bluuebunny/crossref_metadata_2025 huggingface dataset:
10–16 μm Broadband quantum well infrared photodetector
Authors: Bandara S. V. , Gunapala S. D. , Liu J. K. , Luong E. M. , Mumolo J. M. , Hong W. , Sengupta D. K. , McKelvey M. J.
Published: July 2002

A very long wavelength broadband infrared detector, sensitive over a 10–16 μm spectral range, based on GaAs/AlxGa1−xAs quantum wells grown by molecular beam epitaxy, has been demonstrated. Wavelength broadening of Δλ/λp∼42% is observed to be about a 400% increase compared to a typical bound-to-quasibound quantum well infrared photodetector (QWIP). In this device structure, which is different from typical QWIP device structures, two different gain mechanisms associated with photocurrent electrons and dark current electrons were observed and explained. Even with broader response, D*∼1×1010 cmHz/W at T=55 K is comparable to regular QWIPs with similar cutoff wavelengths.
Doping effect of viologen on photoconductive device made of poly (p-phenylenevinylene)
Authors: Park J. Y. , Lee S. B. , Park Y. S. , Park Y. W. , Lee C. H. , Lee J. I. , Shim H. K.
Published: July 2002

We report the photovoltaic properties of the donor–acceptor composite system of poly (p-phenylenevinylene) (PPV) and viologen. We observed the significant enhancement of photocurrent with increasing the doping ratio of viologen. The maximum photocurrent of viologen-doped PPV was nine times as high as that of the pristine PPV. The maximum quantum yield and photosensitivity are 13% (electron/photon) and 0.05 A/W, respectively, at low bias voltage (−2 V). The increase of photocurrent is explained with the efficient charge separation that resulted from the transfer of photoexcited electrons from PPV to viologen. The mixture of PPV and viologen can be used as a sensitive photodiode material.
Picojoule and submillisecond calorimetry with micromechanical probes
Authors: Nakagawa Y. , Schäfer R. , Güntherodt H.-J.
Published: July 2002

The experimental limits of micromechanical calorimetry at ambient conditions are demonstrated. In the investigation of rotator phase transitions of n alkanes we have obtained a heat sensitivity of 500 pJ for a sample mass of about 7 pg with a time resolution of 0.5 ms. The time resolution is restricted by the thermal response of the bimetallic cantilever. A problem in the precise quantification of the transition enthalpies is heat transport through the atmospheric environment. A calibration of the apparatus constant is proposed to overcome this problem.
Incorporation and optical properties of magnesium in cubic GaN epilayers grown by molecular beam epitaxy
Authors: As D. J. , Simonsmeier T. , Schöttker B. , Frey T. , Schikora D. , Kriegseis W. , Burkhardt W. , Meyer B. K.
Published: July 2002

The incorporation and optical properties of Mg in cubic GaN (c-GaN) epilayers grown by rf plasma-assisted molecular beam epitaxy on (100) GaAs are investigated by secondary ion mass spectroscopy and low-temperature photoluminescence (PL). By varying the Mg flux by more than four orders of magnitude, the incorporation of Mg saturates at high Mg flux and is limited to a value of about 5×1018 cm−3 due to the high volatility of Mg at growth temperature. In addition, we observe an accumulation of Mg at the GaN/GaAs interface due to a diffusion of Mg to the GaAs substrate. Low-temperature PL spectra reveal several well-separated lines. Besides a shallow acceptor level at EA≅0.230 eV, additional Mg-related deep defect levels indicate an incorporation of Mg at off-gallium sites or as complexes.
Current-induced local oxidation of metal films: Mechanism and quantum-size effects
Authors: Schmidt Thomas , Martel Richard , Sandstrom Robert L. , Avouris Phaedon
Published: July 2002

A novel route is introduced for oxidizing thin metal films with nanometer-scale resolution. By locally subjecting Ti and Nb films to high in-plane current densities, metal-oxide tunneling barriers are formed in a self-limiting fashion. The oxidation is triggered by current-induced atomic rearrangements and local heating. At the final stages of the barrier formation, when only atomic-scale channels remain unoxidized, the oxidation rate decreases drastically while the conductance drops in steps of about 2e2/h. This behavior gives evidence of ballistic transport and a superior stability of such metallic nanowires against current-induced forces compared with the bulk metal.
Relaxation of the Si lattice strain in the Si(001)–SiO2 interface by annealing in N2O
Authors: Ha Yong Ho , Kim Sehun , Lee Sun Young , Kim Jin Ho , Baek Doo Hyun , Kim Hyun Kyung , Moon Dae Won
Published: July 2002

It was clearly observed with medium energy ion scattering spectroscopy that the strain in the Si(001)–SiO2 interface of thermal oxides is relaxed by annealing in N2O. The strain relaxation could be correlated with the improved hot-electron hardness of the nitrided oxides compared with the thermal oxides. Based on the direct observation of the strain relaxation, it is suggested that the incorporated N atoms at the interface release the strain and increase the immunity of trap generation under the current stress.
Direct nanoimprint of submicron organic light-emitting structures
Authors: Wang Jian , Sun Xiaoyun , Chen Lei , Chou Stephen Y.
Published: July 2002

We have demonstrated a method to directly pattern organic light-emitting structures with a submicron resolution without any degradation in optical properties. Both small molecules and polymer-based light-emitting structures were patterned by nanoimprint at 150 °C in a vacuum. The comparison of luminescence efficiency before and after patterning shows that nanoimprint did not cause degradation in the optical property of the materials. Nanoimprint offers a low-cost, high-throughput, high-resolution patterning technique that opens a way for realizing novel photonic devices based on organic light-emitting materials.
Switching properties of SrBi2Ta2O9 thin films produced by metalorganic decomposition
Authors: Chen X. B. , Yan F. , Li C. H. , Zhang Z. G. , Zhu J. S. , Wang Y. N.
Published: July 2002

The process of polarization reversal in a SrBi2Ta2O9 (SBT) thin film capacitor produced by a metalorganic decomposition method was investigated. The switching time (100–600 ns) and the reversible polarization have been measured at different voltages in the range of 0–5 V. It was found that by annealing the SBT sample in AR atmosphere at 400 °C, the switching times were significantly reduced. This indicates that oxygen vacancies in the film speed up nucleation and growth of new domains.
Nature of the fundamental band gap in GaNxP1−x alloys
Authors: Shan W. , Walukiewicz W. , Yu K. M. , Wu J. , Ager J. W. , Haller E. E. , Xin H. P. , Tu C. W.
Published: July 2002

The optical properties of GaNxP1−x alloys (0.007⩽x⩽0.031) grown by gas-source molecular-beam epitaxy have been studied. An absorption edge appears in GaNxP1−x at energy below the indirect ΓV–XC transition in GaP, and the absorption edge shifts to lower energy with increasing N concentration. Strong photomodulation signals associated with the absorption edges in GaNxP1−x indicate that a direct fundamental optical transition is taking place, revealing that the fundamental band gap has changed from indirect to direct. This N-induced transformation from indirect to direct band gap is explained in terms of an interaction between the highly localized nitrogen states and the extended states at the Γ conduction-band minimum.
Experimental determination of gap scaling in a plasma opening switch
Authors: Black D. C. , Commisso R. J. , Ottinger P. F. , Swanekamp S. B. , Weber B. V.
Published: July 2002

Experiments were performed to investigate the coupling between a ∼0.5 μs conduction-time, ∼0.5 MA conduction-current plasma opening switch (POS), and an electron-beam (e-beam) diode. Electrical diagnostics provided measurements of the voltage at the oil-vacuum insulator and at the diode as well as anode and cathode currents on the generator and load sides of the POS. These measurements were combined with a flow impedance model to determine the POS gap over a range of conduction times and e-beam diode impedances, and for two POS-to-load distances. A comparison of the inferred POS gap at peak power with the critical gap for magnetic insulation indicates that the POS gap is always saturated in both switch-limited and load-limited regimes. This POS gap-size scaling with load impedance is consistent with an opening mechanism dominated by erosion and not J×B forces.
Classical resistivity in a post-thermal quench disrupting plasma
Authors: Humphreys D. A. , Whyte D. G.
Published: July 2002

Confirmation of the common assumption that post-thermal quench plasma resistivities are classical is a key requirement to enable prediction of peak halo current amplitudes expected in mitigated tokamak disruptions. Using numerical simulations and direct plasma measurements, the present work demonstrates that resistivities of core and halo plasmas are indeed classical following a disruption thermal quench. Simulations of core and halo current evolution in tokamak disruptions mitigated by massive He gas puff are performed which make use of direct measurements of post-thermal quench plasma electron temperature and Zeff.
Codoping characteristics of Zn with Mg in GaN
Authors: Kim K. S. , Han M. S. , Yang G. M. , Youn C. J. , Lee H. J. , Cho H. K. , Lee J. Y.
Published: July 2002

The doping characteristics of Mg–Zn codoped GaN films grown by metalorganic chemical vapor deposition are investigated. By means of the concept of Mg–Zn codoping technique, we have grown p-GaN showing a low electrical resistivity (0.72 Ω cm) and a high hole concentration (8.5×1017 cm−3) without structural degradation of the film. It is thought that the codoping of Zn atoms with Mg raises the Mg activation ratio by reducing the hydrogen solubility in p-GaN. In addition, the measured specific contact resistance of Mg–Zn codoped GaN film is 5.0×10−4 Ω cm2, which is one order of magnitude lower than that of Mg doped only GaN film (1.9×10−3 Ω cm2).
Silicon-based organic light-emitting diode operating at a wavelength of 1.5 μm
Authors: Curry R. J. , Gillin W. P. , Knights A. P. , Gwilliam R.
Published: July 2002

1.5-μm light-emitting diodes which operate at room temperature have been fabricated on silicon substrates. The devices use an erbium-containing organic light-emitting diode (OLED) structure which utilizes p++ silicon as the hole injection contact. The OLEDs use N, N′-diphenyl-N,N′-bis(3-methyl)-1,1′-biphenyl-4,4′-diamine as the hole transporting layer and erbium tris(8-hydroxyquinoline) as the electron conducting and emitting layer.
Picosecond infrared optical parametric amplifier for nonlinear interface spectroscopy
Authors: Bodlaki D. , Borguet E.
Published: July 2002

A tunable, narrow bandwidth, high peak power picosecond infrared (IR) laser system is described. The pump source is a picosecond Ti:sapphire regenerative amplifier seeded by a picosecond Ti:sapphire oscillator. The pump bandwidth and pulse duration are tunable producing 4–5 ps, 5–4 cm−1 pulses at 1 kHz. IR pulses are produced by optical parametric generation (OPG) followed by optical parametric amplification (OPA). Tuning is possible over the entire 1050–3300 nm region of the IR, with energies in excess of 15 μJ over most of the range. The temporal and spectral characteristics of the IR pulses are reviewed with a particular focus on the sources of bandwidth broadening in the OPG/OPA. Bandwidth optimization of the IR output is discussed. A spectral filtering scheme results in less than 15 cm−1 IR bandwidth, suitable for nonlinear optical spectroscopic applications.
Monolithic arrays of absorber-coupled voltage-biased superconducting bolometers
Authors: Gildemeister J. M. , Lee Adrian T. , Richards P. L.
Published: July 2002

We describe a design for bolometric detectors of infrared and mm-wave radiation produced in large-format filled arrays by standard planar lithography. A square grid of metallized silicon nitride absorbs the radiation. The bolometer suspension, sensor, and wiring occupy a small fraction of the area. We have produced a 1024 element array of fully released and suspended 1.5 mm×1.5 mm bolometer micromeshes with a filling factor of 88%. We describe a voltage-biased superconducting bolometer built as a prototype for one array element. We have measured noise equivalent power =2.3×10−17 W/Hz, τ=24 ms, and G=2.7×10−11 W/K at 304 mK.
On the deduction of chemical reaction pathways from measurements of time series of concentrations
Authors: Samoilov Michael , Arkin Adam , Ross John
Published: July 2002

We discuss the deduction of reaction pathways in complex chemical systems from measurements of time series of chemical concentrations of reacting species. First we review a technique called correlation metric construction (CMC) and show the construction of a reaction pathway from measurements on a part of glycolysis. Then we present two new improved methods for the analysis of time series of concentrations, entropy metric construction (EMC), and entropy reduction method (ERM), and illustrate (EMC) with calculations on a model reaction system.
Molecular-beam epitaxial growth and surface characterization of GaAs(311)B
Authors: Wang Z. M. , Däweritz L. , Ploog K. H.
Published: July 2002

Reflection high-energy electron diffraction (RHEED) and scanning tunneling microscopy (STM) are used to study the surface and growth of GaAs (311)B. The RHEED pattern reveals a lateral periodicity of 3.2 nm along the [011̄] direction, which is confirmed in real space by STM images. Pronounced RHEED intensity oscillations during the homoepitaxial growth on GaAs(311)B were observed in a wide substrate temperature range.
Open photoacoustic cell determination of the thermal interface resistance in two layer systems
Authors: Pichardo J. L. , Alvarado-Gil J. J.
Published: July 2002

In this article the transport of modulated heat is studied in a system consisting of two layers with a thermal resistance between them. It is shown that in a photothermal experiment, the measurement of the temperature in the rear face can be used for the determination of the thermal resistance of the inner layer. The system consisting of two foils of aluminum joined by a glue of different thickness is studied using the open photoacoustic cell technique. Our approach shows that it can be used for the determination of the thermal resistance and thermal conductivity of the buried layer.
Dielectric nonlinearities in ferroelectric thin-film heterostructures
Authors: Tyunina M. , Levoska J. , Leppävuori S. , Sternberg A.
Published: July 2002

The influence of the film–electrode interface on the ac-electric field dependence of the dielectric permittivity in ferroelectric thin-film heterostructures has been studied. The dielectric nonlinearities in epitaxial thin-film heterostructures of ferroelectric PbZr0.65Ti0.35O3 and relaxor ferroelectric (PbMg1/3Nb2/3O3)0.68–(PbTiO3)0.32 were measured at subswitching fields. A dramatic difference between the dielectric nonlinearities possessed by ferroelectric films and those exhibited by the corresponding heterostructures was revealed both by model evaluations and experimental observations. In the heterostructures, due to the presence of an interface layer the dielectric nonlinearities can be considerably suppressed and their type can be changed compared to those in the films.
Apparent viscosity of suspensions of rods using falling ball rheometry
Authors: Powell Robert L. , Morrison Thomas G. , Milliken William J.
Published: July 2002

Falling ball rheometry measurements of the apparent relative viscosity of suspensions of randomly oriented rodlike particles are reported for rods having a wide range of aspect ratios and concentrations. These data, combined with those from earlier studies [Milliken et al., J. Fluid Mech. 202, 217 (1989); Powell et al., J. Rheology 33, 1173 (1989)], provide a master curve describing the apparent viscosity in terms of a single parameter, nL3, where L is the length of the particle and n is the number density.
Defect-induced tetragonalization of the orthorhombic TiSi2 C49 phase: X-ray diffraction and first principles calculations
Authors: La Via F. , Grimaldi M. G. , Migas D. B. , Miglio Leo
Published: July 2002

We discuss the rather scattered predictions on the orthorhombic lattice parameters for the C49 TiSi2 phase, which are reported in the literature. New and accurate x-ray diffraction measurements and ab initio calculations are presented. Both agree in indicating that the density is higher than the one commonly assumed and it is close to the stable C54 phase. An unexpected tetragonalization of the orthorhombic cell has been observed and the theoretical analysis shows that this phenomenon could be originated by the massive presence of stacking faults.
Fine-structure state resolved rotationally inelastic collisions of CH(A 2Δ,v=0) with Ar: A combined experimental and theoretical study
Authors: Kind M. , Stuhl F. , Tzeng Yi-Ren , Alexander Millard H. , Dagdigian Paul J.
Published: July 2002

A collaborative experimental and theoretical study of rotationally inelastic collisions of CH(A 2Δ,v=0) resolved rotational/fine-structure levels with argon is presented. Experimental state-to-state rate constants were extracted from CH A 2Δ→X 2Π fluorescence spectra upon laser excitation to individual levels in the A 2Δ state in the presence of Ar. Fluorescence detection of the collision-induced population permits resolution of the fine-structure levels at low N, but no Λ-doublet discrimination. For the lowest value of N (N=2), the dominant process is the ΔN=0 fine-structure-changing transition, and the efficiency of this transition decreases markedly with increasing N. There is an increasing preference for conservation of the fine-structure label in ΔN≠0 transitions as N increases. These rate constants have been compared to and interpreted with theoretical rate constants computed through quantum coupled-states calculations of cross sections based on ab initio CH(A 2Δ)–Ar potential energy surfaces determined by Kaledin and Heaven (to be published). The tendency to conserve the fine-structure label is attributed to the Hund’s case (b) nature of the CH(A 2Δ) state, for which the electron spin is a spectator in the collision.
Vortex channeling and the voltage–current characteristics of YBa2Cu3O7 low-angle grain boundaries
Authors: Hogg M. J. , Kahlmann F. , Tarte E. J. , Barber Z. H. , Evetts J. E.
Published: July 2002

We have performed (voltage–current) V–I measurements on a thin film YBa2Cu3O7 4° [001] tilt low-angle grain boundary over an extensive range of temperatures and fields, verifying the presence of a linear characteristic. We report on the occurrence of the linear characteristic in its basic form and on the observation of V–I kinking into several, and in some cases numerous, linear segments. We interpret these findings in terms of a variation in the dissipative width at the grain boundary. Kinking from one linear V–I section to another of different gradient is described in terms of a change in the number of vortex rows being viscously channeled along the boundary.
The cooling rate dependence of crystallization for liquid copper: A molecular dynamics study
Authors: Liu C. S. , Xia Junchao , Zhu Z. G. , Sun D. Y.
Published: July 2002

The constant-pressure molecular dynamics simulations based on the second-moment approximation of tight-binding scheme have been performed to study the relationship between the resulting crystallization microstructure of the liquid copper and the cooling rate. Below the glass-forming critical cooling rate, the metastable hcp phase and the stable fcc phase can coexist in the resulting configuration with all sorts of proportion and various forms such as layering and phase separation. The sizes and the distributions of the two crystalline phases depend on the cooling rate: the faster the cooling rate the larger percents of the metastable hcp phase and the more easily the layering take place. From the split of peaks of the angular distribution function, for both hcp and fcc phase the faster the cooling rate is the more imperfect they are, which may be considered as the precursor of glass-forming.
Temperature dependence of laser threshold in an InGaAsN vertical-cavity surface-emitting laser
Authors: Schneider H. C. , Fischer A. J. , Chow W. W. , Klem J. F.
Published: July 2002

We present theoretical and experimental results for the temperature dependence of threshold current in an InGaAsN/GaAs vertical-cavity surface-emitting laser (VCSEL) operating at 1.3 μm under continuous-wave current injection. Using a microscopic many-body laser theory, good agreement with experimental data is obtained. The influence of radiative and nonradiative recombination processes on the threshold current–density is investigated theoretically. Also, comparison to a GaAs/AlGaAs VCSEL emitting at 850 nm is made.
Application of a high speed, low temperature infrared pyrometer in pulsed power experiments
Authors: Partouche-Sebban David , Holtkamp David B. , Bartsch R. Richard , Lee Huan , Schmitt Gerald G.
Published: July 2002

A new, multichannel infrared (IR) pyrometer has been constructed that is capable of measuring low temperature changes (<100 °C) at high speed (multi-MHz). The pyrometer has four IR channels spanning 1–11 μm and uses 1 mm optical fibers to relay the light from the center of a pulsed power experiment to the detectors. Broad IR passbands are used to increase sensitivity while preserving high accuracy in the temperature measurement (<15% for the absolute temperature (K) up to 1300 K). An application of the pyrometer in material property measurements of high strain at high strain rate is described.
Virtual mesa and spoiler midinfrared angled-grating distributed feedback lasers fabricated by ion bombardment
Authors: Bartolo R. E. , Bewley W. W. , Felix C. L. , Vurgaftman I. , Lindle J. R. , Meyer J. R. , Knies D. L. , Grabowski K. S. , Turner G. W. , Manfra M. J.
Published: July 2002

It is demonstrated that the suppression of parasitic Fabry–Perot-like lasing modes substantially enhances the beam quality and brightness of wide-stripe angled-grating distributed feedback lasers emitting in the midwave infrared. The direct facet-to-facet gain path is blocked by loss regions that are created by ion bombardment with 900 keV silicon ions. Both virtual mesa structures, in which loss regions bound both sides of the 300-μm-wide angled gain path, and spoiler structures, in which loss is induced only near the facets, decrease the etendue of the output by nearly an order of magnitude, and increase the brightness by up to a factor of 3.
In situ transmission electron microscopy study of Ni silicide phases formed on (001) Si active lines
Authors: Teodorescu V. , Nistor L. , Bender H. , Steegen A. , Lauwers A. , Maex K. , Van Landuyt J.
Published: July 2002

The formation of Ni silicides is studied by transmission electron microscopy during in situ heating experiments of 12 nm Ni layers on blanket silicon, or in patterned structures covered with a thin chemical oxide. It is shown that the first phase formed is the NiSi2 which grows epitaxially in pyramidal crystals. The formation of NiSi occurs quite abruptly around 400 °C when a monosilicide layer covers the disilicide grains and the silicon in between. The NiSi phase remains stable up to 800 °C, at which temperature the layer finally fully transforms to NiSi2. The monosilicide grains show different epitaxial relationships with the Si substrate. Ni2Si is never observed.
Effects of an ultrathin silicon oxynitride buffer layer on electrical properties of ferroelectric Bi4Ti3O12 thin films on p-Si(100) surfaces
Authors: Rokuta E. , Hotta Y. , Kubota T. , Tabata H. , Kobayashi H. , Kawai T.
Published: July 2002

Electrical properties of ferroelectric Bi4Ti3O12 (BiT) films on Si(100) using a 1 nm thick silicon oxynitride (SiON) buffer were investigated. The capacitance–voltage (C–V) characteristics of Au/BiT/SiON/Si(100) exhibited hysteresis loops with a memory window of 2 V due to the ferroelectricity, and did not show large carrier injections. The effects of the SiON buffer were demonstrated in current–voltage characteristics. In the reverse bias region, a leakage current density of the specimen without the SiON buffer was much larger than that of the specimen with the buffer. Apart from these electrical measurements, anomalous features appeared in C–V characteristics of the illuminated specimen, which were likely to be due to the ac response of the optically generated electrons in some trap states at the interface.
Infrared and visible emission from organic electroluminescent devices based on praseodymium complex
Authors: Hong Ziruo , Liang Chunjun , Li Ruigang , Zang Faxin , Fan Di , Li Wenlian , Hung L. S. , Lee S. T.
Published: July 2002

Praseodymium(dibenzoylmethanato) 3 (bathophenanthroline )  [Pr(DBM)3bath] was employed as an emitting and electron transport layer, and N,N′-diphenyl-N,N′-bis(3-methylphenyl)-1, 1′-biphenyl-4,4′-diamine (TPD) as a hole transport layer in organic electroluminescent (EL) devices. Bilayer device TPD/Pr(DBM)3bath and trilayer devices TPD/TPD:Pr(DBM)3bath/Pr(DBM)3bath with a different ratio of TPD to the Pr-complex were fabricated. Emission bands at 608 nm (1D2→3H6), 890 nm (1D2→3F2), 1015 nm (1D2→3F3), 1065 nm (1D2→3F4) and 1550 nm (1D2→1G4) originating from the internal f–f transitions of a Pr3+ ion were observed from EL devices using both bilayer and trilayer structures. Decreasing the ratio of TPD to the Pr-complex, the emission of the D21→H63 transition was promoted and that from the exciplex suppressed, which was explained in terms of energy transfer from the ligand to the central ion.
Charge-imaging field-effect transistor
Authors: Chen L. H. , Topinka M. A. , LeRoy B. J. , Westervelt R. M. , Maranowski K. D. , Gossard A. C.
Published: July 2002

Charge-imaging field-effect transistors (FETs) were fabricated from a GaAs/AlGaAs heterostructure containing a near-surface two-dimensional electron gas. These FETs have quantum point contact geometries to minimize the size of the channel and to improve the spatial resolution. The charge noise at T=4.2 K has a 1/f behavior and reaches values ≪1e/Hz1/2 at 30 kHz. The spatial resolution of the FET was measured at liquid He temperatures using a scanned probe microscope with a charged tip. The charge sensitivity of the FET is confined to a disk with full width at half maximum 340 nm. These FETs are suitable for integration onto a GaAs/AlGaAs scanned probe microscopy cantilever.
Self-assembled AlInGaN quaternary superlattice structures
Authors: El-Masry N. A. , Behbehani M. K. , LeBoeuf S. F. , Aumer M. E. , Roberts J. C. , Bedair S. M.
Published: July 2002

When an AlInGaN quaternary alloy is grown by metalorganic chemical-vapor deposition under certain growth conditions, a self-assembled superlattice structure is obtained. The superlattice structure is made of quaternary layers with different AIN and InN compositions. Transmission electron microscopy data show that the superlattice periodicity is regular with an individual layer thickness that depends on the growth conditions. Secondary ion mass spectrometry measurements show that the layers’ composition alternate between high-AIN and InN content and low-AlN and-InN content, while the in-plane lattice constant remains constant for both layers. A model is presented as a preliminary effort to explain these results.
High-quality distributed Bragg reflectors based on AlxGa1−xN/GaN multilayers grown by molecular-beam epitaxy
Authors: Fernández S. , Naranjo F. B. , Calle F. , Sánchez-Garcı́a M. A. , Calleja E. , Vennegues P. , Trampert A. , Ploog K. H.
Published: July 2002

Distributed Bragg reflectors based on AlxGa1−xN/GaN multilayer stacks have been grown by plasma-assisted molecular-beam epitaxy on GaN templates. The nominal Al composition ranged from 30% to 45%, and the layer thicknesses of the ten-period stack were designed for a target wavelength of 510 nm. Transmission electron microscopy data reveal periodic structures where (Al,Ga)N on the GaN interface is sharper than GaN on the (Al,Ga)N one. X-ray diffraction spectra fitted to a dynamic diffraction simulation model yield an estimate of the layer thicknesses, Al%, and lattice strain. Reflectivity values above 50% at 510 nm have been reproducibly achieved, in very good agreement with the results of the matrix-method simulation.
Electrohydrodynamically driven chaotic mixing in a translating drop
Authors: Ward Thomas , Homsy G. M.
Published: July 2002

When a liquid drop of given dielectric constant, resistivity and viscosity is translating in the presence of an electric field in a liquid of different dielectric constant, resistivity and viscosity, the resulting internal circulation is a superposition of the Hadamard–Rybcynski and Taylor (quadrapole) circulations. For sufficiently large field strengths, the quadrapole structure of the Taylor flow causes an internal stagnation disk to occur. When this flow field is modulated by an ac component of the electric field, at low Stokes numbers, the resulting Lagrangian particle trajectories can exhibit chaotic behavior. This Lagrangian chaos is examined via Poincaré maps and both the degree and rates of mixing are studied for a wide range of field strengths and modulation frequencies.
Electronic spectroscopy of jet-cooled vinyl radical
Authors: Pushkarsky Michael B. , Mann Aaron M. , Yeston Jake S. , Moore C. Bradley
Published: July 2002

The spectra of the X̃2A′-Ã2A″ electronic transition of jet-cooled vinyl radical, C2H3, and its partially deuterated isotopomer, CD2CH, have been obtained via action spectroscopy. The appearance of the H or D fragment was monitored via 1+1′ resonance enhanced multiphoton ionization through the Lyman-α transition. The vibrational structure of the Ã2A″ state was fully resolved and analyzed. The experimental vibrational frequencies agree well with ab initio calculations. The simulation of the rotational structure of several bands reinforces vibrational assignments.
Composition dependence of Raman intensity of the nitrogen localized vibrational mode in GaAs1−xNx
Authors: Seong M. J. , Hanna M. C. , Mascarenhas A.
Published: July 2002

We have investigated the nitrogen-localized vibrational mode (LVM) Raman intensity and its frequency (ωLVM) as a function of nitrogen concentration in GaAs1−xNx (x⩽0.04) at 300 K. The normalized Raman intensity of nitrogen LVM with respect to that of GaAs–LO phonon as well as ωLVM exhibits a remarkable linear dependence on the nitrogen concentration for x⩽0.03. This provides excellent calibration to determine nitrogen composition not only in the ternary alloy GaAs1−xNx but also in the quaternaries such as Ga1−yInyAs1−xNx.
Voltage shift phenomena in a heteroepitaxial BaTiO3 thin film capacitor
Authors: Abe K. , Yanase N. , Yasumoto T. , Kawakubo T.
Published: July 2002

Voltage shift phenomena of the hysteresis loop were characterized for a c-axis oriented heteroepitaxial BaTiO3 film by means of switching current measurements using various types of pulse sequences. During application of voltage, the hysteresis loop gradually shifted along the voltage axis according to the polarity of the voltage. Even after the application of voltage, while the top and bottom electrodes were short-circuited, the hysteresis loop continued to move. Under certain conditions, a part of the hysteresis loop shifted back, whereas the rest shifted forward. These results were explained, assuming that there is a nonswitching layer between the ferroelectric layer and the bottom electrode, and that the discontinuity of polarization can be compensated by injection of negative charges from the electrode. It was suggested that the nonswitching layer is possibly formed by relaxation of lattice misfit strain in the heteroepitaxial ferroelectric thin film.
Self-assembled patterns of iron oxide nanoparticles by hydrothermal chemical-vapor deposition
Authors: Zhang Zhengjun , Wei B. Q. , Ajayan P. M.
Published: July 2002

Here, we report a hydrothermal chemical-vapor deposition process, which produces self-assembled patterns of iron oxide nanoparticles. By exposing a planar silica substrate to a prevaporized mixture of water, ferrocene [Fe(C5H5)2] and xylene (C8H10), at temperatures of ∼1000 °C, Fe2O3 nanoparticles are deposited on the substrate surface, in regular circular patterns. The particle sizes are less than 100 nm, and are organized into submicron-size patterns. The same process without water produces arrays of carbon nanotubes catalyzed by iron nanoparticles that are formed by the decomposition of ferrocene molecules.
Continuous wave waveguide laser at room temperature in Nd3+-doped Zn:LiNbO3
Authors: Di Paolo R. E. , Cantelar E. , Pernas P. L. , Lifante G. , Cusso F.
Published: July 2002

This work reports continuous laser action at room temperature in LiNbO3:Nd3+ channel waveguides, fabricated by Zn diffusion. The absorbed pump power at the threshold was 1.25 mW and a slope efficiency of 20% was obtained. With our available pump power the laser could emit up to 0.14 mW without exhibiting any photorefractive damage.
Quantum dots in suspended single-wall carbon nanotubes
Authors: Nygård Jesper , Cobden David H.
Published: July 2002

We present a simple technique which uses a self-aligned oxide etch to suspend individual single-wall carbon nanotubes between metallic electrodes. This enables one to compare the properties of a particular nanotube before and after suspension, as well as to study transport in suspended tubes. As an example of the utility of the technique, we study quantum dots in suspended tubes, finding that their capacitances are reduced owing to the removal of the dielectric substrate.
Study of Schottky barrier of Ni on p-GaN
Authors: Yu L. S. , Qiao D. , Jia L. , Lau S. S. , Qi Y. , Lau K. M.
Published: July 2002

The Schottky barrier characteristics of Ni on p-GaN have been investigated using current–voltage–temperature (I–V–T) and capacitance–voltage characteristics (C–V) measurements. Barrier height values ranging from 2.68 to 2.87 eV were obtained from C–V measurements. The temperature dependence of I–V characteristics clearly indicated the dominance of tunneling current in the transport mechanism of the diodes, therefore, barrier height determination using I–V measurements can lead to erroneous results, as indicated by the wide range of barrier heights reported in the literature. Acceptor concentration, deduced from C–V measurements, was found to be of 1019/cm3 within 200 Å of the sample surface, and tapered off to be ∼1018/cm3. These values are 10–100 times higher than the hole concentration of ∼1017/cm3 obtained from Hall measurements.
Electrical characterization of stressed and broken down SiO2 films at a nanometer scale using a conductive atomic force microscope
Authors: Porti M. , Nafrı́a M. , Aymerich X. , Olbrich A. , Ebersberger B.
Published: July 2002

A conductive atomic force microscope (C-AFM) has been used to investigate the degradation and breakdown of ultrathin (<6 nm) films of SiO2 at a nanometric scale. Working on bare gate oxides, the conductive tip of the C-AFM allows the electrical characterization of nanometric areas. Due to the extremely small size of the analyzed areas, several features, which are not registered during macroscopic tests, are observed. In particular, before the oxide breakdown, switchings between different conduction states and sudden changes of conductivity have been measured, which have been related to the prebreakdown noise observed in conventional metal–oxide–semiconductor structures. Moreover, similar switchings have been also measured after the oxide breakdown, which have been related to the opening or closure of conduction channels between the electrodes. The C-AFM has also allowed the determination of the areas in which the degradation and breakdown take place. The results have shown that, although degradation takes place in areas of few hundreds of nm2, breakdown is laterally propagated to neighbor spots, affecting areas of thousands of nm2. The size of the affected area has been found to be strongly related to the hardness of the breakdown event. The phenomenology observed with the C-AFM provides experimental evidence of the local nature of the degradation and breakdown processes in ultrathin SiO2 films. Therefore, the C-AFM is a powerful tool to analyze the microscopic physics of these phenomena at the same dimensional scale at which they take place.
Performance of high-sensitivity quantum Hall far infrared photodetectors
Authors: Kawaguchi Y. , Hirakawa K. , Saeki M. , Yamanaka K. , Komiyama S.
Published: July 2002

We have characterized the performance of a newly developed far-infrared photodetector which utilizes the cyclotron resonance of two-dimensional electrons in modulation-doped Al0.3Ga0.7As/GaAs heterojunctions in the integer quantum Hall regime. It is found that the responsivity and the detectivity of the present device reach as high as 1.1×107 V/W and 4.0×1013 cm Hz1/2/W, respectively, at 4.2 K. The quantum Hall photodetector is very promising as a high-sensitivity, narrowband tunable photodetector in the far infrared range.
Direct determination of atomic structure in multiple quantum wells InGaN/GaN
Authors: Watanabe K. , Yang Jer-Ren , Nakanishi N. , Inoke K. , Shiojiri M.
Published: July 2002

We present an analysis of a homogeneous cellular interface between ultrathin InGaN and GaN layers in a high-resolution high-angle annular dark field scanning transmission electron microscopy image processed by the deconvolution technique. The method provides high-quality atomic-resolved images uniquely from the experimental images without preassumed models and image simulations. The processed image, then, shows definitely the position of interface between InGaN and GaN layers at atomic scale and an evidence of coherent epitaxial growth.
X-ray diffraction of polymer under load at cryogenic temperature
Authors: Nishino Takashi , Miyazaki Hideyuki , Nakamae Katsuhiko
Published: July 2002

An x-ray diffraction apparatus, consisting of a load cell, a stretching device, and a cryogenic cell, has been constructed to observe the mechanical deformation of the crystal lattice of polymers at cryogenic temperature. The polymer specimen could be cooled down to 18 K using this apparatus. The equatorial diffraction profiles of polymer specimens were observed under various temperatures and loads with 18 K minimum temperature. The equatorial lattice spacings of polyethylene (PE) decreased at 18 K compared with those at 300 K; however, they were elongated by a constant applied tensile stress. The elastic modulus Et of the crystalline regions of PE in the direction perpendicular to the chain axis was evaluated. The Et value for the (200) plane of PE increased to 11 GPa at 18 K from 3.2 GPa at 300 K. This is considered to be due to the increase of the van der Waals interactions between the chains in the crystal lattice at very low temperature, where the incoherent thermal vibrations of the atoms can be ignored.
Photonic band gap formation by microstrip ring: A way to reduce the size of microwave photonic band gap structures
Authors: Kee Chul-Sik , Jang Mi-Young , Park Ikmo , Lim H. , Kim Jae-Eun , Park Hae Yong , Lee Jung Il
Published: July 2002

We show that a microstrip ring with a narrow gap exhibits a photonic band gap (PBG) that is generally believed to be created by a periodic array. The discontinuity of impedance at the narrow gap induces multiple reflections with fixed phase correlation necessary to make the PBG. We also discuss the PBG frequency tuning achieved by applying an external voltage on a varactor mounted on the gap and applications of this PBG ring as a tunable band stop filter and a microwave switch.
Tunable negative differential resistance controlled by spin blockade in single-electron transistors
Authors: Ciorga M. , Pioro-Ladriere M. , Zawadzki P. , Hawrylak P. , Sachrajda A. S.
Published: July 2002

The single-electron transistors containing a few electrons and spin polarized source and drain contacts were formed in GaAs/GaAlAs heterojunctions using metallic gates. Coulomb blockade measurements reveal well-defined regimes where a decrease in the current is observed with increasing bias. We establish that the origin of the negative regime is the spin-polarized detection of electrons combined with a long spin relaxation time in the dot.
Anomalous isotopic effect on vibrational properties of HXeOH
Authors: Khriachtchev Leonid , Lundell Jan , Pettersson Mika , Tanskanen Hanna , Räsänen Markku
Published: July 2002

We study IR absorption spectra of various isotopomers of the HXeOH molecules isolated in solid Xe at 7 K. Upon the 16O to O18 isotopic substitution, we found experimentally an anomalous shift of the H–Xe stretching vibration frequency of HXeOD and HXeOH. This spectral behavior, when the fundamental frequency shifts up in energy for the heavier oxygen isotope, cannot be understood on the basis of the harmonic approximation for fundamental vibrations, and the anharmonic coupling between normal modes is presumably responsible for this anomalous effect. Our anharmonic CC-VSCF calculations describe correctly the experimental observations.
Periodic elastic domains of coexisting phases in epitaxial MnAs films on GaAs
Authors: Plake T. , Ramsteiner M. , Kaganer V. M. , Jenichen B. , Kästner M. , Däweritz L. , Ploog K. H.
Published: July 2002

The surface topography of epitaxial MnAs films on GaAs(001) is studied by scanning probe microscopy. We provide direct experimental evidence for temperature-dependent elastic domains of the coexisting ferromagnetic αMnAs and paramagnetic βMnAs phases. The results agree well with a theoretical model for the elastic equilibrium of periodic domains.
Stimulated terahertz emission from group-V donors in silicon under intracenter photoexcitation
Authors: Shastin V. N. , Zhukavin R. Kh. , Orlova E. E. , Pavlov S. G. , Rümmeli M. H. , Hübers H.-W. , Hovenier J. N. , Klaassen T. O. , Riemann H. , Bradley I. V. , van der Meer A. F. G.
Published: July 2002

Frequency-tunable radiation from the free electron laser FELIX was used to excite neutral phosphorus and bismuth donors embedded in bulk monocrystalline silicon. Lasing at terahertz frequencies has been observed at liquid helium temperature while resonant pumping of odd parity impurity states. The threshold was about two orders of magnitude below the value for photoionization pumping. The influence of nonequilibrium intervalley TO phonons on the population of excited Bi impurity states is discussed.
Abnormal dependence of contact resistivity on hole concentration in nonalloyed ohmic contacts to p-GaN
Authors: Kwak Joon Seop , Nam Ok-Hyun , Park Yongjo
Published: July 2002

The dependence of contact resistivity on hole concentration has been investigated for nonalloyed Pd contacts to p-GaN. The hole concentration was varied by changing the Mg concentration, [Mg], in p-GaN. The p-GaN having the [Mg] of 4.5×1019 cm−3 showed the hole concentration of 2.2×1017 cm−3, where contact resistivity was measured as 8.9×10−2 Ω cm2. When the [Mg] increased to 1.0×1020 cm−3, the hole concentration was significantly reduced to 2.0×1016 cm−3. Nevertheless, the Pd contacts on the p-GaN displayed contact resistivity as low as 5.5×10−4 Ω cm2. The abnormal dependence of contact resistivity on hole concentration may be explained by predominant current flow at the Pd/p-GaN interface through a deep level defect band, rather than the valence band.
Electrical turbulence as a result of the critical curvature for propagation in cardiac tissue
Authors: Cabo Cándido , Pertsov Arkady M. , Davidenko Jorge M. , Jalife José
Published: July 2002

In cardiac tissue, the propagation of electrical excitation waves is dependent on the active properties of the cell membrane (ionic channels) and the passive electrical properties of cardiac tissue (passive membrane properties, distribution of gap junctions, and cell shapes). Initiation of cardiac arrhythmias is usually associated with heterogeneities in the active and/or passive properties of cardiac tissue. However, as a result of the effect of wave front geometry (curvature) on propagation of cardiac waves, inexcitable anatomical obstacles, like veins and arteries, may cause the formation of self-sustained vortices and uncontrolled high-frequency excitation in normal homogeneous myocardium.
Measurement of Hall scattering factor in phosphorus-doped silicon
Authors: del Alamo Jesús A. , Swanson Richard M.
Published: July 2002

A combination of measurements of electrical resistivity, Hall mobility, and phosphorus concentration by secondary ion mass spectroscopy has determined the value of the Hall scattering factor in P-doped epitaxially grown Si. The Hall scattering factor is found to increase from a value close to unity at a P concentration around 1017 cm−3, to ∼1.3 in the 1018–1019 cm−3 doping range. At higher doping levels it decreases and saturates to a value of ∼0.9 for doping levels higher than 1020 cm−3.
Statistics of deep-level amphoteric traps in insulators and at interfaces
Authors: White M. H. , Chao C. C.
Published: July 2002

An analysis is presented on the statistics of deep-level amphoteric traps in insulators and at interfaces (insulator boundaries) in terms of the occupancy functions for neutral and ionized trapping centers as a function of free carrier injection and spatial distribution of centers. Amphoteric traps are postulated for bulk insulator traps, such as silicon nitride in nonvolatile, erase-write, semiconductor memory structures. In addition, amphoteric traps may be observed at interfacial boundaries, such as SiO2-Si3N4, SiO2-Al2O3, Si-SiO2, and insulator-liquid with site bonding. Under free carrier electron injection we find the characteristic trapped-charge centroid movement decreases for small values of Rn=σ+n/σ0n, the ratio of capture cross sections for ionized to neutral trapping centers. These results reduce to Arnett’s treatment for single-occupancy trapping centers (Rn≫1). Efficient conversion of positive (unoccupied) to negative (double-occupancy) charged centers occurs under free carrier electron injection with Rn≂1. An analogous situation exists for the reverse operation, namely, conversion of negative to positive charged centers under free carrier hole injection.
Self-consistent domain theory in soft ferromagnetic media. I. Solenoidal distributions in elliptical thin-film elements
Authors: van den Berg H. A. M.
Published: July 2002

The basic concepts of a self-consistent domain theory for the class of ideal soft ferromagnetic media is unfolded. The treatment is based on micromagnetic equilibrium and stability equations which are simplified by removing the contributions due to the intrinsic anisotropy and the spatial variation term in the exchange energy density. Only the solenoidal magnetization distributions in thin film objects are investigated. This limitation makes confinement to two-dimensional dipole distributions possible. Cauchy’s method of characteristics is employed to derive dipole distributions that satisfy the relation ∇⋅M=0, where M is the magnetization, and the constitutive equation. It is proved that the characteristic base curves are straight lines perpendicular to the magnetization vector. In an ellipse, four different characteristic base curves that do not coincide intersect at one single point in certain regions, thus generating ambiguities in the magnetization direction. It is demonstrated that these ambiguities originate in the incompatibility of the magnetization distribution that is imposed by the various segments of the edge of the ellipse. Based on differential geometrical principles, a partitioning of the edge into segments is introduced, and along with it two adjoining regions in which the magnetization direction is uniquely determined are defined. It is proved that a domain wall is required in the cross section of both regions to accomplish a dipole configuration in stable equilibrium. Experimental confirmation is given by means of the ferrofluid pattern of a 3500-Å-thick Permalloy layer with elliptical geometry.