bluuebunny/crossref_metadata_2025 huggingface dataset:
Resonant tunneling of electrons of one or two degrees of freedom
Authors: Chou S. Y. , Wolak E. , Harris J. S.
Published: July 2002

Analytical expressions for the tunneling current of electrons with one or two degrees of freedom (DOF), due to additional quantum confinement transverse to the electron transport direction, are explicitly derived, analyzed, and implemented into computer simulations. The results are compared with the well-known case in which 3-DOF electrons tunnel through a one-dimensional double-barrier well. The results show that the singularity of the density of states in a one-dimensional system will not manifest sharp features in tunneling current, and that when the spacing between the Fermi energy and bottom of conduction band is the same, the tunneling current peak becomes broader and the peak-to-valley ratio becomes smaller as the number of degrees of freedom of the electrons is reduced. The results also show that when scattering is neglected, the energy quantization due to transverse confinement in 1- or 2-DOF systems will not contribute any additional peaks to the tunneling current.
Direct laser writing of superconducting patterns of Y1Ba2Cu3O7−δ
Authors: Gupta Arunava , Koren Gad
Published: July 2002

Direct argon ion laser writing of 40-μm-wide and 3-μm-thick superconducting lines of Y1Ba2Cu3O7−δ on magnesium oxide (MgO) substrate is reported. A 514.5 nm laser beam of 300 mW was focused onto a thin sprayed film of the nitrate precursors (Y:Ba:Cu=1:2:3) and scanned over it. The laser beam converted oxides along the irradiation path. The film was then ‘‘developed’’ by rinsing it in ethanol, which removed the unirradiated areas, but left the irradiated areas intact. This was followed by the standard oven annealing procedure in flowing oxygen. The resistivity of the laser-written lines as a function of temperature showed an onset and completion of the superconducting transition at 82 and 69 K, respectively. The corresponding values of the unpatterned, blanket film were 82 and 77 K, respectively.
High-speed performance of InP/In0.53Ga0.47As/InP double-heterojunction bipolar transistors
Authors: Won T. , Morkoç H.
Published: July 2002

A theoretical comparison of In0.53Ga0.47As/InP and Al0.3Ga0.7As/GaAs heterojunction bipolar transistors has been undertaken in an effort to determine the relative merits of these material systems. The analysis uses a compact transistor model and considers devices with self-aligned geometries including both extrinsic and intrinsic parameters. The high electron mobility in the In0.53Ga0.47As base layer and high peak velocity of electrons in the collector depletion layer result in a current gain cutoff frequency in excess of 150 GHz for an InP/In0.53Ga0.47As transistor with base thickness of 0.1 μm. Calculations revealed, however, that maximum oscillation frequency is strongly dependent on the contact resistance of the p-type base layer, even for a self-aligned base transistor. A maximum oscillation frequency of 138 GHz is theoretically predicted for an InP/In0.53Ga0.47As transistor with base thickness of 0.06 μm, base doping of 1×1020 cm−3, a p-type contact resistance of 1.0×10−7 Ω cm2, a current density of 5×104 A/cm2, and a VCB of 5 V.
Entropy factor of donor level in gold implanted silicon
Authors: Coffa S. , Calleri G. , Calcagno L. , Campisano S. U. , Ferla G.
Published: July 2002

Rutherford backscattering spectrometry and spreading resistance techniques have been used to determine the concentration profiles of gold implanted on and diffused in p-type silicon. The measurement of the gold concentration is possible by the solution of the charge neutrality condition and thus it requires the determination of the entropy factor X associated with the ionization of the gold donor level. The value of 28±2 compares well with the latest determination of X obtained by using rather complex techniques.
Surface-emitting second-harmonic generator for waveguide study
Authors: Vakhshoori D. , Wu M. C. , Wang S.
Published: July 2002

A novel surface emission of coherently generated second-harmonic wave is reported for the first time.The technique is used for the observation of the difference in propagation constant of the TE0 and TM0 mode of GaAs/AlGaAs waveguide cavity to a high degree of accuracy. In this technique the second-harmonic signal propagates out from the top surface of the waveguide structure, converting the modal phase difference between TE0 and TM0 modes into intensity variation along the waveguide length. The second-harmonic signal is easily observable by the naked eye, and the technique does not require wavelength tuning or mechanical movement for the measurement of birefringence.
Anisotropy dependence of the focused field radiated into a piezoelectric half-space by an interdigital transducer
Authors: Zhang Y. W. , Planat M.
Published: July 2002

Bulk waves radiated into an anisotropic half-space by an interdigital transducer are studied theoretically. The mechanical displacements of excited waves are obtained by asymptotic evaluation of an integral whose singularities give rise to both Rayleigh surface waves and angularly focused bulk waves. By applying the analysis to singly rotated Y cuts of quartz with propagation normal to the x axis, it is shown that the orientation and width of the main lobe are very sensitive to frequency and crystal orientation.
Reaction mechanisms in the organometallic vapor phase epitaxial growth of GaAs
Authors: Larsen C. A. , Buchan N. I. , Stringfellow G. B.
Published: July 2002

The decomposition mechanisms of AsH3, trimethylgallium (TMGa), and mixtures of the two have been studied in an atmospheric-pressure flow system with the use of D2 to label the reaction products which are analyzed in a time-of-flight mass spectrometer. AsH3 decomposes entirely heterogeneously to give H2. TMGa decomposes by a series of gas-phase steps, involving methyl radicals and D atoms to produce CH3D, CH4, C2H6, and HD. TMGa decomposition is accelerated by the presence of AsH3. When the two are mixed, as in the organometallic vapor phase epitaxial growth of GaAs, both compounds decompose in concert to produce only CH4. A likely model is that of a Lewis acid-base adduct that forms and subsequently eliminates CH4.
Sequential tunneling due to intersubband scattering in double-barrier resonant tunneling devices
Authors: Eaves L. , Toombs G. A. , Sheard F. W. , Payling C. A. , Leadbeater M. L. , Alves E. S. , Foster T. J. , Simmonds P. E. , Henini M. , Hughes O. H. , Portal J. C. , Hill G. , Pate M. A.
Published: July 2002

Magnetoquantum oscillations in the tunnel current of double-barrier n-GaAs/(AlGa)As/GaAs/(AlGa)As/GaAs resonant tunneling devices reveal evidence of sequential tunneling in the voltage range corresponding to the resonance when electrons tunnel into the second subband of the GaAs quantum well. The sequential tunneling arises from intersubband scattering between two quasi-bound states of the well. Near this resonance, the charge buildup in the well can be estimated from the magnetoquantum oscillations.
Deep levels in semiconducting In-alloyed bulk n-GaAs and its resistivity conversions by thermal treatments
Authors: Kitagawara Y. , Noto N. , Takahashi T. , Takenaka T.
Published: July 2002

Thermal conversions of resistivities have been studied for In-alloyed semiconducting (100–106 Ω cm) n-GaAs grown by the liquid-encapsulated Czochralski method. These dislocation-free as-grown crystals are converted into semi-insulating (>107 Ω cm) crystals by annealing at 950  °C for 2 h followed by a fast cooling. Such semi-insulating crystals can be converted further into crystals with lower resistivities (∼106 Ω cm) by treating them at 470  °C for 100 h. In the analysis of the as-grown samples by the temperature-dependent Hall measurements, four levels have been found with activation energies 0.13, 0.20, 0.42, and 0.50 eV. It has been shown that these resistivity conversions are induced by concentration changes of the deep states other than the midgap donor EL2, some of them being the levels found in this study.
n-Si/p-Si1−xGex/n-Si double-heterojunction bipolar transistors
Authors: Xu Dan-Xia , Shen Guang-Di , Willander M. , Ni Wei-Xin , Hansson G. V.
Published: July 2002

Two different structures of n-Si/p-Si1−xGex/n-Si double-heterojunction bipolar transistors have been fabricated by molecular beam epitaxy. A common emitter current gain β of about 15 was demonstrated in one kind of structure and the β-IC curve has been investigated. In the other structure, a novel multistep collector current IC vs collector-emitter voltage VCE characteristic together with a strong negative resistance behavior was observed at room temperature. In this letter the basic experiments are described; a comparison and a discussion of the two kinds of devices are presented.
Experimental characterization of two-dimensional dopant profiles in silicon using chemical staining
Authors: Subrahmanyan Ravi , Massoud Hisham Z. , Fair Richard B.
Published: July 2002

An experimental technique for the measurement of two-dimensional impurity diffusion profiles in silicon has been developed. Both the lateral and in-depth extent of dopant diffusion under a mask edge are magnified mechanically by sawing and angle lapping, and the magnified junction contour is delineated by chemical staining. The two-dimensional shape of the junction is reconstructed from the measured stained contour. A complete diffusion profile consisting of several isoconcentration contours can be obtained by measuring the junction shape on a series of samples with increasing substrate resistivities, provided the doping level in the substrate does not affect the diffusion of the impurity under study. Results of the two-dimensional diffusion of boron in silicon at 1050 °C are presented.
Experimental determination of the relation between modal gain and current density for AlGaAs single quantum well lasers grown by metalorganic vapor phase epitaxy
Authors: Hagen S. H. , van ’t Blik H. F. J. , Boermans M. J. B. , Eppenga R.
Published: July 2002

The relation between modal gain and current density has been determined for a 20 Å single quantum well in a graded-index separate-confinement heterostructure laser. The measurements were performed at 153 K in order to avoid leakage of carriers out of the wells which might cause an apparent saturation of the gain. A sublinear dependence of gain on current density has been observed, agreeing rather well with theoretical calculations taking band mixing into account.
Efficiency enhancement in quantum well lasers via tailored doping profiles
Authors: Waters R. G. , Hill D. S. , Yellen S. L.
Published: July 2002

Reduced doping concentrations proximate to the active region in single quantum well (Al)GaAs lasers have been used to improve the external quantum efficiency. A substantial enhancement is observed in a structure in which, by design, the mode overlap with the doped confining regions is large. For a structure in which the mode is tightly confined, doping modifications have no measurable effect.
Realization of high mobility in inverted AlxGa1−xAs/GaAs heterojunctions
Authors: Cho N. M. , Kim D. J. , Madhukar A. , Newman P. G. , Smith D. D. , Aucoin T. , Iafrate G. J.
Published: July 2002

We report reproducible realization of GaAs/Al0.25 Ga0.75 As(100) inverted heterojunctions with liquid-nitrogen electron mobilities in excess of 105 cm2 /(V s). This is made possible through use of reflection high-energy electron-diffraction (RHEED) intensity dynamics determined optimized growth conditions, but without the use of short-period superlattices as buffers or spacer layers.
Heteroepitaxy of GaAs on Si: The effect of i
n s
i
t
u thermal annealing under AsH3
Authors: Freundlich A. , Grenet J. C. , Neu G. , Leycuras A. , Vèrié C.
Published: July 2002

This letter shows that an in situ thermal annealing step in AsH3/H2 during the metalorganic vapor phase epitaxy of GaAs on Si(001) improves the crystalline quality. The dislocation density is reduced (below 107 cm−2) without affecting the Si diffusion across the heterointerface or the strain level in the epilayer. The nature of the various near-band-gap recombinations present in the unannealed and annealed samples is discussed in light of selective photoluminescence experiments.
Laser direct writing of aluminum conductors
Authors: Cacouris T. , Scelsi G. , Shaw P. , Scarmozzino R. , Osgood R. M. , Krchnavek R. R.
Published: July 2002

We report, for the first time, the laser direct writing of high-conductivity aluminum interconnects from dimethylaluminum hydride (DMAlH). These lines were deposited from this metalorganic gas using a focused deep-ultraviolet laser beam, and the deposition process was studied as a function of several process parameters. Electrical measurements and Auger electron spectroscopy were used to characterize the quality of the laser-deposited films.
Stimulated emission in ultrathin (20 Å) AlxGa1−xAs-GaAs single quantum well heterostructures
Authors: Lo Y. C. , Hsieh K. Y. , Kolbas R. M.
Published: July 2002

Spontaneous and stimulated emission spectra from a series of AlxGa1−xAs-GaAs single quantum well heterostructures are demonstrated for well widths as thin as 20 Å. These undoped samples, grown by molecular beam epitaxy, are the thinnest single quantum wells ever reported to support stimulated emission. Laser thresholds are generally quite low (1.2 kW/cm2) despite the fact that the single well is undoped and of dimensions which were previously thought to be too small to effectively collect excess carriers (Lz≪scattering path length). A simple model based on the spatial extent of the wave function, rather than the well width, is proposed to explain the experimental results.
New accurate exploitation of thermally stimulated ionic current curves for alkali contaminated metal-oxide-silicon structures
Authors: Choquet C. , Balland B.
Published: July 2002

Hazardous contamination may appear at different steps of metal-oxide-silicon (MOS) process. It is thus important to identify the ionic species and to evaluate their concentrations. This is often studied by the means of thermally stimulated ionic current (TSIC). But up to now TSIC curve interpretations use more or less justified approximations. A new numerical technique of TSIC curves exploitation is reported. Our method is accurate because it is not based on approximations; moreover, this one is well adapted to computer processing. We present results on K+ contaminated MOS structures. Our results are compared with those obtained by other methods.
Prediction of a large polar Kerr angle in NiUSn
Authors: Daalderop G. H. O. , Mueller F. M. , Albers R. C. , Boring A. M.
Published: July 2002

An ab initio calculation of the magneto-optic Kerr effect is reported for ferromagnetic NiUSn. The double polar Kerr angle 2Φ is predicted to be about 10° in the infrared and about 4° in the visible spectrum. It is concluded that substitutional alloys involving Clb phase materials and uranium atoms are important candidates for device applications.
Depletion charge measurements by tunneling spectroscopy GaAs-GaAlAs field-effect transistors
Authors: Smoliner J. , Gornik E. , Weimann G.
Published: July 2002

The subband energies on GaAs-GaAlAs field-effect transistor samples were measured by tunneling spectroscopy using a structure, where the tunneling process starts from an accumulation layer, and a conventional structure, where the electrons tunnel from a metal electrode into the two-dimensional electron gas. Self-consistent calculations were performed to determine the depletion charge from the measured subband energies. Furthermore, the influence of a backgate voltage was investigated both experimentally and theoretically. As far as we know, this is the first direct determination of the depletion charge in GaAs-GaAlAs field-effect transistor structures.
Superconductive aluminum/YBa2Cu3Ox composites stabilized by oxygen/fluorine donors
Authors: Chen In-Gann , Sen S. , Stefanescu D. M.
Published: July 2002

A fabrication technique for producing bulk and irregular-shaped aluminum/high Tc superconductor composites by traditional powder metallurgy method is reported. During the course of fabrication, the oxygen stoichiometry can be maintained at the desired level in order to retain superconductivity by addition of certain metal oxides or metal carbonates with low oxidation tendency, e.g., Ag2O, Ag2CO3, and CdO. Moreover, replacement of oxygen with fluorine is also possible by addition of certain unstable fluorides like AgF. Superconductivity measurements of Al/YBa2Cu3Ox random composites with additions of different oxygen or fluorine donors show a similiar or improved Tc in comparison to the pure 123 compound. The concept of oxygen/fluorine donor can also be used in other fabrication techniques of metal/123 composites to stabilize the oxygen/fluorine stoichiometry.
Study of oxygen transport in Ba2YCu3O7−δ using a solid-state electrochemical cell
Authors: O’Sullivan Eugene J. M. , Chang Bertha P.
Published: July 2002

A solid-state electrochemical oxygen concentration cell with yttria-stabilized zirconia (YSZ) as electrolyte was employed to study oxygen transport in Ba2YCu3O7−δ . On one side of the YSZ [a single-crystal wafer oriented in the (100) plane], reference and counter electrodes were fabricated from sputtered Au. A ceramic pellet of Ba2YCu3O7−δ was pressed against the other side of the electrolyte. Using this cell, it was possible to electrochemically drive oxygen into and out of the Ba2YCu3O7−δ oxide electrode under controlled conditions. Current versus time and open circuit potential recovery data were analyzed according to various models. The diffusivity of oxygen in 94% dense Ba2YCu3O7−δ was estimated to ≂5×10−8 cm2 s−1 at 550 °C.
Y-Ba-Cu-O films by rf magnetron sputtering using single composite targets: Superconducting and structural properties
Authors: Liou S. H. , Hong M. , Kwo J. , Davidson B. A. , Chen H. S. , Nakahara S. , Boone T. , Felder R. J.
Published: July 2002

High Tc superconducting Y-Ba-Cu-O films have been produced by rf magnetron sputtering using single unreacted composite targets of Y2O3, BaF2, and CuO powders. Transport measurements of the films showed a sharp resistive Tc (R=0) at 89 K and a metallic behavior of resistivity versus temperature. The films show preferred orientations when deposited on single-crystal SrTiO3 substrates (100) or (110) and followed by a post O2 annealing at 825 °C.
New 120 K Tl-Ca-Ba-Cu-O superconductor
Authors: Sheng Z. Z. , Kiehl W. , Bennett J. , El Ali A. , Marsh D. , Mooney G. D. , Arammash F. , Smith J. , Viar D. , Hermann A. M.
Published: July 2002

Electronic and magnetic properties of the new 120 K Tl-Ca-Ba-Cu-O superconductor are presented. Resistance-temperature variations, ac susceptibility, and the (low) magnetic field dependence of the critical current density are reported. The new superconductor can be prepared in a molten state, which should allow processing leading to high critical current density. Preliminary x-ray diffraction data are also presented.
Transmission electron microscopy studies of superconducting Y-Ba-Cu-O films prepared by laser deposition
Authors: Hwang D. M. , Nazar L. , Venkatesan T. , Wu X. D.
Published: July 2002

The polycrystalline structure of pulsed-laser deposited Y-Ba-Cu-O thin films was studied using transmission electron microscopy. Many grains of the superconducting films on (001) SrTiO3 substrates had the c axis normal to the surface, while grains on (110) facets had their c axis oriented preferentially along the interface. Observation of an amorphous layer of thickness ∼6 nm at some oriented grain-substrate interfaces suggests that the amorphous layer is formed subsequent to the formation of the crystalline structures. Surface ridges were found to facilitate the formation of triple points where the grain boundaries between nonepitaxial grains were pinned.
Bi-Sr-Ca-Cu-O superconductor system with critical temperatures of 80 and 107 K
Authors: Kugimiya K. , Kawashima S. , Inoue O. , Adachi S.
Published: July 2002

Critical temperatures (Tc) of 80 and 107 K are confirmed by resistivity and inductance measurements and also by the Meissner effect. X-ray/electron diffraction and electron microscopy analyses on a single phase material of 80 K show that its structure is orthorhombic with a unit cell of a=5.407 Å, b=27.011 Å, c= 30.588 Å composed of a pseudotetragonal cell of a=b=5.41 Å. The pseudotetragonal cell size and cleavage behavior to very thin flakes strongly indicate that the oxide is basically a lamellar oxide of the Aurivillius phase type, i.e., a stacked layer structure of (Bi2O2)2+ sheets and distorted perovskite cell sheets.
Plasma polymerization for high T
c oxide superconductors
Authors: Morohashi Shin’ichi , Tamura Hirotaka , Yoshida Akira , Hasuo Shinya
Published: July 2002

Plasma polymerization using CHF3 gas, which prevents the degradation of high Tc oxide superconductors due to moisture and annealing, was developed. The resistive transitions of Y1Ba2Cu3Ox thin films, coated by plasma polymerization, did not change before and after soaking in 20 °C water for 60 min. In addition, the critical temperature of those films was unchanged after annealing at 200 °C for 30 min in air, and then soaking in 90 °C water for 10 min. We confirmed that the polymer film grown by this method was dense, and the Y1Ba2Cu3Ox film reacted with the fluorine at the interface using x-ray photoelectron spectroscopy.
Preparation, patterning, and properties of thin YBa2Cu3O7−δ films
Authors: de Vries J. W. C. , Dam B. , Heijman M. G. J. , Stollman G. M. , Gijs M. A. M. , Hagen C. W. , Griessen R. P.
Published: July 2002

High TC superconducting thin films were prepared on (100) SrTiO3 substrates by dc triode sputtering and subsequent annealing. In these films Hall-bar structures having a width down to 5 μm were patterned using a reactive ion etching technique. Superconductivity above 77 K was observed. When compared with the original film there is only a small reduction in TC. The critical current density determined by electrical measurements is substantially reduced. On the other hand, the critical current density in the bulk of the grains as measured by the torque on a film is not reduced by the patterning process. It is suggested that superconductor-normal metal-superconductor junctions between the grains account for this difference.
Reactive ion beam deposition of thin films in the bismuth-calcium-strontium-copper oxide ceramic superconductor system
Authors: Harker A. B. , Kobrin P. H. , Morgan P. E. D. , DeNatale J. F. , Ratto J. J. , Gergis I. S. , Howitt D. G.
Published: July 2002

Reactive ion beam deposition has been used to grow c-axis-oriented superconducting thin films in the Bi-Ca-Sr-Cu-O (BCSCO) system around the cation ratio 1:1:1:1 on single-crystal (001) MgO. The films show a single superconducting transition with an initial onset near 85 K and a critical current of 5×104 A/cm2 at 10 K. Two different BCSCO-containing phases have been identified in the thin films: one with a tetragonal pseudo-body-centered subcell, c=24.4 Å, which is not superconducting above 28 K, and a second with c=30.6 Å, which is responsible for the superconductivity. Electron diffraction measurements on the 30.6 Å phase are consistent with those previously reported for the bulk ceramic.
Impurity-induced-disordered phase modulators in AlGaAs/GaAs quantum well and double-heterostructure waveguides
Authors: Hausken T. , Huang T. C. , Lee K. W. , Simes R. J. , Dagli N. , Coldren L. A.
Published: July 2002

Results for two new channel waveguide phase modulators formed by impurity-induced disordering (IID) with Zn diffusion in AlGaAs/GaAs are presented. One device utilizes side diffusion into ridges of single quantum well material and the other utilizes surface diffusion into double-heterostructure material. Waveguide loss for both TE and TM polarizations, and effective index steps calculated from observed mode profiles are reported. TE/TM mode conversion was observed without bias under certain conditions. Initial results for phase modulation are in agreement with expectations and appear not to be affected by the IID.
Electrically tunable Fabry–Perot mirror using multiple quantum well index modulation
Authors: Simes R. J. , Yan R. H. , Geels R. S. , Coldren L. A. , English J. H. , Gossard A. C. , Lishan D. G.
Published: July 2002

We describe a novel surface normal Fabry–Perot multiple quantum well index reflection modulator which may be tuned electrically. The Fabry–Perot étalon, composed of two AlAs/Ga1−xAlxAs quarter-wavelength grating mirrors separating a multiple quantum well GaAs/Ga1−xAlxAs active medium, is formed in a single growth by molecular beam epitaxy. Contrast ratios of up to 8:1 at 873 nm have been measured.
Local modes of vibration in proton- and deuteron-implanted InP
Authors: Tatarkiewicz J. , Clerjaud B. , Côte D. , Gendron F. , Hennel A. M.
Published: July 2002

The results of optical absorption experiments on proton- and deuteron-implanted indium phosphide are reported. It is shown that these implantations produce two types of hydrogen- (or deuterium-) related defects. These defects also exist in as-grown material not intentionally doped with hydrogen. The nature of these two defects is discussed.
Demonstration of an AlGaAs/GaAs integrable optical correlator using surface-emitting second-harmonic generation
Authors: Vakhshoori Daryoosh , Wang Shyh
Published: July 2002

A broadband optical correlator with the potential for integration is demonstrated in a GaAs/AlGaAs waveguide for the first time. The correlator uses the surface emission of a coherently generated second-harmonic signal propagating perpendicularly to the direction of propagation of the fundamental beam. While the fundamental pulse propagates back and forth in a waveguide cavity, it emits a second-harmonic signal normal to the top surface of the waveguide. The variation of this second-harmonic signal across the waveguide cavity yields the pulse width of the fundamental beam. Using this technique, a pulse width of 7 ps is measured for a 100 μW(140 mW) average (peak) power of a compressed mode-locked neodymium:yttrium aluminum garnet laser in a GaAs/AlGaAs ridge waveguide cavity. This integrable technique of correlation has the potential of measuring subpicosecond pulses.
Mechanism for ion-induced mixing of GaAs-AlGaAs interfaces by rapid thermal annealing
Authors: Kahen K. B. , Rajeswaran G. , Lee S. T.
Published: July 2002

A mechanism for the transient-enhanced interdiffusion of GaAs-AlGaAs interfaces during rapid thermal annealing of ion-implanted heterostructures is proposed. The model is based on the solution of the coupled diffusion equations involving the excess vacancies and the post-implantation Al distribution following ion implantation. Both initial distributions are obtained from the solution of a three-dimensional Monte Carlo simulation of ion implantation into a heterostructure sample. In general, the model is valid for time frames within which impurity diffusion does not occur appreciably so that impurity-enhanced diffusion remains a weak effect.
Annealing behavior of bound exciton lines in high quality CdTe
Authors: Seto S. , Tanaka A. , Masa Y. , Dairaku S. , Kawashima M.
Published: July 2002

Annealing behavior of shallow donor-bound and acceptor-bound exciton lines in CdTe crystals grown by the Bridgman method was investigated by high-resoluton photoluminescence measurements at 4.2 K. The intensity of the neutral acceptor-bound exciton line (A0X) at 1.5896 eV, which was commonly observed in p-type CdTe, drastically decreased by annealing under Cd saturated atmosphere. By further subsequent annealing under Te saturated atmosphere, the A0X line recovered again. On the other hand, the intensity of the neutral donor-bound exciton line increased by annealing under Cd saturated atmosphere and decreased by the further annealing under Te saturated atmosphere. It was found that this change in the photoluminescence lines by the annealing process was reversible. These results strongly suggest that the A0X line at 1.5896 eV can be ascribed to the recombination of excitons trapped at Cd-vacancy/donor-impurity complexes.
Threshold transport of high-mobility two-dimensional electron gas in GaAs/AlGaAs heterostructures
Authors: Jiang C. , Tsui D. C. , Weimann G.
Published: July 2002

We have measured the conductivity of the high-mobility two-dimensional electron gas (2DEG) in GaAs/AlGaAs heterostructures at low temperatures as a function of the carrier density (n) close to the conduction threshold. We find that metallic conduction persists to n∼1.4×1010 cm−2, and that the mobility drops much faster than that expected from impurity scattering for n below a characteristic density, which decreases with increasing AlGaAs spacer thickness. This behavior is explained by the effect of the 2DEG density fluctuations at low n close to the conduction threshold, first predicted by Brews’ theory [J. Appl. Phys. 46, 2181 (1975)].
Ultrafast diffusion of a defect in indium-doped silicon introduced by chemomechanical polishing
Authors: Zundel T. , Weber J. , Benson B. , Hahn P. O. , Schnegg A. , Prigge H.
Published: July 2002

The deactivation of the acceptor indium after chemomechanical polishing of p-type silicon is shown to result from the formation of a complex involving the indium atom and a positively charged, extremely fast-diffusing defect X. In the temperature range from 220 to 280 K, the dissociation frequency ν of this complex and the diffusion coefficient D of the defect X are thermally activated and satisfy the expressions ν=ν0 exp(−Ed/kT) and D=D0 exp(−Ea/kT) with ν0=2.6×1012 s−1, Ed=0.690 eV, D0=5×104 cm2/s, and Ea=0.665 eV.
Fast beam switching in surface-emitting distributed Bragg reflector laser
Authors: Kojima Keisuke , Kyuma Kazuo
Published: July 2002

A surface-emitting distributed Bragg reflector (DBR) laser with a mode control section was fabricated. By increasing the injection current into the control section, the lasing wavelength switches from the DBR mode to the Fabry–Perot mode. The lasing wavelength difference was about 13 nm, and the output beam was deflected by 3° while the beam width was kept less than 0.3°. The time required for the switching was as short as 60 ps. The maximum output power exceeded 8 mW under a cw condition at room temperature.
Shallow donors in very pure GaAs grown by gas source molecular beam epitaxy
Authors: Cunningham J. E. , Chiu T. H. , Timp G. , Agyekum E. , Tsang W. T.
Published: July 2002

We report photoluminescence and transport studies of a very pure GaAs layer grown by gas source molecular beam epitaxy methods. A peak mobility of 300 000 cm2/V s is observed at 60 K. A very shallow donor with binding energy less than 2 meV is observed.
Functional annotation of mammalian genomic DNA sequence by chemical mutagenesis: A fine-structure genetic mutation map of a 1- to 2-cM segment of mouse chromosome 7 corresponding to human chromosome 11p14-p15
Authors: Rinchik Eugene M. , Carpenter Donald A. , Johnson Dabney K.
Published: July 2002

Eleven independent, recessive,
N
-ethyl-
N
-nitrosourea-induced mutations that map to a ≈1- to 2-cM region of mouse chromosome (Chr) 7 homologous to human Chr 11p14-p15 were recovered from a screen of 1,218 gametes. These mutations were initially identified in a hemizygous state opposite a large
p
-locus deletion and subsequently were mapped to finer genomic intervals by crosses to a panel of smaller
p
deletions. The 11 mutations also were classified into seven complementation groups by pairwise crosses. Four complementation groups were defined by seven prenatally lethal mutations, including a group (
l7R3
) comprised of two alleles of obvious differing severity. Two allelic mutations (at the
psrt
locus) result in a severe seizure and runting syndrome, but one mutation (at the
fit2
locus) results in a more benign runting phenotype. This experiment has added seven loci, defined by phenotypes of presumed point mutations, to the genetic map of a small (1–2 cM) region of mouse Chr 7 and will facilitate the task of functional annotation of DNA sequence and transcription maps both in the mouse and the corresponding human 11p14-p15 homology region.
Robustness of circadian rhythms with respect to molecular noise
Authors: Gonze Didier , Halloy José , Goldbeter Albert
Published: July 2002

We use a core molecular model capable of generating circadian rhythms to assess the robustness of circadian oscillations with respect to molecular noise. The model is based on the negative feedback exerted by a regulatory protein on the expression of its gene. Such a negative regulatory mechanism underlies circadian oscillations of the PER protein in
Drosophila
and of the FRQ protein in
Neurospora
. The model incorporates gene transcription into mRNA, translation of mRNA into protein, reversible phosphorylation leading to degradation of the regulatory protein, transport of the latter into the nucleus, and repression of gene expression by the nuclear form of the protein. To assess the effect of molecular noise, we perform stochastic simulations after decomposing the deterministic model into elementary reaction steps. The oscillations predicted by the stochastic simulations agree with those obtained with the deterministic version of the model. We show that robust circadian oscillations can occur already with a limited number of mRNA and protein molecules, in the range of tens and hundreds, respectively. Entrainment by light/dark cycles and cooperativity in repression enhance the robustness of circadian oscillations with respect to molecular noise.
Twenty-first aminoacyl-tRNA synthetase–suppressor tRNA pairs for possible use in site-specific incorporation of amino acid analogues into proteins in eukaryotes and in eubacteria
Authors: Kowal Anne K. , Köhrer Caroline , RajBhandary Uttam L.
Published: July 2002

Two critical requirements for developing methods for the
site-specific incorporation of amino acid analogues into proteins
in vivo
are (
i
) a suppressor tRNA that is
not aminoacylated by any of the endogenous aminoacyl-tRNA
synthetases (aaRSs) and (
ii
) an aminoacyl-tRNA
synthetase that aminoacylates the suppressor tRNA but no other tRNA in
the cell. Here we describe two such aaRS–suppressor tRNA pairs, one
for use in the yeast
Saccharomyces cerevisiae
and
another for use in
Escherichia coli
. The “21st
synthetase–tRNA pairs” include
E. coli
glutaminyl-tRNA synthetase (GlnRS) along with an amber suppressor
derived from human initiator tRNA, for use in yeast, and mutants of the
yeast tyrosyl-tRNA synthetase (TyrRS) along with an amber suppressor
derived from
E. coli
initiator tRNA, for use in
E. coli
. The suppressor tRNAs are aminoacylated
in vivo
only in the presence of the heterologous aaRSs,
and the aminoacylated tRNAs function efficiently in suppression of
amber codons. Plasmids carrying the
E. coli
GlnRS gene
can be stably maintained in yeast. However, plasmids carrying the yeast
TyrRS gene could not be stably maintained in
E. coli
.
This lack of stability is most likely due to the fact that the
wild-type yeast TyrRS misaminoacylates the
E. coli
proline tRNA. By using error-prone PCR, we have isolated and
characterized three mutants of yeast TyrRS, which can be stably
expressed in
E. coli
. These mutants still aminoacylate
the suppressor tRNA essentially quantitatively
in vivo
but show increased discrimination
in vitro
for the
suppressor tRNA over the
E. coli
proline tRNA by factors
of 2.2- to 6.8-fold.
Identification of a natural soluble neuropilin-1 that binds vascular endothelial growth factor:In vivoexpression and antitumor activity
Authors: Gagnon Michael L. , Bielenberg Diane R. , Gechtman Ze'ev , Miao Hua-Quan , Takashima Seiji , Soker Shay , Klagsbrun Michael
Published: July 2002

Neuropilin-1 (NRP1) is a 130-kDa transmembrane receptor for semaphorins, mediators of neuronal guidance, and for vascular endothelial growth factor 165 (VEGF165), an angiogenesis factor. A 2.2-kb truncated NRP1 cDNA was cloned that encodes a 644-aa soluble NRP1 (sNRP1) isoform containing just the a/CUB and b/coagulation factor homology extracellular domains of NRP1. sNRP1 is secreted by cells as a 90-kDa protein that binds VEGF165, but not VEGF121. It inhibits125I-VEGF165binding to endothelial and tumor cells and VEGF165-induced tyrosine phosphorylation of KDR in endothelial cells. The 3′ end of sNRP1 cDNA contains a unique, 28-bp intron-derived sequence that is absent in full-length NRP1 cDNA. Using a probe corresponding to this unique sequence, sNRP1 mRNA could be detected byin situhybridization differentially from full-length NRP1 mRNA, for example, in cells of liver, kidney, skin, and breast. Analysis of blood vesselsin situshowed that NRP1, but not sNRP1, was expressed. sNRP1 was functionalin vivo. Unlike control tumors, tumors of rat prostate carcinoma cells expressing recombinant sNRP1 were characterized by extensive hemorrhage, damaged vessels, and apoptotic tumor cells. These results demonstrate the existence of a naturally occurring, soluble NRP1 that is expressed differently from intact NRP1 and that appears to be a VEGF165antagonist.
Physical contact between lipopolysaccharide and Toll-like receptor 4 revealed by genetic complementation
Authors: Poltorak A. , Ricciardi-Castagnoli P. , Citterio S. , Beutler B.
Published: July 2002

Some mammalian species show an ability to discriminate between different lipopolysaccharide (LPS) partial structures (for example, lipid A and its congener LA-14-PP, which lacks secondary acyl chains), whereas others do not. Using a novel genetic complementation system involving the transduction of immortalized macrophages from genetically unresponsive C3H/HeJ mice, we now have shown that the species-dependent discrimination between intact LPS and tetra-acyl LPS partial structures is fully attributable to the species origin of Toll-like receptor 4 (Tlr4), an essential membrane-spanning component of the mammalian LPS sensor. Because Tlr4 interprets the chemical structure of an LPS molecule, we conclude that LPS must achieve close physical proximity with Tlr4 in the course of signal transduction.
Integration of foreign DNA during natural transformation of
Acinetobacter
sp. by homology-facilitated illegitimate recombination
Authors: de Vries Johann , Wackernagel Wilfried
Published: July 2002

The active uptake of extracellular DNA and its genomic integration is termed natural transformation and constitutes a major horizontal gene-transfer mechanism in prokaryotes. Chromosomal DNA transferred within a species can be integrated effectively by homologous recombination, whereas foreign DNA with low or no sequence homology would rely on illegitimate recombination events, which are rare. By using the
nptII
+
gene (kanamycin resistance) as selectable marker, we found that the integration of foreign DNA into the genome of the Gram-negative
Acinetobacter
sp. BD413 during transformation indeed was at least 10
9
-fold lower than that of homologous DNA. However, integration of foreign DNA increased at least 10
5
-fold when it was linked on one side to a piece of DNA homologous to the recipient genome. Analysis of foreign DNA integration sites revealed short stretches of sequence identity (3–8 bp) between donor and recipient DNA, indicating illegitimate recombination events. These findings suggest that homologous DNA served as a recombinational anchor facilitating illegitimate recombination acting on the same molecule. Homologous stretches down to 183 nucleotides served as anchors. Transformation with heteroduplex DNA having different nucleotide sequence tags in the strands indicated that strands entered the cytoplasm 3′ to 5′ and that strands with either polarity were integrated by homologous recombination. The process led to the genomic integration of thousands of foreign nucleotides and often was accompanied by deletion of a roughly corresponding length of recipient DNA. Homology-facilitated illegitimate recombination would explain the introgression of DNA in prokaryotic genomes without the help of mobile genetic elements.
Models of the extracellular domain of the nicotinic receptors and of agonist- and Ca
2+
-binding sites
Authors: Le Novère Nicolas , Grutter Thomas , Changeux Jean-Pierre
Published: July 2002

We constructed a three-dimensional model of the amino-terminal extracellular domain of three major types of nicotinic acetylcholine receptor, (α7)
5
, (α4)
2
(β2)
3
, and (α1)
2
β1γδ, on the basis of the recent x-ray structure determination of the molluscan acetylcholine-binding protein. Comparative analysis of the three models reveals that the agonist-binding pocket is much more conserved than the overall structure. Differences exist, however, in the side chains of several residues. In particular, a phenylalanine residue, present in β2 but not in α7, is proposed to contribute to the high affinity for agonists in receptors containing the β2 subunit. The semiautomatic docking of agonists in the ligand-binding pocket of (α7)
5
led to positions consistent with labeling and mutagenesis experiments. Accordingly, the quaternary ammonium head group of nicotine makes a π-cation interaction with W148 (α7 numbering), whereas the pyridine ring is close to both the cysteine pair 189–190 and the complementary component of the binding site. The intrinsic affinities inferred from docking give a rank order epibatidine > nicotine > acetylcholine, in agreement with experimental values. Finally, our models offer a structural basis for potentiation by external Ca
2+
.
Different sensitivity to receptor editing of B cells from mice hemizygous or homozygous for targeted Ig transgenes
Authors: Braun Uschi , Rajewsky Klaus , Pelanda Roberta
Published: July 2002

Ig knock-in mice have been used to study the relative contribution of receptor selection versus clonal selection in the control of autoreactive B cells. The anti-MHC class I 3-83Ig knock-in (3-83Igi) mice manifest extensive receptor editing in the presence of H-2b. However, receptor editing is also observed on the H-2dbackground, although reactivity toward this antigen is below detection and its presence does not affect the generation of 3-83Ig+mature B cells in classical 3-83Ig transgenic mice. In this study we have analyzed the contribution of genetic background, B cell receptor signaling, and transgene copy number on the initiation and extent of receptor editing in the 3-83Igi;H-2dmice. Crossing the 3-83Ig insertion into either CD45-deficient H-2dmice or onto the BALB/c background reduces the extent of receptor editing and increases the fraction of 3-83Ig-expressing B cells, indicating that in the original line editing depends on B cell receptor signaling induced by cross-reacting antigen(s). However, receptor editing is still detectable in hemizygous 3-83Igi mice even on the BALB/c background, on which the 3-83 antibody was originally raised, whereas it is abrogated in homozygous 3-83Igi;H-2danimals. This latter observation indicates that immature B cells expressing immunoglobulin from single heavy and light chain loci, as they do physiologically, utilize receptor editing for an exquisite quality control of their antigen receptor that may only partly be based on self-reactivity.
The targeting pathway of
Escherichia coli
presecretory and integral membrane proteins is specified by the hydrophobicity of the targeting signal
Authors: Lee Hin C. , Bernstein Harris D.
Published: July 2002

Previous studies have demonstrated that presecretory proteins such
as maltose binding protein (MBP) and outer membrane protein A (OmpA)
are targeted to the
Escherichia coli
inner membrane by
the molecular chaperone SecB, but that integral membrane proteins are
targeted by the signal recognition particle (SRP).
In
vitro
studies have suggested that trigger factor binds to a
sequence near the N terminus of the mature region of OmpA and shunts
the protein into the SecB pathway by blocking an interaction between
SRP and the signal peptide. By contrast, we have found that the
targeting pathway of a protein under physiological conditions is
dictated by the composition of its targeting signal. Replacement of the
MBP or OmpA signal peptide with the first transmembrane segment of AcrB
abolished the dependence on SecB for transport and rerouted both
proteins into the SRP targeting pathway. More modest alterations of the
MBP signal peptide that simply increase its hydrophobicity also
promoted SRP binding. Furthermore, we obtained evidence that SRP has a
low affinity for typical signal peptides
in vivo
. These
results imply that different classes of
E. coli
proteins
are targeted by distinct pathways because bacterial SRP binds to a more
restricted range of targeting signals than its eukaryotic counterpart.
The role of the dynamics of focal adhesion kinase in the mechanotaxis of endothelial cells
Authors: Li Song , Butler Peter , Wang Yingxiao , Hu Yingli , Han Dong Cho , Usami Shunichi , Guan Jun-Lin , Chien Shu
Published: July 2002

The migration of vascular endothelial cells (ECs) is critical in vascular remodeling. We showed that fluid shear stress enhanced EC migration in flow direction and called this “mechanotaxis.” To visualize the molecular dynamics of focal adhesion kinase (FAK) at focal adhesions (FAs), FAK tagged with green fluorescence protein (GFP) was expressed in ECs. Within 10 min of shear stress application, lamellipodial protrusion was induced at cell periphery in the flow direction, with the recruitment of FAK at FAs. ECs under flow migrated with polarized formation of new FAs in flow direction, and these newly formed FAs subsequently disassembled after the rear of the cell moved over them. The cells migrating under flow had a decreased number of FAs. In contrast to shear stress, serum did not significantly affect the speed of cell migration. Serum induced lamellipodia and FAK recruitment at FAs without directional preference. FAK(Y397) phosphorylation colocalized with GFP-FAK at FAs in both shear stress and serum experiments. The total level of FAK(Y397) phosphorylation after shear stress was lower than that after serum treatment, suggesting that the polarized change at cell periphery rather than the total level of FAK(Y397) phosphorylation is important for directional migration. Our results demonstrate the dynamics of FAK at FAs during the directional migration of EC in response to mechanical force, and suggest that mechanotaxis is an important mechanism controlling EC migration.
Immunoglobulin heavy chain constant regions regulate immunity and tolerance to idiotypes of antibody variable regions
Authors: Reitan Solveig Klaebo , Hannestad Kristian
Published: July 2002

Particular syngeneic adjuvant-free monoclonal antibodies are immunogenic and elicit antibody responses against the variable region idiotypes (Ids). We here study how heavy-chain constant regions (CH) regulate immune responses to Ids of free, uncomplexed monoclonal antibodies. To this end, we selected two hybridomas, called Id3and IdA.01, that produce immunogenic IgMλ2 directed toward 2,4,6-trinitrophenyl, and subcloned rare IgG1, IgG3, IgE, or IgA class switch variants. The purified switch variants, which possessed the Ids of their IgM progenitors, were injected repeatedly without added adjuvant into BALB/c mice, and anti-Id IgG responses were determined. These repeated injections revealed that the immunogenicity of Ids was lost by switching to IgG1 and IgG3, restored when the Fc piece of IgG1 was removed, maintained by switching to IgE and monomeric IgA, and lost in polymeric IgA. Loss of immunogenicity was associated with acquisition of Id-specific tolerogenicity, as determined by immunization challenge with Id borne by IgM. An Id borne by IgG induced tolerance when injected at least 90 days before or 3–21 days after immunization with IgM Id was begun. Ids of IgG were also tolerogenic in mice deficient in FcγRIIB or FcγRI + III. The results suggest that Ids that have switched to IgG and pIgA negatively control immune responses to shared Ids, including the Ids of their IgM progenitors.