the DLTS spectrum revealed the presence of deep traps. This signature was absent in those specimens where no deep traps were indicated by DLTS. Correlating these facts, we have concluded that the observed peak at Ec−0.71 eV was due to vanadium atoms in silicon carbide.
Simulation of elastic-network relaxation: Islands in semiconductor heterojunctions
Authors: Biagini M. , Catellani A.
Published: July 2002

The results of a computer simulation for islands in semiconductor strained heterojunctions are presented. The atomic positions are individually calculated by means of a minimization procedure of the total elastic energy, evaluated within Keating’s model. Results obtained for Ge/Si and InAs/GaAs systems differ substantially from the ones obtained via the macroscopic theory, which largely overestimates the elastic relaxation energy. The results are applied to evaluate the critical thickness corresponding to undislocated island nucleation in epitaxial strained heterostructures.
Experimental evidence for statistical-inhomogeneous distributed dopant atoms in a Si metal-oxide-semiconductor field-effect transistor
Authors: Mizuno Tomohisa , Toriumi Akira
Published: July 2002

We have experimentally and analytically studied the statistically inhomogeneous distributed dopant atoms in silicon metal-oxide-semiconductor field-effect transistors (MOSFETs) with very small dimensions, by investigating the threshold voltage characteristics. By interchanging the source and the drain terminals of an individual MOSFET in an 8192 MOSFET array within an area of less than 0.7 mm2, it was found that the asymmetry of the threshold voltage Vth of a MOSFET is observed only at high drain bias, and continues to increase with increasing the drain bias and scaling the channel length down. These results can be quantitatively explained by our analytical model in which the dopant atoms are inhomogeneously distributed along the channel and statistically fluctuated in the local region of the channel among 8192 MOSFETs.
Effect of rapid thermal annealing on the strain relaxation in heavily boron doped silicon epitaxial layer
Authors: Wang Jiangbao , Xu Qiang , Yuan Jian , Lu Fang , Sun Henghui , Wang Xun
Published: July 2002

The lattice parameters of heavily boron doped silicon homoepitaxial layers before and after rapid thermal annealing at different temperatures (800–1100 °C) are characterized by x-ray double crystal diffraction. The results illustrate that although the strain relaxation occurs after the rapid thermal annealing (RTA) treatment, the improvement of the crystalline quality of epitaxial layers are observed at RTA temperatures higher than 1000 °C. The proportional relationship between the lattice mismatch and the substitutional boron concentration with the lattice contract coefficient β=5.3 (in units 10−24 cm3) is valid up to the concentration of 3×1020 cm−3.
Spectroscopic characterization of phases formed by high-dose carbon ion implantation in silicon
Authors: Serre C. , Pérez-Rodríguez A. , Romano-Rodríguez A. , Morante J. R. , Kögler R. , Skorupa W.
Published: July 2002

High-dose carbon-ion-implanted Si samples have been analyzed by infrared spectroscopy, Raman scattering, and x-ray photoelectron spectroscopy (XPS) correlated with transmission electron microscopy. Samples were implanted at room temperature and 500 °C with doses between 1017 and 1018 C+/cm2. Some of the samples were implanted at room temperature with the surface covered by a capping oxide layer. Implanting at room temperature leads to the formation of a surface carbon-rich amorphous layer, in addition to the buried implanted layer. The dependence of this layer on the capping oxide suggests this layer to be determined by carbon migration toward the surface, rather than surface contamination. Implanting at 500 °C, no carbon-rich surface layer is observed and the SiC buried layer is formed by crystalline β-SiC precipitates aligned with the Si matrix. The concentration of SiC in this region as measured by XPS is higher than for the room-temperature implantation.
Study on sputtering power dependence of the structural properties of sputtered CoCrPt
Authors: Yumoto S. , Ohshima N.
Published: July 2002

The structural properties of sputtered CoCrPt at levels of 0.1, 0.3, 0.5, and 1 kW sputtering power were measured by x-ray diffraction, small-angle x-ray scattering (SAXS), x-ray absorption fine structure, and transmission electron microscope (TEM). XRD indicated a continuous increase in the intensity of the peak of hcp(002) orientation, as sputtering power increases. SAXS, on the other hand, in the range of 0.5°–3°, indicated the presence of scattering center for the 0.1 and 0.3 kW samples alone, not for the others. Similarly discontinuous were the spectra produced by x-ray absorption near-edge structure of Co K edge. Only the 0.1 kW sample showed a spectrum different from the others. This means that the electronic state for 0.1 kW sample is different from that for the others. TEM observation, carried out to examine the structural morphology, indicated a clear small-grain structure for the 0.1 kW sample, less clearly defined grains for the 0.3 kW sample, and no observable grains for the others after chemical etching pretreatment. The grain size for the 0.1 and 0.3 kW samples are about 100 and 200 Å, respectively. The grain is considered to consist of a Co-rich boundary and Pt-rich inside region from the chemical analysis of the solvent. The sputtering power dependence of the structural properties is able to be explained consistently as follows. As the sputtering power decreases, the segregated grain structure appears and affects SAXS. When the segregated grain size becomes smaller, it affects the electronic state. The different sputtering power dependence of structural properties comes from the different correlation lengths included in the measurements. Therefore, to understand the relationship between the magnetic and structural properties, structural investigation should include various range orders.
Rapid thermal annealing of Sn-implanted InP
Authors: Ridgway M. C. , Kringho/j P.
Published: July 2002

Rapid thermal annealing (RTA) of Sn-implanted InP has been investigated with a variety of analytical techniques including electrical measurements, Rutherford backscattering spectrometry, transmission electron microscopy and secondary-ion-mass spectrometry. RTA is shown to yield high electrical activation with negligible dopant diffusion. Incomplete electrical activation can result from dopant/defect complexes, native defect compensation, nonstoichiometry and dopant precipitation where the relative contribution of a given factor is dependent on both ion dose and annealing temperature. P coimplantation is shown to increase electrical activation through an increase in the number of ionized donors via a decrease in the number of dopant/defect complexes (in contrast to lattice site switching characteristic of dopant amphoteric character).