bluuebunny/crossref_metadata_2025 huggingface dataset:
Investigation of the role of ZnSe films on GaAs using acoustoelectric voltage spectroscopy
Authors: Han K. J. , Abbate A. , Bhat I. B. , Akram S. , Das P.
Published: July 2002

The electrical properties of the ZnSe/GaAs heterostructure have been investigated using the acoustoelectric voltage spectroscopy technique, and in particular, the role of the high-resistivity ZnSe on the surface passivation of the GaAs substrate has been evaluated. From the transverse acoustoelectric voltage (TAV) spectra, the carrier type and concentration, energy band offsets, and the energy levels of various trap states at the heterostructure interface have been found. The effect of heterostructure epitaxial layer on the surface properties of GaAs has been studied by comparing the normalized changes in TAV amplitude for samples of various epitaxial layers and different thicknesses. From all these measurements, surface recombination velocities (S) have been evaluated. For the pseudomorphic ZnSe films (thickness ≤0.15 μm) on GaAs, a reduction in S has been found. As the thickness of the ZnSe film was increased, the presence of a large number of interface states due to the introduction of misfit dislocations was detected using TAV measurements.
Preparation and characterization of Cd3P2 thin films
Authors: Rao D. R. , Nayak A.
Published: July 2002

Cd3P2 thin films have been deposited on glass substrates by electron beam evaporation technique. The amorphous as-deposited films, after annealing in vacuum in the temperature range 180–200 °C, developed the crystallinity. Such polycrystalline films exhibited preferential orientation along the (224 400) direction. The films indicated n-type conductivity with resistivity value around 1.6 Ω cm at room temperature. The composition, ionicity, and Madelung constant, calculated from x-ray photoelectron spectroscopy data, are Cd3P1.7, 0.08, and 1.68, respectively. From the optical (R and T) and modulation (derivative) spectra, the different parameters, namely, refractive index (n∼3.83), band gap (Eg=0.61 eV), band splitting parameters (crystal field ΔCF=0.03 eV, spin-orbit Δ0=0.08 eV) have been obtained. Photoluminescence emission related to band-to-band transition is observed around 0.51 eV.
Magneto-optic channel waveguides in Ce-substituted yttrium iron garnet
Authors: Shintaku Toshihiro , Uno Takehiko , Kobayashi Morio
Published: July 2002

This article describes the demonstration of magneto-optic channel waveguides in Ce-substituted yttrium iron garnet which has a very large Faraday rotation. The rib waveguides are successfully produced by employing a new etching technique, which is a reactive ion etching method using BCl3 gas. The nonreciprocal phase shift in the magneto-optic waveguides is measured by using a new improved measurement method. These waveguides exhibit the largest nonreciprocal phase shift ever reported of 21.1 rad/cm (λ=1.55 μm).
Thermo-optic effects of 4-alkoxy-3-chlorobenzoic acids in polymeric matrices
Authors: Takahashi Yasushi , Tamaoki Nobuyuki , Komiya Yuichi , Hieda Yoshihiro , Koseki Kenichi , Yamaoka Tsuguo
Published: July 2002

Polymer films containing 4-alkoxy-3-chlorobenzoic acid heated to 140 °C showed two optically different states at room temperature depending on the cooling rate. If after heating the film cooled rapidly, it froze in the transparent state. In contrast, it reverted to the light scattering state when cooled slowly. Cycles between the two optical states were reproducible, therefore these films may offer potential as a rewritable recording material. The reversibility in the optical transmittance may be caused by a reversible change in the crystal size of the acid in the polymer matrix.
Third-order nonlinear optical properties of As2S3 chalcogenide glass
Authors: Kobayashi Hideki , Kanbara Hirohisa , Koga Masanori , Kubodera Ken’ichi
Published: July 2002

Third-order nonlinear optical properties of As2S3 chalcogenide bulk glass are studied experimentally. Third-harmonic generation (THG) measurement shows the value of the third-order susceptibility χ(3) to be 5.6×10−19 m2/V2 (1.0×10−11 esu) at the funda- mental-light wavelength of 2.1 μm. This value is more than 300 times that for a silica glass. The dependence of the THG χ(3) on the wavelength has been measured, which indicates the χ(3) enhancement effect due to three-photon resonance. Optical Kerr shutter operation is also achieved using the chalcogenide glass. The nonlinear refractive index of n2=6.8×10−18 m2/W is obtained at the gate-light wavelength of 700 nm. This value of n2 agrees with that estimated from the THG measurement. Other nonlinear effects such as two-photon absorption for the gate light are discussed.
Electromagnetic-wave absorption in highly collisional plasmas
Authors: Santoru Joseph , Gregoire Daniel J.
Published: July 2002

An experimental and theoretical investigation of basic plasma-physics processes relating to the absorption of electromagnetic waves in collisional plasmas is described. One-way absorption of 63 dB at 4 GHz was demonstrated in a section of plasma-loaded rectangular C-band waveguide. The ultraviolet photoionization plasma-production technique employed permits independent variation of the plasma-density profile and electron-collision frequency. A theoretical model for the absorption and scattering processes, which includes scattering contributions from the plasma-vacuum interface, and partial reflections and collisional absorption in the bulk plasma, is in reasonable agreement with the experimental results.
Improved coupled-mode theory for parallel electron waveguides
Authors: Yuen Wu-Pen
Published: July 2002

A coupled-mode theory for parallel electron waveguide structures is developed and presented. This theory takes the nonorthogonality between the modes belonging to different electron waveguides, which is neglected in the first-order theory, into consideration and reflects this fact by including the modal overlap integral terms in the resultant coupled-mode equations. Various numerical examples are presented, using this improved theory and the first-order theory, and are compared with those of the exact calculations. It is shown that the improved theory can give more accurate results than the conventional first-order theory. Therefore, it is very useful for analyzing the quantum field-effect directional couplers and similar quantum interference devices.
Structure order-disorder and dielectric response in perovskite-related SrO-Fe2O3-Nb2O5 system: The oxygen-deficient composition Sr(Fe1−xNbx)O2.5+y (x=0.17, 0.25, 0.50)
Authors: Liu Donhang , Yao Xi , Smyth Donald M. , Bhalla Amar S. , Cross L. E.
Published: July 2002

Microstructures of materials with composition Sr(Fe1−xNbx)O2.5+y (x=0.17, 0.25, 0.50) have been investigated by powder x-ray diffraction (XRD) and transmission electron microscopy (TEM) examinations. XRD data for ceramic samples annealed under different P(O2) conditions could be indexed to a cubic perovskite. Superlattice diffraction spots have been observed by TEM investigation for the samples which were re-equilibrated by a subsequent annealing in a reducing atmosphere. A microdomain model is proposed to describe the observed phenomena. According to the model, the structure of reduced Sr(Fe1−xNbx)O2.5+y with x=0.17, 0.25 is considered to be a result of disordered intergrowth between brownmillerite-type Sr2Fe2O5 domains and perovskite-type Sr(Fe0.5Nb0.5)O3 domains. Brownmillerite-type domains are formed because of vacancy ordering. The order is extremely short ranged. Electron diffraction patterns constructed in terms of this microdomain model agree well with the observed results. The order-disorder transition can be altered by varying ambient oxygen partial pressures. For both ordered and disordered samples the frequency-domain dielectric response as a function of temperature is reported. Directly measured complex conductivity Y(ω) has been transformed by Kramers–Kronig relations in order to subtract the disturbances of G(0) and C(∞). The transformed temperature dependences have been further normalized and re-expressed in the form of master curves. For all the samples measured, the frequency-domain dielectric response deviates from the Debye model but follows the so-called universal power law behavior. The dielectric spectra for the samples in which brownmillerite-type ordering is identified show an anomalous low-frequency dispersion, while for the disordered samples, the relaxation behavior with a loss peak is observed. The variation of spectra shape with composition indicates that increasing Nb content tends to weaken the cooperative many-body interactions and causes the dielectric behavior to approach the Debye model.
Crystallization of arsenic-doped hydrogenated amorphous silicon deposited on gallium arsenide
Authors: Yokota Katsuhiro , Sakaguchi Masanori , Uchida Daidou , Kuroda Takafumi , Tamura Susumu
Published: July 2002

Crystallization temperatures of a-Si:H films deposited on GaAs substrates by a glow-discharge method increased with an increase in the concentration of As doped into the a-Si:H films. Crystallization of a-Si:H films with an As concentration of 2×1020 cm−3 started at a temperature of about 1050 °C. These a-Si:H films also had a large amounts of H and SiH3.
Nonlinear Fabry–Perot cavity with chalcogenide glass thin films
Authors: Bertolotti M. , Chumash V. , Fazio E. , Ferrari A. , Sibilia C.
Published: July 2002

An investigation of a chalcogenide glass thin-film Fabry–Perot cavity has been performed in picosecond time regime. Nonlinear behavior of the structure derives from both nonlinear absorption of the material and changes in the resonance conditions of the cavity. The conditions to have bistable behavior have been established.
A laminated plate theory for high frequency, piezoelectric thin-film resonators
Authors: Yong Y.-K. , Stewart J. T. , Ballato A.
Published: July 2002

A high frequency, piezoelectric, laminated plate theory is developed and presented for the purpose of modeling and analyzing piezoelectric thin-film resonators and filters. The laminated plate equations are extensions of anisotropic composite plate theories to include piezoelectric effects and capabilities for modeling harmonic overtones of thickness-shear vibrations. Two-dimensional equations of motion for piezoelectric laminates were deduced from the three-dimensional equations of linear piezoelectricity by expanding the mechanical displacements and electric potential in a series of trigonometric function, and obtaining stress resultants by integrating through the plate thickness. Relations for handling the mechanical and electrical effects of platings on the top and bottom surfaces of the laminate are derived. A new matrix method of correcting the cutoff frequencies is presented. This matrix method could also be used to efficiently correct the cutoff frequencies of any nth order plate laminate theories which employ Mindlin’s form of polynomial expansion of mechanical displacements and electric potential through the plate thickness. The first order laminated plate theory with correction factors for cutoff frequencies and slope of the flexural branch at large frequencies was applied to a 2-layer, zinc oxide-silicon strip, and a 3-layer, zinc oxide-zinc oxide-silicon strip. Open circuit, dispersion relations were generated for a range of volume fractions and compared to the exact dispersion relations.

Both the 2-layer and 3-layer strip show similar qualitative comparison: The present theory compares fairly well with the exact dispersion relation for real wave numbers and nonpropagating (imaginary) wave numbers which are smaller than 0.5i. The extensional branch, and thickness-shear branches begin to deviate from the exact solution when the nondimensionalized frequency is greater than one. Consequently, to maintain accuracy of solutions when using the present first order laminated plate theory, one should limit the calculation of resonant, nondimensionalized frequencies to less than 1.1. Results for the frequency spectrum in the vicinity of the open circuit, fundamental thickness-shear frequency, and modes shapes were presented for the 2-layer strip with fixed edges, and a volume fraction of silicon equal to 0.2. The technically important fundamental thickness-shear mode is found to have the shear component strongly coupled with extensional component.
Effects of cluster ionization and acceleration on photoluminescence spectra of CdTe films grown by ionized cluster beam deposition
Authors: Akinaga H. , Ando K. , Yoshida S.
Published: July 2002

Single crystal epitaxial films of CdTe were grown on (0001) sapphire and (100) GaAs substrates by the ionized cluster beam deposition method. The low-temperature photoluminescence spectra depend strongly on the ionization current and the acceleration voltage of the CdTe cluster. The change of the excitonic edge emission can be explained by the improvement of the stoichiometry due to the ionization. The crystalline quality of the epitaxial film was also improved by the small acceleration (0.5 kV).
Study of residual strains in horizontal-Bridgman-grown gallium arsenide wafers by a high-sensitivity computer-controlled infrared polariscope
Authors: Yamada Masayoshi
Published: July 2002

Residual strains in GaAs (100) wafers, sliced from single-crystal ingots grown by the horizontal Bridgman technique, have for the first time been characterized by measuring strain-induced birefringence with a high-sensitivity computer-controlled infrared polariscope. The residual strain components: ‖Sr−St‖, ‖Syy−Szz‖, and 2‖Syz‖ are quantitatively characterized from the measured birefringence values. It was found that the maximum strain value was as low as about 5×10−6 and the corresponding stress value was about 3×105 N/m2. Also there was no strong correlation between the measured residual strain and etch pit density distributions. The origin of residual strains is discussed in conjunction with the correlation between the residual strain and dislocation distributions.
Two-dimensional distributions of ions implanted in channeling and random directions of Si single crystals
Authors: Privitera V. , Raineri V. , Rimini E.
Published: July 2002

P and B ions with energies of 400 keV and 1 MeV were implanted in a random direction and along the [100] axis of single-crystal Si samples patterned with oxide stripes. The lateral spread of the implants was determined by using two-dimensional spreading resistance measurements. The lateral distribution of ions implanted in a random direction is always broader than that of ions implanted with the same energy along the [100] axis. With increasing dose the lateral penetration for channeled implants increases too due to the disorder introduced in the samples. The experimental results are discussed and then compared with Monte Carlo simulations obtained by the marlowe code.
Stability analysis of numerical solutions of wave propagation when pseudoviscosity is replaced by real viscosity
Authors: Fanget A.
Published: July 2002

The artificial viscosity method was originally introduced by Von Neuman and Richtmeyer [R. D. Rchtmeyer and K. W. Morton, in Difference Methods for Initial Value Problem (Interscience, New York, 1967)] to spread the shock over several cells of the mesh. This method is useful if the physical phenomena present in the shock front are not of interest. In some cases, such as polymer, the amount of viscosity is large enough to spread the front shock naturally. In our problem, pseudoviscosity is ignored. A stability analysis shows that the time step restrictions for stability can be less stringent than with pseudoviscosity.
Confined and interface phonon scattering in finite barrier GaAs/AlGaAs quantum wires
Authors: Jiang W. , Leburton J. P.
Published: July 2002

We report on the calculation of the total scattering rate in finite barrier GaAs/AlGaAs quantum wires based on the interaction Hamiltonian of confined longitudinal optical (LO) phonon and surface (SO) phonon modes. With multisubband processes being properly taken into account, our calculation indicates that for GaAs type of phonons the high-frequency symmetric (s+) branch plays an important role among all the other SO phonon branches; it can even dominate over confined LO phonons in highly confined quantum wires as observed by K. W. Kim, M. A. Stroscio, A. Bhatt, R. Mickevicius, and V. V. Mitin [J. Appl. Phys. 70, 319 (1991)]. Our results also demonstrate that the total contributions of confined LO and SO phonon scattering resemble closely to GaAs bulk LO phonon scattering. Selection rules between intersubband transitions for SO modes suggest the possibility of a bottle-neck effect for carrier relaxation in square wires compared with rectangular wires.
Distribution of barrier traversal times in numerical simulations
Authors: Hagmann Mark J.
Published: July 2002

Barrier penetration is attributed to energy fluctuations expected from the uncertainty principle. Numerical simulations are made by calculating the traversal time and action for a large number of possible velocity profiles. Distributions of traversal time are determined by assuming that the probability of each velocity profile decreases exponentially with the action of the fluctuation it requires. Distributions of traversal times are reported for rectangular barriers having different sizes. For large barriers the distributions are leptokurtic and centered at the semiclassical traversal time T0 = d√m/[2(V0−E)], where d and V0 are the length and height of the barrier and m and E are the mass and energy of the particle. The kurtosis decreases and the mode shifts to shorter durations with decreasing barrier size.
Theoretical and experimental studies of fanning effects in photorefractive crystals
Authors: Xie Ping , Hong Yan-Hua , Dai Jian-Hua , Zhu Yong , Zhang Hong-Jun
Published: July 2002

A model to describe fanning effects in photorefractive crystals is presented. The intensity distribution of the fanning beam for one incident beam and two incident beams in 45°-cut BaTiO3 crystals has been studied theoretically and experimentally, with good agreement. The theoretical results suggest that, when there is a fanning effect, only at intermediate effective interaction lengths does the signal beam have a high gain in two-wave mixing. The transient behavior of the fanning beam in a 45°-cut BaTiO3 crystal has also been studied theoretically and experimentally.
Smoothing of the Si surface using CF4/O2 down-flow etching
Authors: Nishino H. , Hayasaka N. , Horioka K. , Shiozawa J. , Nadahara S. , Shooda N. , Akama Y. , Sakai A. , Okano H.
Published: July 2002

Changes in surface morphology have been studied for Si surfaces treated with CF4/O2 down-flow etching. It has been found that rough Si surfaces can be smoothed and Si trench corners can be rounded off using this CF4/O2 down-flow etching. A SiFxOy layer is formed on the Si surface etched by a down-flow discharged CF4/O2 gas mixture in high O2 concentration. A thick SiFxOy layer is formed at the concave part of the surface, which prevents fluorine atoms from reacting with Si. On the other hand, Si etching proceeds fast at the convex part covered with a thin SiFxOy layer. As a result, a rough Si surface is smoothed and trench corners are rounded off. By applying this treatment to a polycrystalline silicon surface, the leakage current of a SiO2 film grown on it is much reduced.
Effect of hydrogenation on the luminescence of strained Si1−xGex alloy layers grown by molecular beam epitaxy
Authors: Tijero J. M. G. , Arbet-Engels V. , Manissadjian A. , Wang K. L. , Higgs V.
Published: July 2002

Thick Si1−xGex strained alloy layers grown by molecular beam epitaxy (MBE) are investigated using photoluminescence (PL) spectroscopy. Near-band-edge luminescence with well resolved phonon structures is observed for both as-grown and deuterated samples. The low energy broad band frequently encountered in MBE-grown alloy layers is shown to be annihilated by deuteration, giving rise to the no-phonon and phonon-assisted near-band-edge PL peaks. The broad band recovers by annealing at T≥360 °C while the intensity of the near-band-edge luminescence vanishes. Secondary ion mass spectroscopy and the effect of deuterium passivation are used to help locate and assign the defects responsible for the low PL efficiency of MBE-grown thick SiGe layers.
Optical properties of conjugated polymer superlattices prepared by potential-programmed electropolymerization
Authors: Fujitsuka Mamoru , Nakahara Reiko , Iyoda Tomokazu , Shimidzu Takeo , Tsuchiya Hajime
Published: July 2002

Photoluminescence (PL) spectra of thin films of pyrrole and bithiophene copolymer with an alternating layered structure prepared by the potential-programmed electropolymerization method were investigated. Band edges of thin films of the copolymers were estimated by optical and electrochemical method, and they were adopted to design a superlattice potential profile of the alternating layered film. PL spectra of the copolymer films shifted to higher energies as thiophene fraction in the film decreased. This shift corresponded to band structure change. PL spectra of the layered films, designed to be type II superlattices structure, shifted to higher energies as periodicity of the layered structure became shorter. This phenomenon is to be interpreted as a quantum size effect. The observed shift was explained by the Kronig–Penney model with 0.6me (electron mass) of effective mass.
Hall electron density and mobility in HgCdTe alloys with anomalous behavior of the Hall coefficient in weak magnetic field
Authors: Rafol S. B. , Wijewarnasuriya P. S. , Sivananthan S. , Faurie J. P.
Published: July 2002

It is difficult to extract unique values of the Hall electron density, nH, and Hall mobility, μH, in indium-doped Hg1−xCdxTe alloys usually in a region of weak magnetic fields where the Hall coefficient RH behaves anomalously. Since this is also the field region where the weak magnetic field condition of semiclassical transport equations is valid, derivatives of Hall resistivity ρxy(B) and the ratio (Hall angle) of ρxy(B) and transverse resistivity ρxx(B) with respect to the field are used to determine unique values of the weak magnetic field nH and μH. This analysis reduces to calculating the slopes of ρxy(B) and ρxy(B)/ρxx(B) in the region of weak magnetic fields where linearity is observed. The conductivity, σ0, determined from the ratio of the slopes is compared to the zero magnetic-field experimental value. The conductivities agree to within 10%, indicating that nH is constant in contrast to the RH result. The values of nH determined from the slopes are in very good agreement with the values of nH determined from a high magnetic field RH and from the period of Shubnikov–de Haas oscillations in heavily doped alloys.
Low-voltage, low-chirp, absorptively bistable transmission modulators using type-IIA and type-IIB In0.3Ga0.7As/Al0.33Ga0.67As/ In0.15Ga0.85As asymmetric coupled quantum wells
Authors: Trezza J. A. , Larson M. C. , Lord S. M. , Harris J. S.
Published: July 2002

Coupled InGaAs quantum-well systems which use field-induced spatial separation of electron and hole states to modulate the magnitude of exciton optical absorption, and hence transmission have been theoretically analyzed and experimentally demonstrated. The samples consisted of p-i-n diodes with an active region of 20 coupled wells, each coupled well containing a 50 Å In0.3Ga0.7As well and a 30 Å In0.15Ga0.85As well separated by a 10 Å Al0.33Ga0.67As barrier. One structure was grown with the thinner well on the n-type side of each coupled quantum well while in the other sample the thinner well was oriented toward the p-type side. By applying bias to the structures, either the lowest electron or hole states effectively switch wells, thereby enhancing certain exciton resonances and quenching others. The two devices grown, despite their similar structure, operate through the field-induced switching of opposite carrier types. Because this method of modulation does not require excitons to Stark shift, the device can produce large absorption/transmission changes with zero refractive index change under bias. These first nonoptimized samples produce changes in absorption per applied bias three times larger than single-well systems. In addition, optical bistability is realizable in these structures. In addition to their presently displayed use, the coupled quantum-well structure has numerous applications for waveguide or Fabry–Perot optical modulator systems.
Rectifying contact formation with indium on polycrystalline p-type hot-filament chemical vapor deposited diamond utilizing molecular ion implantation
Authors: Stacy T. , Zhao G. , Liaw B. Y. , Charlson E. J. , Charlson E. M. , Meese J. M. , Popovici G. , Prelas M.
Published: July 2002

Novel low energy molecular implantation of trimethyl phosphite was used to make rectifying junctions with indium contacts on boron doped diamond grown on silicon substrates. Diamond growth utilized hot-filament enhanced chemical vapor deposition (HFCVD). To our knowledge, this is the first report of molecular implantation on diamond and the only implanted junction with HFCVD polycrystalline diamond on silicon as the starting material. The rectifying contacts have high current density in forward bias and exhibit linear inverse squared capacitance-voltage characteristics at high frequencies, unlike other rectifying diamond junctions.
Dependence of impact ionization coefficients on electric-field profile and injection mode approximations
Authors: Luquet H. , Gouskov L. , Pérotin M. , Karim M. , Rmou A. , Mottet S.
Published: July 2002

The validity of the determination of the ionization coefficients deduced from the experimental measurement of photocurrent multiplication Mph in GaAlSb diodes, in which the electric-field profile is nonlinear and successive pure hole and pure electron injections cannot be made, are compared and discussed. Three methods for determining ionization coefficients kp and kn are discussed: (i) adjusting kp and kn in order to fit the experimentally observed Mph values using an approximated linear electric field; (ii) the same method applied to a more realistic electric-field profile; (iii) the Grant method in which kp and kn are calculated from the results of multiplication measurements in the case of pure injections and using this last electric-field profile. It is concluded that when the condition of pure injection is not fulfilled the Grant method may lead to erroneous k values and the adjustment method is more valid.
Remodified Kondorsky function and asymmetric wall structure
Authors: Lee Tao Yuan , Huang Huei Li
Published: July 2002

We study the ground state of magnetization orientation inside domains on both sides of a domain wall. In a uniaxial ferromagnet, the orientation becomes asymmetric with respect to the normal to domain-wall plane when a magnetic field is applied. As a result, the Kondorsky switching-field distribution function is modified when the magnetization distribution across the wall proper becomes asymmetric and the wall is charged. The variation of the domain-wall structure, wall-shape function, and wall-energy density function γ versus the magnitude and orientation of the external field is examined and discussed. The extra energy associated with the charged wall is expected to enhance domain-wall relaxation, generating wiggles in thin-film head readout waveforms.
Numerical study of relativistic electron-beam electrostatic pumping by anode aperture
Authors: Yovchev I. G. , Spassovsky I. P. , Nikolov N. A. , Kostov K. G. , Spassov V. A.
Published: July 2002

Quality and pumping of an electron beam by means of the inhomogeneous electrostatic field at an anode aperture are investigated using numerical simulation. A specially developed relativistic particle-in-cell code, described here, is employed. The longitudinal velocity spread and the pitch ratio of the beam electrons are derived for different aperture radii and cathode-aperture distances. The presented results could be practically useful for operation of high-power cyclotron resonance masers with a sharpened cathode and an anode aperture. The beam pumping in such devices is typically achieved by more complicated systems (pumping magnet, adiabatically changing magnetic field, etc.). The numerical simulations show a significant pumping effect coming from the anode aperture itself.
Measurement of thermal stress and stress relaxation in confined metal lines. II. Stress relaxation study
Authors: Moske M. A. , Ho P. S. , Mikalsen D. J. , Cuomo J. J.
Published: July 2002

The bending beam technique and stress analysis developed for stress measurements of fine line structures have been applied to investigate stress relaxation in confined Al (2 at. % Cu) line structures on a Si substrate. The observed relaxation of the line structure is compared with corresponding unpassivated and passivated layered film structures. The overall behavior of all structure is similar, showing an initial plastic deformation, then a fast relaxation in sequence with a log(time) slow relaxation. The kinetics of these relaxation processes are found to decrease due to the presence of the passivation and a higher degree of dielectric layer confinement. In addition, the relaxation behavior of the principal stress components of the line structure is anisotropic and does not vary monotonically with the annealing temperature. The results are attributed to the relaxation mechanism and interaction at the metal/dielectric interface.
Density of states of a two-dimensional electron gas including nonparabolicity
Authors: López-Villanueva J. A. , Gámiz F. , Melchor I. , Jiménez-Tejada J. A.
Published: July 2002

An expression for the density of states per unit area and unit of total energy in a two-dimensional electron gas is proposed. This density of states depends on the energy of the electrons in each subband and on the potential well function averaged in the subband by the envelope function. The resulting expression is shown to approach the density of states per unit area and unit energy, which is obtained by integrating the three-dimensional density in the potential well, in the limit of the high levels. This fact is then used for checking the validity of the expression, obtaining excellent results.
Beam trapping in a modified betatron accelerator with a localized bipolar electric-field pulse
Authors: Mathew J. , Marsh S. J. , Dialetis D. , Kapetanakos C. A.
Published: July 2002

A trapping concept for the injected beam in a modified betatron accelerator using a localized inductive electric-field pulse generated by a charged coaxial pulse line is described. The average canonical angular momentum 〈Pθ〉 is not conserved in this scheme. Numerical simulations demonstrate trapping over a wide range of operating parameters. A 0.84 Ω, 45 ns water dielectric pulse line has been built for testing the trapping concept. The pulse line is switched using triggered vacuum surface flashover switches. Test results show that a single bipolar output pulse can be obtained if the pulse line is crowbarred. For 15–40 kV pulses, the observed rise time is roughly 20 ns.
Characterization of Czochralski- and hydrothermal-grown Bi12SiO20
Authors: Leigh W. B. , Larkin J. J. , Harris M. T. , Brown R. N.
Published: July 2002

An investigation was made of the defect characteristics of single-crystal Bi12SiO20 (BSO) grown by both Czochralski (Cz) and hydrothermal transport methods. Only Cz-grown BSO is photorefracting using a coherent pumping source centered around 500 nm, as undoped hydrothermal BSO is transparent throughout the visible spectrum. Thermally stimulated current (TSC) studies in conjunction with temperature-dependent optical-absorption measurements and room-temperature photoconductivity data all indicate that the hydrothermal material is near intrinsic in terms of its low defect content. TSC measurements made below room temperature indicate that concentrations of traps of activation energy <0.7 eV are a factor of 103 smaller in hydrothermal than in Cz BSO. At least six different defects were identified in the TSC measurements. Temperature-dependent optical-absorption measurements indicate two Urbach band tails for Cz-grown materials that are not observed in the hydrothermal materials. Cz material of lower purity also possesses an impurity band tail which can be observed through temperature dependent optical absorption measurements. Comparison of Cz material with hydrothermal BSO of similar impurity content suggests that the BiSi defect responsible for the 500 nm absorption may be complexed with an impurity such as Fe or V. In the absence of this defect, however, these impurities have no effect on the absorption. An additional defect in the TSC data is also related to a transition metal impurity. The results indicate that the photoconductivity associated with the photorefractive effect in Cz material must proceed via a trap-hopping conduction mechanism that is missing in the intrinsic material.
Excitation spectra of 1200 and 1320 nm photoluminescence lines in annealed gallium arsenide doped with silicon
Authors: Suezawa M. , Kasuya A. , Nishina Y. , Sumino K.
Published: July 2002

Excitation spectra have been measured at various temperatures for the intense luminescence lines at 1200 and 1320 nm which are observed in samples of Si-doped GaAs annealed at 900 and 500 °C, respectively. The excitation spectra of the 1200 nm line at 6 K has a sharp peak at 1.490 eV followed by a broad background component on the lower-energy side. This peak corresponds to the electronic excitation of donor (Si at Ga atom site) -acceptor (Si at As atom site) pairs in the crystal. The excitation spectra of the 1320 nm line at 6 K shows two peaks at 1.49 and 1.39 eV. The experimental results show clearly that the creation of donor-acceptor pairs in the above Si impurity complex is the major process of electronic excitation which leads to the intense luminescence at longer wavelengths such as 1200 and 1320 nm.
Critical behavior of the two-dimensional easy-plane ferromagnet
Authors: Cuccoli Alessandro , Tognetti Valerio , Verrucchi Paola , Vaia Ruggero
Published: July 2002

The critical behavior of 2D magnetic easy-plane systems has mainly been studied by the classical XY model. However, the z components of the spins have to be considered in order to describe real systems, and their fluctuations cannot positively be neglected when quantum effects are to be included, quantum spins being intrinsically three-component objects. Therefore, Monte Carlo simulations are performed for the Heisenberg ferromagnet with easy-plane anisotropy (XXZ model) on a two-dimensional square lattice with a twofold aim: first, to obtain accurate quantitative results about the critical behavior of the classical model, showing the relevant role played by the out-of-plane fluctuations; second, to open the way for approaching the quantum thermodynamics by means of the effective Hamiltonian method that reduces the quantum thermodynamics of the XXZ ferromagnet to the investigation of an effective classical model with temperature-dependent renormalized interaction parameters. Specific heat, magnetic susceptibility, and correlation length are calculated in the critical region for lattice sizes up to 128×128. These quantities preserve the Kosterlitz–Thouless behavior of the XY model. For the transition temperature of the classical XX0 model we obtain the estimate kBTc/(JS̃2)=0.69±0.01.
Growth characteristics of single-crystal rods and fibers of Bi12SiO20 by the floating zone method
Authors: Fu Senlin , Ozoe Hiroyuki
Published: July 2002

Single-crystal rods and fibers of Bi12SiO20 were grown directly from a rod of mixed Bi2O3 and SiO2 powders in a floating zone device heated by an infrared source with a special light shutter. The source rod was pressed from the mixed powders at room temperature; to avoid possible contamination, hot pressing and melting with die or crucible were not employed. The following are the main results: (a) The length of the stable molten zone is an increasing function of the diameter of the grown crystal rod. (b) Non-transparency is a major defect in a small diameter crystal rod or fiber. The growth velocity should be less than the critical transparent velocity (the critical value below which the grown crystal is transparent throughout). For growth of a large diameter crystal rod (diameter ≥3 mm), the growth velocity should be less than both the critical transparent velocity and the critical cracking velocity (the critical value below which the grown crystal is free of cracks). In general, both the critical transparent and critical cracking velocities are decreasing functions of the diameter of the grown crystal rod. (c) Use of a single crystal with high thermal conductivity (e.g., Al2O3) as a seed can dramatically increase both the critical transparent and critical cracking velocities for growth of a single crystal of diameter larger than 3 mm. (d) The pulling down floating zone method is more suitable than the pedestal growth method because of the low surface tension and high density of the Bi12SiO20 melt. (e) The grown Bi12SiO20 crystal is a pure body-centered cubic γ phase and has a good infrared transmission spectrum.
Properties of the nonequilibrium plasma from a pulsed sliding discharge in a hydrogen gas layer desorbed from a metal hydride film
Authors: Laqua H. , Bluhm H. , Buth L. , Hoppé P.
Published: July 2002

The properties of a proton source developed for use in a pulsed high power magnetically insulated ion diode have been determined. The source is created from a sliding gas discharge on the surface of a thin double layer of TiH and Pd deposited on an insulating substrate. By driving a short (<20 ns) high current pulse through the metal films hydrogen is released from the Ti store and a multichannel electrical breakdown is created in the desorbed gas layer. The uniformity of this breakdown depends on the capacitance per area of the multilayer setup. It has been found that the breakdown always occurs after the same areal gas density has been released. The density as well as the temperature of the plasma depend on a continuous influx of hydrogen from the reservoir and on the expansion. The electron temperature decreased from up to 12 early in the pulse to less than 3 eV late in the pulse. Over a distance of 1 mm the plasma density falls from 1017 at the surface to 1015 cm−3. The plasma expansion is stopped by a strong magnetic field parallel to the surface; however, the neutral hydrogen density increased with increasing magnetic field. Carbon ions are the most important contaminant of the hydrogen plasma. However, their fraction is smaller than 10%.
Epitaxial growth and photopolymerization of diacetylene films on oriented layers of poly(tetrafluoroethylene)
Authors: Meyer Sébastien , Smith Paul , Wittmann Jean-Claude
Published: July 2002

Highly oriented polydiacetylene thin films were prepared by epitaxial polymerization of vacuum deposited monomer 5,7-dodecadiyne 1,12 diol bis{[(4-butoxy carbonyl) methyl] urethane} (4-BCMU) on friction-transferred poly(tetrafluoroethylene) (PTFE) substrates. Detailed structural and morphological studies were performed using UV-dichroism measurements, optical and transmission electron microscopy, and suggest that 4-BCMU grows epitaxially on PTFE as a result of a near perfect lattice matching. The oriented monomer layer is converted by photopolymerization into a highly oriented polymer layer with a unique poly(4-BCMU)/PTFE chain to chain orientation and a well defined ac contact plane. Annealing of the monomer layer, prior to photopolymerization, is shown to strongly enhance the final degree of ordering of the polydiacetylene thin films.
Metallization of electronic polymers: A comparative study of polyvinylidene fluoride, polytetrafluoroethylene, and polyethylene
Authors: Chen Tim C. S. , Mukhopadhyay Sharmila M.
Published: July 2002

The initial stages of bond formation between a piezoelectric fluropolymer, polyvinylidene fluoride (PVDF) (-CF2-CH2−)n and a moderately reactive metal (Ni) has been studied using x-ray photoelectron spectroscopy. In order to understand the two different types of C bond present (-CF2− and -CH2−), these studies have been extended to include the polymers poly-tetrafluoroethylene (PTFE) (-CF2−)n and polyethylene or PE (-CH2−)n. It is found that Ni attacks the C and F in the -CF2− component, forming nickel fluoride and at least two new C containing products (perhaps a combination of -CFNi- and graphite-like C). On the other hand, the -CH2− species is unaffected by Ni. Therefore, among the polymers studied, the PTFE surface is most severely altered by Ni followed by PVDF. PE is totally inert to this metal. A semiquantitative analyses of these interactions has been presented in light of the information currently available in the literature.
Magnetic characteristics of RCo13−xSix alloys (R=La, Pr, Nd, Gd, and Dy)
Authors: Huang M. Q. , Wallace W. E. , Obermyer R. T. , Simizu S. , McHenry M. , Sankar S. G.
Published: July 2002

The magnetism of LaCo13-type alloys such as LaCo13, PrCo13−xSix, etc., has recently received considerable attention as potentially useful magnetic materials. The present study is concerned with RCo13−xSix where R=La, Pr, Nd, Gd or Dy.
Low-field magnetoresistance in magnetic tunnel junctions prepared by contact masks and lithography: 25% magnetoresistance at 295 K in mega-ohm micron-sized junctions (abstract)
Authors: Parkin S. S. P. , Fontana R. E. , Marley A. C.
Published: July 2002

Interest in magnetic tunnel junctions (MTJ) has been increased by the recent observation of large room-temperature magnetoresistance (MR) in structures comprised of sandwiches of two ferromagnetic (FM) layers with different coercivities separated by a thin Al2O3 tunnel barrier. Using an ultrahigh vacuum dc magnetron sputtering system, a variety of MTJ structures have been explored. Junctions were fabricated directly using computer-controlled placement of successive contact masks, and by patterning using optical lithography techniques and processes, compatible with MR head structures. Structures were prepared in which one of the FM layers is exchange biased with an antiferromagnet MnFe layer. Thus, the junctions exhibit two well-defined magnetic states in which the FM layers are either parallel or antiparallel to one another. The tunnel barrier was prepared by in situ plasma oxidation of thin Al layers sputter deposited at room temperature. Using FM layers comprised of Co or permalloy (Ni81Fe19), MR values exceeding 30% were obtained at room temperature in low fields (5–10 Oe). Similar magnetic and MR properties are found in junctions ranging in size from 80×80 μm2 (contact masks) to 2×2 μm2 (lithography). The smallest junctions, of area 2×2 μm2, have resistance values of ∼5 MΩ and 25% MR. Thus, changes in resistance of ∼1 MΩ are found. Such structures may form an excellent magnetic switch. However, for many device applications, much lower resistances are required. The junction resistance can readily be varied by several orders of magnitude by varying the thickness and oxidation conditions of the Al layer as well as by varying the thicknesses and nature of the layers within the electrodes themselves. Note that it is important that the voltage drop laterally across the electrodes must be small compared to that across the tunnel barrier itself. This requires measurement of junctions with low sheet resistances in lithographically patterned devices.
Effect of oxygen to argon ratio on properties of (Ba,Sr)TiO3 thin films prepared by radio-frequency magnetron sputtering
Authors: Tsai M. S. , Sun S. C. , Tseng T. Y.
Published: July 2002

Thin films of (Ba,Sr)TiO3 on Pt/SiO2/Si substrates were deposited using rf magnetron sputtering at various substrate temperatures and O2/(Ar+O2) mixing ratios (OMR). The crystallinity of the films improved significantly as the OMR increased. The dielectric constant increased with increasing OMR and reached a maximum value at 50% OMR. The leakage current density decreased with increasing oxygen flow, but had a minimum value at 40% OMR. The results for the dielectric constant and the leakage current were interpreted in terms of polarization effect and loss theory. The film deposited at 450 °C and 50% OMR exhibited good surface morphology and had a dielectric constant of 375, a tangent loss of 0.074 at 100 kHz, a leakage current density of 7.35×10−9 A/cm2 at an electric field of 100 kV/cm with a delay time of 30 s, and a charge storage density of 49 fC/μm2 at an applied field of 150 kV/cm. The 10 yr lifetime of time-dependent dielectric breakdown studies indicate that a 50% OMR sample has a longer lifetime than the 0% OMR sample.
Laser diode cavity ring-down spectroscopy using acousto-optic modulator stabilization
Authors: Paldus B. A. , Harris J. S. , Martin J. , Xie J. , Zare R. N.
Published: July 2002

By using an acousto-optic modulator, we have stabilized a free-running continuous wave (CW) laser diode in the presence of strong reflections from a high finesse Fabry–Perot resonator. The laser diode linewidth can be stabilized from several MHz, for high resolution spectroscopy of species at low pressures, to several hundred MHz, for lower resolution spectroscopy of species at atmospheric pressures. We demonstrated CW cavity ring-down spectroscopy of water vapor at both 1 atm and 5 Torr. We achieved ring-down repetition rates of 10–50 kHz, and a noise level of 2×10−8 cm−1.
Soft-hard magnetic composite wires obtained by a modified current annealing technique
Authors: Sinnecker J. P. , Tiberto P. , Vázquez M. , Hernando A.
Published: July 2002

A current annealing procedure is proposed. Heat sinks were positioned along the sample, while annealing with current, inducing tailored temperature profile along its axis. Fe77.5Si7.5B15 amorphous wires were current annealed with this technique so that, magnetically soft and hard local regions develop. As a measure of this magnetic composite character, the coercivity profile was verified after annealing. Results are explained in terms of different power losses along the sample. Some possible applications as security labels of the annealed material are discussed.
Growth kinetics of intermediate compounds at a planar solid-solid or solid-liquid interface by diffusion mechanisms
Authors: Coulet André , Bouche Karine , Marinelli Francis , Barbier Francoise
Published: July 2002

A diffusional model of interface displacement kinetics is proposed for the growth of n intermediate compounds at an initially planar interface between two semi-infinite phases. The model is based on the solution of Fick’s equations with the restrictive assumptions of simultaneous growth of n intermediate phases, unidirectional diffusion flow, and local equilibrium conditions. The velocity of each interface follows the parabolic law and the (n+1) kinetic coefficients are expressed as a function of boundary concentrations and diffusion coefficients of all the phases via (n+1) nonlinear equations. A parametric study of the kinetic coefficients, corresponding to realistic situations of initial solid-solid or solid-liquid interface, is developed for systems with one or two intermediate layers. If two interacting initial phases α and β are such that the chemical diffusion coefficient Dα (in α) is smaller than Dβ (in β), it is found that the interface velocities are enhanced by: (a) increases in Dβ, (b) increases in the solubility limit in β, and (c) reduced miscibility gaps at the interfaces. Moreover, the widths of the intermediate layers are increased by: (a) decreases in Dβ and (b) increases in the diffusion coefficients and solubility limits in these intermediate phases.