bluuebunny/crossref_metadata_2025 huggingface dataset:
Supermode analysis of electron wave directional coupling using a multilayer waveguide approach
Authors: Wilson Daniel W. , Glytsis Elias N. , Gaylord Thomas K.
Published: July 2002

Electron wave directional coupling between parallel semiconductor electron waveguides is analyzed by calculating the supermode (superposition of eigenmodes) of the complete structure. A transfer-matrix technique is used to calculate the eigenmodes of multiple-layer structures having arbitrary potential-energy/effective-mass profiles that form the coupled electron waveguides. It is shown that the eigenmodes of electron waveguides having spatially varying effective mass satisfy an orthogonality relation that involves the effective-mass profile. As determined by supermode simulations, coupled-mode analyses give inaccurate directional-coupling transfer lengths when the coupling is strong or the electron energy is low. The influence of various parameters and structure asymmetries on the directional coupling process is investigated. It is shown that asymmetry severely decreases the ability to transfer current from one waveguide to the other, except in the case of effective-mass asymmetry where it causes an energy-dependent transfer efficiency. Switching by modulation of the transfer length and the transfer efficiency are discussed and simulated.
Effects of ozone oxygenation of YBa2Cu3O7−δ thin crystals
Authors: Zhu Yimei , Sabatini R. L. , Wang Y. L. , Suenaga M.
Published: July 2002

We report the effects of ozone oxygenation of YBa2Cu3O7−δ thin crystals for eliminating local fluctuation in oxygen content, which is known to exist even in normal δ≊0 samples. An optimum ozone annealing condition was determined based on the microstructure and the uniformity of oxygen content of the samples. We observed the formation of fine-spacing twins running perpendicular to the original twins at thin areas and attributed this to accommodating the local strain due to oxygen uptake during ozone annealing.
Two-temperature fluid model for high-pressure plasmas in contact with cooled electrodes
Authors: Meeks E. , Cappelli M. A.
Published: July 2002

A detailed continuum model of a weakly ionized plasma in stagnation flow against an electrically biased surface is presented. The model includes bulk fluid motion as well as electron and ion momentum and continuity equations in both general and quasineutral formulations. The inclusion of both electron and bulk energy equations allows for the determination of electron temperature and deviations from local thermodynamic equilibrium. Results for the quasineutral model demonstrate the importance of considering thermal nonequilibrium in the evaluation of plasma properties in the presheath region. The evaluation of transport properties is self-consistent with properties for the charged species depending on the calculated electric field and neutral number density. In addition to investigating thermal boundary layer and fluid flow effects, a finite electron-ion recombination rate at the electrode surface is employed and the consequences of varying this boundary condition are explored.
Distribution functions of coexisting phases in a complete solid solution system
Authors: Cao Wenwu , Cross L. Eric
Published: July 2002

In the phase diagram of a binary system one often encounters a compositional region in which two phases coexist. A common practice is to use the Lever Rule to describe the distributions of the two coexisting phases. However, if the binary system is a complete solid solution system, the Lever Rule cannot be used. A new type of distribution has been derived for a solid solution system without solubility gap. Application of the theory to pure and modified lead-zirconate–lead-titanate (PZT) systems shows excellent agreement with the experimental data. Several disputed facts about PZT are also explained satisfactorily.
Atomic-layer epitaxy by a flux-interruption and annealing method and the analysis of reflection high-energy electron diffraction oscillation overshoot in the molecular-beam epitaxy growth of GaAs
Authors: Lee Kun-jing , Lee Joseph Ya-min , Chang Yu-jeng
Published: July 2002

Atomic-layer epitaxy is relatively rare in molecular-beam epitaxy (MBE). This is because the precise control of atomic layer deposition is difficult. Shutter mechanical delay and signal transit delay times are main reasons for the difficulty of precise control. A flux-interruption and annealing method has been used to grow atomic-layer epitaxy by MBE. Exact shutter action has been achieved to avoid an excess fractional layer and keep the reflection high-energy electron diffraction (RHEED) oscillation indefinitely. The RHEED oscillation profile, including the overshoot, has been studied as a function of substrate temperature and excess deposition of As4.
The effect of temperature and doping on the segregation of In during solid-phase-epitaxial crystallization of Si
Authors: Elliman R. G. , Fang Z. W.
Published: July 2002

The segregation of In during solid-phase-epitaxial crystallization of amorphous Si layers is examined as a function of annealing temperature and the presence of p- and n-type dopants. The fraction of In segregated to the sample surface decreases with increasing temperature from 38% at 450 °C to 18% at 600 °C. This is shown to be consistent with changes in the crystallization velocity and In diffusivity in amorphous Si. The addition of p- and n-type dopants is shown to increase the crystallization velocity and further decrease the segregation of In. The reduced segregation is shown to be inconsistent with the dopant-induced velocity changes alone, and it is speculated that dopant-induced solubility changes are responsible for this effect.
The hysteresis and transient behavior of Si metal-oxide-semiconductor transistors at 4.2 K. II. Prekink clockwise hysteresis regime
Authors: Simoen E. , Claeys C.
Published: July 2002

The experimental transient behavior of Si metal-oxide-semiconductor transistors (MOSTs) operated at liquid-helium temperatures in the prekink, clockwise hysteresis regime is described in detail. As is shown, the drain current exhibits a pronounced decay, after switching on the device, both in n- and pMOSTs. The effective ‘‘exponential’’ time constant τ of the transient is in the order of seconds to hundreds of seconds and is a strong function of the gate, the drain, and the substrate biases. Generally, a strong reduction of τ with increasing drain current is observed. These findings are critically discussed in view of the dominant dopant ionization mechanisms at 4.2 K. It is demonstrated that the reported drain current dependence of the transient time constant can be interpreted by considering shallow-level impact ionization by the channel carriers. It is furthermore demonstrated that the anomalous transients occasionally observed point to the occurrence of the reverse process, namely capture of free carriers. By considering these two mechanisms, a comprehensive model for the prekink hysteresis behavior is established.
Transverse structures in resonatorless absorptive switching in bulk ZnSe
Authors: St. John W. D. , Wicksted J. P. , Cantwell G.
Published: July 2002

Absorptive switching and transverse structures in resonatorless optical bistability in bulk ZnSe is observed upon exciting the sample at the Urbach edge. The switching is due to nonlinear absorption induced by a temperature tuning of the band gap. We have modeled such behavior by numerically solving for the intensity, carrier density, and temperature, including both longitudinal and transverse fluctuations. Simulations show that both longitudinal and transverse heat diffusion are necessary in producing specific structures, as well as in determining the time of switching. During switching, the experiment and theory illustrate for the intensity profile the formation of a local minima on axis (hole) followed by a local maxima on axis and local minima off axis (rings). Such beam distortions and bistable switching are the result of the spatial and temporal dynamics and positive feedback existing among the absorption and heating.
Anomalous magnetoelastic waves in nickel wire
Authors: Liu Yuan , Viens M. , Cheeke J. D. N. , Jen C. K.
Published: July 2002

An anomalous magnetoelastic wave is observed in a 0.127-mm-diam nickel wire using ordinary magnetostrictive transducers. The group velocity of this wave is constant over a frequency range from 1 to 3.5 MHz, and equal to the shear wave velocity of the material (3 km/s for nickel). However, the phase velocity of this wave is extremely high (up to 416 km/s was observed) and in the opposite direction to the group velocity. This anomaly disappears when an axial dc current is applied; a normal Wiedemann-type signal appears at the same group delay. This wave is different from previously known backward waves. This wave also appears in some other magnetostrictive materials.
Charge sensitivity of the single electron tunneling transistor with discrete energy spectrum
Authors: Averin D. V.
Published: July 2002

The charge sensitivity of the single electron tunneling transistor is calculated in the regime when the discreteness of electron energy spectrum of its middle electrode is important.
Criterion analysis on nonpoisoning of the target surface
Authors: Wang Jingyi , Chen Wei , Wang Yu , Zhao Ning , He Xiaoming , Rodionov Yuri A.
Published: July 2002

The mechanism of target poisoning in reactive sputtering is analyzed. The methods used to reduce the target poisoning are discussed. According to the transport mechanism of the particles of reactive gas in a chamber, a criterion for nonpoisoning of the target is given and two mathematical expressions are respectively derived for ‘‘absolute nonpoisoning’’ and ‘‘experimental nonpoisoning.’’
High temperature characterization of the GaCl3/SiH4/H2 chemical vapor deposition reaction mixture
Authors: Alfano Angelo J. , Benard David J.
Published: July 2002

The GaCl3/SiH4/H2 system is of interest for the production of high quality epitaxial gallium doped silicon. A thermodynamic analysis and spectroscopic investigation of this chemical vapor deposition (CVD) chemistry is described over the temperature range 800–1200 K. GaCl is identified as the major gallium transport species once the flowing reaction mixture reaches gas phase equilibrium in a small CVD chamber.
Studies of bonding and valence states in YBa2Cu3O7−x through secondary-ion-mass spectrometry
Authors: Rajasekar P. , Chakraborty P. , Ray N. , Dey S. D. , Bandyopadhyay S. K. , Barat P. , De Udayan
Published: July 2002

The superconducting compound YBa2Cu3O7−x (YBCO) and oxides of copper (CuO and mixture of CuO and Cu2O) have been examined by secondary-ion-mass spectrometry (SIMS). The results support the fact that two kinds of copper (from the consideration of bonding and valence state) exist in YBCO—one in the basal planes of charge reservoir layers and the other in the conduction layers. The bonding of Cu-O in the CuO2 conduction layer unit is similar to that in CuO. No Cu+++ secondary ions could be detected in the SIMS spectrum, in agreement with electron spectroscopy for chemical analysis, extended x-ray-absorption fine structure, and electron probe microanalysis measurements reported earlier. A plausible explanation has been given for this.
Effects of inert gas dilution of 1,3 butadiene on plasma deposition of a-C:H films
Authors: Seth Jayshree , Babu S. V.
Published: July 2002

Amorphous hydrogenated carbon films (a-C:H) were deposited by the plasma decomposition of mixtures of 1,3 butadiene with different inert gas diluents (Ar, Ne, and He). Several characteristics of the plasma and the deposited films were investigated for deposition gas mixtures ranging in concentration from 0% to 90% of the diluent. Measurement of the optical emission from the plasma indicated the presence of the same dominant species from the hydrocarbon source (CH 430 nm system, and hydrogen Balmer lines) for all the mixtures containing the diluents, although the relative intensities were markedly different. The H*/H*2 and H*/CH* emission intensity ratios increased with the concentration of butadiene in argon- and neon-diluted mixtures while remaining relatively constant in butadiene/helium plasmas. Details of some the bonding configurations were determined from an analysis of the various IR-absorption bands. Film characterization included etch rate measurements in an oxygen plasma as well as the determination of the density and the optical band gap for different deposition gas mixtures. All the measurements suggest that, when the diluent concentration exceeds about 75%, the film structure undergoes a well-defined transition to a predominantly sp2 structure. The relation between the film properties and the deposition gas mixture and the reactive species present in the plasma is discussed.
Time- and space-resolved radio-frequency plasma electron energy distributions from a displacement-current-based electric probe diagnostic
Authors: Turkot R. B. , Ruzic D. N.
Published: July 2002

A displacement-current-based, Langmuir probe technique was developed to measure time-resolved and spatially dependent electron energy distribution functions (EEDF) in a 100 mTorr argon plasma. This diagnostic was designed to take consideration of the changes in displacement current collected at the probe due to changes in the probe tip sheath potential. The EEDFs collected displayed evidence of a time-dependent as well as spatially dependent electron beam component. The beam energy was seen to increase with the instantaneous plasma potential. The electron density was found to be 3.2±1.1×109, 1.0±1.5×1010, and 2.7±1.0×109 cm−3 at locations 0.6, 1.8, and 2.8 cm from the grounded electrode, respectively. Mean electron energies were also spatially dependent, measuring 8.7±0.8, 11.3±1.0, and 6.9±0.7 eV at locations 0.6, 1.8, and 2.8 cm from the grounded electrode, respectively.
Low-frequency noise in electron irradiated n-GaAs epitaxial layers
Authors: Ren L. , Baucour P. , Hooge F. N. , Luthjens L. H. , Leijs M. R.
Published: July 2002

We report on the results of measurements of temperature dependent Hall-effect and low-frequency noise of molecular-beam-epitaxy-grown n-GaAs layers irradiated by 3 MeV electrons. The results of Hall-effect measurements agree with the literature for the electron traps E1 and E2. Besides 1/f noise, an additional generation-recombination (g-r) noise is observed. We attribute the observed g-r noise to an unknown deep level induced by the electron irradiation, which is about 0.18 eV below the conduction band. Its capture cross section is extremely small and thermally activated. The irradiation does not cause a significant change in the 1/f noise parameter α at high temperatures. Possible roles of the defect motion 1/f noise sources are discussed.
Thin YBa2Cu3O7−δ films by electron-beam coevaporation— Growth and in situ characterization
Authors: Maul M. , Schulte B. , Häussler P. , Adrian H.
Published: July 2002

We report on the in situ preparation and characterization of thin YBa2Cu3O7−δ films on various substrates. The films were grown by electron-beam coevaporation of the pure metals Y, Ba, and Cu. A pressure stage is used to obtain high partial O2 pressure close to the substrate and low pressure in the main vacuum system. In situ resistance measurements were performed to investigate growth dynamics during film formation, and temperature dependence during the cool down from T≊1000 K to T≤Tc. The films were analyzed by R(T), ac susceptibility, x-ray diffraction, and critical transport current measurements.
Perturbative integral expansion method of analyzing crosstalks in volume holographic interconnections
Authors: Lee Hyuk
Published: July 2002

Crosstalk effects in high-density volume holographic interconnections are investigated using the perturbative iteration method for the integral equation of Maxwell’s wave equation. In this method, paraxial approximation introduced in conventional coupled-mode theory is not assumed. Interaction geometries consisting of noncoplanar light waves and multiple index gratings are studied. Arbitrary light polarizations are considered and backward diffraction is included. Systematic analysis of crosstalk effects due to multiple index gratings is performed in increasing level of diffraction orders corresponding to successive iterations.
Growth of a Cr oxide layer on GaAs(100) by oxidation with condensed water
Authors: Ettedgui E. , Park Ken T. , Cao Jianming , Gao Y. , Ruckman M. W.
Published: July 2002

A Cr oxide layer produced by the deposition of Cr onto a condensed H2O overlayer on a GaAs(100) wafer at T=90 K was studied using synchrotron radiation photoemission. Cr reacted with H2O immediately upon deposition at low temperature resulting in the synthesis of hydrated Cr oxide compounds, and then formed a stable dry oxide as the temperature of the substrate was raised toward room temperature and eventually 400 °C. The oxide was stable up to a temperature of at least 400 °C, although the substrate did show changes indicative of reaction.
A closed form analytic model for separation by implantation of oxygen oxide growth using a joined-Gaussian approximation
Authors: Hosack H. H. , El-Ghor M. K. , Hollingsworth J. , Joyner K.
Published: July 2002

A closed form analytic solution to the growth characteristics of the separation by implantation of oxygen (SIMOX) buried oxide and silicon film, based on a two-sided Gaussian approximation to the oxygen implant profile in the SIMOX process, is presented. The model used includes the effects of substrate swelling and sputtering due to the implanted oxygen, as well as the effects of saturation of the oxygen density at the stoichiometric SiO2 level in the implanted region. The results of this investigation show that for typical SIMOX implant conditions currently used in high-current implanters, the total dose of oxygen required to first reach the saturation level is only slightly dependent on the swelling and sputtering effects associated with the oxygen implantation, and that the deviation of the location of the first saturation point from the commonly used implant range can be significantly affected by the implant profile. In addition, it is shown that a ‘‘natural parameter’’ GNsat, where G is the net growth of the substrate per implanted oxygen atom and Nsat is the saturation level of oxygen atoms in the buried oxide, can be used to characterize the magnitude of the effects of the implant parameters on the final SIMOX material. It is also shown that the parameter GNsat can be easily obtained from the slope of a Tox vs Tsi plot.
A novel quantum wire formed by lateral p–n–p junctions between quasi-two-dimensional electron and hole systems at corrugated GaAs/AlGaAs interfaces
Authors: Harbury Henry K. , Porod Wolfgang , Goodnick Stephen M.
Published: July 2002

Numerical modeling of a novel quantum wire structure formed by the confinement of electrons between lateral quasi-two-dimensional (Q2-D) p–n junctions in a corrugated GaAs/AlGaAs heterostructure is reported on. Such a quantum wire may be realized at the tip of a Si-doped AlGaAs overgrown V groove in a SI–GaAs substrate due to the surface orientation dependence of Si doping. The two-dimensional conduction and valence band potential profiles for the electron and hole charge densities are solved within a semiclassical Thomas–Fermi screening model. The quantized electronic wire states at the heterointerface are then obtained by solving the two-dimensional effective mass Schrödinger equation using the calculated potential profile. The parameter space of the one-dimensional electron system is explored to establish which features of the structure are dominant factors in controlling the electronic states. It is demonstrated that the energy level spacing of the quantum wire depends primarily on the lateral confinement width in the n-type region at the tip of the V groove. The ground state energy of the wire is shown to depend on both the lateral confinement width and the vertical heterointerface confinement width. The results of our initial calculations are also reported on to incorporate lateral gates on the surface to obtain direct control of the quantum wire transport properties. The advantages of fabricating quantum wires with this structure compared to conventional methods of electrostatic confinement are discussed.
An improved space-dependent-mass Hamiltonian for use in the tight-binding Green’s function method
Authors: Henrickson Lindor E.
Published: July 2002

An improved ‘‘tight-binding’’ Hamiltonian matrix which accounts for the effects of a space-dependent effective-mass is formulated. The matrix elements are derived for a single-particle effective-mass Hamiltonian in the nearest-neighbor approximation. In contrast to previous work, the derivation given here strictly preserves the Hermicity of the original continuous-space Hamiltonian from which the tight-binding Hamiltonian is derived.
High resolution carrier temperature and lifetime topography of semi-insulating GaAs using spatially and spectrally resolved photoluminescence
Authors: Wang Z. M. , Windscheif J. , As D. J. , Jantz W.
Published: July 2002

The temperature Te of photoexcited carriers in semi-insulating GaAs wafers is determined with high lateral resolution by topographic measurement of the spectrally selected band-to-band recombination luminescence intensity IPL. It is also calculated from a detailed balance between carrier excitation and recombination, taking into account optical phonon, electron-electron, piezoelectric, and acoustic phonon scattering processes. Comparison of the experimental and theoretical Te data yields the lifetime τ of the photoexcited carriers, which is thus obtained without time-resolved measurement. The lifetime results are corroborated by comparison of the measured and calculated dependence of Te on the laser excitation power. The relation between Te and τ for given excitation power allows for the generation of two-dimensional high-resolution lifetime topograms. The correlation with conventional IPL topograms is direct, duplicating the lateral cellular pattern with comparable fluctuation amplitude. From these properties, it is inferred that τ is dominantly determined by inhomogeneously distributed nonradiative recombination centers. Their concentration is low in the walls of the dislocation network and high in the interior of the cells. The statistical evaluation of Te topograms allows for an application-oriented comparison of the quality and homogeneity of GaAs wafers.
Characteristic dispersion frequencies of resonance type soft mode system of lead titanium oxide in far infrared
Authors: Gupta Tapan K. , Permbhusri Kulapat
Published: July 2002

Lead titanate films were fabricated from their oxides and spin coated over alumina (Al2O3) and zinc sulphide (ZnS) substrates. Compositional analysis and surface morphology studies are done by x rays and the scanning electron microscope, respectively. The studies on electrical behavior of these films reveal the existence of phase transition at ∼850° K, which is confirmed as the Curie temperature (Tc) by temperature dependence of the dielectric constant. At room temperature, the real and imaginary parts of the permittivity exhibit a relaxation dispersion up to 100 GHz and indications of a resonance around 200 GHz.
Formation of epitaxial β-FeSi2 films on Si(001) as studied by medium-energy ion scattering
Authors: Konuma K. , Vrijmoeth J. , Zagwijn P. M. , Frenken J. W. M. , Vlieg E. , van der Veen J. F.
Published: July 2002

Ultrathin (∼1.3 nm) epitaxial films of β-FeSi2 were grown on Si(001) by room temperature (RT) deposition of Fe followed by annealing. During the various stages of the growth process, the lattice structure, composition, and morphology of the films were investigated by medium-energy ion scattering in conjunction with shadowing and blocking. At RT, the deposited Fe reacts with the Si(001) substrate and forms a continuous film of average composition FeSi. After annealing to 670 K, a conversion into β-FeSi2 has taken place and the film is no longer continuous. Further annealing at higher temperatures results in the formation of islands of increasing height. The β-FeSi2 films grown are composites of two azimuthal orientations with respect to the substrate: The predominant A orientation with β-FeSi2 [010]∥ Si〈110〉 and the B orientation with β-FeSi2 [010] ∥ Si〈100〉. The lattice strain in the films is partially relaxed. At the interface, the Fe atoms are found to be displaced from bulk lattice sites. These displacements are thought to be associated with the formation of atomic bonds at the interface of the dissimilar β-FeSi2(100) and Si(001) lattices.
Characterization of He/CH4 dc glow discharge plasmas by optical emission spectroscopy, mass spectrometry, and actinometry
Authors: de la Cal E. , Tafalla D. , Tabares F. L.
Published: July 2002

The gas-phase kinetics responsible for the formation of some electronically excited radicals (CH) and atoms (H, He, Ar) in glow discharge plasmas of He-methane admixtures has been investigated under several conditions of gas composition and discharge current at a total initial pressure of 0.01 Torr. Actinometry has been used to characterize the microscopic plasma parameters and, in combination with mass spectrometry and optical emission spectroscopy, to establish the formation mechanism of excited species. A very good correlation between CH emission intensity and carbon deposition rate has been found under all conditions. The effective cracking kinetic constant for methane molecules depends on plasma conditions, its value ranging from ≊1 to 5×10−10 cm3 s−1 as plasma current is increased, in good agreement with the expected value according to the actinometric results. A simplified kinetic model, accounting for all the observations reported in this work, is proposed.
Photon assisted reduction of interface charge between CdTe substrates and metalorganic chemical vapor deposition CdTe epilayers
Authors: Ruzin A. , Bezinger A. , Nemirovsky Y.
Published: July 2002

A novel photothermal in situ surface pretreatment that reduces the interface charge between p-type CdTe substrates and CdTe epilayers grown by metalorganic chemical vapor deposition is reported. Prior to the actual growth of the epilayers, the substrates are held at 450 °C in hydrogen atmosphere and simultaneously exposed to ultraviolet radiation. The interface charge is determined by the modified builtin potential derived from capacitance–voltage characteristics of Schottky contacts formed on the epilayers. The interface charge density is reduced from a typical value of ∼1×1012 cm−2 to a practically negligible value.
The role of high-pressure sintering in preparing high-quality n-type Nd2−xCexCuO4−y superconductors
Authors: Jin C. Q. , Zhou W. L. , Gu H. , Yao Y. S. , Wang W. K.
Published: July 2002

High-pressure high-temperature sintering (HHS) under 1.0, 2.0, 4.0, and 6.0 GPa for a short time was directly used to reduce n-type Nd1.85Ce0.15CuO4−y. Superconductors with higher critical temperature were obtained. The samples display lower resistivity, and more metalliclike resistance behavior at normal state with increasing sintering pressure. A small increase of lattice parameter and cell volume was observed via electron and x-ray diffraction. Transmission electron microscopy studies also show that there is a preferential orientation along the c axis among the neighboring grains. This helps get well-matching grain boundaries in the HHS samples. A Cu2O high-pressure phase was found via energy-dispersive x-ray analysis in the sample treated above 5.0 GPa, which is thought to be a deleterious factor for superconductivity. Sintering under suitable pressure not only enhances the intragranular conductivity, but also effectively improves the intergranular links. Finally, via the assumed interstitial oxygen ordering in the T′ structure, a possible mechanism of n-type reduction is proposed based on the change in tolerance factor under reduction atmosphere and high pressure.
Oxygen distribution in a thin epitaxial silicon layer
Authors: Wijaranakula W.
Published: July 2002

An empirical solution for the oxygen distribution in a thin epitaxial silicon layer and substrate wafers heavily doped with boron and antimony accounting for the prebake step is developed. The results from secondary ion mass spectroscopy analysis indicate that heavily doping with either boron or antimony has no effect on oxygen outdiffusion during epitaxial deposition. In contrast to slow-diffusing species such as boron and antimony, the plane at which the oxygen concentration is equal to one-half of the bulk oxygen concentration does not remain at the epitaxial layer/substrate interface, but is a function of the effective diffusion length of oxygen atoms for the total epitaxial process.
Thin film devices based on second order Bragg interaction
Authors: Kasraian Masoud , Seshadri S. R.
Published: July 2002

For an active dielectric film waveguide with different passive dielectrics for the cover and the substrate and with periodic corrugations on the upper surface, the characteristics of the second order Bragg interaction are investigated for the transverse electric mode with reference to such applications as frequency-selective reflector, couplers, and distributed feedback laser. Four canonical equations that are valid for a distributed coupler with two guided waves and two free waves, incident and radiated, one each in the cover and the substrate, are deduced systematically. The coupling coefficients are shown to satisfy all the requirements imposed by symmetry, reciprocity, and power conservation. From the solution of the canonical equations with the appropriate terminal conditions, the characteristics of the devices are obtained. The design parameters leading to the optimum performance of these devices are determined. For the distributed feedback laser, the frequencies of oscillation, the corresponding gain thresholds, and the relative power distributions in the two guided and the two radiated waves are obtained for the first few modes in the neighborhood of the second order Bragg frequency. The modifications of the characteristics introduced by the presence of a phase shifting section on the grating structure are analyzed for symmetrical and asymmetrical grating configurations. The effects of radiation and the phase shifting section on the separation of the gain thresholds of the neighboring lower order laser modes are investigated with a view to increasing the possibility of a single mode oscillation. Selected numerical results are presented to illustrate the important characteristics of the various thin film devices based on the second-order Bragg interaction.
Optical and compositional studies of SiN thin films with conventional and synchrotron radiation ellipsometry
Authors: Logothetidis S. , Petalas J. , Markwitz A. , Johnson R. L.
Published: July 2002

Conventional spectroscopic ellipsometry (SE) and synchrotron radiation spectroscopic ellipsometry (SRE) measurements were used to study SiN thin films grown with different techniques. The SiN films were chosen to have low oxygen and hydrogen content (<5%). Analysis of the dielectric function (ε) spectra measured by SE and SRE with the tetrahedron model and in conjunction with Rutherford backscattering spectroscopy (RBS) results shows that the dielectric function can be indicative of the stoichiometry of the materials. In addition, we apply a theoretical model to estimate major optical parameters characterizing the materials, such as the fundamental and Penn gaps, the refractive index and the thickness. Furthermore, the film thickness estimated by SE coincides within 10% with that calculated by other techniques, while the stoichiometry is found almost identical with the one estimated from RBS and systematically smaller than the one calculated by Auger sputter profiling.
Characteristics of a GaAs-InGaAs delta-doped quantum-well switch
Authors: Hsu Wei-Chou , Guo Der-Feng , Liu Wen-Chau , Lour Wen-Shiung
Published: July 2002

A new delta-doped quantum well negative-differential-resistance (NDR) switch, grown by molecular beam epitaxy, has been fabricated and demonstrated. A delta-doped sheet [δ(p+)], located at the center of InGaAs quantum well, provides a potential barrier for electron injection. When a sufficiently high voltage bias is applied, an interesting S-shaped NDR phenomenon can be observed due to the avalanche multiplication and potential redistribution process. From experimental results, it is known that the environmental temperature plays an important role on the NDR performance. The influence of temperature on the device performance is studied.
Sequential tunneling versus resonant tunneling in a double-barrier diode
Authors: Hu Yuming , Stapleton Shawn
Published: July 2002

The proposed sequential tunneling model predicts that the negative differential resistance in a double-barrier diode (DBD) can exist independent of a resonant Fabry–Perot effect. The proof is based on the existence of quasi-two-dimensional states for electrons in the quantum well. However, we find that existence of the quasi-two-dimensional states depends on a Fabry–Perot resonance. Therefore, if coherence of electrons in the quantum well is completely lost, then the quasi-two-dimensional states will also disappear. We find that the damped Fabry–Perot model can provide a unified formula for electron transport in the DBD from pure coherent tunneling to pure incoherent tunneling. In the latter case, the negative differential resistance disappears.
57Fe hyperfine fields and magnetic anisotropy of Sm2Fe17Nx interstitials with intermediate nitrogen concentrations
Authors: Rosenberg M. , Zhou R. J. , Katter M. , Schultz L. , Filoti G.
Published: July 2002

From the compositional dependence of the hyperfine fields at the iron sites of Sm2Fe17Nx alloys with 0≤x≤2.94, evidence for cone anisotropy for x=0.4 and uniaxial anisotropy at least for x≥0.81 has been found. The analysis of the volume dependence of the hyperfine fields allowed us to separate contributions arising from the anisotropy of the orbital component of the iron moment and from the change of the spin moments owing to magnetovolume and bonding effects. The temperature dependence of the average hyperfine field and of the hyperfine fields at the four different nonequivalent Fe sites suggests weak itinerant ferromagnetism for Sm2Fe17N2.94.
Spin lifetimes in diluted magnetic semiconductor spin superlattices
Authors: Jonker B. T. , Fu L. P. , Yu W. Y. , Chou W. C. , Petrou A. , Warnock J.
Published: July 2002

The growth of spin superlattice structures in which spin-up and spin-down carriers occupy alternating layers in the structure has recently been reported. We report here measurements of electron and hole spin lifetimes and spin relaxation processes. In dc photoluminescence spectra, both the higher-energy (+1/2,+3/2) and lower-energy (−1/2,−3/2) heavy-hole exciton interband transitions are observed, even though it would be energetically favorable for the spin-up carriers to first relax to the spin-down state before radiative recombination. From the field dependence of the intensity ratios of these components and a rate equation model, we determine the heavy hole τhs and electron τes spin lifetimes relative to the radiative lifetime τr, with τhs/τr≊4.5 and τes/τr≊0.08, so that the heavy-hole spin lifetime is ≊50 times longer than that of the electron. This is attributed to the strain-induced splitting of the heavy- and light-hole bands, which prevents fast spin relaxation of the holes.
Neutron study of crystal-field transitions in ErPO4
Authors: Loong C.-K. , Soderholm L. , Hammonds J. P. , Abraham M. M. , Boatner L. A. , Edelstein N. M.
Published: July 2002

The crystal-field splitting of the Er3+ ground multiplet 4I15/2 in ErPO4 is investigated by inelastic neutron scattering. Four excitations from the Γ7 ground state to the excited states and several transitions between the excited states have been identified. The observed transition energies and intensities are used to refine the parameters of the crystal-field potential. The calculated magnetic susceptibility χ(T) agrees well with experimental values from single-crystal measurements. A comparison of the neutron data with optical absorption and both nonresonance and resonance Raman scattering measurements has been made.
Pair interactions in Pr:La(CF3SO3)3⋅9H2O
Authors: Petasis D. T. , VanderVen N. S. , Simizu Satoru , Friedberg S. A.
Published: July 2002

The rare-earth trifluoromethanesulfonates R(CF3SO3)3⋅9H2O (RTFMS), form hexagonal crystals structurally similar to the ethylsulfates R(C2H5SO4)3⋅9H2O (RES). While the magnetic properties of RTFMS and RES analogs for many different rare earths are quite similar, PrTFMS has been found to exhibit long-range spin order (TN=0.31 K) while PrES shows only short-range order. In an attempt to understand this difference, electron paramagnetic resonance has been studied at and below 4.2 K at X band (∼9.2 GHz) and K band (∼23.5 GHz) for 1% and 3% concentrations of Pr3+ substituted in LaTFMS crystals. For Pr3+, the crystal field of C3h symmetry leaves an accidentally degenerate ground doublet and a singlet first excited state. There is a magnetic dipole moment along the c axis while an electric dipole moment develops in the perpendicular (xy) plane. Random distortions split the doublet by an amount Δ. Its Hamiltonian, in an effective spin S=1/2 formalism, is H= g∥βHzSz +AIzSz+(ΔxSx+ΔySy) +γ(ExSx+EySy), where Δ2=Δx2+Δy2. Single-ion resonances are well described by this Hamiltonian with g∥≊1.74, g⊥≊0, A≊0.086 cm−1, γ≊2.3×10−31 C m, and Δ/k=0.224 K. At K band, additional lines due to interacting pairs of Pr3+ ions have been identified and analyzed using an interaction Hamiltonian of the form Si⋅K⋅Sj.

Coupling constants have been determined for both nearest-neighbor (nn) pairs along the c axis and next-nearest-neighbor (nnn) pairs whose axis makes an angle of ∼65° with the c direction. nn and nnn interactions are of opposite sign, comparable magnitudes, and much larger than those of magnetic dipoles. They explain the Weiss constant observed in concentrated PrTFMS. The nnn coupling, which appears to be more important than in PrES, probably accounts for long-range ordering seen in PrTFMS although the detailed mechanism is unclear.
Quasielastic and inelastic neutron-scattering studies of [(CD3)3ND]FeCl3⋅2D2O: A one-dimensional Ising ferromagnet
Authors: Rosov N. , Lynn J. W. , Williams J. J. M. , Landee C. P.
Published: July 2002

Ferrous trimethylammonium chloride (FeTAC) is a member of a family of compounds that consist of chains of bihalide-bridged metal ions aligned along the b axis of the orthorhombic unit cell. The chains are connected in (bc) planes by hydrogen bonding of the waters of hydration; these planes are isolated along the a axis by the large trimethylammonium groups. FeTAC orders three dimensionally below TN=3.16 K, with the moments, which are canted from the b axis by ∼20°, coupled ferromagnetically along the b axis and staggered along the [101] direction. The order parameter follows the exact Onsager solution for a rectangular 2D Ising lattice below T/TN≊0.95, with ratio for the two in-plane magnetic exchange constants of 1.3×10−3. However, quasielastic scattering measurements in the vicinity of TN show that the correlations do not evolve as do those of the 2D Ising antiferromagnets ErBa2Cu3O7 and K2CoF4. Above TN, magnetic excitations along the b axis that follow the dispersion relation of a 1D ferromagnet with a gap of 3.7 meV have been observed.
The role of dipole coupling in multilayers
Authors: Hill E. W. , Tomlinson S. L. , Li J. P.
Published: July 2002

This work considers the role of magnetic dipole coupling in multilayers typically comprising a ferromagnetic material separated by a nonmagnetic metal spacer, e.g., NiFe/Ta. It has been suggested previously that dipolar fields may play an important role in the coupling of magnetic layers through a nonmagnetic spacer, in particular where surface roughness at the interface is significant. This work considers a first-order calculation of the dipole coupling energy and magnetic field between two adjacent layers. It is shown that dipole coupling energies originating from surface corrugations can behave in the same manner as a coercive field in terms of dependence on nonmagnetic spacer thickness and the degree of roughness. Interestingly, calculations also show that both ferromagnetic- and antiferromagnetic-like coupling can be explained in terms of this model. The role of the spacer thickness, magnetic layer thickness, and the form of the roughness is discussed.
Brillouin light scattering on Fe/Cr/Fe films (abstract)
Authors: Dima M. O. , Church D. B. , Kabos P. , Patton C. E.
Published: July 2002

Brillouin light scattering (BLS) was used to measure magnon frequency versus in-plane field H and in-plane wave vector for a Fe(38 Å)/Cr(13 Å)/Fe(38 Å) thin film. Both easy [100] and hard [110] field directions were used. The in-plane wave vector was perpendicular to the field. The data were obtained with a high contrast multipass tandem 3+3 Sandercock Fabry–Perot interferometer, with a free spectral range of 30 GHz. The results were correlated with previous hysteresis measurements.1 In the low field region (H<200 Oe), the magnon frequency decreases with increasing field in a manner consistent with soft mode behavior. The mode frequency in this low field region is slightly different on the Stokes (S) and anti-Stokes (AS) sides of the BLS spectra. At fields of 150–200 Oe one sees a cusp-like minimum in the magnon frequency field dependence. The S and AS frequencies also converge at this point. This is evidence for the antiparallel alignment of the two magnetizations of the Fe layers and the onset of canting.2 At higher fields (250–350 Oe) one sees a jump in frequency, followed by an apparent approach to saturation, and a further increase above 2 kOe. This behavior is basically the same for both field directions, but the frequencies are downshifted somewhat for the hard [110] direction. The wave-number k dependence is weak in the low field region. Above the jump, the magnon frequency shows a distinct increase with k. The substantial frequency increase with field above 2 kOe may indicate conversion to a surface mode character. This is consistent with the k dependence seen at high fields.
Magnetic properties of R2Co14(B,C) compounds (R=Y, Sm)
Authors: Zhao Z. G. , Wang J. Y. , Ge Y. P. , Yang F. M. , Sun X. K. , Chuang Y. C. , Buschow K. H. J. , de Boer F. R.
Published: July 2002

The magnetic properties of R2Co14B1−xCx (x=0, 0.5 and R=Y, Sm) compounds have been studied by measuring the temperature dependence of the easy- and hard-magnetization curves on magnetically aligned samples between 1.5 and 300 K for Y2Co14(B,C) and at 4.2 K for Sm2Co14(B,C). The magnetic anisotropy of Y2Co14B increases due to the substitution of C for B, whereas the saturation magnetization decreases. Between 1.5 and 300 K, the anisotropy field of Y2Co14B0.5C0.5 increases about 2 T and the Co moment decreases about 0.05μB. The anisotropy field of Sm2Co14B also increases upon C substitution and the saturation magnetization decreases slightly. The ac susceptibilities of both Sm2Co14B and Sm2Co14B0.5C0.5 exhibit anomalies that may arise from a spin reorientation within the basal plane.
Magnetic dichroism in core-level photoemission from fcc Fe/Cu(100) films
Authors: Pappas D. P. , Waddill G. D. , Tobin J. G.
Published: July 2002

Circularly polarized x rays from a synchrotron light source were used in photoemission from thin fcc Fe films on Cu(100). The measured exchange splitting for the 3s and 2p levels in these films is reduced from that measured in bulk bcc Fe. For both the 2p3/2 and the 2p1/2 levels the splittings are about half that of bulk bcc Fe. This trend is in line with measurements of the 3s exchange splitting from fcc Fe compared to bcc Fe as well. A 0.25 eV exchange splitting is observed in the 3p core level.
Experimental verification of a micromagnetic model of dual-layer magnetic films
Authors: Oti John O. , Russek Stephen E.
Published: July 2002

A micromagnetic model has been used to characterize the magnetic properties of dual-layer magnetic films. The model calculations, with experimentally determined input parameters, have been compared with measurements on fabricated Co alloy dual-layer films. Calculations are done with a more general version of a previous micromagnetic model, modified to allow the variation of certain media parameters which were previously held constant. Each of the experimental media consists of a bilayer of 30-nm-thick Co0.75Ni0.25 magnetic films separated by a Cr decoupling layer. The calculations predict a split in the coercivities of the layers for small Cr thicknesses which is observed experimentally. The model correlates an observed increase in media squareness ratio and coercivity, as the Cr thickness is increased, with diminishing exchange and magnetostatic interactions between the magnetic layers.
Magnetic properties of the diluted magnetic semiconductor Zn1−xCrxSe
Authors: Twardowski A. , Fries T. , Shapira Y. , Eggenkamp P. , Swagten H. J. M. , Demianiuk M.
Published: July 2002

The magnetic properties of a new diluted magnetic semiconductor Zn1−xCrxSe are reported. Specific heat was measured for 1.5 K≤T≤20 K and B≤3 T, whereas magnetization data were taken at T=2 and 4.2 K for magnetic field (B≤6 T) along (100), (110), and (111) crystallographic directions. The data are interpreted using the crystal-field model for the Cr++ ion, including static Jahn–Teller distortion.
Coercivity mechanisms in magneto-optical recording media (abstract)
Authors: Fu Hong , Mansuripur M. , Giles R.
Published: July 2002

This paper is devoted to an explanation of why the coercivity is much lower than the macroscopic anisotropy field (Brown paradox). The effect of nanostructure on the nucleation coercivity Hnc and wall motion coercivity Hwc is investigated using connection machine simulations. The magneto-optical (MO) thin films with inhomogeneities are modeled by a patchy two-dimensional lattice. The patches have random shape and size; the anisotropy constant fluctuates randomly from patch to patch; the exchange strength is uniform within the patches, but fluctuates at the patch borders. We have simulated various nucleation and wall motion processes on more than a dozen patchy lattices, with the average patch size d ranging from 60 to 300 Å. In all cases the average domain wall width δ≂120 Å. Some of our results are the following: (i) As d increases, Hnc decreases fast for d<δ, but slowly for d≳δ. (ii) Hnc decreases with decreasing exchange strength at the patch borders. (iii) Hwc in films of small patches (d≤δ) is greater than that in films of large patches (d≳δ). (iv) Hwc increases with decreasing exchange coupling at the patch borders. Our results show that the scale of inhomogeneity responsible for the low Hnc in MO thin films ranges from 100 to 200 Å. This work extends the previous work on nucleation1 and pinning2 mechanisms in several respects.
Effect of canting on the measured magnetic anisotropy constant for magneto-optical recording media (abstract)
Authors: Wu Te-Ho , Fu Hong , Hajjar R. A. , Suzuki T. , Mansuripur M.
Published: July 2002

Experimental data of the intrinsic perpendicular magnetic anisotropy energy constant Ku are presented for amorphous rare-earth–transition metal (RE–TM) Tbx(FeCo)1−x and multilayered Co/Pt thin film samples. These data were independently measured using five techniques based on torque magnetometry, the extraordinary Hall effect, and the magneto-optic Kerr effect. In the Hall effect measurement, the external field was applied to the sample in three different ways: fixed at 45° from the film normal; rotating around the sample; and fixed along the in-plane direction. The results obtained with these techniques agree with each other for the Co/Pt samples. However, we do find systematic differences in the measured Ku for the Tbx(FeCo)1−x samples. For example, Ku given by the Hall effect and Kerr effect is always larger (by up to a factor of 3) than that given by torque technique. Another interesting fact is that Ku given by the Hall effect technique drops as x approaches the compensation point xc in the TM-dominant case, but increases as x approaches xc in the RE-dominant case. These experimental results are explained by taking into account the canting between RE and TM subnetworks.