bluuebunny/crossref_metadata_2025 huggingface dataset:
Thermoelectric power of ternary semiconductor Se10Sb10Te80 thin films
Authors: Damodara Das V. , Raju K. S. , Aruna S.
Published: July 2002

Thin films of different thicknesses in the range 400–1600 Å have been vacuum deposited on clean glass substrates held at room temperature by very fast evaporation of the Se10Sb10Te80 bulk alloy. The thermoelectric power of these films has been measured as a function of temperature during heating and cooling cycles by the integral method. It is found that the thermoelectric power of these films is slightly different during the heating and the cooling cycles which is ascribed to slight reorientation of microcrystallites as also evidenced by x-ray diffraction. It is further found that the thermoelectric power (at a constant temperature) is a function of film thickness; it varies nearly linearly with reciprocal thickness. From these data, the nature of carrier scattering in the films has been ascertained. From the energy-dispersive x-ray analysis patterns of the films the semiquantitative content of the constituent elements Sb, Se, and Te has been determined.
Theoretical analysis of noise-reduction effect in semiconductor lasers with help of self-sustained pulsation phenomena
Authors: Yamada Minoru
Published: July 2002

The excess noise of semiconductor lasers has been reduced mostly with the help of the self-sustained pulsation in experiments. A theoretical analysis is given for mechanisms of the noise generation and the reduction method basing on the mode competition theory. Basic equations of these dynamics are given in the form of rate equations, where the nonlinear gain terms and Langevin noise sources are included. Effect of the optical feedback is introduced to give changes in the threshold gain level and by building of external cavity modes formed by the facet of the laser and the reflecting surface. The self-sustained pulsation phenomena are examined in the narrow stripe structure geometries having the saturable absorbing region beside the active region. The amount of noise is expressed relating to time averaged values of the photons and the carrier densities in laser, whose levels are modified by the pulsation. It is shown that the excess noise due to mode competition among the internal cavity modes is well suppressed by the self-sustained pulsation, while noise due to mode competition among the external cavity modes is mostly suppressed but sometimes not suppressed. More general discussions for the mechanisms are also given qualitatively by help of topological charts for dynamics. Three types of mode competitions are indicated. The first one is with a bistable state due to the symmetric profile of gain suppression for wavelength distribution. The second one is with an elliptic eddy motion of dynamics due to weak asymmetric property of the gain suppression. The third one is with a circular eddy motion of dynamics due to the strong asymmetric property of the gain suppression. Some difficulty in the noise reduction in the case of the strong asymmetric property of the gain suppression is mentioned.
Light-absorption effect on Bragg interference in multilayer semiconductor heterostructures
Authors: Kavokin Alexey V. , Kaliteevski Mikhail A.
Published: July 2002

The transfer matrix method has been employed to study the effect of light absorption on optical spectra of GaAs/AlAs Bragg reflectors. Substantial distortion of the Bragg plateau due to the absorption in GaAs layers is found. The saturation value of the reflection coefficient of the infinite Bragg reflector is shown to change nonmonotonically with the absorption coefficient showing a minimum. The particular case of low-temperature light reflection from the Bragg mirror in the vicinity of the exciton resonance frequency in GaAs layer is considered. The dramatic enhancement (by a factor of 50) of the exciton resonance amplitude in comparison with the reflectivity spectrum of GaAs is found in case of GaAs/AlAs Bragg structure covered by λ/2-thick GaAs caplayer.
Parallel conduction in GaAs heterostructures modified by low-energy ion exposure
Authors: Li F. , Wang T. , Spencer G. F. , Andrews C. C. , Kirk W. P.
Published: July 2002

We present a systematic study of the low-temperature magnetotransport properties of modulation-doped GaAs heterostructures to examine the device isolation mechanism in a low-energy (150 eV) Ar+ ion exposure process. Measurements were carried out repeatedly on the same Hall bar as a function of the ion exposure time. A gradual evolution from parallel conduction to strictly single-channel conduction was observed. The carrier density of the upper channel was depleted by ion surface milling, while the lower channel two-dimensional electron gas was essentially unaffected. The data indicated that carrier depletion and the subsequent breakdown in electron screening of the long-range random potential was the main reason for device isolation during the low-energy ion exposure process, in agreement with recent theoretical work.
Reduction of low frequency ac losses in coaxial cables of type II superconductors by a steady bias current
Authors: LeBlanc M. A. R. , Cameron Daniel S. M. , LeBlanc David , Meng Jinglei
Published: July 2002

Hysteresis losses, Wac, in the core of a monolithic coaxial cable carrying an alternating current of fixed amplitude Iac are predicted to trace a valley as a steady bias current Ibias is superimposed on Iac, when (a) the critical current density jc diminishes with increasing magnetic field H, and/or (b) a Meissner current IM or a surface barrier current ISB opposing flux entry play a role. The predicted Ibias,min where the valley minimum occurs and the value of Wac at the minima are displayed for various IM≥0 and ISB≥0 when jc=α (Bean) and jc=α/H (Kim approximation).
Effect of pyrolysis temperature on the magnetic properties of the carbon materials prepared from trialkylboranes
Authors: Murata Kazuhisa , Ushijima Hirobumi
Published: July 2002

Magnetic properties of amorphous carbon materials, prepared by pyrolysis of trialkylboranes, were examined using the superconducting quantum interference device and the electron spin resonance (ESR) methods. It is found that the observed magnetic properties could be greatly affected by pyrolysis temperature and the analysis of the magnetization curves observed in the magnetic field range 0–1 T has shown that these samples are basically of a superparamagnetic character. The ESR measurements have also supported these aspects.
Submillimeter radiation effect in the tunnel-coupled quantum wells under infrared excitation: A theoretical study
Authors: Soldatenko Yu. N. , Vasko F. T.
Published: July 2002

Redistribution of the electrons between the tunnel-coupled ground levels in the double quantum wells (DQWs) due to the resonant infrared excitation on the next level is considered. Intra- and interwell relaxation of electrons is studied self-consistently, taking into account both modification of the electron states in DQWs due to the upper ground states occupation and heating of the electrons due to IR pumping. The electron populations in the tunnel-coupled states is presented as a function of the DQWs parameters, doping, and IR pumping characteristics. The condition for origin of the inversive electron population, and possible value of the negative absorption coefficient in the submillimeter spectral region is found.
Refractive index change in Al+-ion-implanted silica glass
Authors: Fukumi Kohei , Chayahara Akiyoshi , Kitamura Naoyuki , Nishii Junji , Kadono Kohei , Makihara Masaki , Fujii Kanenaga , Hayakawa Junji
Published: July 2002

Al+ ions have been implanted in silica glass at an acceleration energy of 200 eV and doses ranging from 1×1013 to 1×1017 ions cm−2. Infrared reflection spectra and ultraviolet, visible, and near-infrared absorption spectra have been measured. It was found that refractive index of silica glass increased by 6%–10% after implantation of 1×1017 Al+ ions cm−2. It was deduced that this refractive index change is caused by the formation of Si—Si homobonds, but not by the decrease in Si—O—-Si bond angle which leads to compaction.
Photostimulated anodic oxide film formation
Authors: Goossens Albert , Macdonald Digby D.
Published: July 2002

An explanation of the photostimulated formation of anodic oxide films is given in terms of the photoquenching of the internal electric field. A description of this phenomena is presented within the framework of the point defect model. For the stationary situation, concentration profiles for oxygen ion vacancies across the film, in the dark and under super band-gap illumination conditions, are derived. For the nonstationary case a transport model is presented that accounts for the observed linear dependence of the film thickness L(t) on √t for the photostimulated formation of anodic films on zirconium in (1 M) H3PO4. This model can be used to derive the product of the diffusion coefficient of the oxygen vacancies DO and changes in their concentration at the metal/film interface. With DO=5×10−17 cm2 s−1, a change in the m/f interfacial oxygen vacancy concentration of 3.4×1022 cm−3 is found.
Image charge method for electrostatic calculations in field-emission diodes
Authors: Mesa G. , Dobado-Fuentes E. , Sáenz J. J.
Published: July 2002

We present a method to calculate the electrostatic field between a metallic tip of arbitrary shape and a sample surface. The basic idea is to replace the electrodes by a set of ‘‘image’’ charges. These charges are adjusted in order to fit the boundary conditions on the surfaces. As an application of the method, we describe the field characteristics of a field-emission diode as a function of the gap between electrodes for different tip shapes. A comparison between numerical and analytical results is presented. The results do not depend on the overall tip geometry only for gap distances smaller than ≊1/2 the tip radius. The field enhancement factor due to the presence of small protrusions on the tip apex is calculated and their influence in near-field-emission scanning tunneling microscopy is also discussed. We show that the electron-field emission from the sample is stable against tip-shape changes due to adsorbate diffusion or atomic rearrangements.
Thickness vibrations of thin piezoelectric plates and the determination of the electro-elastic constants of Al0.88Ga0.12PO4 crystal
Authors: Wang Jin-Feng , Liu Shi-Fang , Zhou Zhi-Qiang , Zhong Wei-Lie , Qin Zi-Kai , Han Jian-Ru
Published: July 2002

Thickness vibrations of thin piezoelectric plates excited by perpendicular or parallel fields have been investigated. In the analyses of the resonances of the thin plates, both the stress boundary and the electrical conditions have been considered. The theory has been applied to the determination of the electro-elastic constants of a newly developed piezoelectric crystal, Al0.88Ga0.12PO4.
Effect of temperature on capacitance of Al/microcrystalline chlorophyll a/Ag sandwich cells
Authors: Taleb Toufik , Nasr Chouhaid , Hotchandani Surat , Leblanc Roger M.
Published: July 2002

The capacitance measurements of Al/microcrystalline Chl a/Ag sandwich cells have been carried out as a function of temperature. The results show that while the capacitance at 0.10 Hz is voltage dependent at 20 °C, it is totally voltage invariant at −150 °C. This is explained in terms of thermal mobilization of the trapped charges which are responsible for the depletion layer. The temperature supplies the necessary energy and brings the response time of the trapped charges within the modulation frequency of the applied voltage and makes capacitance voltage dependent. Further, depending upon the frequency, f, of the applied voltage, the temperature needed to mobilize the carriers to the same extent is also different. This is due to the fact that when the frequency is high, the trapped carriers are not able to respond as well to the variations in voltage compared to when the frequency is low. As a result, a comparatively high temperature is needed to mobilize the carriers to the same degree at high frequency than at low frequency. The same reasoning, i.e., the response behavior of the trapped charges with respect to the frequency of the probe voltage can also explain the onset of the geometric capacitance at much lower temperatures at lower frequencies than those at high frequencies. From the plot of log f versus reciprocal characteristic temperature, an activation energy of ∼0.2 eV has been calculated, and corresponds to the energy necessary to detrap and mobilize the charge carriers from the traps. The structural imperfections in the electrodeposited Chl a film are most probably the source of traps.
The effect of annealing and crystallization on the magnetoelastic properties of Co-Si-B amorphous wires
Authors: Atkinson D. , Squire P. T. , Atalay S.
Published: July 2002

The field dependence of Young’s modulus (ΔE effect) of amorphous wires of Co72.5Si12.5B15 has been studied in the as-quenched state, and after annealing for times between one and several hundred minutes at temperatures of 450, 480, and 500 °C. In the as-quenched state the ΔE effect is small (<2%), because the quenching stresses couple with the magnetostriction (λs∼−3×10−6) to produce significant anisotropy. Annealing for short times (≳1 min) is sufficient to relieve much of the quenching stress, resulting in magnetic softening and enhanced ΔE effect (at best ≳30%). Further annealing eventually reverses these trends, increasing coercivity and anisotropy, and reducing the ΔE effect. This behavior is attributed to surface crystallization. The results are interpreted in terms of the core-shell domain model, and are consistent with the retention of a significant component of circumferential domain structure in the shell in all annealing conditions studied.
Influence of ion irradiation damage on properties of porous silicon
Authors: Bao Xi-Mao , Yang Hai-Qiang , Yan Feng
Published: July 2002

We have studied the effect of ion-irradiation on porous Si formation, microstructure, and optical properties. Porous Si was first self-implanted and then fabricated by anodization. With increasing implantation dose, the photoluminescence (PL) intensity decreased, and the PL spectra were also red shifted. Porous Si formed from crystal Si emitted light, while that from preamorphized Si did not. Porous Si luminescent patterns with a resolution of 2 μm features were formed by selective ion implantation.
Magnetic-field dependence of the electronic charge density, cyclotron mass, and Fermi energy in a GaAs–Ga1−xAlxAs heterojunction
Authors: Sabín del Valle J. , de Dios-Leyva M.
Published: July 2002

A theoretical study of the magnetic-field dependence of the two-dimensional electron-gas density, cyclotron mass, and Fermi energy in a modulation-doped GaAs–Ga1−xAlxAs heterojunction under a magnetic field applied perpendicular to the interface is presented. The study, based on both the envelope-function approximation and a parametric self-consistent approach, takes into account band-nonparabolicity effects. Numerical results show that the above-mentioned quantities as well as a screening parameter introduced in the calculations are oscillatory functions of the magnetic field. Our results are in good agreement with experimental data previously reported and show that nonparabolicity may significantly influence the behavior of the cyclotron-resonance parameters.
A modified model in positive temperature coefficient of resistance BaTiO3 ceramics
Authors: Zhang Feng , Cao Zechun
Published: July 2002

Current–voltage and resistivity–voltage measurements have been made on donor-doped BaTiO3 ceramics in order to investigate the nature of the current transport processes. It is found that the characteristics of the current–voltage do not follow the Heywang model [Solid-State Electron. 3, 51 (1961)]. Hence, the classical Heywang model is modified and the thermionic emission model with smoothly changed barrier resulted from image force is suggested. The modified theory coincides better with the experimental observation. The dependence between current density and voltage is J=J0 exp[−(φ0−βV1/2)/kT]. The relationship between resistivity and voltage (voltage effect) is ρ=(2kT/dJ0β)V1/2 exp[(φ0−βV1/2)/kT . From the modified model, the maximum barrier height of BaTiO3 ceramics can be deduced experimentally as well.
Time dependence and mechanisms of magnetization reversal in Tb–Fe–Co films
Authors: Brown S. D. , Street R. , Chantrell R. W. , Haycock P. W. , O’Grady K.
Published: July 2002

Measurements have been made of the time dependence of magnetization, under constant applied field conditions, of thin films of Tb–Fe–Co alloy. The results have been analyzed using a model in which it is assumed that magnetization proceeds by thermal activation of irreversible switching of the magnetic moments of identical elementary volumes. It is assumed that the activation energy for switching, intrinsically the same for all elementary volumes, varies according to local interactions. Values of the fluctuation field Λ are derived and it is found that for the films examined, Λ is independent of magnetization M and the applied field. It is shown that information about the magnetic interactions between the elementary volumes, due to exchange and demagnetizing fields, may be derived from graphs of dM/dt vs M.
Origin of the charge carriers accumulation/depletion in porous silicon contacted by a liquid phase
Authors: Bsiesy A. , Gelloz B. , Gaspard F. , Muller F.
Published: July 2002

The origin of the electronic free charge carriers accumulation/depletion in the porous silicon skeleton contacted with an electrolyte is determined by investigating the limiting step in the conduction process. Experimental evidences, supported by quantitative considerations, show that the porous silicon can be conductive if the free charge carriers flux supplied by the substrate is not the rate determining step of the electrical current conduction.
A surface modification study of InGaP etched with an electron cyclotron resonance source at variable microwave powers
Authors: Cole M. W. , Han W. Y. , Pfeffer R. L. , Eckart D. W. , Ren F. , Hobson W. S. , Lothian J. R. , Lopata J. , Caballero J. A. , Pearton S. J.
Published: July 2002

Etch-induced surface modifications, utilizing an electron cyclotron resonance source, have been studied as a function of controllable etch parameters. InGaP was etched with BCl3 at a constant substrate temperature (100 °C) and bias voltage (−145 V) using microwave powers varying between 250 and 1000 W. The surface morphology, residual etch damage, and surface stoichiometry were strongly influenced by changes in ion flux. The etch-induced lattice damage and surface smoothness increased as the ion energy was elevated. Low ion flux etching resulted in an In-enriched P-depleted surface suggesting nonuniform desorption of InClx which gave rise to the surface roughness observed at the low microwave powers. The smooth surfaces, achieved at the higher microwave power levels, were attributed to either efficient sputter-assisted desorption of the InClx etch products or to InClx desorption via plasma-induced surface heating. Results of this study demonstrate that etching at microwave powers between 500 and 750 W induce low residual damage and smooth surfaces while maintaining a reasonable etch rate for device processing.
Carrier dynamics and photodetection in charge injection transistors
Authors: Frankel Michael Y. , Belenky Gregory L. , Luryi Serge , Carruthers Thomas F. , Dennis Michael L. , Cho Alfred Y. , Hamm R. A. , Sivco Deborah L.
Published: July 2002

We study picosecond carrier transport dynamics induced by 200 fs 1.55 μm optical pulses in charge injection transistor structures. We propose and demonstrate a new optoelectronic method for exploring the interactions of hot majority carriers and cold minority carriers, as well as the optical control of real space transfer in these devices. The minority holes photogenerated in the channel produce substantial cooling of the hot-electron majority carriers and lead to the reduction of the real space transfer. The new method also provides a direct measure of the minority carrier lifetime in the transistor channel. These effects are demonstrated in InGaAs-channel devices with both InAlAs and InP barriers. The similarities in the device characteristics are explained in terms of the interaction of photogenerated minority holes with majority electrons in the channel leading to a photoconductor-like drain current and to a reduction in the real space transfer collector current. The differences are attributed to the different conduction and valence band energy offsets between the wide band gap barrier and the low band gap collector and channel layers. Furthermore, the InAlAs-barrier device shows a capability of serving as a practical photodetector with the measured, system-limited recovery speed of ∼5 ps.
Au-mediated low-temperature solid phase epitaxial growth of a SixGe1−x alloy on Si(001)
Authors: Ramanath G. , Xiao H. Z. , Lai S. L. , Allen L. H. , Alford T. L.
Published: July 2002

The evolution of microstructure during Au-mediated solid phase epitaxial growth of a SixGe1−x alloy film on Si(001) was investigated by in situ sheet resistance measurements, x-ray diffraction, conventional and high-resolution transmission electron microscopy, energy dispersive x-ray spectroscopy, and Rutherford backscattering spectrometry. Annealing amorphous-Ge/Au bilayers on Si(001) to temperatures below 120 °C caused changes primarily in the microstructure of the Au film. Near ≊130 °C, Ge from the top layer diffused and crystallized along the grain boundaries of Au. The Ge that had reached the Au/Si (001) interface mixed with Si from the substrate, to form epitaxial SixGe1−x islands on Si (001). Si from the substrate had dissolved into Au before entering the growing epitaxial islands. Meanwhile, the Au that was displaced by Ge that filled the Au grain boundaries, diffused into the top layer along columnar voids in the amorphous Ge film. With increasing temperature, more Au was displaced to the top by the flux of Ge towards the substrate, facilitating further epitaxial growth and the coalescence of epitaxial SixGe1−x islands. At 310 °C, the initial Au film was displaced completely to the top by a laterally continuous SixGe1−x epilayer of uniform composition (x≊0.15). The epilayer thickness was limited by that of the initial Au film. Twins and residual amounts of Au trapped near the SixGe1−x/Si(001) interface were the predominant defects observed in the completely strain-relaxed SixGe1−x epilayer.
Time dependence of ac losses in multifilamentary NbTi superconductors due to proximity coupling
Authors: Schmidt Curt
Published: July 2002

The ac losses of NbTi multifilamentary superconducting wires with a copper matrix were measured using a sensitive calorimetric method. The wire samples, which have filament diameters in the micrometer range and interfilament spacings of a few tenths of a micrometer, exhibit a strong proximity effect. The measured power loss at a constant ac field amplitude and frequency showed an unexpected time dependence on a very long time scale of hours, compared to the ac field cycle time being in the millisecond range. The measured loss vs time curves were found to depend sensitively on amplitude and frequency of the ac field, and in the way the ac field was switched on. The effect can be rather large, changes of losses by up to 60% were found. Losses can increase or decrease with time depending on the structure of the sample. As an independent check losses were also measured by a magnetization method. The results were in very good agreement with the calorimetric measurement. The effect of time dependent losses is qualitatively explained by the slow decay of proximity currents which are induced when the ac field is switched on. They create a time dependent bias magnetization, which influences the ac losses.
Optical spectroscopy of epitaxial Ga2Se3 layers from the far infrared to the ultraviolet
Authors: Morley S. , von der Emde M. , Zahn D. R. T. , Offermann V. , Ng T. L. , Maung N. , Wright A. C. , Fan G. H. , Poole I. B. , Williams J. O.
Published: July 2002

Epitaxial Ga2Se3 layers were grown on GaP (100) and GaAs (100) by metal–organic chemical vapor deposition and the heterovalent exchange reaction, respectively. Measurements of the sample reflectance were carried out in the spectral range from 70 to 50 000 cm−1 (∼10 meV−6.2 eV). The dielectric functions in the far infrared were determined from the reflectance measurements and are dominated by strong phonon features of the substrate and the layers. Substrate related multiphonon absorbances and Fabry–Perot interference dominate the mid infrared range. The spectra in the visible spectral range reveal Fabry–Perot interferences up to 2.6 eV indicating a fundamental band gap energy in the blue spectral range in contrast to the previously reported lower value of 2 eV. Further electronic transition energies were observed at 3.9, 4.7, and 5.0 eV.
Weak ferromagnetic resonance of Gd2CuO4 small particles (abstract)
Authors: Mira J. , Rivas J. , Butera A. , Tovar M. , Vázquez-Vázquez C. , Mahía J. , López-Quintela M. A. , Oseroff S. B.
Published: July 2002

RE2CuO4 (RE=rare earth) compounds constitute an interesting family of materials because of their magnetic complexity as well as for their relationship with high Tc superconductors. For R=Gd and heavier rare earths, a weak ferromagnetic component (WF) is observed below ∼280 K. This component has its origin in a canting of the Cu antiferromagnetic (AF) structure due to a distortion of the oxygen ions between the copper atoms. In this contribution we center our work in Gd2CuO4, which is the only WF compound in the series that can be synthesized at normal pressures. WF can be easily detected by microwave absorption, due to the presence of an associated low field absorption signal. We have synthesized the material by sol-gel methods, allowing a control of grain size, dependent on time and the final calcination temperature. The microwave absorption results show that the onset point for the WF component shifts from 290 K for the bulk material, to 210 K for particles with an average size of 120 nm. For particles less than 90 nm the WF resonance line is not observed, suggesting the absence of weak ferromagnetism. As the WF component is associated to the AF ordering, we interpret our results as the elimination of the long range order motivated by the particle size reduction.
Electrical resistivity of bismuth implanted into silicon
Authors: Ferreira da Silva A. , Sernelius Bo E. , de Souza J. P. , Boudinov H.
Published: July 2002

We investigate the electrical properties of Bi-doped Si samples, prepared by ion implantation, in a range of concentrations around and above the metal–nonmetal transition. Comparison between experimental and theoretical values of the resistivity brought out that in these samples a similar behavior is observed as for other n-doped Si, thus confirming the results obtained in the same range of impurity concentration, i.e., ρ(Sb)≤ρ(P)≤ρ(As)≤ρ(Bi).
Exchange effect on the space charge in nanostructure resonances
Authors: Price Peter J.
Published: July 2002

It is elucidated that, for nanostructures in which the electron states are filled though the energy range of a transmission or reflection peak (quasilevel resonance), the electron pair distribution has a variety of the Fermi exchange hole in which, to a good approximation, just half of the electron pairs contribute to the Coulomb interaction energy. The displacement of current characteristics due to the space-charge interaction should be calculated with this 1/2 factor.
Magnetoresistance oscillation in window-coupled wire systems
Authors: Zhu Ningjia , Lei Ming , Guo Hong
Published: July 2002

We present a theoretical investigation of electron magnetotransport characteristics of a window-coupled quantum wire system which is treated as a four-terminal device. Different shapes and sizes of the coupling region are studied and various four-terminal Büttiker resistances are computed as a function of an external magnetic field. Our results are consistent with those of the experimental measurements of Hirayama, Tokura, Wieck, Koch, Haug, von Klitzing, and Ploog [Phys. Rev. B 48, 7991 (1993)] on similar device structures. In particular we have observed the fine interference pattern in the magnetoresistance and negative values of certain four-terminal resistance at low-magnetic-field strength. These features are found to be sensitive to the incoming electron energy.
Photoluminescence of GaInP under high pressure
Authors: Dong Jianrong , Li Guohua , Wang Zhanguo , Lu Dacheng , Liu Xianglin , Li Xiaobing , Sun Dianzhao , Kong Meiying , Wang Zhijie
Published: July 2002

Photoluminescence of GaInP under hydrostatic pressure is investigated. The Γ valley of disordered GaInP shifts sublinearly upwards with respect to the top of the valence band with increasing pressure and this sublinearity is caused by the nonlinear relationship between lattice constant and hydrostatic pressure. The Γ valleys of ordered GaInP rise more slowly than that of the disordered one and the relationship between the band gap and the pressure can not be explained in the same way. Taking into account the interactions between the Γ valley and the folded L valleys, as well as, the X valleys, the experimental pressure dependences of the band gap of ordered GaInP epilayers are calculated and fitted quite well using first order perturbation theory. The results indicate that simultaneous ordering along 〈111〉 and 〈100〉 directions can occur in ordered GaInP.
Determination of nonlinear susceptibilities by transforming random variables
Authors: Murray J. T. , Bonilla S. , Jiang J. P. , Tajalli H. , Powell Richard C. , Peyghambarian N.
Published: July 2002

High-peak-power lasers are typically used as pump sources in nonlinear optical measurements. Intrinsic to these sources are pulse-to-pulse output energy instabilities. We report on how pump instabilities affect the measurement of nonlinear susceptabilites and develop theory which relates statistical parameters of a general, arbitrary pump energy probability distribution to those of its corresponding mth harmonic output. The relation leads to a simple method of data analysis whereby pump instabilities can be used as a tool in determining the nonlinear susceptibility of a material.
X-ray truncation rod study of Ge(001) surface roughening by molecular beam homoepitaxial growth
Authors: Hong Hawoong , Aburano R. D. , Chung Ki-Sup , Lin D.-S. , Hirschorn E. S. , Chiang T.-C. , Chen Haydn
Published: July 2002

Surface roughness was determined by x-ray diffraction for Ge films on Ge(001) grown by molecular beam epitaxy at room temperature. The truncation rod intensities and transverse-scan line profiles were measured as a function of perpendicular momentum transfer. Depending on the initial morphology of the surface, the same growth condition resulted in very different surface morphologies. Two types of initial surfaces were used. One was an atomically flat surface with very large terraces. The other, characterized by a roughness exponent α=1, had a high density of steps. Deposition on the flat surfaces resulted in a fairly smooth surface, but with a graded crystalline density below the surface. Deposition on the α=1 surfaces resulted in a more jagged surface characterized by an increase in the average height–height correlation function and a final roughness exponent of α=1/2. Additional and complementary information about the surface structure was obtained by scanning tunneling microscopy observations.
Giant magnetoimpedance effect in soft and ultrasoft magnetic fibers
Authors: Ciureanu P. , Rudkowski P. , Rudkowska G. , Menard D. , Britel M. , Currie J. F. , Ström-Olsen J. O. , Yelon A.
Published: July 2002

Soft NiFe-Permalloy fibers and ultrasoft NiCo-based amorphous fibers, having a circular cross section with 30–40 μm in diameter, have been cast by melt extraction. The fibers have been driven by a sinusoidal current with 20 mApp constant amplitude and frequencies from 0.1 to 100 MHz. Both longitudinal and transverse giant magnetoimpedance effects have been observed in these fibers. The longitudinal GMI effect at 30 MHz was 60% in ultrasoft (NiCo)70FeSiBMn fibers for a saturating field of about 7 kA/m. The same effect was found at 10 MHz frequency for Permalloy fibers for a larger saturating field (20 kA/m). The transverse GMI effect was smaller (≊35% for NiCo and ≊20% for NiFe), and showed a maximum at low frequency (3 MHz). The magnetic field responses of the fibers are quadratic. An inverse effect of 10%–30% was observed for both types of fibers in longitudinal as well as in transverse field responses at high current frequency and low field strength. All of the observed effects could prove to be very useful for a new generation of high-sensitivity magnetic field sensors.
Uniaxial magnetic anisotropy of iron thin films deposited by oblique incidence of deposition particles
Authors: Hoshi Y. , Suzuki E. , Naoe M.
Published: July 2002

Uniaxial magnetic anisotropy induced in iron thin films by the oblique incidence of deposition particles was investigated by computer simulation. In the simulation, the self-shadowing effect was considered mainly to explain changes in the magnetic anisotropy of iron films with the incident angle. The simulated grain shape in the film changes significantly with the incident angle of deposition particles. Grains elongated in a direction normal to the incidence direction of depositing particles were clearly observed in simulated film deposited with an incidence angle around 60°. However, the film deposited at an incidence angle of 80° had columnar grains separated from each other and inclined to the incidence direction. Changes in the uniaxial magnetic anisotropy of the film can be qualitatively explained by the shape anisotropy of these grains in the film. The decrease in saturation magnetization of these films with incident angle seems to be mainly caused by the formation of a porous film.
Deuteride absorption and desorption effects on magnetic properties of YFe2Dx
Authors: Paul-Boncour V. , Escorne M. , Mauger A. , Latroche M. , Percheron-Guegan A.
Published: July 2002

Magnetic properties of YFe2 and its deuterides YFe2Dx have been investigated in the temperature range 300<T<800 K. Both the lattice structure and the Curie temperature Tc show strong dependence on the deuterium composition x. The deuterides have a C15 cubic structure up to x=2.7 and Tc rises from 560 K for x=0, up to 715 K for x=2.2 and 2.7. Amorphous YFe2D2.3 was also synthetized and surprisingly displays a lower Tc=330 K. For larger deuterium concentrations, the deuterides show a rhombohedral distortion and the Curie temperature is reduced. Rhombohedral deuteride YFe2D3.6 has Tc=345 K. Heating this deuteride above 410 K leads to partial desorption of the deuterium and transformation of the material into a cubic phase. This is evidenced by the fact that a spontaneous magnetization at these high temperatures is restored, characteristic of cubic deuterides with x≤2.7. On the other hand, additional desorption upon heating above 700 K induces a transition to the amorphous state, as a result of the weakening of the Y—Fe bond. The time dependence of the magnetization at a given temperature shows that the desorption at high temperature is due to the diffusion of deuterium through the surface.
Effective boundary conditions in optics of thin surface overlayers
Authors: Rashba Emmanuel I.
Published: July 2002

Narrow surface overlayers contribute significantly to optical spectra. Effective boundary conditions are derived which generalize the usual boundary conditions of crystal optics and include corrections to them up to the order (kd)2, where k is the wave vector, and d is the layer width. Electromagnetic response functions of a surface layer as well as a bulk dielectric function enter as parameters into these boundary conditions. Equations are presented which relate parameters of the reflected waves and their angular dependences to the surface response functions. These equations indicate that the spectral shapes of those features in reflection spectra which originate from the surface absorption depend critically on the bulk dielectric function. The proposed formalism can be used for reconstructing surface response functions from experimental data and applied for investigating surface layers, surface-controlled ordering effects, etc.
Two-photon-induced optical switching using a CdTe etalon
Authors: Hamad A. Y. , Wicksted J. P.
Published: July 2002

We report the observation of optical switching in a CdTe etalon at room temperature using 11 ns pulses at 1.064 μm radiation. We have also observed optical limiting and pulse compression. The switching was attributed to the nonlinear change in the index of refraction induced by two-photon absorption. The thermal effect was found to have a negligible contribution to the nonlinearity. The transmission of the etalon was studied as a function of its initial detuning and the input intensity. The switching times were found to be in the range of 1.5–3.7 ns. A theoretical model was developed in the plane-wave approximation assuming both nonlinear absorption and refraction and was found to be in good agreement with the experimental results.
Raman-scattering assessment of Si+-implantation damage in InP
Authors: Cuscó R. , Talamàs G. , Artús L. , Martin J. M. , González-Díaz G.
Published: July 2002

We studied the lattice damage caused by Si+ implantation into semi-insulating InP, with doses in the range of 1012 to 5×1014 cm−2, and the subsequent lattice recovery achieved by rapid thermal annealing (RTA), by means of Raman spectroscopy. With increasing implantation dose, an intensity reduction of the first- and second-order Raman peaks characteristic of crystalline InP is observed, together with the enhancement of disorder-activated modes. In samples implanted with doses higher than 1014 cm−2 the Raman spectra resembles that of amorphous InP, and the samples can be considered as fully amorphized. By RTA at 875 °C for 10 s, sample crystallinity is recovered, even in the case of those samples implanted with the highest dose. After annealing, the Raman spectra show no evidence of disorder-activated modes, and the intensity of the characteristic second-order peaks approaches the value found in unimplanted InP.
Stress dependence of magnetostrictions and strains in 〈111〉-oriented single crystals of Terfenol-D
Authors: Zhao Xuegen , Wu Guangheng , Wang Jinghua , Jia Kechang , Zhan Wenshan
Published: July 2002

Single crystals of Tb0.27Dy0.73Fe2 were prepared successfully by the Czochralski (CZ) method using a magnetic levitation cold crucible first in this laboratory. The Laue x-ray technique and metallography verified its twin-free single crystalline perfection. Weidmanstatten precipitation (WSP) slightly exists in these specimens with WSP platelets parallel to the {111} planes of Terfenol-D. Saturation magnetostriction were measured along three principal crystallographic axes [111], [11 2̄] and [1 1̄0] of single crystal Tb0.27Dy0.73Fe2 with magnetic field parallel and then perpendicular to the measuring direction, which gives an intrinsic magnetostriction λ111=1640×10−6 for 〈111〉 easy axis. Stress dependence of magnetostrictions and strains measured along three mutually perpendicular axes, [111], [11 2̄], and, [1 1̄0] on 〈111〉-oriented single crystalline rods of Terfenol-D indicates that apparent magnetostriction for 〈111〉 direction increases with increasing applied compressive stress along the rod axis, and reaches 2375×10−6 under a stress of 24 MPa and a magnetic field of 20 kOe. Maximum d33 decreases with the increment of applied stresses and the amount of WSP, and ranges from 1×10−6/Oe to 6×10−6/Oe at about 500 Oe. Less hysteresis appears. Magnetizing processes are calculated and compared with the measured changes of magnetostriction. The authors conclude that 〈111〉-oriented single crystal of Terfenol-D is suitable for precise displacement actuators although it would be much better to eliminate the WSP precipitation.
A scanning microscope using a magnetoresistive head as the sensing element
Authors: O’Barr R. , Lederman M. , Schultz S.
Published: July 2002

We have built a scanning probe microscope where a magnetoresistive (MR) head is used as the sensing element. The sensor region of the MR head is about 5 μm wide and 30 nm thick giving high resolution in one direction and poor resolution in the other. As MR heads with smaller cross-track dimensions are developed, submicron resolution in this direction should also be realizable. The MR head is mounted on a piezoelectric bimorph and vibrated perpendicular to the plane of the sample, and the ac voltage output of the head is detected with a lock-in amplifier. We have imaged pre-recorded bits as a function of scan height, and also imaged bits that were written using the write element of the same MR head. The typical voltage change through a transition is ∼4 μV, two orders of magnitude smaller than signals generated on a spin stand. This is most likely due to the difficulty in setting the slider parallel to within submilliradian accuracy to the plane of the sample. Our positioning system does not have this capability at this time. With modifications in the positioning stage, this device should provide a valuable tool for investigating the recording and playback processes, and for characterizing media and heads at ever decreasing track widths, bit separations, and flying heights.
High sensitivity to temperature and quantum effects in vanadium oxide diodes
Authors: Kuznetsov V. A. , Haneman D.
Published: July 2002

We have fabricated vanadium oxide films by the method of oxidation of evaporated vanadium films, and found it possible to achieve a temperature coefficient of the resistance for a diode structure of 25% and more per degree Celsius—much higher than any previous result. The nature of the contact influences the sensitivity. The structures showed switching behavior, and also in some cases apparent quantum effects. The films are composed of mixed oxides.
Mechanism of texture formation during heat-treatment process in Ag-sheathed Bi-2223 tapes
Authors: Zhang C. , Wang Y. Z. , Zeng D. C. , Yang Z. Q. , Li Z. R. , Qiao G. W. , Fang J. R. , Chuang Y. C.
Published: July 2002

The texture formation and development of Bi-based superconducting phases during the heat-treatment process are carefully investigated. With increasing anneal time, the (001) texture of both 2212 and 2223 phases is enhanced. The reduction of the interfacial energy due to a Bi2O2 layer in the cell is the main factor contributing to the texture formation. The texture of the parent phase and the interface parallel to the (001) plane are beneficial factors. The effects of sheathed Ag, second phases, and voids on the texture formation are studied.
Electrical inactivation in Pb-implanted InP
Authors: Ridgway M. C. , Kringho/j P. , Johnson C. M.
Published: July 2002

Pb-implanted InP has been characterized with electrical measurements, Rutherford backscattering spectrometry combined with channeling (RBS/C), and transmission electron microscopy (TEM). Although donor activation can be achieved in InP with implantation and annealing of all group-IV elements of lesser mass, the n-type conductivity measurable in Pb-implanted InP is attributed not to ionized Pb donors but to implantation-induced disorder. The latter was verified with samples implanted with the isoelectronic group-V element Bi which yielded both comparable disorder and conductivity. Furthermore, RBS/C measurements indicate that for impurity concentrations of ∼1×1020 atoms/cm3, only ∼5% and ∼17% of Pb and Bi atoms, respectively, occupy substitutional or near-substitutional lattice positions following rapid thermal annealing. Pb precipitates, as evident with TEM, comprise a significant component of the post-anneal, nonsubstitutional atom fraction. Conversely, the as-implanted, substitutional fractions of Pb and Bi atoms are both ∼85%.
Tunnel barrier properties of oxidized bismuth droplets on Al2O3
Authors: Ekkens T. B. , Nolen S. , Ruggiero S. T.
Published: July 2002

We discuss the tunnel barrier properties of oxidized ultrasmall Bi droplets on and sandwiched within Al2O3. Scanning tunneling microscopy (STM) and standard sandwich junction studies of these systems have revealed a consistent evolution of tunnel barrier parameters for the composite Bi2O3/Al2O3 barrier. We observe a systematic decrease in barrier height with increasing barrier width similar to the results for progressively oxidized pure Al2O3 and other refractory oxides. Our results suggest that STM, under appropriate conditions, can be employed to evaluate and view the time evolution of the local characteristics of selected tunnel-barrier systems.
High memory effects in polymer dispersed chiral liquid crystals with negative dielectric anisotropy
Authors: Magnaldo A. , Nourry J. , Sixou P.
Published: July 2002

A cholesteric liquid crystal with negative dielectric anisotropy and with a visible wavelength selective reflection presents high memory effects when encapsulated in a polymer film. Three states at least are accessible within the same film: a reflecting state, a memory state, and a refreshed state. The last two states are stable over months. When the samples were annealed, the memory state was no longer stable. By comparing samples as they were annealed we tried to understand how the memory effect was removed.
Tunable infrared detector with epitaxial silicide/silicon heterostructures
Authors: Schwarz C. , von Känel H.
Published: July 2002

Epitaxial CoSi2/Si/CoSi2 and CoSi2/Si/poly-PtSi heterostructures were grown with molecular-beam epitaxy onto Si(111). Characterization of the heterostructures with scanning tunneling microscopy, Rutherford backscattering spectrometry, and transmission electron microscopy revealed very high structural quality. We report on the application of these heterostructures to a wavelength-tunable infrared detector. It consists of two back-to-back Schottky contacts separated by the thin (1000–2000 Å) undoped Si spacer layer. The different Schottky barrier heights which photocreated charge carriers in the silicides have to surmount can be used to control the cutoff wavelength by simply varying the applied bias across the structure. Photoelectric measurements of so-called symmetrical sensors made of CoSi2/Si/CoSi2 where both silicides contribute equally to the photocurrent, yielded a bias dependence of the cutoff energy three times as large as predicted by the conventional Schottky effect. In this case, the observed tunability of the cutoff energy can be explained only by considering ballistic transport of photocreated carriers (holes and electrons) in the silicon. Different mean free path lengths of hot electrons and holes in Si lead to a strongly bias-dependent ratio of the collected photoelectrons and photoholes. Photocurrents measured in asymmetrical sensors made of CoSi2/Si/PtSi were found to change phase as a function of light energy at a constant bias. This change of photocurrent direction can also be understood with the proposed energy band diagram and ballistic transport of hot carriers in Si. This kind of device showed a tunability of the cutoff energy between 0.3 and 0.5 eV.
First-principles calculation of the hexagonal-closed-packed Ni75Ta25 metastable phase
Authors: Huang X. Y. , Pan J. S. , Fan Y. D.
Published: July 2002

The total energy and electronic structure of the hcp Ni76Ta25 metastable phase have been obtained by the self-consistent discrete variational method and the local Xα exchange approximation. The total energy of this phase was calculated as a function of lattice constant, a, for a fixed c/a of 1.63, which is determined experimentally. The predicted lattice constant, i.e., a=4.93 a.u. is very close to the experimental value of a=4.97 a.u. In addition, the density of states and bonding charge density were used to clarify the characteristics of bonding in this metastable crystalline phase.