bluuebunny/crossref_metadata_2025 huggingface dataset:
Effects of in situ doping from B2H6 and PH3 on hydrogen desorption and the low-temperature growth mode of Si on Si(100) by remote plasma enhanced chemical vapor deposition
Authors: Doris B. , Fretwell J. , Erskine J. L. , Banerjee S. K.
Published: July 2002

We demonstrate that di- and monohydride desorption peak temperatures are shifted lower for boron-doped films and higher for phosphorus-doped films compared to intrinsic Si(100). This observation is exploited to show that the shifts in di- and monohydride desorption peak temperatures with doping are accompanied by shifts in the growth mode transition temperatures, with one exception which is discussed. This work suggests that dihydrides lead to breakdown of epitaxial growth while monohydrides promote three-dimensional epitaxial growth.
Hole trap generation by thermal treatment of nitrogen doped p-type ZnSe on GaAs characterized by deep level transient spectroscopy
Authors: Hellig K. , Prösch G. , Behringer M. , Fehrer M. , Beyer R. , Burghardt H. , Hommel D. , Zahn D. R. T.
Published: July 2002

The influence of a thermal treatment on hole trap generation in nitrogen doped ZnSe on GaAs was investigated by means of deep level transient spectroscopy. The samples were grown by molecular beam epitaxy (MBE) on highly zinc doped p-type GaAs(001) substrates. p-type doping of the ZnSe epilayer was performed by applying a rf nitrogen plasma during MBE growth. In order to carry out electrical measurements Schottky contacts were prepared by evaporating gold (Au) on top of the ZnSe. A dominant hole trap with a thermal activation energy of about 0.65 eV, showing a strong temperature dependence of the hole capture cross section, was determined. It was found that the concentration of this trap increases by more than one order of magnitude after an annealing of the Au/ZnSe/GaAs samples at 550 K and that it is responsible for hole compensation effects.
Dislocation-free InSb grown on GaAs compliant universal substrates
Authors: Ejeckam F. E. , Seaford M. L. , Lo Y.-H. , Hou H. Q. , Hammons B. E.
Published: July 2002

An innovative compliant GaAs substrate was formed by wafer bonding a 30 Å GaAs layer to a bulk GaAs crystal with a large angular misalignment inserted about their common normals. InSb epitaxial layers, which is about 15% lattice mismatched to GaAs, have been grown on both compliant substrates and conventional GaAs substrates. Transmission electron microscopy studies showed that the InSb films grown on the compliant substrates have no measurable threading dislocations, whereas the InSb films on the conventional GaAs substrates exhibited dislocation densities as high as 1011 cm−2. The observations made here suggest that the defect-free heteroepitaxial growth of exceedingly large lattice-mismatched crystals can be achieved with compliant universal substrates.
Metallic single electron devices fabricated using a multilayer technique
Authors: Weimann Th. , Wolf H. , Scherer H. , Niemeyer J. , Krupenin V. A.
Published: July 2002

Single electron tunneling (SET) transistors were fabricated for the electrodes and for the island using two different resist masks. This technology overcomes disadvantages of the well-established shadow evaporation method usually used for the fabrication of SET devices and offers more flexibility in materials and layouts.
Giant magnetoresistance in La0.8Sr0.2FexCo1−xO3 (0.025⩽X⩽0.3)
Authors: Barman A. , Ghosh M. , Biswas S. , De S. K. , Chatterjee S.
Published: July 2002

The electrical resistance and magnetoresistance of La0.8Sr0.2FexCo1−xO3 (0.025⩽X⩽0.3) have been studied at low temperatures down to 1.5 K and magnetic fields up to 7.5 T. The large magnetoresistance at high and low temperature has been found. The magnetoresistance exhibits a broad minima in the temperature interval between 150 and 50 K for all the compositions. The large magnetoresistance and its broad minima may be interpreted as the interplay of spin state transition, Jahn–Teller distortion, and orbital ordering of Co ions.
Magnetic domains of cobalt ultrathin films observed with a scanning tunneling microscope using optically pumped GaAs tips
Authors: Suzuki Y. , Nabhan W. , Tanaka K.
Published: July 2002

We performed scanning tunneling microscope experiments with GaAs tips under illumination by circularly polarized light in ultrahigh vacuum at room temperature. Ferromagnetic ultrathin cobalt layers with perpendicular magnetization were employed as test samples. The spin-polarized tunneling current was shown to exhibit a magnetic sensitive component, changing its sign from one place to another on the sample surface. Reproducible images exhibiting magnetic domains of a 300–500 nm characteristic size were obtained. These results agree with magnetic force microscope measurements performed on a typical sample.
Reduction of flux creep by heat pulses
Authors: Levin G. A. , Almasan C. C. , Gajewski D. A. , Maple M. B.
Published: July 2002

We investigated the possibility of reducing the giant flux creep in high-temperature superconductors by temporarily increasing the temperature T above the operating temperature T0, after the critical state is created at T0. This T increase leads to a supercritical state which decays rapidly so that, when T is decreased back to T0, the superconductor is in the subcritical state. We have found that both the maximum persistent current and the relaxation rate are hysteretic and differ substantially from the corresponding quantities measured after flux annealing.
Uniform multiatomic step arrays formed by atomic hydrogen assisted molecular beam epitaxy on GaAs (331) substrates
Authors: Schönherr Hans-Peter , Fricke Jörg , Niu Zhichuan , Friedland Klaus-Jürgen , Nötzel Richard , Ploog Klaus H.
Published: July 2002

Coherently aligned multi-atomic step arrays are naturally formed during molecular beam epitaxy (MBE) of GaAs/(AlGa)As heterostructures on GaAs (331) substrates. The step height systematically increases with the substrate temperature while the lateral periodicity remains almost unchanged. With atomic hydrogen irradiation the step height is larger by more than a factor of 2 compared to that in conventional MBE which is attributed to a larger surface migration length of adatoms. The higher uniformity in atomic hydrogen assisted MBE allows the formation of step arrays, 12–13 nm high with a lateral periodicity around 250 nm, and straight step edges over 10 μm length. The step arrays reveal a strong influence on the electron transport of Si-modulation-doped GaAs/(AlGa)As heterostructures with the conductivity parallel to the step edges at cryogenic temperatures more than one order of magnitude larger than that perpendicular to the steps.
Microstructure and hard magnetic properties of nanocomposite Sm2Fe15Ga2Cx permanent magnets with an excess of Fe prepared directly by melt spinning
Authors: Cheng Zhao-hua , Zhang Jun-xian , Guo Hui-qun , van Lier J. , Kronmüller H. , Shen Bao-gen
Published: July 2002

The nanocomposites Sm–Fe–Ga–C with low Sm contents have been prepared directly by melt spinning without a subsequent heat treatment. A highly isotropic remanence to saturation magnetization ratio of 0.6–0.7 and a relatively high coercivity of 5.5 kOe were achieved from the as-quenched ribbons. X-ray diffraction and thermomagnetic analyses show that the as-quenched ribbons consist of a magnetically hard phase Sm2Fe15Ga2Cx and a magnetically soft phase α-Fe. Transmission electron microscopy observation demonstrates that the major 2:17-type phase has larger crystallites and the crystallites of the minor phase α-Fe are smaller and located separately at the grain boundaries of the major phase. The remanence enhancement is attributed to the exchange coupling between the intergrains. Both microstructure and magnetic properties are found to depend sensitively on the substrate velocity. The effect of the microstructure on hard magnetic properties has been discussed.
Boron segregation in As-implanted Si caused by electric field and transient enhanced diffusion
Authors: Chang R. D. , Choi P. S. , Kwong D. L. , Wristers D. , Chu P. K.
Published: July 2002

Boron segregation in an implanted arsenic profile in Si during annealing was investigated under various annealing conditions. It was found that both the implant damage created by arsenic implantation and arsenic deactivation enhance the diffusion of the embedded boron layer toward the shallow As implanted profile. The segregation phenomenon was observed in both 650 °C furnace annealed (FA) and 1000 °C rapid thermally annealed (RTA) samples. For the 650 °C FA sample, the boron segregation peak was located at the junction formed by implanted As, where residual dislocation loops at the original amorphous/crystalline (a/c) interface were also observed. However, no a/c interface dislocation loops were found to be present for the RTA samples. Additional anomalous boron segregation was observed for the 1000 °C RTA+750 °C FA samples. The additional boron segregation is not correlated with defect layers. It is, therefore, concluded that the anomalous boron segregation is caused by the electric field resulting from the formation of a p-n junction.
Unusual polarization dependent optical erasure of surface relief gratings on azobenzene polymer films
Authors: Jiang X. L. , Li L. , Kumar J. , Kim D. Y. , Tripathy S. K.
Published: July 2002

Direct formation of surface relief gratings at modest intensities on azobenzene polymers has recently been accomplished. Unusual optical erasure of these surface relief gratings was observed. It is found that the optical erasure is dependent on the polarization state of the erasing beam as well as that of the recording beams used to form the gratings. Thus, gratings when formed memorize the polarization states which created them and influence their erasure behaviors accordingly. Direct transfer of patterns by photoprinting through a phase mask has been achieved.
Thermal stability of the negative electron affinity condition on cubic boron nitride
Authors: Loh Kian Ping , Nishitani-Gamo Mikka , Sakaguchi Isao , Taniguchi Takashi , Ando Toshihiro
Published: July 2002

We have verified that the condition of negative electron affinity (NEA) exists on hydrogen-terminated polycrystalline cubic boron nitride (c-BN) grown by the high-pressure high-temperature method using ultraviolet photoelectron spectroscopy. The NEA condition is thermally stable to 950 °C. At higher temperatures, the surface reverts to a positive electron affinity condition due to the desorption of surface bound hydrogen. Repeated annealing at high temperatures results in the degradation of the surface crystallinity, which manifests in the growth of a π→π* feature attributable to sp2-type bonds. Complete regeneration of initial valence band features and NEA conditions along with the suppression of the π→π* features can be achieved by subjecting the surface to atomic hydrogen etching. It is discovered that the He (II)-excited valence band spectra of c-BN display significant differences between the hydrogen-terminated and hydrogen-free surface.
Helicon plasma deposition of a TiO2/SiO2 multilayer optical filter with graded refractive index profiles
Authors: Wang Xinrong , Masumoto Hiroshi , Someno Yoshihiro , Hirai Toshio
Published: July 2002

Thirty one layer TiO2/SiO2 optical filters with graded refractive index profiles were fabricated by helicon plasma sputtering at room temperature. Multilayer films were deposited on glass (BK7) and Si (100) substrates simultaneously and sequentially. The measured transmittance spectrum exhibited a reflectance of 99.8% at a central wavelength of 730 nm and high transmittance over the wavelength region outside of the reflected band as a result of the suppression of the sidelobes. The experimental transmittance spectrum corresponded almost completely with that calculated based on the optical multilayer film theory and using the measured refractive indices of TiO2, SiO2 and TiO2–SiO2 composite films. Transmission electron microscopic observations confirmed the expected microstructure of the filter.
Nonlinear electron transport characteristics in ultrathin wires of recrystallized hydrogenated amorphous silicon
Authors: Ng V. , Ahmed H. , Shimada T.
Published: July 2002

Electron transport in nanowires of hydrogenated amorphous silicon recrystallized by electron beam annealing has been studied. Evidence for single electron effects with I–V characteristics at low temperature has been presented. The region of the nonlinear I–V characteristics is modeled as a hopping conduction between limited number of trapping sites in amorphous regions or at grain boundaries with Coulomb blockade effect. Transmission electron microscopy is used to support this hypothesis.
Elimination of spectral shifts associated with tip-induced band bending in scanning tunneling spectroscopy of lightly doped silicon
Authors: Lin Hai-An , Jaccodine Ralph J. , Freund Michael S.
Published: July 2002

It has been demonstrated in this letter that spectral shifts arising from the tip-induced band bending on the lightly doped silicon can be eliminated by forming an accumulation layer in p-type silicon or an inversion layer in n-type silicon by using a Pt–Ir tip. Illumination is also required for n-type silicon in order to eliminate shifts associated with deep depletion caused by tunneling leakage currents. Using the approaches described herein, energy gaps of approximately 1.1 eV are determined for both p-type and n-type silicon. Furthermore, identical bias polarity is observed in current–voltage curves for both n-type and p-type silicon, and can be explained by the direction of the band bending induced by Pt–Ir on lightly doped samples. These results suggest that scanning tunneling spectroscopy can be used to reveal various features associated with surface states and bulk properties in lightly doped samples by using high work function metals such as Pt–Ir in place of lower work function metals such as W.
High-frequency permeability of (Fe, Co, Ni)62Nb8B30 amorphous alloys with a wide supercooled liquid region
Authors: Itoi Takaomi , Inoue Akihisa
Published: July 2002

The increase in B content to 30 at. % for Fe–Co–Ni–Nb–B amorphous alloys caused an extension of a supercooled liquid region before crystallization, and a wide supercooled liquid region greater than 80 K was obtained in a wide composition range of 5–42 at. % Co and 0–10 at. % Ni for Fe62−x−yCoxNiyNb8B30 amorphous alloys. High B-containing amorphous alloys with low Ni content below 10 at. % have high electrical resistivity above 230 μΩ cm and exhibit good soft magnetic properties, i.e., a low coercive force of 2 A/m, low saturated magnetostriction of 2–7×10−6, and high permeability of 21 000–29 300 at 1 kHz and 5000–7500 at 1 MHz. The high-frequency permeability characteristics at 1 MHz are much better than those for any kind of soft magnetic materials, and have been presumed to result from the decrease in eddy current loss caused by the increase in electrical resistivity. The synthesis of the Fe–Co–Nb–B amorphous alloys exhibiting a wide supercooled liquid region greater than 80 K and high-frequency permeability of 7500 at 1 MHz is promising for future development as a type of soft magnetic material.
Fabry–Perot cavity chemical sensors by silicon micromachining techniques
Authors: Han Jaeheon
Published: July 2002

Micromachined Fabry–Perot microcavity structures filled with polymeric layers composed of poly(3-dodecylthiophene) have been fabricated and studied for use as chemical sensors. The polymer-filled microcavity devices show reversible sensing behavior in response to the exposure of molecular iodine. Here, the chemical dosing results in a dramatic change in the fraction of transmitted light which passes through the microcavity structure (up to 50% at 633 nm). Importantly, the Fabry–Perot microcavity structure produces a significantly larger change in transmitted light intensity compared to a single membrane structure.
Low dielectric constant Parylene-F-like films for intermetal dielectric applications
Authors: Hanyaloglu Bengi , Aydinli Atilla , Oye Michael , Aydi Eray S.
Published: July 2002

We report on the dielectric properties and thermal stability of thin polymer films that are suitable candidates for replacing silicon dioxide as the intermetal dielectric material in integrated circuits. Parylene-F-like films, (–CF2–C6H4–CF2–)n, were produced by plasma deposition from a mixture of Ar and 1,4-bis(trifluoromethyl)benzene (CF3–C6H4–CF3) discharges and characterized using infrared absorption spectroscopy, spectroscopic ellipsometry, and capacitance measurements. The dielectric constant and the magnitude of the electronic and ionic contributions to the dielectric constant were determined through capacitance measurements and Kramers–Kronig analysis of the infrared absorption data. The film’s dielectric constant ranges between 2 and 2.6 depending on the deposition conditions and the largest contribution to the dielectric constant is electronic. The films deposited at 300 °C are stable above 400 °C and further optimization could push this limit to as high as 500 °C.
Full-band Monte Carlo model of electron and hole transport in strained Si including inelastic acoustic phonon scattering
Authors: Fischer Björn , Hofmann Karl R.
Published: July 2002

Full-band Monte Carlo simulations of electron and hole transport in strained Si on Si0.7Ge0.3 have been performed with a transport model which includes a wave-vector-dependent inelastic acoustic phonon scattering rate. Only two unambiguously determined deformation potentials are needed to achieve excellent agreement with experimental drift velocity versus electric field data for unstrained Si over the very wide temperature range from 20 to 430 K. For strained Si, this model yields in-plane lattice mobilities of 3490 cm2/(V s) for electrons and 1760 cm2/(V s) for holes at 300 K. Drift velocity and energy versus electric field characteristics are given as reference for conventional device simulations. In contrast to simpler transport models, we do not find a pronounced Gunn effect at 77 K.
Ultrafast (GaIn)(NAs)/GaAs vertical-cavity surface-emitting laser for the 1.3 μm wavelength regime
Authors: Ellmers C. , Höhnsdorf F. , Koch J. , Agert C. , Leu S. , Karaiskaj D. , Hofmann M. , Stolz W. , Rühle W. W.
Published: July 2002

(GaIn)(NAs) vertical-cavity surface-emitting lasers for room-temperature emission at 1.3 μm wavelength are designed and grown by metal-organic vapor-phase epitaxy using dimethylhydrazine and tertiarybutylarsine. Room-temperature operation at wavelengths up to 1.285 μm is achieved with low optical pumping thresholds between 1.6 and 2.0 kW/cm2. Stimulated emission dynamics after femtosecond optical pumping are measured and compare favorably with results on (GaIn)As/Ga(PAs)-based structures.
In situ growth of an epitaxial CoSi2 layer on a Si (100) substrate by reactive chemical-vapor deposition using a cobalt metallorganic source
Authors: Rhee Hwa Sung , Ahn Byung Tae
Published: July 2002

Uniform epitaxial CoSi2 layers have been grown in situ on a (100) Si substrate at temperatures above 600 °C by reactive chemical-vapor deposition of cyclopentadienyl dicarbonyl cobalt, Co(η5-C5H5)(CO)2. Co-rich phases such as Co2Si and CoSi were suppressed during cobalt metallorganic chemical-vapor deposition at substrate temperatures above 500 °C. A thin carbon layer was found on the top of the epitaxial CoSi2 layer grown on the Si substrate due to incomplete decomposition of the cobalt metallorganic source and diffusion of Co into the Si substrate. In spite of the existence of a surface carbon layer, an ion channeling minimum yield, χmin, of 8% in Rutherford backscattering/channeling spectrometry has been achieved in the epitaxial layer, indicating a nearly perfect epitaxial order. The carbon pileup on the surface of the CoSi2 layer at the initial stage of Co deposition seems to play the role of a cobalt diffusion barrier, avoiding the formation of Co-rich phases.
Domain configuration in pulsed laser deposited films of rhombohedral PbZr0.65Ti0.35O3
Authors: Tyunina M. , Wittborn J. , Rao K. V. , Levoska J. , Leppävuori S. , Sternberg A.
Published: July 2002

Highly oriented perovskite ferroelectric films of rhombohedral PbZr0.65Ti0.35O3, with [001] direction normal to the substrate surface, have been produced by pulsed laser deposition on La0.5Sr0.5CoO3/MgO (100). The domains in the films were detected using atomic force microscopy, registering the electromechanical response of the films in the presence of a low ac field. We observe a direct correlation between domain configuration and the microstructural features in the as-deposited films. In the large (∼200 nm) grains, an ordered polydomain configuration with {100} domain boundaries and “puckering” of the top surface of the grains are observed. The smaller grains are found to be single domain. The observed domain configuration in our films is significantly different from the lamellar pattern reported recently.
Nitrogen incorporation kinetics in metalorganic molecular beam epitaxy of GaAsN
Authors: Jin C. , Qiu Y. , Nikishin S. A. , Temkin H.
Published: July 2002

Epitaxial layers of GaAsN were grown on GaAs by metalorganic molecular beam epitaxy using dimethylhydrazine, triethylgallium, and conventional arsenic sources. The nitrogen incorporation in GaAsN was studied by varying the arsenic and gallium fluxes, at growth temperatures between 430 and 500 °C. The nitrogen incorporation kinetics and growth mechanism have been modeled by assuming formation of an adduct of trimethylgallium and dimethylhydrazine. The model accounts for experimentally observed relationships between growth rates and incorporation of N into GaAsN, fluxes of Ga, As, and N, and the growth temperature.
Spectrum control of THz radiation from InAs in a magnetic field by duration and frequency chirp of the excitation pulses
Authors: Izumida Shinji , Ono Shingo , Liu Zhenlin , Ohtake Hideyuki , Sarukura Nobuhiko
Published: July 2002

The THz radiation spectrum from InAs in a magnetic field irradiated with femtosecond pulses can be controlled by varying the excitation pulse width and chirp direction of the excitation pulse. A longer excitation pulse width produces lower-frequency THz radiation. Also, positively chirped pulse excitation will generate higher power and higher frequency THz radiation.
Characterization of the silicon on insulator film in bonded wafers by high resolution x-ray diffraction
Authors: Cohen G. M. , Mooney P. M. , Jones E. C. , Chan K. K. , Solomon P. M. , Wong H-S. P.
Published: July 2002

High resolution x-ray diffraction (HRXRD) is proposed as a nondestructive tool for the characterization of the silicon on insulator (SOI) film in bonded wafers. Although the bonded stack may consist of many amorphous layers, the measured diffraction spectra only show the crystalline SOI layer, thus providing a direct measurement of the film. We have demonstrated that HRXRD is capable of accurately measuring the film thickness, the tilt of the film planes with respect to the substrate planes, and the rotation misalignment of the bonded film with respect to the carrier substrate. SOI films with thicknesses down to 30 nm were readily measured with accuracy better than 1%. It is shown that an angular separation between the layer and the substrate diffraction peaks is maintained due to an unintentional miscut which usually exists in the starting wafers used for bonding. This angular separation is unique to bonded wafers as opposed to separation by implanted oxygen (SIMOX) wafers where the layer and substrate peaks are nonseparable. Calculated diffraction spectra based on the kinematic approach showed excellent agreement with the measured diffraction.
Refractive index and hygroscopic stability of AlxGa1−xAs native oxides
Authors: Hall D. C. , Wu H. , Kou L. , Luo Y. , Epstein R. J. , Blum O. , Hou H.
Published: July 2002

We present prism coupling measurements on AlxGa1−xAs native oxides showing the dependence of refractive index on composition (0.3⩽x⩽0.97), oxidation temperature (400⩽T⩽500), and carrier gas purity. Index values range from n=1.490 (x=0.9, 400 °C) to 1.707 (x=0.3, 500 °C). The oxides are shown to adsorb moisture, increasing their index by up to 0.10 (7%). Native oxides of AlxGa1−xAs (x⩽0.5) have index values up to 0.27 higher and are less hygroscopic when prepared with a small amount of O2 in the N2+H2O process gas. The higher index values are attributed to a transition from a hydroxide to a denser (AlxGa1−x)2O3 oxide phase.
Subpicosecond carrier dynamics in low-temperature grown GaAs on Si substrates
Authors: Kadow C. , Fleischer S. B. , Ibbetson J. P. , Bowers J. E. , Gossard A. C.
Published: July 2002

This letter describes time-resolved differential reflection measurements on low-temperature grown GaAs on (100) Si substrates. The carrier recombination depends sensitively on growth and anneal conditions. The differential reflectance signals of samples annealed at 600 °C are dominated by an exponential subpicosecond transient, which can be as short as 370 fs. Optical microscopy and atomic force microscopy show that the films are comparably smooth or smoother than other GaAs material grown on Si. X-ray diffraction indicates tensile strain in the films, which is explained by the different thermal expansion coefficients of GaAs and Si.
Molecular organic light-emitting diodes with temperature-independent quantum efficiency and improved thermal durability
Authors: Murata Hideyuki , Merritt Charles D. , Inada Hiroshi , Shirota Yasuhiko , Kafafi Zakya H.
Published: July 2002

We demonstrate efficient and thermally stable molecular light-emitting diodes based on a multilayered structure that consists of two hole transport layers with high glass-transition temperatures, and an emitting layer doped with a highly fluorescent dye. Stable device operation was achieved up to 90° C. By using an emitting layer doped with a highly fluorescent dye, the electroluminescence quantum efficiency reaches 2.2% and does not decrease with increasing temperature in contrast to a device with an undoped emitting layer. The luminous efficiency at 100 cd/m2 increases from 4.6 lm/W at room temperature to 7.5 lm/W at 90 °C. The half-decay time of the initial luminance (∼800 cd/m2) of the devices is 3200 h (room temperature) and 200 h (80 °C) at constant dc current (10 mA/cm2).
Lateral electric-field effects on excitonic photoemissions in InGaAs quantum disks
Authors: Gotoh Hideki , Kamada Hidehiko , Ando Hiroaki , Temmyo Jiro
Published: July 2002

Electric-field effects on excitons in zero-dimensional InGaAs quantum disks have been examined at low temperature. Photoluminescence from a single isolated disk was measured under the application of a lateral electric field by using the microphotoluminescence technique. A redshift of sharp excitonic luminescence and a decrease in its intensity under increasing electric field were observed. These were found to distinctively depend on the lateral extent of the disks: these were much more prominent in the larger disk. The exciton luminescence was found to be highly polarized along the direction of the field in the larger disk.
A full-band Monte Carlo model for the temperature dependence of electron and hole transport in silicon
Authors: Fischer Björn , Hofmann Karl R.
Published: July 2002

A full-band Monte Carlo transport model for silicon is presented that achieves excellent quantitative agreement with the temperature, field, and crystal direction dependences of experimental electron and hole drift velocities from 20 to 500 K. The model is based on wave-vector-dependent phonon scattering rates, for which a unique set of only two empirical deformation potentials for each carrier type has been determined from the experiments. Numerical accuracy is obtained by a variable Brillouin zone discretization. We discuss discrepancies between different experimental low-field electron mobilities at 77 K showing that the value should be 26 100 cm2/(V s) instead of the often quoted 20 800 cm2/(V s). For holes, we show that the inclusion of inelastic intravalley acoustic phonons cannot be restricted to low temperatures, but is essential for a correct transport description even at room temperature.
Strain-dependent electrical resistance of tin-doped indium oxide on polymer substrates
Authors: Cairns Darran R. , Witte Richard P. , Sparacin Daniel K. , Sachsman Suzanne M. , Paine David C. , Crawford Gregory P. , Newton R. R.
Published: July 2002

The increase in sheet resistance of indium–tin–oxide (ITO) films on polyethylene terephthalate with increasing tensile strain is reported. The increase in resistance is related to the number of cracks in the conducting layer which depends upon applied strain and film thickness. We propose a simple model that describes the finite but increasing resistance in the cracked ITO layer in terms of a small volume of conducting material within each crack.
Effect of phase transformations on the shape of the unloading curve in the nanoindentation of silicon
Authors: Domnich Vladislav , Gogotsi Yury , Dub Sergey
Published: July 2002

Silicon wafers subject to depth-sensing indentation tests have been studied using Raman microspectroscopy. We report a strong correlation between the shape of the load-displacement curve and the phase transformations occurring within a nanoindentation. The results of Raman microanalysis of nanoindentations in silicon suggest that sudden volume change in the unloading part of the load-displacement curve (“pop-out” or “kink-back” effect) corresponds to the formation of Si–XII and Si–III phases, whereas the gradual slope change of the unloading curve (“elbow”) is due to the amorphization of silicon on pressure release. The transformation pressures obtained in nanoindentation tests are in agreement with the results of high pressure cell experiments.
Effect of sp2-phase nanostructure on field emission from amorphous carbons
Authors: Ilie A. , Ferrari A. C. , Yagi T. , Robertson J.
Published: July 2002

Electron field emission from amorphous carbon is found to depend on the clustering of the sp2 phase. The size of the sp2 phase is varied by thermal annealing and it dominates the effect of other parameters, such as chemical composition, surface termination, sp3 content, or conductivity. The optimum size of the sp2 phase is determined by Raman spectroscopy and is of the order of 1 nm. The field emission originates from the sp2 regions and is facilitated by the large field enhancement from more conductive sp2 clusters in an insulating sp3 matrix.
Comparison of Si doping effect in optical properties of GaN epilayers and InxGa1−xN quantum wells
Authors: Oh Eunsoon , Sone Cheolsoo , Nam Okhyun , Park Hyeongsoo , Park Yongjo
Published: July 2002

Micro-photoluminescence (PL) spectra of Si-doped GaN epilayers and three-period In0.1Ga0.9N/In0.02Ga0.98N:Si quantum-well (QW) structures were studied and compared with macro-PL spectra. The shift of the macro-PL peak with increasing Si concentration was found to be similar to that with increasing excitation density in both GaN:Si and InxGa1−xN QWs. Also, it was observed that the macro-PL intensity increased with increasing Si concentration in GaN:Si and InxGa1−xN QWs, but the micro-PL intensity was independent of doping concentration. These results indicate that the changes of PL spectra with Si doping are mainly due to the increase of carriers.
Conductivity and Hall-effect in highly resistive GaN layers
Authors: Kordoš P. , Javorka P. , Morvic M. , Betko J. , Van Hove J. M. , Wowchak A. M. , Chow P. P.
Published: July 2002

Highly resistive GaN layers grown by molecular beam epitaxy are characterized by temperature dependent conductivity and Hall effect measurements. Samples with ρ300≅3×103 Ω cm show room temperature Hall mobility of 22 and 35 cm2 V−1 s−1 and have a temperature dependence μH∼Tx with x=0.9 and 0.5. This is in contradiction to a sample with ρ300≅32 Ω cm which has a room temperature mobility of 310 cm2 V−1 s−1 and a μH∼Tx with x=−1.4. The same activation energy of 0.23 eV, attributed to donor-like defects, is found for all three samples investigated. Temperature dependent conductivity data can be reasonably fitted considering band conduction. The presence of various hopping mechanisms is discussed.
Smeared heat-kernel coefficients on the ball and generalized cone
Authors: Dowker J. S. , Kirsten Klaus
Published: July 2002

We consider smeared zeta functions and heat-kernel coefficients on the bounded, generalized cone in arbitrary dimensions. The specific case of a ball is analyzed in detail and used to restrict the form of the heat-kernel coefficients An on smooth manifolds with boundary. Supplemented by conformal transformation techniques, it is used to provide an effective scheme for the calculation of the An. As an application, the complete A5/2 coefficient is given.
Modular localization of elementary systems in the theory of Wigner
Authors: Ramacher Pablo
Published: July 2002

Starting from Wigner’s theory of elementary systems and following a recent approach of Brunetti, Guido, and Longo also taken up by Schroer we define certain subspaces of localized wave functions in the underlying Hilbert space with the help of the theory of modular von-Neumann algebras of Tomita and Takesaki. We characterize the elements of these subspaces as boundary values of holomorphic functions in the sense of distribution theory and show that the corresponding holomorphic functions satisfy the sufficient conditions of the theorems of Paley–Wiener–Schwartz and Hörmander.
Hyperspherical theory of anisotropic exciton
Authors: Muljarov E. A. , Yablonskii A. L. , Tikhodeev S. G. , Bulatov A. E. , Birman Joseph L.
Published: July 2002

A new approach to the theory of anisotropic exciton based on Fock transformation, i.e., on a stereographic projection of the momentum to the unit four-dimensional (4D) sphere, is developed. Hyperspherical functions are used as a basis of the perturbation theory. The binding energies, wave functions and oscillator strengths of elongated as well as flattened excitons are obtained numerically. It is shown that with an increase of the anisotropy degree the oscillator strengths are markedly redistributed between optically active and formerly inactive states, making the latter optically active. An approximate analytical solution of the anisotropic exciton problem taking into account the angular momentum conserving terms is obtained. This solution gives the binding energies of moderately anisotropic exciton with a good accuracy and provides a useful qualitative description of the energy level evolution.
Highly accurate smoothing filter
Authors: Flores-Llamas H.
Published: July 2002

An iterative filter for the smoothing of degraded line shapes with Gaussian noise, and a signal to noise ratio defined in a wide interval 20–∞ is presented. The smoothed line shape shows errors below 2% in its height and width parameters with regard to the true values used in the simulation. The filter reduces the Gaussian noise to less than a half. In the tests carried out with a PC computer, the new filter proved that the original line shape for a linewidth of 5–30 is better recovered than the Savitsky–Golay filter. The application of the procedure only involves the use of the linewidth parameter of the degraded peak which simplifies the smoothing process. The filter follows two basic operations: first, an oversmoothing is applied to the degraded line and second, an iterative enhancement on the oversmoothing spectrum is made. The resulting spectrum of these two operations is compared with the degraded line by a least squares fitting method. When the filter is used in a simplified version, it is useful not only to determine very overlapped lines in a degraded spectrum, but also to calculate the background in an experimental spectrum.
A liquid nitrogen immersion cryostat for optical measurements
Authors: Andrews Steven S. , Boxer Steven G.
Published: July 2002

A liquid nitrogen immersion cryostat has been developed for optical measurements. Liquid nitrogen in the sample chamber is kept free of bubbles and schlieren by the use of a cooling jacket of sacrificial liquid nitrogen, which is kept at a lower pressure than the sample chamber. This design has proven especially useful for volatile liquid samples, where direct electrical connections, rapid sample freezing, and efficient heat dissipation are required, but it can also accommodate a wide variety of other samples. The cryostat is compact, easy to use, and relatively inexpensive.
A class of colliding waves in metric-affine gravity, nonmetricity and torsion shock waves
Authors: Macı́as Alfredo , Lämmerzahl Claus , Garcı́a Alberto
Published: July 2002

By using our recent generalization of the colliding waves concept to metric–affine gravity theories, and also our generalization of the advanced and retarded time coordinate representation in terms of Jacobi functions, we find a general class of colliding wave solutions with fourth degree polynomials in metric-affine gravity. We show that our general approach contains the standard second degree polynomial colliding wave solutions as a particular case.
Colored solutions of the Yang–Baxter equation from representations of Uqgl(2)
Authors: Luan Pi-Gang , Lee H. C. , Zhang R. B.
Published: July 2002

We study the Hopf algebra structure and the highest weight representation of a multiparameter version of Uqgl(2). The Hopf algebra maps of this algebra are explicitly given. We show that the multiparameter universal ℛ matrix can be constructed directly as a quantum double intertwiner without using Reshetikhin’s twisting transformation. We find there are two types highest weight representations for this algebra: type a corresponds to the qeneric q and type b corresponds to the case that q is a root of unity. When applying the representation theory to the multi-parameter universal ℛ matrix, both standard and nonstandard colored solutions of the Yang–Baxter equation are obtained.
A new stretching apparatus for applying anisotropic mechanical strain to bone cells in-vitro
Authors: Grabner B. , Varga F. , Fratzl-Zelman N. , Luegmayr E. , Glantschnig H. , Rumpler M. , Tatschl A. , Fratzl P. , Klaushofer K.
Published: July 2002

Bone is adapting to in-vivo loading by modeling and remodeling processes. The sensors of the external forces acting on the bone matrix seem to be the bone cells. Osteocytes, osteoblasts, and bone lining cells have been shown to respond to mechanical forces in-vitro. In this work, we describe a new in-vitro system which applies anisotropic stress conditions to MC3T3-E1, osteoblast-like mouse calvaria derived cells. The system allows stretching of cell cultures under well-defined stretching conditions. Cells are grown on an elastic polyurethane culture support (PUCS) that is subjected to uniaxial tensile stress using a direct current (dc) motor-driven linear positioning stage, situated within the incubator. The physical stretching parameters, the maximum elongation of the PUCS (the maximum strain applied to the cells), the strain rate, and the number of cycles, can be varied. First, the actual strains occurring at different locations of the PUCS were determined using optical methods. The surface strain appeared to be uniform over the PUCS and biaxial with a Poisson contraction nearly 80% in magnitude to the axial extension. Second, we tested the behavior of the MC3T3-E1 cells on PUCS compared to the cells grown in petridishes (PD). After 11 days of culture, cell number per dish on PUCS was significantly reduced to PD cultures (20% of control). At that time, cultures on PUCS reached confluency as compared to day 4 for the PD cultures. However, histochemical staining of alkaline phosphatase (ALP) and multilayer formation of the PUCS cultures appeared to be not significantly different from PD cultures. We also looked at the cytoskeleton by phalloidin staining, at vinculin, a protein of the cell–matrix and cell–cell interaction, and at fibronectin, a protein of the extracellular matrix using immuno staining methods. All these features tested so far seemed not to be different in cells cultured on PUCS compared to cultures in PD. Third, the responsiveness to the external force was tested using confluent cells on PUCS. A strain of 6.8 millistrain (6800 microstrain) was applied to the cells, using a strain rate of 4.9 millistrain/s and 350 cycles/h for a period of 48 h. These loading conditions led to significantly decreased cell proliferation, as measured by [3H] deoxythymidine ([3H] dT) incorporation, and significantly increased ALP activity. These data show that the stretching device introduced in this paper offers new possibilities to study the response of osteoblast-like cells to anisotropic forces.
Easy-to-use unglued tip replacement near-field optical microscope with piezoelectric shear force detection
Authors: Mulin D. , Vannier C. , Bainier C. , Courjon D. , Spajer M.
Published: July 2002

A new nonoptical shear force detection for scanning near-field optical microscopes is proposed in this article. Its main characteristic is a simple and fast tip replacement. The probe is mechanically in contact with a piezoelectric plate of a homemade tuning fork, ensuring a direct coupling and avoiding the fiber tip gluing.
Low-cost setup for generation of 3 GHz frequency difference phase-locked laser light
Authors: Buchkremer F. B. J. , Dumke R. , Buggle Ch. , Birkl G. , Ertmer W.
Published: July 2002

We have devised an all-optical setup for the generation of two phase-locked laser fields with a frequency difference of 3 GHz using only standard optics and two acousto-optical frequency shifters, that are operated at 253 MHz in sixtupel pass. The spectral width of the beat frequency is measured to be 300 Hz (full width at half maximum) limited by the resolution bandwidth of the spectrum analyzer. We routinely obtain an overall efficiency of more than 15% and demonstrate that the frequency shifted light can be further amplified by injecting it into additional “slave” lasers. This setup provides a low-cost alternative over conventional methods to generate laser fields with difference frequencies in the GHz domain, as for example, used in laser spectroscopy, laser cooling and trapping, and coherent manipulation of atomic quantum states.
Three particle description of H-inert gas interaction
Authors: Patil S. H.
Published: July 2002

We have analyzed the H-inert gas atom pair as a three particle system, an electron, a proton, and an inert gas atom. The interaction of the electron and the proton with the inert gas atom is described by a model potential with polarizability and nonadiabatic terms. This potential allows us to obtain reliable values for the energies at different separations, in particular dissociation energy at equilibrium separation. It also provides an alternate approach to the usual two body analysis in terms of van der Waals type of potential, and an insight into the physical structure of the system.
Pseudosolid nuclear magnetic resonance approach to poly(ethylene-oxide) chain dynamics in the melt
Authors: Guillermo Armel , Addad Jean-Pierre Cohen , Bytchenkoff Dimitri
Published: July 2002

Residual spin–spin interactions of protons attached to highly entangled chains in molten polymers give rise to a time reversal effect detected from solidlike spin–echoes formed from the transverse magnetization. The quantitative analysis of such pseudosolid spin–echoes, observed on molten poly(ethylene-oxide), reveals that the transverse relaxation curve is the product of two contributions: MxR(t), mainly sensitive to the existence of a temporary network and ΦR(t) arising from fast anisotropic segmental motions which give rise to residual spin–spin interactions. It is shown that the analysis provides a suitable method for distinguishing the two components from each other. The molecular weight was varied over the range 12–450 K. The description of MxR(t) is based on the assumption that there exists two stochastically independent effects. In accordance with a previous study [J. P. Cohen Addad and A. Guillermo, J. Chem. Phys. 111, 7131 (1999)], the first process is interpreted in terms of exponential relaxation modes resulting from the partition of one chain into Gaussian submolecules. In addition to the effect of long-range fluctuations on the magnetization, an orientational memory effect is introduced along the chain. The proposed relaxation function accounts for the very specific shapes of both the experimental curves and of the ln(MxR(t))/t plots; the minimum number of parameters required to describe such complex curves is 4. The analysis provides a coherent set of numerical values: the mean square spin–spin interaction and the correlation time τs, assigned to one submolecule are equal to 5×105 (rad s−1)2 and 0.002 s, respectively. Proton relaxation rates of end submolecules (≈70 s−1) and of short free chains (12 K) in the melt (≈20 s−1) have about the same order of magnitude. © 2000 American Institute of Physics.
Huge differences between low- and high-angle twist grain boundaries: The case of ultrathin (001) Si films bonded to (001) Si wafers
Authors: Rouviere J. L. , Rousseau K. , Fournel F. , Moriceau H.
Published: July 2002

Ultrathin (001) Si films bonded onto (001) Si wafers, inducing grain boundaries with twist angles varying from 0.5° to 12°, were studied by transmission electron microscopy. A great structural difference between low (ψ<5°) and high (ψ>6°) twist angles was observed. In low twist angle grain boundaries, “twist interfacial dislocations” are dissociated and produce rough interfaces with no oxide precipitates. It is the opposite in high-angle grain boundaries: there is no dissociation, the interfaces are smoother but contain oxide precipitates. These differences are not attributed to the thin thickness of one grain, but to the large atomic differences between high- and low-angle twist grain boundaries, which is not the case for tilt grain boundaries.
Diffusion of small probe molecule in oligomers
Authors: Park Ha Seon , Chang Taihyun , Lee Song Hi
Published: July 2002

Diffusion of methyl yellow (MY) in the oligomeric host of n-alkanes and n-alcohols was studied by forced Rayleigh scattering as a function of the molecular weight and the viscosity of the medium. We observe that the diffusivity of the probe molecule follows a power law dependence on the molecular weight of the oligomers, DMY∼M−α well. As the molecular weight of the oligomers increases, the exponent α shows a sharp transition from 1.88 to 0.91 near docosane (C22) in n-alkanes and from 1.31 to 0.60 near 1-hexadecanol (C16OH) in n-alcohols at 45 °C. A similar transition is also found in the molecular-dynamics simulation for the diffusion of a Lennard-Jones particle of a size similar to MY in n-alkanes. This transition deems to reflect a change of the dynamics of oligomeric chain molecules that the motion of the segments, not the entire molecules, becomes responsible for the transport of the probe molecule as the molecular weight of the oligomer increases. However, a weak but finite molecular weight dependence is observed above the transition point. Since it is well established that this molecular weight dependence disappears at high molecular weight medium, this indicates that the molecular weight of the oligomers is not high enough and the motion of the entire chain molecule is still correlated with the probe motion to some extent. The transition points in n-alkane and n-alcohol are in good agreement with the molecular weight at which the diffusional activation energy of MY reaches the asymptotic value, ∼20 kJ/mol near n-docosane and ∼30 kJ/mol near 1-hexadecanol.