bluuebunny/crossref_metadata_2025 huggingface dataset:
The bonding of protective films of amorphic diamond to titanium
Authors: Collins C. B. , Davanloo F. , Lee T. J. , Jander D. R. , You J. H. , Park H. , Pivin J. C.
Published: July 2002

Films of amorphic diamond can be deposited from laser plasma ions without the use of catalysts such as hydrogen or fluorine. Prepared without columnar patterns of growth, the layers of this material have been reported to have ‘‘bulk’’ values of mechanical properties that have suggested their usage as protective coatings for metals. Described here is a study of the bonding and properties realized in one such example, the deposition of amorphic diamond on titanium. Measurements with Rutherford backscattering spectrometry and transmission electron microscopy showed that the diamond coatings deposited from laser plasmas were chemically bonded to Ti substrates in 100–200-Å-thick interfacial layers containing some crystalline precipitates of TiC. Resistance to wear was estimated with a modified sand blaster and in all cases the coating was worn away without any rupture or deterioration of the bonding layer. Such wear was greatly reduced and lifetimes of the coated samples were increased by a factor of better than 300 with only 2.7 μm of amorphic diamond.
Reflection high-energy electron diffraction study of growth and interface formation of the Ga1−xInxSb/InAs strained-layer superlattices
Authors: Fan W. C. , Zborowski J. T. , Golding T. D. , Shih H. D.
Published: July 2002

Growth and interface formation of the Ga1−xInxSb/InAs (x≤0.4) strained-layer superlattices (SLSs) on GaSb(100) substrates have been studied by the reflection high-energy electron diffraction (RHEED) during molecular beam epitaxy. A number of surface atomic structures were observed in the growth of the SLS: a (1×3) phases from the InAs epilayer surface; a (2×3) phase, a (2×4) phase and diffuse (1×1)-like phases from the InAs epilayer surface. The RHEED intensity variations in the formation of the interfaces have been discussed in terms of interface chemical reactions.
On the image brightness of the trench bottom surface in a scanning electron microscope
Authors: Harafuji Kenji , Nomura Noboru , Kouda Tatsuro
Published: July 2002

Trajectories of secondary electrons (SEs) as an image signal in scanning electron microscopy are studied analytically in high-aspect-ratio submicron wide infinite trenches. It is found that the SE collection efficiency ξ, which is defined as the ratio of the number of SEs exiting the trench top surface divided by that emitted from the trench bottom surface, has the following dependencies on magnetic flux density Bz generated by an objective lens and electric field Ez. Here, Bz and Ez are the components perpendicular to the wafer surface. For a low Bz region corresponding to a usual operation condition, ξ decreases gradually but is almost constant at about 0.15 with increasing Bz for trenches 0.5 μm wide and 5 μm deep. For a high Bz region over 3 T, ξ increases with increasing Bz, since the cyclotron radius is fairly small compared with the trench width. A special superconducting magnet is necessary to reach the high Bz region. An external electric field Ez brings about the larger increase of ξ for the smaller SE energy εs. For εs=1 eV, Ez as large as 106 V/m is necessary to realize a 50% increase of ξ compared with the case of Ez=0 V/m, whereas for εs=0.1 eV, only about Ez=105 V/m is sufficient to realize the increase of 50%. On the other hand, experiments show that an electric field as large as 105 V/m is effective to achieve bright and clear observations of the bottom image.
Ion emission from ferroelectric media
Authors: Rosenman G. , Urbakh M.
Published: July 2002

Field ion emission from flat polar surfaces of ferroelectric crystals is predicted. Fast reorientation of the polar axis of such crystals may produce surface electrostatic fields of about 108 V cm−1. The induced nonequilibrium charge may be screened by ion emission. The experimental conditions under which various kinds of ion emission may be observed are discussed.
Optical study of Ar+ implantation-induced damage in GaAs/GaAlAs heterostructures
Authors: Kieslich A. , Straka J. , Forchel A.
Published: July 2002

We have investigated the depth range of ion implantation damage in GaAs/GaAlAs quantum wells. The photoluminescence emission intensity of single quantum wells was used as a local probe for the study of the damage created by an Ar+ ion beam at energies up to 170 keV. The range of the damage was examined for implantations under different angle of incidence. Even for random orientation we observe a wide extension of the defect profiles, which can be described by a characteristic decay length of 90 nm at an ion energy of 70 keV. Ion implantation along the major crystallographic axes leads to effective extensions of the damage, which are larger by more than a factor of 2 due to ion channeling.
Growth of epitaxial TiN films on MgO(100) by high current discharge plasma
Authors: Miyagawa S. , Saitoh K. , Niwa H. , Ikeyama M. , Massouras G. , Miyagawa Y.
Published: July 2002

High current discharge plasma produced by the Ta-LaB6 cathode was used for ion plating of TiN films. The dependence of the deposition rate of TiN on the substrate dc bias potential was measured using the Rutherford backscattering spectrometry (RBS) method. The results indicate that the particles impinging on the substrate, i.e., vaporized particles (Ti) and reactive gases (N2), could be ionized at high efficiency of 70%. Epitaxial TiN films were grown at temperatures between 400 and 700 °C on cleaved MgO(100) substrate supplied with negative dc anf rf potential. Structures of the epitaxial film were investigated by RBS and ion channeling, and scanning tunneling microscope (STM). The minimum channeling yields for (100) and (110) axes obtained at a substrate temperature of 550 °C with the growth rate of 40 Å/s were found to be 1.4% and 2.0%, respectively, which were almost the same as that of the MgO substrate. Atomically resolved tunneling images of (100) surface of the epitaxial TiN film were observed by STM.
Effects of doping variations on electron transport in GaAs n+-n-n+ structures
Authors: Tian H. , Kim K. W. , Littlejohn M. A. , Mishra U. K. , Hashemi M.
Published: July 2002

Electron transport properties in GaAs n+-n-n+ structures with varying doping profiles (in the direction of electron transport) are investigated using self-consistent ensemble Monte Carlo simulations. In particular, we study the effects of ramp doping [i.e., linearly increasing (ramp-up) or decreasing (ramp-down) doping density within the n region] and spike doping (i.e., introducing one or more n+ spikes in the n region) on electron transport to assess differences, advantages, and potential applications of these doping profiles on device performance. Underlying physical mechanisms for electron transport in these structures are analyzed. Simulation results reveal that overall electron transport can be improved significantly by employing ramp-up (rather than ramp-down) doping and multispike (instead of single-spike) doping schemes. Potential advantages of variable doping in field-effect transistor applications include enhanced current drive capability, reduced source resistance, and improved breakdown characteristics.
Coherent and escape tunneling processes in asymmetric coupled quantum wells
Authors: Cruz H. , Muga J. G.
Published: July 2002

The dynamics of electrons in a Ga1−xAlxAs-GaAs asymmetric double quantum well heterostructure under an external applied electric field is studied. In particular, we examine the coupling between two different tunneling processes: coherent tunneling between the two quantum wells and escape tunneling to the GaAs bulk. Simple hydrodynamical models are proposed to account for the observed features. The effect of a longitudinal magnetic field on both tunneling processes is also analyzed. In this coupled quantum well system, the possibility exists of a direct observation of an oscillating luminescence signal with a quantum tunneling period modulated by means of an applied longitudinal magnetic field.
Inversion layer instability in a P(VDF/TriFE)/silicon transistor structure: Effect of oxygen exposure/polymer porosity, interface, and bulk trapping
Authors: Ledoyen F. , Andry P. , Rambo A. , Lauzon J.
Published: July 2002

A copolymer of vinylidene fluoride and trifluoroethylene is solution cast onto a p-type silicon wafer to produce an enhancement field-effect transistor structure. A strong inversion is induced at the silicon surface when an aluminum gate electrode is suitably biased. When the gate is left floating, the inversion layer decays with time. Both the initial strength of this inversion and its rate of decay are found to depend on the charging history of the device, as well as the oxygen concentration in the surrounding gas ambient. The oxygen sensitivity is due to the large gas permeability of the cast film as compared to commercially extruded film. A novel method for directly measuring these permeabilities is presented. All aspects of device behavior may be treated in a single model where the inversion layer decay is shown to be a function of interface and bulk trapping in the polymer as well as ambient oxygen concentration. Excellent agreement between theory and results is obtained. The physical significance of all model parameters and their relation to device behavior under different experimental conditions is discussed.
Picosecond YBa2Cu3O7−δ detector for far-infrared radiation
Authors: Nebosis R. S. , Steinke R. , Lang P. T. , Schatz W. , Heusinger M. A. , Renk K. F. , Gol’tsman G. N. , Karasik B. S. , Semenov A. D. , Gershenzon E. M.
Published: July 2002

We report on a picosecond YBa2Cu3O7−δ detector for far-infrared radiation. The detector, consisting of a current carrying structure cooled to liquid-nitrogen temperature, was studied by use of ultrashort laser pulses from an optically pumped far-infrared laser in the frequency range from 25 to 215 cm−1. We found that the sensitivity (1 mV/W) was almost constant in this frequency range. We estimated a noise equivalent power of less than 5×10−7 W Hz−1/2. Taking into account the results of a mixing experiment (in the frequency range from 4 to 30 cm−1) we suggest that the response time of the detector was few picoseconds.
Light power dependence of electro-optical transmission in InGaAs/AlGaAs multiple quantum wells
Authors: Giugni Stephen , Kawashima Kenji , Sano Naokatsu , Fujiwara Kenzo
Published: July 2002

Dual wavelength electro-optical bistability in resistor-biased and symmetric self electro-optic effect devices and their input power dependence are investigated using InGaAs/AlGaAs multiple quantum wells. A new principle to drive the devices using two widely separated wavelengths for signal and control light is proposed and demonstrated to obtain optical transmission bistability between two stable states of the signal light at λ1 by varying the control beam power at λ2. Detailed experimental results of signal power dependencies of the optical responses are presented and rigorously explained based on the voltage switching mechanism of the signal diode by the second control light using a simple load line analysis. Advantages of this new device configuration are discussed taking full merit of the transparency of the GaAs substrate to the signal wavelength.
The effects of As overpressure and diffusion source on the diffusion of Mn in GaAs
Authors: Wu C. H. , Hsieh K. C.
Published: July 2002

Data are presented to show the effect of As overpressure on the diffusion of Mn in GaAs using four different Mn sources. These sources include solid Mn thin film deposited directly on the GaAs substrate and Mn vapors from pure Mn, MnAs, and Mn3As solids. In the circumstance for which a solid Mn film is used as the diffusion source, a nonuniform doping distribution and poor surface morphology is obtained due to a reaction between the Mn film and the GaAs matrix. The degraded surface consists of a layer of polycrystalline cubic alloy having a lattice constant of nearly 8.4 Å and a composition close to MnGa2 with a small amount of As. Of the remaining diffusion sources (Mn, MnAs, and Mn3As), only MnAs consistently produces a uniform doping distribution and smooth surface morphology. For diffusions at 800 °C, a uniform surface hole carrier concentration as high as 1020/cm3 can be obtained using MnAs as the source. The As overpressure is found to drastically alter the Mn diffusion profile, and Mn, like Zn, may diffuse in GaAs interstitial-substitutionally. Vapor from both the Mn and Mn3As solids degrade the GaAs surface. Mn3As, however, uncharacteristically degrades the surface more rapidly although the details of such are not well understood. With the presence of a high As overpressure, however, both surfaces of the Mn and Mn3As sources are converted to (Mn,As) compounds, the compositions being close to MnAs. High enough As overpressures are shown to completely suppress the GaAs surface degradation which is evident when Mn3As alone is used as the diffusion source.
Electron tunneling spectroscopy and defects in GaAs/AlGaAs/GaAs heterostructures
Authors: Magno R. , Spencer M. G.
Published: July 2002

Electron tunneling spectroscopy has been used to study the phonon modes of the GaAs electrode and the AlGaAs barrier of single barrier GaAs/AlGaAs/GaAs heterostructures. The barriers were spiked doped with Si or Be to determine whether defects or impurities in the barrier have an effect on the measured line shapes. The phonon line shapes and intensities have been observed to change after shining light on the devices to photoionize defects in the barrier. The results demonstrate that the charge state of defects in a heterostructure barrier can affect the interaction between a tunneling electron and the phonon modes of a tunnel barrier.
Infrared study of OH− defects in KTiOPO4 crystals
Authors: Morris P. A. , Crawford M. K. , Jones B.
Published: July 2002

Variations in the concentrations and distributions of the OH− defects present in flux and hydrothermal KTiOPO4 (KTP) crystals, measured by infrared spectroscopy of single crystals, are attributed to differences in the growth environments and other nonhydrogenic defects present in the crystals. The concentrations of OH− have been estimated from the infrared data to be approximately 400 ppma (parts per million atomic) (3.0×1019 cm−3) in the flux crystals, 1100–1500 ppma (0.74–1.1×1020 cm−3) in the high-temperature hydrothermal and 600 ppma (4.3×1019 cm−3) in the low-temperature hydrothermal crystals. A 3566 cm−1 peak and a 3575 cm−1 band are observed in all crystals. The integrated intensity of the OH− absorption band at 3566 cm−1 increases at the expense of the 3575 cm−1 band at higher temperatures in the high-temperature hydrothermal crystals. Several OH− peaks (3490, 3455, 3428, 3420, and 3333 cm−1), which have strongly temperature-dependent linewidths, are present in the hydrothermally grown KTP crystals. The temperature dependencies of their peak frequencies and widths are consistent with the presence of mobile protons in the lattice. The protons located in the 3490 and 3428 cm−1 sites are believed to contribute to the ionic conductivity of the high-conductivity high-temperature hydrothermal crystals.
Disorder/order/disorder Ga0.5In0.5P visible light-emitting diodes
Authors: Lee M. K. , Horng R. H. , Haung L. C.
Published: July 2002

A new Ga0.5In0.5P light-emitting diode (LED) with order/disorder/order (DOD) structure has been fabricated by metalorganic chemical vapor deposition. Growth temperature and dopant concentration were successfully used as growth parameters to obtain a heterojunction-equivalent structure. From the 77 K photoluminescence measurement, three peak energies of the DOD structure can be resolved clearly. It is shown that the DOD structure is equivalent to the double-heterojunction structure. The wavelength of the LED occurring at 667 nm coincides with the ordered active layer emission. The light intensity of the DOD LED is seven times stronger as compared with that of the homojunction disordering LED at the injection current of 10 mA. These results demonstrate that the DOD structure can provide good electrical and optical confinements and can be served as heterojunction-equivalent applications.
Thomas–Fermi theory of n-i-p-i doping superlattices
Authors: Schrüfer K. , Eckl S. , Metzner C. , Beyer H. J. , Döhler G. H.
Published: July 2002

We apply the nonlinearized Thomas–Fermi approximation for the calculation of the electronic structure of n-i-p-i doping superlattices at finite temperatures. A comparison with the solutions determined by the self-consistent Hartree model shows that the deviations are negligibly small even for the extreme cases of δ-doped n-i-p-i and uniformly doped n-p-n-p doping structures at any degree of excitation, whereas the computing time is reduced by orders of magnitude.
High temperature characteristics of amorphous TiWSix nonalloyed ohmic contacts to GaAs
Authors: Papanicolaou N. A. , Jones S. H. , Jones J. R. , Anderson W. T. , Sillmon R. S.
Published: July 2002

This article reports on amorphous (α) nonalloyed TiWSix ohmic contacts on n-GaAs using an intervening graded-band-gap layer of InxGa1−xAs grown by the low pressure organometallic chemical vapor deposition method. The metal silicide contacts consisted of extremely thin alternating layers of TiW and Si sequentially deposited by rf magnetron sputtering to a total thickness of 500 Å. The as-deposited contacts exhibited ohmic behavior without requiring post-deposition heat treatment, and yielded specific contact resistivity values as low as 9×10−7 Ω cm2. These contacts were shown to be stable and retained excellent surface morphology after 600 °C thermal annealing. Rutherford backscattering and Auger electron spectroscopy investigations revealed no apparent interdiffusion at the metal/semiconductor interface under the above annealing conditions.
Effects of oxygen pressure and substrate temperature on the in situ grown BiSrCaCuO films by laser ablation
Authors: Lin Wen-Tai , Wu Kuo-Chung
Published: July 2002

The dependence of the phase formation on the oxygen pressures ranging from 2.0×10−2 to 20 Torr and the substrate temperatures ranging from 500 to 750 °C in the in situ grown BiSrCaCuO (BSCCO) films prepared by laser ablation from a Bi1.5Pb0.5Sr2Ca3Cu4Ox target was presented. The crystallization of significant amounts of the Bi2Sr2CaCu2Ox (2212) and Bi2Sr2CuOx (2201) phases appeared at 510 °C. The minimum oxygen pressure required for the formation of the 2212 phase was 2.0×10−2 Torr. Higher oxygen pressure enhanced the nucleation and growth of the 2201 phase and shifted the formation temperatures of the 2212 and Bi2Sr2Ca2Cu3Ox (2223) phases to be higher. The decomposition temperature of the perovskite structure increased with increasing the oxygen pressure. In this study, nearly pure 2212, 2223, and Bi2Sr2Ca3Cu4Ox (2234) films with the corresponding Tc (zero) of 73, 83, and 71 K were, respectively, grown by varying the oxygen pressure and substrate temperature. The oxygen pressure and substrate temperature also played important roles in determining the microstructures and hence the superconductivity in the in situ grown BSCCO films. Conditions required for the successful in situ growth of the BSCCO films are prone to the higher oxygen pressure and lower substrate temperature in comparison with those of the YBa2Cu3Ox (123) films.
Optical properties of the magnetic monopole field applied to electron microscopy and spectroscopy
Authors: Kruit P. , Lenc M.
Published: July 2002

An analytical treatment of the electron’s motion in a magnetic monopole field results in useful expressions for both the lens action and the mirror action of the field. Using an appropriate definition of the magnetic moment of the electron, it is shown that there is an exact conservation of this parameter in the monopole field, implying that the motion is perfectly adiabatic. This property is important when the field is used for directing Auger electrons from a target to a detector; that is, when it is used as a parallelizer in a through-the-lens detection scheme. Regarding the monopole field as an electron lens, the image position and magnification are derived for an arbitrary object position. Expressions for both the axial aberrations (chromatic and spherical) and the image aberrations (coma, field curvature, astigmatism, distortion, and transverse chromatic) are derived for an arbitrary number of intermediate images between object and final image. The chromatic aberration turns out to be independent of the number of intermediate images and the spherical aberration decreases slightly with this number. This property is important when an electron beam must be focused to a small probe in a strong magnetic field. It is shown that if a certain combination of deflectors is used in conjunction with the monopole field, an ideal swinging objective lens is obtained: All image aberrations except field curvature disappear. Designs are presented in which the monopole field is used in the objective lenses of a transmission electron microscope and a scanning electron microscope.
Mechanical properties of epitaxial TiN/(V0.6Nb0.4)N superlattices measured by nanoindentation
Authors: Hubbard Kevin M. , Jervis Thomas R. , Mirkarimi Paul B. , Barnett Scott A.
Published: July 2002

We have used nanoindentation to measure the mechanical properties of epitaxial TiN/(V0.6Nb0.4)N superlattices, grown on MgO(100), as a function of the wavelength λ. The V/Nb ratio within the VNbN layers was chosen to provide a lattice match with TiN, minimizing effects resulting from coherency strains. For λ≥4 nm, the hardness was found to be significantly enhanced relative to a homogeneous reference film of the same average composition. For λ<4 nm, the hardness decreased to a value close to that of the reference film. The elastic modulus was found to be constant for λ≥4nm, at a value close to that predicted by the law of mixtures. For samples with λ=2.3 and 2.8 nm, there was a 15% decrease in modulus. The observed variations appear not to be an effect of interfacial strain. Possible mechanisms are discussed.
A new maskless selective-growth process for InP on (100) Si
Authors: Tang G.-P. , Peiner E. , Wehmann H.-H. , Lubnow A. , Zwinge G. , Schlachetzki A. , Hergeth J.
Published: July 2002

We have developed a new selective-growth process of InP on exactly (100)-oriented Si substrate in a conventional low-pressure metal-organic vapor-phase-epitaxy system. In this process, the InP epitaxial layer was deposited on a photolithographically patterned InP-buffer film without an additional dielectric mask during the growth. Under our experimental conditions, the InP growth has a very high selectivity and the InP epitaxial layer is antiphase-domain free. Experimental results show that the undesirable sidewall-growth interaction in conventional dielectric mask selective-growth processes is effectively suppressed. Spatially resolved photoluminescence displayed very high optical quality of patterned InP layers compared to those grown on blanket substrates.
Dispersion analysis of symmetric transverse magnetic modes in a split cavity oscillator
Authors: Lemke Raymond W.
Published: July 2002

The split cavity oscillator (SCO) is a resonant cavity device which can be used to modulate the current of a relativistic electron beam. The modulated electron beam can be used to produce high-power microwaves. The SCO consists of a cylindrical cavity resonator which has been divided (split) into two identical regions (cavities) by a thin conducting foil (or screen) whose radius is less than the inner radius of the cylinder. The resulting gap between the foil perimeter and inner cylinder wall forms an annular slot which couples the cavities electromagnetically. The SCO supports standard transverse magnetic (TM) modes, in addition to split cavity TM modes. The latter will interact unstably with an electron beam, thereby producing a high degree of current modulation in a short distance. For the purpose of characterizing split cavity TM modes, and developing a design tool for SCOs, a cold dispersion relation (no electron beam) is derived by solving Maxwell’s equations for the fields in an equivalent periodic structure. The dispersion relation and field solutions are employed to numerically calculate eigenfrequencies and eigenfunctions for the SCO. The latter are used to develop predictive formula for the SCO oscillation frequency, and to gain some understanding of the interaction that takes place when an electron beam is present.
Phosphor currents in ZnS:Mn ac thin film electroluminescent display devices
Authors: Singh Vijay P. , Xu Qin , McClure John C. , Morton David C.
Published: July 2002

Conduction current in the phosphor layer of ZnS:Mn ac thin film electroluminescent (ACTFEL) display device was measured as a function of the amplitude and the rise time of the bipolar voltage pulses: electric field in the phosphor was also determined. It was found that the clamping field of an ac thin film electroluminescent device is not a single-valued device characteristic. The clamping field depended upon the rise time of the applied voltage pulse increasing in value as the rise time became smaller. These data were interpreted in terms of a nonideal breakdown of the insulator-phosphor interface and a time delay involved in the emission of electrons from this interface. To account for the nonideality in the interface breakdown the ac equivalent circuit of the ACTFEL device was modified by including a voltage-dependent resistor in parallel with the phosphor capacitance. The phosphor current measurements, transferred charge measurements and tunneling current calculations indicate that in the nonhysteretic ZnS:Mn devices under investigation, impact ionization in the phosphor layer, if present at all, leads to relatively small carrier multiplication factors.
High resolution time-of-flight analysis of photon stimulated ion desorption from chemically treated silicon surfaces
Authors: Mochiji K. , Lee K. , Ma C. I. , Kim D. Y. , Mahalingam M. , Hanson D. M. , Johnson E. D.
Published: July 2002

The native oxide, thermally oxidized and hydrofluoric acid rinsed surfaces of Si(100) have been characterized by photon stimulated ion desorption (PSD), and both photoelectron and Auger electron spectroscopies. The only species detected by PSD were H+ ions with different kinetic energies. Low kinetic energy H+ ions were detected only from the HF rinsed surfaces presumably arising from scission of Si—H bonds while higher kinetic energy ions attributed to adsorbed hydrocarbon dissociation were observed for all of the surfaces.
Thermally stable non-gold Ohmic contacts to n-type GaAs. II. NiSiW contact metal
Authors: Takata H. J. , Tanahashi Kiwamu , Otsuki A. , Inui H. , Murakami Masanori
Published: July 2002

New ‘‘nongold’’ NiSiW Ohmic contacts to n-type GaAs have been developed using an electron beam evaporator and a rapid thermal annealer. Ohmic behavior was found to have dependencies on the Si concentrations of the NiSiW contacts and the annealing condition. The Ohmic contacts with 40 at. % Si, prepared by annealing at 650 °C, had smooth surfaces and yielded excellent thermal stability during subsequent annealing at 400 °C after contact formation. The microstructural analysis of the interface between the contact metal and the GaAs substrate was carried out using x-ray diffraction, Auger electron spectroscopy, and cross-sectional transmission electron microscopy. It was concluded that the key parameter that influenced the electrical properties was the NiAs compounds formed during contact annealing.
Wet oxidation of amorphous Si-Ge layer deposited on Si(001) at 800 and 900 °C
Authors: Rai A. K. , Prokes S. M.
Published: July 2002

Amorphous Si0.56G0.44 films were deposited on (001)Si by electron beam evaporation in a vacuum having a base pressure of 10−7 Torr. They were then wet oxidized at 800 and 900 °C in an open tube furnace for various times. Cross (x)-sectional and plan view transmission electron microscope techniques were employed to characterize the samples. At 800 °C, 30 min of wet oxidation produced a continuous polycrystalline Si-Ge layer, whereas 60 min of wet oxidation produced a discontinuous polycrystalline layer. After 100 min of wet oxidation at 800 °C, the Si-Ge layer was almost completely oxidized and no observable evidence of the epitaxial Si-Ge layer was found. Wet oxidation at 900 °C for 10 min produced a bilayer structure; one epitaxial and one polycrystalline layer separated by a contamination layer initially present on the substrate prior to deposition. A mostly epitaxial Si-Ge layer was obtained after 30 min of wet oxidation at 900 °C. These results will be discussed in terms of a previously suggested epitaxial growth model. The failure to obtain an observable epitaxial Si-Ge layer by wet oxidation at 800 °C will be discussed by consideration of changes in the kinetics and the stability of both SiO2 and GeO2 at this temperature.
Morphology of oxide precipitates in Czochralski silicon degenerately doped with boron
Authors: Wijaranakula W.
Published: July 2002

The morphology of oxide precipitates in Czochralski-grown silicon degenerately doped with boron to the concentration of 1019 atoms/cm3 was studied after a two-step anneal using the transmission electron microscope. In addition to square-shaped precipitate platelets having {100}-type habit planes commonly observed in lightly doped silicon, it was found that the oxide precipitates in degenerately doped silicon also exhibited a disk-shaped morphology with both {110}- and {111}-type habit planes. Based upon the result from the calculation of the elastic stress field around the precipitate disk, it is hypothesized that the formation of the disk-shaped precipitates on the planes other than the {100}-type planes could be facilitated by localized reduction in the elastic moduli of silicon resulting from degenerately doping with boron. This finding is significant for understanding the fundamental differences in the oxygen precipitation mechanisms between silicon lightly and degenerately doped with boron.
Resonant tunneling through low dimensional quantum structures
Authors: Sa’ar A. , Feng J. , Gravé I. , Yariv A.
Published: July 2002

Tunneling mechanism in zero- and one-dimensional quantum structures is studied. Several new results, peculiar to low dimensions, are predicted. We find that subband mixing and multichannel tunneling induce the appearance of new tunneling channels with unusual interference patterns, and allow for longer lifetime of the resonances at higher energies in various channels. It is shown that in low dimensions, there exists a critical size of the structure below which the resonance nature of the tunneling process is diminished. In zero and one dimensions, there exists a critical magnitude of the confinement potential, below which there are no resonances in the transmission function for any size of the well. Negative differential resistance and other phenomena related to the resonance characters of the tunneling will not appear in this case. We also develop a generalized transfer matrix method that takes into account subband mixing; this formalism can be used to describe any transport problem in low dimensions.
Residual hydrogen gas induced defects in heteroepitaxial Y1Ba2Cu3O7−x films
Authors: Kamei Masayuki , Yoshida Isao , Takahashi Hiromi , Itti Rittaporn , Morishita Tadataka
Published: July 2002

Heteroepitaxial Y1Ba2Cu3O7−x films were grown on MgO(100) substrates by using the reactive evaporation method. The in situ streak reflection high energy electron diffraction pattern of films gradually faded with an increasing thickness when the hydrogen partial pressure during growth was higher than 1×10−6 Torr. Transmission electron microscope observation revealed that the film showed heteroepitaxial growth in the early stage of coevaporation. However, as with the increase of film thickness, the quality of the film became worse and finally reached polycrystalline. These results are attributed to the accumulation of hydrogen induced defects during heteroepitaxial growth of Y1Ba2Cu3O7−x films.
Intersubband optical absorption in Si/Si1−xGex superlattices
Authors: Cho Sung M. , Lee Hong H.
Published: July 2002

The absorption coefficient for intersubband transitions in Si/Si1−xGex superlattices have been calculated using multicomponent envelope functions. The results are compared with those obtained by a single wave function approximation. Large differences in the absorption coefficient were found between the two cases. The differences are analyzed in terms of the charge densities of the wave functions and the momentum matrix elements for the intersubband transitions involved. It is found that the absorption coefficient estimate is much lower and most of the transition peaks are absent when the single wave function approximation is used.
Microwave magnetic resonances in the one-dimensional ferromagnet FeTAC
Authors: Rubins R. S. , Black T. D. , Ravindran K. , Grigereit T. E. , Drumheller John E.
Published: July 2002

Magnetic resonance spectra at 12–36 GHz are reported for Fe2+(3d6, S=2) in the 1D Ising ferromagnet [(CH3)3NH]FeCl3⋅2H2O, known as FeTAC, between 2.5 and 13 K. The main signal was identified as a spin-cluster resonance (SCR) because of its characteristic temperature dependence below 6 K and the linear dependence of the resonance field on the microwave frequency for H parallel to the easy axis at 4.2 K. The temperature dependence of the line intensity was fitted to the function exp[−(2J+hν)/kBT], with 2J/kB=(41±1) K, and the linear field dependence was explained by the value gz=8.26±0.05 in the S=1/2 representation and a demagnetization field of several hundred gauss, which depended on the sample shape. A frequency-dependent satellite structure observed on the low field side of the main line, which appeared as a broad line below 15 GHz, is tentatively associated with energy shifts of the lower-lying spin-cluster states produced by the magnetic dipole interaction and other terms in the Hamiltonian.
Thermodynamics and correlations of the easy-plane ferromagnet CsNiF3
Authors: Cuccoli Alessandro , Tognetti Valerio , Verrucchi Paola , Vaia Ruggero
Published: July 2002

The ferromagnetic compound CsNiF3 has been the object of plenty of experimental and theoretical research, due to its apparent 1D behavior (above the 3D ordering temperature TN=2.7 K). Indeed it can be modeled as a ferromagnetic chain with single-site easy-plane anisotropy. Its quantum Hamiltonian can also be expressed in terms of canonically conjugate operators by means of the Villain transformation, allowing us to apply a recently devised method, called the pure-quantum self-consistent harmonic approximation (PQSCHA), in order to obtain a classical effective Hamiltonian, by means of which the quantum thermal averages can be approximated by phase-space integrals. Comparing with the available quantum techniques, it is apparent that this is an enormous simplification. These classical-like formulas are able to describe the quantum behavior of the system, provided that the quantum coupling is not too strong. This is indeed the case for CsNiF3, whose effective Hamiltonian bears the form of the classical counterpart of the original one, but with suitably renormalized values of applied field, anisotropy, and exchange constant. An interesting feature is the appearance of an exchange anisotropy in competition with the easy-plane one, reflecting the effect of the quantum out-of-plane fluctuations. The classical transfer-matrix method is finally used in calculating various thermodynamic quantities, static structure factors and correlation lengths. Comparison with the available experimental data for CsNiF3, as well as with existing numerical simulation results, generally shows very good quantitative agreement.
Hysteresis model using distribution of susceptibility
Authors: Morita T. , Ishii Y. , Fukai I.
Published: July 2002

We prepare hysteresis model that reproduces smooth hysteresis curves including minor loops from several experimental hysteresis loops. In this model, the distribution of susceptibility on the M-H plane obtained from the experimental hysteresis loops is used. The hysteresis loop in the demagnetizing process is computed with this model, and found to converge on the original point of the M-H coordinates. The computed hysteresis curves including minor loops agree well with the experimental results. In addition, the present hysteresis model is used for a numerical analysis of magnetic fields with the spatial network method. In this simulation, the magnetization and magnetic fields inside and outside an infinite magnetic cylinder in an external pulse field are obtained. The results are in fair agreement with theoretical results.
Circular magnetic x-ray dichroism and local magnetic fields (invited) (abstract)
Authors: Carra Paolo
Published: July 2002

Circular magnetic x-ray dichroism (CMXD) experiments on ferromagnetic systems are discussed on the basis of a new magneto-optical sum rule, which relates the edge-integrated CMXD signal to the ground state expectation value of the operators (spin, orbit, and dipole) of the magnetic field generated by the atomic electrons. Results for 3d transition metals, rare earth, and actinide systems will be presented.
Electronic structure of Sm2Fe17Nx compounds
Authors: Woods J. P. , Fernando A. S. , Jaswal S. S. , Patterson B. M. , Welipitiya D. , Sellmyer D. J.
Published: July 2002

Sm2Fe17Nx is of considerable current interest as a permanent-magnet material because of its enhanced Curie temperature and uniaxial anisotropy. The electronic structures of Sm2Fe17Nx for x=0 and x∼2.6 have been studied with photoemission and spin-polarized calculations. The materials are prepared by arc melting and nitrogen is introduced by ion implantation. The nitrogen concentration is quantified with Auger electron spectroscopy. The Sm 4f electrons with binding energies between 6 and 10 eV are investigated with resonant photoemission using photon energies near 140 eV. The major features of ultra-violet photoemission spectra include the Fe 3d band with a strong peak at 0.8 eV and a small peak at 2.9 eV below the Fermi energy which agree quite well with the theoretical density of states calculation. The modification of the electronic structure with nitrogen concentration is studied to understand the effect of nitrogen addition on the magnetic properties.
Photoemission studies of Co- and Fe-based compounds with the ThMn12 structure
Authors: Fernando A. S. , Woods J. P. , Jaswal S. S. , Patterson B. M. , Welipitiya D. , Nazareth A. S. , Sellmyer D. J.
Published: July 2002

The electronic structures of NdFe11Ti, NdCo10V2, and YCo10Cr2 have been studied with photoemission and spin-polarized calculations. The changes in these electronic structures upon nitrogenation have also been investigated. In the Fe compound, the Fe 3d states dominate the calculated density of states near the Fermi-edge, and the N(2p) peak is evident at around 6.3 eV. There is no shift in Fe 3d peaks visible in these compounds upon nitrogenation. Other than small energy shifts in the peak positions, there is an overall agreement between experimental data and the calculated density of states. The calculated density of states in the local-density approximation for YCo10V2 is broadened to account for the well-known many-body effects and compared with the photoemission data.
One-dimensional energy barrier model for coercivity
Authors: Jatau James A. , Torre Edward Della
Published: July 2002

The one-dimensional energy well model used to compute the coercivity of magneto-optic media based on domain-wall motion has been extended to include energy barriers. It is found that a local increase in either the anisotropy or exchange energy causes local wall pinning. In this case the wall is pinned in front of the energy barrier. When an applied field is gradually increased, the wall is pushed closer to the energy barrier, until the critical field is exceeded and a Barkhausen jump occurs. Variation of coercivity with barrier height was studied, and the results show that the coercivity increases essentially linearly with increase in height of the energy barrier. The effect of well shape on the coercivity was studied using several types of barriers of varying widths. Finally, the coercivity as a function of temperature for exchange and anisotropy barriers was computed and compared to the measured values. The results obtained in these calculations are compared with those for the well model, and it is found that by a proper choice of the dimensions of the energy barrier the coercivity obtained can approximate measured values, as in the case for the energy well model.
Biased differential relay with digital real time integration
Authors: Hijazi M. E. A. , Basak A.
Published: July 2002

A new algorithm has been developed for a single phase relay. This can be used for the protection of both single phase and three phase transformers. This paper presents a new scheme using the real time integration technique to determine the differential current caused by either magnetizing inrush current or an internal fault. The proposed relay has been simulated for continuous monitoring of the behavior of the differential current wave form, through the peak value of each current cycle and digital integration of that cycle. The trip of the relay depends on any changes in the area under the differential current wave form. The magnetizing inrush current waveform has less area under its curve than the fault current wave form which contains a sinusoidal wave and a transient dc component. This phenomenon has been used to block the operation of the relay on ‘‘not true’’ fault current seen as differential current.
A new two-dimensional coil system compensating X,Y,Z components of leakage field from an aperture of magnetic shields (abstract)
Authors: Sasada I. , Harada K.
Published: July 2002

Lightweight magnetic shields of high performance are required for the precise magnetic field measurements such as brain field measurement by SQUID magnetometers. Most of the present magnetic shielding rooms rely on ferromagnetic enclosures to isolate the area to be shielded from outside and, as a result, become heavy and expensive. In this paper, we propose a new method to compensate the leakage of noise field from an aperture of magnetic shields. The proposed method exploits the flux pattern just inside the aperture of the shielding body. Figure 1 depicts this method. Three flat coils mutually orthogonal and to be placed parallel inside the aperture are driven by the feedback signal to cancel each of three components detected by a three-axis fluxgate magnetometer. The important point of this method is that the coil system is flat unlike other active shielding system employing Helmholtz coils. Numerical results obtained for a superconductor vessel showed that the transverse field was attenuated by a factor of 100. Applying this method, the cylindrical shielding case of superconductor or of ferromagnet can be shortened with high shielding factor.
Numerical representation for anisotropic materials based on coenergy modeling
Authors: Péra Thierry , Ossart Florence , Waeckerle Thierry
Published: July 2002

The aim of this paper is to propose a numerical model of the magnetic behavior law of soft materials based on the equivalence between the vectorial B(H) relationship and a compact representation using the coenergy density w′ as a function of the magnetic field H. The experimental data needed are the B(H) curves in the rolling and transverse directions (respectively x and y). The anisotropic behavior of the sheet is taken into account by a proper modeling of the contours of equal coenergy over the Hx–Hy plan. Then the curves along any direction are derived from the coenergy density. This model is built to reproduce the decoupling of the magnetization processes at low field as well as the emergence of the difficult direction at 54.7° at higher fields, which is a typical behavior of grain-oriented silicon steel sheets. This model has been implemented and tested with experimental data for Goss textured sheets. The B(H) curves in various directions are computed with an error less than 5% over the whole range of magnetization.
The electromagnetic behaviors of a large object under high power microwave environment (abstract)
Authors: Governal Michael C. , Pan C. Y.
Published: July 2002

In order to investigate the behavior of aerospace asserts exposed to high power microwaves (HPM) or electromagnetic pulse (EMP) modeling the time-dependent electromagnetic (EM) behavior of the flying body before or after actual experiments has received significant attentions. The present available finite-difference time-domain method have fundamental difficulties in meeting the needs when the object has either complex geometry (including complex composite materials) or large physical size. We have proposed a method which can overcome the above-mentioned difficulties in EM modeling. The method is based upon a card-shuffle parallel algorithm used on a massive parallel computer. The initial test has been carried out on several types of computers. It enhances the modeling ability significantly. To model a large size flying body under HPM environment is no longer an impossible thing. The method can also provide effective approach to deal with many other important applications whenever it involves solving a wavelike equation and demanding giant computer storage. Examples are antenna design, aerodynamics, upset and damage model of semiconductor devices, turbulence, neural network, etc. Further improving the method is ongoing.
Erasure and noise study in barium-ferrite tape media
Authors: Lekawat L. , Spratt G. W. D. , Kryder M. H.
Published: July 2002

Erasure and noise characteristics of longitudinally oriented Ba-ferrite tape media with different squareness values were examined. The study showed media with higher squareness values were easier to erase using longitudinal erase fields and were harder to erase using vertical erase fields than media with lower squareness values. However, at the highest squareness value, longitudinal dc fields were superior to longitudinal ac fields in erasing the signal, suggesting that there is an optimum squareness value for longitudinal ac erasure. This trend was consistent with the observed increase in positive interactions with the increasing squareness ratios. Results of the noise study showed a dip in the reverse dc demagnetization noise plot for all samples studied. Strong positive interactions caused the ac demagnetization remanence noise to reach its final level more slowly than the isothermal remanence noise. This result is in agreement with the fact that the strong positive interactions render dc fields superior to ac fields in erasing the remanent signal levels.
Dependence of magnetic losses in Fe73.5 Cu1Nb3Si16.5B6 on annealing temperature
Authors: del Real R. P. , Prados C. , Pulido E. , Hernando A.
Published: July 2002

It is shown that heat treatments at different annealing temperatures change drastically the magnetic power losses of Fe-Cu-Nb-Si-B samples. This behavior is compared with the coercive force change. Two regions are described, the former, up to the nanocrystalline state, with a small diminution in losses, and the latter, coinciding with the hardening, with one order of magnitude increase in magnetic losses.
Two-impurity Anderson model: Variational wave functions with electron-hole excitations
Authors: Andreani Lucio Claudio , Beck Hans
Published: July 2002

A variational study of the two-impurity Anderson model is performed. Renormalization effects associated with electron-hole (EH) excitations are studied. For an interimpurity distance R≥Rc≂2.5/kF, the effect of EH excitations is to renormalize the f-level position and to add a magnetic interaction −JS1⋅S2, where the coupling J can be calculated in perturbation theory. For R≤Rc, the magnetic coupling itself is renormalized by the collective Kondo effect.