bluuebunny/crossref_metadata_2025 huggingface dataset:
Metastable phase formation in mechanically alloyed and ball milled Ti–Si
Authors: Yan Z. H. , Oehring M. , Bormann R.
Published: July 2002

The Ti–Si system is chosen as a model system to study the mechanical alloying of elemental powder blends in which one of the components is brittle. In addition, the stability of intermetallic compounds upon milling is investigated, in order to interpret the metastable phase formation during the milling process. Detailed x-ray investigations show that during mechanical alloying, a solution of Si in Ti preceeds the formation of amorphous or intermetallic phases. The selection of phases formed, as well as the final state, are influenced by the overall composition of the powder blends, and by the milling procedure applied. However, predictions can be made by considering the free energy curves of the stable and metastable phases in the Ti–Si system, which have been calculated by the CALPHAD method. Milling of the intermetallic compounds generally leads to the same final state as the mechanical alloying of elemental powder blends with corresponding compositions. These results demonstrate that the intermetallic compounds are energetically destabilized upon milling, allowing for phase transformations into metastable phases, such as amorphous or metastable crystalline alloys. The energetic destabilization of intermetallic compounds mainly originates from the chemical disorder caused by the milling process.
A study of interband magneto-optical transition in semiconductors
Authors: Chakraborty P. K. , Ghatak K. P.
Published: July 2002

An attempt is made to study the magneto-optical absorption for direct band parabolic semiconductors considering the k̄ dependence of the optical matrix element. Thus we have obtained the generalized expressions for the same coefficients which are consistent with the experimental results.
Gyrotron-backward-wave-oscillator experiments utilizing a high current, high voltage, microsecond electron accelerator
Authors: Spencer Thomas A. , Gilgenbach Ronald M. , Choi Jin J.
Published: July 2002

We report the first gyrotron-backward-wave-oscillator experiments to produce high power (tube power of ∼1–8 MW), long-pulse (0.3–1.2 μs) microwaves at high currents (0.1–2 kA) and high voltages (650–750 kV). Experiments were performed in the TE11 fundamental backward-wave mode, with efficiencies of ∼1%–2%. Mode competition was observed which is believed to originate from the TE21 absolute instability.
Thin film reaction kinetics of niobium/aluminum multilayers
Authors: Coffey K. R. , Barmak K. , Rudman D. A. , Foner S.
Published: July 2002

Phase formation kinetics in Nb/Al multilayered thin films having overall compositions of 25, 33, 50, and 75 at. % Al have been investigated using scanning calorimetry, x-ray diffraction, and cross-sectional transmission electron microscopy. The first phase to form upon annealing the Nb/Al layered structure of all samples is the NbAl3 intermetallic. Calorimetry clearly identifies the NbAl3 formation to be a two-stage process. The first stage is the formation of a planar layer by nucleation and growth to coalescence while the second stage is the thickening of the planar layer. The large amount of heat released (and hence large volume fraction of NbAl3 formed) during the first reaction stage is consistent with heterogeneous nucleation at well-isolated sites in the Nb/Al interface. This is surprising in light of the large thermodynamic driving force expected for nucleation and suggests that the local nonequilibrium nature of the Nb/Al interface greatly reduces the driving force for nucleation. The next phase observed in samples of 25 and 33 at. % Al is the A15 superconducting phase, Nb3Al. The Nb3Al growth completes a first reaction stage similar to the NbAl3, but the subsequent thickening reaction stage is not observed without simultaneous Nb2Al growth. The high interface velocities derived from the calorimetry for formation of both NbAl3 and the A15 Nb3Al indicate that atomic transport must be by grain boundary diffusion.
Electroluminescence in Tb-doped Gd2O2S phosphor
Authors: Shanker V. , Chatterjee S. , Ghosh P. K.
Published: July 2002

We report a strong ac green electroluminescence (EL) in powder layers of terbium doped gadolinium oxysulfide (Gd2O2S:Tb) with methyl methacrylate as binder. An intensity of the order of 30 nits (Cd/m2) has been achieved. The EL emission spectra shows line emissions corresponding to 5D3 and 5D4 fluorescing levels of Tb3+ ions. A very sharply rising B-V curve normally related to insulator-phosphor interface properties of a thin film electroluminescent device has been observed in these cells. This indicates the possibilities of barrier formation due to the localized space charge region in the absence of any prominent interfaces leading to impact excitation of Tb3+ ions. This has further been confirmed by the excitation spectrum of Gd2O2S:Tb phosphor, which reveals Tb3+ impurity absorption bands related to 4f8 shell transitions.
Laser chemical vapor deposition of TiN dots: A comparison of theoretical and experimental results
Authors: Conde O. , Kar A. , Mazumder J.
Published: July 2002

A mathematical model for the concentration profile of TiCl4 at the top surface of an Incoloy 800H substrate placed inside a laser chemical vapor deposition (LCVD) reactor is developed by using the three-dimensional transient mass diffusion equation. The model is used for studying the spatial variation of the thickness of TiN dots deposited by LCVD, using a CO2 laser and a reactive atmosphere consisting of TiCl4, N2, and H2. By assuming that the chemical reaction is first-order with respect to TiCl4, and that the sticking coefficient of TiN at the substrate surface is temperature dependent, the deposited TiN film is found to have a volcanic profile under certain conditions, which is in good agreement with experimental results.
Transmission electron microscopy and scanning transmission electron microscope analysis of the effects of thermal processing on the structural integrity of GaInAs/GaInAsP multilayers
Authors: Mallard R. E. , Long N. J. , Thrush E. J. , Scarrott K. , Norman A. G. , Booker G. R.
Published: July 2002

The structural characterization by electron microscopy of a metalorganic chemical vapor deposition grown GaInAs/GaInAsP multiple-quantum-well structure undergoing a ‘‘blue shift’’ in luminescence wavelength upon thermal processing, as part of a multistage epitaxial growth process, is reported. The sample exhibits a shift toward shorter wavelengths of greater than 100 nm upon annealing at 750 °C. The structural modifications that result in the blue shift include an elimination of atomic ordering in the quaternary barrier layers of the material, and significant layer interdiffusion. A method is described whereby quantitative analyses of the layer composition and lattice parameter with a spatial resolution better than 2 nm can be obtained from semiconductor multilayers using energy-dispersive x-ray microanalysis in the scanning transmission electron microscope. Such an analysis shows that the interdiffusion occurs along a nonlinear (that is, nonlattice matched) path where the group-V element diffusivities exceed those of the group-III elements, and results in the incorporation of excess coherency strains of up to 0.5% in the quantum-well regions.
Modulated photocarrier grating technique for diffusion length measurement in amorphous semiconductors
Authors: Hattori K. , Koji Y. , Fukuda S. , Ma W. , Okamoto H. , Hamakawa Y.
Published: July 2002

This article presents a method for investigating the diffusion of photocarriers in semiconductors by the analysis of phase shift between a temporally modulated illumination grating and its inducing photocurrent. Experiments on hydrogenated amorphous silicon prove that an accurate measurement of the diffusion length as well as an identification of ambipolar or nonambipolar diffusion can be acquired by using this technique.
Enhanced ambipolar in-plane transport in an InAs/GaAs hetero-n-i-p-i
Authors: McCallum D. S. , Cartwright A. N. , Huang X. R. , Boggess Thomas F. , Smirl Arthur L. , Hasenberg T. C.
Published: July 2002

In-plane transport in an InAs/GaAs semiconductor hetero-n-i-p-i has been investigated using picosecond transient grating techniques and an order-of-magnitude enhancement of the ambipolar transport relative to that measured in a similar undoped sample has been demonstrated. Both the magnitude and the density dependence of this enhanced transport are consistent with an additional driving force that is associated with an in-plane modulation of the screened n-i-p-i field. This modulation is the result of the spatial separation by perpendicular transport of electrons and holes that also have an in-plane density modulation.
The C49 to C54-TiSi2 transformation in self-aligned silicide applications
Authors: Mann R. W. , Clevenger L. A. , Hong Q. Z.
Published: July 2002

We show that when processing conditions closely mimic those used in the conventional self-aligned silicide process, the effective activation energy for the polymorphic phase transformation of orthorhombic base-centered (oC12) C49-TiSi2 to the low-resistivity orthorhombic face-centered (oF24) C54-TiSi2 phase is over 1 eV higher than previously reported literature values where a 1-step heat cycle was used. For C49-TiSi2 films formed at temperatures of 600 and 625 °C on (100) single crystal silicon substrates, the activation energy was determined to be 5.6±0.3 and 5.7±0.13 eV, respectively, for the transformation of this phase into C54-TiSi2 in the temperature range of 625–700 °C. The higher activation energy obtained with the simulation of the self-aligned silicide processing conditions suggests that the conventional processing may need to be modified for future semiconductor applications.
Amorphization of Ti/Cr multilayer films by solid-state reaction
Authors: Michaelsen C. , Yan Z. H. , Bormann R.
Published: July 2002

The solid-state reaction in Ti/Cr multilayered diffusion couples at temperatures between 450 and 650 °C was investigated by x-ray diffraction. It was found that an amorphous phase is formed at a concentration around 55 at. % Cr. The results are in excellent agreement with those obtained from mechanically alloyed powders, and with the corresponding metastable phase diagram calculated by the calculation of phase diagrams method. The observations strongly suggest that inverse melting is possible in the Ti-Cr system at around 55 at. % Cr.
Precursor to paramagnetic centers induced in gamma-irradiated doped silica glasses
Authors: Awazu Koichi , Kawazoe Hiroshi , Harada Koushi , Kido Kazuhiro , Inoue Satoru
Published: July 2002

We prepared silica glasses having various concentrations of chlorine, fluorine, hydrogen (SiO2−x), and oxygen (SiO2+x) to examine the precursors of paramagnetic centers induced by γ rays. In the case of glasses sintered under chlorine and hydrogen ambients, the concentration of the E′ center induced by γ-ray irradiation scaled with the partial pressure of chlorine and hydrogen. In contrast, the E′ center was suppressed in glasses doped with fluorine. Stress in the glasses was also found to enhance formation of the E′ center. Planar ring structures in the glass are influenced by stress and are proposed as precursors to the E′ center.
Femtosecond spectroscopic study of ultrafast carrier relaxation in hydrogenated amorphous silicon a-Si:H
Authors: Esser A. , Heesel H. , Kurz H. , Wang C. , Parsons G. N. , Lucovsky G.
Published: July 2002

We present a study of spectral- and time-resolved reflectivity and transmission measurements on hydrogenated amorphous silicon (a-Si:H) with 50 fs time resolution. Electron-hole pairs are photoexcited into the extended states of a-Si:H by an ultrashort pump pulse at ℏω=2 eV. The temporal evolution of the photoinduced optical response is studied for a broad range of excitation densities from 1018 up to 1020 cm−3, and on a timescale of up to 200 ps. The temporal evolution of the optical response is discussed in terms of recombination and trapping mechanisms for carriers in the extended states of a-Si:H.
Polarizing effects of magnetization on the crystallization of Co100−xPx amorphous alloys
Authors: Cebollada F. , González J. M. , González J.
Published: July 2002

Results are presented on the microstructure and magnetic properties of samples obtained by crystallization of amorphous films of composition Co100−xPx (10<x<25). Microstructural studies by means of scanning electron microscopy, x-ray diffractometry, as well as magnetic measurements showed the presence of a well-defined texture in the cobalt crystallites of samples of composition given by x<21, the easy axis direction of the amorphous samples being preserved during the crystallization. In these samples the thermal dependence of the coercive and anisotropy fields and of the remanence was studied. The crystallization product of samples of composition given by x≳21 is characterized by the presence of cobalt crystallites with random easy axes distribution. The polarizing effect of the magnetization was evidenced through the correlation between the two different types of microstructure and the presence or absence of magnetic order during the crystallization of the samples.
Detection wavelength of quantum-well infrared photodetectors
Authors: Choi K. K.
Published: July 2002

Long wavelength infrared detection using intersubband transitions has been progressing rapidly in recent years. One advantage of the quantum-well infrared photodetectors is the wavelength tunability as a function of their structural parameters. In this work, we have performed a systematic calculation on the detection wavelength, the absorption linewidth, and the oscillator strength of a typical GaAs/AlxGa1−xAs multiple-quantum-well photodetector, with aluminum molar ratio in the barriers ranging from 0.14 to 0.42 and the quantum-well width ranging from 20 to 70 Å. We found that within these material parameters, the detection wavelength can be varied from 5 to over 25 μm. In addition, we also discuss the photoconductive gain of the detectors with respect to the energies of the final state of the optical transition and the satellite valleys of the detector material.
Residual strain measurements in thick InxGa1−xAs layers grown on GaAs (100) by molecular beam epitaxy
Authors: Westwood D. I. , Woolf D. A.
Published: July 2002

The final stages of strain relief in the lattice mismatched InxGa1−xAs/GaAs(100) system are addressed by the examination of the residual strain in thick films (∼3 μm), grown by molecular beam epitaxy, across the entire compositional range. These results are compared with the observed variations in both the growth mode and material quality, and related to available theories. It is found that measurements are not consistent with a degradation of material quality that is simply misfit dependent, or to an abrupt change from two-dimensional (2-D) to three-dimensional (3-D) growth in the relaxation stage. Instead, the results seem to be more consistent with a continuous change from 2-D to 3-D growth between x=0 and x=0.4 (it is wholly 3-D above x=0.4). In addition, the large residual strains observed around x=0.5 are related to the poor material quality (possibly through work hardening) at this composition, which is in turn due to problems peculiar to the growth of mismatched alloys.
High-frequency terminal resonance in ZnO-Bi2O3-based varistors
Authors: Alim Mohammad A.
Published: July 2002

The negative terminal capacitance at high frequencies (usually f≫106 Hz) in ZnO-Bi2O3–based varistor systems is attributed to two possible sequential and/or combined sources: (a) piezoelectric grain resonance; and (b) electrode-lead or contact impedance. These sources are examined using a variation in the geometry of the varistor material and its electrode-lead configuration. The approximate values of the resonating parameters, designated by an equivalent series lumped inductance-capacitance-resistance (Lr-Cr-Rr) circuit in parallel with materials’ characteristic capacitance C0, are extracted employing lumped parameter/complex plane analysis technique for these ac electrical data. At the resonating frequency, the lumped reactance of this series circuit nullifies yielding a resistance Rr referred to the lumped ZnO grains.
Defects in electron irradiated n-type GaP
Authors: Zaidi M. A. , Zazoui M. , Bourgoin J. C.
Published: July 2002

The characteristic ionization energies, barriers associated with capture, energy levels, and introduction rates of the various electron and hole traps introduced by electron irradiation in n-type GaP are determined using deep level transient spectroscopy. The same traps are created after 4 or 300 K irradiation. Their introduction rates correspond to those expected for primary displacements. From the similarity to the case of GaAs, we conclude that the corresponding defects are intrinsic defects (isolated vacancies and vacancy interstitial pairs) associated with the P (electron traps) and Ga (hole traps) sublattices.
Quenching of porous silicon photoluminescence by deposition of metal adsorbates
Authors: Andsager D. , Hilliard J. , Hetrick J. M. , AbuHassan L. H. , Plisch M. , Nayfeh M. H.
Published: July 2002

Various metals were deposited on luminescent porous silicon (PS) by immersion in metal ion solutions and by evaporation. The photoluminescence (PL) was quenched upon immersion in ionic solutions of Cu, Ag, and Au but not noticeably quenched in other ionic solutions. Evaporation of 100 Å of Cu or 110 Å of Au was not observed to quench PL. Auger electron spectroscopy performed on samples quenched and then immediately removed from solution showed a metallic concentration in the PS layer of order 10 at.%, but persisting to a depth of order 3000 Å.
Observation of ion-implantation-damage-created n-type conductivity in InP after high-temperature annealing
Authors: Molnar B. , Kennedy T. A. , Glaser E. R. , Dietrich H. B.
Published: July 2002

The conductivity and the impurity profiles of InP implanted with dopant ions (Si,Be) or non dopant ions (B,H,N,O,P) have been investigated. Experiments have been done in substrates with and without Fe doping. Low-temperature, short-time annealing of implanted Si and Be reveals an n-type distribution of carriers which cannot be accounted for on the basis of implant activation. In order to examine the contribution without the carriers originating from the dopant ions, the behavior of electrically inactive B, H, N, O, and P, implants was investigated. Implantation of these ions into semi-insulating InP introduced n-type doping in the 1×1016 cm3 range after an anneal above 450 °C. For H, O, N, and P ions, the n-type conductivity could be eliminated by annealing at higher temperatures. However, boron anneals up to 750 °C did not eliminate the n-type conductivity. The n-type carrier profiles tracked the ion profiles. The carrier profile is influenced by the redistribution of the Fe during annealing; however, the Fe motion cannot explain the n-type conductivity. This conductivity may be due to a complex formed above 450 °C.
Structural and electrical properties of reactively sputtered InN thin films on AlN-buffered (00.1) sapphire substrates: Dependence on buffer and film growth temperatures and thicknesses
Authors: Kistenmacher T. J. , Ecelberger S. A. , Bryden W. A.
Published: July 2002

An extensive investigation of InN overlayers on AlN-buffered (00.1) sapphire by reactive magnetron sputtering has been undertaken and the dependencies of several basic materials properties (film thickness, development and quality of heteroepitaxy, film morphology, and electrical transport) on such key deposition parameters such as the growth temperatures of the insulating AlN buffer layer and the InN overlayer and their thicknesses have been determined. Three prominent effects of the AlN buffer layer are (1) the stabilization of heteroepitaxial growth over a broad range of film and buffer layer growth temperatures; (2) the attainment of a higher Hall mobility (up to 60 cm2/V s) over much of the same range; and, (3) the retention of heteroepitaxial growth, higher Hall mobility, and pseudo-two-dimensional growth even in the limit of an InN layer of ∼40 Å. In the context of a structure-zone model, the AlN buffer layer is projected to effectively raise the growth temperature of the InN thin film. The increase in effective growth temperature is, however, insufficient to overcome low atomic and cluster mobility and to achieve single-crystal InN thin film growth.
Intersubband stimulated emission and optical gain by ‘‘phonon pumping’’ in quantum wires
Authors: Leburton J. P.
Published: July 2002

We present an analysis of the optical gain for stimulated emission between subbands in quasione-dimensional structures. The population inversion is induced by a new pumping mechanism involving phonon absorption during transport in quantum wires. The population inversion threshold for far infrared stimulated emission is calculated in a generic GaAs quantum wires laser structure and shows a quite reasonable minimum value of Δn=3×105/cm at λ=100 μm. For longer wavelengths, free carrier absorption is the main limitation to stimulated emission.
A neutron diffraction and Mössbauer effect study of the magnetic properties of Pr2Fe17 and Pr2Fe17N2.6
Authors: Long Gary J. , Pringle O. A. , Grandjean F. , Yelon W. B. , Buschow K. H. J.
Published: July 2002

The neutron diffraction powder patterns of Pr2Fe17 and Pr2Fe17N2.6 have been measured at 295 K and refined within the Th2Zn17 structure type to give the structural changes which occur upon nitrogenation; changes which include a 6.4% expansion of the unit cell volume. The Mössbauer spectra of Pr2Fe17 and Pr2Fe17N2.6 have been measured between 85 and 295 K and analyzed with a model which is based on the Wigner–Seitz cell environment of each iron site, the basal orientation of the magnetization, and a comparison with the Mössbauer spectra of Nd2Fe17 and Nd2Fe17N2.6. Upon nitrogenation of Pr2Fe17, the 85 K-weighted average isomer shift increases from 0.049 to 0.156 mm/s, and the isomer shifts of the four crystallographically distinct iron sites increase in agreement with the increase found in their Wigner–Seitz cell volumes and the presence of a nitrogen near-neighbor for the 18f and 18h sites. The temperature dependence of the isomer shifts indicates an increase in covalency upon the formation of the nitride. Upon nitrogenation of Pr2Fe17, the 85 K-weighted average hyperfine field increases from 286.2 to 331.9 kOe, however, as is the case for the nitrogenation of Nd2Fe17, the increases on the 6c and 18f sites are much smaller than those observed on the 9d and 18h sites. These changes provide support for the band structure calculations of changes in the magnetic moments upon nitrogenation of Y2Fe17, Nd2Fe17, and Gd2Fe17.
Comment on ‘‘Steady-state temperature profile for a thin-film resistor under bias’’ [J. Appl. Phys. 72, 3862 (1992)]
Authors: Lu Yong-Feng
Published: July 2002

This comment is to point out that the temperature dependence of the thermal conductivity should be taken into account in computing the temperature distribution in substrate material (such as silicon) with temperature-dependent thermal conductivity. Ignoring the temperature dependence of the thermal conductivity will lead to a large error in the calculated results.
Grain boundary diffusion and its effects on the magnetic properties of Co/Cu and Co/Cr thin film bilayers
Authors: Pellerin J. G. , Anderson S. G. H. , Ho P. S. , Wooten C. , Coffey K. R. , Howard J. K. , Barmak K.
Published: July 2002

X-ray photoelectron spectroscopy has been used to investigate grain boundary diffusion of Cu and Cr through 1000-Å-thick Co films in the temperature range of 300–400 °C. Grain boundary diffusivities were determined by modeling the accumulation of Cu or Cr on Co surfaces as a function of time at fixed annealing temperature. The grain boundary diffusivity of Cu through Co has a diffusion coefficient D0,gb of 2×104 cm2/s and an activation energy Ea,gb of 2.4 eV. Similarly, Cr grain boundary diffusion through Co thin films exhibits a D0,gb of 6×10−2 cm2/s and an Ea,gb of 1.8 eV. The Co film microstructure has been investigated before and after annealing by x-ray diffraction and transmission electron microscopy. Extensive grain growth and texturing of the magnetic film occurred during annealing for Co deposited on a Cu underlayer which is believed to influence the grain boundary diffusion process. In contrast, the microstructure of Co deposited on a Cr underlayer remained relatively unchanged upon annealing. The difference in grain growth between the two bilayers has been suggested to account for the large disparity in activation energies. Magnetometer measurements have shown that increased in-plane coercivity Hc, reduced remanence squareness S, and reduced coercive squareness S* result from grain boundary diffusion of Cu and Cr into the Co films.
Inhomogeneous laminar conductor model for dielectric relaxation in oxide glasses
Authors: Saha S. K. , Chakravorty D.
Published: July 2002

An inhomogeneous conductor model involving laminae has been delineated. The ratio of conductivities of two successive laminae is assumed to have a sinusoidal variation with the spatial coordinate. The model has been used to calculate the dielectric modulus values as a function of frequency for two glass systems, viz., an alkali-containing silicate and V2O5-P2O5, respectively. Theoretical values are found to be in good agreement with the experimental data. The Kohlrausch–Williams–Watts exponent β appears to be correlated with the ratio of the maximum and minimum values of the conductivity ratio between two successive layers.
Acousto-optic modulation using a new chlorotellurite glass
Authors: Abdulhalim I. , Pannell C. N. , Wang J. , Wylangowski G. , Payne D. N.
Published: July 2002

Results of initial tests on an acousto-optic modulator using a new chlorotellurite glass with a composition of 3TeO2-2ZnCl2 are reported. This glass exhibits a wider transparency range (0.42–5.7 μm) than other tellurite glasses, with approximately the same figures of merit. We obtained an 80% diffraction loss and a 65% diffraction efficiency at a 633 nm wavelength, using an 80 MHz 15×1 mm piezoelectric transducer with 1.5 W of rf power. This glass is able to convert to fiber form, thus allowing efficient in-fiber acousto-optic modulators to be built.
Adding elements to the Co layer in Co/Pt multilayers
Authors: Hashimoto S.
Published: July 2002

In order to determine which elements added to the Co layer in Co/Pt multilayers will reduce the Curie temperature with no significant decrease in the Kerr rotation angle, the magneto-optical (MO) properties of (Co1−XMX)/Pt (M: additional elements) multilayers were studied, taking into account the change in MO properties brought about by varying Pt layer thickness while maintaining Co layer thickness. The addition of any element simultaneously decreases Curie temperature Tc and Kerr rotation angle almost proportionately to the reduction in saturation magnetization Ms . The perpendicular magnetic anisotropy energy Ku also declines with the addition of another element, but for Ni, Fe, and Tb the change is small. The ratio Q of Ku to demagnetization energy, which represents the effective perpendicular magnetic anisotropy, degrades with X, due to the enlargement of Ms for Fe-added films, while Ni and Tb preserve a large Q. Although the Tb-added films distinctly change their interfacial structure above X=0.2, Ni and Fe hardly change the structure even if added in large amounts. As a result, the addition of Ni can control MO properties, keeping large effective perpendicular magnetic anisotropy without influencing the structure. (Co,Ni)/Pt films possess lower Tc than Co/Pt films with the same Ms , and the disks using (Co,Ni)/Pt films achieved higher write sensitivity and higher write/erase cyclability owing to the lower Tc than Co/Pt disks with the same carrier-to-noise ratio. It would seem, therefore, that (CoNi)/Pt multilayers are more promising for practical MO disks.
Selective formation of luminescent porous silicon by photosynthesis
Authors: Noguchi Nobuaki , Suemune Ikuo
Published: July 2002

Selective patterning of luminescent porous silicon was achieved by a photosynthesis method. Porous silicon was selectively formed on boron-doped p+ regions patterned on a n-type silicon surface with spatially uniform photoirradiation. Porous silicon stripes formed on 7-μm-wide p+ stripes exhibited a clear visible photoluminescence image with the same dimension. The mechanism of the selective formation is discussed from the viewpoint of how photogenerated holes are accumulated. The resolution limit in the present method is given by the sharpness of the potential distribution at the p-n junctions, i.e., the widths of depletion layers, which reduce for highly doped p-n junctions.
Magnetic-field-effect three-terminal device based on YBa2Cu3O7−δ grain boundary junctions
Authors: Alff L. , Mayer B. , Schuster S. , Fröhlich O. , Gerdemann R. , Beck A. , Gross R.
Published: July 2002

Three-terminal YBa2Cu3O7−δ thin film devices based on a parallel array of grain boundary Josephson junctions were fabricated. The multiloop superconducting device is controlled by the magnetic field generated by a current Ictrl, which modulates the critical current Ic through the Josephson junction array. For these devices a current gain ranging between 2 at 77 K and 5 at 30 K was obtained. The spatial homogeneity of the device properties was studied by low-temperature scanning electron microscopy and by the determination of the supercurrent correlation function. Comparing the measured and the theoretically expected supercurrent correlation function shows that the device characteristics agree well with those expected from the device geometry.
Deep electron states in chlorine-doped ZnSe films grown by molecular beam epitaxy
Authors: Karczewski G. , Hu B. , Yin A. , Luo H. , Furdyna J. K.
Published: July 2002

Deep level transient spectroscopy (DLTS) was used to investigate defect centers in chlorine-doped ZnSe epitaxial films grown by molecular beam epitaxy on (100) n+-GaAs substrates. The resulting carrier concentrations were in the range from 8×1015 to 3.8×1018 cm−3. In low and moderately doped samples two isolated point defects are found, with energy levels at 0.30 and 0.51 eV below the conduction band. The concentration of the dominant trap (at 0.51 eV) is relatively low— of the order of 1015 cm−3—and does not depend on the Cl-doping level. The trap exhibits a strong electric field dependence, indicating its donorlike character. Heavily doped samples reveal a single thermal emission peak. The DLTS amplitude of this peak changes as a logarithm of the filling pulse duration, suggesting that the emission originates from spatially extended defects. We compare the DLTS behavior observed on ZnSe:Cl to earlier studies of Ga-doped ZnSe. Our results clearly indicate that Cl is far superior to Ga as an n-type dopant, because—unlike Ga—Cl does not of itself introduce any detectable deep defects into ZnSe.
Relaxation of SiGe thin films grown on Si/SiO2 substrates
Authors: LeGoues F. K. , Powell A. , Iyer S. S.
Published: July 2002

The relaxation in Si/SiGe bilayers grown on top of SIMOX wafers has been studied. By judiciously choosing the thickness ratios of the Si and SiGe, it is possible to relax the bilayer through the glide of dislocations exclusively in the Si layer, leaving the top SiGe layer relaxed and (mostly) dislocation free. This approach is completely different from previously proposed ways of reducing the number of threading dislocations in SiGe films because at no stage during the relaxation process are new threads introduced in the top SiGe layer. It is shown that the Si/SiGe bilayer behaves as a free-floating foil constrained to remain flat by the substrate, even at temperatures as low as 700 °C. The relaxation is shown to proceed until the strain left in the Si layer is too low for dislocations to glide. When the temperature is raised to 1050 °C, interdiffusion between the two layers forces the dislocation network to move into the SiGe through glide. The original network of 60° dislocations can then react to form a network of edge dislocations, which had never been observed before in this system. At such high temperature, glide is no longer the limiting factor for relaxation, and almost complete relaxation is attained.
Alkali metals impurities influence on the magnetic and electrical properties of YBCO
Authors: Nedkov I. , Veneva A.
Published: July 2002

The samples were prepared using the primary batch formula Y1−0.2xBa2−0.2xCu3+0.3x(Me)0.4xOy, where x=0–1.20 and Me denotes alkali metals (Na, K, Rb) introduced as carbonates. The presence of alkali carbonates during the process allows one to influence the YBCO grain size and improve the homogeneity of the structure. Despite the large concentration range of additives in the initial batch after the second treatment at temperatures higher than 900 °C, only traces of the alkali metals were revealed. The influence of Na, K, and Rb impurities in concentrations for Na about 5×10−1 wt %; for K to 4×10−2 wt %, and Rb to 2×10−2 wt % in polycrystalline YBCO was investigated. For Rb and K, the R(T) transitions were comparable to that of pure YBCO. The microwave surface resistance Rs was least influenced by the impurities, but strongly depended on the polycrystalline microstructure. Each impurity had a specific effect on χ(T) and Jc which are related to the grain-boundary structure.
Observation of CuPt-ordered CuInSe2
Authors: Bode Michael H.
Published: July 2002

Long-range order of the CuPt type has been observed in the I-III-VI2 material CuInSe2. The ordering was observed by transmission electron diffraction and by high resolution transmission electron microscopy. Comparison with simulated images confirms the CuPt-type ordering and shows the high degree of ordering. Extrinsic stacking faults were found near domain boundaries of the CuPt-type ordered material. During irradiation with the electron beam, the transformation from CuPt ordered to the sphalerite or chalcopyrite phase of CuInSe2 was observed.
Density of glow discharge amorphous silicon films determined by spectroscopic ellipsometry
Authors: Haage T. , Schmidt U. I. , Fath H. , Hess P. , Schröder B. , Oechsner H.
Published: July 2002

The dielectric function of thin-film hydrogenated amorphous silicon (a-Si:H) grown on fused silica at different substrate temperatures has been investigated by spectroscopic ellipsometry. An improved interpretation of the ellipsometric data is based on a tetrahedron model that takes into account the influence of hydrogen incorporation in the amorphous network. It is shown that the film density can be derived from an accurate data interpretation, whereas the maximum value of the imaginary part of the dielectric function ε2max and the void volume fraction are not proportional to the density of a-Si:H films.
Combined transmission electron microscopy and x-ray study of the microstructure and texture in sputtered Mo films
Authors: Karpenko O. P. , Bilello J. C. , Yalisove S. M.
Published: July 2002

The microstructure and texture of thin Mo films sputtered onto the native oxide of Si(100) wafers were investigated with both conventional reflection x-ray pole figures, and transmission electron microscopy and diffraction. Films were grown at two deposition rates (powers), 34 nm/min (1.5 kW) and 67 nm/min (3.9 kW), onto both moving and stationary substrates, under otherwise identical experimental conditions. The microstructure of the Mo films evolved into a zone 2 microstructure within the first 2 μm of growth. The development of both out-of-plane and in-plane textures was found to be influenced by deposition rate and geometry. Films grown at the lower deposition rate exhibited predominantly {110} textures, while films grown at the higher rate exhibited predominantly {110} textures up to a film thickness of ∼0.5 μm and {111} textures above a film thickness of ∼1 μm. Films with the {110} textures developed grains with elongated footprints and faceted surfaces, while films with the {111} textures developed grains with elongated triangular footprints and faceted surfaces. In all of the films deposited onto moving substrates, an alignment of the grains normal to the tangent plane (defined by the substrate normal and the direction of platen rotation) was observed. In all of the films deposited onto stationary substrates, the development of an in-plane texture was suppressed. These results suggest that a combination of geometric, energetic, and kinetic mechanisms are contributing to the evolution of the microstructure and texture in the Mo films.
Monte Carlo studies of ohmic hole mobility in silicon and germanium: Examination of the optical phonon deformation potential
Authors: Hinckley J. M. , Singh J.
Published: July 2002

Monte Carlo methods which have been widely used for studying high field electron and hole transport, so far have not been applied to the complex problem of Ohmic hole transport. We present a versatile generalization of the Monte Carlo approach for Ohmic hole mobility studies and apply it to pure silicon and germanium. In particular, we examine the role of the optical phonon deformation potential d0 in controlling the temperature dependence of the mobility.
Reverse I-V and C-V characteristics of Schottky barrier type diodes on Zn doped InP epilayers grown by metalorganic vapor phase epitaxy
Authors: Singh A. , Cova P. , Masut R. A.
Published: July 2002

Mesa etched, Au/p-InP Schottky diodes with a thin interface layer [metal-thin interface layer-semiconductor (MIS) diodes] were fabricated by evaporation of Au onto a Zn doped epitaxial layer of InP grown by low pressure metalorganic vapor phase epitaxy, on a highly doped InP substrate. The reverse current-voltage (Ir-Vr) and 1 MHz capacitance-voltage (C-V) characteristics of the Au/p-InP MIS diodes were measured in the temperature range 220–393 K. The reverse current in the MIS diodes did not saturate but increased with the increase in the reverse bias voltage. The soft Ir-Vr characteristics of the epitaxial Au/p-InP MIS diodes were very well described by the interface layer thermionic emission theory of Wu [J. Appl. Phys. 51, 3786 (1980)] for reverse bias voltages in the range 0–5 V and over the temperature range 300–393 K. In this temperature range, the values of the zero bias barrier height (φb0) obtained from the analysis of the Ir-Vr/T characteristic using the self-consistent iterative least square fitting method of Tseng and Wu [J. Appl. Phys. 61, 299 (1987)] agreed very well with those obtained from the C-V/T data. The analysis of the Ir-Vr/T data provided the values of (7.5±1.7)×10−3 and (45±22) Å for the transmission coefficient and the thickness of the interface layer, respectively. The capacitance-frequency (C-f) data for frequencies in the range 1 kHz up to 1 MHz and for bias voltages between −0.2 and 4.0 V, justify the assumption of voltage independence of the charge trapped in the states localized at the interface layer, made in the analysis of both the Ir-Vr/T and C-V/T data.
Electrical and structural studies of copper and nickel precipitates in a Σ=25 silicon bicrystal
Authors: Rizk R. , Portier X. , Allais G. , Nouet G.
Published: July 2002

Deep-level transient spectroscopy measurements of electronic defect states in a coincidence Σ=25 grain boundary (GB) in silicon have been performed after quenching of heat-treated samples (900 °C, 2 h) containing copper and/or nickel. These elements are usually suspected to be nonintentionally contaminating impurities. The special care supplied to get processed samples free from transition-metal impurities (particularly copper) has led to the measurement of a continuous distribution of boundary levels between Ec−0.20 eV and Ec−0.38 eV which shift toward deeper energies (between Ec−0.25 eV and Ec−0.42 eV) with a gradual increase of the copper content. The heavily contaminated samples with Cu or with both Cu and Ni exhibit a single interface trap at Ec−0.54 eV. According to transmission electron microscopy (TEM) and energy-dispersive x-ray (EDX) analyses, as well as comparable data reported earlier, this interface state seems characteristic of copper precipitates obtained by quenching from 900 °C; however, the incorporation of nickel in a ‘‘copper-free’’ bicrystal has been found to result in an almost discrete boundary level at Ec−0.50 eV. From TEM, EDX, and electron-diffraction investigations, the corresponding precipitates were geometrically shaped, intersecting the GB plane, and identified as two types (A and B) of NiSi2 silicide.
High electric-field amorphous silicon p-i-n diodes: Effect of the p-layer thickness
Authors: Chévrier J. B. , Equer B.
Published: July 2002

We present amorphous silicon p-i-n diodes able to sustain a reverse bias corresponding to 106 V/cm with a reasonably low leakage current. The influence of the p-layer thickness on the reverse bias current and the breakdown voltage is investigated. The high-voltage reverse current at room temperature is attributed to two different mechanisms: field enhanced thermal generation in the p-i interface region and, at the highest bias, electron injection through the p layer. Variable range hopping is also contributing to the low-temperature reverse current. Charge collection measurements after pulsed photogeneration were also performed up to the maximum voltage. No evidence for signal amplification is found, which sets a lower limit of 106 V/cm for impact ionization and avalanche phenomena.
Theory of transport in inhomogeneous systems and application to magnetic multilayer systems
Authors: Butler W. H. , Zhang X.-G. , Nicholson D. M. C. , MacLaren J. M.
Published: July 2002

A theory of the electrical conductivity of homogeneous random alloys based on the Korringa—Kohn—Rostoker coherent potential approximation (KKR-CPA) is generalized to treat an inhomogeneous alloy in which the concentrations of the constituent atoms can vary from site to site. A special case of such a system is an epitaxial multilayer system. We develop the theory for such systems and show how it can be implemented by using the layer Korringa—Kohn—Rostoker technique to calculate the electronic structure. Applications to magnetic multilayers and to the calculation of the giant magnetoresistance are discussed.
Observation and computer simulation of static magnetization process in soft magnetic thin film
Authors: Wang Zhigang , Tagawa Ikuya , Nakamura Yoshihisa
Published: July 2002

We have developed a new numerical micromagnetic simulator based on the curling particle assembly model and the finite element method which describes the static magnetization process in soft magnetic films successfully, and which exhibits many details of that process. Compared to conventional numerical treatment of the magnetic structure in soft magnetic films, e.g., the Landau–Lifshits–Gilbert equation method or recently proposed Monte Carlo method, our approach has the following advantages: high calculation speed, arbitrary meshing size, and good agreement with experiment.
Magnetic and structural instabilities of ferromagnetic and antiferromagnetic Fe/Cu(100)
Authors: Li Dongqi , Freitag M. , Pearson J. , Qiu Z. Q. , Bader S. D.
Published: July 2002

Fe wedges epitaxially grown on Cu(100) have been employed to investigate the interplay between magnetic and structural instabilities. 2–4 monolayer (ML) clean Fe films grown at room temperature are ferromagnetic with perpendicular easy axes. bcc Fe films≳11 ML thick are ferromagnetic with in-plane easy axes. Most importantly, 6–11 ML fcc Fe films are antiferromagnetic and have a ferromagnetic surface. Films grown below 200 K and annealed to room temperature do not exhibit the antiferromagnetic phase, but remain ferromagnetic and undergo a spin-reorientation transition from perpendicular to in plane at ∼6 ML. A new phase diagram for Fe/Cu(100) is proposed as a function of thickness and growth temperature. In addition, an impurity-stabilized layer-by-layer growth that persists to 30–40 ML Fe is also reported.
Finite-time thermodynamics: Exergy and optimization of time-constrained processes
Authors: Mironova Valentina A. , Tsirlin Anatolii M. , Kazakov Vladimir A. , Berry R. Stephen
Published: July 2002

The criterion of thermodynamic ideality is introduced. It is the ratio of actual rate of entropy production to the minimal rate of entropy production. It is closely related to the exergy approach but incorporates the irreversible losses due to finite-time and nonzero rates. No explicit reference need be made to the average values of the thermodynamic potentials of the environment. The regimes with minimal entropy production for the system where chemical reaction occurs and for the set of parallel heat engines are found.
On the mechanical strength of free-standing and substrate-bonded Al thin films
Authors: Heinen D. , Bohn H. G. , Schilling W.
Published: July 2002

Free-standing 4-μm-thick Al thin films were prepared by partially removing the Si substrate after deposition of the film. The tensile film stress was determined from the membrane vibration frequency in the temperature range between room temperature and −190 °C. The flow stress of the free-standing films is compared to that of substrate-bonded ones deposited under identical conditions thus having basically the same microstructure. It was found that the flow stress of the unsupported films agrees with that of bulk Al (when extrapolated to the same grain size of 1–4 μm) being considerably smaller than that of substrate bonded films. This directly proves the importance of the film/substrate interface for the yield strength of substrate-bonded metallic films supporting existing theoretical models.
Reply to ‘‘Comment on ‘Temperature limits on infrared detectivities of InAs/InxGa1−xSb superlattices and bulk Hg1−xCdxTe’ ’’ [J. Appl. Phys. 74, 4774 (1993)]
Authors: Grein C. H. , Flatté M. E. , Ehrenreich H. , Miles R. H.
Published: July 2002

Our calculations of the relative performance of ideal superlattice and HgCdTe (MCT) photovoltaic long wave infrared detectors, criticized in the previous Comment, do include radiative lifetimes. They also find the Auger-7 lifetime to be shorter than the radiative lifetime for p doping levels of 1017 cm−3. This agrees with other calculations and with the majority of experiments. The issue of detector thickness raised in the comment is not relevant since neither MCT nor the superlattices has an intrinsic advantage in this respect. The superlattices are further favored relative to MCT by lower tunneling currents, higher uniformity, and materials processing advantages. We suggest the superlattice system to be promising for the eventual realization of high-performance detectors.
Effects of local ambient atmosphere on the stability of electroluminescent porous silicon diodes
Authors: Zhang Libing , Coffer Jeffery L. , Gnade Bruce E. , Xu DaXue , Pinizzotto Russell F.
Published: July 2002

In this work, the influence of surrounding ambient atmosphere on the stability of electroluminescent (EL) porous Si (PS) diodes is examined. We have fabricated electroluminescent porous Si layers from anodic oxidation of (1) epitaxially grown p-type layers on n-type Si substrates; (2) n-type substrates with Au/Pd contacts; (3) p-type substrates with Au/Pd contacts. These structures are characterized using photoluminescence (PL), EL, and infrared (IR) spectroscopies, as well as scanning electron microscopy (SEM). In the case of the porous Si structures fabricated from p-n junctions, such structures yield orange emission with maxima near 620 nm upon the application of moderate applied voltages (3–7 V). For each type of diode, it is found that in strong oxidizing environments, EL intensity degrades completely within 30 min; in contrast, the integrated intensity remains essentially unchanged in the same time frame in the presence of a vigorous flow of inert gases such as nitrogen and argon. Infrared spectroscopic studies strongly suggest that electroluminescence degradation is related to porous silicon surface oxidation.