bluuebunny/crossref_metadata_2025 huggingface dataset:
New photoluminescence defect at 1.0192 eV in silicon molecular beam epitaxy layers ascribed to Cu
Authors: Sauer R. , Asom M. , People R. , Lang D. V. , Kimerling L. C. , Bean J. C.
Published: July 2002

We report a new photoluminescence defect spectrum with a no-phonon transition at 1.0192 eV which emerges in many silicon layers grown on silicon substrates by molecular beam epitaxy. Comparison of the no-phonon transition and the chief local mode (ℏω=7.5 meV) to the well-established Cu-related spectrum at 1.0145 eV suggests that the new defect incorporates Cu as well. Deep level transient measurements support the presence of Cu in the epilayers. We suggest that this defect spectrum has recently been observed by others but was not identified and associated with Cu.
Analysis of initial stage of Pd-Si (111) and Au-Si (111) interface reactions by means of high-resolution proton energy-loss spectroscopy
Authors: Kanasaki Jun-ichi , Itoh Noriaki , Matsunami Noriaki
Published: July 2002

High-resolution spectroscopy of protons incident under nonchanneling condition and scattered by 12° on Pd- and Au-deposited Si(111) surfaces has been employed for direct layer-by-layer analysis of the depth distribution of Au and Pd. For nearly monolayer deposition at room temperature, most of the Au atoms are found to remain on the silicon surface, while the Pd atoms are found to be distributed over the two layers, with the palladium composition not exceeding that of Pd2Si. We also found that the most probable energy loss of protons scattered from the Au-deposited surface is sensitive to the depth of the Au atoms on the surface.
High-speed operation of InP metal-insulator-semiconductor field-effect transistors grown by chloride vapor phase epitaxy
Authors: Antreasyan A. , Garbinski P. A. , Mattera V. D. , Temkin H. , Abeles J. H.
Published: July 2002

We report the millimeter-wave performance of enhancement mode InP metal-insulator-semiconductor field-effect transistors grown by chloride vapor phase epitaxy. For a gate length of 1 μm we have measured a current gain cutoff frequency of 29 GHz and an electron velocity of 2.5×107 cm/s, close to a theoretical current gain cutoff frequency of 40 GHz. This represents the fastest InP-based field-effect transistor ever demonstrated, and surpasses state-of-the-art AlGaAs/GaAs modulation-doped field-effect transistors having similar gate length.
Depth profiles of defects in CdTe(100) overlayers grown by molecular beam epitaxy on GaAs(100)
Authors: Rauhala E. , Keinonen J. , Rakennus K. , Pessa M.
Published: July 2002

The quality of crystal structure of CdTe(100) overlayers grown by molecular beam epitaxy on GaAs(100) substrates has been studied by x-ray diffraction and Rutherford backscattering/channeling methods. The depth distribution of displaced atoms in the overlayers with thicknesses varying from 1.6 to 3.1 μm shows that a defect-free surface is obtained when the film thickness is about 2 μm.
Electrical determination of the valence-band discontinuity in HgTe-CdTe heterojunctions
Authors: Chow D. H. , McCaldin J. O. , Bonnefoi A. R. , McGill T. C. , Sou I. K. , Faurie J. P.
Published: July 2002

Current-voltage behavior is studied experimentally in a Hg0.78Cd0.22Te-CdTe-Hg0.78Cd0.22Te heterostructure grown by molecular beam epitaxy. At temperatures above 160 K, energy-band diagrams suggest that the dominant low-bias current is thermionic hole emission across the CdTe barrier layer. This interpretation yields a direct determination of 390±75 meV for the HgTe-CdTe valence-band discontinuity at 300 K. Similar analyses of current-voltage data taken at 190–300 K suggest that the valence-band offset decreases at low temperatures in this heterojunction.
Ultranarrow conducting channels defined in GaAs-AlGaAs by low-energy ion damage
Authors: Scherer A. , Roukes M. L. , Craighead H. G. , Ruthen R. M. , Beebe E. D. , Harbison J. P.
Published: July 2002

We have laterally patterned the narrowest conducting wires of two-dimensional electron gas (2DEG) material reported to date. The depletion induced by low-energy ion etching of GaAs-AlGaAs 2DEG structures was used to define narrow conducting channels. We employed high voltage electron beam lithography to create a range of channel geometries with widths as small as 75 nm. Using ion beam assisted etching by Cl2 gas and Ar ions with energies as low as 150 eV, conducting channels were defined by etching only through the thin GaAs cap layer. This slight etching is sufficient to entirely deplete the underlying material without necessitating exposure of the sidewalls that results in long lateral depletion lengths. At 4.2 K, without illumination, our narrowest wires retain a carrier density and mobility at least as high as that of the bulk 2DEG and exhibit quantized Hall effects. Aharonov–Bohm oscillations are seen in rings defined by this controlled etch-damage patterning. This patterning technique holds promise for creating one-dimensional conducting wires of even smaller sizes.
Evaluation of second-order hyperpolarizabilities for systems with low-energy double transitions by the three-level model
Authors: Suzuki H. , Sukegawa K.
Published: July 2002

Second-order hyperpolarizabilities have been calculated by the three-level model for systems having low-energy double transitions. In the three-level model, the second excited state, in addition to the first excited and ground states, is explicitly taken into account. The second-order hyperpolarizabilities calculated by the three-level model gave excellent agreement with the observed values determined by the dc induced second harmonic generation technique, for 4-aminostilbene, p-nitroaniline, and 2-methyl-4-nitroaniline.
New resonant tunneling superlattice avalanche photodiode device structure for long-wavelength infrared detection
Authors: Summers C. J. , Brennan K. F.
Published: July 2002

A new device structure useful in detecting long-wavelength radiation which uses a variably spaced superlattice both to inject photogenerated electrons into a wider band-gap semiconductor and to promote efficient impact ionization and gain at low bias voltages is proposed. Both of these properties result directly from the unique property of the superlattice structure to create a near monoenergetic stream of electrons and inject them at high energy into an adjacent semiconducting layer. Thus for the first time it should be possible to obtain long-wavelength sensors with high gain (∼103), ultralow noise, and high detectivity.
How perfect is a perfect crystal? Part-per-billion level mosaicity measurements in silicon
Authors: Deutsch Moshe , Hart Michael , Cummings Stewart
Published: July 2002

A novel method for measuring mosaicity in nearly perfect crystals is presented. The method employs a monolithic Laue case two-crystal diffractometer cut entirely from the crystal under investigation for measuring rocking curve broadening. The advantages of this arrangement as compared to conventional separate crystal diffractometers are greatly enhanced stability and complete elimination of alignment problems. High-order or symmetry-forbidden reflections having a very narrow intrinsic half-width of ∼8 ms of arc are used to obtain high resolution. Measurements on a commercial dislocation-free silicon crystal show line broadening of (54±16)×10−9 rad at different points on the crystal. The broadening is discussed in terms of mosaic tilt and block size. The method is shown to have an ultimate resolution of a few parts in 1010 for these quantities.
Magnetization of sinter-forged YBa2Cu3O7−δ
Authors: Song S. N. , Robinson Q. , Hwu S.-J. , Johnson D. L. , Poeppelmeier K. R. , Ketterson J. B.
Published: July 2002

The magnetization and critical magnetization current densities of sinter-forged YBa2Cu3O7−δ samples of nearly theoretical density have been studied. The magnetization curves can be well fitted by Kim’s critical state equation Jc(B)=α/(B0+B) with a flat plate geometry. The results revealed an anisotropy in the lower critical field and the critical current density with respect to the forging axis. The deduced critical current density in the favorable direction is larger than the value of isotropic polycrystalline samples.
Electron beam electroreflectance studies of GaAs and CdTe surfaces
Authors: Raccah P. M. , Garland J. W. , Buttrill S. E. , Francke L. , Jackson J.
Published: July 2002

An electron beam electroreflectance (EBER) system has been built and used to investigate GaAs and CdTe sample surfaces. EBER permits one to go to low temperatures, to couple electroreflectance with high-vacuum techniques and to study the effects of different surface treatments. Analysis shows that the EBER technique leads to only negligible thermal modulation of the sample for low-energy electron beams such as that used here. The results of our measurements before and after sputtering with 2 keV O+2 ions clearly show that the measurements are very sensitive to the condition of sample surfaces. Analysis of the results shows the existence of distinct surface and bulk signals and gives detailed information about the surface before and after sputtering. Comparison of our EBER results with room-temperature electrolyte-electroreflectance results on both GaAs and CdTe confirms the analysis and establishes that electroreflectance is a surface-sensitive technique.
Tight-binding study of the strained monolayer superlattices (Si)1/(Si1−xGex)1 (100)
Authors: Shen Dingli , Lu Fen , Zhang Kaiming , Xie Xide
Published: July 2002

Electronic structures of strained monolayer superlattices (Si)1/(Si1−xGex)1 are calculated within the tight-binding scheme by taking into consideration the effect of stress on atomic interactions. Gap variations with composition and band offsets for different strain conditions are obtained. When Si(Ge) is grown on cubic Ge(Si) with stress to match the lattice constant of the substrates, the calculated nonmonotonic behavior of the superlattice Eg(x) curve can be attributed to a similar phenomenon of the bulk alloy under the same strain. The energy band of the monolayer Si/Ge shows the zinc blende symmetry with a Si-like feature. Starting with the virtual crystal approximation for the calculation of gap and density of states, it is found that further modification using coherent potential approximation is not very significant.
High-performance InGaAsP/InP buried-heterostructure lasers and arrays defined by ion-beam-assisted etching
Authors: Yap D. , Liau Z. L. , Tsang D. Z. , Walpole J. N.
Published: July 2002

Ion-beam-assisted etching has been used to fabricate mass-transported InGaAsP/InP buried- heterostructure lasers. These lasers have a novel, deeply etched rectangular mesa that results in reduced current leakage. Both single-stripe lasers and Y-junction-coupled multiple-stripe laser arrays have been demonstrated. The single-stripe lasers have 12 mA cw threshold currents, differential quantum efficiencies of 32%–34% per facet, and smooth single-lobe far-field patterns. The multiple-stripe arrays lase in-phase with strong phase coherence.
Very shallow acceptors in neutron transmutation doped silicon
Authors: LaBrec C. R. , Udo M. K. , Ramdas A. K.
Published: July 2002

We report the far-infrared absorption spectra of very shallow acceptor centers in silicon consisting of boron (aluminum) introduced during growth complexed with defect centers produced during neutron transmutation doping followed by a partial high-temperature anneal. As annealing of the samples removes radiation damage and activates isolated boron (aluminum) centers, a new acceptor series, attributed to this acceptor complex, is observed with a ground-state binding energy significantly lower than that of the known impurity.
GaAs semiconductor-insulator-semiconductor field-effect transistor with a planar-doped barrier gate
Authors: Figueredo Domingo A. , Zurakowski Mark P. , Elliott Scott S. , Anklam William J. , Sloan Susan R.
Published: July 2002

A new GaAs field-effect transistor structure is proposed and demonstrated. The gate electrode consists of an asymmetric planar-doped barrier (PDB) diode which behaves like a metal-semiconductor Schottky contact. The device allows engineering of the gate energy barrier, and optimization of transconductance and gate capacitance for a given application. The larger gate energy barrier in conjunction with the self-aligned nature of the process holds promise for both large signal analog and digital switching applications.
Lateral diffusion of mercury during laser annealing of HgMnTe and other HgTe-based materials
Authors: Moore F. G. , Kremer R. E.
Published: July 2002

The diffusion of mercury in HgTe-related materials has played a major role in the development of devices utilizing the unique features of this family of materials. The study of mercury diffusion, however, has been limited to diffusion perpendicular to the surface (i.e., into or out of the sample). We present evidence showing that when samples undergo laser annealing, substantial amounts of mercury are displaced laterally as well. This lateral diffusion was observed with the use of thermal probe mapping of slices of HgMnTe and HgCdTe.
Carrier confinement photoconductive detector
Authors: Jalali B. , Evans H. L. , Yang E. S.
Published: July 2002

We propose a new device, the carrier confinement photoconductive detector, in which an improvement in performance over a conventional photoconductor is achieved by confinement of photogenerated carriers in the active channel. The confinement can be realized by placing a layer of a wide band-gap semiconductor between the channel and ohmic contact. Analytical and numerical analyses show that gain-bandwidth improvement of 100% can be achieved by using the GaAl/AlGaAs system.
Superpolished single-crystal molybdenum mirror for x-ray ultraviolet to x-ray radiation
Authors: Yoshida K. , Takeuchi K. , Kato Y. , Fujiwara E. , Yamashita K. , Kurosawa K. , Sasaki W. , Okamoto H.
Published: July 2002

High-quality single-crystal molybdenum (Mo) fabricated by a secondary recrystallization method has been polished with a new polishing technique. The superpolished surface of the Mo single crystal has a surface roughness of 3.5 Å rms. This superpolished single-crystal Mo has been used for a high-power electron beam pumped argon excimer laser (λ=126 nm) as a cavity mirror. Peak output power of more than 3 MW has been achieved without any damage to the mirror surface.
Demonstration of all-optical modulation in a vertical guided-wave nonlinear coupler
Authors: Berger Paul R. , Chen Yi , Bhattacharya Pallab , Pamulapati Jagadeesh , Vezzoli G. C.
Published: July 2002

The performance characteristics of an AlGaAs dual waveguide vertical coupler with a nonlinear GaAs/AlGaAs multiquantum well coupling medium are demonstrated. The structure was grown by molecular beam epitaxy and fabricated by optical lithography and ion milling. The nonlinear coupling and modulation behavior is identical to that predicted theoretically. The nonlinear index of refraction and critical input power are estimated to be n2=1.67×10−5 cm2/W and Pc=170 W/cm2, respectively. This device also allows reliable measurement of the nonlinear refractive index for varying quantum well and optical excitation parameters.
Row-backlight, column-shutter display concept
Authors: Nelson T. J. , Patel J. S. , Ngo P. D. T.
Published: July 2002

Feasibility of a novel flat-panel liquid-crystal display that is capable of full-motion video but does not require active elements at each pixel is shown. It can use the wide viewing angle, high contrast, and high speed of ferroelectric liquid crystals while being insensitive to their lack of a definite field threshold. To demonstrate this display, a linear array of ferroelectric liquid-crystal shutters is driven to permit or block the view of rows that are illuminated one after another on an oscilloscope. At 15 V drive the shutters respond in 100 μs, which would be within the time available per row if the top and bottom halves of a display were to be addressed separately.
Electron beam oxidation of semiconductors (one mechanism of electron beam processes)
Authors: Wada Takao
Published: July 2002

New types of oxide layers of Si and GaAs were grown on (100) Si and (100) GaAs substrate by using an electron beam doping technique at 40–50 °C. The surfaces of the semiconductors were irradiated with a fluence of ∼5×1017 electrons cm−2 at 7 MeV. The samples were put in an isothermal circulating water bath with a thermoregulator. The electronic structure of the oxide layers was observed by an x-ray photoelectron spectroscopy (XPS). The chemical shifts between oxidized and nonoxidized Ga signals for Auger electron spectra and 3d XPS spectra were nearly equal to that in the conventional plasma-grown oxidation. It was suggested that the electron beam doping, oxidation, and epitaxy were caused by plasma reaction.
Investigation of point-defect generation in silicon during oxidation of a deposited WSi2 layer
Authors: Fahey P. , Dutton R. W.
Published: July 2002

In recent letters it was reported from separate studies that silicidation of the refractory metals TaSi2 or TiSi2 on a silicon surface gives rise to injection of vacancy point defects into the underlying substrates. In both studies it was proposed that a vacancy influx may be induced by a silicon efflux during silicidation. In this work we show that during oxidation of WSi2 on silicon, which also causes an efflux of silicon atoms from the underlying substrate, no vacancy excess is observed based on monitoring diffusion of P and B buried marker layers. We also propose a possible explanation that reconciles the apparently discrepant findings that B diffusion is always retarded during vacancy supersaturations resulting from thermal nitridation reactions, while vacancy supersaturations observed during silicidation of TaSi2 result in enhanced diffusion of B.
Ultrafast switching characteristics of a bistable surface-emitting multiple quantum well distributed Bragg reflector laser
Authors: Kojima Keisuke , Kyuma Kazuo , Noda Susumu , Ohta Jun , Hamanaka Koichi
Published: July 2002

We describe an ultrafast switching operation of a bistable surface-emitting distributed Bragg reflector laser. The rise time was as small as 12 ps and the fall time was 90 ps. Both are much smaller than those of conventional bistable laser diodes. Ths was realized by the effect of the multiple quantum well structure and a strong detuning.
Optoelectronic transient response of an n-channel double heterostructure optoelectronic switch
Authors: Crawford D. L. , Taylor G. W. , Simmons J. G.
Published: July 2002

A bistable n-channel double heterostructure optoelectronic switch, fabricated as a three-terminal light-emitting diode, is reported and results are presented demonstrating simultaneous electrical and optical switching characteristics. Electrical turn-on times of 2 ns are observed, with corresponding optical turn-on times of 60 ns. Electrical turn-off transition times of 6 ns are observed with optimum optical turn-off times of 20 ns. These times are consistent with the device size and the measurement impedance.
Low-temperature oxidation of implanted TiSi2 films
Authors: Holland O. W.
Published: July 2002

The effect of implantation on the low-temperature oxidation behavior of TiSi2 films is investigated. Critical fluences of implanted impurities, such as As and Ga, are shown to alter the dominant oxidation mechanism in the films and greatly enhance oxidation. Different techniques, such as inert- and self-ion implantation, and implantation at elevated temperatures, were done so that ion-induced damage and chemical effects could be separately studied. These results, along with the dose dependency of the film’s behavior, will be presented. A model is proposed which accounts for these observations.
Characterization of beam-recrystallized Si films and their Si/SiO2 interfaces in silicon-on-insulator structures: The very thin Si film case
Authors: Vu D. P. , Pfister J. C.
Published: July 2002

A depletion-mode field-effect transistor is used to determine the electrical characteristics of a silicon-on-insulator (SOI) structure such as the Si film doping, the fixed oxide charges, and the interface trapped charges at the Si/SiO2 interfaces. We consider the case of a very thin Si film, i.e., the Si film thickness is smaller than the maximum depletion layer width. The electrical parameters of the SOI structure are derived from drain-source current versus gate voltage and transconductance characteristics, provided that the back Si surface is accumulated by a proper substrate biasing. The device can be of small dimensions and can be used in process control without any extra process step.
New type of solid phase epitaxy of alloy semiconductors by electron beam irradiation
Authors: Wada Takao , Maeda Yoshinobu
Published: July 2002

Solid phase epitaxial layers of AlxGa1−xSb (x≂0.26–0.28) were grown by using an electron beam doping method on a (100)GaSb substrate at 50 °C. Surfaces of Al layers deposited by vacuum evaporation on GaSb wafers were irradiated with a fluence of 0.1–5.6×1018 electrons cm−2 at 7 MeV. After the irradiation and removal of the Al layer, the epilayers with a thickness of ∼400 Å before annealing were measured by a secondary-ion mass spectrometer, an x-ray photoelectron spectroscope, an x-ray diffractometer, and a reflection high-energy electron diffractometer.
A 35 ps, Q-switched, continuous-pumped neodymium:yttrium aluminum garnet laser using an electro-optic ring modulator
Authors: Marchetti R. , Penco E. , Giuliani G. , Palange E.
Published: July 2002

An electro-optical ring modulator, used as a cavity output coupler, has permitted simultaneous active mode locking, Q switching, and cavity dumping of a continuous-pumped neodymium:yttrium aluminum garnet laser. We have obtained output pulses of 4 MW peak power and 35 ps duration at repetition rates up to 2.5 kHz.
Ultrafast optoelectronic ferromagnetic semiconductor CdCr2Se4 switch
Authors: Walser Ardie D. , Alfano R. R.
Published: July 2002

A new ultrafast optoelectronic switch that utilizes the ferromagnetic semiconductor CdCr2Se4 is demonstrated to have a scope-limited sampled response time of approximately 90 ps.
Use of Raman spectroscopy to examine the effects of growth stops on the interfaces of superlattices grown by molecular beam epitaxy
Authors: Wicks G. W. , Bradshaw J. T. , Radulescu D. C.
Published: July 2002

Raman spectroscopy of confined longitudinal optical phonons is used to examine the abruptness of the interfaces in a sequence of (GaAs)7(AlAs)7 superlattice samples grown by molecular beam epitaxy. The duration and stoichiometry of growth stops at each heterointerface was varied throughout the sequence. Arsenic rich, group III rich, and approximately stoichiometric growth stops were investigated. It was found that, when the arsenic beam remains on during the growth stops, the surfaces continuously become smoother, resulting in more abrupt interfaces. However, when growth is stopped under group III rich or stoichiometric conditions, an initial roughening of the surface occurs, followed by gradual smoothing.
Interface degradation in Si-metal-oxide-semiconductor structures by homogeneous, microwave heating of channel carriers
Authors: Qiu-Yi Ye , Zrenner A. , Koch F. , Zeller C. , Dorda G.
Published: July 2002

Using a pulsed microwave source to provide a strong lateral electric field in a Si-metal-oxide-semiconductor structure, we observe interface degradation by hot holes. Damage occurs in a two-step process. Holes are trapped in the oxide at low temperature and are subsequently converted into interface states in an annealing step at room temperature.
AlGaAs graded index lens for integrated optics
Authors: Yoshida I. , Bedair S. M.
Published: July 2002

We propose a new approach for two-dimensional graded index lenses, based on the one-dimensional compositional grading in III-V compounds ternary alloys. Compositional grading is achieved by the direct writing of these alloys, using the laser-assisted chemical vapor deposition technique. The desired compositional grading and the corresponding variations in the refractive index are achieved by changing the mole fraction of the reactants in the gas phase while scanning the laser beam. An AlGaAs graded index lens structure has been demonstrated for the first time. This III-V compound graded index lens can have potential applications in the field of integrated optics.
Diffuse discharge interruption induced by excimer lasers in a pure N2 gas
Authors: Sasagawa Teruo , Kawahara Akihiro , Obara Minoru
Published: July 2002

We observed the complete discharge opening by ArF (193 nm) or KrF (248 nm) laser irradiation in pure N2 gas of a few Torr, using a discharge tube with a pair of cylindrical electrodes. With an ArF laser fluence of 27 mJ/cm2(10 ns FWHM: full width at half-maximum) and at a N2 pressure of 1.0 Torr, a discharge current of 1.1 mA(0.156 mA/cm2) was interrupted completely, and the observed maximum current decay rate was 770 A/s at a sustaining discharge current of 0.75 mA. The discharge interruption depended on the laser wavelength. The discharge current over 0.27 mA could not be interrupted by a KrF laser irradiation of 27 mJ/cm2 (20 ns FWHM). The discharge current was not interrupted at all by XeF (351 nm) laser irradiation. Moreover, the discharge interruption strongly depended on the laser fluence, total pressure, and discharge current. Finally, we addressed the kinetic processes relevant to the discharge interruption in pure N2 ArF or KrF laser irradiation.
Solid phase recrystallization of implanted III-V semiconductors under uniaxial stress
Authors: Licoppe C. , Savary H. , Allovon M.
Published: July 2002

The effect of mechanical stress on solid phase epitaxial recrystallization of implanted amorphous III-V semiconductor layers is studied for the first time. A dual approach is used, involving either direct application of uniaxial stress on whole GaAs samples or the use of strained InGaAs layers deposited on InP substrates with indium composition as a stress control parameter. Observations show that with high applied stresses up to a few kilobars recrystallization kinetics remain unaltered. While homogeneous coherent strain does not bear any influence on interface roughness during regrowth, inhomogeneous strains due to defects greatly enhance the growth front roughness. This last result is interpreted in terms of defects acting as generating sources of additional defects during recrystallization.
Noise voltage and instability in GaAs devices
Authors: Day D. J. , Trudeau M. , McAlister S. P. , Hurd C. M.
Published: July 2002

Low-frequency current noise that has a f−3/2 power law noise spectrum is observed in GaAs field-effect devices. We show that this is due to modulation by a thermally activated and bias-independent voltage noise in the gate. We identify this noise with generation recombination associated with the semi-insulating substrate supporting these devices. Lowering the activation energy of these traps by the drain bias causes instability in this noise process, leading to low-frequency oscillations that have previously been identified with high field domains in the substrate material.
All-optical switching between modes of a GaAs/GaAlAs multiple quantum well waveguide
Authors: Li Kam Wa P. , Robson P. N. , Roberts J. S. , Pate M. A. , David J. P. R.
Published: July 2002

A strain-induced multiple quantum well waveguide which can support two transverse modes in the plane normal to the quantum wells was excited in the TE mode by 350-ns-long probe pulses of low intensity from a dye laser tuned to a wavelength of 886 nm. A laser diode emitting at 850 nm was used in order to switch the 886 nm light between two positions at the exit facet of this overmoded waveguide. The switching mechanism has been shown to be non-thermal in origin.
Photocurrent spectroscopy in a sawtooth doping superlattice
Authors: Ullrich B. , Zhang C. , Fronius H. , Klitzing K. v.
Published: July 2002

We report an investigation of photocurrent response in a sawtooth doping superlattice which is composed of alternating n(Si) and p(Be) δ-doping layers with undoped GaAs layers (7–15 nm) between them. A selective contact method is used to measure the interesting behavior of the subband gap compared with the gap of the host semiconductor. The photocurrent is finite even when the photon energy is below the gap of GaAs and can be varied by applying a bias voltage between the n and p contacts. The observed phenomena can be explained by a generalized Franz–Keldysh model which takes into account the finite voltage drop inside the sample.
Investigations of the preparation process for efficient second-harmonic generation in optical fibers
Authors: Krotkus Arūnas , Margulis Walter
Published: July 2002

Experiments were carried out to elucidate aspects of the preparation process, which enables glass fibers to efficiently frequency-double infrared laser light. A picture is described which is consistent with most of the observations.
Growth of CdSxSe1−x thin films by laser evaporation deposition
Authors: Kwok H. S. , Zheng J. P. , Witanachchi S. , Shi L. , Shaw D. T.
Published: July 2002

Optical quality thin films of CdSxSe1−x were deposited on quartz for various values of x. It was found that these films were polycrystalline in structure, with a high degree of orientation of the c axis. An empirical relationship between the band gap and the lattice constant for the binary system CdS-CdSe was also obtained.
1.06–0.53 μm second harmonic generation using congruent lithium niobate
Authors: Ammann E. O. , Guch S.
Published: July 2002

Efficient 1.06–0.53 μm second harmonic generation (SHG) has been achieved using congruent lithium niobate. Index inhomogeneities are induced in the lithium niobate, but saturate in the vicinity of the beam thereby allowing efficient SHG to occur. SHG conversion efficiencies of 50% were typically achieved with crystal lengths as short as 9 mm and fundamental power densities of 35 MW/cm2.
Calculations on the flux pinning action of ferroelectric crystals inside a ceramic superconductor
Authors: Schilling Osvaldo F.
Published: July 2002

Taking as an example the case of BaTiO3 ferroelectric crystals inside an Y1Ba2Cu3Ox matrix (1-2-3 compound) it is shown that the ferroelectric properties of such crystals can generate an extra flux pinning effect, in addition to the expected pinning due to the normal character of such inclusions. The calculations show that the additional pinning force attainable depends strongly on the spontaneous polarization and the piezoelectric strain tensor components of the ferroelectric material.
High critical field anisotropy of superconducting Bi-Sr-Ca-Cu oxide from highly oriented thin films
Authors: Kang J. H. , Kampwirth R. T. , Gray K. E.
Published: July 2002

Superconducting films of Bi-Sr-Ca-Cu oxides have been grown on MgO substrates by multitarget magnetron sputtering. After post-annealing in an oxygen atmosphere, zero resistance was obtained at about 80 K. X-ray diffraction indicates a high degree of preferential growth of the c axis perpendicular to the substrate. Measurements of the upper critical field Bc2(T) of these films show critical field slopes (B′c2≡−dBc2/dT at Tc) of 8.5 and 0.56 T/K, respectively, for B parallel and perpendicular to the substrate. The anisotropy of ∼15 and the Bc2∥ are the highest values yet reported for high-temperature superconductors.
Field-induced spin reorientation in erbium-iron-manganese-boron
Authors: Pinkerton F. E. , Fuerst C. D. , Meisner G. P.
Published: July 2002

Er2Fe14B undergoes a spin reorientation from a low-temperature planar moment orientation to a high-temperature axial orientation at Ts=336 K. We report for the first time that the planar to axial reorientation can be induced at temperatures below Ts by applying a sufficiently large magnetic field. Room-temperature reorientation in Er2Fe14B is induced at a field of about 120 kOe. Investigation of Er2Fe14−xMnxB compounds shows that the field required to induce reorientation decreases as T approaches Ts from below. The magnetic moment is larger in the axial orientation than in the planar orientation; hence, reorientation occurs at the field where the magnetostatic energy gained by the larger axial moment overcomes the unfavorable anisotropy energy at temperatures near Ts.
Probing of basal planes of MoS2 by scanning tunneling microscopy
Authors: Sarid Dror , Henson Tammy D. , Armstrong Neal R. , Bell L. Stephen
Published: July 2002

Atomically resolved images of MoS2 have been obtained using scanning tunneling microscopy with both positively and negatively biased samples yielding the hexagonal symmetry of the surface of the crystal. Also measured were curves of tunneling current as a function of bias voltage, from which the density of states of the valence and conduction bands can be inferred.
High-temperature processing of cuprate oxide superconductors
Authors: Wu M. K. , Ashburn J. R. , Higgins C. A. , Fellows C. W. , Loo B. H. , Burns D. H. , Ibrahim A. , Rolin T. D. , Peters P. N. , Sisk R. C. , Huang C. Y.
Published: July 2002

Superconducting Y1Ba2Cu3O7 (‘‘123’’) films were fabricated on the Y2BaCuO5 (‘‘211’’) phase substrate. The superconducting characteristics of these films, in terms of superconducting transition temperature (Tc) and width, are better than those using other oxide compounds as substrates. In addition, using high-temperature processing, the bulk 211 phase was converted into the 123 phase. A new high Tc copper oxide material with non-rare-earth elements (Bi-Sr-Cu-O) was prepared using similar high-temperature processing. High-temperature processing presents an alternative synthetic route in the search of new high Tc superconductors.
Picosecond all-optical switching in single-mode GaAs/AlGaAs strip-loaded nonlinear directional couplers
Authors: Jin R. , Chuang C. L. , Gibbs H. M. , Koch S. W. , Polky J. N. , Pubanz G. A.
Published: July 2002

Room-temperature all-optical switching has been achieved in single-mode strip-loaded nonlinear directional couplers with a GaAs/AlGaAs multiple quantum well guiding layer. For low input intensities nearly complete (1:3–1:5) cross coupling occurs, while at high input intensities switching occurs so that most (>3:1) of the light stays in the input guide. The effects of carrier diffusion are minimized by the use of picosecond pulses. The response time and the recovery time of the nonlinear directional couplers are measured with pump-probe experiments, and the origin of the nonlinearity is attributed to fast electronic effects.
Influence of striations in GaAs on the activated profiles of implanted silicon
Authors: Schink H. , Schnell R. D.
Published: July 2002

Liquid-encapsulated Czochralski-grown GaAs is known to show radial striations. In GaAs:In they can be seen by x-ray topography via fluctuations of the In concentration which cause variations in the lattice constant. Using a high spatial resolution capacitance-voltage technique we investigated In-doped as well as undoped GaAs. It is demonstrated that both materials exhibit striations in the activation behavior of implanted silicon and hence in the threshold voltage of ion-implanted metal-semiconductor field-effect transistors. In the cases investigated here the threshold voltage varied by up to ±12 mV from peak to peak. To our knowledge this is the first time that the effect of striations on the activation behavior of implanted Si is demonstrated in both kinds of substrate material.
Nonlinear mixing and phase conjugation in broad-area diode lasers
Authors: Lucente M. , Carter G. M. , Fujimoto J. G.
Published: July 2002

Four-wave mixing is investigated in broad-area diode lasers. Two external fields are injected into the device using a phase conjugation geometry and the nonlinear four-wave mixing signal observed by performing spectrally resolved measurement of the far field. By varying the injection geometry, the excited mode of the broad-area diode can be controlled. The gain nonlinearity generates a conjugate wave within the device, thereby resulting in far-field emission in the conjugate direction. Nonlinear conjugate signal efficiency levels of ∼−21 dB have been observed.
Experimental observation of the dynamical image potential in extremely low GaAs/AlxGa1−xAs/GaAs tunnel barriers
Authors: Guéret P. , Marclay E. , Meier H.
Published: July 2002

Tunneling in low-barrier GaAs/AlxGa1−xAs/GaAs heterostructures has been systematically investigated over a wide range of barrier heights and thicknesses. Measured conductance data have been compared with tunnel conductances calculated with and without the image potential correction. Experimental evidence is found for the validity of the static image correction in the limit of large tunneling times, and for the occurrence of dynamic effects in the limit of short tunneling times.
Effect of ion mixing on the depth resolution of sputter depth profiling
Authors: Cheng Yang-Tse , Dow Audrey A. , Clemens Bruce M.
Published: July 2002

The effect of ion mixing on the depth resolution of sputter depth profiling has been studied using x-ray photoelectron spectroscopy and Ar+ sputtering of Pt/Zr, Pt/Mo, Pt/Ti, and Pt/Ni interfaces. It is shown that the heat of mixing of binary alloys plays an important role in depth resolution. The measured interface width of bilayers consisting of metals with a large and negative heat of mixing (Pt/Zr and Pt/Ti) is significantly broader than that of metals with a nearly zero heat of mixing (Pt/Mo and Pt/Ni). This observation suggests that the mechanism of ion mixing during sputter depth profiling with ion energies of a few keV is the same as ion mixing with ion energies of several hundred keV; diffusion in thermal spikes is the dominant contribution to ion mixing in the systems investigated.
High repetition rate operation of a photoinitiated impulse-enhanced electrically excited CO2 laser discharge using a burst-mode technique
Authors: Nikumb S. K. , Seguin H. J. J. , Seguin V. A. , Presakarchuk D.
Published: July 2002

The incorporation of a gating signal into the trigger circuit of a photoinitiated, impulse-enhanced, electrically excited (PIE) laser system has permitted high-power, pulsed operation of a normally cw CO2 discharge. The 40 liter gain medium has been run at repetition rates approaching 1 kHz utilizing this approach. Plasma uniformity and stability have been significantly enhanced, such that a factor of two increase in electrical power deposition into the excited volume has been achieved. Results suggest that pulsed performance considerably in excess of that achievable under cw operating conditions can be realized through the adoption of this simple modification to the PIE ionization process.
Evolution of germ-line signals that regulate growth and aging in nematodes
Authors: Patel Mavji N. , Knight Christopher G. , Karageorgi Constantina , Leroi Armand M.
Published: July 2002

We show that a signal from the germ line represses growth in the nematodeCaenorhabditis elegans. Laser-microbeam ablation of cells that give rise to the germ line causes adults to become giant. Ablation of these cells in self-sterile mutant worms also causes gigantism, suggesting that the germ line represses growth because it is the source of a growth-antagonizing signal rather than because of a sink of resources required for reproduction. TheC. elegansgerm line also emits a signal that represses longevity. This longevity-repressing signal requires the activity of DAF-16, a forkhead/winged-helix transcription factor, but we find that that the growth-repressing signal does not. The growth-repressing signal also does not require the activity of DBL-1, a transforming growth factor β-related protein that promotes growth in worms. By ablating the germ-line precursors of other species of free-living nematodes, we also found that both the growth-repressing and longevity-repressing signals are evolutionarily variable. Some species have both signals; others have just one or the other. We suggest that variation in germ-line signaling contributes to body size and life-history diversity in the nematodes.
Enhanced ADP-glucose pyrophosphorylase activity in wheat endosperm increases seed yield
Authors: Smidansky Eric D. , Clancy Maureen , Meyer Fletcher D. , Lanning Susan P. , Blake Nancy K. , Talbert Luther E. , Giroux Michael J.
Published: July 2002

Yield in cereals is a function of seed number and weight; both parameters are largely controlled by seed sink strength. The allosteric enzyme ADP-glucose pyrophosphorylase (AGP) plays a key role in regulating starch biosynthesis in cereal seeds and is likely the most important determinant of seed sink strength. Plant AGPs are heterotetrameric, consisting of two large and two small subunits. We transformed wheat (
Triticum aestivum
L.) with a modified form of the maize (
Zea mays
L.)
Shrunken2
gene (
Sh2r6hs
), which encodes an altered AGP large subunit. The altered large subunit gives rise to a maize AGP heterotetramer with decreased sensitivity to its negative allosteric effector, orthophosphate, and more stable interactions between large and small subunits. The
Sh2r6hs
transgene was still functional after five generations in wheat. Developing seeds from
Sh2r6hs
transgenic wheat exhibited increased AGP activity in the presence of a range of orthophosphate concentrations
in vitro
. Transgenic
Sh2r6hs
wheat lines produced on average 38% more seed weight per plant. Total plant biomass was increased by 31% in
Sh2r6hs
plants. Results indicate increased availability and utilization of resources in response to enhanced seed sink strength, increasing seed yield, and total plant biomass.
Mice transgenically overexpressing sulfonylurea receptor 1 in forebrain resist seizure induction and excitotoxic neuron death
Authors: Hernández-Sánchez Catalina , Basile Anthony S. , Fedorova Irina , Arima Hiroshi , Stannard Bethel , Fernandez Ana M. , Ito Yutaka , LeRoith Derek
Published: July 2002

The ability of the sulfonylurea receptor (SUR) 1 to suppress
seizures and excitotoxic neuron damage was assessed in mice
transgenically overexpressing this receptor. Fertilized eggs from FVB
mice were injected with a construct containing SUR cDNA and a
calcium-calmodulin kinase IIα promoter. The resulting
mice showed normal gross anatomy, brain morphology and histology, and
locomotor and cognitive behavior. However, they overexpressed the SUR1
transgene, yielding a 9- to 12-fold increase in the density of
[
3
H]glibenclamide binding to the cortex, hippocampus, and
striatum. These mice resisted kainic acid-induced seizures, showing a
36% decrease in average maximum seizure intensity and a 75% survival
rate at a dose that killed 53% of the wild-type mice. Kainic
acid-treated transgenic mice showed no significant loss of hippocampal
pyramidal neurons or expression of heat shock protein 70, whereas
wild-type mice lost 68–79% of pyramidal neurons in the CA1–3
subfields and expressed high levels of heat shock protein 70 after
kainate administration. These results indicate that the transgenic
overexpression of SUR1 alone in forebrain structures significantly
protects mice from seizures and neuronal damage without interfering
with locomotor or cognitive function.
Two different neurodegenerative diseases caused by proteins with similar structures
Authors: Mo Huaping , Moore Richard C. , Cohen Fred E. , Westaway David , Prusiner Stanley B. , Wright Peter E. , Dyson H. Jane
Published: July 2002

The downstream prion-like protein (doppel, or Dpl) is a paralog of
the cellular prion protein, PrP
C
. The two proteins have
≈25% sequence identity, but seem to have distinct physiologic roles.
Unlike PrP
C
, Dpl does not support prion replication;
instead, overexpression of Dpl in the brain seems to cause a completely
different neurodegenerative disease. We report the solution structure
of a fragment of recombinant mouse Dpl (residues 26–157) containing a
globular domain with three helices and a small amount of β-structure.
Overall, the topology of Dpl is very similar to that of
PrP
C
. Significant differences include a marked kink in one
of the helices in Dpl, and a different orientation of the two short
β-strands. Although the two proteins most likely arose through
duplication of a single ancestral gene, the relationship is now so
distant that only the structures retain similarity; the functions have
diversified along with the sequence.
Crystal structure of an anti-carbohydrate antibody directed againstVibrio choleraeO1 in complex with antigen: Molecular basis for serotype specificity
Authors: Villeneuve S. , Souchon H. , Riottot M.-M. , Mazié J.-C. , Lei P.-s. , Glaudemans C. P. J. , Kováč P. , Fournier J.-M. , Alzari P. M.
Published: July 2002

The crystal structure of the murine Fab S-20-4 from a protective anti-cholera Ab specific for the lipopolysaccharide Ag of the Ogawa serotype has been determined in its unliganded form and in complex with synthetic fragments of the Ogawa O-specific polysaccharide (O-SP). The upstream terminal O-SP monosaccharide is shown to be the primary antigenic determinant. Additional perosamine residues protrude outwards from the Ab surface and contribute only marginally to the binding affinity and specificity. A complementary water-excluding hydrophobic interface and five Ab–Ag hydrogen bonds are crucial for carbohydrate recognition. The structure reported here explains the serotype specificity of anti-Ogawa Abs and provides a rational basis toward the development of a synthetic carbohydrate-based anti-cholera vaccine.
Single-molecule height measurements on microsomal cytochrome P450 in nanometer-scale phospholipid bilayer disks
Authors: Bayburt Timothy H. , Sligar Stephen G.
Published: July 2002

The architecture of membrane proteins in their native environment of the phospholipid bilayer is critical for understanding physiological function, but has been difficult to realize experimentally. In this communication we describe the incorporation of a membrane-anchored protein into a supported phospholipid bilayer. Cytochrome P450 2B4 solubilized and purified from the hepatic endoplasmic reticulum was incorporated into phospholipid bilayer nanostructures and oriented on a surface for visualization by atomic force microscopy. Individual P450 molecules were observed protruding from the bilayer surface. Problems associated with deformation of the protein by the atomic force microscopy probe were avoided by analyzing force-dependent height measurements to quantitate the height of the protein above the bilayer surface. Measurements of the atomic force microscopy cantilever deflection as a function of probe-sample separation reveal that the top of the P450 opposite the N-terminal membrane anchor region sits 3.5 nanometers above the phospholipid–water boundary. Models of the orientation of the enzyme are presented and discussed in relation to membrane interactions and interaction with cytochrome P450 reductase.