bluuebunny/crossref_metadata_2025 huggingface dataset:
Prediction of the phase behavior of acetonitrile and methanol with ab initio pair potentials. I. Pure components
Authors: Sum Amadeu K. , Sandler Stanley I. , Bukowski Robert , Szalewicz Krzysztof
Published: July 2002

In a recent publication [J. Chem. Phys. 113, 5401 (2000)], we investigated the applicability of ab initio pair potentials for acetonitrile and methanol for predicting their phase behavior using Gibbs ensemble Monte Carlo simulations. Here, we extend this study by introducing improvements to the pair interactions to better represent the phase behavior of acetonitrile and methanol. The first adjustment was a scaling of the interaction energies so that the calculated second virial coefficient matches the measured values. Excellent agreement was obtained for the second virial coefficient by scaling of the pair potentials by 1.08 and 1.20 for acetonitrile and methanol, respectively. The predicted phase behavior with these scaled potentials results in a large improvement for methanol, and slightly poorer predictions for acetonitrile. Next, to account for multibody effects, a classical polarizable model was added to the simulations, which showed that multibody interactions do not contribute to the equilibrium properties of acetonitrile, but play an important role in the molecular interactions of methanol. The addition of polarization terms to the simulations for methanol results in vapor pressures in good agreement with experimental values.
Simulation studies of energy transfer to heavy ions by strong current-driven instabilities
Authors: Toida Mieko , Sugishima Akihiro , Ohsawa Yukiharu
Published: July 2002

Energy transfer from electrons to heavy ions via the nonlinear evolution of instabilities driven by strong currents is studied by means of a two-dimensional, electrostatic, particle simulation code with full ion and electron dynamics. Electrons are assumed to drift along a uniform magnetic field with an initial speed larger than the electron thermal speed in a plasma consisting of hydrogen (H) and helium (He) ions and electrons. Simulations then show that, because of the electron trapping by the Buneman waves which grow at first, the shape of the electron velocity distribution function fe(v∥) is drastically changed and is significantly broadened. Then, H cyclotron waves, which are marginal in the initial state, are destabilized and grow to large amplitudes. Their frequencies are higher than those predicted by the linear theory. Although He ions do not gain energies from the Buneman waves, they are heated by H cyclotron waves. As a result of the instabilities, fe(v∥) has a plateau region with its width much greater than the initial thermal speed.
Performance projections for ballistic carbon nanotube field-effect transistors
Authors: Guo Jing , Lundstrom Mark , Datta Supriyo
Published: July 2002

The performance limits of carbon nanotube field-effect transistors (CNTFETs) are examined theoretically by extending a one-dimensional treatment used for silicon metal–oxide–semiconductor field-effect transistors (MOSFETs). Compared to ballistic MOSFETs, ballistic CNTFETs show similar I–V characteristics but the channel conductance is quantized. For low-voltage, digital applications, the CNTFET with a planar gate geometry provides an on-current that is comparable to that expected for a ballistic MOSFET. Significantly better performance, however, could be achieved with high gate capacitance structures. Because the computed performance limits greatly exceed the performance of recently reported CNTFETs, there is considerable opportunity for progress in device performance.
Toroidal flow stablization of disruptive high β tokamaks
Authors: Kleva Robert G. , Guzdar Parvez N.
Published: July 2002

Disruptive high β tokamak plasmas can be stabilized by the addition of a sheared toroidal flow. Nonlinear simulations demonstrate that confinement in flow-free high β tokamaks is rapidly destroyed by growing fingers of hot plasma that jet out from the center of the discharge to the wall. The added toroidal flow eliminates the growing fingers, maintaining confinement. As β increases further, the toroidal flow becomes less effective at maintaining a stable plasma. But a sound speed toroidal flow increases the critical value of β below which confinement is maintained without disruptions.
High-piezoelectric behavior of c-axis-oriented lead zirconate titanate thin films with composition near the morphotropic phase boundary
Authors: Fu Desheng , Suzuki Hisao , Ogawa Takeshi , Ishikawa Kenji
Published: July 2002

The piezoelectric responses of c-axis-oriented Pb(Zr0.53Ti0.47)O3 (PZT) thin films have been studied by measuring the stress-induced charge with an accurate charge integrator. These measurements reveal that the c-axis-oriented PZT films have high values of d33, which are several times those of ceramic materials. The intrinsic d33 values of poled films are about 680 and 800 pC/N for the c-axis-oriented films on Si and MgO single-crystal substrates, respectively. It shows that the thin-film deposition technique opens an approach for exploring the potential superior properties of PZT near the morphotropic phase boundary.
Field emission from crystalline copper sulphide nanowire arrays
Authors: Chen Jun , Deng S. Z. , Xu N. S. , Wang Suhua , Wen Xiaogang , Yang Shihe , Yang Chunlei , Wang Jiannong , Ge Weikun
Published: July 2002

Straight crystalline copper sulphide (Cu2S) nanowire arrays have been grown by using a simple gas–solid reaction at room temperature. These were demonstrated to exhibit semiconductor properties. Field emission was observed at a field of ∼6 MV/m, and its current-field characteristics deviate from Fowler–Nordheim theory, i.e., showing a nonlinear Fowler–Nordheim plot. The uniform emission from the whole arrays was observed using transparent anode technique, and their variation with applied field was recorded. The emission from individual nanowires was also studied using a field emission microscope, and was found to consist of a number of spatially resolved diffuse spots. Finally, stable emission current at different levels and over time was recorded. These findings indicate that semiconductor nanowires as cold cathode have a potential future, worthy of further comprehensive investigation. The technical importance of using semiconductor nanowires as cold cathode emitter is given.
Photoabsorption spectra of icosahedral fullerenes: A semiempirical approach
Authors: Iglesias-Groth S. , Ruiz A. , Bretón J. , Gomez Llorente J. M.
Published: July 2002

A semiempirical model has been used to predict the electronic photoabsorption spectra of five icosahedral fullerenes in the range C60–C720. The model parameters are first fixed in C60 by fitting its calculated spectrum to the available experimental data, and then conveniently adjusted to describe the larger fullerenes. The structures observed in the calculated spectra show a tendency to smooth π and σ plasmons as the fullerene size increases; however other finite-size features related to the particular geometry of these molecules are still visible at higher resolution. Some consequences of the strong electron screening effects on these spectra are discussed.
Equilibrium size of circular domains in Langmuir monolayers
Authors: Wurlitzer S. , Fischer Th. M. , Schmiedel H.
Published: July 2002

Calculations of equilibrium radii of Langmuir-monolayer domains are compared with measurements in the liquid expanded/gas coexistence region of myristic acid. This paper extends the works of McConnell [Proc. Natl. Acad. Sci. 86, 3452 (1989)] to higher area fractions Φ of the domain phase. A maximum increase of the equilibrium radius of gas-domains by the factor of 26 on expansion (from dilute towards closely packed domains) is predicted, while the number-density of the domains passes through a maximum at Φ=0.35. The experiments agree with these predictions. Some domains disappear on expansion before domains actually touch each other. A quantitative analysis shows that the line tension between both phases increases by about 10% on expansion.
Entropy computing via integration over fractal measures
Authors: Słomczyński Wojciech , Kwapień Jarosław , Życzkowski Karol
Published: July 2002

We discuss the properties of invariant measures corresponding to iterated function systems (IFSs) with place-dependent probabilities and compute their Rényi entropies, generalized dimensions, and multifractal spectra. It is shown that with certain dynamical systems, one can associate the corresponding IFSs in such a way that their generalized entropies are equal. This provides a new method of computing entropy for some classical and quantum dynamical systems. Numerical techniques are based on integration over the fractal measures.
On accurate integration algorithms
Authors: Folland N. O.
Published: July 2002

Algorithms are described for the accurate evaluation of a class of integrals, including Fourier transforms, in which the integral is well represented as the product of an exponential and a polynomial. The algorithms treat all or a part of the exponential dependence exactly and represent the polynomial by conventional finite difference. Numerical examples of Fourier transforms are given that illustrate the accuracy and stability of the algorithms and include a case in which the evaluation of Fourier transforms does not require the evaluation of any sine, cosine, or complex exponential functions.
A transfer-matrix approach to one-dimensional quantum mechanics using Mathematica
Authors: Walker James S. , Gathright J.
Published: July 2002

A variety of one-dimensional quantum systems is studied using a transfer-matrix formalism. In particular, only two matrices are required: one to propagate a wave function over a region of constant potential, the other to connect wave functions at a discontinuity in the potential. Using these simple matrices as building blocks, we constructed complex transfer matrices with which to study energy bands in one-dimensional crystals, as well as the intriguing characteristics of transmission through multiple barriers. All systems studied here are modeled with finite-step potentials, not delta functions. The solutions obtained are exact and can be generated with relative ease using Mathematica. The method is thoroughly explicated and several coding examples are provided.
A computational model of exploding metallic fuses for multimegajoule switching
Authors: Lindemuth Irvin R. , Brownell John H. , Greene Arthur E. , Nickel George H. , Oliphant Thomas A. , Weiss Douglas L.
Published: July 2002

A new model for determining the time-dependent behavior of exploding metallic fuses is formulated. The model draws on an atomic data base and gives insight into the temporal behavior of the material density and temperature of the fuse as well as the nonlinear electrical circuit interaction. The model includes an embedding insulating tamper and leads to a plausible explanation of fuse ‘‘restrike.’’ The model predicts time-scale compression of 500 for inductive store systems powered by explosive driven magnetic flux compression generators. A scenario for achieving multimegajoule foil implosions is predicted.
rf sputtering of highly Bi-substituted garnet films on glass substrates for magneto-optic memory
Authors: Gomi M. , Tanida T. , Abe M.
Published: July 2002

Using glass substrates, we have prepared highly Bi-substituted garnet [(Bi,Y,Fe,Al)8O12 and (Bi,Gd,Fe,Al)8O12] films by rf diode sputtering which was followed by annealing for crystallization. The films were polycrystalline garnet with no preferred orientation and no impurity crystalline phase detectable by x-ray analysis, which contained 1.1–1.4 Bi ions per formula unit. The films crystallized directly during the sputter deposition when the substrate temperature Ts>440 °C. Film quality was best when Ts was just below 440 °C. The film had magnetic anisotropy perpendicular to the film plane, which is stress induced. In the (Bi,Gd,Fe,Al)8O12 film, the magnetization normal to the film plane exhibited strong coercivity, Hc=600 Oe, and a large remanence-to-saturation ratio, Mr/Ms=1, while in the (Bi,Y,Fe,Al)8O12 film, weak Hc (200 Oe) and small Mr/Ms (0.47). These films had large Faraday rotation, typically 1.5×104 deg/cm at λ=633 nm, owing to the high Bi substitution.
Time resolved transmission studies of poly(methyl methacrylate) films during ultraviolet laser ablative photodecomposition
Authors: Davis G. M. , Gower M. C.
Published: July 2002

The time resolved transmission of P10, poly(methyl methacrylate), resist films has been studied during ablative photodecomposition (APD) by KrF, KrCl, and ArF excimer lasers. The observed changes in transmission are most likely due to scattering and absorption by the ablated material during and after the laser pulse. Measurements of the energy per unit volume absorbed at the threshold laser fluence for APD as a function of wavelength indicate that APD is not caused solely by a thermal degradation process.
Switching kinetics in KNO3 ferroelectric thin-film memories
Authors: Dimmler K. , Parris M. , Butler D. , Eaton S. , Pouligny B. , Scott J. F. , Ishibashi Y.
Published: July 2002

The time dependence of the current transient i(t) produced by the reversal of domains in ferroelectric potassium nitrate thin-film memories of 75–300 nm is analyzed as a function of temperature and of thickness using the Avrami theory. For all the films the kinetics confirm the low-dimensional nature of the system
Ordering by disorder: Ground-state selection in fcc vector antiferromagnets
Authors: Henley Christopher L.
Published: July 2002

Face-centered-cubic antiferromagnets with vector spins (including those representing MnO and Cd1−xMnxTe) have nontrivial continuous degeneracies of their ground states which are broken by effects of either thermal or quenched disorder. Using spin-wave calculations near T=0, it is argued that thermal fluctuations select the collinear states. On the other hand, dilution favors noncollinear (yet long-range ordered) states.
Analytical electron microscopy of Al/TiN contacts on silicon for applications to very large scale integrated devices
Authors: Armigliato A. , Valdré G.
Published: July 2002

Aluminum contacts on silicon have been realized for very large scale integration applications by interposing a titanium nitride diffusion barrier. The TiN films have been prepared by implanting nitrogen ions on silicon wafers coated with titanium layers of different thicknesses (60, 80, and 100 nm); by a subsequent annealing in vacuum, films of TiSi2, from 0 up to 75 nm thick, grow at the nitride/silicon interface. The Al/TiN/Si and Al/TiN/TiSi2/Si structures have been annealed at 600 °C for 30 and 10 min, which in some cases results in barrier degradation, due to Al/Si interdiffusion. Cross-sectioned specimens of these contacts have been studied by analytical transmission electron microscopy (AEM) employing a Philips EM 400T microscope, equipped with a field emission gun. It has been found that, contrary to previous suggestions, the TiN film does not dissolve during the failure process, but retains its morphology and a sharp interface with the neighboring layers. Unlike scanning electron microscopy experiments, where the Al film must be removed after the barrier degradation to investigate the pits formed by the massive migration of Si into Al, AEM on cross sections allowed us to make an accurate structural and compositional characterization of the material filling these pits. In addition, when the annealing treatment does not result in the barrier failure, the migration of Al, Ti, and Si throughout the structure can be easily detected and analyzed.
Surface anisotropy in micromagnetics
Authors: Aharoni Amikam
Published: July 2002

The conventional boundary conditions of micromagnetics are modified to include surface anisotropy of crystallographic nature, which is possible in epitaxy, or when surfactants are chemisorbed on the surface of ferromagnetic particles. As an illustration, the nucleation by the magnetization curling mode is calculated for a sphere with such a surface anisotropy.
Crystallization of amorphous silicon during thin-film gold reaction
Authors: Hultman L. , Robertsson A. , Hentzell H. T. G. , Engström I. , Psaras P. A.
Published: July 2002

The crystallization of a-Si in a-Si (50-nm) and Au (5-nm) thin-film bilayers has been investigated during heat treatment in a transmission electron microscope. When crystallization of a-Si first begins at 130 °C, the Au-Si alloy (Au and a precursor phase) reflections observed at lower temperatures vanish, and several new reflections from metastable Au-Si compounds occur. Dendritically growing islands of poly-Si are observed after heating at 175 °C. If the samples are held at a constant temperature of 175 °C for 10 min, the poly-Si islands coalesce. The formation of poly-Si depends on the diffusion of Au into a-Si and the formation of metastable Au-Si compounds, which act as transport phases for both Si and Au. After crystallization Au segregates to the front and back surfaces of the poly-Si film. The result of this work and earlier diffraction investigations are interpreted in terms of superlattices based on a sublattice. A fundamental body-centered-cubic structure with a=5.52 Å and composition Au4Si is suggested for the Au-Si compounds.
X-ray determination of alignment in FeNdB magnets
Authors: Zhou S. Z. , Zhou Y. X. , Graham C. D.
Published: July 2002

The preferred orientation or crystallographic alignment of several NdFeB magnets has been studied by x-ray diffraction. A simple diffraction geometry was used, corresponding to a single line section across a pole figure. The (006) reflection was found to be the most useful for this purpose. Some practical problems of sample preparation and reproducibility are discussed, and results are presented. As expected, the magnetic properties of the magnets improve with the degree of alignment, although the dependence is relatively weak. Extrapolation suggests that significant further increases in magnet properties could be achieved by better crystallographic alignment.
Scattering of electromagnetic waves from a periodic surface with random roughness
Authors: Yueh H. A. , Shin R. T. , Kong J. A.
Published: July 2002

The scattering of electromagnetic waves from a randomly perturbed periodic surface is formulated by the extended boundary condition (EBC) method and solved by the small perturbation method (SPM). The scattering from periodic surface is solved exactly and this solution is used in the SPM to solve for the surface currents and scattered fields up to the second order. The random perturbation is modeled as a Gaussian random process. The theoretical results are illustrated by calculating the bistatic and backscattering coefficients. It is shown that as the correlation length of the random roughness increases, the bistatic scattering pattern of the scattered fields show several beams associated with each Bragg diffraction direction of the periodic surface. When the correlation length becomes smaller, then the shape of the beams become broader. The results obtained using the EBC/SPM method is also compared with the results obtained using the Kirchhoff approximation. It is shown that the Kirchhoff approximation results show quite a good agreement with EBC/SPM method results for the hh and vv polarized backscattering coefficients for small angles of incidence. However, the Kirchhoff approximation does not give depolarized returns in the backscattering direction, whereas the results obtained using the EBC/SPM method give significant depolarized returns when the incident direction is not perpendicular to the row direction of the periodic surface.
Study of the decomposition of wet SF6, subjected to 50-Hz ac corona discharges
Authors: Derdouri A. , Casanovas J. , Hergli R. , Grob R. , Mathieu J.
Published: July 2002

Mixtures of SF6 (100 kPa≤PSF6≤400 kPa) and water (concentrations ranging from 240 to 2000 vpm) have been submitted to point-plane 50-Hz ac corona discharges. The only stable gaseous by-products detected, either by gas-phase chromatography or gas chromatography–mass spectrometry techniques, were SOF2 and SO2F2. The variation of their concentrations as a function of the discharge current value (3–25 μA rms), the charge transported (0.15–1.5 C), the water content, the SF6 pressure, and the gap spacing (1.5–5 mm) have been studied. The results indicate that, at least in our experimental conditions, the 50-Hz ac corona discharges behave more like negative than like positive dc corona.
Magnetic properties of LaCo13-based systems
Authors: Ido H. , Sohn J. C. , Pourarian F. , Cheng S. F. , Wallace W. E.
Published: July 2002

Four polycomponent systems based on LaCo13 have been studied: La(Co1−xFex)13, La(Co1−xAlx)13, La(Co1−x−yFexAly)13, and La0.7Nd0.3(Co0.7Fe0.3)13. These 1:13 systems were studied because of their potential for permanent magnet fabrication. LaCo13 has a high 3d metal content, the highest for any known rare-earth intermetallic, a 13 kG saturation induction, and a high Tc (1318 K). Unfortunately, it is cubic and lacks anisotropy. The substituted systems were examined as a portion of a program to find LaCo13-based systems of applications significance. Replacement of Co in LaCo13 by Fe and/or Al leads to a rapid decline in Tc for all systems studied. Replacement of Co by Al results in a decline in moment, whereas replacement by Fe leads to a rise in moment to 2.39μB/3d atom for La(Co0.4Fe0.6)13, as compared to 2.46μB/3d atom for Fe0.7Co0.3. Analysis of the magnetic data shows that vacancies occur in both half-bands for LaCo13 (4.8 spin up and 3.24 spin down) but in only one half-band in La(Co1−xAlx)13 for x≥0.2. Nd in these systems carries a moment of 3μB, close to that of the free tripositive ion.
Metalorganic deposition of high-J
c Ba2YCu3O7−x thin films from trifluoroacetate precursors onto (100) SrTiO3
Authors: McIntyre Paul C. , Cima Michael J. , Ng Man Fai
Published: July 2002

Superconducting thin films of Ba2YCu3O7-x were prepared on (100) SrTiO3 substrates by metalorganic deposition (MOD) of trifluoroacetate precursors. The best electrical transport properties were measured in films annealed at 750 °C in a humid, low PO2 gas mixture followed by slow cooling in oxygen. These specimens had sharp resistive transitions with Tc above 90 K and zero-field critical current densities in excess of 106 A/cm2 at 77 K. Critical current densities of this magnitude have not previously been reported in films produced by MOD. The highest Jc obtained in films fired only in humid oxygen was 3×105 A/cm2. Annealing at high temperature in the low PO2 atmosphere also resulted in a smoother surface morphology than was observed in the oxygen-fired films. Use of the low PO2 furnace gas appeared to suppress the formation of b-axis normal oriented grains in the superconducting films and to strengthen c-axis normal texture. X-ray powder diffraction indicated the presence of a-axis normal textured material in the films, although it was not present as separate microstructural features which could be identified by scanning electron microscopy.
Low-temperature specific heat and thermal expansion in the frustrated garnet Gd3Ga5O12
Authors: Ramirez A. P. , Kleiman R. N.
Published: July 2002

Gd3Ga5O12 is strongly frustrated, possessing a Curie–Weiss temperature of −2.3 K, while not ordering down to 25 mK, in zero field. Specific-heat and thermal-expansion measurements were made as a function of temperature (0–2 K) and magnetic field (0–2 T). The unusual phase diagram of this material, reported in the susceptibility measurements of Hov, Bratsberg, and Skjeltorp [J. Magn. Magn. Mater. 15–18, 455 (1980)], is verified by the present thermal behavior.
Angular ion and neutral energy distribution in a collisional rf sheath
Authors: Manenschijn A. , Goedheer W. J.
Published: July 2002

A numerical study on the ion and neutral angular impact energy distribution at the rf-driven electrode of a reactive ion etcher is presented. The calculations for the ions are performed using a Monte Carlo method that includes charge exchange and elastic scattering. The contribution of both collision processes to the angular ion impact energy distribution is studied. For the case that charge exchange is the only collision process, the Monte Carlo results can be checked against those of a method based on a spatially uniform and time independent collision rate. In that case, both methods yield the same ion impact energy distribution. The position, velocity, and propagation angle of the energetic neutrals created in collisions of ions with the background gas are stored. These are used as input data for a separate code that follows the evolution of the angular neutral energy distribution, taking into account (multiple) neutral elastic scattering. From the ion and neutral distributions, the number of neutrals per ion, the average impact energy, and the energy-weighted average impact angle have been derived. It is shown that these parameters are well described by simple expressions. Finally, the sputter yield is calculated. The results show that the contribution of the angular distributions of both ions and neutrals to the yield can be neglected.
Deposition of device quality, low H content amorphous silicon
Authors: Mahan A. H. , Carapella J. , Nelson B. P. , Crandall R. S. , Balberg I.
Published: July 2002

Device-quality hydrogenated amorphous silicon containing as little as 1/10 the bonded H observed in device-quality glow discharge films have been deposited by thermal decomposition of silane on a heated filament. These low H content films show an Urbach edge width of 50 mV and a spin density of ∼1/100 as large as that of glow discharge films containing comparable amounts of H. High substrate temperatures, deposition in a high flux of atomic H, and lack of energetic particle bombardment are suggested as reasons for this behavior.
In situ observation of fractal growth during a-Si crystallization in a Cu3Si matrix
Authors: Russell S. W. , Li Jian , Mayer J. W.
Published: July 2002

We observe that the crystallization of amorphous Si thin films in contact with a copper silicide layer occurs at a temperature of around 485 °C in the form of dendrites with a fractal dimension of 1.7. The in situ observation of both the silicidation reaction, forming Cu3Si, and the subsequent crystallization of the remaining amorphous silicon in the silicide matrix, were observed during annealing in a transmission electron microscope. We estimate the radial growth rate of these crystallites at 5 nm/s at this temperature. The fractal dimension of the dendrites indicates a growth process similar to one known as diffusion-limited aggregation.
Improved value for the silicon intrinsic carrier concentration from 275 to 375 K
Authors: Sproul A. B. , Green M. A.
Published: July 2002

A recent review has suggested that the commonly cited value of 1.45×1010 cm−3 for the silicon intrinsic carrier concentration at 300 K is inconsistent with the best experimental data. An alternate value of 1.08×1010 cm−3 was proposed. From measurements of the current-voltage characteristics of p-n junction diodes, this paper reports a new and more accurate determination of this parameter over the 275–375 K temperature range which supports such lower values. The one-standard-deviation uncertainty in the measurement of the intrinsic carrier concentration is estimated to lie in the 3%–4% range, about three times smaller than previous measurements at these temperatures. Additionally, this technique provides information on the minority carrier electron diffusivity in silicon.
Doping of In0.53Ga0.47As and In0.52Al0.48As by Si+ and Be+ ion implantation
Authors: Hailemariam E. , Pearton S. J. , Hobson W. S. , Luftman H. S. , Perley A. P.
Published: July 2002

The activation kinetics and diffusion characteristics of Si+ and Be+ ions implanted into InAlAs and InGaAs were investigated for rapid thermal annealing in the temperature range 600–900 °C. The apparent activation energies for electrical activation of Be are 0.43±0.03 eV in InAlAs and 0.38±0.03 eV in InGaAs, and for Si are 0.58±0.05 eV in InAlAs and 0.64±0.06 eV in InGaAs. Higher activation efficiencies are obtained for both dopants in InGaAs relative to InAlAs and anomalously low activation for low dose Be implants is observed due to a lack of vacant sites for the Be to occupy. Extensive redistribution of Be after annealing at 750 °C, 10 s is observed in both materials, whereas Si shows no motion even for annealing at 850 °C.
dc reactive magnetron sputtering of titanium-structural and optical characterization of TiO2 films
Authors: Suhail M. H. , Rao G. Mohan , Mohan S.
Published: July 2002

This paper deals with the reactive sputtering of titanium in an argon and oxygen mixture. The variation in cathode potential as a function of oxygen partial pressure has been explained in terms of cathode poisoning effects. The titania films deposited during this process have been studied for their structural and optical characteristics. The effect of substrate temperature (from 25 to 400 °C) and annealing (from 250 to 700 °C) on the packing density, refractive index, extinction coefficient, and crystallinity has been investigated. The refractive index varied from 2.24 to 2.46 and extinction coefficient from 2.6 × 10−3 to 10.4× 10−3 at 500 nm as the substrate temperature increased from 25 to 400 °C. The refractive index increased from 2.19 to 2.35 and extinction coefficient changed from 3.2× 10−3 to 11.6 × 10−3 at 500 nm as the annealing temperature was increased from 250 to 700 °C. Anatase and rutile phases have been observed in the films deposited at 400 °C substrate temperature and annealed at 300 °C. The changes in the optical constants at higher substrate temperature have been attributed to an increase in packing density, oxygen content, and crystallinity of the films.
Luminescent properties of ZnS:Mn films deposited by spray pyrolysis
Authors: Falcony C. , Garcia M. , Ortiz A. , Alonso J. C.
Published: July 2002

The room temperature, photoluminescent properties of manganese-doped zinc sulfide films deposited by spray pyrolysis are reported. These films were deposited on Pyrex glass substrates at atmospheric pressure using air as a carrier gas. All films were polycrystalline with a wurtzite (hexagonal) structure. The manganese doping was achieved by mixing MnCl3 with the starting solution to deposit ZnS. The photoluminescence spectra was measured at room temperature as a function of the different deposition parameters and the Mn concentration. Besides the characteristic light emission associated with Mn impurities in a ZnS matrix, a peak associated with the self-activated emission was also observed under certain deposition conditions (low substrate temperatures and/or long deposition times). The presence of chlorine impurities in the films is suggested to be associated with this emission. The Mn luminescence presents a quenching effect with the Mn concentration. This quenching effect is similar to the one reported on films deposited by other techniques. The light emission at this center has an activation energy of 0.71±0.05 eV with the deposition temperature. This energy is proposed to be related with the energy required by the Mn atoms to find a proper site during the growth process to form a Mn2+ center.
An inorganic anti-reflective coating for use in photolithography
Authors: Tompkins Harland G. , Sellers James A. , Tracy Clarence
Published: July 2002

In this work, we discuss the design of an inorganic anti-reflective coating for use in photolithography at a wavelength of 3650 Å (I line). We consider the effect of the optical constants on the reflectance and show that when the extinction coefficient, k, of the film has a value near 0.8, the reflectance will be very small for a range of values of the thickness and the index of refraction, n. We illustrate the principle with one example where we use a combination of TiW and the oxide of TiW. Reflectance is measured for a range of thicknesses of both the TiW and the oxide of TiW. Values less than 5% were obtained.
Effects of electric and magnetic fields on confined donor states in a coupled double quantum well
Authors: Cen J. , Lee S. M. , Bajaj K. K.
Published: July 2002

We have developed a variational formalism to calculate the effects of electric and magnetic fields on confined hydrogenic donor states in asymmetric coupled double quantum well structures. It is demonstrated that an electric field applied along the growth axis can easily shift the electron wave function from one quantum well across the center barrier into the neighboring well, without ejecting the electron from a confined donor state. Depending on donor location in the structure, binding energy can either increase or decrease under the applied electric field, as had been found in the case of single quantum wells, but with significantly greater rates of change in response to the external field. The magnetic field applied along the growth axis of the quantum well structure leads to additional quantum confinement, increasing both the donor binding energies and the transition energy between the 1s and 2p+ donor states. Effect of the relative size of the two coupled quantum wells on the donor binding energy is also discussed. Dipole moment and polarizability of the confined donor states are obtained simultaneously as well.
Electrical properties of a silicon quantum dot diode
Authors: Nicollian Edward H. , Tsu Raphael
Published: July 2002

The electrical properties of a diode consisting of nanoscale silicon quantum dots embedded in an amorphous silicon dioxide matrix are presented in terms of an equivalent circuit. The division of the applied bias between the quantum dot-silicon dioxide layer and the nondegenerate silicon substrate, and the magnitude of the coulomb blockade due to charged electron traps in the quantum dots are determined from the equivalent circuit. Coulomb blockade is important because it contributes to both the large energy separation between successive quantum states, and the applied bias at which quantum effects are first observed.
Epitaxial yttrium iron garnet films grown by pulsed laser deposition
Authors: Dorsey P. C. , Bushnell S. E. , Seed R. G. , Vittoria C.
Published: July 2002

Epitaxial Y3Fe5O12 (YIG) films have been grown by the pulsed laser deposition (PLD) technique on (111) gadolinium gallium garnet substrates. The effect of substrate temperature and oxygen partial pressure on the structure, composition, and magnetic properties of the films was investigated and compared to liquid phase epitaxy YIG films. The results demonstrated that epitaxial YIG films could be prepared under a wide range of deposition conditions, but narrow linewidth (ΔH≂1 Oe) films were producible only at low oxygen partial pressures (O2<250 mTorr) and relatively high substrate temperatures (Ts≳800 °C). Since the linewidth of single-crystal YIG is dominated by surface and volume defects and/or impurities, the narrow linewidth indicated that PLD is a viable technique for producing high-quality ferrite films for microwave device applications. In addition, under all deposition conditions (50–1000 mTorr and 700–850 °C) there is a uniaxial axis perpendicular to the film plane. However, at low oxygen pressure the uniaxial anisotropy energy constant Ku is negative while at high oxygen pressure Ku is positive.
Die-upset PrCo5-type magnets from melt-spun ribbons
Authors: Fuerst C. D. , Brewer E. G.
Published: July 2002

We used consolidation (hot pressing) and deformation (die upsetting) techniques to produce magnetically anisotropic permanent magnets from melt-spun PryCo100−y−xCx alloys. The primary phase in these magnets (16≤y≤24 at. %) was the hexagonal PrCo5 phase. The greatest alignment was obtained for low-carbon compositions (0≤x≤2 at. %); in particular, Pr18Co81C produced a remanence of ∼8.7 kG and an energy product of 16.9 MG Oe. The addition of 1 at. % carbon enhanced the coercivity (Hcl≊5.9 kOe) by nearly 80% compared to the carbon-free composition (Hcl≊3.3 kOe). Higher carbon levels (x≥4 at. %) resulted in even larger coercivities (Hcl≥10 kOe) but were accompanied by significantly lower remanences (Br≊6 kG).
Elastic waves in plates with periodically placed inclusions
Authors: Sigalas M. M. , Economou E. N.
Published: July 2002

We calculated the frequency ω versus the wave vector K for elastic waves propagating in both thick and thin plates consisting of solid inclusions placed periodically in the host material. We were particularly interested in the possible creation of spectral gaps (stop bands) in all directions of propagation for bending waves. We found that Mo, Fe, steel, or Pb inclusions forming a two dimensional hexagonal lattice in a Lucite host give rise to spectral gaps.
Characterization of a thickness-shear mode quartz resonator with multiple nonpiezoelectric layers
Authors: Granstaff Victoria Edwards , Martin Stephen J.
Published: July 2002

This paper describes a one-dimensional analysis of a piezoelectric resonator with multiple nonpiezoelectric layers of arbitrary thickness and complex shear modulus. This analysis shows under what conditions the simple Butterworth–Van Dyke (BVD) lumped-element equivalent circuit can be used to extract the properties of these layers, and provides a theoretical basis for doing so under these conditions. The method presented here is based on the physics of wave propagation, but uses transmission line techniques to transform the equations and boundary conditions into a simple string of 2×2 matrix multiplications, which easily accommodate any number of layers. An analysis of the AT-cut quartz/polymer/liquid composite resonator is presented in detail because of its importance in sensor and electrochemical quartz crystal microbalance applications. The analysis shows that adding the polymer and liquid impedances in the motional arm of a BVD circuit is not valid near the polymer film resonance, but is appropriate for a thin, rigid polymer film in contact with a liquid.
Local magnetic moments and intermediate valence state of cerium impurities in ferromagnetic rare-earth metals
Authors: Leal C. E. , Troper A.
Published: July 2002

Using experimental data for magnetic hyperfine fields obtained through the technique of time differential perturbed γ–γ angular correlations (TDPAC) in the highly diluted system CeR (R=Gd, Tb), we theoretically discuss the formation of the local moments at the Ce impurity site, adopting an intermediate valence model. The ferromagnetic rare-earth host is considered as a transition metal in the beginning of a 5d series and the CeR alloys are described as strongly d–f correlated and hybridized systems. One calculates the hyperfine field which reproduces quite well the experimental data. The Ce valence in these systems is also obtained, giving values in good agreement with experimental results and theoretical claims involving Gd and Tb band calculations. Some comments about the diluted systems YbR (R=Gd, Tb) are also made.
Ytterbium-doped apatite-structure crystals: A new class of laser materials
Authors: Payne Stephen A. , DeLoach Laura D. , Smith Larry K. , Kway Wayne L. , Tassano John B. , Krupke William F. , Chai Bruce H. T. , Loutts George
Published: July 2002

A new class of Yb-lasers is summarized in this article. The apatite family of crystals, based on the hexagonal structure of the mineral fluorapatite, has been found to impose favorable spectroscopic and laser properties on the Yb3+ activator ion. Crystals of Yb-doped Ca5(PO4)3F, Sr5(PO4)3F, CaxSr5−x(PO4)3F, and Sr5(VO4)3F have been grown and investigated. Several useful laser crystals have been identified which offer a variety of fundamental laser parameters for designing diode-pumped systems. In general, this class of materials is characterized by high emission cross sections (3.6–13.1×10−20 cm2), useful emission lifetimes (0.59–1.26 ms), a strong pump band (σabs=2.0–10.0×10−20 cm2), and pump and extraction wavelengths near 900 and 1045 nm, respectively. Efficient lasing has been demonstrated for several of the members of this class of materials, and high optical quality crystals have been grown by the Czochralski method. A summary of the laser parameters and a discussion of the Yb:apatite class of lasers is presented.
Very large magneto-impedance in amorphous soft ferromagnetic wires (invited)
Authors: Rao K. V. , Humphrey F. B. , Costa-Krämer J. L.
Published: July 2002

Changes in the impedance, ≊600%/Oe, at axial fields less than 1 Oe have been observed in the presence of a 90 kHz, few mA current through a soft, nearly zero magnetostrictive wire. In this 125 μm diameter CoFeSiB amorphous wire we observe a total change of 160% at the maxima of the impedance in dc fields less than 2 Oe. A systematic study of the role of induced anisotropy in the axial, circumferential, and helical directions on the magneto-impedance shows that the largest effect is seen in the wire annealed to obtain circumferential easy axis using a 15 mA ac current passing through it. Both the axial hysteresis loops and the observed I-V characteristics reflect the induced anisotropies. The observed dependence of the inductance change on the type of the anisotropy induced in these wires can be modeled in terms of an interplay between the induced anisotropy, reversing ac field, and the axial dc field.
Plasma-assisted low-pressure metalorganic chemical vapor deposition of GaN on GaAs substrates
Authors: Sato Michio
Published: July 2002

Cubic and hexagonal GaN films were grown on (001) GaAs substrates by plasma-assisted low-pressure metalorganic chemical vapor deposition using triethylgallium and nitrogen radicals. The GaN growth rate was lower than that of GaAs and it was enhanced by simultaneous supply of hydrogen radicals to the growing surface. Cubic GaN was grown epitaxially under Ga-rich conditions, and c-axis oriented hexagonal GaN was grown under N-rich conditions.
Alloying behavior induced by ion mixing in a system with positive heat of formation
Authors: Jin O. , Zhang Z. J. , Liu B. X.
Published: July 2002

Amorphous alloys were formed by room-temperature 200 keV xenon ion mixing of multilayered films in the Zr-Nb system that has a positive heat of formation (+6 kJ mol−1) and the ion-induced amorphization was found to be discontinuous in the composition range. Thermodynamic calculation was conducted and a free-energy diagram, which concerns free-energy curves of the amorphous phase and the initial state of the multilayered films with excess interfacial energy, was constructed. An interpretation to the observed discontinuity of amorphization range was presented based on the calculated free-energy diagram. In addition, two new metastable crystalline phases both of fcc structure with different lattice parameters were formed in the composition range close to pure zirconium and niobium, respectively.
Properties of the shallow O-related acceptor level in ZnSe
Authors: Chen J. , Zhang Y. , Skromme B. J. , Akimoto K. , Pachuta S. J.
Published: July 2002

Zinc selenide layers grown by molecular beam epitaxy (MBE) and doped with ZnO have been characterized using low temperature photoluminescence (PL) measurements as a function of excitation level, temperature, and laser energy (i.e., selectively excited donor-acceptor pair luminescence or SPL), as well as reflectance measurements. An O-related donor-to-acceptor (D0−A0) pair band is clearly observed in all of the ZnO-doped layers, whose position varies from 2.7196 to 2.7304 eV, depending on the excitation level. The same peak occurs in a number of undoped, As-doped, and Ga-doped MBE samples, showing that O can occur as a residual impurity. Temperature-dependent measurements reveal the existence of a corresponding conduction band-to-acceptor (e−A0) peak at 2.7372 eV (39.8 K), confirming the existence of the acceptor level. The binding energy of this acceptor is about 84±2 meV, which is 27 meV shallower than that of N. The SPL measurements reveal four excited states of the shallow acceptor level, separated from the 1s3/2 ground state by 48.2 (2p3/2), 57.1 (2s3/2), 64.3 (2p5/2:Γ7), and 67.7 meV (3p3/2:Γ8), respectively (all values ±1 meV). These energies fit well to conventional effective mass theory, which demonstrates that this O-related acceptor level is effective-mass-like. However, luminescence and secondary ion mass spectrometry show that the ZnO doping technique introduces shallow donor impurities into the material in addition to O acceptors, specifically high levels of chemical contaminants (mainly B and Ga) originating from the doping source. This effect may account for the lack of reproducibility in obtaining p-type conduction with ZnO doping, and suggests that more effective O incorporation methods should be devised.
Single electron–photon logic device using coupled quantum dots: Computation with the Fock ground state
Authors: Nomoto K. , Ugajin R. , Suzuki T. , Hase I.
Published: July 2002

We propose a novel logic device using coupled quantum dots (CQDs) in which single-electron tunneling is influenced by electron–electron interaction. If occupation/unoccupation by a single electron in a quantum dot is viewed as a bit 1/0, we can say that the device can perform (N)AND and (N)OR operations simultaneously. Data input/output is performed by irradiation/absorption of photons. The (N)AND and (N)OR operations are performed by the relaxation of the electronic system to the (Fock) ground state which depends on the number of electrons in CQDs. When the device is constructed of semiconductor nanostructures, phonon emission from an electron is the main contributor to the energy dissipation process. We also present results of a theoretical analysis of the device performance. These results show that (i) the error probability at the final state depends on only the dissipation energy and becomes smaller as the dissipation energy becomes larger, and (ii) the speed of operation depends on the dissipation energy and dissipative interactions and becomes slower as the dissipation energy becomes larger if LA-phonon emission is taken into account. If the size of the dot is 10 nm, the speed of operation and the error probability are estimated to be about 10 ps and about 0.2 at 77 K, respectively. The basic idea of the device is applicable to two-dimensional (2D) pattern processing if the devices are arranged in a 2D array.
Effect of substrate orientation and crystal anisotropy on the thermally oxidized SiO2/SiC interface
Authors: Shenoy J. N. , Das M. K. , Cooper J. A. , Melloch M. R. , Palmour J. W.
Published: July 2002

We use the high–low capacitance–voltage technique and the conductance–frequency techniques to characterize the SiO2/SiC interface formed by thermal oxidation of the silicon-face (0001) c-axis, the (112̄0) a-axis, and the (11̄00) a-axis orientations of 6H–SiC. The oxidation rate of the a-axis orientations is 3–5 times higher than that of the silicon face. Interface state densities on the a-axis orientations are a factor of 4–10 times higher than the Si-face c-axis orientation for both n-type and p-type dopings. Maximum oxide electric breakdown fields are about 10–11 MV/cm for both a-axis and c-axis orientations for an oxide thickness of about 60 nm.
Analysis and optimization of synchronous magnetic couplings
Authors: Furlani E. P.
Published: July 2002

Permanent magnet couplings made from rare-earth materials such as NdFeB render substantial torque, and are used in numerous applications that cannot be addressed with conventional materials. Moreover, formulas have been developed for designing and optimizing these devices. In this paper, the design formulas are summarized and demonstrated via optimization of practical coupling geometries.