bluuebunny/crossref_metadata_2025 huggingface dataset:
Fabrication of nanometer flat areas onto YBa2Cu3O7−x thin film surfaces by scanning tunneling microscope
Authors: Virtanen J. A. , Suketu P. , Huth G. C. , Cho Z. H.
Published: July 2002

A scanning tunneling microscope was used to mechanically ‘‘mill’’ nanometer flat areas of up to 1600 μm2 on high temperature superconducting (HTS) films of YBa2Cu3O7−x which were originally formed by laser ablation. Flatness to a standard deviation of 2 nm in height was found to be characteristic of milled areas. It was subsequently possible to mill trenches and ditches onto these flat areas. Scanning tunneling measurements of the exposed layered structure of the milled HTS surface are also reported. Surface modifications are also possible by the application of voltage pulse to the tunneling tip. The combination of electrical pulses and milling offer a possibility of mixed electromechanical patterning of the film.
Measurements of etch rate and film stoichiometry variations during plasma etching of lead-lanthanum-zirconium-titanate thin films
Authors: Poor M. R. , Fleddermann C. B.
Published: July 2002

Plasma etching of lead-lanthanum-zirconium-titanate (PLZT) ferroelectric thin films using a dc hollow-cathode discharge in CF4 and dilute HCl has been investigated. The etch rate and film stoichiometry variations of sputter-deposited lead-lanthanum-titanate (PLT), and solution-deposited PLT, lead-zirconium-titanate (PZT), and PLZT have been measured over a wide range of etching conditions. Etching does not take place at room temperature, but requires substrate heating above 200 °C. Etch rate and film stoichiometry varied strongly as a function of substrate temperature, gas mixture, and film preparation method. For sputter-deposited films, the fastest etch rates of 6500 Å/h were obtained using a mixture of CF4 and dilute HCl. Etch rates for solution-deposited films were higher than for sputter-deposited films, reaching 2000 Å/min in CF4.
The I-V characteristics of double-barrier resonant tunneling diodes: Observation and calculation on their temperature dependence and asymmetry
Authors: Chen J. , Chen J. G. , Yang C. H. , Wilson R. A.
Published: July 2002

We report a calculation on the current-voltage characteristics of double-barrier resonant tunneling diodes. The results can well-explain experimental observations, in particular, the temperature dependence of both the absolute current amplitude and the resonant voltage positions. Our calculation takes into account the self-consistent Poisson equation, quantum mechanical tunneling, and band-gap offset at the heterojunctions. In addition, the calculation includes a realistic scattering-induced broadening effect, with which a quantitative fit throughout the entire negative differential resistance (NDR) region is demonstrated. Finally, we show that the differential resistance in the NDR region can be tuned, when asymmetrical tunneling barriers are used.
The stability of the self-consistently determined current of a double-barrier resonant-tunneling diode
Authors: Joosten H. P. , Noteborn H. J. M. F. , Kaski K. , Lenstra D.
Published: July 2002

Double-barrier resonant-tunneling devices belong to a novel class of nanoelectronic devices with great potential applications. In these devices, the self-consistent buildup of charge due to resonant carriers in the well may lead to bistability and hysteresis. To investigate aspects of dynamical (in)stability, a simple set of equations is derived from an extension of the static theory. These dynamic equations adequately describe small and slow (<100 GHz) deviations from the stationary state. This approach is viewed more as being more satisfactory than an equivalent-circuit analysis, but its limitations are also discussed.
The ferroelectric domain layer interface wave in multiple domain layered structure
Authors: Li Xingjiao , Li Yibing , Lei Yiwu , Cross L. E.
Published: July 2002

A theoretical analysis is made of the ferroelectric domain layer interface wave in a multiple domain layered structure. The dispersion equation is presented and the dispersion curves are obtained. When domain layer N is large, the multimode dispersion curves form a dispersion band. There are three kinds of dispersion curves corresponding to different field distributions, which describe the different cases of interaction of multiple domains.
Electro-optic effects of electron cyclotron resonance plasma-sputtered Bi12SiO20 thin films on sapphire
Authors: Nomura Koji , Ogawa Hisahito
Published: July 2002

Bi12SiO20 thin films have been prepared on (0001) sapphire substrates by electron cyclotron resonance plasma sputtering with a Bi and Si multitarget system. The epitaxial thin films of γ-phase Bi12SiO20 have been obtained at the substrate temperature of 600 °C during the sputtering process. Excellent quadratic electro-optic effects for these epitaxial thin films were successfully observed for the first time. The details of the preparation, structure, and electro-optic properties of the Bi12SiO20 thin films are precisely described.
Above-band-gap linear electro-optic coefficients of GaAs
Authors: Acosta-Ortiz S. E.
Published: July 2002

We report an easy way to obtain the linear electro-optic coefficients of zinc-blende crystals from experimental results of optical anisotropies. Above-band-gap linear electro-optic coefficients of GaAs (380–450 nm) are determined.
Spectra of χ(3)(−3ω;ω,ω,ω) in poly(2,5-thienylenevinylene) thin films with controlled conjugation lengths
Authors: Murata Hideyuki , Takada Noriyuki , Tsutsui Tetsuo , Saito Shogo , Kurihara Takashi , Kaino Toshikuni
Published: July 2002

The spectra of the third-order nonlinear optical susceptibility, χ(3), of poly(2,5-thienylenevinylene) thin films were measured with third-harmonic generation in the fundamental wavelength range of 1475–2100 nm. The effect of increasing the π-conjugation length for χ(3) was investigated for both samples with varied extents of conversion and uniaxially stretched sample. The χ(3) values were intensively increased with the progress of conversion. Two χ(3) peaks at the fundamentals of 1600 and 1950 nm were found in the fully converted film. The largest χ(3) value reached 4.5×10−10 esu at 1950 nm, and the origin of the peak was ascribed to three-photon resonance with the lowest π-π* transition band. It was also found that the overall profile of the χ(3) spectrum drastically changed through uniaxial stretching. The reason for the observed change is discussed in terms of the difference in average π-conjugation length.
Interaction of ambient gas and meniscus surface during growth of edge-defined film-fed growth polycrystalline silicon samples
Authors: Pivac B. , Borghesi A. , Sassella A. , Ottolini L. , Kalejs J.
Published: July 2002

We studied interaction of molten silicon at the meniscus surface with CO and CO2 gas added to an inert argon atmosphere during the growth of edge-defined film-fed growth polycrystalline silicon nonagons. Both gases caused a significant increase in carbon content of the nonagons. However, most carbon remained trapped in the layer close to the surface and made a SiC-like contribution to the infrared spectrum. In addition, CO2 gas caused enhanced surface oxidation (with respect to CO gas).
Optimization of the channeling radiation source crystal to produce intense quasimonochromatic x rays
Authors: Gary C. K. , Pantell R. H. , Özcan M. , Piestrup M. A. , Boyers D. G.
Published: July 2002

Channeling radiation is a source of intense, tunable, quasimonochromatic x rays produced by electrons traveling along a direction of symmetry through a crystal. We analyze the effect of the channeling source crystal, through both its composition and lattice structure, on the number of photons emitted per electron, the linewidths of the radiation, and the maximum sustainable currents to identify the crystal which will yield the most photons in a bandwidth of less than 10% full width at half maximum (FWHM). Although high atomic number (Z) crystals produce a greater number of photons per electron, given their poor thermal properties these crystals cannot sustain as high average currents as lower Z crystals. The linewidths of the channeling radiation emitted from high Z crystals are as large as 50% FWHM, and electrons are dechanneled more quickly in these crystals, limiting their use as a quasimonochromatic x-ray source. Given its exceptional thermal conductivity and narrow linewidths, diamond is the optimal source crystal for the production of intense, quasimonochromatic x rays using channeling radiation.
Ruby laser-induced linear dichroism of F
A(II) centers in KCl:Li crystals
Authors: Peiponen K.-E. , Silfsten P. , Raerinne P.
Published: July 2002

The FA(II) color centers of KCl:Li crystals were observed to reorientate at room temperature under the exposure of a ruby laser. The linear dichroism of the centers, induced by ruby laser light, is presented here.
Effect of temperature and atmosphere on stability of high-T
c phases in Bi–Pb–Sr–Ca–Cu–O superconductors
Authors: Lee H. K. , Park K. , Ha D. H.
Published: July 2002

The effect of temperature and atmosphere on stability of the high-Tc phase in Bi1.8Pb0.4Sr2Ca2Cu3Ox superconductors has been investigated using ac susceptibility, x-ray diffraction, and Raman measurements. It is found that the high-Tc phase becomes unstable due to the structural phase transformation at temperature above 650 °C in O2 of 1 atm. The formation and the decomposition of the high-Tc phase is found to occur through the medium of the Ca2PbOx phase and depends on oxygen partial pressure at temperature above 650 °C.
Aperture effects on charged particles in a magnetic field
Authors: Wang E. Y. , Li W. Q. , Lam S. W. , Hershkowitz N.
Published: July 2002

In this paper we present investigations of two kinds of aperture effects on charged particles in the presence of a magnetic field. The first is an experimental study of the plasma potential profiles in front of an aperture. Plasma potential is measured with emissive probes. The interaction between the plasma and the solid wall of the aperture results in a nonuniform potential profile. Our experiments show that suitable aperture biasing can smooth the electric field in front of the aperture. The second is the transmission of electrons through a finite thickness aperture. The experimental results of the latter are compared to results of a single particle orbit model. It is shown that transmission measurements can be used to estimate the electron perpendicular energy in the magnetic field.
Temperature and impact ionization effects on f
T of advanced bipolar transistors
Authors: Yuan J. S. , Yeh C. S. , Gadepally B.
Published: July 2002

Modeling the cutoff frequency (fT) of the advanced bipolar transistors at avalanche breakdown has been developed. The analytical equations developed account for temperature and impact ionization effects on fT and high-current effects at the collector-base junction. Process sensitivity of the collector charging time at the avalanche breakdown regime has been examined. The experimental data reported in the literature are compared in support of the model. Good agreement between the model prediction and measurement is obtained.
Scanning magnetic force microscopy on Co/Pt magneto-optical disks
Authors: van Kesteren H. W. , den Boef A. J. , Zeper W. B. , Spruit J. H. M. , Jacobs B. A. J. , Carcia P. F.
Published: July 2002

High resolution images of domains written on Co/Pt magneto-optical disks were obtained using scanning magnetic force microscopy. The key advantage of this technique compared to electron microscopy techniques is that regular disks can be used. We studied the influence of the multilayer composition and preparation conditions as well as the effect of the pregrooves and the incorporation of a Pt underlayer, on the structure of the domains on the disk. Marks were written both with laser and magnetic field modulation, and carrier and noise levels were determined. Regularly shaped and subdomain-free marks for both writing schemes were obtained for multilayers with about 4-Å-thick Co layers and Pt layers thinner than 15 Å. The magnitude of the coercive field had only a minor influence on the domain structure. On the other hand, the preparation conditions and the structure of the base layers have a large impact on the recording conditions and domain structures.
On the intrinsic bistability in resonant tunneling structures: Observation of area and temperature dependence of hysteresis
Authors: Chen J. G. , Chen J. , Wilson R. A. , Johnson W. , Yang C. H.
Published: July 2002

We report experimental observation on the area and temperature dependence of the hysteresis in the current-voltage characteristics of double-barrier resonant tunneling diodes. Our observation shows that the hysteresis can simply result from a load line effect, since (1) the hysteresis will disappear when the the device area is reduced, and (2) the hysteresis becomes wider at lower temperatures. We compare the data with theoretical predictions from the intrinsic bistability picture, and obtain the criterion for observing current bistability.
Unmonochromatized synchrotron radiation promoted silicon oxynitridation at room temperature
Authors: Glachant A. , Soukiassian P. , Kim S. T. , Kapoor S. , Papageorgopoulos Aris , Baros Y.
Published: July 2002

We investigate the effect of unmonochromatized synchrotron radiation (USR) on the room-temperature reaction between a Si(111)2×1 surface and adsorbed NO. After exposition to NO of the Si(111) surface, an oxynitride/silicon interface is formed. The evolution with irradiation time of the oxynitridation of silicon and of the oxynitride/silicon interface formation is examined in situ by means of core-level and valence-band photoemission spectroscopy, Auger electron spectroscopy, and work-function measurements. The analysis of the valence band and Si 2p, O 2s, and N 2s photoelectron spectra, together with the Auger Si transition, clearly exhibits additional oxynitride formation under USR. The oxygen-to-nitrogen ratio, O/N, from the thin oxynitride layer increased after a long time (≳10 min) of exposure to USR. Hence, USR appears to have the opposite effect to the one produced by thermal activation in which the nitridation is favored.
High-pressure oxygen-induced bulk superconductivity in 1222 structure Tl-Pb-Sr-Eu(Ce)-Cu-O
Authors: Iqbal Z. , Sinha A. P. B. , Morris D. E. , Barry J. C. , Auchterlonie G. J. , Ramakrishna B. L.
Published: July 2002

A primarily single phase compound of composition (Tl0.5Pb0.5)Sr2(Eu2−xCex)Cu2O9 (where x ranges from ≪0.1 to 0.17) has been synthesized in the so-called 1222 structure. Bulk superconductivity near 40 K is induced via oxygen doping in 100 bar of oxygen at 550 °C. The material has been structurally characterized by diffraction and high-resolution lattice imaging techniques. The lattice images reveal the presence of some intergrowths of a 1212 structure nonsuperconducting (Tl0.5Pb0.5)Sr2EuCu2O7 phase in the bulk 1222 structure of the parent compound and indications of periodicity that suggest the possible existence of a new, composite structure 2434 phase.
Time-resolved emission spectra following the 193-nm photoablation of CuO, BaO2, Y2O3, and YBa2Cu3O7−x in vacuum and oxygen
Authors: Fried Daniel , Reck Gene P. , Kushida Toshimoto , Rothe Erhard W.
Published: July 2002

Gated diode array spectra were taken after the photoablation of CuO, BaO2, Y2O3, and YBa2Cu3O7 in vacuum and oxygen. An ArF excimer laser (193 nm) was used for photoablation. The spectra were resolved on a microsecond time scale. Emission from ions, atoms, and molecular oxides are observed and identified. Observations in the first microsecond represent phenomena produced in the primary ablation process. This emission is due primarily to high-energy collisions of ejected atoms and ions. Later time observations reveal that excited metal oxides are formed from reactive collisions of barium and yttrium atoms with the background oxygen, when present. Such species may be responsible for improved film quality in the presence of oxygen.
Thermal reaction of WSix thin films with underlying Al films
Authors: Kikkawa Takamaro , Endo Nobuhiro
Published: July 2002

Thermal reactions of tungsten silicide (WSix) thin films with underlying aluminum (Al) films in WSix/Al bilayers after heat treatments were investigated. Rutherford-backscattering and x-ray-diffraction results indicated that excess Si atoms in the Si-rich WSix (x=2.3) film diffused into the Al film, while Al atoms diffused into the WSix film at 450 °C, resulting in the formation of hexagonal WSi2 or W(Si,Al)2. After 500 °C annealing, tetragonal WSi2 was formed and the diffused Al atoms piled up in the WSi2 film surface layer. Morphological deformations, such as protrusions, occurred in the WSix/Al bilayer after annealing at above 450 °C. Transmission electron microscopy revealed that the deformation was caused by Si nodule growth at the interface between the WSi2 and Al films as a result of the excess Si atom outdiffusion from the Si-rich WSix (x=2.3) and precipitaion at the interface.
Plasma etching of polymers: A reinvestigation of temperature effects
Authors: Pons M. , Joubert O. , Paniez P. , Pelletier J.
Published: July 2002

Temperature effects in plasma etching of polymers have been reinvestigated. A stepwise increase in the etch rate is observed when the substrate temperature exceeds the glass transition temperature Tg of the polymer. This phenomenon is correlated with changes in the physical properties of the polymer such as heat capacity. Furthermore, whereas temperature is usually assumed to enhance the etching process, the ion-induced etching component, below Tg, is shown to increase when the polymer temperature is decreased. This behavior can be explained by a simple model involving changes in the adsorption rate of reactive species with temperature.
Impact ionization rates of holes in AlxGa1−xSb
Authors: Kuwatsuka H. , Mikawa T. , Miura S. , Yasuoka N. , Ito M. , Tanahashi T. , Wada O.
Published: July 2002

This paper discusses the impact ionization rates of holes in AlxGa1−xSb near x=0.06, where resonance impact ionization is expected. Resonance impact ionization did not occur in a range of the electric field we studied. We evaluate the impact ionization of holes at the spin-split-off band theoretically, and show that our measured result is reasonable.
Deposition of textured yttria-stabilized ZrO2 films on oxidized silicon
Authors: Stamper A. K. , Greve D. W. , Schlesinger T. E.
Published: July 2002

Highly textured yttria-stabilized ZrO2 films have been deposited using rf diode sputtering from a single polycrystalline target. These films were deposited on oxidized silicon substrates at substrate temperatures up to 600 °C using a 90% argon and 10% oxygen sputtering gas mixture. The films crystallized in the cubic phase and were large grained (d∼500 Å). Films deposited above 500 °C exhibit extreme (100) texturing while films deposited without substrate heating exhibit extreme (111) texturing. The effect of oxygen annealing on the crystal structure of these films has also been studied.
The characteristics of polytropic magnetic refrigeration cycles
Authors: Yan Zijun , Chen Jincan
Published: July 2002

The concepts of generalized polytropic processes and polytropic refrigeration cycles, which consist of two isothermal processes and two generalized polytropic processes of paramagnetic salt, are introduced. It is shown that such a class of general magnetic refrigeration cycles, which includes the Carnot, Stirling, and other useful magnetic refrigeration cycles, possesses the conditions of perfect regeneration and can have the same coefficient of performance as the Carnot cycle for the same temperature range. Thus, they have many applications in the research and manufacture of magnetic refrigerators.
Stripe waveguides with matched refractive index profiles fabricated by ion exchange in glass
Authors: Poszner Torsten , Schreiter Gerd , Müller Rudolf
Published: July 2002

The paper presents both the theoretical investigation of refractive index profile generation of stripe waveguides by field-assisted ion exchange in glass and a comparison with experimental results obtained by Ag+-Na+ exchange in a special glass type. The internal electric field of a periodical structure of mask openings is calculated and applied to ion-exchange processes through a single mask opening as well as to processes using so-called guard bands. These guard bands are especially suitable for profile matching to step index fibers. Matching to graded index fibers requires burying steps, which are modeled with plausible physical assumptions.
Effects of phosphorus doping level and the annealing treatment on the oxidation kinetics of tungsten silicide
Authors: Lee Chong Mu , Han Suk Bin , Im Ho Bin , Lee Jong Gil
Published: July 2002

Effects of phosphorus doping level in the polycrystalline silicon (polysilicon) layer of unannealed as well as annealed WSi2.6−polysilicon composite films on the growth kinetics of the tungsten silicide have been investigated by analyses of growth rate of the thermal oxide, depth profiles, and microstructures of the polycide cross section. The behaviors of excess silicon in the tungsten silicide layer during annealing and oxidation as well as the oxidation mechanisms of the silicon-rich tungsten silicide have also been studied. For the unannealed tungsten polycides, the activation energy of the linear rate constant decreases with an increase in the phosphorus doping level while the parabolic rate constant is independent of the phosphorus concentration. For annealed tungsten polycides, however, both rate constants are doping level independent. Analysis of the microstructures of the cross section taken by scanning electron microscope clearly indicates that the excess silicon in the silicide layer is consumed first and then the silicon from the underlying polysilicon is consumed during oxidation.
Metalorganic chemical vapor deposition of Bi-Sr-Ca-Cu-O ultrathin films
Authors: Sugimoto Tsunemi , Yoshida Masaji , Yuhya Sigenori , Baar David J. , Shiohara Yuh , Tanaka Shouji
Published: July 2002

The metalorganic chemical vapor deposition of Bi-Sr-Ca-Cu-O ultrathin films was performed on single-crystal (100) MgO substrates. An ultrathin film with average thickness of 3.5 nm was found to exhibit the full superconductivity at more than 60 K. The critical current density of a 5-nm-thick film was found to be strongly anisotropic for applied fields parallel and perpendicular to the film plane. In particular, in a field parallel to a film plane, the critical current density was constant up to 6 T even at 50 K, near the critical temperature. From the x-ray-diffraction patterns of ultrathin films prepared at the deposition rates of 3 and 0.3 nm/min, a Bi2Sr2CaCu2Ox (2212) phase was found to be formed at the first stage of deposition regardless of the deposition rate, while a Bi2Sr2Ca2Cu3Ox (2223) phase was found to be formed in the earlier stage of deposition when the deposition rate was decreased.
Removal of fluorocarbon residue on Si with an electron cyclotron resonance excited Ar plasma
Authors: Delfino M. , Salimian S. , Hodul D.
Published: July 2002

An electron cyclotron resonance-excited Ar plasma completely removes CFx residue on Si resulting in a clean surface that is free of native Si oxide. In situ x-ray photoelectron spectroscopy verifies the absence of C and F on the surface, and the presence of what is thought to be a small amount of adsorbed or interstitially implanted O. Mechanistically, the Ar ion bombardment affects a nearly instantaneous ablation of F from the CFx surface followed in succession by a low average energy (100 eV) sputtering of the C-rich remnant, the native Si oxide, and the Si substrate. The etching rate of thick CFx residue is approximately 15 nm/min without any heat applied to the substrate.
Influence of growth temperature upon the In solid composition in InxGa1−xSb epilayers grown by metalorganic chemical vapor deposition
Authors: Su Y. K. , Juang F. S. , Wu T. S.
Published: July 2002

InxGa1−xSb epitaxial layers have been grown on (100) GaSb substrates under TMSb/(TMIn+TEGa) ratio of 5.8 and system pressure of 170 Torr at different substrate temperatures (550–630 °C). From analyses of both the x-ray diffraction and photoluminescence (PL), the indium solid mole fraction was found to decrease with increasing growth temperature. The x-ray diffraction patterns degrade with increasing lattice mismatch (or with increasing In mole fraction in the solid). The photoluminescence peak wavelength decreases with increasing growth temperature. The optimum growth temperature is about 600 °C at which the good quality InxGa1−xSb epilayers with the most mirrorlike surface, narrowest PL full width at half-maximum (9 meV) and strongest PL intensity can be obtained. The narrowest full width at half-maximum value is only 9 meV. The full width at half-maximum of 10-K photoluminescence peaks was found to increase with increasing lattice mismatch due to the increasing misfit dislocations and strains existing at the lattice-mismatched interface.
Correlation between resistivity and diffusion in thin films
Authors: Madakson Peter
Published: July 2002

Empirical equations have been derived that relate resistivity to diffusivity and activation energies for thin films. Results for the Cr/Au structure agree with published data on the interdiffusion of Cr and Au thin films at temperatures ranging from 350 to 450 °C. The following relationships were determined: Δρ/ρ0C=λ exp(−Q/kT); Δρ/ρ0C=nDt/χ2; Δρ/ρ0C=ψ exp(−χ2/4Dt) and D=Nχ2 /t exp(−Q/kT), where Nχ2/t=D0(cm2/s) is the frequency factor; Δρ is the change in resistivity; ρ0 is the room temperature resistivity; χ is film thickness; T is temperature; t is time; C is concentration; k is Boltzman’s constant; D is diffusion coefficient; and n, ψ, N, and λ are constants. Analysis of the Cr/Au resistivity data using these equations gives D0=1.2×10−7 cm2/s and Q=1.00 eV for Cr diffusion in Au; and D0=1.0×10−7 cm2/s and Q=1.06 eV for Au diffusion in Cr. These values were verified by analysis using Rutherford backscattering spectroscopy and, they are consistent with those reported for grain boundary diffusion.
Dopant deactivation and annealing characteristics of metal-oxide-semiconductor structures on germanium/boron-doped silicon after gamma irradiation or Fowler–Nordheim charge injection
Authors: Hashemipour O. , Ang S. S. , Brown W. D. , Yeargan J. R. , West L.
Published: July 2002

Dopant deactivation and thermal annealing characteristics of metal-oxide-semiconductor capacitors fabricated on Ge/B-doped silicon after gamma irradiation or Fowler–Nordheim injection were investigated for the first time. A decrease of about 30% in active acceptor concentration was observed immediately after gamma irradiation or Fowler–Nordheim injection. Further deactivation of boron (∼20%) occurred with annealing for temperatures of 80 °C and higher. Hydrogen for the deactivation, which occurred during annealing, is thought to come from dissociation of weakly bonded Ge—H formed during the gamma irradiation or Fowler–Nordheim injection. Capacitors fabricated on conventional boron-doped substrates do not exhibit acceptor deactivation as a result of annealing following irradiation or injection. For annealing temperatures of 110 °C and higher, the boron is first deactivated by the process noted above, and then is apparently reactivated by the dissociation of B—H bonds with hydrogen evolution from the structure.
Gel layers for inducing adjustable pretilt angles in liquid crystal systems
Authors: Hikmet R. A. M. , de Witz C.
Published: July 2002

A novel system for inducing pretilt angles in liquid crystals (LC) was devised. For this purpose thin layers of a LC mixture containing diacrylate molecules were spun on glass plates covered with rubbed polyimide films. The layers were then photopolymerized to form a gel by UV radiation. In this way the orientation within the pregel layer was frozen. In a cell construction when another LC system was brought in between such gel layers, it was found that a pretilt angle within the LC system was induced. The induced pretilt angle was found to be easily controllable by varying the thickness of the gel layer and a wide range of pretilt angles could be induced.
Diffusion, solubility, and thermodynamic properties of gold in solid germanium studied by means of radiotracer and spreading-resistance analysis
Authors: Almazouzi A. , Bernardini J. , Moya E. G. , Bracht H. , Stolwijk N. A. , Mehrer H.
Published: July 2002

Diffusion profiles of gold in germanium were measured using the radiotracer as well as the spreading-resistance technique. Diffusion coefficients from both experimental methods show a good agreement and are described by an activation energy of 147±2 kJ mol−1 and a preexponential factor in the range (1–2)×10−6 m2 s−1. The results are interpreted within a generalized dissociative model allowing for interstitial-substitutional exchange with the aid of vacancies or interstitial Au-vacancy pairs. Solubility data taken from the boundary concentration of the penetration profiles reveal nonsignificant differences between the total Au concentration and that of substitutional Au atoms. Basic thermodynamic quantities were calculated from the temperature dependence of the solid solubility using data from the Au-Ge phase diagram.
Change in refractive index for p-type GaAs at λ=1.06, 1.3, and 1.55 μm due to free carriers
Authors: Huang H. C. , Yee S.
Published: July 2002

A quantum-mechanical approach used to calculate the refractive-index change Δn due to free carriers for various doped p-type GaAs is presented. The approach makes use of a numerical Kramers–Kronig analysis to analyze a carrier-related optical-absorption spectrum in which various important carrier effects have been considered including the band-filling effect, the band-tailing effect, the band-gap-shrinkage effect, the direct optical transition of carriers between subvalence bands, and the indirect intravalence-band absorption due to phonons and impurities. Values of Δn have been obtained for various doping and carrier concentrations at wavelength λ=1.06, 1.3, and 1.55 μm. These Δn data are directly applicable to both the injection- and depletion-type optical-switching and modulation applications, and the optical-probing application. A comparison of various index-changing effects for GaAs at wavelength λ=1.06, 1.3, and 1.55 μm is also listed.
The effects of crystallography and lateral intermixing on the subband structure and optical anisotropy in quantum-wire arrays on vicinal substrates in the layer-orbital model
Authors: Citrin D. S. , Chang Yia-Chung
Published: July 2002

Materials-characterization studies indicate a high degree of lateral intermixing in quantum-wire arrays grown on vicinal substrates. In this study, the layer-orbital model, which is a variation of the effective bond-orbital model, is used to calculate the zone-center conduction- and valence-subband energies and optical matrix elements of recently grown epitaxially buried GaAs/AlGaAs/AlAs quantum wires and quantum-wire arrays. The lateral intermixing effect is included using a compositional profile that has the merit that it is appropriate for the entire range of lateral intermixing, whether small or large, and that is computationally efficient. The effect of the orientation of the quantum wires on the subbands and the optical matrix elements is also investigated. For uncoupled quantum wires, the orientation mainly affects the positions of the valence subbands; the optical matrix elements are nearly independent of the orientation. The situation for the quantum-wire array is more complicated due to its intermediate nature between one and two dimensional. It is found that only the position of the light-hole subband is appreciably affected. It is also found that quantum-wire arrays misoriented from the [011] ([01̄1]) direction display somewhat reduced optical anisotropy.
Space-charge-limited current investigation of the density of states in a-SiGe:H alloys
Authors: Labdi A. , Godet C. , Equer B. , de Rosny G.
Published: July 2002

Space-charge-limited current measurements have been carried out on undoped amorphous silicon-germanium alloys as a function of the Ge content in the range 0%–36%. The scaling law is checked for different series of samples with varying thickness, and the J-V data consequently analyzed by using the Weisfield method. The position of the Fermi level EF is obtained from the activation energy of the ohmic conductivity Ea. The deduced value of the density of states (DOS) near EF increases as a function of the Ge content in the range 1016 to 4×1017 cm−3 eV−1. A factor of 7 improvement of the Si0.74Ge0.26:H alloy DOS is clearly evidenced when the material is prepared using high H2 dilution of GeH4–SiH4 mixtures, leading to a DOS value of 1×1016 cm−3 eV−1.
Theoretical study of a high-energy pulsed H2/F2 chain first vibrational overtone HF chemical laser
Authors: Ashidate Shu-ichi , Takashima Toshiaki , Kannari Fumihiko , Obara Minoru
Published: July 2002

We have theoretically investigated the output performance of a pulsed discharge initiated H2/F2 chain first vibrational overtone HF chemical laser. The model used included rotational relaxation processes and simulated simultaneously both fundamental and overtone oscillations since the undesirable fundamental lasing cannot completely be suppressed during the experimental overtone oscillation. In the H2/F2 chain reaction system, which can produce the highly vibrationally excited HF(v) molecules up to vibrational level v=6, higher overtone output energy can be expected due to the increased vibrational overtone P branches, the increased transition probability of the first overtone transitions at the higher vibrational levels, and to the relative longer gain duration of a few tens microseconds. The overtone output energy of 2.73 J/ℓ is obtainable from the laser gas mixture of F2/H2/He=10/4/786 (Torr). Adding He diluent gas can effectively suppress the undesirable fundamental lasing by pressure broadening; the overtone output energy is much less sensitive to this effect. The overtone output energy is found to be dependent on the residual reflectivity of the resonator mirrors over the fundamental lasing band.
Reply to ‘‘Comment on ‘Coupled ac photocurrent and photothermal reflectance response theory of semiconducting p-n junctions I and II’ ’’[J. Appl. Phys. 70, 1087 (1991)]
Authors: Mandelis A. , Ward A. A. , Lee K. T.
Published: July 2002

The particular character of a focused laser-excited p-n junction as a strongly forward-biased depletion device, corroborated by experimental frequency responses, was found to yield a photothermal behavior different from the expected conventional photoacoustic response.
Pulsed laser heating of thin films: An efficient algorithm for computing temperature profiles
Authors: Cheung Nai-Ho
Published: July 2002

An efficient algorithm for computing temperature profiles of pulsed laser heating of thin metal films was developed, based on a semianalytical approach when temperature-dependent absorptance and arbitrary laser pulse wave forms can be realistically modeled. Frequency doubled Nd:YAG laser heating of thin aluminum films on glass was used as a test case. For film thickness ranging from 35 to 300 nm, the present algorithm demonstrates far superior accuracy and speed than conventional finite difference approaches.
Generation of intense pulsed microwave from a high-density virtual cathode of a reflex triode
Authors: Hanjo Hideshi , Nakagawa Yoshiro
Published: July 2002

A virtual cathode reflex triode was operated using a low-voltage (∼100 kV), low-impedance Marx generator, thus maintaining a low electron reflex frequency. The electron current density was increased from ∼80 A/cm2 to ∼3.2 kA/cm2 by changing the shape of the cathode. The observed frequency at peak power increase from 2 to 20 GHz along with an increase in the electron current density, being in good agreement with the frequency of the calculated values of virtual cathode oscillations. The highest peak power (approximately 100 MW) in the 5-GHz band was generated from the virtual cathode in the triode, measured by a crystal detector and the calorimeter of a graphite sheet absorber. Readings taken at a distance of 37 cm from the side of the triode resulted in microwave power measurements of 60 kW received by the waveguide of 5 GHz.
Spontaneous photon-emission spectrum of tunneling electrons in a double-barrier structure
Authors: Bai Guang , Shum Kai , Alfano R. R.
Published: July 2002

The spontaneous photon-emission spectrum of an electron tunneling through a semiconductor double-barrier structure is calculated using the Green’s-function approach. The peak photon-emission power of 10−7 W per well can be achieved for an injected current of 50 mA. Our results are consistent with the measured value of 10−9 W per well by Helm et al. [Phys. Rev. Lett. 63, 74 (1989)] when the nonradiative process and other factors are taken into account.
Superposition principle in small-angle light scattering at high frequency
Authors: Chen Tuan W.
Published: July 2002

For the small-angle scattering of light at high frequency, the sum rule reported earlier implies that a system of N uniform dielectrics of different sizes may be superimposed to form a single scatterer of nonuniform dielectric. Numerical tests are given for the cases of forward end-on scatterings by two dielectric spheres of different sizes which are embedded into a layered sphere.
Polar Kerr effect in high-T
c superconductors: The rotating-plate technique
Authors: Lyons K. B. , Dillon J. F. , McGlashan-Powell M.
Published: July 2002

A technique recently used for measurement of the complex polar Kerr rotation in high-Tc superconductors is discussed in a tutorial manner, making use of a Jones matrix approach. The primary concern addressed is the rejection of effects caused by linear birefringence and dichroism effects in the sample and associated optical components. This rejection is accomplished by the use of a rotating half-wave retardation plate.
Improved surface properties of InP through chemical treatments
Authors: Paul T. K. , Bose D. N.
Published: July 2002

Chemical treatment is a very effective method for passivation of semiconductor surfaces. HF and sulfide (Na2S⋅9H2O) pretreatments of InP have been shown to improve the properties of BaF2/InP interface significantly. The interface state density as obtained from C–V (1 MHz) measurements of metal-insulator–semiconductor structures was found to be reduced from 5.8×1010 cm−2 eV−1 to 2.1×1010 cm−2 eV−1 after HF treatment. The reduced interface state density resulted in increased photoluminescence intensity. X-ray photoelectron spectroscopy studies revealed that the formation of InF3 and P2S3 after HF and sulfide treatments, respectively, are responsible for better interfacial behavior.
Effect of surface on the conductance characteristics of Au-Bi2Sr2CaCu2O8+δ (single crystal) point contact junctions
Authors: Srikanth H. , Raychaudhuri A. K.
Published: July 2002

We report the effect of surface treatments on the dynamic conductance curves (G=dI/dV-V) of Au-Bi2Sr2CaCu2O8+δ (single crystal) point contact junctions of variable junction conductances (100 mS≳G≳100 μS). We find that if the crystal surface is cleaved freshly just prior to making contacts, all irreproducible sharp multiple features often observed in tunneling data of Bi(2212) oxide superconductors disappear. If the cleaved crystal surfaces are left under ambient conditions for a few days and the tunneling experiments are repeated, these multiple features reappear. We also find that if the current in the junction is made to pass predominantly through the bulk (and not along the surface), gap features are sharper. The observed conductance curves are fitted to a modified model [G. E. Blonder et al., Phys. Rev. B 25, 4515 (1982)] and estimated gap values are Δ≂28 to 30 meV corresponding to the ratio 2Δ/kBTc ≂ 7.5 with lifetime broadening Γ/Δ≂0.2. We conclude that the sharp multiple features observed in Bi(2212) tunneling curves has no intrinsic origin in the bulk and they arise from the surface only.
Impurities and defects of high-J
c YBa2Cu3Ox superconductor with flux jumps
Authors: Wang L. P. , Chen K. , Lin C. L. , Chu J. J.
Published: July 2002

The microstructures of a highly textured bulk YBa2Cu3 (123) superconductor, with the high-transport Jc (≥ (R18)3.7×104 A/cm2 at 77 K) and flux jumps (T<15 K), were characterized by x-ray diffraction, polarized optical microscopy, scanning electron microscopy, and scanning transmission electron microscopy. It was found that the grains of the 123 superconductor are preferentially oriented along the a-b plane of the 123 phase. The Y2BaCuO5 and CuO particles are dispersed uniformly in the 123 matrix, and the highly dense twins run through the matrix everywhere. Some irregular-shaped BaCuO2-CuO mixtures were observed at the 123 grain boundaries. The aligned 123 crystals and microstructural defects in the 123 matrix are considered to be responsible for the high-Jc value and flux jumps.
A photoemission study of cluster growth and morphology on inert substrates
Authors: Haugstad G. , Caprile C. , Franciosi A. , Wieliczka D. M. , Olson C. G.
Published: July 2002

Thin metal overlayer growth on solid xenon was characterized by means of synchrotron radiation photoemission spectroscopy. We employed a simple experimental arrangement in which a closed-cycle refrigerator and in situ evaporated metal films were used as substrate for Xe condensation. A ‘‘sandwich’’ geometry, in which the overlayer metal was used as substrate for Xe condensation, simplified the isolation of metal and Xe emission features. The evolution of the Xe and metal photoemission intensity and the line shape of core and valence states, as a function of metal coverage, were used to estimate average particle size and nucleation site density. The coverage dependence of the Sm 4f binding energy, as well as a number of newly identified spectral fingerprints of particle coalescence, support the measured film morphology and particle size.
Experimental investigation of a plasma edge cathode scheme for pulsed electron beam extraction
Authors: Grothaus M. G. , Zieher K. W.
Published: July 2002

A scheme with the potential for generation of an electron beam with high brightness and several μs pulse duration for microwave generation, electron accelerators, or free electron lasers is investigated experimentally. An electron beam is extracted transversely to the flow of a plasma jet. The transverse boundary of the plasma allows extraction of a space charge limited electron current for 7 μs at a current density of 18 A/cm2. At present, a normalized microscopic brightness of 8×108 A m−2 rad−2 is achieved. Closure of the extraction gap by invasion of plasma has been observed with a typical velocity of 0.1 cm/μs. Higher current density and higher brightness are expected for higher plasma densities and larger extraction fields.
Raman spectra and electric resistance of thermally treated In/GaAs structures
Authors: Yano Koji , Katoda Takashi
Published: July 2002

Thermally treated In/GaAs structure was characterized with laser Raman spectroscopy. InxGa1−xAs, which was formed during the thermal treatment, had two kinds of compositions, that is, In-rich and Ga-rich InxGa1−xAs were formed epitaxially with GaAs. In-rich InxGa1−xAs formed by thermal treatment at 400 °C was n type and had a carrier concentration of (1–2)×1018 cm−3. The amount of In-rich InxGa1−xAs, which determines electric resistance of In/GaAs structures, increased with an increase in the annealing temperature. Graded regions with different compositions could not be detected in the direction of depth.
Characterization of an individual Si thin layer buried in GaAs (001) using Raman spectroscopy
Authors: Tanino H. , Amano S. , Kawanami H. , Matsuhata H.
Published: July 2002

We present a Raman scattering study of a 10-layer-thick Si film grown epitaxially on GaAs (001) and covered by GaAs. The downshift of the optical phonon mode of Si is described in terms of the planar biaxially tensile strains in the Si thin layer. The broadening to the lower-energy side is explained by the confinement effect. The layer is grown pseudomorphic on GaAs with partial lattice relaxation.
Attenuation length measurements in lead arachidate Langmuir Blodgett films
Authors: Sastry Murali , Ganguly P. , Badrinarayanan S. , Mandale A. B. , Sainkar S. R. , Paranjape D. V. , Patil K. R. , Chaudhary S. K.
Published: July 2002

In this paper the attenuation length measurements in lead arachidate [(C19H39COO)2Pb] Langmuir Blodgett films deposited on copper was reported on. A discrete layer model appropriate to the ordered structure of these built up films is applied to the measurements of the x-ray photoemission intensity variation with electron takeoff angle and attenuation lengths for Pb 4f7/2, O 1s and Cu 2p3/2 electrons (kinetic energy range 550–1350 eV) are determined. There is evidence for the dependence of the attenuation length on the dielectric properties of the film/substrate interface through a variation of the film thickness.
Numerical simulation of magnetic liquid-encapsulated Czochralski growth of gallium arsenide
Authors: Chan Y. T. , Grubin H. L.
Published: July 2002

The present study involves the development and application of numerical techniques to simulate a liquid-encapsulated Czochralski growth chamber. The equations solved are the conservation equations for the mass, momentum, and energy for the melt and encapsulant. For the crystal, crucible liner, and support, the energy conservation equation is solved. The shapes of the melt-crystal interface, meniscus and crystal diameter are determined through solutions to the differential equations which are derived from the kinematic conditions at the specific interface, and are not assumed a priori. Calculations have shown that the influence of fluid motion of the encapsulant upon the temperature gradient along the crystal surface cannot be ignored even if the system is operating with a dc magnetic field. This is because the encapsulant, boric oxide, is not electrically conductive, so that the magnetic field does not inhibit convection. Depending on the depth of the melt, a 0.2-T magnetic field may or may not be strong enough to completely suppress the bulk melt motion. The locations of the flow vortices have direct influence on the shape of the melt-crystal interface. In the present studies, simulations have been performed for a process time of about 2 h. In these calculations, the crucible translating rate at each time step is calculated based on the global mass balance condition while the heater power is maintained constant. Calculations were also performed to investigate a technique to control the crystal diameter by adjusting the crucible translating rate. It was found that the crucible translating rate is a slow response parameter but if used in an increase/decrease sequence, it can be an efficient way to control the size of the crystal.
Effect of high current density and doping concentration on the characteristics of GaAs/AlAs vertically integrated resonant tunneling diodes
Authors: Park B. G. , Wolak E. , Harris J. S.
Published: July 2002

The influence of high current density and doping concentration on the current-voltage (I-V) characteristics of vertically integrated resonant tunneling diodes (VIRTDs) is experimentally determined. Room-temperature peak current densities as high as 2.7×104 A/cm2 and first and second peak-to-valley ratios of 3.6 and 2, respectively, are achieved in VIRTDs with 6-monolayer (ML) (17 Å) barrier RTDs and 600 Å separation between them. Symptoms of degradation in the I-V characteristics of these devices, which are attributed to insufficient longitudinal momentum relaxation in the region between RTDs, turn into a serious problem when the separation between RTDs is decreased to 500 Å. Through the variation of doping in the separation region, higher doping (3×1018 cm−3) between RTDs is proposed to remedy this problem and demonstrated to be quite effective in restoring the I-V characteristics.