bluuebunny/crossref_metadata_2025 huggingface dataset:
Structural properties of magnetically anisotropic SmCo5 ribbons
Authors: Li Long , Zhang Wen-yong , Zhou Yu-qing , Li Jian-qi , Shen Bao-gen , Zhang Jian
Published: July 2002

The structural features of magnetically anisotropic SmCo5 ribbons prepared by melt spinning have been investigated by means of transmission electron microscopy. It was determined that this kind of SmCo5 ribbons are made up of structural domains, which consist of layered grains regularly packed along the c-axis direction. The c axes of the grains generally lie in the ribbon plane and along the length. The SmCo5 grains are well crystallized in the conventional hexagonal structure (P6/mmm). The properties of grain boundaries either with or without impurity phases have been systematically analyzed.
Local membrane ordering of sponge phases at a solid–solution interface
Authors: Hamilton W. A. , Porcar L. , Butler Paul D. , Warr Gregory G.
Published: July 2002

We report a study of the ordering of the surfactant membranes of cetylpyridiniumchloride–hexanol in heavy brine sponge phase solutions in the proximity of a quartz surface by simultaneous neutron reflectometry (NR) and “near surface” small angle neutron scattering (NS-SANS) measurement in a reflection geometry sample cell. The NR results indicate layered surface ordering correlated with the solid–solution interface and decaying exponentially with depth over distances corresponding to a few membrane separations. The absolutely normalized NS-SANS results are consistent with conventional bulk SANS measurements, also indicating that the layered ordering established very near the surface does not constitute a phase of significant volume. We have compared this local surface ordering with the dilution behavior observed for sponge and lamellar phases in the bulk. At low membrane volume fraction the surface layering periodicity corresponds to the bulk sponge correlation peak, but approaches the smaller periodicities measured for lamellar systems at the same membrane volume fraction at higher concentrations.
Josephson device for quantum experiments
Authors: Granata C. , Corato V. , Longobardi L. , Russo M. , Ruggiero B. , Silvestrini P.
Published: July 2002

We present the characterization of a fully integrated Josephson device consisting of a rf-superconducting quantum interference device (SQUID) coupled to a readout system based on a dc SQUID sensor. Particular care in the design is devoted to minimize the electromagnetic noise coming from the environment. We report data on dc SQUID performances as well as on the decay rate from metastable flux states of the rf SQUID in the classical regime. The low dissipation level and the good insulation of the probe from the external noise are encouraging in view of macroscopic quantum experiments.
Oxygen-deficiency-activated phase transition in a long-aged La0.8Ca0.2MnO3 film
Authors: Prokhorov V. G. , Kaminsky G. G. , Komashko V. A. , Lee Y. P. , Park J. S. , Ri H. C.
Published: July 2002

The magnetic and transport properties of as-deposited and long-aged La0.8Ca0.2MnO3−δ films have been investigated in a wide temperature range. The x-ray diffraction data have shown separation of the film into two crystalline phases of the cubic and rhombohedral symmetry with different oxygen contents of δ≃0.08 and 0.16, respectively, after a half-year aging at room temperature in air. Both phases testify two different electronic (metal–insulator) and magnetic transitions with similar maximum values of magnetoresistance. The possible mechanism for such a type of structural transition in the aged manganite thin films is discussed.
Optical switching in midinfrared light-emitting diodes
Authors: Krier A. , Sherstnev V. V. , Gao H. H. , Monahov A. M. , Hill G.
Published: July 2002

We report on the optical quenching of electroluminescence in midinfrared light-emitting diodes operating at 3.0 μm. The source is based on a symmetrical double heterostructure with large band offsets and is effectively switched off using coherent visible light.
Microscopic evidence of point defect incorporation in laterally overgrown GaN
Authors: Gradečak S. , Wagner V. , Ilegems M. , Riemann T. , Christen J. , Stadelmann P.
Published: July 2002

Electron microscopy techniques are applied to investigate structural and optical properties of GaN layers selectively grown by hydride vapor phase epitaxy on crystalline GaN seed layers deposited on (0001)Al2O3 substrates. Regions with different optical properties are observed in the cross- sections of the layers. They are defined by the crystallographic planes that serve as growth facets. We give a simple geometrical explanation of point defect incorporation occurring more easily for the {112̄2} GaN than for the {0001} GaN growth facets. Microscopic evidences supporting the model are higher concentrations of point-like defects and local strain variations in laterally grown regions that are revealed by high-resolution electron microscopy.
Coupled-stripe quantum-well-assisted AlGaAs–GaAs–InGaAs–InAs quantum-dot laser
Authors: Walter G. , Chung T. , Holonyak N.
Published: July 2002

Data are presented on the coupled-stripe laser operation (continuous wave, 300 K) of a single InAs quantum-dot (QD) layer coupled via a thin (5 Å) GaAs barrier to an auxiliary strained InGaAs quantum well (QW) grown (confined) in an AlGaAs–GaAs heterostructure. Because of strain-induced (QW strain) improvement of the QD growth and QD alignment along diagonal (reflecting) ridges, the InGaAs-QW+InAs-QD crystals exhibit high gain along and across laser stripes, which is advantageous for coupled-stripe laser operation. A twin-stripe single-QD-layer QW+QD laser (4 μm stripes on 6 μm centers) of usual cleaved length, < 500 μm, is capable of continuous 300 K operation, with only probe heat-sink clamping and testing, at >50 mW.
Surface segregation of bulk oxygen on oxidation of epitaxially grown Nb-doped SrTiO3 on SrTiO3(001)
Authors: Chen Fan , Zhao Tong , Fei Y. Y. , Lu Huibin , Chen Zhenghao , Yang Guozhen , Zhu X. D.
Published: July 2002

We studied the epitaxy of 10 mol % Nb:SrTiO3 on a SrTiO3(100) substrate under an interrupted pulsed-laser-deposition condition. By monitoring the recovery behaviors of reflection high-energy electron diffraction intensity and an optical reflectance difference signal from the growth surface, we observed that, at temperatures above 630 °C, the oxidation of an as-deposited Nb:SrTiO3 monolayer was achieved by the diffusion of oxygen vacancies in the monolayer into the substrate.
The melting lines of model systems calculated from coexistence simulations
Authors: Morris James R. , Song Xueyu
Published: July 2002

We have performed large-scale molecular dynamics simulations of coexisting solid and liquid phases using 4ε(σ/r)n interactions for n=9 and n=12, and for Lennard-Jones systems, in order to calculate the equilibrium melting curve. The coexisting systems evolve rapidly toward the melting temperature. The P–T melting curves agree well with previous calculations, as do the other bulk phase properties. The melting curve for the Lennard-Jones system, evaluated using various truncations of the potential, converges rapidly as a function of the potential cutoff, indicating that long-range corrections to the free energies of the solid and liquid phases very nearly cancel. This approach provides an alternative to traditional methods of calculating melting curves.
Large changes in refractive index by synchrotron-radiation-driven compaction of hydrogenated silicon nitride films
Authors: Akazawa Housei
Published: July 2002

When high-energy photons (hν>100 eV) irradiate films of hydrogenated silicon nitride (a-SiNx:H) that have been deposited by plasma-enhanced chemical-vapor deposition, the network of atoms in the material is photolytically rearranged. This reaction proceeds rapidly from the instance the radiation is applied and is almost completed within 200 s. This is in striking contrast with the continuing effect of such radiation on SiO2 (defect formation and decomposition). The result was a change (Δn) in the refractive index at 633 nm of as much as 0.04, and this effect was independent of the temperature of irradiation. By using an effective medium mixture of Si3N4, crystalline Si, and voids to model the dielectric function of a-SiNx:H films, the volumes of the voids and of crystalline Si components were found to be reduced by 44% and 49%, respectively, from their initial values. The resulting 9.8% reduction in the thickness (compaction) is the origin of the large Δn.
High-sensitivity quantum Hall far-infrared photodetector integrated with log-periodic antenna
Authors: Kawaguchi Y. , Hirakawa K. , Komiyama S.
Published: July 2002

We have developed a high sensitivity far-infrared (FIR) photodetector by using the two-dimensional electron gas system in modulation-doped Al0.3Ga0.7As/GaAs heterojunctions. A FIR-sensitive channel was formed by applying a negative bias voltage to the split gates which were integrated with a log-periodic antenna. Although the area of sensitivity was small (850×850 μm2), the responsivity reached as high as 5.8×104 V/W. It is also demonstrated that the responsivity can be modulated by a factor of 10 by changing the split-gate voltages. The capability of ON/OFF switching of photoresponsivity suggests that this detector is suitable for realizing high sensitivity FIR imaging arrays.
Determination of the optimal cation composition of ferroelectric (ZnxCd1−x)S thin films for applications to silicon-based nonvolatile memories
Authors: Hotta Y. , Rokuta E. , Jhoi J.-H. , Tabata H. , Kobayashi H. , Kawai T.
Published: July 2002

Thin films of ferroelectric binary mixed II–VI compounds such as (ZnxCd1−x)S, as well as (ZnyCd1−y)Te and (ZnzCd1−z)Se (0⩽x,y,z⩽1), were examined from the standpoint of the application to Si-based nonvolatile memories. Electronic-band discontinuities at the ferroelectric–Si interface decreased significantly with increase in the atomic number of the constituent chalcogenide atoms, which favored (ZnxCd1−x)S as the most potential gate ferroelectrics among the three compounds. Polarization-field (P–E) characteristics of the (ZnxCd1−x)S films were found to largely depend on the cation composition. No hysteretic behaviors in the P–E curves were observed for high-Zn concentrations above x=0.5, while the P–E curves traced hysteretic loops due to the ferroelectricity for x<0.5. The remnant polarization was greatly dependent on the Zn concentration, and yielded a maximum of 0.03 μC/cm2 for x=0.3. On the other hand, the coercive field was not composition dependent, and was approximately 12 kV/cm.
Band line-up and mechanisms of current flow in n-GaN/p-SiC and n-AlGaN/p-SiC heterojunctions
Authors: Polyakov A. Y. , Smirnov N. B. , Govorkov A. V. , Kozhukhova E. A. , Luo B. , Kim J. , Mehandru R. , Ren F. , Lee K. P. , Pearton S. J. , Osinsky A. V. , Norris P. E.
Published: July 2002

The properties of n-GaN/p-SiC and n-AlGaN/p-SiC heterojunctions (HJ) prepared by hydride vapor phase epitaxy (HVPE) on 4H SiC substrates are reported. It is shown that the GaN/p-SiC HJ is staggered type II with the conduction bandoffset and the valence bandoffset values, respectively, ΔEc=−0.49 eV and ΔEv=0.65 eV. When changing GaN for AlGaN with Al mole fraction of x=0.25–0.3 the band alignment becomes normal type I with ΔEc=0.2 eV and ΔEv=0.6 eV. Current–voltage characteristics of both heterojunctions bear evidence of strong tunneling via defect states. The tunneling was found to be more pronounced in the AlGaN/SiC HJs even though these showed no evidence of formation of dark line defects at the interface, in contrast to GaN/SiC.
Microscopic correlation of redshifted luminescence and surface defects in thick InxGa1−xN layers
Authors: Bertram F. , Srinivasan S. , Geng L. , Ponce F. A. , Riemann T. , Christen J.
Published: July 2002

A direct correlation between the structural and luminescence properties of thick InxGa1−xN layers has been achieved on a microscopic scale using highly spatially resolved cathodoluminescence. Surface roughening is typically observed in growth by metalorganic vapor phase epitaxy of thick InxGa1−xN layers for x⩾0.1. Although the film remains highly planar, craters and protrusions appear on the surface. These surface defects are associated with redshifted luminescence indicative of indium segregation, and are related to threading dislocations in the films.
Depth and range shifting of a focal spot using a time-reversal mirror in an acoustic waveguide
Authors: Conti Stéphane , Roux Philippe , Fink Mathias
Published: July 2002

We present a technique based on time reversal to focus an acoustic field at any depth and range in a waveguide. We take advantage of the signal received from an acoustic beacon in the guide to build a time-reversed modified version of the field that refocuses at a different point in the waveguide. This method is based on the application of the images theorem in the guide and, as in the classical time-reversal experiment, it does not require a priori knowledge of the waveguide characteristics. Ultrasonic laboratory experiments and underwater acoustic simulations are presented and compared to classical time-reversal focusing results.
Determination of the barrier-height fluctuations based on the parallel-noninteracting diode model
Authors: Alonso C. F. , Hernández M. P. , Casielles E. , Peña J. L.
Published: July 2002

A model is proposed to determine the barrier height fluctuations of the inhomogeneous Schottky junction from the experimental current–voltage (I–V) characteristics. This model is based on noninteracting parallel diodes. In it, the current of each diode responds to a general expression of the thermionic theory where the Schottky effect and the series resistance are enclosed. The barrier height fluctuations are an effective-area barrier-height distribution which is defined as the probability of occurrence of the barrier height for each diode. The model is applied to the experimental I–V curves reported by I. M. Dharmadasa et al., Solid-State Electronics 42, 595 (1998).
Impact of metal–oxide gate dielectric on minority carrier lifetime in silicon
Authors: Hegde Rama I. , Hobbs Christopher C. , Dip LuRae , Schaeffer Jamie , Tobin Philip J.
Published: July 2002

Using a deposition followed by high-temperature drive in, we have examined the impact of metalorganic chemical vapor deposition (MOCVD) HfO2, MOCVD ZrO2, and physical vapor deposition (PVD) ZrxSi1−xO2 gate dielectrics on the minority carrier lifetime. The lifetime measurement utilized electrolytical metal tracer (ELYMAT)™. The minority carrier lifetime in Si after a 1025 °C anneal is comparable for HfO2 and the SiO2 control. The ELYMAT™ data showed similar results for the MOCVD ZrO2. However, a highly reduced lifetime was measured for the PVD ZrxSi1−xO2 films due to the presence of significant levels of Fe and Ni in the PVD Zr-silicate wafers as detected by VPD-TXRF. Thus, we saw no impact on minority carrier lifetime in Si for the HfO2 and ZrO2 formed by MOCVD methods, while there was a major impact on minority carrier lifetime in Si for the ZrxSi1−xO2 produced by the PVD method.
Direct measurement of the fine-structure interval in alkali atoms using diode lasers
Authors: Banerjee Ayan , Rapol Umakant D. , Natarajan Vasant
Published: July 2002

We demonstrate a technique for directly measuring the fine-structure interval in alkali atoms using two frequency-stabilized diode lasers. Each laser has a linewidth of order 1 MHz and precise tunability: one laser is tuned to a hyperfine transition in the D1 line, and the other laser to a hyperfine transition in the D2 line. The outputs of the lasers are fed into a scanning Michelson interferometer that measures the ratio of their wavelengths accurately. To illustrate the technique, we measure the fine-structure interval in Rb, and obtain a value of 237.6000(3)(5) cm−1 for the hyperfine-free 5P3/2–5P1/2 interval.
Spontaneous planarization of nanoscale phase separated thin film
Authors: Saraf Ravi F. , Niu Sanjun , Stumb Eric
Published: July 2002

Structure of complex fluid at mesoscales is influenced by interfacial effects. We describe the dynamic response in such films to sudden change in interfacial tension. In a self-assembled block copolymer film, the monolayer of 15 nm diam cylindrical discrete phases close to the surface commence to sink at an average rate of 0.16 nm/day in response to the interfacial tension change. Surprisingly, this spontaneous planarization occurs, even though the cylinders are covalently stitched to the matrix. A simple model explains the observed behavior. The observation may lead to approaches to tailor the structure of mesoscale thin films of complex fluids for long-range order that are desirable for nanoscale device fabrication.
Investigation of the interfacial properties of laminated polymer diodes
Authors: Guo Tzung-Fang , He Gufeng , Pyo Seungmoon , Yang Yang
Published: July 2002

Plastic lamination has become a promising approach to the fabrication of high-performance polymeric electronic devices. [T.-F. Guo, S. Pyo, S.-C. Chang, and Y. Yang, Adv. Funct. Mater. 11, 339 (2001)]. The major challenge for achieving high-performance laminated devices is the control of interface quality. In this letter, the various interfaces between two organic films have been investigated using atomic force microscope and impedance spectroscopy. Our results indicate the device performance is directly related to the formation of the interface. We attribute the better charge injection and higher electroluminescence efficiency of a laminated polymer light-emitting diode is due to the nanoscale surface roughness at the laminating interface.
Piezoelectric in situ transmission electron microscopy technique for direct observations of fatigue damage accumulation in constrained metallic thin films
Authors: Tan X. , Du T. , Shang J. K.
Published: July 2002

A piezoelectric in situ transmission electron microscopy (TEM) technique has been developed to observe the damage mechanism in constrained metallic thin films under cyclic loading. The technique was based on the piezoelectric actuation of a multilayered structure in which a metallic thin film was sandwiched between a piezoelectric actuator and a silicon substrate. An alternating electric field with a static offset was applied on the piezoelectric actuator to drive the crack growth in the thin metallic layer while the sample was imaged in TEM. The technique was demonstrated on solder thin films where cavitation was found to be the dominant fatigue damage mechanism.
Near-electrode model of transient currents in (Ba,Sr)TiO3 thin film capacitor structures
Authors: Boikov Yu. A. , Goltsman B. M. , Yarmarkin V. K. , Lemanov V. V.
Published: July 2002

Transient currents in SrRuO3//Ba0.8Sr0.2TiO3//SrRuO3 thin film capacitors, in the time interval 0.05–2.5 s after the step voltage switching on and off, have been investigated. The origin of these currents is proposed, including two components: charging (discharging) of near-electrode potential barriers and oxygen vacancy migration. It has been established that transient current of capacitor charging is mainly determined by the barrier component, and transient current of capacitor discharging is due to both components. The parameters of potential barriers were determined giving a good agreement of the calculated time dependences of transient currents with the experimental data.
Heterotic phase in ferromagnetism of a nerve-cell-like fractal-based complex
Authors: Ugajin Ryuichi
Published: July 2002

Ferromagnetism of a nerve-cell-like fractal-based complex, in which the dendritic portions characterized by a small fractal dimension are grown on a somatic nucleus characterized by a large fractal dimension, is investigated using Monte Carlo simulations of the octahedron-spin model. The difference in fractal dimensions in a nerve-cell-like fractal-based complex creates a heterotic phase in which the spin-ordered states of a somatic nucleus and the spin-disordered states of dendritic portions are orchestrated in the vicinity of the junction of the two regions.
Artificially induced reduction of the dissipation anisotropy in high-temperature superconductors
Authors: Gonzalez E. M. , Villegas J. E. , Varela M. , Santamaria J. , Prieto P. , Schuller Ivan K. , Vicent J. L.
Published: July 2002

Bi 2 Sr 2 CaCu 2 O 8 ,  RBa2Cu3O7 (R=Y, Eu) thin films and RBa2Cu3O7/PrBa2Cu3O7 superlattices have been fabricated by sputtering technique. The anisotropic dissipation was measured close to the critical temperatures with high applied magnetic fields rotating from parallel to perpendicular to the substrate. In multilayers, in a large magnetic field interval, the dissipation anisotropy is reduced as much as 60% in comparison with the most anisotropic system (Bi2Sr2CaCu2O8) and at least 50% at 30 kOe in comparison with 123 films. This strong anisotropy reduction is discussed taking into account the role played by the superlattice modulation lengths on magnetic matching effects and coupling between the superconducting layers.
Quasistationary states of a relativistic field-emission-limited diode employing a high-transparency mesh anode
Authors: Lin Ming-Chieh , Chuu Der-San
Published: July 2002

A relativistic field-emission-limited diode employing a high-transparency mesh anode is investigated via a self-consistent approach. The field emission process is described quantum mechanically by the Fowler–Nordheim equation. The cathode plasma and surface properties are considered within the framework of the effective work function approximation. Space-charge effects are described by Poisson’s equation including relativistic effects. Ionization effects at the high-transparency mesh anode are ignored. The numerical calculations are carried out on a time scale much shorter than the emergence of the gap closure. The quasistationary state of the diode exhibits a cutoff voltage. The electric field on the cathode surface is found to be saturated in the high-voltage regime and determined by the effective work function only.
Bulk carrier lifetime measurement by the microwave reflectance photoconductivity decay method with external surface electric field
Authors: Ichimura Masaya , Tada Atsushi , Arai Eisuke , Takamatsu Hiroyuki , Sumie Shingo
Published: July 2002

We attempted to measure the bulk carrier recombination lifetime of Si wafers by the microwave reflectance photoconductivity decay (PCD) method. Voltage was applied between an external electrode and a Si wafer to suppress surface recombination. Before the measurement, the surface state density was reduced by a chemical treatment using NH4OH–H2O2–H2O and diluted HF solutions. Carrier lifetime as long as 1 ms was measured by the present method for a wafer with a bare surface. Comparison with results for oxidized wafers show that the present method can suppress surface recombination more effectively than thermal oxidation, which has been often used for surface passivation in PCD measurements.
Plasma chemistry fluctuations in a reactive arc plasma in the presence of magnetic fields
Authors: Rosén Johanna , Anders André , Schneider Jochen M.
Published: July 2002

The effect of a magnetic field on the plasma chemistry and pulse-to-pulse fluctuations of cathodic arc ion charge state distributions in a reactive environment were investigated. The plasma composition was measured by time-of-flight charge-to-mass spectrometry. The fluctuation of the concentrations of Al+, Al2+, and Al3+ was found to increase with an increasing magnetic field strength. We suggest that this is caused by magnetic field dependent fluctuations of the energy input into cathode spots as seen through fluctuations of the cathode potential. These results are qualitatively consistent with the model of partial local Saha equilibrium and are of fundamental importance for the evolution of the structure of films deposited by reactive cathodic arc deposition.
Antiferromagnetically coupled hard/Ru/soft layers and their applications in spin valves
Authors: Wu Yihong , Li Kebin , Qiu Jinjun , Guo Zaibing , Han Guchang
Published: July 2002

Antiferromagnetically coupled hard/Ru/soft triple layers have been fabricated using ultrahigh vacuum sputtering. A series of experiments has been carried out to study the dependence of the exchange-coupling field on the thickness of the soft layer which is either CoFe or NiFe, while the hard layer is CoCrPt. It was found that this type of synthetic structure exhibits a significantly higher exchange coupling field as compared to the conventional antiferromagnet/ferromagnet exchange coupled structures. The use of these synthetic structures as the pinned and pinning layers has resulted in spin valves with a magnetoresistance ratio in the range of 5%–7% and a thermal stability that is better than those of IrMn-based and comparable to those of PtMn-based spin valves.
Anisotropic surface etching of 6H–SiC(0001) induced by reaction with oxygen molecules
Authors: Kubo O. , Kobayashi T. , Yamaoka N. , Itou S. , Katayama M. , Oura K. , Ohkado H.
Published: July 2002

The initial reactions of the 6H–SiC(0001)∛×∛ surface with O2 molecules at elevated temperatures have been investigated by scanning tunneling microscopy. As a result, two types of anisotropic features were observed. One was a dark parallelogram with ∛×∛ periodic protrusions elongated in the 〈11̄00〉 direction, which was observed only in the case of O2 exposure at 700 °C. The other was a dark line in the 〈11̄00〉 direction, which was observed with O2 exposure at temperatures above 500 °C. Both features manifest anisotropic surface etching induced by O2 molecules.
Effects of ultrathin tantalum seeding layers on sol-gel-derived SrBi2Ta2O9 thin films
Authors: Leu Ching-Chich , Chien Chao-Hsin , Yang Ming-Jui , Yang Ming-Che , Huang Tiao-Yuan , Lin Hung-Tao , Hu Chen-Ti
Published: July 2002

The effects of Ta seeding layer on the crystalline microstructure and ferroelectric properties of sol-gel-derived strontium–bismuth–tantalate (SBT) thin films were investigated. Ultrathin (<1 nm) Ta seeding layers were deposited onto Pt/TiO2/SiO2/Si substrates by magnetron sputtering at room temperature before spin coating of SBT thin films. Our results show that the presence of Ta seed layer induces significant changes in the crystallinity and microstructure of the resultant SBT thin films, resulting in improved ferroelectric properties in terms of spontaneous polarization. Interestingly, the remanent polarization (2Pr) of SBT films is also found to initially increase and then decrease with increasing Ta thickness, showing a peak value of 18.8 μC/cm2 (@5 V) at Ta thickness of around 0.23 nm. Since a stoichiometric SBT solution was employed in our experiments, the additional Ta on the Pt surface is thought to provide a Ta-rich environment for SBT films, which, in turn, results in lower nucleation activation energy for crystallization.
Dynamic diseases in neurology and psychiatry
Authors: Milton John , Black Deborah
Published: July 2002

Thirty-two (32) periodic diseases of the nervous system are identified in which symptoms and/or signs recur. In 10/32, the recurrence of a symptom complex is one of the defining features of the illness, whereas in 22/32 oscillatory signs occur in the setting of an ongoing nervous system disorder. We discuss the possibility that these disorders may be dynamic diseases.
‘‘Θ physics’’ and quantum spin chains (abstract)
Authors: Haldane F. D. M.
Published: July 2002

The one-dimensional quantum Heisenberg antiferromagnet with exchange alternation is related to the nonlinear 0(3) sigma model with a topological ‘‘θ term’’ in the Langrangian. The models with parity symmetry (θ=0 or π) are respectively related to the integer and half-integer spin models without exchange alternation. The nonlinear quantum physics of Heisenberg antiferromagnets is placed in a wider context of topologically nontrivial field theories and the spin-Peierls effect in half-integer spin systems (and its absence in integer spin systems) explained.
3s–3p laser gain and x-ray line ratios for the carbon isoelectronic sequence
Authors: Feldman U. , Seely J. F. , Doschek G. A. , Bhatia A. K.
Published: July 2002

The populations of the 46 levels belonging to the configurations 2s22p2, 2s2p3, 2p4, 2s22p3s, 2s22p3p, and 2s22p3d have been calculated for ions in the C I isoelectronic sequence with atomic numbers Z=18, 22, 26, 30, 34, and 36 and for electron densities from 1015 to 1022 cm−3. The populations of the 2s22p3p levels are relatively large owing to the large electron collisional monopole excitation rates from the ground configuration and to the small radiative decay rates to the ground configuration. This results in population inversions for transitions of the type 3s–3p. The gain coefficients for these transitions are determined and are compared to previous calculations. At high electron densities where collisional mixing of the excited levels becomes important, the intensities of the x-ray transitions from the 2s22p3p levels to the 2s2p3 levels decrease relative to the x-ray transitions from the 2s22p3s and 2s22p3d levels. The density dependence of these x-ray line ratios is also presented. These line ratios represent a promising diagnostic for electron density.
The effect of the two-dimensional gas degeneracy on the I-V characteristics of the modulation-doped field-effect transistor
Authors: Grinberg A. A.
Published: July 2002

A modulation-doped field-effect transistor model for long-channel devices is discussed and results presented. The model takes into account an arbitrary electrons degeneracy condition. It describes the drain current accounting for both the drift and the diffusion components in the constant mobility approximation. The additional effective electric field that arises due to dependence of the subband bottom on concentration is also taken into account. It is shown that at low temperatures (78 K) the degeneracy of the channel electrons plays an important role in the entire region of the device operation including the saturated region of the I-V characteristics.
Computer simulation of high-resolution transmission electron microscope images based on ball-and-spoke models of (100) Si/SiO2 interface
Authors: Ohdomari Iwao , Mihara Tohru , Kai Kazuhiko
Published: July 2002

Cross-sectional high-resolution transmission electron microscope (HRTEM) images of the (100) Si/SiO2 interface have been computer simulated on the basis of various interface models in order to facilitate the interpretation of experimental HRTEM images. Continuous random networks of SiO2 have been constructed on crystalline Si substrates with different surface morphologies using plastic balls and spokes. The distortion energy introduced by artifacts in the model has been relaxed by modifying the atom positions. HRTEM images of the models have been calculated using the multislice dynamical electron scattering method. Although models with pyramidlike or edgelike protuberances delineated by {111} facets on the (100) interface give HRTEM lattice images close to the observed ones, the undulation in the calculated images is always smaller than that in the actual atomic arrangement. A rough (100) interface with a suboxide region is an alternative to the energetically favorable interface structure which gives a very flat calculated image.
Thermal annealing and photoluminescence measurements on AlxGa1−xAs-GaAs quantum-well heterostructures with Se and Mg sheet doping
Authors: Kaliski R. W. , Nam D. W. , Deppe D. G. , Holonyak N. , Hsieh K. C. , Burnham R. D.
Published: July 2002

Transmission electron microscopy and photoluminescence data are presented, before and after sample thermal annealing, on AlxGa1−xAs-GaAs quantum-well heterostructures with Se or Mg sheet doping in the barriers or in the quantum wells (QWs). The crystals are grown by metalorganic chemical vapor deposition and, in the active regions, have four GaAs wells and three Al0.2Ga0.8As barriers (or in one case five QWs and four barriers). The confining layer regions are bulk doped n type on the substrate side and p type on top (p-n). The sheet doping of the QW active region is obtained by deposition of Se from H2Se or Mg from bis(cyclopentadienyl) Mg in the middle of the layers (GaAs wells or Al0.2Ga0.8As barriers). The impurity sheet deposition (δ-Se or δ-Mg) is carried out in an arsine-rich atmosphere. Photoluminescence data on the as-grown crystals show that the energy (ℏω) of the recombination radiation is not shifted by the sheet doping and that the samples behave as ordinary photopumped QWH lasers. Annealing the crystals in an As4 atmosphere (provided by excess As in a quartz ampoule) causes layer intermixing of the Se sheet-doped samples and very little disordering in the case of the Mg sheet-doped samples. Magnesium sheet doping leads to layer disordering only in the case of sample annealing with no excess As in the annealing ampoule. Disordering of both Se and Mg sheet-doped crystals occurs for Si3N4 capped but not for SiO2 capped samples.
Emitter-collector reciprocity of double heterojunction bipolar transistors: A three-dimensional analysis and proof for transistors with heavily doped and degenerate base regions
Authors: Mohammad S. Noor
Published: July 2002

A three-dimensional analysis of the validity of emitter-collector reciprocity of double heterostructure bipolar transistors with heavily doped and degenerate base regions has been carried out. The analysis is based on current-density relations of Marshak and Van Vliet [Solid-State Electron. 21, 429 (1978)], an effective intrinsic carrier concentration derived in terms of band-gap narrowing and the effect of Fermi–Dirac statistics, and p-n product derived by considering potential barriers and related properties of heterostructures. The analysis indicates that the bipolar reciprocity holds under low-level injection even when parameters such as band-gap narrowing of the band, and mobility, diffusivity, lifetime, and recombination velocity, etc., of the carriers depend on spatial coordinates, and the carrier concentrations are described by Fermi–Dirac statistics. The analysis is general enough to be applicable to both n-p-n and p-n-p heterostructure and homostructure bipolar transistors.
Low effective back-surface recombination velocity by boron implantation on 0.3-Ω-cm p-type silicon solar cells
Authors: Verhoef Leendert A. , Zondervan Albert , Lindholm Fredrik A. , Spitzer Mark B. , Keavney Christopher J.
Published: July 2002

We measure a low effective back-surface recombination velocity S≊1000 cm/s on boron-implanted back-surface field (BSF) silicon solar cells with a p-base resistivity of 0.3 Ω cm. For cells of this low resistivity, this velocity is much lower than values obtained with the conventional Al-alloying technique. Our data indicate that the low surface recombination velocity results from a low value of minority electron diffusivity D≊0.45 cm2/s peculiar to these highly doped implanted regions at the back of the solar cells. The effect of a thermal anneal at 950 °C, prior to emitter fabrication, on the solar cell parameters was also investigated. The short-circuit current density, minority-carrier diffusion length, and back-surface recombination velocity, improved with a 1- and 2-h anneal of the BSF cells, and either degraded or remained unchanged on ohmic back contact cells. The decrease in S is due to widening of the BSF region during anneal, deduced from spreading resistance measurements.
The investigation of the process of microwave bubble generation in bubble material
Authors: Dong J. L. , Artman J. O. , Charap S. H.
Published: July 2002

The first observation of the process of magnetic bubble generation by microwave excitation has been made. A high-speed optical sampling microscope system was employed to sense the evolution of the bubble with time. The dependence of the microwave power threshold for bubble generation on experimental conditions such as microwave frequency, microwave pulse width, and dc bias field are reported. Magnetic garnet films with the composition Y1.92Sm0.1Ca0.98Fe4.02Ge0.98O12 were used as our experimental samples. The experimental results are not consistent with the theoretical predictions of the Seagle coherent rotation model for bubble generation. We find that the magnetization in the area driven by the strong microwave field responds rather noncoherently. Nucleation centers, which may be too small to be seen optically, are formed first. Under appropriate conditions, these centers may enlarge to a full size bubble. As examples, with a bias field setting of 65 Oe and the microwave frequency set at 550 MHz, about 25.5 dBm of power are required to generate a bubble from a 60-ns-long pulse; only 20.5 dBm are required if a 12-μs pulse is applied.
Early-time behavior of spectral emissions from the interaction of intense relativistic electron beams with initially neutral gases
Authors: Ono K. , Oomori T. , Fujita S.
Published: July 2002

Visible-UV spectral emissions from electron beam (700 kV, 12 kA, 50 ns) generated neon plasmas have been investigated over a wide pressure range (0.1–100 Torr) with and without axial magnetic guide field. The Ne I–III line emissions were viewed perpendicular to the beam propagation; attention was focused on a time delay of the onset of the emission with respect to the beam front arrival in the field of view, and on correlations of the subsequent intensity evolution with that of the beam and net (beam plus plasma) currents. The observations are interpreted as direct confirmation of the major processes for gas ionization and excitation that occur depending on the gas pressure and magnetic guide field.
Electron beam induced decomposition of cadmium chloride thin films with potential resist application
Authors: Aidinis C. J. , Green Mino
Published: July 2002

The radiolysis of vacuum-deposited thin films of cadmium chloride on silicon substrates has been studied in the energy range from 1 to 5 keV. Phase-locked mass spectrometry was used for the in situ monitoring of the evolution of the decomposition products, chlorine and cadmium, as a function of substrate temperature, current density, film thickness, and beam energy, for pure and doped films. Ellipsometric thickness and refractive index measurements, and scanning electron microscope examination of the morphology of irradiated films were carried out. The decomposition kinetics have been analyzed in terms of a model in which a fraction of the radiation-generated electron-hole pairs participate in the thermally activated decomposition reaction sequence at the film surface. A potential lithographic scheme has been established, in which decomposition patterns generated onto the films are transferred onto the SiO2 underlayer by reactive ion etching, with a resolution approximately equal to the film grain size.
The effective second-order elastic constants of a strained crystal using the elastic wave propagation in a homogeneously deformed material
Authors: Rao R. Ramji , Padmaja A.
Published: July 2002

The equation for elastic wave propagation in a homogeneously deformed crystal has been used to obtain the expressions for the effective second-order elastic constants of the seven crystal systems in terms of their natural second- and third-order elastic constants. These expressions are employed to obtain the pressure derivatives of the effective second-order elastic constants of some cubic crystals for which experimental data are available.
Formation of some hierarchy in amorphous structure during the crystallization of vacuum-deposited amorphous semiconductor films
Authors: Maki Kunisuke , Shigeta Yukichi
Published: July 2002

A view that some hierarchy in the structure of the amorphous phase surrounding the crystallized phase is formed, is proposed from the investigation of the kinetics in the crystallization process of amorphous semiconductor films. The view is discussed from some results in the study of solid-phase epitaxial growth of the amorphous Si film deposited on the Si(111) surface by low-energy electron diffraction. Some comment on the formation of the amorphous structure is also given from a viewpoint of thin-film growth.
Hyperfine structures of Mössbauer spectra of trivalent 167Er and 151Eu in an hydroxide host
Authors: Dasgupta Sulava , Ghosh D.
Published: July 2002

The hyperfine energy levels of 167Er in Er(OH)3 have been deduced using the crystal field parameters and wave functions obtained from an analysis of the experimental results of magnetic susceptibilities, their anisotropies, and optical absorption spectra of pure Er(OH)3. The hyperfine splitting of the energy levels corresponding to the ground (Ig =7/2) and first excited (Ie =9/2) 79.32-keV nuclear manifolds of 167Er in Er(OH)3 are 0.124 and 0.152 cm−1, respectively. The expected Mössbauer spectra were also predicted. No experimental data is available for comparison. The width of Mössbauer spectra for 166Er in Er(OH)3 was found to be 10.56 mm/s, which is close to the reported value. The Mössbauer spectra of 151Eu(OH)3 was studied between 300 and 4.2 K and the results were compared with the spectra calculated using similar magnetic and optical data of Eu(OH)3. The observed isomer shift (with respect to EuF3) remained temperature independent, having a value of 1.04±0.02 mm/s. The nuclear specific heat (Chf) was calculated for the 151Eu isotope in Eu(OH)3 and it showed a peak at about 0.9 mK. It was observed that Chf varied as 1/T2 between 1 K and 10 mK, which is a property that may be utilized in millikelvin thermometry.
A two-dimensional analytical solution of the diffusion equation for silicon-on-insulator structures
Authors: Sweid I. , Guillemot N. , Kamarinos G.
Published: July 2002

The two-dimensional (2D) redistribution of impurities in semiconductors is an important step for the research and fabrication of modern very large scale integrated (VLSI) circuits. The diffusion profiles must be known precisely if one is to master and model correctly the operations of integrated circuits. Many two-dimensional (2D) simulators for processing bulk silicon do actually exist; however, they are not general enough to be able to simulate these processes on other structures, e.g., SOI (silicon on insulator). In this paper we present a two-dimensional (2D) analytical solution of the diffusion equation for the case of the SIMOX (separation by implanted oxygen) structure. The model developed takes into account the limiting conditions associated with this material as well as the initial ion implantation profile. The program has been developed in this way and will be included in a complete 2D simulator for SOI process. As an example, we simulate the formation of the isolation zone in the SIMOX structure, i.e., the implantation and redistribution of boron during the formation of field oxide.
An exact wave-optics analysis of optical waveguide with anisotropic and gyrotropic materials
Authors: Chung Sang-Koo , Kim Sang-Seol
Published: July 2002

An exact wave-optics analysis of wave propagation in thin-film optical waveguide with gyrotropic and anisotropic materials as waveguide film or substrate is presented for the first time. Based on the field equation and the boundary conditions of the guide, the field distribution and phase velocity of the eigenmodes of the guide are determined, and the differences in characteristics due to waveguide structure are examined in detail. The relation for TE-TM conversion is derived in the form of the Jones matrix which allows a direct interpretation in the conversion efficiency of the guide.
A comparison using a delta-function model of envelope function approximations for quantum wells
Authors: Cunningham Thomas , Barker R. C. , Chiu L. C.
Published: July 2002

This paper compares four versions of the effective-mass approximation to the exact atomic wave function, as determined from a one-dimensional model potential, in predicting the energies of the bound states in a single quantum well such as those formed in compound semiconductor heterostructures. The model potential consists of delta-function barriers between atoms with a ‘‘well’’ in the strength of these delta functions. The model yields simple, closed-form solutions for the bound state energy. The corresponding effective-mass model that uses the calculated dispersion relations and continuity of the wave function derivatives at the well boundaries predicts bound state energies indistinguishable from those predicted by the exact solution. The parabolic-band model overestimates the lowest energy level of a 0.4-eV-deep, 50-Å-wide well by about 15%. Continuity of 1/m* times the wave functions underestimates the energies by about the same amount.
rf-induced steps in intermediate length Josephson-tunnel junctions
Authors: Costabile G. , Monaco R. , Pagano S.
Published: July 2002

We report on the behavior of the rf-induced steps in unidimensional Josephson-tunnel junctions having overlap geometry and length L≊λj. We have investigated the response of the junctions to the rf current in a range of frequency covering the geometrical resonance of the junctions. The experimental data are compared with the predictions of a model based on a multimode expansion of the junction phase.
Optical studies of misfit strain effects in GaxIn1−xP epitaxial layers on (001) GaAs substrates
Authors: Moroni D. , Dupont-Nivet E. , André J. P. , Patillon J. N. , Delalande C.
Published: July 2002

The effect of interfacial elastic strain due to lattice mismatch in GaxIn1−xP layers grown on (001) GaAs substrates by atmospheric pressure metalorganic vapor-phase epitaxy has been studied by photoluminescence and photoluminescence excitation spectroscopy. Strong excitonic features have been observed in the 2-K excitation spectra. They provide the first direct observation of the valence-band splitting due to the misfit strain in accordance with theoretical calculations.
Magnetoresistivity in NiFeCo multilayer films
Authors: Hur J. H. , Comstock C. S. , Pohm A. V. , Pearey L. A.
Published: July 2002

Magnetoresistive (MR) ratios of NiFeCo multilayer films were investigated before and after annealing with temperatures up to 300 °C for 1, 2, 4, and 8 h. MR ratios increased by an average of 15%–22% and resistance decreased by an average of 7%–10%.
The effect of C13O162 on the performance of a sealed-off pulsed CO2 laser
Authors: Marcus S. , Stein D. T.
Published: July 2002

A sealed-off rf-excited CO2 waveguide laser was used to investigate the relative performance with C13O2 and C12O2 mixtures. By modulating the laser at various rates and pulse widths, we demonstrated effects due to both temperature and upper level relaxation rates. In addition, replacing N142 with N152 was shown to improve the performance of C13O2.
Reversible color changes in sputter-deposited Ag-Zn alloy films
Authors: Minemura T. , Andoh H. , Maeda Y.
Published: July 2002

Reversible color-change behavior and its relation to phase transformations in sputter-deposited Ag-50 at. % Zn alloy films were investigated by spectral reflectivity measurements, x-ray diffractometry, and transmission electron microscopy. The color of the alloy film is pink at room temperature when produced by quenching above 560 K, and is silver by quenching between 425 and 545 K. By alternately repeating these heat treatments, the two colored states can be reversibly produced. This color change is observed as reflectivity changes at wavelengths below 600 nm and in the near-infrared range for spectral reflectivity. The pink-colored film has a CsCl-type ordered structure (β′ phase), while the silver colored has a hexagonal one (ζ phase). Therefore, the reversible color change is due to the solid-state phase transformations between the β′ and ζ phases. By irradiation with semiconductor laser beam, the color change can be induced in a microarea (about 1 μm) of the film.
Comment on ‘‘Engineered Schottky barrier diodes for the modification and control of Schottky barrier heights’’ [J. Appl. Phys. 6
1, 5159 (1987)]
Authors: Horváth Zs. J.
Published: July 2002

Eglash and co-workers [J. Appl. Phys. 61, 5159 (1987)] studying the dependence of the voltage intercept of the C−2-V plots for the engineered Schottky barrier diodes on the thickness of the p+ region used an analytical expression of the C-V characteristics for the prediction of the voltage intercept. In this communication, a general expression is derived for the C-V characteristics of the Schottky contacts with changing dopant concentration at the M-S interface. It is shown that the expression used by Eglash and co-workers is a special case of this general expression.