bluuebunny/crossref_metadata_2025 huggingface dataset:
Temperature dependence of optical properties of the deep sulfur center in silicon
Authors: Pettersson H. , Pässler R. , Blaschta F. , Grimmeiss H. G.
Published: July 2002

Spectral distributions of the hole photoionization cross section of the deep sulfur center in silicon have been measured at 10 different temperatures within the range 75 K≤T≤297 K applying the steady-state photocurrent technique. Zero-phonon hole binding energies of the deep donor level have been determined at these temperatures by using a detailed numerical fitting procedure. The temperature dependence of the hole binding energy is well described by a novel analytical formula with a zero-temperature binding energy of 557 meV. Further analysis of our data resulted in a lattice adjustment energy (Franck–Condon shift) of 51 meV and an associated average phonon energy of 33 meV.
The role of defects in the rear side laser ablation of MgO at 308 nm
Authors: Webb R. L. , Langford S. C. , Dickinson J. T.
Published: July 2002

Over a range of fluences, 308 nm pulsed laser irradiation of cleaved, single crystal MgO can produce a fluorescent plume on the side of the specimen opposite the incident light (rear side ablation). After one or two laser pulses, linear patterns of melted material are often observed. We present evidence that Fresnel diffraction from heterogeneous features on the front surface control the formation of the localized interactions on the rear side. At fluences below the threshold for plume formation on the front surface, large areas of the rear surface can be melted and ablated without fracture and pitting. At higher fluences, defect accumulation on and near the front surface eventually yields a front surface plume and eliminates the rear surface plume. Mechanical treatments strongly affect the evolution of these features due to the production of strongly absorbing point defects.
Red electroluminescence from a thin organometallic layer of europium
Authors: Campos R. A. , Kovalev I. P. , Guo Y. , Wakili N. , Skotheim T.
Published: July 2002

We investigate the performance of organic electroluminescence devices employing a fluorinated diketone complex of europium (ETP) as the emitter material. The architecture of our devices isolates the emitter from the injecting contacts by sandwiching a thin layer of ETP between a hole-transporting diamine layer and an electron-transporting aluminum complex layer. The organic layers are deposited in high vacuum with rate-controlled sources onto glass substrates coated with indium–tin–oxide, and the cell is completed by evaporation of aluminum or calcium–aluminum cathodes. By varying the thickness of ETP layer in increments of 6 Å we demonstrate spatial confinement of the electroluminescence emission zone and optimal performance for an ETP thickness of 50 Å. Both the optical and electrical characteristics of these cells follow steep power-law relationships with voltage, which are indicative of trap-modified, space-charge-limited conduction. With aluminum cathodes we routinely achieve luminances up to 10 cd/m2 with direct current densities near 40 mA/cm2. The electroluminescence has a red–orange color and exhibits a narrow spectrum that is characteristic of trivalent europium ions.
Transmission electron microscopy studies of crystal-to-amorphous transition in ion implanted silicon
Authors: Ishimaru Manabu , Harada Shinsuke , Motooka Teruaki
Published: July 2002

The microstructure of 5 MeV ion implanted silicon at room temperature has been investigated in detail by means of cross-sectional transmission electron microscopy. Buried amorphous layers were observed in the specimens obtained by ion doses of 1×1017 and 2×1017/cm2 with abrupt amorphous-to-crystal interfaces. Damaged layers adjacent to the amorphous layers included many dislocation loops and the concentration increased toward the amorphous region. Microdiffraction patterns and high-resolution images showed that this damaged region is defective crystalline silicon, suggesting that homogeneous amorphization occurs due to an accumulation of defects. The atomistic structure of the damaged regions was analyzed by comparing high-resolution electron microscopy images with those calculated on the basis of a model for amorphization processes proposed previously [T. Motooka, Phys. Rev. B 49, 16 367 (1994)].
Shear stress measurements in copper, iron, and mild steel under shock loading conditions
Authors: Millett J. C. F. , Bourne N. K. , Rosenberg Z.
Published: July 2002

A series of experiments have been conducted on metals subjected to planar impact loading in which a biaxial stress state and a uniaxial strain state is induced. Longitudinal and transverse stresses have been measured in copper, iron, and mild steel, using manganin stress gauges. The results have been used to calculate shear stress from the difference between the stress components. Results indicate that copper displays an increase in shear stress with pressure, showing similar trends to other work. An increase in dislocation density has been suggested as a possible mechanism. Iron shows a constant shear stress with increasing pressure, again in accordance with other workers. Finally, mild steel has been observed to have a significant increase in shear stress with increasing pressure. The inclusion of a hard second phase in the microstructure is thought to produce a large amount of dislocation debris, again explaining the observed hardening.
Molecular dynamics simulation of electron trapping in sapphire
Authors: Rambaut C. , Oh K. H. , Jaffrezic H. , Kohanoff J. , Fayeulle S.
Published: July 2002

Most phenomenological aspects of energy storage and release in dielectric materials caused by the charge trapping mechanism have recently been rationalized using the space charge model. We studied dynamical aspects by performing molecular dynamics simulations. We show that an excess electron introduced into the sapphire lattice (α-Al2O3) can be trapped only at a limited number of sites. The energy gained by allowing the electron to localize in these sites is of the order of 4–5 eV, agreeing well with the results of the space charge model. Displacements of the neighboring ions due to the implanted charge are shown to be localized in a small region of about 5 Å. Detrapping is observed at 250 K. The ionic displacements turn out to play an important role in modifying the potential landscape by dynamically lowering the barriers that cause localization at low temperatures.
On the origin of the uniaxial anisotropy in nanocrystalline soft-magnetic materials
Authors: van de Riet E. , Klaassens W. , Roozeboom F.
Published: July 2002

Annealing induced uniaxial anisotropy in soft-magnetic nanocrystalline thin films of FeTaN and FeNbSiN has been investigated. It was found that there are several contributions to the uniaxial anisotropy. The direction and the size of some of these can be changed reversibly at temperatures lower than 200 °C, whereas much higher temperatures are needed for others. The nitrogen atoms in these alloys play an important role in the uniaxial anisotropy. The cooling down period is extremely important for the induction of the uniaxial anisotropy. Control of uniaxial anisotropy through appropriate annealing is critical to the application of these materials in magnetic recording devices. Knowledge of the induction, manipulability, and of the thermal stability is essential for finding the optimum annealing procedure and for the application of the material in a magnetic device.
Characterization of hard magnetic two-phase mechanically alloyed Sm2Fe17N3/α-Fe nanocomposites
Authors: O’Donnell K. , Coey J. M. D.
Published: July 2002

A range of exchange-coupled two-phase nanocomposites composed of hard magnetic Sm2Fe17N3 and soft magnetic α-Fe was prepared by mechanical alloying with a view to optimizing the hysteresis loop shape. The main variables were the crystallization conditions, the nitriding treatment, and the chemical additives. A model of the diffusion of nitrogen in the two-phase nanocomposite is proposed that explains how the presence of Fe permits the nitrogenation of samples at lower temperatures than in single phase Sm2Fe17 materials. Studies of samples both resin bonded and cold compacted measured in open and closed circuits revealed that the correct choice of demagnetizing factor used to correct demagnetizing fields depends critically on the sample density. Transmission electron microscopy (TEM) studies of the materials prepared revealed grain sizes in the range 10–50 nm. The shape of the magnetic hysteresis loop and resulting magnetic properties reflects the grain size of both phases. Image analysis of high resolution scanning electron microscopy micrographs of etched samples showed that in general two to three soft grains cluster together and are surrounded by hard grains, but the grain sizes of both phases were found to be the same. The crystallization of the hard phase from the mainly amorphous precursor is the primary factor determining grain size. Zr and Ta were the most successful additives in controlling the grain growth during crystallization, reducing the grain size from 20–30 to 10–20 nm. High resolution TEM indicated the presence of a grain boundary phase between the crystallites of the two phases. This phase was confirmed in Mössbauer studies of samples where it seems to constitute 15 vol % of the samples and has a significant effect on the coupling between the two phases. Susceptibility measurements are an effective indicator of the degree of coupling between the hard and soft magnetic phases.
Magneto-optical spin spectroscopy in hybrid ferromagnetic semiconductor heterostructures
Authors: Crowell P. A. , Nikitin V. , Awschalom D. D. , Flack F. , Samarth N. , Prinz G. A.
Published: July 2002

We report on magneto-optical measurements of diluted magnetic semiconductor quantum well structures in which an iron film is epitaxially grown and subsequently patterned on top of the heterostructure. The magnetic field due to the iron penetrates the quantum well near the edges of the patterned regions. Photoluminescence, Hanle effect, and time-resolved Faraday rotation measurements demonstrate that the films can be processed without significantly altering the electronic properties of the quantum well. The smallest length scale probed in these far-field optical measurements is of order 10 μm, so that no effects of the local magnetic fields at the film edges are observed for structures with 50-nm-thick ferromagnetic layers.
Effects of sample size and field orientation on pseudo-Hall voltage in micronscale nickel thin-film squares
Authors: Jia Y. Q. , Kong Linshu , Shi Rick C. , Chou Stephen Y.
Published: July 2002

Pseudo-Hall effect (PHE) in Ni thin-film squares of 1–5 μm size is measured with a constant current through two leads along one diagonal of the square and the voltage output from leads along the other diagonal. The PHE voltage in response to an in-plane magnetic field depends on the square size and field orientation. The minimum PHE voltage at low field is close to zero only with the 2 μm square containing four symmetrical closure domains leading to a 600% relative change in PHE voltage. The PHE signal is found the largest when the field direction is along the square side while the smallest when along the square diagonal.
Nuclear magnetic resonance investigations of Co nanoclusters in a SiO2 thin film matrix
Authors: Thomson T. , Riedi P. C. , Sankar S. , Berkowitz A. E.
Published: July 2002

Co 59 nuclear magnetic resonance (NMR) spectra of small Co particles in a SiO2 matrix have been measured. In zero field, strong signals were obtained in the frequency range of 210–250 MHz. The Co59 NMR frequency in fcc multi (single) domain particles is 217.4 (223) MHz. Possible contributions to the Co hyperfine field and changes to the Co magnetic moment in nanoclusters are discussed. An enhanced magnetic moment could be responsible for an increase in the hyperfine field leading to the observed NMR line at ∼243 MHz. Measurements of the transverse relaxation time (T2) as a function of temperature and field are reported.
Spin tunneling in Ni–Fe/Al2O3/Co junction devices (invited)
Authors: Miyazaki Terunobu , Kumagai Seiji , Yaoi Toshihiko
Published: July 2002

Magnetoresistive properties of a Ni–Fe/Al–oxide/Co junction with various junction area between 3.5×3.5 μm2 and 85×85 μm2 have been studied. Antiferromagnetic interlayer exchange coupling between magnetic electrodes, which was predicted theoretically by Slonczewski was found experimentally. The antiferromagnetic exchange coupling strength increased with reduction of the junction area. The tunneling magnetoresistive effect tends to increase with reduction of the junction area. However, it was much smaller than the tunneling magnetoresistance ratio expected from the spin polarizations of electrodes. The reasons were discussed briefly.
Elastic properties of single- and multi-domain crystals of LiTaO3
Authors: Kushibiki Jun-ichi , Takanaga Izumi
Published: July 2002

A basic study of the differences in elastic properties between single- and multi-domain crystals of LiTaO3 is conducted by measuring the longitudinal velocities for X-, Y-, and Z-cut specimens and the shear velocities for Z-cut specimens. Two sets of X-, Y-, and Z-cut specimens are prepared: one is for normal (single-domain) specimens and the other is for perfectly depoled (multi-domain) specimens. For the longitudinal wave propagation directions along the Y and Z axes, the velocities for the single-domain specimens, due to piezoelectricity, are 91.5 m/s (1.59%) and 92.7 m/s (1.50%) larger than those for the multi-domain specimens, respectively. On the other hand, for the X-axis longitudinal and Z-axis shear wave propagation directions, piezoelectricity is uncoupled, but the very interesting velocity changes are that the longitudinal and shear velocities for the single-domain specimens are 48.4 m/s (0.87%) and 135.6 m/s (3.79%) smaller than those for the multi-domain specimens, respectively. These differences are introduced by the poling operation for multi-domain LiTaO3 crystals. Four elastic constants of c11, c33, c44, and c14 in the multi-domain state are determined.
The role of NiAl underlayers in longitudinal thin-film recording media
Authors: Ross C. A. , Nolan T. P. , Ranjan R. , Lu D.
Published: July 2002

Longitudinal recording media made with chromium and with nickel-aluminum underlayers are compared. NiAl films have smaller grains and random crystallographic orientation, while Cr films have larger grains and either a {110} or a {200} preferred orientation depending on the deposition conditions. Cobalt alloy films grown on NiAl have nearly random crystallographic orientation and significant c-axis out-of-plane component, and lower hysteresis loop squareness and coercivity than films made on a heated Cr underlayer. NiAl-underlayer media do not exhibit in-plane magnetic anisotropy induced by the substrate texture lines. CoCrTaPt/Cr/NiAl media had higher overwrite than CoCrTaPt/Cr media due to higher coercive squareness and a sharper hysteresis loop closure.
Decreased electrochromism in Li-intercalated Ti oxide films containing La, Ce, and Pr
Authors: Kullman L. , Azens A. , Granqvist C. G.
Published: July 2002

Films of Ti–La oxide, Ti–Ce oxide, and Ti–Pr oxide were produced by reactive dc magnetron sputtering. Their composition was determined by Rutherford backscattering spectrometry. X-ray diffractometry and infrared absorption spectroscopy indicated that the microstructure was heavily disordered and in most cases Ti-oxidelike. Electrochemical Li intercalation/deintercalation was studied by cyclic voltammetry, and ensuing optical data were recorded by spectrophotometry. Ce addition diminished the electrochromism, and films with Ce/Ti atom ratios exceeding 0.3 were almost fully transparent irrespective of their lithiation, pointing at the potential applications of such films as counter electrodes in transparent electrochromic devices. The optical and electrochemical data were discussed in terms of a model based on electron insertion/extraction in 4f states located in the gap between the valence and conduction bands of CeO2.
Hall effect in a highly inhomogeneous magnetic field distribution
Authors: Bending S. J. , Oral A.
Published: July 2002

Two-dimensional Hall probes are becoming increasingly popular as “local” magnetometers for ferromagnetic and superconducting materials. In many applications, the magnetic field at a sample surface varies on a length scale much smaller than the Hall probe dimensions, and data interpretation requires a quantitative model of the Hall voltage in this situation. We present here a classical numerical model of the Hall effect in a strongly inhomogeneous magnetic field and show how a response function can be defined to calculate the Hall voltage for an arbitrary magnetic-field distribution. The results are successfully applied to recent scanning Hall probe microscopy data on superconducting vortices.
Transport properties of intrinsic hydrogenated amorphous silicon produced by the hot-wire technique investigated by the photomixing technique
Authors: Dong S. , Tang Y. , Liebe J. , Braunstein R. , Crandall R. S. , Nelson B. P. , Mahan A. H.
Published: July 2002

The transport properties of hydrogenated amorphous silicon (a-Si:H) with a hydrogen content ranging from 12% to less than 1%, which were produced by the hot-wire technique, varying the deposition substrate temperature, 290 °C<TS<400 °C, were systematically studied by the photoconductive frequency mixing technique. With an increase of the deposition substrate temperature, and consequent decrease of hydrogen content, the photoconductivity, σpc, and the drift mobility, μd, are found to decrease, while the width of the conduction band tail, ε, increases. Continuous degradations of photoconductivity, drift mobility, and photomixing lifetime, τ, were found during light soaking experiments. In addition, it was found that the drift mobility increases and the photomixing lifetime decreases with an increase of the applied electric field, while the photoconductivity is essentially independent of the electric field within the range of 1000–10 000 V cm−1. Furthermore, the electric field dependence of the drift mobility in the annealed state is always larger than in the light-soaked state. The results for the electric field dependence are explained using the model of long-range potential fluctuations, whose range can be determined by employing an analysis previously developed.
Bias formation in a pulsed radiofrequency argon discharge
Authors: Smith H. B. , Charles C. , Boswell R. W. , Kuwahara H.
Published: July 2002

A one dimensional (1D) particle-in-cell (PIC) computer simulation has been used in conjunction with a small experimental plasma reactor, to investigate the effects of pulsing on a low pressure, capacitively coupled, rf argon plasma. In particular this article investigates the time-constants involved in the development and evolution of the bias voltage in asymmetric reactor geometry. Surprisingly, the charging time for the blocking capacitor does not occur on electron time scales, but is influenced primarily by the ambipolar drift of ions to the earthed electrode. It is shown that following plasma breakdown there is a net current flow in the system which charges the blocking capacitor in the external matching circuit and produces the bias voltage. Both the PIC simulation and the experimental measurements show that a net current flow is produced by a delay in the onset of the electron current to the earthed electrode, which is correlated to the charging time of the capacitor. From the simulation it can be seen that during this period the plasma potential in the center of the discharge is higher than one would expect, preventing electrons from reaching the earthed electrode.
Low voltage electron emission from calcium carbonate whiskers coated with a thin layer of gold
Authors: Zhou D. , Krauss A. R. , Gruen D. M.
Published: July 2002

Cold cathode electron field emission from aragonite CaCO3 whiskers coated with 10-nm-thick gold has been observed. The microstructure of the whiskers grown on a Ni substrate by electrochemical deposition has been examined by scanning electron microscope, energy dispersive x-ray spectrometer, x-ray diffraction spectrometer, and Raman spectroscopy. For a 220 μm anode-cathode gap, emission current densities in excess of ∼2×10−6 A/cm2 are observed for applied voltages of 660 V or greater. Although it is believed that the electric field is locally enhanced by the geometry of the whiskers, the voltage required increases roughly linearly with the anode-cathode spacing, corresponding to a turn-on field of approximately 3 V/μm, and an emission current density of 0.4 mA/cm2 has been obtained for an applied field of 5.5 V/μm.
Broadening of x-ray powder diffraction lines under nonhydrostatic stress
Authors: Funamori Nobumasa , Funamori Miho , Jeanloz Raymond , Hamaya Nozomu
Published: July 2002

Examining the effect of nonhydrostaticity on the shape of x-ray powder diffraction lines shows that a uniaxial stress field, as is generated in opposed-anvil type high-pressure apparatuses, can result in lines being split or having an asymmetric shape. The distribution of local stresses, generated by mismatches in shapes of neighboring grains within the specimen, results in orientation-dependent broadening: diffraction linewidths are generally proportional to 1/E(hkl), where E(hkl) is Young’s modulus for plane hkl. Since anomalous diffraction patterns can be misinterpreted as indicating a phase transformation, the occurrence of new phases under nonhydrostatic pressure should be carefully confirmed.
Properties of epitaxial ferroelectric PbZr0.56Ti0.44O3 heterostructures with La0.5Sr0.5CoO3 metallic oxide electrodes
Authors: Wang Fan , Leppävuori Seppo
Published: July 2002

PbZr 0.56 Ti 0.44 O 3 (PZT) epitaxial ferroelectric thin films on a LaAlO3 (100) substrate, covered by a metallic oxide electrode La0.5Sr0.5CoO3 (LSCO) are demonstrated in this work. The films are fabricated by the sol–gel method and effort was focused on thermal processing to obtain the desired epitaxial heterostructure. The dielectric and ferroelectric properties of PZT thin films were measured and it was found that they are as good as in the films deposited by other thin-film methods. The dielectric constant and the dissipation factor of the PZT films are, respectively, about 500 and 0.06 below 20 kHz. The remanent polarization Pr is about 27 μC/cm2 and the coercive field Ec is about 50 kV/cm. It was found that the ferroelectric properties were significantly influenced by the microstructure of the PZT layers. The present study also showed that the fatigue characteristics of the epitaxial heterostructure with LSCO electrodes under a reversed electrical field are far superior to those obtained with a polycrystalline ferroelectric layer on Pt bottom electrodes. A low leakage current, about 3 μA/cm2 at 0.5 MV/cm, was obtained for these epitaxial films. In addition, the optical transmittance spectrum of PZT thin films was also measured and the conduction mechanism is discussed.
Dimension scaling of 1/f noise in the base current of quasiself-aligned polysilicon emitter bipolar junction transistors
Authors: Llinares P. , Celi D. , Roux-dit-Buisson O. , Ghibaudo G. , Chroboczek J. A.
Published: July 2002

Experimental results on low frequency noise in quasiself-aligned bipolar n-p-n junction transistors, with widely varying emitter/base junction dimensions are presented and compared with former results obtained on devices of the same type. The power spectral density of base current fluctuations was found to depend linearly on the inverse of the area of the emitter/base interface junction, implying localization of the low frequency noise sources on the interface, rather than on the transistor perimeter. An application of current-to-voltage converters for studies of current fluctuations is also discussed and compared with a more conventional technique.
High-energy electron–electron interactions in silicon and their effect on hot carrier energy distributions
Authors: Chang M. Y. , Dyke D. W. , Leung C. C. C. , Childs P. A.
Published: July 2002

This paper presents results from the calculation of the high-energy electron–electron scattering rate in silicon based on a full energy-band structure obtained by the pseudopotential technique. The effects on the scattering rate of the overlap integrals, wave-vector-dependent dielectric function and umklapp processes are described and the transition rate is compared with that obtained using a semiclassical analysis based on a parabolic energy dispersion. A hybrid Monte Carlo/iterative technique for solving the Boltzmann transport equation is used to obtain the electron energy distribution function generated by binary particle interactions in a one-dimensional system.
Substitutional carbon in Si1−yCy alloys as measured with infrared absorption and Raman spectroscopy
Authors: Meléndez-Lira M. , Menéndez J. , Kramer K. M. , Thompson M. O. , Cave N. , Liu R. , Christiansen J. W. , Theodore N. D. , Candelaria J. J.
Published: July 2002

We present a study of the infrared absorption and Raman scattering intensity of the local carbon mode in Si1−yCy alloys grown by direct carbon implantation followed by different recrystallization procedures. For the case of laser-induced recrystallization, the integrated infrared absorbances are found to agree with an extrapolation of the calibration curve previously determined for very low substitutional carbon concentrations in Si. We argue that this finding provides strong evidence for the achievement of nearly perfect substitutionality in laser-recrystallized films, even though their carbon concentrations are three orders of magnitude beyond the solubility limit of carbon in Si. This conclusion is found to be consistent with measurements of the intensity of defect-induced Si Raman scattering relative to the Raman intensity of the local carbon mode. The Raman intensity of the local carbon mode at 605 cm−1 relative to the first-order Si Raman line at 521 cm−1 provides an ideal spectroscopic tool for the determination of substitutional carbon concentrations. By correlating Raman and infrared measurements, we find that for laser excitation at 488 nm the intensity ratio is given by I605/I521=(3.7±0.2)ysub.
Transient input of an electromagnetic wave into an open waveguide coated with nonstationary plasma film
Authors: Bakunov M. I. , Maslov A. V.
Published: July 2002

A technique to input an electromagnetic wave into an open planar waveguiding structure is presented. That is, we propose to coat the waveguide with a thin semiconductor film whose free electron density is controlled by laser pulses. The transient processes which follow the electron density shift in the coating film give rise to the effective coupling between the incident electromagnetic wave and local modes of the waveguiding structure and consequently lead to the creation of the guided modes. We analyze frequencies and amplitudes of the excited guided waves as well as characteristics of the transient radiation from the coating film. Due to the relative simplicity, small size, and as a result fast temporal response, the proposed scheme may find application as a part of integrated circuits for the submillimeter wave range.
Graded InxGa1−xAs/GaAs 1.3 μm wavelength light emitting diode structures grown with molecular beam epitaxy
Authors: Bulsara M. T. , Yang V. , Thilderkvist A. , Fitzgerald E. A. , Haüsler K. , Eberl K.
Published: July 2002

In this study 1.1–1.3 μm wavelength light emitting diode (LED) structures with InxGa1−xAs compositionally graded buffers were grown on GaAs substrates with molecular beam epitaxy and characterized using microstructure and discrete device characterization techniques. The growth temperature and design of the graded buffer greatly affect the luminescence properties of the active device region above the graded buffer. These effects were most prominent in LED structures which incorporated a quantum well in the active device region. In quantum well devices, bright luminescent bands in the 〈110〉 directions are revealed under cathodoluminescence investigation and the number of bands and their intensity depend on grading rate. This study shows that a high threading dislocation density generated at or above the quantum well region has an adverse effect on the I–V characteristics of the diodes, but did not hinder luminescence from the quantum well LEDs.
The effects of the magnitude of the modulation field on electroreflectance spectroscopy of undoped-n+ type doped GaAs
Authors: Wang D. P. , Huang K. M. , Shen T. L. , Huang K. F. , Huang T. C.
Published: July 2002

The electroreflectance (ER) spectra of an undoped-n+ type doped GaAs has been measured at various amplitudes of modulating fields (δF). Many Franz–Keldysh oscillations were observed above the band gap energy, thus enabling the electric field (F) in the undoped layer to be determined. The F is obtained by applying fast Fourier transformation to the ER spectra. When δF is small, the power spectrum can be clearly resolved into two peaks, which corresponds to heavy- and light-hole transitions. When δF is less than ∼1/8 of the built-in field (Fbi∼77 420 V/cm), the F deduced from the ER is almost independent of δF. However, when larger than this, F is increased with δF. Also, when δF is increased to larger than ∼1/8 of Fbi, a shoulder appears on the right side of the heavy-hole peak of the power spectrum. The separation between the main peak and the shoulder of the heavy-hole peak becomes wider as δF becomes larger.
Relationship between plasma parameters and film microstructure in radio frequency magnetron sputter deposition of barium strontium titanate
Authors: Panda B. , Dhar A. , Nigam G. D. , Bhattacharya D. , Ray S. K.
Published: July 2002

Radio frequency magnetron sputtered Ba0.8Sr0.2TiO3 thin films have been deposited on silicon and Si/SiO2/SiN/Pt substrates. The analysis of plasma discharge has been carried out using the Langmuir probe technique. Both the pressure and power have been found to influence the ion density and self-bias of the target. Introduction of oxygen into the discharge effectively decreases the ion density. The structural and electrical properties have been investigated using x-ray diffraction, atomic force microscopy of deposited films and capacitance–voltage, conductance–voltage, and current density–electric field characteristics of fabricated capacitors. The growth and orientation of the films have been found to depend upon the type of substrates and deposition temperatures. The 〈100〉 texture in the film is promoted at a pressure 0.25 Torr with a moderately high value of ion density and low ion bombardment energy. Films deposited on Si/SiO2/SiN/Pt substrate have shown higher dielectric constant (191) and lower leakage current density (2.8×10−6 A/cm2 at 100 kV/cm) compared to that on silicon.
Reflection-mode scanning near-field optical microscopy: Influence of sample type, tip shape, and polarization of light
Authors: Durkan C. , Shvets I. V.
Published: July 2002

A reflection-mode aperture-type scanning near-field optical microscope (R-SNOM) based on the external collection of the reflected light is presented. The light detection is based on an elliptical mirror setup, with the tip and sample at one focus, and a photomultiplier tube at the other. Results are presented on the general imaging properties of this microscope. The results presented concentrate on an analysis of the gap-width dependence of the optical signal, on resolution and on the contrast mechanisms which may be used in R-SNOM imaging, including reflectivity, polarization/magneto-optics, and topographic effects.
Frequency response of molecularly thin alternating current light-emitting diodes
Authors: Österbacka R. , Pal A. J. , Källman K.-M. , Stubb H.
Published: July 2002

The frequency response of molecularly thin alternating-current polymeric light-emitting diodes has been studied. Langmuir–Blodgett (LB) films of poly(3-hexylthiophene) (PHT) were used as the active emitting material and the device was formed by sandwiching PHT films between LB films of emeraldine base polyaniline. As a step towards molecular electronic devices, we have shown that even two molecular layers of PHT (≈6 nm) are sufficient for light emission. The high frequency operation limit of the device has been discussed in terms of a charge accumulation process at the polymer–polymer interface. The electroluminescence (EL) spectra of different structures have been compared with corresponding photoluminescence spectra. A significant blueshift in EL has been observed in thinner structures and its origin has been discussed.
Thermal expansion of LaAlO3 and (La,Sr)(Al,Ta)O3, substrate materials for superconducting thin-film device applications
Authors: Chakoumakos B. C. , Schlom D. G. , Urbanik M. , Luine J.
Published: July 2002

The thermal expansion for the perovskite (La,Sr)(Al,Ta)O3, i.e., LSAT, grown from the formulation 0.29(LaAlO3):0.35(Sr2AlTaO6), was determined by Rietveld refinement of neutron powder diffraction data over the temperature range of 15–1200 K. In comparison to LaAlO3 the relative volume thermal expansion is the same, although the cell volume of LSAT is slightly larger. Site occupation refinement for LSAT gives a structural formula of (La0.29(5)Sr0.71(5))A site(Al0.65(1)Ta0.35(1))B siteO3. At and below 150 K, LSAT shows a small distortion from cubic symmetry. Unlike the cubic-to-rhombohedral transition (800 K) observed in LaAlO3, the low temperature structural phase transition in LSAT appears to be cubic-to-tetragonal or cubic-to-orthorhombic. The rms displacement of the A site in LSAT is significantly larger than that for LaAlO3, and about half of the difference can be accounted for by a static displacement component.
X-ray diffraction measurement of segregation-induced interface broadening in InxGa1−xAs/GaAs superlattices
Authors: Yashar P. , Pillai M. R. , Mirecki-Millunchick J. , Barnett S. A.
Published: July 2002

X-ray diffraction θ–2θ scans and kinematical simulations were used to study the composition modulation in coherently strained ten period InxGa1−xAs/GaAs superlattices grown by molecular beam epitaxy (MBE). Superlattice periods (Λ) were 34.5, 55.0, and 92.0 Å, each with nominally 10-Å-thick In0.5Ga0.5As layers. Simulations using exponential In composition modulations, as expected due to In segregation during MBE growth, produced the best fits to the experimental data. The measured 1/e segregation lengths ranged from 11 to 16 Å. Since this distance was larger than the InGaAs layer thickness, the maximum In concentration reached in the samples was only x≈0.20–0.25. For the Λ=34.5 Å sample, since the GaAs layer was also thin, the composition modulation varied with each period of the superlattice. The simulation also showed layer thickness fluctuations which varied from 1.0 to 2.0 Å for the InGaAs layers and 2.0–3.0 Å for the GaAs layers.
Reasons for lower dielectric constant of fluorinated SiO2 films
Authors: Han Sang M. , Aydil Eray S.
Published: July 2002

The electronic and ionic contributions to the overall dielectric constant of fluorinated SiO2 films deposited in SiF4 and O2 plasmas were quantified from the refractive index measured by in situ spectroscopic ellipsometry in the visible-to-UV range and the infrared spectra taken by in situ attenuated total reflection Fourier transform infrared spectroscopy. The Kramers–Kronig dispersion relations facilitated the computation of ionic contribution to the dielectric constant from the IR absorbance spectra. The dependence of the dielectric response of SiO2 films on the SiF4-to-O2 ratio (R) in the feed gas mixture revealed that F incorporation leads to a decrease in both electronic and ionic contributions, thus reducing the overall dielectric constant. The electronic component, for instance, comprised 1/3 of the total dielectric constant above the vacuum level and decreased with increasing F content until SiF4-rich plasma resulted in a-Si incorporation. The rate of decrease, however, showed a sudden change at R=0.25. Below the ratio, the Si–O–Si bond angle relaxation in the SiO2 matrix and the subsequent density reduction were largely responsible for a moderate rate of decrease in the electronic contribution. Above this ratio, inclusion of voids caused a more pronounced decrease in the electronic contribution. The ionic component, which comprised less than 1/3 of the total dielectric constant, similarly decreased with increasing F content. This decrease was attributed to the replacement of more ionic Si–O bonds with Si–F bonds. The ionic contributions, whose characteristic vibrational frequencies appear below our experimentally observable range, constituted the remaining 1/3 and remained constant at 1, independent of fluorine concentration. Based on these observations, we propose a method to predict the total dielectric constant of SiOF films deposited in SiF4/O2 plasmas from a combination of ellipsometric and infrared absorption measurements.
Mechanism of exchange anisotropy and thermal stability of spin valves biased with ultrathin TbCo layers
Authors: Redon O. , Freitas P. P.
Published: July 2002

A sharp transition from unidirectional to uniaxial behavior is observed in magnetization measurements performed either at high temperatures or after annealing, on spin valves exchange biased by a thin TbCo layer. Magneto-optic Kerr-effect measurements with in-plane or out-of-plane magnetic fields were carried out on annealed samples to demonstrate that the weakening of the TbCo perpendicular anisotropy is responsible for this transition. The uniaxial characteristic is achieved when the exchange coupling between the in-plane and out-of-plane magnetization components of TbCo (J∥⊥) overcomes the perpendicular anisotropy of the TbCo layer (K⊥TbCo). A modified spin-valve structure with reinforced perpendicular anisotropy, developed to shift the transition to high temperature, shows a blocking temperature as high as 270 °C and an exchange field at room temperature larger than 500 Oe. Unshielded sensors were implemented with this structure showing good thermal stability up to 280 °C.
Demagnetizing factors for rectangular ferromagnetic prisms
Authors: Aharoni Amikam
Published: July 2002

An analytic expression is given for the magnetometric demagnetizing factors of the general rectangular prism, with special emphasis on the particular case of a square cross section. It is argued that this demagnetizing factor should be used in numerical computations that assume such prisms, when comparing the results with the theoretical study of ellipsoids.
Determination of valence band splitting parameters in GaN
Authors: Yamaguchi A. A. , Mochizuki Y. , Sunakawa H. , Usui A.
Published: July 2002

Valence band structures and their strain effects in GaN have been investigated by optical spectroscopy for thick GaN films with high optical quality grown by hydride vapor phase epitaxy. Excitons associated with the A, B, and C valence bands are clearly observed in reflectance measurements without modulation techniques. It is found that the exciton energies shift with the film thickness because of the relaxation of the residual strain. From the quantitative analysis of this behavior, we have precisely determined the valence band splitting parameters in GaN as Δ1=10 meV, Δ2=5.5 meV, and Δ3=6.0 meV.
SrTiO
3
films epitaxially grown by eclipse pulsed laser deposition and their electrical characterization
Authors: Tachiki Minoru , Noda Makoto , Yamada Kenji , Kobayashi Takeshi
Published: July 2002

Epitaxial (100)SrTiO3 (STO) thin films were grown successfully on (100)La0.7Sr0.3MnO3/(100)MgO substrates by eclipse pulsed laser deposition in (O2+Ar) ambient gas. The droplet-free STO films showed marked improvement in their permittivity, 320ε0 at room temperature. Time-resolved plume observation and spectrum measurement suggested that the improvement was due to effective excitation and ionization of growth species in the plume with the aid of coexisting Ar. The STO films also showed nonlinear permittivity against the applied field, as commonly observed in single-crystal bulk STO. Using this nonlinear characteristic and hole injection into the STO valence band, we deduced the hole trap concentration in the STO films. The concentration was on the order of 1018 cm−3.
Experimental studies and modeling of Pb–Zr–Ti–O film growth in pulsed laser deposition
Authors: Tyunina M. , Levoska J. , Leppävuori S.
Published: July 2002

The growth rate and composition of films deposited by laser ablation of Pb(Zr0.65Ti0.35)O3, both in vacuum and in ambient oxygen and argon with laser fluences in the range 0.3–3.0 J/cm2, were studied experimentally with using energy dispersive x-ray analysis. The film growth rate increased in vacuum with an increase in laser fluence and demonstrated two modes of behavior upon adding gas: decreased at low laser fluence and increased nonmonotonously at high laser fluence. Deposition in vacuum resulted in Pb-deficient films with the Pb/Ti ratio decreasing with an increase in laser fluence, while deposition in a gas resulted in a strong increase of Pb content with the Pb/Ti ratio increasing faster under strong laser irradiation. Changes in the film growth rate and composition were similar upon adding either oxygen or argon. No noticeable changes in the spatial distribution of the film growth rate and composition were found in a gas ambient with respect to a vacuum ambient. The obtained experimental results were analyzed using a phenomenological description of the film growth as a sorption of ablated species on the substrate surface and assuming the species retarding in a gas, with respect to vacuum, due to collisions of the species with gas molecules.
Bright red electroluminescence from a dye/copolymer blend
Authors: Hu B. , Zhang N. , Karasz F. E.
Published: July 2002

The low molecular weight dye [2-methyl-6-[2-(2,3,6,7-tetrahydro-1H,5H-benzo[ij]quinolizin-q-yl)ethenyl]-4H-pyran-4-ylidene]-propane dinitrile (MPD) dispersed in an alternating block copolymer containing thiophene vinylene chromophore units and inert oligomethylene spacer units provided bright red luminescent emission centered at 645 nm. The strength of the emission was based, in part, on efficient energy transfer between the copolymer matrix and the dye. The characteristics of an electroluminescent diode based on this blend, and provided also with a hole transport layer and additional carrier transport components, were investigated.
Soft magnetic properties of as-deposited Fe–Hf–C–N and Fe–Hf–N nanocrystalline thin films
Authors: Song J. Y. , Lee J. J. , Han S. H. , Kim H. J. , Kim J.
Published: July 2002

As-deposited Fe–Hf–C–N nanocrystalline thin films are investigated to improve soft magnetic properties by controlling both the compositions of the films and sputtering conditions, such as input power and N2 partial pressure. As-deposited Fe–Hf–N thin films are also investigated for the applications to simplify the fabrication of magnetic devices. The as-deposited Fe–Hf–C–N and the as-deposited Fe–Hf–N thin films are fully nanocrystallized during deposition by controlling the composition and sputtering condition. The thin films show the excellent soft magnetic properties of saturation magnetization ∼17.5 kG/∼16.5 kG and coercivity ∼0.5 Oe/∼0.5 Oe when the compositions of each film are 6.8–7.2 at % Hf, 2.0–2.5 at % C, 9.8–10.8 at % N, and balanced Fe/6.3–7.0 at % Hf, 13.2–14.0 at % N, and balanced Fe, respectively. Both of the thin films exhibit an outstanding frequency dependence of permeabilities, i.e., the effective premeabilities of the films remain flat over 3000 up to 100 MHz.
Magnetic properties of RFe10.5Mo1.5Cx and their nitrides
Authors: Mao Weihua , Yang Jinbo , Cheng Benpei , Yang Yingchang
Published: July 2002

RFe 10.5 Mo 1.5 C x (R=Y, Nd, and Dy; x=0–0.6) have been successfully synthesized by arc melting, then followed by a gas-solid phase reaction to form YFe10.5Mo1.5CxNy. X-ray diffraction studies and thermomagnetic analysis show that all samples are of good single phase, and carbides and carbon nitrides remain the ThMn12-type structure. The addition of carbon atoms has an effect of improving the magnetic properties. Both Curie temperature and saturation magnetization are found to be increased with carbon concentration x in the RFe10.5Mo1.5Cx series, and the carbon atoms also affect the magnetocrystalline anisotropy. The easy axial anisotropy of NdFe10.5Mo1.5Cx and DyFe10.5Mo1.5Cx is enhanced. As to the nitrides, the results indicate that YFe10.5Mo1.5CxNy possess a higher Curie temperature and a larger saturation magnetization than YFe10.5Mo1.5Ny, which is of significance in the technical applications.
Magneto-optical properties of MnPt3: LDA+U calculations
Authors: Sabiryanov R. F. , Jaswal S. S.
Published: July 2002

Our recent first-principle LMTO-ASA calculations of the magnetooptical properties of MnPt3 are in good agreement with the experimental data. However, the calculated positions of the Kerr-rotation peaks are shifted by approximately 0.5 eV with respect to the experimental results. An interesting feature in the density of states (DOS) of MnPt3 is the narrow unoccupied peak in the minority Mn d DOS signifying the fairly localized nature of these states. Usually the study of the optical properties of systems with localized states requires the inclusion of correlation effects beyond the LDA through the Hubbard U parameter. We report here a study of the effect of U and the energy expansion parameter Eν in LMTO for the minority Mn d states on the calculated Kerr spectra of MnPt3. It is found that a reasonable choice of Eν gives the Kerr effect in good agreement with the experimental data without the need to use the U parameter.
Effects of plasma and bias power on magnetic properties of sendust films
Authors: Araghi M. S. , Hurley R. E. , Gamble H. S. , Dodd P. M. , Atkinson R.
Published: July 2002

Thin films of Sendust (wt. 85% Fe, 10% Si, 5% Al) were deposited by rf diode and dc magnetron sputtering. The objective was the production of soft magnetic films without post deposition annealing. Films were deposited on both glass and Al2O3/TiC(AlTiC) ceramic substrate. Coercivities in the range 45 to 50 Oe for rf diode deposited and 25 to 35 Oe for dc magnetron deposited films were measured. Application of rf bias to the substrate reduced this further to 7 to 12 Oe together with a reduction in grain size. The value of magnetic flux at the surface of the sputtering target was controlled by an electromagnet and found to be an important parameter for the production of soft magnetic films, low flux densities producing the softest layers. This may be related to deposition rate and/or the influence of the target magnetic field penetrating into the substrate region. The softer magnetic layers produced as a result of applying a low rf bias to the substrate is believed to be due to ion bombardment during deposition resulting in smaller grain size.
Spin-dependent tunneling junctions with hard magnetic layer pinning
Authors: Bobo J. F. , Mancoff F. B. , Bessho K. , Sharma M. , Sin K. , Guarisco D. , Wang S. X. , Clemens B. M.
Published: July 2002

We have fabricated ferromagnet-insulator-ferromagnet tunnel junctions with Co and NiFe electrodes, where the Co electrodes are pinned with a hard magnetic Co81Pt19 alloy layer. This approach gives a coercivity of about 300 Oe for the Co layer, while that of the NiFe is about 80 Oe, so we obtain antiparallel magnetization over a wide field range. The Al2O3 tunneling barrier layers were formed by in situ plasma oxidation of elemental Al layers with thicknesses from 10 to 25 Å. For the junctions, we find room temperature magnetoresistance ratios as high as 13% and nonlinear current–voltage curves that are well fit by the Simmons tunneling theory. Depth profiling x-ray photoelectron spectroscopy of oxidized Al barrier layers on Co underlayers reveals a stoichiometry of nearly Al2O3.