bluuebunny/crossref_metadata_2025 huggingface dataset:
Atomic disorder and phase transformation in L12-Ni3Al by mechanical milling
Authors: Zhou G. F. , Zwanenburg M. J. , Bakker H.
Published: July 2002

Mechanical milling of the ordered L12-Ni3Al was performed in a high-energy ball mill. The milling process was monitored by x-ray diffraction, high-field magnetization, ac magnetic susceptibility, and differential scanning calorimetry (DSC). It was found that antisite disorder is generated in the early stages of milling and a phase transformation from the disordered L12 compound to the disordered fcc solid solution of Al in Ni was observed after milling for long periods. Partial amorphization occurs after prolonged periods of milling. The long-range-order parameter decreases monotonously with time in the early stages of milling and attains a zero value after relatively short periods of milling. The lattice parameter and the relative strain increase with milling time. Magnetic properties of ball-milled Ni3Al at 4.2 K differ markedly from those of the ordered state. Based on the magnetic data, it is concluded that short-range disordering could be the third source of energy storage during milling of Ni3Al. Exothermic heat effects resulting from atomic reordering and phase restoration and/or crystallization are evident in the DSC scans of ball-milled Ni3Al.
Influence of prebonding cleaning on the electrical properties of the buried oxide of bond-and-etchback silicon-on-insulator materials
Authors: Ericsson Per , Bengtsson Stefan , Södervall Ulf
Published: July 2002

Three different groups of metal-oxide-semiconductor devices were manufactured of bond-and-etchback silicon-on-insulator wafers where the buried oxide functioned as the gate dielectric. The groups differed in the procedure used to clean the surfaces prior to bonding and in the location of the bonded interface. The surfaces were cleaned using either the standard RCA cleaning procedure without HF dip or by rinsing in de-ionized water only. The location of the bonded interface was in the buried oxide or at its interface toward a silicon wafer. The RCA-cleaned devices with the bonded interface within the buried oxide were found to degrade severely under bias temperature stress. This degradation was evident from both oxide charging and an increase in the density of states at the Si/SiO2 interface for negative gate biases. For positive biases the most prominent effect was lateral nonuniform charging of the oxide. The lateral nonuniformities might be connected to voids formed by ammonia desorption during postbonding annealing. Devices rinsed in de-ionized water prior to bonding and devices with a homogeneous oxide showed only slight degradation after bias temperature stress. Electron injection by internal photoemission showed that the buried oxides contained electron traps with capture cross sections corresponding to Coulomb attractive traps. The different processing conditions did not affect the trap cross section but influenced the trap density Nt. Devices with the bonded interface within the buried oxide had Nt≊1×1011 cm−2 in the case of RCA cleaning and Nt≊4×1010 cm−2 for de-ionized water rinsing. The devices with a homogeneous oxide had Nt≊4×109 cm−2. Electron trapping in the Coulomb attractive traps was accompanied by a corresponding increase in the density of states at the thermally grown Si/SiO2 interface for devices with a bonded buried oxide. SIMS investigations revealed a correlation between the degradation upon stress and hydrogen concentration in the devices with bonded oxides.
Tensile-strained GaAsP/AlGaAs quantum wells grown by low-pressure metalorganic vapor phase epitaxy
Authors: Pan Wugen , Yaguchi Hiroyuki , Onabe Kentaro , Ito Ryoichi , Shiraki Yasuhiro
Published: July 2002

Tensile-strained GaAs0.89P0.11/Al0.33Ga0.67As quantum wells have been grown by low-pressure metalorganic vapor phase epitaxy. The grown samples have been studied by low-temperature (6 K) and 100 K in-plane photoluminescence as well as cross-sectional polarized photoluminescence measurements. The experimental results show the coincident or reversal point of the light- and heavy-hole related transitions at the well width of 60 Å, above which the low-temperature photoluminescence is given by the transition 1e–1lh. A theoretical calculation agrees well with the experimental results.
Homoepitaxial diamond films codoped with phosphorus and nitrogen by chemical-vapor deposition
Authors: Cao G. Z. , Driessen F. A. J. M. , Bauhuis G. J. , Giling L. J. , Alkemade P. F. A.
Published: July 2002

Homoepitaxial diamond films codoped with phosphorus and nitrogen have been grown on natural diamond substrates using phosphine and nitrogen as doping sources by hot-filament chemical-vapor deposition. The experiments show that the incorporation of nitrogen enhances both the phosphorus incorporation and the film growth rate. The former is attributed to the local lattice dilatation caused by nitrogen, while the latter can be explained by defects induced on the surface of diamond by nitrogen or by a change in gaseous composition. The highest concentrations of phosphorus and nitrogen in the epitaxial films are approximately 3×1019 and 6×1019 atoms/cm3, respectively, determined by secondary-ion-mass spectrometry; however, these epilayers are highly resistive. Furthermore, cathodoluminescence and photoluminescence studies show that despite the variety of luminescence features related to nitrogen, no indication of phosphorus induced luminescence is found in the infrared to visible range.
Detection of very small defects and tiny inclusions just under mirror polished surfaces of silicon wafers by inside total reflection
Authors: Nango Nouhito , Furuya Hisashi , Furukawa Jun , Ogawa Tomoya
Published: July 2002

Defects just beneath the mirror surface of a silicon wafer grown by the Czochralski method is critical for epitaxial growth on the surface. Inside total reflection of an IR laser beam allowed only the light scattered by defects and tiny inclusions in the denuded zone of the wafer to be observed through the surface, because part of the scattered light makes an incident angle smaller than the critical one while the unscattered part of the beam is totally reflected and thus does not emerge from the wafer. Background brightness was caused by light scattering from tiny inclusions such as interstitial oxygen atoms in the zone, and inhomogeneity of this background indicated distribution fluctuation of the defects with gettering function.
Picosecond pulse shaping using dynamic carrier heating in a gain-switched semiconductor laser
Authors: Tolstikhin V. I. , Willander M. , Mamaev A. N.
Published: July 2002

Complementary modulation involving dynamic carrier heating along with carrier injection is reported as means for the picosecond pulse shaping in a gain-switched semiconductor laser. By numerical simulation based on the earlier developed model of a GaInAsP/InP laser operating in the 1.55 μm wavelength, we show that it is possible to produce the ideal-shaped low-chirp high-intensity optical pulses at multi-Gbit/s repetition rate. A three-terminal laser device, in which hot electrons with well-defined and tunable energy are injected by resonant tunneling, is proposed to implement this modulation technique.
Effect of water impurity on the crystallization of vacuum evaporated Se
Authors: Legros Anne , Shi Ming-Kun , Mouton André , Selmani Amine
Published: July 2002

The crystallization of vacuum evaporated selenium on glass, indium tin oxide, and polycarbonate substrates held at 70 °C was investigated using a near infrared microscope and scanning electron microscopy. The crystallization, observed merely at the Se/substrate interface, was found greatly enhanced and the structure of the crystallites modified if the substrates are soaked in water prior to the selenium deposition. It is suggested that residual water molecules entrapped in the substrate act as additional nucleation sites and induce the crystallization of selenium. The morphology of the individual crystallite depends only on the type of substrate; it is not affected by water-soaking or by post-annealing of the Se films in air. For all surface compositions and treatments, no chemical bond was formed between the substrate and the selenium film as revealed by x-ray photoelectron spectroscopy.
Electrostatics of anisotropic inclusions in anisotropic media
Authors: Helsing Johan , Samuelsson Klas
Published: July 2002

There are many ways to solve potential problems for anisotropic mixtures. The finite element method is a popular choice with great flexibility that gives the solution in the entire computational domain. Effective medium approximation is a simpler option that only estimates effective properties. Interface integral equation methods is a third class of methods that has been studied for a long time, but only rarely been used for computations. In this paper, electrostatics of anisotropic two-dimensional composites is discussed. The geometry under consideration is a periodic composite of arbitrarily shaped anisotropic inclusions in an anisotropic matrix. The location of the inclusion interfaces are given on analytic form as to facilitate reproduction of results. Highly accurate calculations are performed with a coupled first and second kind Fredholm integral equation method. Comparison is made with finite element method calculations.
Atomic force microscope patterning on plasma deposited polyacetylene film
Authors: Gorwadkar S. , Vinogradov G. K. , Senda K. , Morita S.
Published: July 2002

Nanometer-thick hydrocarbon films were plasma polymerized in a rf pulse discharge in an acetylene/argon mixture and were mechanically patterned by the AFM (atomic force microscope). In addition a dc bias voltage was applied to the gold coated Si3N4 AFM tip. Depending on the experimental conditions, different patterns have been observed: mechanical indentation, electric charge, and material deposition. The viscous properties of the plasma deposited film affects the movement of the AFM tip while it is scanning the surface in a contact mode, and also affects the size and shape of the patterned area. Spikes of about 25–72 nm height and 60–200 nm width were formed from gold transferred from the newly mounted gold coated tips. The mechanism of gold deposition could be assigned to the Joule heating of the tip, resulting from the electric breakdown of underlying dielectric layers.
Bounds on the effective thermal-expansion coefficient of a polycrystalline aggregate
Authors: Ghorai Asis Kumar , Dutta Tapati
Published: July 2002

The present study establishes bounds on the effective thermal-expansion coefficient α* of a monocomponent isotropic polycrystalline aggregate with crystallites having a preferred uniaxial axis (tetragonal, trigonal, and hexagonal) using multiple scattering theory (MST). The expression of α* using MST for such polycrystals is a monotonically increasing (or decreasing) function of the effective bulk modulus K*. This provides a means to predict bounds on α* using the bounds on K*. MST has been used to derive, first, the third-order bounds on K* and, second, the t-matrix bounds, which are even stricter. Both these bounds are, however, narrower than the Voigt–Reuss bounds. Finally, the formulas obtained have been used to calculate bounds on α* in the case of polycrystals belonging to symmetry classes listed above, and the results obtained are found to compare favorably with observations.
Optical absorption of microcrystalline Si-Ni thin films
Authors: Schoenfeld Olaf , Zhao X. , Hempel T. , Aoyagi Y.
Published: July 2002

The optical absorption behavior of microcrystalline Si-Ni thin films (μc-Si-Ni) with a homogeneous and heterogeneous microstructure is investigated. In the homogeneous μc-Si-Ni specimens the structural phase consists of nanometer-sized grains. The heterogeneous specimens consist of a network of micrometer-sized crystalline Si needles. The meshes of the Si needle network are filled with nanometer-sized grains. The optical absorption of the specimens is compared with the structural properties. The optical absorption of μc-Si-Ni below an energy of 1.7 eV is described by a model of grain boundary states while above 2.0 eV optical transitions in the nanometer sized grains are discussed.
Deep-center oxygen-related photoluminescence in GaAs doped with dimethylaluminum methoxide during organometallic vapor phase epitaxy
Authors: Yu P. W. , Park Y. , Skowronski M. , Timmons Michael L.
Published: July 2002

Infrared oxygen-related photoluminescence measurements of GaAs epilayers grown by organometallic vapor phase epitaxy (OMVPE) and intentionally doped with dimethylaluminum methoxide (DMALMO) are reported. The photoluminescence emissions at 1.08, 0.95, 0.81, and 0.62 eV are detected. The centers responsible for the emissions at 0.81 and 0.62 eV are tentatively attributed to Al–O–Ga and Al–O–Al, respectively, based on correlation with layer growth conditions. The growth parameters lead to information on the number of nearest-neighbor aluminum associated with oxygen. The Al–O–Ga center responsible for the 0.81 emission is also studied for AlxGa1−xAs(x=0.05–0.9) layers grown by OMVPE without DMALMO. The 0.81 eV emission is attributed to the transition from the conduction band to the Al–O–Ga center. The center shows the lattice relaxation exemplified by the Franck–Condon shift of 0.15 eV. The thermal ionization energy of this acceptor is determined to be 0.56 eV above the valence band.
Hot-electron impact excitation of ZnS:Tb alternating-current thin-film electroluminescent devices
Authors: Streicher K. , Plant T. K. , Wager J. F.
Published: July 2002

A new method of performing hot-electron impact excitation experiments using alternating-current thin-film electroluminescent (ACTFEL) devices is reported. This method relies on the use of a field-control circuit to control the magnitude of the phosphor field and consists of plotting the intensity of a given electroluminescence transition, normalized by the amount of conduction charge which flows while the field-control circuit is asserted (the electroluminescence intensity of a given transition divided by the conduction charge is denoted the impact excitation quantum yield ηie), as a function of the phosphor field Fp. ηie vs Fp is measured for ZnS:Tb ACTFEL devices fabricated by atomic layer epitaxy (ALE) and by sputtering. ηie exhibits a threshold at approximately 0.5 MV/cm and saturation at approximately 1.5 MV/cm. The magnitude of ηie for the ALE ACTFEL device depends strongly on temperature; in contrast, ηie for the sputtered ACTFEL device is virtually temperature independent.
Structural changes of multiphase bismuth-oxide superconductor thin films at high temperatures
Authors: Ishibashi Teruo , Yoda Osamu
Published: July 2002

Structural changes during heat treatment have been studied for multiphase bismuth-oxide thin film superconductor in terms of desorption of oxygen. Desorption of oxygen by heating has made the low Tc 2212 phase thermally unstable. About 90% of the 2212 phase has melted at 800 °C. In the course of cooling, the high Tc, oxygen stoichiometric 2223 phase has recrystallized from the melted 2212 phase. After heat treatment, the thin film has been composed mostly of the 2223 phase with a Tc onset and a Tc endpoint of 107 and 83 K, respectively.
p-doped GaAs/Ga0.51In0.49P quantum well intersub-band photodetectors
Authors: Hoff J. , He X. , Erdtmann M. , Bigan E. , Razeghi M. , Brown G. J.
Published: July 2002

Lattice-matched p-doped GaAs–Ga0.51In0.49P quantum well intersub-band photodetectors with three different well widths have been grown on GaAs substrates by metal-organic chemical-vapor deposition and fabricated into mesa structures. The photoresponse cutoff wavelength varies between 3.5 and 5.5 μm by decreasing the well width from 50 down to 25 Å. Dark current measurements as a function of temperature reveal activation energies for thermionic emission that closely correspond to measured cutoff wavelengths. Experimental results are in reasonable agreement with Kronig–Penney calculations.
Small-signal analysis of novel semiconductor superlattice electron-wave interference devices
Authors: Yahya C. B. , Martin K. P. , Higgins R. J.
Published: July 2002

The linear response of a novel AlxGa1−xAs superlattice electron-wave interference diode (EWID) is numerically investigated using the time-dependent Schrödinger equation. This device is based on analogies between electromagnetic waves in dielectrics and quantum mechanical electron waves in semiconductors. These analogies provide a basis for a new class of highly functional devices which use above-band transport. Recent experimental results and theoretical models showed that the EWID has direct current characteristics similar to the resonant tunneling diode. This first quantum mechanical calculation for the EWID alternating current characteristics shows that the device negative differential resistance (NDR) persists up to about 10 THz. By examining three different EWID designs, it is shown that device parameters, such as the number of layers, have a strong effect on high-frequency performance. The important property of NDR, combined with expected high current densities and possible integration with optoelectronic devices, makes the EWID a good candidate for high speed applications.
Electron–electron interaction in three-dimensional model quantum box
Authors: Yang R. , Ruden P. P.
Published: July 2002

The effects of electron–electron interaction on the electronic structure of a three-dimensional model quantum box are studied. The potential in the lateral plane of the quantum box is taken to be a two-dimensional harmonic potential, and an infinite quantum-well potential is used to represent the confinement in the vertical direction. Three-dimensional two-electron wave functions are constructed by the configuration interaction technique. Exchange and correlation are found to affect the electronic structure strongly. The effect of the finite thickness of the quantum box on the electronic structure is examined. It is shown that the electron–electron interaction can induce interesting polarization effects not only in the lateral plane but also in the vertical direction of the quantum box. Numerical results based on GaAs material parameters are presented.
Phosphorus-enhanced diffusion of antimony due to generation of self-interstitials
Authors: Pichler P. , Ryssel H. , Ploss R. , Bonafos C. , Claverie A.
Published: July 2002

In a series of experiments, the influence of phosphorus diffusion at high concentrations on the diffusion of an antimony marker layer was investigated. The marker layer was separated from the surface by a 4 μm layer of epitaxially grown silicon. To reduce the effects of implantation enhancement and phosphorus precipitation on the diffusion of the antimony marker layer, the phosphorus was implanted into a polysilicon layer deposited on top of the single-crystalline substrate. It was found that the diffusion of the antimony marker layer is already reduced by the epilayer. From these diffusion coefficients, upper limits for the fractional diffusivity of antimony were derived. In contrast to previous investigations, the diffusion of the antimony marker layer was found to be enhanced below regions where phosphorus was implanted. A comparison of diffusion in FZ and CZ samples shows that this enhanced diffusion of antimony can be explained only by an injection of self-interstitials from the phosphorus-doped region. Since the polysilicon layer was found to recrystallize, this self-interstitial injection can be the result of phosphorus diffusing mainly via self-interstitials, phosphorus precipitation, or both. Using Boltzmann–Matano analysis and antimony diffusivity data, the fractional diffusivity of phosphorus via self-interstitials was estimated to be lower than 0.71 at 950 °C. Possible sources of error in this estimation are discussed.
Effects of crystallinity and thickness of silicide layer and substrate orientation on the oxidation of NiSi2 on silicon
Authors: Huang G. J. , Chen L. J.
Published: July 2002

Oxidation kinetics for both dry and wet oxidation of epitaxial NiSi2 (200 nm)/(001)Si samples as well as dry oxidation of polycrystalline NiSi2 (200 nm)/(111)Si and epitaxial NiSi2 (600 nm)/(111)Si samples have been studied by transmission electron microscopy. Comparing oxidation kinetics data of 200-nm-thick epitaxial NiSi2 on (001) and (111)Si, activation energies of the parabolic rate constants are rather close, whereas those for linear rate constants are substantially different. The orientation dependence of the linear activation energies is explained in terms of the total number of available Si atoms for oxidation as a function of the substrate orientation. Oxide growth rate was found to be higher in polycrystalline NiSi2/(111)Si samples than that in epitaxial NiSi2/(111)Si samples. Strong influence of the grain boundaries of NiSi2 on oxidation kinetics was observed with the grain boundaries serving as fast paths for oxidation. For dry oxidation of epitaxial NiSi2 (600 nm)/(111)Si samples, both parabolic and linear activation energies are higher than those of Ni(200 nm)/(111)Si samples.
Effects of gas flame annealing on the structure of poly-Si films
Authors: Masaki Y. , Suzumi M. , Qu W. F. , Kakimoto Y. , Kitagawa A. , Suzuki M.
Published: July 2002

The effect of gas flame annealing on the structure of polycrystalline Si films was studied using transmission electron microscopy and electron spin resonance. This annealing technique involved heating the sample surface to more than 1100 °C using flat gas flames with a scan rate of 1 mm/s and a heating rate of about 260 °C/min. Electron microscopy images revealed that the secondary grain growth proceeded with increasing the number of annealing times (annealing frequency) and that the grain size for samples annealed at 1360 °C was more than 1 μm whereas secondary grain growth was not significant for samples annealed at 1150 °C. Further, it was found that the spin density in the samples annealed at 1360 °C decreased from 1.5×1018 cm−3 to 3.8×1017 cm−3. It was concluded from the spin resonance results and the electron microscopy images that the secondary grain growth consists of two processes, the initial structural rearrangement of the grain boundaries and the subsequent grain growth.
Optimized resonant tunneling light emitting diodes
Authors: Kindlihagen A. , Willander M. , Chao K. A.
Published: July 2002

We have programmed a self-consistent calculation to theoretically model a resonant tunneling light emitting diode of optimized performance, using a bipolar AlGaAs double barrier resonant tunneling structure as an illustrating example. The principle of optimization is under a nearly zero-field bias, electrons and holes resonantly tunnel into the well simultaneously, and are trapped completely in the well to recombine to emit light with maximum intensity.
Electron depletion due to bias of a T-shaped field-effect transistor
Authors: Georgakis G. A. , Niu Qian
Published: July 2002

A T-shaped field-effect transistor, made out of a pair of two-dimensional electron gases, is modeled and studied. A simple numerical model is developed to study the electron distribution versus applied gate voltage for different gate lengths. The model is then improved to account for depletion and the width of the two-dimensional electron gases. The results are then compared to the experimental ones and to some approximate analytical calculations and are found to be in good agreement with them.
Pinning via intrinsic weak links: A possible scenario and applications
Authors: Sergeenkov Sergei A.
Published: July 2002

A possible scenario for critical current enhancement via intrinsic weak-link-assisted pinning in isotropic high-Tc superconductors is proposed. By considering a subtle balance between different forces acting upon extended (dislocations) and point (oxygen vacancies) defects to stabilize their equilibrium position inside a crystal, the model suggests a qualitative description of some anomalous magnetic properties observed in dislocated, deoxygenated, and irradiated YBa2Cu3O7−δ superconductors.
Angular dependence of the coercivities of Ba ferrite particulate samples: A comparison between theory and experiments
Authors: Han D. H. , Luo H. L. , Yang Z.
Published: July 2002

We measured the angular dependence of coercivity Hc and remnant coercivity Hr of oriented, Co-Ti substituted, plate-like barium ferrite particulate samples. The experimental results were compared to those modelled by a chain of two oblate ellipsoids. By including the effects of the uniaxial magnetocrystalline anisotropy, the shape anisotropy, and the interaction anisotropy of the ellipsoids, the model of a chain of oblate ellipsoids can explain the magnetization reversal mechanism of the barium ferrite samples. In an individual particle, the magnetic moment tends to reverse coherently because of it large uniaxial magnetocrystalline anisotropy, while in the oriented particulate samples the magnetization reversal of the stacked assembly occurs by symmetrical fanning, consistent with the larger uniaxial magnetocrystalline anisotropy and stronger interaction anisotropy.
Low-temperature photoluminescence from bulk CdTe and Cd0.967Zn0.033Te
Authors: Lee Jaesun , Giles N. C.
Published: July 2002

Low-temperature photoluminescence spectroscopy has been performed on bulk CdTe and Cd1−xZnxTe (x=0.033) in the emission region near the band-gap energy. A method to distinguish the free-exciton recombination band from other close-lying emissions in these materials is described. From the identification of the free-exciton emission, the band-gap energy was then determined. Radiative emission bands involving exciton–phonon quasiparticles were clearly observed above the band-gap energy at temperatures up to 40 K. The sample variation in the above-band-gap emission indicates that the exciton–phonon quasiparticle energy depends on the presence of donors and/or acceptors. In addition, the above-band-gap emission related to (e,h) transitions between the conduction band and valence band in these materials is observed.
Influence of light soaking on surface- and bulk-spin densities in hydrogenated amorphous silicon
Authors: Zhang Qing , Kumeda Minoru , Shimizu Tatsuo
Published: July 2002

To study the influence of light soaking on surface- and bulk-spin densities in hydrogenated amorphous silicon (a-Si:H) films, we have measured the spin density in the films with thicknesses ranging from 0.02 to 3.9 μm as a function of light-soaking time. It is found that the total spin density in films thinner than 0.1 μm approaches the steady state spin density within a few hours and is increased by only a factor of 2–3 after 12 h strong illumination with light intensity of 3.8 W/cm2. However, for the films thicker than 0.7 μm, the total spin density does not show a tendency to approach the steady state value and is increased by a factor of 10. Furthermore, in order to describe the spatial distribution of the spin density, a simulation, in which the total spin density is composed of the surface-spin density and the bulk-spin density, is carried out. The calculated spatial distribution further supports that light soaking increases the surface-spin density by a factor of 2–3, but the bulk-spin density by a factor of 25 if the bulk-spin density for the annealed state is taken as 4×1015 cm−3.
Characterization of GaAs/GaAsP quantum wire structures fabricated by atomic layer epitaxy
Authors: Isshiki Hideo , Aoyagi Yoshinobu , Sugano Takuo , Iwai Sohachi , Meguro Takashi
Published: July 2002

We have developed a fabrication process for isolated quantum wire structures, using advanced atomic layer epitaxy (ALE) techniques based on the self-limiting effect. We describe several advantages of the self-limiting effect for the fabrication process of quantum nanostructures. We also present characterizations of the quantum wire structures including photoluminescence (PL) measurements. Due to the fact that the ALE is localized on a nanometer scale and that a growth mode switching technique between isotropic and anisotropic ALE was used, well-defined GaAs quantum wires with structure control were successfully realized. Also, quantum confinement effects of one-dimensional systems have been observed clearly in the quantum wires via PL measurements.
Preparation of b-axis oriented YBa2Cu3Oy thin film by laser metalorganic chemical vapor deposition and successive annealing
Authors: Kawamoto Koji , Hirabayashi Izumi
Published: July 2002

We have recently succeeded in preparing b-axis oriented YBa2Cu3Oy thin films by laser metalorganic chemical vapor deposition and special thermal treatments. The b-axis orientation means that the CuO chain in the basal plane is perpendicular to the film surface, while the a-axis orientation is parallel to the film surface. When oxygen is introduced after complete oxygen depletion around y≊6, we always obtain a-axis oriented films. On the other hand, b-axis oriented films can be obtained if the film whose c-axis is parallel to the film surface is refilled with oxygen after medium oxygen reduction close to the orthorhombic-tetragonal phase boundary. These phenomena can be explained by oxygen arrangement in Cu-O basal planes caused by the stress between the deposited films and the substrates due to the difference in their thermal expansion coefficients.
Exact moduli of phase interchange invariant composites with coupled fields
Authors: Chen Tungyang
Published: July 2002

A concise and general solution is derived for the exact effective moduli of a two-phase medium for multifield, coupled linear responses, in which the medium is insensitive to phase interchange. The constituents are isotropic and statistically symmetric. The solutions follow from a compatibility condition together with a phase interchange theorem recently discovered by Milgrom and Shtrikman [Phys. Rev. A 40, 1568 (1989)]. Due to the mathematical analogy, the obtained results are applicable to certain classes of two-dimensional polycrystals. The solution extends the corresponding results for two-coupled field media obtained by Benveniste [J. Mech. Phys. Solids 43, 553 (1995)].
Interface reactions between manganese zinc ferrite single crystals and SiO2-PbO-ZnO ternary systems
Authors: Je Hae June , Kim Do Kyung , Kim Chong Hee , Hong Kug Sun , Park Byeong Won , Jang Sung Do
Published: July 2002

Interface reactions between Mn-Zn ferrite single crystals and SiO2-PbO-ZnO ternary systems were investigated using scanning electron microscope, electron probe microanalyzer, and x-ray diffractometer. The addition of ZnO in a SiO2-PbO glass resulted in inhibition of formation of the intermediate phase and suppression of dissolution of the ferrite. The interface morphology became a dendrite structure with increase of ZnO concentration in the glass. Appearance of the anomalous concentration profile of Zn and Mn at the ferrite adjacent to the interface was investigated in terms of the dissolved quantity of Mn and Zn ion. It was found that Zn ion has dissolved from the ferrite into the glass melt during the reaction and Mn ion in the ferrite dissolved into the glass melt more than Zn ion, resulting in changing the undissolved ferrite adjacent to the interface into a Zn-rich Mn-Zn ferrite, Mn0.16Zn0.85Fe2O4, after reaction with 57SiO2-38PbO-5ZnO (in mol %) glass. It was considered that the source of the hump of Zn concentration at the ferrite adjacent to the interface would be the undissolved, remaining Zn ions. A microscopic model was suggested.
Electrical characteristics of diamond films synthesized from methane/hydrogen and acetone/hydrogen mixtures
Authors: Hung Chung-Chih , Valco George J. , Aithal Shashikant M. , Subramaniam Vish V.
Published: July 2002

We present the results of detailed electrical measurements on diamond films grown by hot-filament chemical-vapor deposition. Two different mixtures of reaction species, hydrogen/methane and hydrogen/acetone, were utilized to grow diamond films. The latter is useful for introducing dopants during growth in a relatively safe manner. For the diamond films grown using hydrogen and methane, a high-temperature anneal increased the resistivity of the films by seven orders of magnitude to about 1012 Ω cm while the I–V characteristics retained the same qualitative shape. Further annealing was found to change the I–V characteristics of the film itself, not the contacts. Spatial variation of the electrical characteristics is also reported. In addition, for the diamond films grown using the hydrogen and acetone, a variety of different results was obtained. Electrical measurements and Raman spectroscopy suggest that some areas of these films were high-resistivity diamond while other areas may contain nondiamond carbon at grain boundaries.
Modification on the surface of superconducting YBa2Cu3O7−x films by microwave plasma-enhanced pulsed laser deposition
Authors: Huang C. S. , Tseng T. Y. , Chung B. C. , Tsai C. H. , Hsu S. S. , Lin I. N.
Published: July 2002

Superconducting YBa2Cu3O7−x films having smooth surfaces were successfully synthesized by a microwave plasma-enhanced pulsed laser deposition (PEPLD) process. Particulates that frequently occurred in films grown by the conventional PLD process were effectively eliminated. The films were epitaxial-like. The c-axis was perpendicular to the films’ surface, the a and b axes were aligned in the films’ plane, and the onset and zero of the transition temperature were at Tc=90 K and Tc0=86 K, respectively. Optical emission spectroscopy indicated that the presence of oxygen plasma not only reexcited the laser-induced species via the collision process, but also produced abundant atomic oxygen in PEPLD process. The surface reaction kinetics for the formation of the thin-film process was thus greatly enhanced, which substantially improved the thin-film quality.
Interface intermixing and magnetoresistance in Co/Cu spin valves with uncoupled Co layers
Authors: Willekens M. M. H. , Rijks Th. G. S. M. , Swagten H. J. M. , de Jonge W. J. M.
Published: July 2002

The interpretation of experiments on the effect of interface intermixing on the giant magnetoresistance (GMR) effect in antiferromagnetic-coupled multilayers can be complicated by the fact that interface intermixing also changes the coupling strength; therefore, we have grown an artificially intermixed region in Co/Cu spin valves with uncoupled Co layers. The structure we used was a newly engineered spin valve composed of 100 Å Co+6 Å Ru+25 Å Co+40 Å Cu+100 Å Co. Here the Ru layer provides an antiparallel alignment of the Co layers and the Cu layer decouples the upper two Co layers. An intermixed CoCu region has been grown at the Cu/Co interface and in some cases also at the Co/Cu interface by alternately sputtering 1 Å Co and 1 Å Cu. X-ray measurements confirm the existence of an intermixed region, although no reduction of magnetic moment is observed as is reported for homogeneous sputtered Co0.5Cu0.5 alloys. This indicates the existence of Co clusters in the intermixed regions. There is no difference in GMR between an intermixed layer of thickness t at one Co/Cu interface or two intermixed layers of thickness t/2 at both Co/Cu interfaces. Thus, it seems that the total thickness of the intermixed regions is decisive for the magnitude of the GMR. Because G, ΔG, and ΔG/Gap all show a gradual decrease when the nominal thickness of the intermixed region increases from 0 to 36 Å, this indicates that there is no strong spin-dependent scattering in this region. This is in agreement with calculations on a model bilayer Co/Cu/Co with the Camley–Barnas model.
Gate pulse frequency-dependent kink effects in GaAs metal-semiconductor field-effect-transistors with a low-temperature-grown buffer layer
Authors: Haruyama Junzi , Negishi Hitoshi
Published: July 2002

Two kinds of kink effects (abrupt increases in pulsed drain current ID) were observed in GaAs metal–semiconductor field-effect transistors (MESFETs) with a buffer layer grown by molecular-beam epitaxy at low substrate temperature of 300 °C (LTB), when pulsed voltage trains with swinging range from threshold voltage to 0 V and 10% duty cycle were applied to gate electrode. These kink effects were not observed in dc measurement; however, they appeared with increasing frequency of those gate pulse trains. One kink effect is observed at 100 μs of the pulse width of high level Wh with the pulsed ID increase of 11% (first kink). Overshoot observed in the transient wave form suggests that this first kink effect is attributed only to capture of excess number of holes, which are generated by impact ionization along channel, into the hole trap of LTB. The lifetime of these holes is estimated to be 100 μs at least in the LTB from the discussion. The other kink effect is observed at 10 μs of Wh with the pulsed ID increase of 47% (second kink). This second kink effect is attributed to elimination of anomalous transient response of self-side-gating effect (SSGE) (side-gating effect caused by negative voltage applied to gate pad) by excess holes mentioned above.

The mechanism of this transient of SSGE is discussed based on the observation of anomalous transient wave-form change with as small a time constant as 200 ns, using the model of side-gating effect proposed by Goto, Ohno, and Yano [IEEE Trans. Electron Devices ED-34, 1821 (1990)]. The lifetime of holes is estimated to be 10 μs at least in VD region smaller than the VD at which impact ionization occurs from the discussion. The observation of these kink effects gives evidence that the elimination of kink effects in dc was due to perfect recombination of free holes in all VD regions in this LTB MESFET. On the other hand, pulsed ID–VD curves were the same as ID–VD measured at 0 V of Vg in dc of the MESFETs with an i-GaAs buffer layer grown at 600 °C. Therefore, it is concluded that these frequency-dependent kink effects result only from the LTB. All kink effects are eliminated in dc measurement of this LTB MESFET. The kink effect related to SSGE, however, is observed at higher frequency in the LTB MESFET rather than in the MESFET with normal temperature grown buffer.
Direct current and microwave characterization of integrated resonant tunneling diodes
Authors: Janes D. B. , Webb K. J. , Carroll M. S. , Starnes G. E. , Huang K. C. , Shenoy J. , Melloch M. R.
Published: July 2002

We present an integrated device fabrication sequence for GaAs/AlGaAs resonant tunneling diodes and the results of dc and microwave characterization of the integrated devices. The development of an integrated structure represents a first step toward monolithic or hybrid integration for applications such as oscillators in the 100 GHz–1 THz range. The use of a proton implant in addition to a mesa etch for device isolation allows low parasitic capacitance connections to bond pads, interconnects, or radiating elements. The dc and microwave measurement procedures and an equivalent circuit topology for the integrated device are described. Several features associated with the integrated geometry, including a decrease in peak current density with increasing device diameter, are observed in a study of the current-voltage characteristics of devices with various diameters. Contact resistances are determined from a physically based model and compared with experimental results. Microwave characterization techniques are used to obtain microwave equivalent circuit parameters for the integrated diodes. Device capacitance and conductance are presented as functions of device dimension and bias voltage. Parasitic circuit elements, including series resistance and a capacitance and resistance associated with the proton bombardment used to define the mesa, are also determined from the microwave analysis. The results obtained from microwave impedance measurements are compared with parameters obtained from dc characterization and numerical simulations of comparable device structures.
Chaos in superconducting tunnel junctions
Authors: Xu Huifen , Fan Minhua , Yang Senzu , Wu Peiheng
Published: July 2002

All currents through a superconducting tunnel junction, including quasiparticle tunneling current and Josephson current, are modeled on an electronic simulator. The current–voltage (I–V) curves of a rf-driven superconducting tunnel junction are generated using this simulator so that the ranges of dc bias and rf excitation, over which chaotic behavior occurs, can be found for a given rf frequency. It turns out that the ranges are roughly the same as those for the resistively shunted junction model, although some differences exist. Contributions of either quasiparticle current or Josephson current to chaos in the junction are evaluated briefly.
Field emission from a silicon surface potential well through a thin oxide
Authors: Huang Qing-An
Published: July 2002

The quantum well arises because band bending of silicon confines the electrons to a narrow surface region during field emission. Field emission from the surface potential well of silicon covered with a thin oxide layer is modelled using a conduction band effective potential and effective mass of the oxide. Based on an exact solution of the Schrödinger equation under the application of a linear potential field, the emission current density is represented by an Airy function. The results show that the existing thin oxide always reduces the emission current and the standard Fowler–Nordheim plot gives the two distinct linear regions.
Growth mode of epitaxial Si0.5Ge0.5 layer grown on Si(100) by ion-beam-assisted deposition
Authors: Park S. W. , Shim J. Y. , Baik H. K.
Published: July 2002

The nucleation and growth of the Si0.5Ge0.5 alloy layer on Si (100) substrate during ion-beam-assisted deposition (IBAD) have been investigated by atomic force microscopy, reflection high-energy electron diffraction, transmission electron microscopy, and double-crystal rocking diffraction. We confirmed that Si0.5Ge0.5 nucleates on Si (100) via the Stranski–Krastanov (SK) mechanism by IBAD, and Ar-ion bombardment suppressed SK growth mode as well as improved crystalline perfection. The epitaxial temperature was observed at 200 °C, and it was much lower than the growth temperature (550–600 °C) in molecular-beam epitaxy (MBE). The χmin value (the ratio of channeling to random backscattering yields) was 10.5% lower than the obtained MBE value. The effect of ion bombardment on nucleation was explained as the result of ion-bombardment-induced dissociation of three-dimensional islands and enhanced surface diffusion, and appeared only at low deposition temperatures where the dissociation of three-dimensional islands is more favorable than the formation of those islands.
Layer thickness and temperature variation of the giant magnetoresistance in Fe/Ag multilayers
Authors: Li Shuxiang , Yu Chengtao , Lai Wuyan , Wang Yizhong , Yan Minglang
Published: July 2002

The magnetoresistance ratio in Fe/Ag multilayers is increased dramatically with decreasing Fe layer thickness. This behavior can be explained in terms of interference of electronic Bloch waves due to interface roughness and lattice mismatch between Fe and Ag layers. At the same time, the interlayer coupling changes from ferromagnetic to antiferromagnetic when the Fe layer thickness is decreased. An anomalous temperature effect of magnetoresistance is also observed. The magnetoresistance ratio at 4.2 K is smaller than that at 300 K when the Fe layer thickness is larger than about 12 Å, which can be attributed to the presence of some superparamagnetic-like thin pieces at the interfaces.
Effects of phosphorous doping on the stability of coercivity of cobalt-substituted acicular oxides
Authors: Na J. G. , Jeung W. Y. , Han D. H. , Wang G. H. , Luo H. L.
Published: July 2002

The effects of P doping on the coercivity and magnetic-thermal stability of Co-substituted iron oxides have been investigated by means of magnetic measurements, torque curves, and Mössbauer spectra. It is found that the P doping of cobalt-substituted acicular oxides promotes the coercivity and the stability with temperature of the coercivity. Then effects may be attributed to the increase of the cubic magnetocrystalline anisotropy constant K1 and the activation energy E, respectively. The Mössbauer results suggest that P ions are located in the neighborhood of the B-site Fe ions.
Electrical characteristics of (Sr0.2Ba0.8)TiO3 positive temperature coefficient of resistivity materials prepared by microwave sintering
Authors: Chang Horng-Yi , Liu Kuo-Shung , Lin I-Nan
Published: July 2002

The resistivity-temperature (ρ-T) properties of the (Sr0.2Ba0.8)TiO3 materials densified by the microwave sintering process were investigated. Neither the sintering temperature (1100–1180 °C) nor the soaking time (10–40 min) in this process exert a significant influence on the positive temperature coefficient of resistivity behavior of the as-sintered samples. Contrarily, reducing the cooling rate from 154 °C/min to 4 °C/min or post-annealing the samples at 1250 °C for 2 h would increase the maximum resistivity (ρmax) more than three orders of magnitude. Consequently, the resistivity jump with the maximum to minimum resistivity ratio ρmax/ρmin≊107 has been achieved. The increase in maximum resistivity by these processes is ascribed to the increase in cationic vacancies, which subsequently act as effective electronic traps. The trap level (Ese) of the cationic vacancies is estimated from the Arrhenius plots of maximum resistivity (ρmax) and temperature (Tmax) to be Ese≊1.5–2.0 eV. On the other hand, the ρ-T curves in the T≳Tmax regime estimate the intrinsic trap level (Esi) of the (Sr0.2Ba0.8)TiO3 materials to be Esi≊2.46–2.51 eV.
Second harmonic generation in potassium niobate thin films
Authors: Chow A. F. , Lichtenwalner D. J. , Auciello O. , Kingon A. I. , Busch J. R. , Wood V. E.
Published: July 2002

Green light produced by second harmonic generation has been observed in an epitaxial orthorhombic KNbO3 thin film planar waveguide produced by ion-beam sputter deposition on a (100)-oriented MgO single crystal substrate. A Nd:YLF laser beam, with a wavelength of 1.053 μm and ∼80 ps, 100 MHz pulses under mode-locked operation, was coupled into the waveguide using a rutile prism, and a green light streak 3–4 mm long was seen in the guide. The TM0 mode of the input beam was phase matched to the TE1 mode of the second harmonic for a film thickness of 2300 Å. Second harmonic generation was also observed in a nonwaveguided configuration on thicker (4600–6500 Å) films on both MgO and KTaO3 substrates.
Model of the competitive growth of amorphous carbon and diamond films
Authors: Ford I. J.
Published: July 2002

Recent experiments by D. S. Olson, M. A. Kelly, S. Kapoor, and S. B. Hagstrom [J. Appl. Phys. 74, 5167 (1993)] have demonstrated that depending on the ratio of the fluxes of carbon and atomic hydrogen onto a substrate in a chemical vapor deposition reactor, either an amorphous carbon deposit, or a crystalline diamond film, may be produced. A simple interpretation of these findings is proposed, based on a set of phenomenological rate equations for various growth and etching processes. The model is simple enough to admit analytical solutions in certain circumstances, which may provide insights into the optimisation of carbon film deposition methods.
Photoassisted organometallic vapor-phase epitaxy of ZnSe: An ab initio molecular orbital study
Authors: Nakano Takashi , Hirano Tsuneo
Published: July 2002

Surface reactions of photoassisted organometallic vapor-phase epitaxy (OMVPE) of ZnSe using dimethyl selenide (DMSe) and dimethylzinc (DMZn) as source gases were studied by ab initio molecular orbital methods. The (001) face of zinc-blende-type ZnSe was simulated by a Zn4Se4 cluster, and the corresponding surface having a hole generated by photon irradiation was simulated by the Zn4Se4 cluster cation. In the Se-layer growing step, electron transfer occurs from the nonbonding Se orbital of DMSe to the Zn4Se4 cluster in both neutral and cation forms with simultaneous back-donation from the cluster part to the antibonding Se-C orbitals, resulting in the weakening of the Se—C bond of DMSe. In the Zn-layer growing step, electron transfer occurs from the Zn-C σ orbital of the adsorbed DMZn to the Zn4Se4 cation resulting in the weakening of the Zn—C bond, while such electron transfer does not occur to the neutral Zn4Se4. Thus, while the Se layer can grow even under nonirradiation, the Zn layer can grow only under irradiation. These results explain the enhancement of the growth rate observed in the photoassisted OMVPE of ZnSe.