bluuebunny/crossref_metadata_2025 huggingface dataset:
Fundamental electrical properties of fluorinated and N2O plasma-annealed ultrathin silicon oxides grown by microwave plasma afterglow oxidation at low temperatures
Authors: Chen P. C. , Hsu Klaus Y. J. , Hwang H. L. , Lin J. Y.
Published: July 2002

A technique for growing ultrathin silicon oxides of superior quality at low temperatures is indispensable for future submicron device applications. Fundamental characteristics such as the oxide breakdown fields, oxide charges, and interface-state densities of various ultrathin silicon oxides (≤8 nm) grown by microwave plasma afterglow oxidation at low temperatures (400 and 600 °C) were investigated. Fluorination (HF soaking) and low-temperature N2O plasma annealing were employed to improve the properties of the oxides. The breakdown fields of the as-grown silicon oxides were enhanced and the interface-state densities were reduced. The effect of N2O annealing time on the interface-state density was also investigated. A longer annealing time (≳1 h) was required to reduce the interface-state density. The effective oxide charge density of 600 °C as-grown oxide was as low as 6×1010 cm−2. Additionally, the breakdown field of the thin silicon oxide grown at 600 °C with 15 min N2O plasma annealing was 12 MV/cm.
X-ray in situ observation of relaxation and diffusion processes in Si1−xGex layers on silicon substrates
Authors: Zaumseil P. , Jagdhold U. , Krüger D.
Published: July 2002

A method is presented to simultaneously investigate in situ the relaxation and diffusion behavior of Si1−xGex layers on silicon substrates using a conventional x-ray powder diffractometer with a high-temperature attachment. The method allows the direct determination of the time and temperature dependence of the relaxation and of the maximum Ge content. The diffusivity of Ge in silicon was studied by x-ray diffraction and secondary ion mass spectroscopy measurements. A nonlinear dependence of the effective diffusion coefficient on the Ge content was deduced by solving a diffusion equation.
Numerical study of the wave-vector dependence of the electron interband impact ionization rate in bulk GaAs
Authors: Wang Yang , Brennan Kevin F.
Published: July 2002

Ensemble Monte Carlo calculations of the electron interband impact ionization rate in bulk GaAs are presented using a wave-vector (k)-dependent formulation of the ionization transition rate. The transition rate is evaluated through use of numerically generated wavefunctions determined via a k⋅p calculation within the first two conduction bands at numerous points within a finely spaced three-dimensional grid in k space. The transition rate is determined to be greatest for states within the second conduction band. It is found that the interband impact ionization transition rate in bulk GaAs is best characterized as having an exceedingly ‘‘soft’’ threshold energy. As a consequence, the dead space, defined as the distance over which the ionization probability for a given carrier is assumed to be zero, is estimated to be much larger than that estimated using a ‘‘harder’’ threshold. These results have importance in the design of multiquantum-well avalanche photodiodes.
Quantum state transfer in double-quantum-well devices
Authors: Jakumeit Jürgen , Tutt Marcel , Pavlidis Dimitris
Published: July 2002

A Monte Carlo simulation of double-quantum-well (DQW) devices is presented in view of analyzing the quantum state transfer (QST) effect. Different structures, based on the AlGaAs/GaAs system, were simulated at 77 and 300 K and optimized in terms of electron transfer and device speed. The analysis revealed the dominant role of the impurity scattering for the QST. Different approaches were used for the optimization of QST devices and basic physical limitations were found in the electron transfer between the QWs. The maximum transfer of electrons from a high to a low mobility well was at best 20%. Negative differential resistance is hampered by the almost linear rather than threshold dependent relation of electron transfer on electric field. By optimizing the doping profile the operation frequency limit could be extended to 260 GHz.
Ubiquitous nonexponential decay: The effect of long-range couplings? (invited)
Authors: Dahlberg E. Dan , Lottis D. K. , White R. M. , Matson M. , Engle E.
Published: July 2002

Many physical systems exhibit a dynamic response referred to either as slow relaxation, a quasilogarithmic time dependence, or a stretched exponential response. Historically this time dependence has been attributed to the presence of disorder which creates a distribution of relaxation times. In two papers [D. K. Lottis, E. Dan Dahlberg, J. Christner, J. I. Lee, R. Peterson, and R. White, J. Appl. Phys. 63, 2920 (1988); D. K. Lottis, R. M. White, and E. Dan Dahlberg, Phys. Rev. Lett. 67, 362 (1991)], we have shown that this time dependence can alternatively be explained to be a consequence of interactions or couplings. In the model, the interactions between relaxing spins, the dipole–dipole couplings, drive the system from an initial state towards equilibrium. As the system relaxes, the dipolar energy is reduced and the driving force diminishes. This process gives rise to the observed slow relaxation-time dependence in a very natural manner. To guarantee the absence of disorder, the model considers the dipolar coupling or interaction between relaxing spins with a mean-field approximation, the demagnetization field. Another feature observed in physical systems which the model explains is the nonmonotonic temperature dependence of the logarithmic decay slope. In addition to a description of the model, measurements to determine the presence of interactions in some of the systems will be discussed.
Surface modification of magnetic recording heads by plasma immersion ion implantation and deposition
Authors: Komvopoulos K. , Wei B. , Anders S. , Anders A. , Brown I. G.
Published: July 2002

Plasma immersion ion implantation and deposition is a novel process for surface modification. By combining plasma deposition and ion implantation and using filtered vacuum arc plasmas, thin film formation, direct and recoil ion implantation, and ion-beam-assisted intermixing of the film and substrate can be accomplished simultaneously. The implications of this technique in tribology of magnetic recording media have been investigated experimentally. Surfaces of Al2O3-TiC heads were modified with silver, carbon, and titanium ions at doses of 3×1016 ions/cm2 and mean implantation energies between 2.0 and 4.2 keV. Simulation results indicated that the modified regions exhibited high concentrations of implanted species in the top 2–3 nm, atomically mixed interfaces, and thicknesses between 10 and 25 nm. Surface imaging with an atomic force microscope and nanoindentation testing revealed that the modified heads possessed smoother topographies and increased hardnesses. Contact start-stop and continuous sliding experiments with modified heads and carbon-coated magnetic rigid disks and microscopy observations demonstrated that significant enhancement of the friction and wear characteristics can be achieved with the present surface modification technique. The possible reasons for the improved tribological behavior and the predominant mechanisms during contact start-stop and continuous sliding are interpreted in light of the obtained experimental results.
Model of etching profiles for ion energy flux dependent etch rates in a collisionless plasma sheath
Authors: Abraham-Shrauner Barbara , Wang Chungdar Daniel
Published: July 2002

A model for the simulation of etch rates in ion-assisted etching of trenches on semiconductor wafers is developed. The evolution equation for the etched profiles where the etch rate is proportional to the incident ion energy flux is solved by the method of characteristics. The novel feature is that the ions incident on the semiconductor wafer are represented by a drifted Maxwellian distribution function. Approximate analytical expressions for the etch rates are derived and etch profiles are computed by integrating trajectory equations. The anisotropy of the etch improves with increasing values of the parameter U where U is the ratio of the ion beam speed to the ion thermal speed. The deviation from a rectangular trench is shown for different values of U.
Parametric dependence of x-ray laser gain in laser plasmas for 3p-3s transitions in neon-like krypton ions
Authors: Gupta G. P. , Sinha B. K.
Published: July 2002

X-ray laser gain in laser-produced plasmas for collisionally pumped 3p-3s transitions in neon (Ne)-like krypton ions has been estimated. The dependence of the gain on the temperature-dependent average ionic charge state Z̄(Te) and the fraction δ(Te) of Ne-like ions evaluated by using the plasma ionization model has been studied. It is observed that the temperature dependence of these parameters has a profound effect on the gain estimate, showing the importance of including the ionization model precisely for accurate modeling of x-ray laser gain.
Dissociation kinetics of hydrogen-passivated (100) Si/SiO2 interface defects
Authors: Stathis J. H.
Published: July 2002

The activation energy for thermal dissociation of hydrogen from silicon dangling-bond defects (Pb centers) has been measured using both (111)- and (100)-oriented samples. The behavior of each of the three Pb varieties [P111b at the (111) interface, P100b0 and P100b1 at the (100) interface] is compared. For P111b, excellent agreement with previous results by Brower [Phys. Rev. B 42, 3444 (1990)] is obtained. The activation energies of the (100)-interface Pb centers are slightly higher, assuming the same vibrational frequency, and, unlike that of P111b, are affected by a postoxidation anneal.
Detailed investigation of a nonradiative recombination center in Si by electrically detected magnetic resonance
Authors: Xiong Z. , Miller D. J.
Published: July 2002

The variation of the electrically detected magnetic-resonance signal from a Si diode has been studied in detail as a function of forward bias voltage in the temperature range 203–314 K. The results are explained in terms of the model of a weakly exchange-coupled pair of carriers in the ground and excited states respectively of a nonradiative recombination center. It was found that only a small proportion of the recombination centers in the space-charge region was paramagnetic and therefore contributed to the signal. The detailed agreement between theory and experiment enabled the time constants of the processes involved in those recombination centers to be determined as well as their spin-lattice relaxation time. More important, an energy level of the paramagnetic recombination center has been determined to be located 0.36 eV below the conduction-band edge. It is significant that the magnetic-resonance spectrum and the energy level of a recombination center have been determined in the same experiment.
Method of measuring anisotropy field of a recording medium investigated by computer simulation
Authors: Uesaka Yasutaro , Yoshida Kazuetsu , Nakatani Yoshinobu , Hayashi Nobuo
Published: July 2002

Three methods of measuring anisotropy field of a recording medium with the easy directions in the film plane were examined by computer simulation: (1) applying magnetic fields in a direction slightly tilted from the hard direction of the medium, (2) using rotational hysteresis loss, (3) applying perpendicular field to the film plane. The anisotropy fields obtained with these methods were smaller than the intrinsic anisotropy fields of grains (or particles). A method is proposed for obtaining the intrinsic anisotropy field of grains. In this method, magnetic torques are measured out-of-plane of a film. A method, which is an extension of the above method (1), for measuring the distribution of easy direction, is also examined and a revised method is proposed.
Consistent formula for the refractive index of AlxGa1−xAs below the band edge
Authors: Deri R. J. , Emanuel M. A.
Published: July 2002

We present a semi-empirical, analytic formula for the refractive index of AlxGa1−xAs below the band edge. The formula describes experimental refractive indices to within ±0.02 accuracy over the wavelength range from 1.5 to 0.75 μm and over the complete range of AlGaAs compositions.
Electrostriction of the copolymer of vinylidene-fluoride and trifluoroethylene
Authors: Elhami K. , Gauthier-Manuel B. , Manceau J. F. , Bastien F.
Published: July 2002

In the presence of an electrical field, the mechanical behavior of thin films of copolymer of vinylydene-fluoride and trifluoroethylene varies with the annealing temperature. If the film is annealed below the melting point of the copolymer, we observe, before any poling, an electrostriction effect mixed with a piezoelectric one. If the film is annealed above the melting point we observe only an electrostriction effect before the poling and a piezoelectric one after. The high value of the electrostriction coefficient obtained before poling could be used for actuators application.
Temperature variation of the refractive index of potassium niobate
Authors: Hsu Chia-Chen , Huang Tzer-Hsiang , Wei Tai-Huei , Chang Springfield , Leaung Chung-Yi
Published: July 2002

Use was made of a novel Fabry–Pérot interferometer technique to determine the variation of refractive index with temperature (∂n/∂T) of a potassium niobate crystal with a GaAlAs diode laser. The transverse intensity distributions of the laser transmitted through the crystal were found to be temperature dependent, and the ∂n/∂T values along crystallographic b and c axes were determined to be −4.5×10−5 and 6.1×10−5 °C−1, respectively. The temperature changes required for tangential tuning from transverse mode TEMqmn to TEMqm+1n and sagittal tuning from TEMqmn to TEMqmn+1 were found to be 0.29 and 1.27 °C, respectively, when the laser was polarized along the b axis. When the laser polarization was along the c axis, the tangential and sagittal tunings were found to be 0.10 and 0.54 °C, respectively. These differences between the tangential and sagittal directions indicated that the monolithic ring resonator used was indeed asymmetric.
Optical spectroscopy study of the role of Ag in laser ablated YBa2Cu3O7−δ thin films
Authors: Pinto R. , Apte P. R. , Adhi K. P. , Ogale S. B. , Kumar D. , Hegde M. S.
Published: July 2002

We have found evidence through optical spectroscopy that AgO is indeed generated in the laser plume during pulsed laser deposition of YBa2Cu3O7−δ (YBCO) thin films using Ag-doped YBCO targets. This supports our earlier conjecture that formation of AgO in the plume and its subsequent dissociation at the elevated substrate temperature (since AgO is unstable above 350 °C) provides active oxygen to the YBCO lattice, thereby increasing oxygen incorporation during growth of YBCO thin films.
An alternative approach to giant magnetoimpedance phenomena in amorphous ferromagnetic wires
Authors: Valenzuela R. , Knobel M. , Vázquez M. , Hernando A.
Published: July 2002

Magnetoimpedance in as-cast, nonmagnetostrictive CoFeBSi amorphous ferromagnetic wires, submitted to an ac electric current of 5 mA in the frequency range 100 Hz–100 kHz, is analyzed in terms of equivalent circuits. It is shown that the equivalent circuit representing the wire frequency behavior can be approximated by a parallel LpRp arrangement with elements Rs and Ls in series; Lp and Rp are associated with circumferential domain wall permeability and wall damping, respectively; Ls is related to the circumferential rotational permeability of the wire, and Rs accounts for the dc resistance in the circuit. When the wire is submitted to a longitudinal dc field high enough to approach saturation (Hdc=3600 A/m), the circuit becomes simply a series RsLs circuit. The various contributions from basic magnetization processes to magnetoimpedance are discussed, as well as deviations from this idealized model.
Quantum-well intermixing for optoelectronic integration using high energy ion implantation
Authors: Charbonneau S. , Poole P. J. , Piva P. G. , Aers G. C. , Koteles E. S. , Fallahi M. , He J.-J. , McCaffrey J. P. , Buchanan M. , Dion M. , Goldberg R. D. , Mitchell I. V.
Published: July 2002

The technique of ion-induced quantum-well (QW) intermixing using broad area, high energy (2–8 MeV As4+) ion implantation has been studied in a graded-index separate confinement heterostructure InGaAs/GaAs QW laser. This approach offers the prospect of a powerful and relatively simple fabrication technique for integrating optoelectronic devices. Parameters controlling the ion-induced QW intermixing, such as ion doses, fluxes, and energies, post-implantation annealing time, and temperature are investigated and optimized using optical characterization techniques such as photoluminescence, photoluminescence excitation, and absorption spectroscopy.
Growth kinetics of oxides during furnace oxidation of Si in N2O ambient
Authors: Bhat M. , Jia H. H. , Kwong D. L.
Published: July 2002

During oxidation of silicon in N2O ambient, the individual products of dissociation of N2O play important roles in the oxidation/nitridation process. In this paper the roles of the individual products of dissociation of N2O are discussed and the reaction mechanisms involving the oxidants and silicon are proposed. It is proposed that oxygen and nitric oxide, the two main dissociation products, participate in the oxidation and nitridation processes, respectively. In this paper, a one-dimensional steady-state model is presented for the growth of a silicon–oxynitride layer from N2O which utilizes the role of individual by-products of dissociation by accounting for the parallel reactions of silicon with oxygen (O2) and nitric oxide (NO). Using the model and the experimental data, values of diffusivities and reaction rates of O2 and NO through the oxynitride have been determined. The model has also been used to predict the functional form of the distribution of nitrogen in the dielectric for the first time.
Statistical thermodynamics foundation for photovoltaic and photothermal conversion. II. Application to photovoltaic conversion
Authors: Badescu Viorel , Landsberg Peter T.
Published: July 2002

The general theory developed in part I was applied to build up two models of photovoltaic conversion. To this end two different systems were analyzed. The first system consists of the whole absorber (converter), for which the balance equations for energy and entropy are written and then used to derive an upper bound for solar energy conversion. The second system covers a part of the absorber (converter), namely the valence and conduction electronic bands. The balance of energy is used in this case to derive, under additional assumptions, another upper limit for the conversion efficiency. This second system deals with the real location where the power is generated. Both models take into consideration the radiation polarization and reflection, and the effects of concentration. The second model yields a more accurate upper bound for the conversion efficiency. A generalized solar cell equation is derived. It is proved that other previous theories are particular cases of the present more general formalism.
Heat-energy deposition in x-ray interaction with materials application to Si and Be
Authors: Tong X. M. , Yamaoka H. , Nagasawa H. , Watanabe Tsutomu
Published: July 2002

Based on the analysis of the micro-processes due to the interaction of synchrotron radiation with materials, we have developed a theoretical method to calculate the heat energy deposited when synchrotron radiation passes through insert devices (filters, mirrors or monochromators). The micro-processes are photoionization, Compton scattering, Rayleigh scattering, electron elastic and inelastic collisions, electron Bremsstrahlung scattering and the Auger process. The energy of x rays is converted into the electrons’ kinetic energy and atomic excitation energy by photoionization and Compton scattering. High-energy photoelectrons slow down mainly through inelastic collisions with the atoms in materials. The energy deposition in a material is simulated according to the x-ray atom interaction cross sections and photoelectron-atom collision cross sections. The results of a calculation for x rays traversing Si and Be plates of 1.0 cm in thickness are presented and discussed as one typical example concerning important materials in optical devices. The dependence of the energy deposition on the angle of incidence of the x rays is also discussed. Both the utility and validity of the present simulation method are discussed.
Ferromagnetic Mn/C/Si films
Authors: Takeuchi T. , Igarashi M. , Hirayama Y. , Futamoto M.
Published: July 2002

Ferromagnetism at room temperature has been found in Mn/C/Si films prepared by sequential deposition of these elements at a substrate temperature about 360 °C by vacuum evaporation. The saturation magnetization increases rapidly with the carbon quantity, and it is about 250 emu/cc for a film with a nominal structure of Mn(6 nm)/C(0.5 nm)/Si(6 nm). The magnetization measurements at low temperatures show that the magnetic moment per ferromagnetic Mn atom corresponds to more than 1.2 Bohr magnetons.
Harmonic heat flow in anisotropic thin films
Authors: Grönbeck Henrik , Reichling Michael
Published: July 2002

The three-dimensional equation of heat conduction is solved to obtain the time-dependent (harmonic) temperature field in an opaque, thermally anisotropic layer on a thermally thick and isotropic substrate when the system is heated by a modulated Gaussian laser beam. The influence of the anisotropy on the amplitude and phase response of the temperature variation as a function of the position on the surface and depth in the layer is studied. The limiting case of one-dimensional heat diffusion in highly anisotropic media is discussed. For one example the influence of thermal anisotropy on the frequency dependence of the surface temperature distribution is studied.
Inhomogeneous strain relaxation and defect distribution of ZnTe layers deposited on (100)GaAs by metalorganic vapor phase epitaxy
Authors: Lovergine N. , Liaci L. , Ganière J.-D. , Leo G. , Drigo A. V. , Romanato F. , Mancini A. M. , Vasanelli L.
Published: July 2002

The structural characterization of ZnTe epilayers grown on (100)GaAs by metalorganic vapor-phase epitaxy is reported. A detailed study of the ZnTe/GaAs heterostructure based on both high-resolution and conventional electron microscopy and ion channeling Rutherford backscattering spectrometry allows correlation of the type and spatial distribution of the extended defects occurring at or close to the ZnTe/GaAs interface with the amount of residual lattice strain into the ZnTe epilayers. Both pure edge Lomer and 60°-mixed misfit dislocations were identified at the interface along with partial dislocations bounding stacking faults, their overall density and distance distribution indicating the occurrence of a residual compressive strain at the heterostructure interface. By comparing this interface strain to the corresponding surface value of the same samples the occurrence of an inhomogeneous strain relaxation along the growth direction is clearly demonstrated. It is shown that such a strain gradient should be entirely ascribed to threading dislocations occurring into the ZnTe epilayers, their distribution being strictly correlated to the amount of residual strain along the epilayer growth direction. The conclusions are further supported by the analysis of the ZnTe surface strain, whose dependence on the epilayer thickness is consistent with that expected on the basis of a phenomenological model for the epilayer residual strain relaxation by threading dislocations.
Identification of the common electrically detected magnetic resonance signal from a Si diode
Authors: Xiong Z. , Miller D. J.
Published: July 2002

Structure in the electrically detected magnetic resonance signal from a common type of Si diode has been resolved and measured for different orientations of the magnetic field in the (111) plane. The center is found to have C3v symmetry with Δg∥≳0 and Δg⊥≊0. It is proposed that the signal is due to a broken bond, or combination of colinear broken bonds, in a vacancy cluster which acts as a recombination center. The most likely cluster is the divacancy which has captured an electron in the antibonding state formed from the most distant broken bonds and a hole in a bonding state associated with the divacancy.
Photoluminescence study on point defects in buried SiO2 film formed by implantation of oxygen
Authors: Seol Kwang Soo , Ieki Akihito , Ohki Yoshimichi , Nishikawa Hiroyuki , Tachimori Masaharu
Published: July 2002

Defects in buried SiO2 films in Si formed by implantation of oxygen ions were characterized by photoluminescence (PL) excited by KrF (5.0 eV) excimer laser and synchrotron radiation. Two PL bands were observed at 4.3 and 2.7 eV. The 4.3 eV band has two PL excitation bands at 5.0 and 7.4 eV, and its decay time is 4.0 ns for the 5.0 eV excitation and 2.4 ns for the 7.4 eV excitation. The decay time of the 2.7 eV PL band is found to be 9.7 ms. These results are very similar to those for the 4.3 eV and the 2.7 eV PL bands, which are observed in bulk silica glass of an oxygen-deficient type and attributed to the oxygen vacancy. Through the change in the PL intensity with the film thickness, the buried SiO2 film is considered to contain the oxygen vacancy defects in a high amount throughout the oxide.
Improved Airy function formalism for study of resonant tunneling in multibarrier semiconductor heterostructures
Authors: Allen Shaune S. , Richardson Steven L.
Published: July 2002

We show that our exact one-dimensional Airy function formalism for studying electron resonant tunneling in multibarrier semiconductor heterostructures is an improvement on a previous calculation of Brennan and Summers [J. Appl. Phys. 61, 614 (1987)]. We also clearly demonstrate that our method gives better agreement with the numerical approach of Vassell, Lee, and Lockwood [J. Appl. Phys. 54, 5206 (1983)] in calculating the transmission coefficient T(E) and current density J(E) for multibarrier semiconductor heterostructures.
Quantized conductance in a split-gate point contact based on a pseudomorphic InGaAs/InP heterostructure
Authors: Tietze M. F. , Schäpers Th. , Appenzeller J. , Engels G. , Hollfelder M. , Lengeler B. , Lüth H.
Published: July 2002

In this paper the fabrication and characterization of split-gate point contacts based on a pseudomorphic InGaAs/InP heterostructure with an indium content of 77% in the strained channel layer is described. Steps in the conductance were observed, which are due to quantized conductance through the quasi one-dimensional constriction formed by the split-gates. Deviations from the ideal quantization are studied by applying differing bias voltages on the two fingers forming the point contact. Since the channel layer of our structure consists of a ternary material it is argued that, beside impurity and interface roughness scattering, alloy scattering processes contribute significantly to the observed deviations of the ideal quantized conductance.
Maximally efficient permanent magnet structures
Authors: Jensen J. H. , Abele M. G.
Published: July 2002

A general theory is developed for determining the most efficient magnetic structure that generates a specified magnetic field. The structure is assumed to be composed of linear, isotropic magnetized material, occupying a given geometrical region. The efficiency is quantified by a figure of merit defined as the external field energy produced by the magnet divided by the maximum amount of energy that can be stored by the magnetized material. A set of equations is derived that determines the distribution of remanence needed to maximize this figure of merit, and several theorems concerning maximally efficient structures are given. Simple two-dimensional examples are used to illustrate the theory.
Thermal runaway and bistability in microwave heated isothermal slabs
Authors: Vriezinga C. A.
Published: July 2002

The dissipation of electromagnetic energy within a microwave heated layer has been analyzed. It has been shown that the dissipation oscillates as a function of temperature, regardless of the specimen material. This oscillation, combined with the heat loss, is found to be responsible for thermal runaway phenomenon in isothermal slabs. Based on such an observation, a general rule to prevent thermal runaway was developed. Slab temperature analysis for time dependent microwave power indicates that the concept of bistability is not the appropriate term to describe the observed jumps in temperature.
Modeling of magnetic trilayers with interlayer coupling: Application to Co/Ru
Authors: Zoll S. , Van den Berg H. A. M. , Ounadjela K. , Stoeffler D. , Dinia A.
Published: July 2002

A magnetization model is presented that is used to cover trilayers containing two magnetic layers that are exchange coupled via an intermediate nonmagnetic layer and that have different crystalline anisotropies. The interfaces are coupled to the bulk by a twisted magnetization configuration which is evaluated using the Ritz method. By minimizing the total energy, experimental magnetization curves of strongly coupled Co/Ru sandwiches can be reproduced with a good precision and with the same set of parameters in two perpendicular field directions. These physical parameters can be determined with a good reliability and are in agreement with the literature except for the bulk anisotropy of the Co layer first deposited, which is twice as large as the known bulk value. This originates in the magnetoelastic contributions due to lattice misfit and interface roughness. It is shown that the interlayer exchange coupling forces the magnetization of both layers to be along the same axis in the low-field range notwithstanding the opposite sign of the anisotropy constants in most stacks. It is also demonstrated that the differences in the orientations of the moments in one Co layer are modest and depend on the various parameters. In particular, the bulk exchange constant is a decisive parameter that makes the calculated curves close to the experimental ones.
Ab initio studies of S chemisorption on GaAs(100)
Authors: Guo-Ping Jing , Ruda Harry E.
Published: July 2002

Ab initio molecular orbital calculations are used to study S chemisorption on reconstructed GaAs(100) surfaces. Three cluster models are developed to simulate the main characteristics of 4×2, 4×6, and 2×6 reconstructed GaAs(100) surfaces. S atoms chemisorb preferentially on bridge sites and dramatically change the surface geometry. If a single S is adsorbed on a bridge site by breaking a Ga—Ga dimer bond, the Ga–Ga separation increases to 4.10 Å (from the dimerized separation of 2.80 Å), with a concomitant lowering of the bond energy by 0.07 (hartree). When two S atoms are adsorbed on adjacent bridge sites, the two S atoms weakly repel each other (establishing themselves ∼10° off from the normal bridge position). At the equilibrium geometry, each S atom retains a charge of −0.6 (e) and increases the net dipole moment perpendicular to the surface by −1.12 (hartree). This dipole moment in turn drives holes (electrons) toward (away) from the surface and increases the photoluminescence efficiency. Calculated electronic structure and local density of states also reflect a large charge accumulation near adsorbed S atoms.
Thermal diffusivity of solids determination by photoacoustic piezoelectric technique
Authors: Blonskij I. V. , Tkhoryk V. A. , Shendeleva M. L.
Published: July 2002

A simple approach for the calculation of photoacoustic piezoelectric response in the case of low modulation frequencies has been proposed. The analytical expressions have been obtained for the amplitude and phase of the photoacoustic signal and their experimental verification has been carried out. The new method for the thermal diffusivity determination from the phase versus frequency characteristics has been developed.
Coefficients of thermal expansion for barium hexaferrite
Authors: Dorsey P. C. , Qadri S. B. , Feldman J. L. , Horwitz J. S. , Lubitz P. , Chrisey D. B. , Ings J. B.
Published: July 2002

The thermal expansion coefficients along the a axis (αa) and c axis (αc) of barium hexaferrite were measured from room temperature to 850 °C. The thermal expansion of barium hexaferrite along the a axis and c axis was found to be linear as a function of temperature, indicating constant thermal expansion coefficients over this temperature range. The specific values for αc and αa were calculated to be 12.2±0.9 and 9.99±1.1 ppm/°C, respectively. The measurements were performed using a standard x-ray powder diffraction setup equipped with a high temperature stage.
Search for high moment soft magnetic materials: FeZrN (abstract)
Authors: Chakraborty A. , Bellesis G. H. , Mountfield K. R. , Lambeth D. N. , Kryder M. H.
Published: July 2002

FeN materials exhibiting high moment, low coercivity and small magnetostriction have previously been reported. Zr has been known to reduce the magnetostriction in other Fe alloys. The criteria for an ideal recording head pole material as well as shields for magnetroresistive sensors include high moment, low coercivity, high permeability, and zero magnetostriction. We present here the properties of half micrometer thick rf sputtered FeZrN films on glass coupons. The films were deposited at a pressure of 3 mTorr using a Perkin–Elmer sputtering system. The target was composed of Fe with Zr chips covering approximately 2% of the surface area. The properties were measured as a function of the N2 partial pressure. The saturation magnetization of the as-sputtered films was approximately 20 kG. The easy axis and the hard axis coercivities show minima at approximately 7%–10% N2 partial pressure of approximately 1.8 and 0.6 G, respectively. The magnetic anisotropy is approximately 5 G yielding a dc permeability of approximately 4000 along the hard axis. X-ray data reveal a systematic change in the ratio of α-Fe and γ-Fe4N; the amount of the γ-Fe4N phase increases with increasing N2 flow rate. The magnetostriction increases with increasing N2 content crossing zero at approximately 6%. The grain size as probed by atomic force microscopy is an increasing function of the N2 partial pressure, from a few nm for a N2 partial pressure of 5% to as large as 50 nm for a N2 partial pressure of 15%.
Ba ferrite films with large saturation magnetization and high coercivity prepared by low-temperature sputter deposition
Authors: Noma K. , Matsushita N. , Nakagawa S. , Naoe M.
Published: July 2002

Ba ferrite films were deposited in a mixture of Xe, Ar, and O2 by using facing targets sputtering apparatus with sintered targets of Fe-excessive BaM ferrite. By using Xe as sputtering gas, the bombardment of energetic Ar atoms recoiled from target to film surface was sufficiently suppressed and Fe content in Ba ferrite crystallites was significantly increased. It was found that the segregation of spinel crystallites among BaM ones were not observed and these BaM crystallites revealed the excellent c-axis orientation normal to film plane and clear perpendicular magnetic anisotropy. At substrate temperature Ts of 600 °C, saturation magnetization 4πMs of 5.1 kG, which is larger than that of BaM ferrite single crystal, and perpendicular coercivity Hc⊥ of 2.4 kOe were obtained. BaM ferrite films composed of well c-axis oriented crystallites with large perpendicular magnetic anisotropy constant, large saturation magnetization 4πMs of 4.7 kG and high perpendicular coercivity Hc⊥ of 2.4 kOe were obtained at substrate temperature Ts as low as 475 °C.
Fourier-transform infrared and Raman spectroscopies of plasmon anisotropy in heavily doped GaAs/AlAs superlattices
Authors: Pusep Yu. A. , Silva M. T. O. , Galzerani J. C. , Milekhin A. G. , Moshegov N. T. , Toropov A. I.
Published: July 2002

The plasmon-longitudinal optical phonon vibrational modes in heavily doped GaAs/AlAs superlattices were analyzed both with Fourier transform infrared and Raman spectroscopies. The modes polarized in the direction normal to the layers were observed in the infrared spectra, while the Raman spectra revealed those modes polarized along the layers. The experimental results allowed us to derive the dispersion relations of the electrons in both directions and these relations were found to be in good agreement with the miniband dispersions calculated in the envelope function approximation when both Γ and X conduction electron states were taken into account. No evidence of the contribution of the two-dimensional electron states to the vertical transport was found.
Time-resolved photoluminescence microscopy of GaInAs/GaInAsP quantum wells intermixed using a pulsed laser technique
Authors: Fancey S. J. , Buller G. S. , Massa J. S. , Walker A. C. , McLean C. J. , McKee A. , Bryce A. C. , Marsh J. H. , De La Rue R. M.
Published: July 2002

High spatial resolution time-resolved photoluminescence has been used to study GaInAs/GaInAsP quantum-well structures selectively intermixed using the pulsed photoabsorption-induced disordering technique. Photoluminescence decay measurements at wavelengths ≳1.3 μm were obtained using novel high-efficiency photon-counting detectors and were found to correlate spatially with the observed luminescence blue shift in these structures. Results indicate a reduction in the nonradiative recombination time of nearly two orders of magnitude as a result of this intermixing technique.
Effect of disorder-induced band mixing on the conduction-band effective mass of InAlGaAs alloys lattice matched to InP
Authors: Fan J. C. , Chen Y. F.
Published: July 2002

It is found that the calculation of the effective mass for InAlGaAs alloys obtained from the five-band k⋅p theory is smaller than the measured value from the optically detected cyclotron resonance reported recently. We point out that the effect of disorder-induced conduction valence band mixing must be considered. This disorder effect which creates potential fluctuations is to reduce the matrix element P2 for the conduction valence band coupling in the k⋅p theoretical expression. The strength of the potential fluctuations can be described in terms of the electronegativity difference related to chemical disorder. The inclusion of the disorder effect in the (In0.53Al0.47As)x(In0.53Ga0.47As)1−x quaternary system gives a very good fit to the measured data.
Structural and optical properties of strain-relaxed InAsP/InP heterostructures grown by metalorganic vapor phase epitaxy on InP(001) using tertiarybutylarsine
Authors: Desjardins P. , Beaudoin M. , Leonelli R. , L’Espérance G. , Masut R. A.
Published: July 2002

A combination of transmission electron microscopy and high-resolution x-ray diffraction analyses has been used to determine the exact strain in each layer of InAsP/InP multiple-quantum-well structures grown by metalorganic vapor phase epitaxy on InP(001) using trimethylindium, tertiarybutylarsine, and phosphine as precursors. The strain-relaxed structures are characterized by misfit dislocations located exclusively at (i) the interface between the buffer layer and the multilayer, and (ii) the interface between the multilayer and the cap layer. The low-temperature optical absorption spectra show well resolved excitonic transitions that are significantly shifted by strain relaxation. The spectra are analyzed with a solution to the Schrödinger equation in the envelope function formalism using the Bastard–Marzin model. The energies for the major transitions involving light- and heavy-holes are predicted accurately for all samples, allowing the determination of the heterojunction band offset. The heavy- and light-hole exciton binding energies deduced from that analysis range from 5 to 7 meV and 2 to 5 meV, respectively. The absolute values of the conduction band offset (expressed in meV) are consistent with the predictions of the quantum dipole model [J. Tersoff, Phys. Rev. B 30, 4874 (1984)] when calculating the midgap energy using a linear interpolation for the InAsP ternary composition between the values for the binaries InAs and InP. The absolute value of conduction band offset (in meV), which is dictated by the composition of the ternary layer, does not significantly depend on the degree of strain relaxation of the multilayer. However, the effect of this strain-relaxation on the InP and InAsP band gaps causes the conduction band offset to apparently increase from 72 to 82% of the partially-strained band gap difference when the strain-relaxation increases from 0 to 17%.
Three-dimensional, particle-in-cell simulations of applied-B ion diodes on the particle beam fusion accelerator II
Authors: Pointon T. D. , Desjarlais M. P.
Published: July 2002

We have used the three-dimensional, particle-in-cell code QUICKSILVER [J. P. Quintenz, et al., Lasers and Particle Beams 12, 283 (1994)] to simulate radial applied-B ion diodes on the particle beam fusion accelerator II at Sandia National Laboratories. The simulations agree well with experiments early in the beam pulse, but differ substantially as the ion-beam current increases. This is attributed to the oversimplified ion emission model. We see the same instabilities seen in earlier simulations with idealized diode geometries; Early in time there is a diocotron instability, followed by a transition to an ‘‘ion mode’’ instability at much lower frequency. The instability-induced beam divergence for the ∼10 MeV beam during the diocotron phase is <10 mrad, significantly less than the total beam divergence in experiments early in the pulse, but increases to ≳25 mrad after the transition. The ion mode has a distinct harmonic structure along the applied field lines, making the instability transition sensitive to the diode geometry. The ion mode instability in our latest simulations is consistent with evidence of instabilities from recent experiments.
Dynamic behavior of quantum cellular automata
Authors: Tougaw P. Douglas , Lent Craig S.
Published: July 2002

We examine the dynamic behavior of quantum cellular automata, arrays of artificial quantum-dot cells that can be used to perform useful computations. The dynamics of the array can be solved directly, retaining the full many-electron degrees of freedom only for small array sizes. For larger arrays, we develop several approximate techniques for reducing the size of the basis set required. We examine the effect of intercellular quantum correlations on the switching response. Several important examples of switching behavior are solved using the techniques developed.
Photothermal depth profiling using only phase data
Authors: Lan T. T. N. , Walther H. G.
Published: July 2002

Surface problems in photothermal depth profiling of technical surfaces arise from the intrinsic optical absorptivity; therefore, it is not appropriate to use amplitude values for profile reconstruction. We report a reconstruction technique based only on phase data. The performance of this approach was tested successfully by measuring the thermal conductivity profiles of laser hardened steel samples.
Local probe measurements of current filamentation in p-germanium
Authors: Muz Ch. , Müller W. , Clauss W.
Published: July 2002

Current filaments formed in the parameter regime of the low-temperature impurity impact ionization breakdown of p-germanium were investigated by measuring tunneling and point contact characteristics of a scanning tunneling microscope tip fixed at certain sample positions under variation of the sample voltage. The results clearly indicate that the tip current is determined by local potential variations which can be interpreted by a model for the filament taking into account the finite electric resistivity of the sample contacts.
Measurements of frequency upconversion and picosecond excitation-correlation luminescence spectra in GaAs quantum wells and determination of time constants describing exciton dynamics
Authors: Kumar Rajesh , Vengurlekar A. S. , Prabhu S. S. , Shah Jagdeep , Pfeiffer L. N.
Published: July 2002

We study the picosecond dynamics of free carriers and excitons in high-quality GaAs quantum wells at 8 K using frequency upconversion (UC) and nonlinear picosecond excitation-correlation (PEC) luminescence spectroscopy under nonresonant excitation conditions with carrier densities of a few 1010 cm−2. We analyze the measurements using a theoretical model of the coupled free electron–hole pair and exciton dynamics, incorporating important density-dependent nonlinear effects, caused by exciton collisions and fermion exclusion. The time-resolved UC photoluminescence (PL) spectra are used to deduce time constants related to free electron–hole (e–h)-pair transformation into excitons and exciton transfer from large K to small K (k is the exciton wave vector). It is shown that unambiguous determination of exciton radiative and nonradiative decay times are possible when both UC and PEC PL spectra are considered simultaneously. We carry out a detailed line-shape analysis of the picosecond UC and PEC PL spectra. We find that while collision broadening is mainly responsible for the observed nonlinear excitonic PEC signal, the PEC PL spectra can be fully explained only when additional nonlinear effects, such as density-dependent radiative lifetime and fermion exclusion, are included in the analysis. From a comparison of the time evolution of both the PEC and UC PL spectra with theory, the time constants introduced in the model to describe free carrier density decay, transfer of excitons with K≠0 to K≊0 states, and their radiative and nonradiative decay are estimated to be about 50, 15, 60, and 1800 ps respectively.
Magnetization reversal in Co/Pd multilayers with varying Co sublayer thickness
Authors: Choe Sug-Bong , Shin Sung-Chul
Published: July 2002

We have investigated magnetization reversal in nanostructured Co/Pd multilayers. Interestingly enough, reversal phenomena in this system were found to be very sensitive to the Co-sublayer thickness and very contrasting reversal behaviors were observed between the samples having 2-Å-thick Co and 4-Å-thick Co sublayers. Direct domain observation and time-dependent magnetization viscosity curves revealed the wall-motion dominant reversal in the former samples, and the nucleation dominant reversal in the latter samples. Magnetization reversal in Co/Pd multilayers was theoretically studied by Monte Carlo simulation and experimentally observed reversal behaviors could be well explained.
Epitaxial Ag templates on Si(001) for bicrystal CoCrTa media
Authors: Yang Wei , Lambeth David N. , Tang Li , Laughlin David E.
Published: July 2002

Ag thin films were sputter deposited on hydrofluoric acid-etched Si(001) single crystal substrates and employed as templates for the epitaxial growth of Cr(001) films and consequently bicrystal CoCrTa(112̄0) films. The orientation relationship was determined to be bicrystal CoCrTa on Cr(001)[100]‖Ag(001)[110]‖Si(001)[110]. X-ray diffraction analysis showed only strong Ag(002) peaks throughout the Ag film thickness range of 50–1500 Å. Atomic force microscopy showed that almost continuous Ag films can be achieved in the thickness range of 500–1000 Å, which result in the highest coercivities in CoCrTa films whose magnetic properties were found to vary with the Ag template thickness and surface morphology. The periodic in-plane angular variation of magnetic properties and torque curves with a four-fold symmetry were confirmed in the bicrystal CoCrTa films.