bluuebunny/crossref_metadata_2025 huggingface dataset:
Second harmonic generation by counter propagating beams in 4-dimethylamino-4′-nitrostilbene side-chain polymer channel waveguides
Authors: Otomo Akira , Mittler-Neher Silvia , Bosshard Christian , Stegeman George I. , Horsthuis Winnfried H. G. , Möhlmann Gustaaf R.
Published: July 2002

We demonstrate surface emitted second harmonic generation due to mixing of counter propagating waves in in-plane poled, low loss, channel waveguides of 4-dimethylamino- 4′-nitrostilbene (DANS) side-chain polymers. Single film DANS side-chain polymer waveguides yield high conversion efficiency devices.
Limits of Si/CoSi2/Si heterotransistors at high frequencies: Measurement and estimation
Authors: Uffmann Dirk , Zaage Stefan
Published: July 2002

We report on the characterization of Si/CoSi2/Si heterotransistors. Transistor action results from the current flow through pinholes in the CoSi2 base layer. By fabricating integrated transistor structures the high-frequency performance of Si/CoSi2/Si heterotransistors was accessed for the first time. The cutoff-frequency fα of a low-α device is higher than 20 GHz. Using a deduced model of the transistor we conclude that this result should be true for high-α devices as well. A maximum oscillation frequency of 45 GHz is calculated for an experimental device with improved contact resistance. The high-frequency performance of the devices is enhanced by increasing the emitter current. Considering the limits for high current operation we propose an advantageous configuration of pinholes and how to achieve this configuration.
Field induced decoupling of superconducting bands in oxygen deficient melt-textured YBa2Cu3O7−x
Authors: Martínez B. , Gomis V. , Piñol S. , Catalán I. , Fontcuberta J. , Obradors X.
Published: July 2002

A layered melt-textured YBa2Cu3O7−x superconductor has been prepared consisting of alternate fully oxygenated and oxygen deficient bands stacked parallel to a-b planes. An anomalous decrease of Jcc(H∥ab) with field is observed at high temperatures. We show that this behavior is a signature of a field induced decoupling of the strong superconducting bands, and provides evidence of a decrease of vortex pinning energy in oxygen deficient regions of melt-textured YBa2Cu3O7−x. A large downwards shift of the irreversibility line results as a consequence of the substantially smaller area where a critical state sets in. When vortices are parallel to the c-axis Jcab(H∥c) decreases due to the smaller condensation energy of the oxygen deficient bands.
Pulsed-laser ablation growth of epitaxial ZnSe1−xSx films and superlattices with continuously variable composition
Authors: McCamy J. W. , Lowndes Douglas H.
Published: July 2002

We describe a new method to grow epitaxial semiconductor alloys with continuously variable composition, while using a single pulsed laser ablation target of fixed composition. Epitaxial ZnSe1−xSx films with continuously variable sulfur content, x, were grown by ablating a ZnSe target through low-pressure ambient H2S gas. The sulfur content was easily controlled in the range 0<x<0.18 by varying the H2S partial pressure from 0 to 45 mTorr, for films grown at 325 °C. ZnSe1−xSx films differing in composition by as much as x=0.52 from the pure ZnSe target have been grown at 400 °C. We have used this method to grow heteroepitaxial structures with continuously graded or periodically repeating, abrupt compositional changes (compositional superlattices). This development removes the principal barrier to pulsed-laser ablation (PLA) growth of compositionally graded semiconductor thin-film materials, namely that the film and target normally have the same composition. The method appears to have broad application for PLA growth of other compound semiconductor films and heterostuctures, as well as to dope individual layers.
Structure of Ga0.47In0.53As epitaxial layers grown on InP substrates at different temperatures
Authors: Zakharov N. D. , Liliental-Weber Z. , Swider W. , Brown A. S. , Metzger R.
Published: July 2002

The evolution of defect structures and surface reconstruction of Ga0.47In0.53As epitaxial layers grown by molecular beam epitaxy on InP substrate have been investigated by TEM and RHEED over a wide growth temperature range (150 °C≤Tg≤450 °C) before and after annealing. In the growth temperature range 400 °C≤Tg≤450 °C, extensive segregation of In near the layer surface takes place. The maximum of In concentration was found to lie under the layer surface. The kinetic of surface pit formation after annealing was explained in terms of surface reconstruction and As clustering. Extensive As clustering was observed after annealing for the growth temperature range 175 °C≤Tg≤300 °C. Analysis of diffuse scattering and dark-field images made it possible to propose an atomic model of cluster structure. Precipitate formation close to the interface was observed only for the samples grown at 150 °C and annealed at 500 °C. Differences in electrical properties between LT-grown GaAs and Ga0.47In0.53As are explained in terms of the structural defects.
Enhancement of critical current density by confinement of vortices in Bi2Sr2CaCu2Ox single crystals and thin films
Authors: Metlushko V. V. , Güntherodt G. , Wagner P. , Adrian H. , Goncharov I. N. , Moshchalkov V. V. , Bruynseraede Y.
Published: July 2002

Thin dc-sputtered Bi2Sr2CaCu2Ox films show very high values of the critical current density jc (jc≊1.7×107 A/cm2 at T=6 K and jc≊1×106 A/cm2 at T=60 K, H=0) which are at low temperature the same as for irradiated Bi-2212 single crystals. In the high temperature range jc of the thin films is enhanced over jc of as-grown and Kr-irradiated single crystals. The magnetically measured current-voltage characteristics E(j) and the pinning potential U(j) have been used to characterize the difference in the pinning regimes in single crystals and thin films.
Novel stable resonator for large-volume TEM00 mode operation
Authors: Takenaka Yushi , Nishimae Jun-ichi , Kuzumoto Masaki , Yoshizawa Kenji
Published: July 2002

A novel stable resonator is developed to realize stable operation at a TEM00 mode with a large beam diameter. The output coupler has a circular partial-reflection region in the center surrounded by an antireflection region. The transverse mode is mainly selected by the partial-reflection region. An amplified diffraction beam is extracted from the outer antireflection region. By combining part of the ‘‘core’’ beam transmitted from the partial-reflection region with the amplified diffraction beam, a diffraction-limited beam with the M2 factor of 1.3 is obtained from a high power continuous wave CO2 laser as predicted theoretically.
Relations between preionization density distribution, electrode design, and efficiency in high-pressure discharge-excited gas lasers
Authors: Turner M. M.
Published: July 2002

Certain types of discharge-excited gas lasers are routinely operated at pressures of 1 atm or more; these high-pressure glow discharges are formed using preionization, which is effective only when uniform and of adequate density. Using experiments and a two-dimensional discharge model, this letter shows that even when these requirements are met, preionization that strays outside the active volume can reduce the laser efficiency by as much as 40%. The use of wide, smoothly contoured electrodes—such as are used in conjunction with high-speed gas flow in high average power lasers—exacerbates this problem. It is therefore important to ensure that preionization is restricted to the active volume of the laser.
Crystallography and electromigration resistance of epitaxial Al films grown on (011)Si substrates by the sputtering method
Authors: Niwa H. , Teramae Satoshi , Kato Masaharu
Published: July 2002

From transmission electron microscopy observations, Al films sputtered on (011)Si substrates were found to grow epitaxially, similar to those on (001)Si and (111)Si substrates. The condition for single crystallization of the epitaxial Al films was obtained by changing heat treatments and Al film thickness. In addition, a simple acceleration test revealed that the electromigration resistance of Al interconnects on a (011)Si substrate was much higher than that on a SiO2 film.
Role of gas-phase adducts in the growth of gallium arsenide by metalorganic vapor-phase epitaxy
Authors: Foster Douglas F. , Glidewell Christopher , Cole-Hamilton David J.
Published: July 2002

Heating hex-5-enylarsine (hexAsH2) in the presence of trimethylgallium in solution produces methylenecyclopentane via a series of intermediates which include the adduct, [Me3Ga⋅As(hex)H2]. In the gas phase, the major C6 product obtained from decomposition of hexAsH2 in the presence or absence of Me3Ga is hex-1-ene, indicating that the adduct is not formed to any significant extent in the gas phase. Calculations on [Me3Ga⋅AsH3] indicate that, whereas at room temperature the formation constant from Me3Ga and AsH3 is 0.24, the unfavorable entropy of this formation reaction ensures that the adduct will not be present to a significant extent in the gas phase at 900 K.
Pulse broadening in picosecond amplification by a 1.3 μm InGaAsP traveling-wave amplifier
Authors: Liu Hai-Feng , Tohyama Masaki , Kamiya Takeshi , Kawahara Masato
Published: July 2002

Pulse broadening is observed for the amplification of a 2.2 ps pulse by a semiconductor laser traveling-wave amplifier when the input pulse energy is well below the saturation energy of the amplifier. The broadening is found to be much larger than that predicted on the basis of homogeneous gain saturation. It is shown from a traveling-wave rate equation simulation that the nonlinear gain compression is responsible for such a large temporal broadening.
Fowler–Nordheim tunneling current in a Mg/polycrystalline Si oxide/n+ polycrystalline Si metal-oxide-silicon structure
Authors: Kong Sik On , Kwok Chee Yee
Published: July 2002

Fowler–Nordheim tunneling from a Mg/polycrystalline silicon oxide/n+ polycrystalline silicon structure has been studied. The Fowler–Nordheim tunneling barrier height is greatly reduced using Mg as the cathode instead of monocrystalline silicon (denoted by Si) or polycrystalline silicon (denoted by polycrystalline Si). A minimum value of 0.94 eV is observed after sintering the sample in N2 at 260 °C for 185 min. The tunneling from the n+ polycrystalline Si side of the structure is also enhanced by the sharp point effect. This may have applications in electrically erasable read only memory, where a lower programing voltage, faster programing time, and enhanced endurance to repeated programing is desirable.
Observation of optically detected magnetic resonance in GaN films
Authors: Glaser E. R. , Kennedy T. A. , Crookham H. C. , Freitas J. A. , Khan M. Asif , Olson D. T. , Kuznia J. N.
Published: July 2002

Optically detected magnetic resonance has been observed from GaN. Two magnetic resonances have been detected on the 2.2 eV-deep photoluminescence band. The first resonance is sharp [full width at half-maximum (FWHM) ∼2.2 mT] with g∥=1.9515±0.0002 and g⊥=1.9485±0.0002 and is assigned to conduction electrons, in agreement with recent electron paramagnetic resonance (EPR) studies of similar samples. The second feature, which has not been seen by EPR, is much broader (FWHM∼13 mT) with g∥=1.989±0.001 and g⊥=1.992±0.001. These parameters indicate a deep state. A tentative assignment is made to a deep state associated with the N vacancy.
Transition time and turn-on jitter of optically triggered bistable lasers incorporating a proton bombarded absorber
Authors: Landais P. , Duan G.-H. , Gaumont-Goarin E. , Garabédian P. , Jacquet J.
Published: July 2002

Recently, we have demonstrated 1 Gbit/s error free operation of an optically triggered bistable laser, which is the highest bit-rate operation ever reported without electrical reset. This is achieved by using proton bombardment of the absorber section which induces a reduction of the carrier lifetime in this section. This letter presents the first experimental results on the transition time and the turn-on jitter of an optically triggered bistable laser incorporating a proton bombarded absorber section. Current injection conditions that minimize the overall transition time have been derived. It has been found that for a constant current in one active section, the rise time decreases, but the fall time increases with injection current in the other section. An optimal current value can then be found to minimize the overall transition time. The measured turn-on jitter and relaxation oscillation period decrease with increasing injection current, in good agreement with theoretical predictions.
Strained single quantum well InGaAs lasers with a threshold current of 0.25 mA
Authors: Chen T. R. , Eng L. E. , Zhao B. , Zhuang Y. H. , Yariv A.
Published: July 2002

Strained layer single quantum well InGaAs lasers with a record low threshold current of 1 mA for as-cleaved facets and 0.25 mA with high reflectivity coated facets have been demonstrated. In addition, these lasers display a weak dependence of threshold current, quantum efficiency, and lasing wavelength on cavity length in comparison with those single quantum well lasers previously reported.
High quality YBa2Cu3O7 films on polyalumina
Authors: Ganapathi L. , Giles S. , Rao Rama , Zheng J. P. , Kwok H. S.
Published: July 2002

Superconducting YBCO (YBa2Cu3O7) films were deposited on polycrystalline alumina substrates with c-normal orientation. YBCO films deposited directly on alumina exhibited a broad transition with Tc, zero=60 K. The Tc, zero increased to 86 K upon the use of a YSZ (yttria stabilized zirconia) buffer layer to stop the interface reaction. However, the Jc of these films was still below 104 A/cm2. A simple surface modification of the polycrystalline alumina substrate by way of depositing a YSZ buffer and repolishing the surface drastically improved the Jc. Values of 104 A/cm2 observed at 30 K is the best Jc reported so far for YBCO films on polycrystalline alumina. We believe further improvements in both the surface finish of the polyalumina and the YSZ buffer layer will lead to microwave quality YBCO films on polyalumina.
Novel self-mode-locking mechanism in narrow-band lasers
Authors: Bai Yingxin , Chen Shisheng , Wang Zhijiang , Zhang Guoqing
Published: July 2002

We suggest a novel self-mode-locking (SML) mechanism in narrow-band lasers. The analysis for such a mechanism show that the gain-line splitting (Stark splitting) induced by an intracavity laser field can form a dip or dips in gain-line shape, and the gain medium with such a gain-line shape can produce short pulses. The further analyses give the rough criterion for SML, i.e., 8 ln(2)Δω2s≳Δω2g for the gas lasers and 12Δω2s≳Δω2g for the solid or liquid lasers.
Raman study of Al0.54In0.46P/GaAs interfaces grown by solid source molecular beam epitaxy
Authors: Johnson F. G. , Wicks G. W.
Published: July 2002

An Al0.54In0.46P/GaAs superlattice was grown using a new approach to molecular beam epitaxy that employs solid-source valved crackers to supply the As4 and P2 group V fluxes. The superlattice sample was characterized by off-resonance Raman backscattering spectroscopy. In comparison to other III-V superlattices, the acoustic region of the Raman spectrum from this superlattice was especially rich, and phonon doublets up to the seventh order were visible in the spectrum. The interface roughness in the superlattice was determined to be two monolayers using a simple photoelastic model to characterize the intensities of the folded longitudinal-acoustic phonon peaks. The minimal intermixing present at the interfaces demonstrates the ability to abruptly switch between group V fluxes using the solid-source valved crackers.
Novel binary buffer layers for applications in the heteroepitaxy of highly mismatched In0.53Ga0.47As epilayers grown on GaAs substrates
Authors: Chang Shou-Zen , Lee Si-Chen , Shiao Hung-Pin , Lin Wei , Tu Yuan-Kuang
Published: July 2002

From the studies of the initial growth mechanisms of ternary InGaAs and binary InAs epilayers on GaAs, a novel binary buffer layer concept is proposed for application in the large lattice-mismatched heteroepitaxy. Although the growth of this binary buffer layer results in three-dimensional island growth at the initial stage, no vertical-type dislocations are generated and most dislocations are annihilated during the island coalescence stage. Using this new concept and selecting InP as the binary buffer, device-quality In0.53Ga0.47As grown on InP-coated GaAs substrates can be achieved. This technique has the potential to be further applied to other heteroepitaxial systems.
Magnetic field dependence of flux pinning force in a high T
c Bi1.6Pb0.4Sr2Ca2Cu3Oy superconductor
Authors: Kim Mun-Seog , Yu Seong-Cho , Oh Suhk-Kun
Published: July 2002

The flux pinning force in a Bi1.6Pb0.4Sr2Ca2Cu3Oy high Tc superconductor was obtained from the angular dependence of magnetization of the sample in fields from 150 to 1000 Oe at 80 K. The pinned fluxoid rotates with the sample like a rigid body at low fields. With further increase in the magnetic field H, the angular dependence of field-cooled magnetization disappears gradually above a critical rotation angle δ. This feature has been analyzed by the balance of two different kinds of opposing torques acting on the flux lines. Flux pinning force per unit length calculated from the torque balance under the external fields between 150 and 1000 Oe is about 10−3 dyn/cm at 80 K.
Can electrical deactivation of highly Si-doped GaAs be explained by autocompensation?
Authors: Schuppler S. , Adler D. L. , Pfeiffer L. N. , West K. W. , Chaban E. E. , Citrin P. H.
Published: July 2002

Using near-edge x-ray absorption fine structure, the first experimental determination of Si atom concentrations occupying As sites in Si-doped GaAs (100) is reported. The measurements reveal that at high doping levels (≳1019 cm−3) in molecular-beam-epitaxy-grown samples, the number of such p-type Si atoms is insufficient to account for the observed large reduction of free-carriers.
Cross-sectional transmission electron microscopy study for Nb/AlOx-Al/Nb, Nb/ZrOx-Zr/Nb, and Nb/HfOx-Hf/Nb Josephson junctions
Authors: Morohashi Shin’ichi , Hasuo Shinya
Published: July 2002

We studied the interfaces of Nb Josephson junctions with an overlayer structure using cross-sectional transmission electron microscopy. We compared Nb/AlOx-Al/Nb, Nb/ZrOx-Zr/Nb, and Nb/HfOx-Hf/Nb junctions. The interface between the Zr layer and Nb base electrode, and that between the Hf layer and Nb were smoother than that between the Al layer and Nb. The roughness of the interface between Al and Nb is caused not by the roughness of the original Nb base electrode but grain boundary diffusion following Al deposition. Annealing increased the roughness of the interface between Al and Nb, but had very little effects on the interfaces between Zr and Nb, or Hf and Nb. We attribute these effects to the differences in the melting points of the various overlayers.
Emission spectra analyses of a new vacuum ultraviolet source
Authors: Sentis M. L. , Chazaud F. , Delaporte Ph. C. , Marine W. I.
Published: July 2002

Spectroscopic study of a formed-ferrite plasma source has been carried out in the 115–210 nm spectral region. Emission spectra are dominated either by atomic lines superimposed on continuum emission depending on the surrounding gas. Atomic lines are related to the emission from electronic transitions of singly or multi-ionized atomic species of the ferrite. The electronic density of such a vacuum ultraviolet source is estimated to 4×1018 cm−3 based on Stark broadening effect.
Free-standing single-crystalline chemically vapor deposited diamond films
Authors: Tzeng Y. , Wei J. , Woo J. T. , Lanford W.
Published: July 2002

Free-standing chemically vapor deposited (CVD) single-crystalline diamond films have been fabricated by a sequence of processes including high energy ion implantation, microwave plasma enhanced chemical vapor deposition, and the final separation of the diamond epilayer from the substrate by heating in oxygen. The homoepitaxial diamond film separated from the substrate is about 15 μm thick. It is flat and transparent, and exhibits a sharp diamond Raman peak without nondiamond background signals. This process is promising for mass production of large-area single-crystalline diamonds with the size restricted only by the availability of one large initial diamond substrate. The initial substrate is expensive but can be used repetitively to produce a large number of single-crystalline CVD diamond films.
Forward delay in scaled Al0.48In0.52As/In0.53Ga0.47As heterojunction bipolar transistors
Authors: Baquedano J. A. , Levi A. F. J. , Jalali B. , Cho A. Y.
Published: July 2002

We present experimental measurements and numerical simulations of the intrinsic forward delay as a function of base thickness in abrupt junction n-p-n Al0.48In0.52As/In0.53Ga0.47As heterojunction bipolar transistors. For base thicknesses up to 1350 Å and impurity concentration p=1.5×1019 cm−3 we find that nonequilibrium electron transport ensures that base transit delay is less than that in the 3000-Å-thick collector space-charge region. This provides an opportunity to increase base thickness and reduce base resistance without sacrificing the intrinsic forward delay time.
Native-oxide-defined low-loss AlGaAs-GaAs planar waveguide bends
Authors: Caracci S. J. , Krames M. R. , Holonyak N. , Herzinger C. M. , Crook A. C. , DeTemple T. A. , Besse P.-A.
Published: July 2002

Data are presented on planar waveguide S-bends fabricated from AlxGa1−xAs-GaAs p-n quantum well heterostructure (QWH) laser material by a wet native oxidation process. The oxide’s low refractive index (n∼1.55) is used to define ∼3.5-μm-wide single mode guides exhibiting low excess losses for raised-cosine S-bends with 100 μm offsets. The waveguide’s routing properties are determined by the lateral effective index profile which is controlled by the native oxide thickness. Guides with two different oxidation depths are investigated. For the deepest oxidation, excess bend losses of 3 dB are measured for transition distances of ∼180 μm and ≲120 μm for transverse electric (TE) and transverse magnetic (TM) polarizations, respectively. Theoretical loss calculations for the guides show good agreement with measured data. Guides with a shallow oxidation exhibit 3 dB transition lengths of ∼260 μm and ∼220 μm for the TE and TM polarizations.
Structure of niobium thin films on sapphire
Authors: McMorrow D. F. , Cowley R. A. , Gibaud A. , Ward R. C. C. , Wells M. R.
Published: July 2002

The structure of Nb thin films grown by molecular beam epitaxy on sapphire has been studied using high-resolution x-ray scattering techniques. Transverse scans of the x-ray wave vector transfer through the (110)Nb Bragg peak show diffuse scattering with a Lorentzian-squared profile, and satellite Bragg peaks for certain orientations of the crystal. The former feature arises from a random, two-dimensional network of Nb domains, and the latter from a periodic distortion of the Nb films at the terrace edges of the vicinal sapphire substrate.
Evidence of hydrogen modulation in amorphous germanium prepared by reactive evaporation
Authors: Houssaïni S. , Vergnat M. , Bruson A. , Marchal G. , Mangin Ph. , Vettier C.
Published: July 2002

Reactive evaporation of germanium under a flow of hydrogen atoms on substrates maintained at liquid-nitrogen temperature allows us to obtain a-Ge:H thin films and multilayers. The material contains a considerable amount of polygermane bonds and is porous, as shown by low angle x-ray scattering performed on Ge/Ge:H/... multilayers. The stability of hydrogen is measured by resistivity measurements and by effusion experiments. Finally, low angle neutron scattering shows that it is possible to modulate hydrogen in Ge/Ge:H/Ge/Ge:D/... multilayers with 40 Å thick layers.
Low-energy Ar+ ion bombardment-induced modification of surface atomic bond lengths on InP(100) wafer
Authors: Mangat P. S. , Soukiassian P. , Huttel Y. , Hurych Z. , Gruzza B. , Porte A.
Published: July 2002

We report the first direct measurement of surface interatomic bond distance modifications due to ion bombardment. The experiments were performed using low energy Ar+ ion on a InP(100) surface by photoemission x-ray absorption fine structure. The structural changes are sputtering time dependent and result in first (P–In) and second (P–P) surface bond distances relaxation approaching the bulk values. It suggests that, prior to bond breaking, the Ar+ ion beam first stretches atomic bond lengths during a precursor stage, with small energy and momentum transfer. This work brings new insights into the knowledge of ion sputtering various micromechanisms which is relevant for surface preparation.
New epitaxial multilayer system for spin-valve magnetic sensors
Authors: Childress J. R. , Durand O. , Schuhl A. , George J.-M.
Published: July 2002

Experimental results on a new ‘‘spin-valve’’ system for magnetoresistive detection, based on epitaxial Fe/Pd bilayers grown on MgO single crystals, are presented. These Fe/Pd bilayers display particularly sensitive magnetic properties, which are shown to result directly from the presence of the polarized Pd interface. A magnetically hard trilayer is subsequently grown onto the soft Fe/Pd bilayer, forming an epitaxial spin-valve system which displays abrupt transitions between states of parallel and antiparallel alignment of the two magnetic layers. The field sensitivity of the resistivity at low temperatures attains 0.3% per Oe in a Fe/Pd/Cu/Co/Cu system.
Traps for excitons and interstitial atoms in edge-defined film-fed growth silicon
Authors: Davies Gordon , Park Seung-Chul , Higgs V. , Sawyer William D.
Published: July 2002

We show from the power dependence of photoluminescence of as-grown edge-defined film-fed film-grown silicon (EFG Si) that there are few additional nonradiative traps for excitons in EFG Si relative to electronic grade silicon. The first application of cathodoluminescence topography to EFG Si reveals only a small (10%) decrease in the luminescence at grain boundaries from radiation-damage centers at grain boundaries. A radiation-damage complex formed by migration of an interstitial carbon atom is shown to be created, and also destroyed, at rates very similar to those in electronic grade silicon, indicating the absence of interstitial traps specific to EFG Si.
Submicron contact hole filling by electron bias sputtering
Authors: Onuki Jin , Nihei Masayasu
Published: July 2002

A new electron bias sputtering method capable of filling Al into submicron contact holes without degradation of the quality has been developed for the formation of reliable interconnections of very large scale integrated circuits. The method features intermittent electron irradiation to enhance Al flow during sputtering. The Al film deposited contains little Ar and has low resistivity. Contact holes with 0.4-μm diameter and 0.9-μm depth can be filled with Al.
Terahertz electromagnetic transients as probes of a two-dimensional electron gas
Authors: Walecki W. J. , Some D. , Kozlov V. G. , Nurmikko A. V.
Published: July 2002

We demonstrate the application of terahertz electromagnetic transients to the study of the dynamical conductivity of a two-dimensional, high mobility electron gas in modulation-doped GaAs quantum wells. The transient response of such an electron system on a picosecond time scale occurs in a regime where the implications of electron–electron interaction effects need be considered.
Broadband, intensity dependent absorption in tetravalent chromium-doped crystals
Authors: Munin Egberto , Villaverde A. Balbin , Zhang X. X. , Bass Michael
Published: July 2002

Cr4+-doped Y2SiO5 and Ca2Al2SiO7 have been investigated as broadband, solid state saturable absorbers. Saturation properties in the near infrared as well as a demonstration of tunable, passive Q switching of a flash-lamp pumped Cr:LiSAF laser are discussed.
Mechanical fatigue monitoring using absorption spectroscopy of infrared fibers
Authors: Barkay N. , Katzir A.
Published: July 2002

The spectral transmittance of infrared fibers was measured while they were undergoing flexing procedures leading to mechanical fatigue. Microscopic mechanical defects were detected through their influence on the optical losses, without interfering with the deformation. Such experiments were successfully carried out on infrared transmitting silver-halide polycrystalline fibers. The spectral characterization provides a probe of the defects, as their effect is related to the ratio between defect size and wavelength. The method discussed here may be used to examine theoretical models concerning the dynamics of mechanical defects and optical guiding in fibers with defects.
Formation of amorphous Ge-S semiconductor alloys by mechanical alloying
Authors: Shen T. D. , Wang K. Y. , Quan M. X. , Hu Z. Q.
Published: July 2002

Amorphous Ge1−xSx(x=0.61, 0.67, and 0.72) semiconductor alloys have been found formed by ball milling of powder mixtures of elemental Ge and S. The amorphization process and the ball-milled products have been investigated by x-ray diffraction, transmission electron microscopy, and Raman scattering. The results show that amorphous Ge-S alloys may form in a self-heating reaction, driven by the large heat of formation of amorphous and/or liquid Ge-S alloys. The structure of mechanically alloyed amorphous Ge-S alloys is similar to but not exactly identical with that of rapidly cooled amorphous Ge-S alloys. The amorphization transition mechanism reported is believed to be suitable for preparing other amorphous semiconductor alloys with similar thermodynamic conditions.
Low-frequency noise behavior of γ-irradiated partially depleted silicon-on-insulator n-channel metal-oxide-semiconductor transistors
Authors: Simoen E. , Claeys C.
Published: July 2002

This letter discusses the low-frequency noise behavior of partially depleted silicon-on-insulator n-channel metal-oxide-semiconductor transistors after γ irradiation to a total dose of 100 krad(Si). The noise characteristics both in linear operation and in saturation are investigated in detail. It is shown that the increase of the noise in the linear region is mainly due to the degradation of the sidewall isolation regions. In contrast, the so-called kink-related noise overshoot is hardly affected by the irradiation, indicating that its origin is most likely not interface related. This result will be discussed in view of a recently proposed model for the noise overshoot.
Anomalous doping behavior of in situ boron-doped polycrystalline silicon deposited by ultrahigh vacuum chemical vapor deposition
Authors: Lin Horng-Chih , Lin Hsiao-Yi , Chang Chun-Yen , Lei Tan-Fu , Wang P. J. , Deng Ray-Chern , Lin Jandel , Chao Chih-Yeh
Published: July 2002

The deposition of in situ heavily boron-doped polycrystalline silicon (poly-Si) films was studied using an ultrahigh vacuum/chemical vapor deposition system. Fully activated carrier concentrations up to 3×1020 cm−3 were obtained for the as-deposited films grown at 550 °C. For boron concentration beyond this level, the crystallinity of poly-Si films degraded with increasing boron concentration, which resulted in an anomalous rise in resistivity. This crystallinity degradation occurred at a higher rate for films grown on a SiO2 surface than those grown on an undoped poly-Si surface. It is attributed to the preferential adsorption of boron atoms on the SiO2 surface. Under a high B2H6 flux condition, a large amount of boron atoms would accumulate on the SiO2 surface before the formation of Si nuclei, and thus disturbs the subsequent film deposition and grain growth processes.
Improved process for high-T
c superconducting step-edge junctions
Authors: Sun J. Z. , Gallagher W. J. , Callegari A. C. , Foglietti V. , Koch R. H.
Published: July 2002

We have developed a process for the fabrication of YBaCuO high-Tc junctions based on the step-edge weak-link concept. The process emphasizes the creation of sharp and straight step edges on a substrate, and the restoration of oxygen content for superconducting materials at the step edges. A diamond-like carbon film is used as an ion milling mask for the creation of steps on substrates such as LaAlO3 and SrTiO3. Room-temperature plasma oxidation is shown to be effective in restoring Tc from processing related degradation for grains residing at the step edge. Using this process, dc SQUIDs were fabricated with 77 K electrical performances matching, and in certain cases exceeding, similar SQUIDs made using bicrystal-based tilt-boundary weak-link junctions.
Real-time control of molecular beam epitaxy by optical-based flux monitoring
Authors: Chalmers S. A. , Killeen K. P.
Published: July 2002

We have developed a real-time molecular beam epitaxy control system based on Al and Ga atomic beam resonant absorption of hollow cathode lamp emission. By continuously monitoring the absorption of Al and Ga beams during growth, this system accurately determines instantaneous growth rates and then integrates these over time to determine layer thickness. This information is used in real time for effusion cell shutter (and hence layer thickness) control. The accuracy and flexibility of this system is demonstrated here by growing AlAs/GaAs distributed Bragg reflectors with consistent layer thicknesses even though effusion cell temperatures were intentionally varied. In each instance the system automatically detected and compensated for the different growth rates, resulting in DBRs with center wavelengths controlled to within 0.3% of the target wavelength.
a-axis oriented epitaxial YBa2Cu3O7−y/PrBa2Cu3−xNbxO7−z multistructures
Authors: Hontsu S. , Mukai N. , Ishii J. , Kawai T. , Kawai S.
Published: July 2002

Using the pulsed laser deposition technique, we have successfully prepared a-axis oriented PrBa2Cu3−xNbxO7−z (PBCNO)(x=0–1.5) films and YBa2Cu3O7−x (YBCO)/PBCNO heterorepitaxial multilayer structures. For the PBCNO films, the resistivity showed semiconducting behavior at all Nb substitution levels investigated. The resistivity increased with increasing Nb content, reaching a value of 104 Ω cm at 100 K for a 300-Å-thick film with x=1.0. Using (100)SrTiO3 (STO) as a substrate, the following multilayer structures were successfully formed: (100)PBCNO(x=1.0)/(100)YBCO/(100)PBCO//STO, (001) YBCO/(100)PBCNO(x=1.0)/(100)PBCO//STO, and (100)YBCO/(100)PBCO/(100)PBCNO(x=1.0)/(100)PBCO//STO. The bottom and top YBCO layers in the (100)YBCO/(100)PBCO/(100)PBCNO/(100)PBCO/(100)YBCO//STO structure showed superconducting transition temperatures (zero resistance) of 82 and 83 K, respectively. The interlayer resistivity of the 200-Å-thick PBCNO(x=1) interlayer was as high as 105 Ω cm at 30 K.
Similarities between the Landau spectra and dispersion relations in Si/Si1−xGex quantum wells investigated by magnetotunneling spectroscopy
Authors: Gennser Ulf , Kesan V. P. , Syphers D. A. , Smith T. P. , Iyer S. S. , Yang E. S.
Published: July 2002

The current-voltage characteristics of Si/Si1−xGex hole resonant tunneling structures in the presence of strong magnetic fields both parallel and perpendicular to the interfaces revealed new resonances for the latter configuration due to Landau level tunneling. However, there are remarkable similarities between the spectra for the two different magnetic field directions. This can be understood in a semiclassical context, and indicates that the broadening of the Landau levels in Si/Si1−xGex hole resonant tunneling diodes is severe.
Reverse current mechanisms in amorphous silicon diodes
Authors: Kramer N. , van Berkel C.
Published: July 2002

We analyze the dark steady-state reverse current of a-Si:H nip diodes at high voltages. The reverse current shows a strong voltage dependence and has a temperature dependence characterized by a voltage-dependent activation energy. A model, based on the simultaneous field enhanced generation of electrons and holes, is developed to describe this voltage and temperature dependence. In this model the effective mass of electrons and holes is a model parameter. Good fits with experimental results are obtained for an effective mass value of 0.05me. The low effective mass value is tentatively explained as a parameter that accounts for a field dependent narrowing of the band gap due to the presence of localized band tail states.
Enhancement of electron-wave reflection by superlattices with multiple stacks of multiquantum barriers
Authors: Yen S. T. , Tsai C. M. , Lee C. P. , Liu D. C.
Published: July 2002

We report a superlattice (SL) with multiple stacks of multiquantum barriers (MQB), which can reflect electrons more effectively than with a single stack of MQB. The reflectivities are calculated and compared with one another for a variety of potential barrier structures. The multistack SL has a wider energy spectrum within which electrons are reflected. Four types of n-GaAs/i-barrier/n-GaAs diodes were fabricated to confirm the calculated results. The current-voltage characteristics measured at 77 K for these diodes show that the turn-on voltage increases with the number of stacks of MQBs in the SL. This is in agreement with our calculated results.
Reactive rapid sequential pulse laser deposition of YBa2Cu3O7: A candidate to eliminate particulate formation
Authors: Watanabe Yukio
Published: July 2002

We propose a modified version of pulsed laser deposition: rapid sequential pulsed laser deposition. In this method, different metal elements are sequentially deposited onto a substrate in a fixed stoichiometric ratio in a very short period of time. This method allows a flexible choice of target material and of laser power density, which may solve the particulate problem intrinsic to a conventional laser ablation. We show results of an in situ formation of YBa2Cu3O7 thin film as a demonstration of this method.
Comparison of graded and abrupt junction In0.53Ga0.47As heterojunction bipolar transistors
Authors: Baquedano J. A. , Levi A. F. J. , Jalali B.
Published: July 2002

We compare calculated intrinsic forward delay as a function of base thickness and p-type doping level in n-p-n heterojunction bipolar transistors with graded as well as abrupt Al0.48In0.52As/In0.53Ga0.47As and InP/In0.53Ga0.47As emitter-base junctions. We find that, for a given p-type concentration and fixed base delay time, an InP/In0.53Ga0.47As abrupt junction design results in high intrinsic speed, low base resistance, and modest power consumption.
Resonance effects in Raman scattering from InAs/AlSb quantum wells
Authors: Wagner J. , Schmitz J. , Ralston J. D. , Koidl P.
Published: July 2002

Resonant Raman scattering has been used to study GaSb-capped AlSb/InAs/AlSb quantum wells, grown by molecular-beam epitaxy with a shutter sequence intended to promote the formation of an InSb-like InAs/AlSb heterointerface. For optical excitation in resonance with the E1 band gap of InAs, scattering by one-longitudinal-optical (LO) phonon and by two-LO phonons from the quantum well layer is strongly enhanced. Excitation close to the E1/E1+Δ1 gap resonance of GaSb, in contrast, enhances the one-LO scattering from the GaSb cap over that from the InAs quantum well. Thus resonance effects in LO phonon Raman scattering allow us to distinguish between scattering from these two materials despite the near-coincidence of the corresponding LO phonon frequencies. Scattering by an InSb-like mode is found to resonate at approximately the InAs E1/E1+Δ1 gap energy, supporting its assignment to an interface mode.
Platinum-enhanced densification of grain-aligned YBa2Cu3O7−x films
Authors: Lewis J. A. , Wegmann M. , Platt C. E. , Teepe M.
Published: July 2002

Grain-aligned YBa2Cu3O7−x films were fabricated by vacuum filtration in an applied magnetic field (7 T). Platinum (Pt) was shown to lower the peritectic temperature of YBa2Cu3O7−x by almost 75 °C, leading to dramatic microstructural differences between films densified on MgO (single crystal) substrates and those densified on Pt foil to a maximum temperature of 1020 °C in oxygen. Superconducting quantum-interference device hysteresis loops measured at 0–5.5 T at 5 K showed that films fired on Pt had ΔM (Happ∥c axis) values 15–60 times larger than those fired on MgO.
Direct observation of ferroelastic phase transition in Er-doped lanthanum pentaphosphate with synchrotron topography
Authors: Hu Z. W. , Jiang S. S. , Huang P. Q. , Huang X. R. , Feng D. , Wang J. Y. , Li L. X.
Published: July 2002

In situ observation of the ferroelastic phase transition in Er-doped lanthanum pentaphosphate has been carried out by real time white-beam synchrotron radiation x-ray topography. The ferroelastic domain walls are found to vanish and reappear with characteristics of a classical second-order transition on topographs on cycling the sample between room temperature and above the transition temperature. However, fluctuations of the number of domain walls with temperature approaching the Curie point are revealed for the first time and shown to be reversible. Discussions on these interesting phenomena are presented.
Low threshold current dual wavelength planar buried heterostructure lasers with close spatial and large spectral separation
Authors: Beernink Kevin J. , Thornton Robert L. , Chung Harlan F.
Published: July 2002

A novel technique for achieving closely spaced laser devices with large, but well-controlled, wavelength separation is described. Preferential population of the lowest band-gap active region in a stacked active layer structure is used along with patterned etching of the long wavelength active regions and regrowth over all devices to achieve the desired wavelength. Dual-wavelength dual-stripe buried heterostructure lasers with 5 μm stripes on 20 μm centers formed by impurity-induced layer disordering in a stacked active layer structure have threshold currents of 9.1 and 10.9 mA for laser wavelengths of 846 and 760 nm, respectively.
Current contribution to electroabsorption of zinc-blende semiconductor quantum wells
Authors: Xue Deng-Ping
Published: July 2002

The coexistence of overlap and gradient terms in electroabsorption of zinc-blende semiconductor quantum wells is discussed. It is illustrated that the gradient is related to the exciton polarization current. A model is proposed to incorporate the current contribution. It is an improved version of the prevailing one. Its conclusions are consistent with experimental observation and the previous theoretical work.
Ultraviolet laser-projection patterning of polymeric materials for electrochemical gas sensors
Authors: Tejedor P. , Briones F.
Published: July 2002

ArF laser ablation has been successfully applied to maskless pattern by projection lithography of two polymeric materials, polysiloxane and polyHEMA (2-hydroxy-ethyl methacrylate), to be used as gas diffusion membranes in electrochemical gas sensors. Etch rates up to 0.65 μm/s with smooth surface morphology, high edge definition, and a resolution of ∼5 μm were obtained using laser fluences between 250 and 400 mJ/cm2 and repetition rates between 1 and 10 Hz in air for polyHEMA films and in nitrogen for polysiloxane films.
Effect of high-temperature annealing on the dissolution of the D-defects in n-type Czochralski silicon
Authors: Wijaranakula W. , Chiou H.-D.
Published: July 2002

The effect of high-temperature annealing on the dissolution of crystal grown-in defects referred to as D-defects in n-type Czochralski silicon under various annealing conditions was studied. Annealing at 1200 °C for 1 h in either oxygen or nitrogen ambient causes the dissolution of the D-defects while in contrast no effect of a high-temperature annealing in hydrogen ambient on the D-defects was observed. Based upon the present analysis, D-defect dissolution during oxygen and nitrogen annealing could be attributed to vacancy undersaturation resulting from silicon interstitial injection from the silicon surface. Annealing in hydrogen ambient is hypothesized to cause vacancies at the surface sites to migrate into the silicon lattice via a Schottky defect formation mechanism.
New method of deposition of biomolecules for bioelectronic purposes
Authors: Morales P. , Sperandei M.
Published: July 2002

A laser induced plasma vaporization and ionization technique is proposed for electric field assisted deposition of proteins. Experiments were carried out depositing thick layers of lysozyme on metal, creating submillimeter patterns. The enzymatic activity of horseradish peroxidase deposited by this method was tested, together with that of the enzyme laccase from a micro-organism. The applicability of this method to the construction of nanometric patterns for bioelectronic purposes is discussed.
Sb-induced interatomic bond distance stabilization on InP(100) surface
Authors: Mangat P. S. , Soukiassian P. , Huttel Y. , Gruzza B. , Porte A.
Published: July 2002

We have studied the formation of the Sb/InP(100) interface by photoemission extended x-ray absorption fine structure. Our results indicate that a low Sb coverage stretches and subsequently weakens P—In surface bonds which appears as a precursor stage prior to In—Sb bond formation. Then, at increasing Sb coverages, the substrate surface reconstructs with bonds approaching the relaxed clean InP(100) surface values. This results from the breakdown of In clusters (formed during surface preparation by ion sputtering) which leads to the formation of an InSb interface layer. The surface is finally found to be stabilized at higher Sb coverages with no more change in the substrate first and second near neighbor bond distances.
Novel self-electro-optic device using bulk Franz–Keldysh effects in an n+-GaAlAs-GaAs-Ag asymmetric Fabry–Perot device structure
Authors: Tayebati Parviz , Jauniskis Linas
Published: July 2002

We report a novel n+-Al0.3Ga0.7As/GaAs/Ag asymmetric Fabry–Perot optical modulator based on bulk Franz–Keldysh (FK) effects in GaAs. The modulator exhibits high on/off contrast (≳120:1) and high reflectivity (33%). Using a pair of these devices in series, we demonstrate a bistable FK self-electro-optic device (FK-SEED) operating at 872.5 nm with on/off contrast ratio of 87:1 and 27% reflectivity under 11-V bias. The negative differential responsivity necessary for SEED operation is provided through a combination of ‘‘normally off’’ mode of the Fabry–Perot and the negative electroabsorption at the wavelength of operation.
Influence of delta-doping profile and interface roughness on the transport properties of pseudomorphic heterostructures
Authors: Fernández de Avila S. , Sánchez-Rojas J. L. , González-Sanz F. , Calleja E. , Muñoz E. , Hiesinger P. , Köhler K. , Jantz W.
Published: July 2002

Thickness effects of the InGaAs channel on photoluminescence and transport properties of δ-doped Al0.3Ga0.7As/In0.3Ga0.7As heterostructures are investigated. The spreading of the Si δ-doping layer is deduced from a comparison of the measured charge with self-consistent calculations assuming a Gaussian Si distribution profile and a definite ionization probability of the Si-related DX centers. With decreasing channel thickness below 80 Å, the effect of the spreading on the sheet carrier concentration increases and the low temperature mobility decreases due to roughness scattering at the In0.3Ga0.7As/GaAs interface. In channels thicker than 80 Å the thickness-independent alloy scattering process dominates.
Broadband response of the quasiparticle current in a superconducting tunnel junction
Authors: Verghese S. , Karadi C. , Mears C. A. , Orenstein J. , Richards P. L. , Barfknecht A. T.
Published: July 2002

We have measured the linear response of the quasiparticle current in a Nb/AlOx/Nb junction from 75–200 GHz. Picosecond pulses of millimeter wave radiation were generated by illuminating a photoconductive switch with a mode-locked laser. The pulses were coupled to the junction through an impedance-mismatched, planar lithographed antenna. The broadband response of the superconductor-insulator-superconductor junction was measured by monitoring changes in the dc current induced by interfering two electrical pulses at the junction as a function of the time delay between them. The data are in agreement with the linear theory of photon-assisted tunneling.