bluuebunny/crossref_metadata_2025 huggingface dataset:
Magnetic buoyancy force acting on bubbles in nonconducting and diamagnetic fluids under microgravity
Authors: Wakayama Nobuko I.
Published: July 2002

The magnetic buoyancy force acting on a bubble in a one-dimensional magnetic field can be represented as F=(χG−χL)∫H(dH/dx)dVolB, where χG and χL are the volume magnetic susceptibilities of the gas and liquid, respectively, and H is the magnetic field strength. Since |χL|≫|χG| and most liquids are diamagnetic, this expression indicates that the magnetic buoyancy forces act in the direction of increasing magnetic field strength. Because the magnetic buoyancy force in a diamagnetic fluid is small, the motion of bubbles under normal gravity is difficult to study, but microgravity offers the possibility of detailed observations. Using a compact permanent magnet under microgravity conditions, N2 bubbles in pure water (0.01 dyne s/cm2) and in a 69:31 glycerol/water mixture (0.21 dyne s/cm2) were found to move in the direction of increasing H, and to be held stationary at the point of maximum H. The motion of the bubbles was also simulated with a theoretical model and was found to agree with measurements made under microgravity conditions. These results indicate that magnetic buoyancy can be used to control bubble motion. Since most fluids are diamagnetic, magnetic buoyancy can be used to control bubbles in many fluidic devices used in space applications.
Anisotropic magnetoresistance and planar Hall effect in magnetic metal-insulator composite films
Authors: Zhao B. , Yan X. , Pakhomov A. B.
Published: July 2002

We studied anisotropic magnetoresistivity (AMR) and planar Hall effect of granular Ni-rich NiFe–SiO2 and Fe–SiO2 films for various metallic volume fraction. Planar Hall resistivity was found to be the same as the magnetoresistivity (MR) difference between the longitudinal and the transverse geometry. As metallic volume fraction decreases, we found that the MR evolves from the AMR domination in the metallic conduction region, through an intermediate conduction region with mixed AMR and negative MR behavior, to the isotropic negative MR in the tunneling conduction region. Plausible explanations to this complicated evolution are discussed.
Magnetomechanical effect in nickel and cobalt
Authors: Devine M. K. , Jiles D. C.
Published: July 2002

The change in magnetization as a result of applied uniaxial stress has been measured in nickel and cobalt. Both tensile and compressive stresses were applied up to 125 MPa. Magnetostriction and anhysteretic magnetization as a function of stress were also measured. The change in magnetization with stress depended on the applied stress and the displacement between the prevailing magnetization and anhysteretic. At the loop tips, nickel showed a +6 mT (compression) and −6 mT (tension) magnetization change while cobalt displayed a +15 mT (compression) and −15 mT (tension) magnetization change. At remanence, nickel decreased in magnetization by 45 mT under either sign of stress, while cobalt decreased by 20 mT also under either sign of stress. Magnetomechanical changes in magnetization near the loop tips were mostly reversible, while at remanence the magnetomechanical change was predominately irreversible. Cobalt generally displayed larger changes in magnetization with stress than nickel at locations close to the loop tips, while the converse was true at locations near remanence. The results confirm the hypothesis that the magnetomechanical effect (dM/dσ) depends on the displacement between the anhysteretic and prevailing magnetization.
Stress dependence of ΔE in amorphous ribbon
Authors: Kim D. Y. , Kim C. G. , Kim H. C. , Sung U. H.
Published: July 2002

The Young’s modulus with magnetic field (ΔE effect) has been investigated by the single domain model in Fe81B13.5Si3.5C2 amorphous ribbon for as-received state. The stress dependence of Young’s modulus was analyzed in terms of the volume fraction of transverse domains, the average internal stress, and transverse anisotropy constant as a function of applied stress, and yielded a good agreement with the experimental results.
Spin polarization at Fe/Cr interfaces
Authors: Pizzagalli L. , Freyss M. , Moraitis G. , Stoeffler D. , Demangeat C. , Dreyssé H. , Vega A. , Miethaner S. , Bayreuther G.
Published: July 2002

It is shown that contradictory experimental data on magnetic moments and spin order at Fe/Cr interfaces can be explained by structural irregularities at the interfaces. The spin-polarized electronic charge distribution was calculated by using a self-consistent tight-binding model combined with a real-space recursion method. It was used to interpret the total magnetic moment of Cr(001) films and of Cr/Fe(001) sandwiches molecular beam epitaxy grown on Fe(001) from in situ measurements with an alternating gradient magnetometer during film growth. While a strong decrease of the sample moment during Cr deposition was observed on a very smooth surface, no moment change occurred for a strongly faceted surface. The different results of both experiments are consistent with the calculations if we take into account (i) a possible ferrimagnetic c(2×2) spin configuration of a Cr monolayer on Fe(001) which might be favorable in clusters of a certain size and for high step densities; (ii) a possible interchange of one Cr and Fe monolayer at the interface; and (iii) a multidomain configuration with zero net moment of a thin Fe layer on a Cr surface due to a high step density.
Magnetostrictive properties of amorphous and partially crystalline TbDyFe thin films
Authors: Speliotis A. , Kalogirou O. , Niarchos D.
Published: July 2002

A series of amorphous and partially crystalline giant magnetostrictive thin films of the composition (Tb0.3Dy0.7)39Fe61 has been prepared by dc magnetron sputtering. The sputtering conditions were the same for all samples apart from the substrate temperature TS, which varied between 330 and 510 °C. The crystalline state and the magnetic properties of the samples were investigated in relation to the substrate temperature. Films deposited at TS=330–400 °C were amorphous. Crystallization started at TS=425 °C. The magnetostrictive coefficient λ, at 4 kOe and at room temperature, increased with increasing TS from 185 to 750 ppm at Hmax. This was related also to an increase of the Curie temperature TC from 35 to 315 °C. The hysteresis loops of the amorphous samples showed coercivities lower than 10 Oe. These samples presented λ≈300 ppm in a field of 1000 Oe. The direction of the magnetic moments in the amorphous films changed gradually from perpendicular to parallel to the film plane with increasing TS. The samples deposited at TS⩾425 °C showed λmax≈350–750 ppm, but the magnetostrictive curve was much broader and shifted to the value of Hc.
Dynamics of optical nonlinearities induced by strong light illumination in CdS nanocrystallites
Authors: Vanagas E. , Moniatte J. , Mazilu M. , Riblet P. , Hönerlage B. , Juodkazis S. , Paille F. , Plenet J. C. , Dumas J. G. , Petrauskas M. , Vaitkus J.
Published: July 2002

Light induced grating experiments have been performed at room temperature in CdS nanocrystallites of 2.5 nm radius embedded in sol-gel glass films of different semiconductor volume concentration. The dynamics of the photogenerated carriers at the lowest transition peak has been shown to depend on the excitation intensity. No spatial diffusion is observed up to a nanocrystallite volume concentration of 35%. Below a few MW/cm2 the carriers relax in each nanocrystallite, either radiatively in ∼150 ps or by trapping the electrons 15 meV below their excitation transition. For higher intensities of excitation, a saturation of the excited states occurs which allows the electrons to be trapped in the glass matrix, inducing a permanent grating by the creation of a static electric field.
Effects of defects on magnetoresistivity in La0.7Sr0.3MnO3
Authors: Wilson M. L. , Byers J. M. , Dorsey P. C. , Horwitz J. S. , Chrisey D. B. , Osofsky M. S.
Published: July 2002

The effects of structural and oxygen site defects on the ferromagnetic phase transition and magnetotransport in doped lanthanum manganite films have been examined. Oxygen defects were introduced through a vacuum annealing process while structural defects were introduced using ion irradiation. The introduction of both defect types strongly suppressed the Curie temperature Tc while increasing the peak resistivity, activation energy, and magnetoresistance ratio. For defect types leading to similar reductions in Tc, structural defects lead to a broader transition regime and a smaller MRR than are found for oxygen defects. Well above Tc both defect types lead to nearly identical resistivity curves. Structural defects are argued to primarily affect the carrier mobility, hence these data provide a clue that the carrier mobility is dominating the transport properties in this regime.
Correlation between crystalline structure and soft magnetic properties in sputtered sendust films
Authors: Dodd P. M. , Atkinson R. , Papakonstantinou P. , Araghi M. S. , Gamble H. S.
Published: July 2002

We have investigated the change in crystallographic and magnetic properties of sendust thin films before/after annealing. Films were deposited by both rf diode and rf magnetron sputtering. rf diode-sputtered films did not possess soft magnetic properties in the as-deposited state. Films were subsequently annealed in both a conventional oven and a rapid thermal annealing (RTA) system. The coercivity of the films decreased from over 10 Oe to a minimum value of 0.6 Oe after annealing and a systematic shift in the (110) x-ray diffraction (XRD) peak position towards that of bulk sendust was observed. No additional XRD peaks were observed after annealing. The optimum annealing conditions to minimize coercivity was found to be 1–2 h at 550 °C. By comparison, films processed by RTA resulted in minimum coercivity of 0.9 Oe obtained after annealing for 2 min at 550 °C. XRD for films annealed by both RTA and oven annealing showed a similar decrease in the d spacing of the (110) plane, indicating a release of tensile strain in the films that had been introduced during deposition. rf magnetron-deposited samples possessed coercivities as low as 1.4 Oe as deposited and showed in-plane anisotropy. XRD again showed a (110) texture, however, the peak intensity of magnetron-sputtered samples was a factor of 20 less than the diode-sputtered films of the same thickness. The position of the (110) peak, however, was close to that of rf diode deposited films after annealing indicating that the magnetron-sputtered films were relatively stress-free.
Magnetic properties and structure of R2Co17−xTix intermetallic compounds (R=Pr or Nd)
Authors: Tang H. , Shen B. G. , Buschow K. H. J. , de Boer F. R. , Wang Y. Z. , Qiao G. W.
Published: July 2002

The effect of substitution of Ti for Co in compounds R2Co17−xTix (R=Pr or Nd, and x=0.0–2.0) on the structure and the magnetic properties has been investigated. It is found that the solubility of Ti in the compounds R2Co17−xTix (R=Pr or Nd) is around 1.0. The R2Co16Ti (R=Pr or Nd) are isostructures to their parent compounds (rhombohedral), and its unit cell is expanded. The spontaneous magnetization μs and Curie temperature TC decrease with increasing Ti content, owing to the magnetic dilution and the reduction of the moment of Co sublattice caused by Ti substitution. Moreover, an anomaly in the M-T curves of Pr2Co16Ti and Nd2Co17−xTix compounds with x=0.5 and 1.0 may be attributed to a spin reorientation. This transition may originate from the change of crystal electron field due to the preferential occupation of Ti atoms on specific crystallographic sites.
Exchange fields at rare-earth sites in Rn+1Co3n+5B2n (R=Gd and Dy) compounds
Authors: Suzuki Y. , Ito T. , Uchida T. , Nashima O. , Hong N. M. , Ido H.
Published: July 2002

Spontaneous magnetization and paramagnetic susceptibility have been measured for the Rn+1Co3n+5B2n compounds with R=Y, Gd, and Dy and n=1, 2, and 3 in the temperature range from 4.2 up to 1000 K in fields up to 20 kOe. The data on the Gd system are analyzed based on the molecular field model. The exchange field at the Gd site from the Co sublattice is estimated. The averaged exchange field at T=0 K is estimated to be 101 T for n=1, 75T for n=2, and 56 T for n=3. Paramagnetic susceptibilities are fairly well expressed by the superposition of the Co- and the Gd-sublattice contributions. Other magnetic properties of the Y and the Gd system are also reported. Only magnetic data are given for the Dy system.
Incorporation and stability of carbon during low-temperature epitaxial growth of Ge1−xCx (x<0.1) alloys on Si(100): Microstructural and Raman studies
Authors: Yang B.-K. , Krishnamurthy M. , Weber W. H.
Published: July 2002

Low-temperature (∼200 °C) molecular beam epitaxy of Ge1−xCx alloys grown on Si(100) have been extensively investigated by in situ reflection high-energy electron diffraction, ex situ x-ray diffraction, transmission electron microscopy, and Raman spectroscopy. Carbon concentrations were nominally varied from 0 up to ∼10 at. %. Selected samples were annealed in an Ar ambient at 750 °C to evaluate the stability of the thin films. A few films were also grown on Ge substrates. With increasing C concentration, the epitaxial growth mode changes from two dimensional layer growth to three dimensional island growth. The surface has a tendency to facet along {311} planes under certain growth conditions. The microstructure shows an increase in planar defect density with increasing C concentration. The x-ray diffraction data show that the lattice parameter decreases with increasing C concentration and that a maximum of 1 at. % C is incorporated substitutionally in Ge. Raman spectroscopy shows no clear Ge–C signal though extra intensity is measured at the energies where Ge–C modes may be expected. Films with nominal C concentrations greater than 2 to 3 at. % show clear evidence for amorphous C. We propose that under our growth conditions, nominal C in excess of about 2 to 3 at. % remains on the surface as amorphous C and plays an important role in 3D islanding, defect formation, and {311} faceting during epitaxial growth.
Relation between electroluminescence and photoluminescence of Si+-implanted SiO2
Authors: Song Hai-Zhi , Bao Xi-Mao , Li Ning-Sheng , Zhang Jia-Yu
Published: July 2002

The electroluminescence (EL) from Si+ implanted SiO2 thin film prepared by thermal oxidation was compared with photoluminescence (PL) properties. Both EL and PL spectra indicate that the luminescence originate from the same three luminescence bands around 470, 600, and 730 nm. Annealing at temperatures below and above 1000 °C makes the 470 and the 730 nm bands dominate in PL spectra, respectively. The 600 nm band, which is weaker in PL, is usually the strongest in EL. The relative contributions from different luminescence bands to EL depend on annealing, but are independent of current density. The different excitation mechanisms of the 470, 600, and 730 nm luminescence bands give rise to the discrepancy between EL and PL.
The magnetic properties of Gd2Co17−xGax compounds
Authors: Liang Bing , Shen Bao-gen , Wang Fang-wei , Zhao Tong-yun , Cheng Zhao-hua , Zhang Shao-ying , Gong Hua-yang , Zhan Wen-shan
Published: July 2002

X-ray diffraction (XRD) and magnetization measurements were performed to investigate the effect of Ga substitution for Co on the structural and magnetic properties of Gd2Co17−xGax compounds (x=0, 1, 2, 3, 4, 5, 6, 7, and 8). Crystal-structure studies indicate that all samples are single phase with the rhombohedral Th2Zn17-type structure except for the samples with x=7 and 8, which contain a small amount of Co. The Ga substitution for Co in Gd2Co17 compounds leads to a monotonic increase in the unit cell volume and an approximately linear decrease in the saturation magnetization. The Curie temperature TC is found to decrease monotonically from 1210 K for x=0 to 30 K for x=8. The compensation points are observed for the Gd2Co17−xGax samples with x=5 and 6. The value of the compensation temperature, determined from the temperature dependence of the magnetization measured in a magnetic field of 1000 Oe, is 120 and 152 K for x=5 and 6, respectively. It is noteworthy that the substitution of Ga for Co in the Gd2Co17 compounds has a remarkable influence on the magnetocrystalline anisotropy. XRD studies on magnetic-field oriented Gd2Co17−xGax powders show that the sample with x=0 exhibits an easy-plane anisotropy, while the samples with x⩾1 exhibit an easy c-axis anisotropy at room temperature. The change of the easy magnetization direction of Gd2Co17−xGax compounds from a basal plane to c axis indicates that the substitution of Ga for Co leads to the decrease of the planar anisotropy of the Co sublattice.
Photoluminescence of the Se and Si DX centers in (AlxGa1−x)0.5In0.5P (x<0.5) grown by metalorganic vapor phase epitaxy
Authors: Lee Kun-Jing , Chen H. K. , Chen J. C.
Published: July 2002

The near-infrared photoluminescence (PL) of Se and Si DX centers in (AlxGa1−x)0.5In0.5P grown by metalorganic vapor phase epitaxy has been demonstrated. Two luminescence peaks with energies of 1.16 and 0.96 eV were observed. The peak positions in PL spectrum were not shifted with various Al compositions (x) in (AlxGa1−x)0.5In0.5P (x<0.5). This phenomenon is similar to that found for the thermal activation energy of Se related traps (Ec−Et=0.28 eV determined by deep level transient spectroscopy measurements) and Si related traps (Ec−Et=0.4 eV) which also do not change with different Al compositions in AlGaInP. The intensity of these peaks increased with increasing net carrier concentration. In addition, the luminescence energies of these peaks are almost identical with those observed in AlxGa1−xAs (x from 0.07 to 0.4) doped with Se and Si. This observation contradicts the large lattice relaxation model, which predicts that the energy of these two luminescence peaks should change with various host materials due to different energy band structures. On the other hand, interpreted by the small lattice relaxation model, the 1.16 and 0.96 eV luminescence bands can be attributed to an internal transition between the excited DX state and its ground state of Se and Si DX centers, respectively. The luminescence energy is dependent on impurities themselves rather than the host materials, such as AlGaAs or AlGaInP.
Step responses of radio-frequency capacitively coupled discharges
Authors: Yang Jing , Ventzek Peter L. G. , Sugawara H. , Sakai Y. , Kitamori K. , Tagashira H.
Published: July 2002

Models capturing the periodic steady-state behavior of rf capacitively coupled discharges are now commonplace. New plasma sources have been motivated by selectivity, charge-damage mitigation, and general process control needs in plasma processing of electronic materials. These new sources require models that can accurately capture the transient behavior of the plasma source. Such models are not commonplace because the behavior of transport parameters in transients is still not well understood and because the problem is inherently stiff, i.e., widely disparate time scales are important. In this paper, we present the results of an investigation of the simplest type of transient, known as a step disturbance, in a 2 cm gap parallel-plate argon discharge at 1 Torr. As examples, two classes of step transients are considered: step increases in the peak-to-peak (pp) applied voltage (300 to up to 450 V pp) and step decreases (300 to as low as 150 V pp). The resulting transients are interpreted in terms of time scales representative of electron and ion motion in the sheath, ionization dynamics, and neutral transport processes. The possibility of using these transients as a means of process identification is discussed.
Radiation response of heteroepitaxial n+p InP/Si solar cells
Authors: Walters R. J. , Messenger S. R. , Cotal H. L. , Xapsos M. A. , Wojtczuk S. J. , Serreze H. B. , Summers G. P.
Published: July 2002

The effect of 1 MeV electron and 3 MeV proton irradiation on the performance of n+p InP solar cells grown heteroepitaxially on Si (InP/Si) substrates is presented. The radiation response of the cells was characterized by a comprehensive series of measurements of current versus voltage (I–V), capacitance versus voltage (C–V), quantum efficiency (QE), and deep level transient spectroscopy (DLTS). The degradation of the photovoltaic response of the cells, measured under simulated 1 sun, AM0 solar illumination, is analyzed in terms of displacement damage dose (Dd) which enables a characteristic degradation curve to be determined. This curve is used to accurately predict measured cell degradation under proton irradiation with energies from 4.5 down to 1 MeV. From the QE measurements, the base minority carrier diffusion length is determined as a function of particle fluence, and a diffusion length damage coefficient is calculated. From the C–V measurements, the radiation-induced carrier removal rate in the base region of the cells is determined. The DLTS data show the electron and proton irradiations to produce essentially the same defect spectra, and the spectra are essentially the same as observed in irradiated homoepitaxial n+p InP. From the DLTS data, the introduction rate of each defect level is determined. From the dark I–V curves, the effect of irradiation on the various contributions to the dark current are determined. The data are analyzed, and a detailed description of the physical mechanisms for the radiation response of these cells is given. The results enable a model to be developed for the radiation response of the cells.
Simulation of microdischarges in a dielectric-barrier discharge
Authors: Li Jing , Dhali Shirshak K.
Published: July 2002

The results of simulation of microdischarges in a dielectric-barrier discharge are presented. These simulations have been carried out in two dimensions with an adaptive grid. The results presented are for oxygen at atmospheric pressure. The dielectric has little effect on the propagation of the streamerlike discharge until the very last stage. The dielectric capacitance has been found to have very little effect on the radical production efficiency.
Mobility spectrum approach in the analysis of the electrical conduction of a GaAs layer grown by molecular beam epitaxy
Authors: Regiński K. , Marczewski J. , Dziuba Z. , Grodzicka E.
Published: July 2002

A new algorithm of the analysis of the electrical conduction in the layers forming a semiconductor structure has been described. In the algorithm, in contrast to earlier approaches, the mobility spectrum technique has been applied to the sheet conductivity tensor. The standard carriers parameters have been estimated from the tensor components in the high and low magnetic field limits. The proposed method has been illustrated by the analysis of results of the electrical conduction measurements versus magnetic field performed up to 1.6 T at 77 K in a molecular beam epitaxial (MBE) grown GaAs:Si structure. The sample was an MBE GaAs:Si layer grown on a buffer epilayer. As a result of the analysis, the mobility spectra and the standard parameters describing carriers in different layers have been found.
Photoacoustic determination of recombination parameters in CdTe/glass system
Authors: Bernal-Alvarado J. , Vargas M. , Alvarado-Gil J. J. , Delgadillo I. , Cruz-Orea A. , Vargas H. , Tufiño-Velázquez M. , Albor-Aguilera M. L. , González-Trujillo M. A.
Published: July 2002

Using the open photoacoustic cell technique we analyze, for a polycrystal CdTe/glass system, the dependence upon the photoacoustic signal phase of both the rear and front semiconductor carrier surface velocities and the bulk recombination time.
Intrinsic exciton transitions in GaN
Authors: Shan W. , Fischer A. J. , Hwang S. J. , Little B. D. , Hauenstein R. J. , Xie X. C. , Song J. J. , Kim D. S. , Goldenberg B. , Horning R. , Krishnankutty S. , Perry W. G. , Bremser M. D. , Davis R. F.
Published: July 2002

Intrinsic excitonic transitions in GaN have been studied using a variety of spectroscopic measurements. Sharp spectral structures associated with intrinsic free excitons could be observed in photoluminescence, reflection, and absorption spectra. The energy positions of excitonic transitions in GaN epitaxial layers were found to be influenced by the residual strain resulting from lattice-parameter and thermal-expansion mismatches between the epilayers and the substrates. The values of the four principal deformation potentials of wurtzite GaN were derived by using the strain tensor components determined by x-ray measurements. The observation of spectral features involving the emission of LO phonons in absorption and photoluminescence excitation spectra at energies above exciton resonances indicate that a phonon-assisted indirect excitation process, which simultaneously generates a free exciton and a LO phonon, is a very significant and efficient process in GaN. The lifetime of the free excitons is found to be longer than the relaxation time of LO-phonon emission but much shorter than that of acoustic-phonon emission.
Capacitance analysis for AlGaAs/GaAs and InAlAs/InGaAs heterostructure barrier varactor diodes
Authors: Fu Y. , Dillner L. , Stake J. , Willander M. , Kollberg E. L.
Published: July 2002

By self-consistently solving Schrödinger and Poissons equations, we have investigated the capacitances of AlGaAs/GaAs and InAlAs/InGaAs heterostructure barrier varactors. When compared with semiclassical particle model, quantum mechanics show that the maximal capacitance of the varactor is saturated when the spacer and barrier are thin. When the spacer and barrier are very thick, both the quantum mechanics and semiconductor particle approach result in the same conclusion, namely, the maximal capacitance is inversely proportional to the sum of the spacer and barrier thicknesses. We have also shown that the maximal capacitance increases for high carrier effective mass.
Time-resolved surface magnetometry in the nanosecond scale using synchrotron radiation
Authors: Sirotti F. , Bosshard R. , Prieto P. , Panaccione G. , Floreano L. , Jucha A. , Bellier J. D. , Rossi G.
Published: July 2002

The dynamics of magnetization reversal at surfaces and interfaces is studied by a specific photoelectric effect experiment with pulsed soft x rays. This novel time-resolved surface magnetometry is based on the measure of the spin polarization of the total photoejected electron yield by means of a Mott scattering experiment and on the acquisition synchronism with the multibunch structure of the positron storage ring SuperAco at Orsay (500 ps pulses at 120 ns intervals). We present results on the fast magnetization reversal of Fe ultrathin films deposited on amorphous low-coercivity ferromagnetic ribbons.
Self-organized InAs/GaAs quantum dots grown by gas source molecular beam epitaxy
Authors: Chen Ming-Chin , Lin Hao-Hsiung , Shie Chih-Wei
Published: July 2002

Self-organized InAs quantum dots (QDs) grown on both (100) exact and (100) misoriented 7° toward (110) GaAs substrates using gas source molecular beam epitaxy with V/III ratio ranging from 1.1 to 20 have been studied by photoluminescence (PL) measurements from 8.5 to 300 K. The QD structures grown on the misoriented substrates show a better uniformity than those grown on the exact substrates at the same growth conditions. Effects of AsH3 flow rate on the PL intensity, peak energy, and linewidth for QDs grown on both types of substrates are presented. Basically, higher AsH3 flow rate gives higher PL intensity at 8.5 K. At room temperature (300 K), on the contrary, lower AsH3 flow rate results in higher PL intensity. The study of thermal quenching energy reveals that the larger the dot size the better the PL intensity at 300 K.
Scanning tunneling microscopy study on void formation by thermal decomposition of thin oxide layers on stepped Si surfaces
Authors: Fujita Ken , Watanabe Heiji , Ichikawa Masakazu
Published: July 2002

We investigate void formation by thermal decomposition of thin oxide layers on stepped Si(001) and Si(111) surfaces by using high-temperature scanning tunneling microscopy. We have found that the surface roughening during void formation on stepped Si surfaces is less than that on on-axis Si surfaces. The Si atoms necessary for oxide decomposition are supplied from step edges on the stepped surface rather than by hole nucleation.
Photoluminescence studies on Si-doped GaAs/Ge
Authors: Hudait M. K. , Modak P. , Hardikar S. , Krupanidhi S. B.
Published: July 2002

Photoluminescence (PL) spectroscopy has been used to study the silicon incorporation in polar GaAs on nonpolar Ge substrates. Shifts of PL spectra towards higher energy with growth temperature, trimethylgallium (TMGa) and arsine (AsH3) mole fractions were observed. The full width at half maximum increases with increasing growth temperature, AsH3 and TMGa mole fractions. The peak at 1.49 eV has been attributed to band-to-acceptor transition involving residual carbon. The PL peak energy shifts towards higher energy with increasing growth temperature due to the increase in electron concentration. A vacancy control model may explain the PL shift towards higher energy with increasing AsH3 mole fraction. The PL peak shifts towards higher energy with increasing TMGa mole fraction. The experimental results about the growth temperature, trimethylgallium, and arsine mole fractions on silicon-doped GaAs on GaAs were presented for comparison. The outdiffusion of Ge into the GaAs epitaxial layer was hardly to be seen from the secondary ion mass spectroscopy result.
Structural analysis and crystallization studies of germanium–antimony– tellurium sputtered films on different underlayers
Authors: Ohshima Norikazu
Published: July 2002

Crystallization processes and structures were analyzed, by using modified dynamical reflectance spectroscopy, x-ray diffraction measurement, and transmission electron microscopy for a germanium–antimony–tellurium (Ge–Sb–Te) ternary amorphous film on different underlayer materials: a silicon nitride (Si3N4) film and a zinc sulfide–20 mol % silicon dioxide (ZnS–SiO2) film. The crystallization temperature was almost the same (about 430 K) for each sample. Above the crystallization temperature, the Ge–Sb–Te film on the Si3N4 underlayer film exhibited a face-centered-cubic (fcc) structure and grains grew gradually with transformation to a hexagonal structure, whereas the film on the ZnS–SiO2 underlayer film exhibited mixed phases of the fcc and hexagonal structure and abrupt grain growth at 520 K. These crystallization differences are thought to be caused by the interfacial atomic arrangement of amorphous Ge–Sb–Te. This arrangement depends on the chemical affinity at the interface.
f
-Electron delocalization/localization and the abrupt disappearance of uranium magnetic ordering with dilution alloying
Authors: Cooper B. R. , Lin Y.-L.
Published: July 2002

We have applied the model and technique previously applied to the change of Curie temperature with pressure for correlated-electron uranium systems to predict the change in Curie temperature and ordered moment with dilution alloying. The theory is remarkably successful in its predictions, and this success has important implications for the overall understanding of magnetic ordering in correlated-electron systems including heavy fermion systems. For US, the dilution alloying behavior found experimentally is dramatic. In UxLa1−xS, the magnetic ordering abruptly disappears at about 55% uranium. Our ab initio-based theory quantitatively predicts this abrupt disappearance while also quantitatively predicting the monotonic decrease of Curie temperature with pressure for undiluted US. In addition, in agreement with experiment, the theory predicts the correct trend for the magnetic ordering to disappear with dilution, while at the same time absolutely and quantitatively predicting the nonmonotonic variation of Curie temperature with pressure, for USe and UTe. The ab initio-based model gives absolute material-specific predictions using input from the local density approximation paramagnetic uranium f-electron-projected density of states plus ab initio calculated values of the correlation energy U. The key physics of the model is the recognition and quantification of the concept that the f spectral density in the vicinity of a specific uranium nucleus (so-to-speak in the muffin-tin sphere) can either be in a stable f3 configuration for a long enough period of time that, through coupling to other such stable f3 sites, it can magnetically order, or can be in a situation such that the configuration fluctuates rapidly between f3 and f2, and for purposes of magnetic ordering acts like a hole in the f-electron lattice in the same way that substitution of lanthanum for uranium creates a hole. Both pressure and dilution alloying, by causing an increase in f-delocalization, increase the fraction of uranium sites in the rapidly fluctuating, magnetically ineffective, condition. For dilution alloying, at a certain point this increase in fluctuations causes the stable f3 component to fall abruptly below a critical value necessary to sustain any magnetic ordering, and hence brings about a catastrophic collapse in magnetic ordering.
Magnetic behavior of the low-dimensional compounds Ba2Cu3O4Cl2 and Ba3Cu2O4Cl2
Authors: Eckert D. , Ruck K. , Wolf M. , Krabbes G. , Müller K.-H.
Published: July 2002

Orthorhombic Ba3Cu2O4Cl2 contains folded Cu2O4 chains along the crystallographic a direction with two different Cu sites. In single crystals of this compound the magnetization measured for a field applied parallel to the a axis shows a spin-flop transition at a threshold field of 2.6 T for temperatures below TN≈20 K. Above TN a Curie–Weiss behavior is found with a paramagnetic moment of 2.0 μB per Cu atom. Parallel to the b or c axis the magnetization increases linearly with the field strength, and no metamagnetic transition was detected for these directions. It is concluded that Ba3Cu2O4Cl2 has localized magnetic moments which, at low temperatures, order antiferromagnetically parallel to the a axis. The bulklike magnetic behavior of this compound is probably caused by a strong coupling between the Cu2O4 chains. Tetragonal Ba2Cu3O4Cl2 is built up of Cu3O4 planes, also with two types of copper atoms (CuA,CuB). Its magnetization increases nearly linearly with the field. Below TNA=337 K single-crystalline Ba2Cu3O4Cl2 shows a spontaneous magnetization and ferromagnetic hysteresis for fields applied parallel to the tetragonal [100] or [110] directions. For T<TNB≈33 K, the coercive field is more than one order of magnitude larger compared to that measured in the temperature range TNB<T<TNA. The spontaneous magnetization observed in TNB<T<TNA may be attributed to weak ferromagnetism of the Dzyaloshinsky–Moriya type. The behavior of Ba2Cu3O4Cl2 at lower temperatures, T⩽TNB, is not yet understood.
Flux patterns of multifilamentary Ag-sheathed (Pb,Bi)2Sr2Ca2Cu3O10+δ tapes
Authors: Koblischka M. R. , Johansen T. H. , Bratsberg H. , Půst L. , Galkin A. , Nálevka P. , Maryško M. , Jirsa M. , Bentzon M. , Bodin P. , Vase P. , Freltoft T.
Published: July 2002

Flux patterns of multifilamentary Ag-sheathed (Pb,Bi)2Sr2Ca2Cu3O10+δ  tapes comprising 19 filaments are visualized by means of magneto-optic imaging. In low fields, the shielding currents are seen to flow mainly in the outermost filaments. With increasing external magnetic field, the inner filaments also contribute to the current flow. To compare the local flux distribution with the integral magnetization values, magnetization loops are measured by a SQUID magnetometer on the same sample following the fields used in the magneto-optic imaging (± 120 mT) and covering fields up to ±5 T at various temperatures. The magnetization loops also reveal that the multifilamentary tapes show the anomalous position of the central peak, but always less pronounced than in monofilamentary tapes.
Phase transformation and magnetic properties of Sm(Co0.91−xFexCu0.07Zr0.02)z powders for bonded magnet applications
Authors: Gong W. , Ma B. M. , Bounds C. O.
Published: July 2002

Structural and magnetic properties of Sm(Co0.91−xFexCu0.07Zr0.02)8.0, where x=0, 0.12, 0.21, 0.29, and 0.32, and Sm(Co0.64Fe0.28Cu0.06Zr0.02)z, where z ranged from 6.9 to 8.6, have been systematically investigated by x-ray powder diffraction. It was found that most of these alloys exhibit a hexagonal Th2Ni17 structure with a space group P63/mmc after an optimum solid solution treatment with rapid quench. The fully processed materials (after precipitation hardening) all exhibit a rhombohedral Th2Zn17 structure with a space group of R3̄m as the primary phase. For Sm(Co0.91−xFexCu0.07Zr0.02)8.0, the lattice parameters, a and c, and cell volume, V, increase slightly at first with increasing x, and then more sharply when x is increased above 0.28. The latter drastic increase in lattice parameters and cell volume results in a sharp decrease in the Hci, Hc, and Hk values of fully processed materials. For Sm(Co0.64Fe0.28Cu0.06Zr0.02)z, the lattice parameters, a and c, and cell volume increase slightly when z is varied from 6.8 to 7.8, then exhibit a drastic decrease when z is further increased beyond 8.0. A Br of 11.3 kG, Hci of 27 kOe, and BHmax of 30 MGOe have been obtained on Sm(Co0.63Fe0.28Cu0.07Zr0.02)8.0 after a solid solution at 1160 °C and aging at 850 °C.
Aggregate linear properties of ferroelectric ceramics and polycrystalline thin films: Calculation by the method of effective piezoelectric medium
Authors: Pertsev N. A. , Zembilgotov A. G. , Waser R.
Published: July 2002

The effective dielectric, piezoelectric, and elastic constants of polycrystalline ferroelectric materials are calculated from single-crystal data by an advanced method of effective medium, which takes into account the piezoelectric interactions between grains in full measure. For bulk BaTiO3 and PbTiO3 polarized ceramics, the dependences of material constants on the remanent polarization are reported. Dielectric and elastic constants are computed also for unpolarized c- and a-textured ferroelectric thin films deposited on cubic or amorphous substrates. It is found that the dielectric properties of BaTiO3 and PbTiO3 polycrystalline thin films strongly depend on the type of crystal texture. The influence of two-dimensional clamping by the substrate on the dielectric and piezoelectric responses of polarized films is described quantitatively and shown to be especially important for the piezoelectric charge coefficient of BaTiO3 films.
Determining 4H silicon carbide electronic properties through combined use of device simulation and metal–semiconductor field-effect-transistor terminal characteristics
Authors: Huang Mingwei , Goldsman Neil , Chang Chien-Hwa , Mayergoyz Isaak , McGarrity James M. , Woolard Dwight
Published: July 2002

A two-dimensional numerical device simulator has been developed specially for the recessed gate 4H silicon carbide(4H-SiC) metal–semiconductor field-effect-transistor (MESFET). By combining numerical techniques, material physics, and measured device characteristics, we are able to use the simulator to extract more information about the new material 4H-SiC, including the mobility, velocity-field curves, and the Schottky barrier height. We have also enabled and used the new simulator to investigate breakdown voltage and thus predict operation limitations of the 4H-SiC device. Simulations indicate that impact ionization is relatively small in 4H-SiC, thereby leading to a very high breakdown voltage of 125 V in a 0.7 μm gate MESFET.
Distribution of 1.68 eV emission from diamond films
Authors: Correia M. R. , Monteiro T. , Pereira E. , Costa L. C.
Published: July 2002

Free-standing polycrystalline chemical vapor deposition diamond films grown on a silicon wafer, with electrical behavior similar to values currently mentioned in the literature, present microheterogeneity. A detailed analysis by micro Raman shows how the diamond and nondiamond phases are distributed within the film and also the distribution of the silicon related luminescence. This luminescence is discussed in terms of two emitting centers close in energy. Absolute intensity of the diamond peak is not correlated with the good quality of the film as assessed by the Raman linewidth and ratio of this line to the nondiamond Raman lines.
Characterization of a graded index of refraction separate confinement heterostructure (GRINSCH) laser structure using contactless electroreflectance
Authors: Krystek Wojciech , Leibovitch M. , Sun W. D. , Pollak Fred H. , Gumbs Godfrey , Burnham G. T. , Wang X.
Published: July 2002

Using contactless electroreflectance we have characterized the complete potential profile of a pseudomorphic 0.98 μm InGaAs/GaAs/GaAlAs graded index of refraction separate confinement heterostructure (GRINSCH) laser fabricated by molecular beam epitaxy. Signals were detected from all three relevant portions of the sample. Comparison of the observed transitions from the InGaAs single quantum well (SQW) section with an envelope function calculation (including the effects of strain and electric field) made it possible to evaluate the In composition and width of the SQW. These values are in good agreement with the intended parameters. The energy of 11HH, the fundamental conduction to heavy-hole level (which is closely related to the lasing frequency) can easily be determined to less than ±1 meV at 300 K. From the period of the observed Franz–Keldysh oscillations (FKOs) originating in the GaAlAs GRINSCH region it was possible to directly evaluate the built-in electric field in the structure. We also present a general expression for FKOs in the presence of both electric fields (due to charge) and the “fields” (potential gradient) due to compositional grading. The temperature dependence (20 K<T<320 K) of quantum transitions from the InGaAs SQW has been evaluated. Our analysis has emphasized the 11HH transition because it can provide information on the operational temperature of the laser.
Texture generation by postdeposition annealing in a temperature gradient
Authors: Rauf I. A. , Egerton R. F. , Sayer M.
Published: July 2002

A simple process for improving texture and electrical mobility of polycrystalline films is described. The technique involves annealing the film in a controlled temperature gradient and produces chainlike structures of impurity-free grains; dopants and other defects are confined to grain boundaries. The diffusion of impurities, as well as self-diffusion of the host lattice, produces a combing effect and causes grain reorientation in the annealed films. The electrical sheet conductance increases with increasing temperature gradient but attains an optimum value, above which the conductance falls due to microcracking of the film.
Nitrogen ion implanted ZnSe/GaAs p-i-n photodetectors
Authors: Hong H. , Anderson W. A. , Haetty J. , Lee E. H. , Chang H. C. , Na M. H. , Luo H. , Petrou A.
Published: July 2002

p-i-n photodiodes were fabricated on nitrogen ion implanted undoped ZnSe/n-type ZnSe epilayers grown on n+GaAs (100) substrates by molecular beam epitaxy. To obtain a quasi-uniform p layer doping profile, nitrogen ions at multiple energies and ion doses were implanted at room temperature. The activation of implanted species was carried out by an optimized post-annealing in a nitrogen ambient. Optical studies were performed on the implanted/annealed devices by photoluminescence spectroscopy at 10 K, which indicated donor–acceptor pairs at an energy of 2.7 eV and its phonon replicas with 30 meV intervals. The circular p-i-n diodes with a 1 mm diam contact area showed a device breakdown voltage to be linearly dependent on the thickness of the undoped ZnSe epilayer. For p-i-n diodes fabricated on an initial 0.5 μm thick undoped ZnSe layer, an ideality factor of 1.19 and a reverse bias breakdown voltage of 12 V was observed. A large photocurrent, good linearity with light intensity, and low dark current were observed. A photocurrent/dark current ratio >105 was obtained at an illumination intensity of 100 mW/cm2. These devices exhibited a responsivity of 0.025 A/W at a wavelength of 460 nm through the top 200 Å thick metal contacts.
Formation effects and optical absorption of Ag nanocrystals embedded in single crystal SiO2 by implantation
Authors: Liu Zhengxin , Li Hao , Feng Xiaodong , Ren Shuguang , Wang Honghong , Liu Zhenghui , Lu Baofu
Published: July 2002

Ag + ions of 200 keV were implanted into single crystal SiO2 at room temperature to five different doses: 5×1015, 2.3×1016, 4.5×1016, 5.6×1016, and 6.7×1016/cm2. With increasing dose, the implanted Ag distributions change from usual Gaussian-type profiles to abnormal bimodal profiles with narrow full width at half maximum, which are associated with Ag nanoparticles forming during high dose implantation. The implanted Ag depth profile evolution with dose can be clearly observed during Rutherford backscattering spectroscopy analysis. The nanoparticles form dual-layer structures at high doses: as far as the dose of 6.7×1016/cm2 is concerned, transmission electron microscopy proves that the shallower implanted layer contains noninteracting small Ag nanoparticles with the diameters of about 7 nm; the deeper layer contains a high density of interacting large nanoparticles with the diameters of about 25 nm. High resolution electron microscopy identifies that the nanoparticles are perfect single crystals. Although plasmon resonance frequency of the Ag nanoparticles formed at relatively low dose agrees well with the Mie’s theoretical prediction, great redshift due to multipole interactions between high density nanoparticles occurs for high doses, moreover, the magnitude of redshift increases with implanted dose.
Electrical characterization of He-plasma processed n-GaAs
Authors: Auret F. D. , Meyer W. E. , Deenapanray P. N. K. , Goodman S. A. , Myburg G. , Murtagh M. , Ye Shu-Ren , Crean G. M.
Published: July 2002

We employed capacitance-voltage (C–V) measurements to determine the free-carrier concentration changes in n-GaAs after processing it in a He plasma, and deep-level transient spectroscopy (DLTS) to study the electrical properties of the plasma-induced defects. C–V measurements indicated that He-plasma processing resulted in a strong carrier reduction up to 1 μm below the GaAs surface. DLTS showed that He-ion processing introduced several prominent defects, including the frequently studied radiation-induced defects E1 and E2, associated with VAs. Current-voltage measurements demonstrated that the He-plasma processing inhibits the fabrication of high barrier Schottky diodes on n-GaAs.
Structural characteristics of CeO2 films grown on biaxially textured nickel (001)
Authors: Wang Rong-ping , Zhou Yue-liang , Pan Shao-hua , Zhang Hao , Guo Xiang-xin , Xiong Xu-ming , Lu Hui-bin , Zhen Zheng-hao , Yang Guo-zhen
Published: July 2002

Preferential (001) orientation CeO2 films have been successfully grown on biaxially textured Ni substrates using ion beam assisted pulsed laser deposition. The x-ray diffraction (XRD) patterns for the CeO2 films and target have been measured. The in-plane lattice constants have been derived from the data of XRD. It was found that the in-plane lattice constants of the films increase with increasing deposition temperature. A reasonable explanation based on the difference of the linear thermal expansion coefficient between the CeO2 film and Ni substrate was proposed. Moreover, Raman spectra of the films and target have been recorded. It was shown that the oxygen deficiency in the CeO2 samples has a great effect on the full width at half maximum of the Raman lines. According to the relation between the Gruneisen shift and the lattice contraction, we have obtained the theoretical Raman shifts which are in good agreement with the experimental results.