bluuebunny/crossref_metadata_2025 huggingface dataset:
Electron trapping process in ferroelectric lead–zirconate–titanate thin-film capacitors
Authors: Chen Hong-ming , Lee Joseph Ya-min
Published: July 2002

The effects of constant voltage stress on ferroelectric lead–zirconate–titanate (PZT) thin-film capacitors have been studied. Two stress effects are observed in the PZT capacitance–voltage (C–V) characteristics. The first is that the capacitance is reduced, and the second is a voltage shift of the C–V curve. These effects are found to depend on the stress electric field and the injected charge fluence. A correlation between the stress effects and the electron trapping inside the films is established. The injected charge fluence can be calculated from the leakage current. The trapping efficiency, electron capture cross section, and the trap density are obtained from the measured injected charge influence and the C–V voltage shift. The calculated value of the electron capture cross-section σT is 1.89×10−19 cm2 and the neutral electron trap density NT is 1.20×1013 cm−2.
Transverse thermoelectric response of a tilted metallic multilayer structure
Authors: Zahner Th. , Förg R. , Lengfellner H.
Published: July 2002

Thermoelectric fields transverse to an applied temperature gradient have been observed in artificially created tilted multilayer structures. Copper-constantan multilayer structures were prepared by sintering of a compressed stack of copper/constantan/copper foils and by cutting the stack obliquely to its axis. Tilted multilayer structures represent a new class of thermoelectric devices, and applications for detection of light are demonstrated.
Room-temperature pulsed operation of an electrically injected InGaN/GaN multi-quantum well distributed feedback laser
Authors: Hofstetter Daniel , Thornton Robert L. , Romano Linda T. , Bour David P. , Kneissl Michael , Donaldson Rose M.
Published: July 2002

We demonstrate room-temperature pulsed operation of an electrically injected InGaN/GaN-based distributed feedback laser with an emission wavelength of 403 nm. The threshold current of a 1000-μm-long and 20-μm-wide device was 3.2 A; corresponding to a threshold current density of 16 kA/cm2. The 3rd order grating providing feedback was defined holographically and dry etched into the upper waveguiding layer by chemically assisted ion beam etching. We observed single mode operation of the laser with a side mode suppression ratio of 15 dB over a temperature range of about 30 K.
Electrically modulated photoluminescence in self-organized InGaAs/GaAs quantum dots
Authors: Babiński Adam , Wysmołek A. , Tomaszewicz T. , Baranowski J. M. , Leon R. , Lobo C. , Jagadish C.
Published: July 2002

Results of photoluminescence (PL) study of the self-organized InGaAs/GaAs quantum dots (QDs) in a field-effect structure grown by metalorganic vapor phase epitaxy are presented. It has been found that the PL from the QDs strongly depends on the bias voltage. No PL from the QDs ground state can be observed from the reverse biased structure, whereas the PL signal recovers in the forward biased structure. It is proposed that the bias dependence of the PL signal results from the QDs electron occupancy changes driven by the electric field within the structure. Due to a long thermalization time, the photogenerated electrons are swept out of the QDs by the electric field before radiative recombination. The electrically modulated PL (e-m PL), making use of the bias dependence of PL signal, is proposed as a tool for QD investigation. The e-m PL spectra at T=300 and T=4.2 K are analyzed and discussed.
Three-dimensional photonic crystal structures achieved with two-photon-absorption photopolymerization of resin
Authors: Sun Hong-Bo , Matsuo Shigeki , Misawa Hiroaki
Published: July 2002

Three-dimensional photonic crystal structures were fabricated with laser microfabrication techniques through two-photon-absorption photopolymerization of resin. Significant band-gap effects in the infrared wavelength region were observed from “layer-by-layer” structures.
Distributed Bragg reflectors based on AlN/GaN multilayers
Authors: Ng H. M. , Doppalapudi D. , Iliopoulos E. , Moustakas T. D.
Published: July 2002

A 20.5 period distributed Bragg reflector stack based on AlN/GaN has been grown on (0001) sapphire by electron cyclotron resonance plasma-assisted molecular-beam epitaxy. Peak reflectance up to 95% was observed at a wavelength of 392.5 nm. Cross-section transmission electron microscopy studies indicate that the interface of GaN-on-AlN is always sharper and more distinct than the interface of AlN-on-GaN. This is attributed to the different growth modes of AlN and GaN. When AlN grows on GaN, it tends to grow in a two-dimensional mode (Frank–van der Merwe mode) whereas GaN grows on AlN in a three-dimensional mode (Stranski–Krastanov mode). Based on these findings, the experimental reflectance data were simulated using the transmission matrix method.
Self-organized In0.4Ga0.6As quantum-dot lasers grown on Si substrates
Authors: Linder K. K. , Phillips J. , Qasaimeh O. , Liu X. F. , Krishna S. , Bhattacharya P. , Jiang J. C.
Published: July 2002

We report growth of self-organized In0.4Ga0.6As quantum dots on Si substrates by molecular-beam epitaxy. Low-temperature (17 K) photoluminescence spectra show that the optical properties of In0.4Ga0.6As quantum dots grown on Si are comparable to quantum dots grown on GaAs substrates. We also present preliminary characteristics of In0.4Ga0.6As quantum-dot lasers grown on Si substrates. Light versus current measurements at 80 K under pulsed bias conditions show that Ith=3.85 kA/cm2. The lasing spectral output has a peak emission wavelength of 1.013 μm and a linewidth (full width at half maximum) of ∼4 Å at the threshold.
High-power GaAs/AlGaAs quantum fountain unipolar laser emitting at 14.5 μm with 2.5% tunability
Authors: Gauthier-Lafaye O. , Julien F. H. , Cabaret S. , Lourtioz J.-M. , Strasser G. , Gornik E. , Helm M. , Bois P.
Published: July 2002

We demonstrate operation of a high-power quantum fountain unipolar laser relying on intersubband emission in optically pumped GaAs/AlGaAs quantum wells. The collected power per facet is as large as 2.3 W at 20 K and 1.5 W at 120 K, which translates into 6.6 W optical power per facet at low temperature accounting for collection efficiency. The maximum operating temperature is 135 K. We also demonstrate that the lasing wavelength can be tuned by as much as Δλ/λ≈2.5% simply by tuning the pump wavelength.
Negative capacitance of GaAs homojunction far-infrared detectors
Authors: Perera A. G. U. , Shen W. Z. , Ershov M. , Liu H. C. , Buchanan M. , Schaff W. J.
Published: July 2002

Bias, frequency and temperature-dependent capacitance characteristics of p-GaAs homojunction interfacial work-function internal photoemission (HIWIP) far-infrared detectors are reported. A strong negative capacitance phenomenon has been observed. Unlike in other devices, even up to 1 MHz in HIWIP, the negative capacitance value keeps increasing with frequency, giving a stronger effect. The origin of this effect is believed to be due to the carrier capture and emission at interface states. Fitting data based on charging-discharging current and the inertial conducting current model show good agreement with the experimental observations.
Direct nanometer-scale patterning by the cantilever oscillation of an atomic force microscope
Authors: Hyon C. K. , Choi S. C. , Hwang S. W. , Ahn D. , Kim Yong , Kim E. K.
Published: July 2002

A resistless nanostructure patterning technique using tip oscillation of an atomic force microscope (AFM) was systematically investigated. Commercial AFM cantilevers are used to successfully generate patterns as narrow as 10 nm on a GaAs surface, without further sharpening of the tips. Reliable patterns with fully controlled width and depth are achieved by adjusting the feedback gain and the scan speed. This process allows nanometer-scale patterning to be performed simply, and is well suited for nanodevice fabrication.
High-Tc second-order gradiometer for magnetocardiography in an unshielded environment
Authors: Kouznetsov K. A. , Borgmann J. , Clarke John
Published: July 2002

We have fabricated planar first-derivative gradiometers in which an asymmetric flux transformer patterned in a single-layer YBa2Cu3O7−x film is permanently bonded to a directly coupled magnetometer based on a dc superconducting quantum interference device (SQUID). The gradiometer base line is 48 mm. The common mode rejection of uniform magnetic fields is typically one part in a few hundred. The outputs of two such gradiometers are subtracted digitally to form a second-derivative gradiometer. Additionally, three orthogonal SQUID magnetometers are used to reduce the residual response to uniform magnetic fields to better than 100 ppm. The system is able to detect magnetic signals from a human heart in an unshielded environment.
Observation of carrier depletion and emission effects on capacitance dispersion in relaxed In0.2Ga0.8As/GaAs quantum wells
Authors: Chen J. F. , Wang P. Y. , Tsai C. Y. , Wang J. S. , Chen N. C.
Published: July 2002

Strong changes in capacitance over frequency are found for highly relaxed In0.2Ga0.8As/GaAs quantum well. The high-frequency dispersion is explained by a resistance–capacitance time constant effect due to the existence of a high resistive layer while the low-frequency dispersion is due to carrier emission from traps. The high-resistance layer is created by carrier depletion when InGaAs thickness increases beyond the critical thickness. Excellent agreement is found between the data from capacitance–frequency spectra and deep-level transient spectroscopy, permitting us to conclude that both the carrier depletion and emission effects observed in capacitance–frequency spectra are due to the existence of an acceptor trap at 0.33 eV. This trap is generated when the InGaAs thickness is beyond its critical thickness and is due to defect states associated with misfit dislocations.
Generation and detection of fluorescent color centers in diamond with submicron resolution
Authors: Martin J. , Wannemacher R. , Teichert J. , Bischoff L. , Köhler B.
Published: July 2002

Fluorescent color-center patterns have been written on surfaces of synthetic type-Ib diamonds with spatial resolution below 180 nm via irradiation with focused ion and electron beams and subsequent annealing. The patterns are detected and spectroscopically analyzed using confocal optical microscopy. From the spatial extent of the color-center distributions, the activation energy for diffusion of vacancies in diamond is determined as (2.55±0.15) eV. Detailed information about the formation of color centers in diamond is obtained employing the three-dimensional spatial resolution of the confocal microscope combined with spectral resolution. In particular, the distributions of two color centers, ascribed to different charge states of the NV defect in diamond, have been spatially mapped and shown to depend strongly on the irradiation dose.
GaN avalanche photodiodes
Authors: Carrano J. C. , Lambert D. J. H. , Eiting C. J. , Collins C. J. , Li T. , Wang S. , Yang B. , Beck A. L. , Dupuis R. D. , Campbell J. C.
Published: July 2002

We report the electrical and optical characteristics of avalanche photodiodes fabricated in GaN grown by metalorganic chemical vapor deposition. The current–voltage characteristics indicate a multiplication of >25. Experiment indicates and simulation verifies that the magnitude of the electric field at the onset of avalanche gain is ⩾3 MV/cm. Small-area devices exhibit stable gain with no evidence of microplasmas.
The electro-optic properties of interdiffused InGaAs/InP quantum well structures
Authors: Weiss Bernard L. , Chan Y. , Shiu W. C. , Li E. Herbert
Published: July 2002

We present a model for the optical properties of interdiffused InGaAs/InP quantum well structures. The structure is investigated in a two-phase group V interdiffusion that is characterized by three parameters: the interdiffusion coefficient in the barrier layer, the well layer, and the concentration ratio of diffused species at the well/barrier interface. The quantum confined Stark effect is considered including the exciton and full subband under an applied electric field. Results show interesting optical properties for the TE and TM polarization and a tunable operation wavelength near 1.55 μm for modulators.
On weakly nonlinear modulation of waves on deep water
Authors: Trulsen Karsten , Kliakhandler Igor , Dysthe Kristian B. , Velarde Manuel G.
Published: July 2002

We propose a new approach for modeling weakly nonlinear waves, based on enhancing truncated amplitude equations with exact linear dispersion. Our example is based on the nonlinear Schrödinger (NLS) equation for deep-water waves. The enhanced NLS equation reproduces exactly the conditions for nonlinear four-wave resonance (the “figure 8” of Phillips) even for bandwidths greater than unity. Sideband instability for uniform Stokes waves is limited to finite bandwidths only, and agrees well with exact results of McLean; therefore, sideband instability cannot produce energy leakage to high-wave-number modes for the enhanced equation, as reported previously for the NLS equation. The new equation is extractable from the Zakharov integral equation, and can be regarded as an intermediate between the latter and the NLS equation. Being solvable numerically at no additional cost in comparison with the NLS equation, the new model is physically and numerically attractive for investigation of wave evolution.
Light-induced softening of azobenzene dye-doped polymer films probed with quartz crystal resonators
Authors: Srikhirin Toemsak , Laschitsch Alexander , Neher Dieter , Johannsmann Diethelm
Published: July 2002

Employing quartz crystal resonators, we have measured the elastic compliance of thin polymers films doped with an azobenzene dye. From a comparison of the shifts of resonance frequency on different acoustic overtones of the resonator, the films’ elastic compliance was derived. Upon irradiation with ultraviolett light at low intensity, the compliance decreases by a few percent. Irradiation with visible light at high intensity, on the contrary, softens the material. We assume that, when irradiated with high-power visible light, the azobenzene molecules undergo rapid cycling through the trans- and cis-states, thereby softening the polymer matrix.
Structural evolution and optical properties of TiO2 thin films prepared by thermal oxidation of sputtered Ti films
Authors: Ting Chu-Chi , Chen San-Yuan , Liu Dean-Mo
Published: July 2002

A dense rutile TiO2 thin film was synthesized by the thermal oxidation of a sputtered titanium metal film in ambient air. The effects on optical properties of TiO2 films of the crystal structure and microstructural evolution at various oxidation temperatures were investigated. The Ti films transformed into single-phase rutile TiO2 at temperatures ⩾ 550 °C without going through an anatase-to-rutile transformation. Instead, an additional crystalline Ti2O phase was detected at 550 °C only. An increase in the oxidation temperatures ranging between 700 and 900 °C led to an increase in both the refractive index and absorption coefficient, but a decrease in the band gap energy (Eg). According to the coherent potential approximation model, the band gap evolution of the oxidized films was primarily attributed to the electronic disorder due to oxygen deficiency at a higher oxidation temperature rather than the presence of an amorphous component in the prepared films.
Full configuration interaction benchmarking of coupled-cluster models for the lowest singlet energy surfaces of N2
Authors: Larsen Helena , Olsen Jeppe , Jørgensen Poul , Christiansen Ove
Published: July 2002

The potential-energy curves for the X 1Σg+, a 1Πg, a′ 1Σu−, w 1Δu, c3 1Πu, and b 1Πu states of N2 have been investigated in full configuration interaction (FCI) and coupled-cluster response calculations. The equilibrium bond lengths, adiabatic excitation energies, and harmonic frequencies have been obtained with the coupled-cluster singles model (CCS), an approximate coupled-cluster singles and doubles model (CC2), the coupled-cluster singles and doubles model (CCSD), and an approximate coupled-cluster singles, doubles, and triples model (CC3), and subsequently compared to FCI results. The weak and strong features of the coupled-cluster models are discussed and illustrated. Overall, improvements towards FCI are obtained in the hierarchy CCS–CC2–CCSD–CC3. CC3 is always consistently better than CCSD, and for all the considered spectroscopic constants CC3 provides excellent results. Examples where the CC3 model fails are also given. The noniterative triples model, CCSDR(3), is compared to the iterative triples models CC3 and FCI. CCSDR(3) recovers the major part of the CC3 correlation contribution and is thus a cheap alternative to the CC3 model.
Magnetoresistive double spin filter tunnel junction
Authors: Worledge D. C. , Geballe T. H.
Published: July 2002

We propose a magnetoresistive tunnel device that takes advantage of the spin filter effect. Two magnetic tunnel barriers are contacted by normal metal electrodes. The resistance of the device is lower (higher) when the magnetic moments of the two barriers are parallel (antiparallel). We present a theoretical calculation of the magnetoresistance. This device has the potential to work above room temperature, in very small fields, and to give a sensitivity orders of magnitude larger than what is possible with standard magnetic tunnel junctions.
Self-amplification of the field of velocity derivatives in quasi-isotropic turbulence
Authors: Galanti B. , Tsinober A.
Published: July 2002

We report results of direct numerical simulations of the Navier–Stokes equations regarding the role of self-amplification of the field of velocity derivatives, i.e., involving both vorticity and strain. The main result is that even at rather moderate values of the Reynolds number the self-amplification totally dominates the process of production of the field of velocity derivatives. The role of external forcing in this process is negligible. This dominance occurs not only in the mean, but practically pointwise throughout the whole flow field. The property of self-amplification possesses a number of quantitative universal properties which are independent of the details of forcing.
Detailed Tabulation of Atomic Form Factors, Photoelectric Absorption and Scattering Cross Section, and Mass Attenuation Coefficients in the Vicinity of Absorption Edges in the Soft X-Ray (Z=30–36, Z=60–89, E=0.1 keV–10 keV), Addressing Convergence Issues of Earlier Work
Authors: Chantler C. T.
Published: July 2002

Reliable knowledge of the complex x-ray form factor [Re(f ) and f″] and the photoelectric attenuation coefficient (σPE) is required for crystallography, medical diagnosis, radiation safety, and XAFS studies. Discrepancies between currently used theoretical approaches of 200% exist for numerous elements from 1 to 3 keV x-ray energies. The key discrepancies are due to the smoothing of edge structure, the use of nonrelativistic wave functions, and the lack of appropriate convergence of wave functions. This paper addresses these key discrepancies and derives new theoretical results of substantially higher accuracy in near-edge soft x-ray regions. The high-energy limitations of the current approach are also illustrated. The energy range covered is 0.1 to 10 keV. The associated figures and tabulation demonstrate the current comparison with alternate theory and with available experimental data. In general, experimental data are not sufficiently accurate to establish the errors and inadequacies of theory at this level. However, the best experimental data and the observed experimental structure as a function of energy are strong indicators of the validity of the current approach. New developments in experimental measurement hold great promise in making critical comparisons with theory in the near future.
Measuring radio frequency properties of materials in pulsed magnetic fields with a tunnel diode oscillator
Authors: Coffey T. , Bayindir Z. , DeCarolis J. F. , Bennett M. , Esper G. , Agosta C. C.
Published: July 2002

Tunnel diode oscillators have been used in many types of experiments that measure the properties of materials. We present the details of an apparatus that extend these tunnel diode techniques to measure the properties of materials in pulsed magnetic fields. In the most common version of this method, a sample is placed in the inductor of a small rf tank circuit powered by a tunnel diode and the conductivity, magnetization, or penetration depth is measured. We explain in this article how the sample and configuration of the radio frequency fields determine which property is measured. Our major innovations are to stabilize the tunnel diode oscillator during a magnet pulse by using compensated coils in the tank circuit and the development of two methods, one digital and one analog, to measure the frequency and amplitude shifts in the oscillator during the short (10 s of ms) magnet pulse. We illustrate the power of this new measurement method by showing preliminary results of the superconducting transition and the Shubnikov–de Haas effect in the organic conductor κ-(ET)2Cu(NCS)2. The Shubnikov–de Haas effect shows particularly high amplitude oscillations due to magnetic breakdown orbits.
Comment on “Does the structural superionic phase transition at 231 °C in CsH2PO4 really not exist?” [J. Chem. Phys. 110, 4847 (1999)]
Authors: Bronowska Wieslawa
Published: July 2002

Single crystal and powder diffraction investigations have revealed that the paraelectric phase of cesium dihydrogen phosphate, CsH2PO4 (CDP), from the crystallographic point of view is stable (space group P21/m) up to ca. 231 °C. At this temperature a structural phase transition from the paraelectric phase to the superionic phase (space group Pm–3m) has been observed. The superionic phase of CDP is unstable under normal air condition due to dehydration process. Under H2O-saturated atmosphere the superionic phase transition of CDP takes place without decomposition and is reversible with a temperature hysteresis on cooling.
A generalized electron-pair density function for atoms
Authors: Koga Toshikatsu
Published: July 2002

For many-electron atoms, we discuss a generalized electron-pair density function g(q;a,b) that represents the probability density function for the magnitude |ari+brj| of the two-electron vector ari+brj to be q, where a and b are real-valued parameters. It is pointed out that the function g(q;a,b) connects smoothly the single-electron density ρ(r), the electron-pair intracule (relative motion) density h(r12), and the electron-pair extracule (center-of-mass motion) density d(R). Moreover, ρ(r) is found to be a local extremum function of g(q;1,b) with respect to the parameter b. Analogously, the single-electron moments 〈rn〉 are local extrema of the moments 〈qn〉(a,b) associated with g(q;a,b). An illustrative example is given for the helium atom within the Kellner approximation.
Study of relativistic effects on nuclear shieldings using density-functional theory and spin–orbit pseudopotentials
Authors: Vaara Juha , Malkina Olga L. , Stoll Hermann , Malkin Vladimir G. , Kaupp Martin
Published: July 2002

We combine a perturbational calculation, using spin–orbit pseudopotentials, of the spin–orbit effect on nuclear shieldings with scalar relativistic effects taken into account self-consistently using quasirelativistic pseudopotentials. The shieldings are calculated for light nuclei in systems containing heavy atoms, for which matching spin–orbit and quasirelativistic pseudopotentials are used. The second-order spin–orbit contribution arising from the magnetic field dependence of the one-electron spin–orbit Hamiltonian is also included. The method is implemented within the framework of density-functional theory and is capable of treating large molecular systems economically. Calculations are presented for the HX and CH3X (X=F, Cl, Br, I) molecules, third-row transition metal carbonyls, and a series of methyl mercury compounds.
Transmissive liquid crystal cell parameters measurement by spectroscopic ellipsometry
Authors: Tang S. T. , Kwok H. S.
Published: July 2002

A powerful method for liquid crystal (LC) cell twist angle and retardation measurement is presented. This method is based on the spectroscopic ellipsometry. By varying the polarizer and analyzer angles only, a transmission spectrum containing null transmission at some particular wavelengths can be obtained. Analytical equations derived bases on a polarization analysis of twisted birefringent layers are then solved to give all the LC cell parameters. In general, the twist angle, twist sense, retardation, and the rubbing direction can all be determined. Measurement results on commercial liquid crystal display panels are presented.
Probing domain microstructure in ferroelectric Bi4Ti3O12 thin films by optical second harmonic generation
Authors: Barad Yaniv , Lettieri James , Theis Chris D. , Schlom Darrell G. , Gopalan Venkatraman , Jiang J. C. , Pan X. Q.
Published: July 2002

The domain microstructure in an epitaxial thin film of Bi4Ti3O12 on a SrTiO3(001) substrate is studied by second harmonic generation measurements. The input polarization dependence of the second harmonic signal exhibits spatial symmetries that reflect the presence of eight different domain variants present in the film. A theoretical model is presented that explains the observed symmetries and extracts quantitative information on the nonlinear optical coefficients of the material and statistics of domain variants present in the film area being probed. The following ratios of nonlinear coefficients and birefringence was determined: d12/d11=−3.498±0.171, |d26/d12|=0.365±0.010, |d26/d11|=1.273±0.036, and |nb−na|=0.101±0.018 (at 532 nm).
Flow coherence in a bead pack observed using frequency domain modulated gradient nuclear magnetic resonance
Authors: Callaghan P. T. , Codd S. L.
Published: July 2002

We have used a new nuclear magnetic resonance (NMR) method based on periodic wave form magnetic field gradients to investigate temporal correlations for flow in porous media. The frequency domain modulated field gradient NMR technique directly yields the frequencydependent-dispersion coefficient, i.e., the spectral density of the velocity autocorrelation function. Our measurements of dispersion spectra have been carried out, in the direction transverse to the mean flow, for water flowing in a monodisperse latex bead pack (diameters 50–136 μm) and at Péclet numbers ranging from 10 to 5000. We observe spectral peaks at a frequency corresponding to the inverse time for flow around a bead, an effect we attribute to coherent meandering flow around the bead. This observation is in close agreement with the recent computer simulations of Maier et al., in which negative velocity autocorrelation function transients are seen.
Molecular interaction maps as information organizers and simulation guides
Authors: Kohn Kurt W.
Published: July 2002

A graphical method for mapping bioregulatory networks is presented that is suited for the representation of multimolecular complexes, protein modifications, as well as actions at cell membranes and between protein domains. The symbol conventions defined for these molecular interaction maps are designed to accommodate multiprotein assemblies and protein modifications that can generate combinatorially large numbers of molecular species. Diagrams can either be “heuristic,” meaning that detailed knowledge of all possible reaction paths is not required, or “explicit,” meaning that the diagrams are totally unambiguous and suitable for simulation. Interaction maps are linked to annotation lists and indexes that provide ready access to pertinent data and references, and that allow any molecular species to be easily located. Illustrative interaction maps are included on the domain interactions of Src, transcription control of E2F-regulated genes, and signaling from receptor tyrosine kinase through phosphoinositides to Akt/PKB. A simple method of going from an explicit interaction diagram to an input file for a simulation program is outlined, in which the differential equations need not be written out. The role of interaction maps in selecting and defining systems for modeling is discussed.
A novel pressure array sensor based on contact resistance variation: Metrological properties
Authors: Del Prete Z. , Monteleone L. , Steindler R.
Published: July 2002

The working principle and the metrological performances of a novel array sensor devoted to pressure map measurements are experimentally analyzed here. The physical principle on which the sensor elements are sensitive to the pressure is the variation of the contact resistance. Pressure maps from 1 up to 500 kPa can be measured. The prototype here utilized for the metrological characterization has been an 8×8 matrix sensor with a 5 mm spatial resolution over both x and y direction and a total thickness of 150 μm. The materials that have been chosen to assemble the prototype yielded to a very flexible and robust sensor which can easily be fitted over round surfaces without being damaged or leading to an alteration of its measuring properties. The static and the dynamic metrological performances of the sensor that have been studied and discussed are the response function and the calibration curve, the repeatability, the sensitivity, the time drift, the hysteresis, and the dynamic response. In spite of its functional and constructive simplicity, the metrological performances the sensor has exhibited, together with its peculiar constructive feature, have indicated the possibility to utilize it to effectively measure pressure maps in every application for which the sensor could be shaped in.
Transient and steady-state behavior of space charges in multilayer organic light-emitting diodes
Authors: Ruhstaller B. , Carter S. A. , Barth S. , Riel H. , Riess W. , Scott J. C.
Published: July 2002

A numerical study of space charge effects in multilayer organic light-emitting diodes (OLEDs) is presented. The method of solving the coupled Poisson and continuity equations, previously established for single-layer polymer LEDs, has been extended to treat internal organic interfaces. In addition, we consider the transient current and electroluminescence response. We discuss the accumulation of charges at internal interfaces and their signature in the transient response as well as the electric field distribution. Comparison to experimental transient data of a typical bilayer LED based on tris(8-hydroxyquinolinato)aluminum (Alq3) is provided and good agreement is found. Our results are consistent with commonly assumed operating principles of bilayer LEDs. In particular, the assumptions that the electric field is predominantly dropped across the Alq3 layer and that the electroluminescence delay time is determined by electrons passing through Alq3 to the internal interface are self-consistently supported by the results of the simulation. Moreover, the creation of emissive singlet excitons is found to be strongly confined to the Alq3 side of the internal interface and the emission zone width is dictated by the exciton diffusion length. Design principles for trilayer LEDs with improved power efficiency are also discussed.
Grain-boundary-controlled impedances of electroceramics: Generalized effective-medium approach and brick-layer model
Authors: Nan Ce Wen , Zhou Xisong , Nan Jun , Wu Junbo
Published: July 2002

In every electroceramic there exist variations throughout the microstructure of both grain size (and shape) and electrical properties of individual grain boundaries. To calculate the effects of such microstructural variations on the frequency-dependent impedance/dielectric behavior, we describe a generalized statistical effective-medium approach based on effective-medium theory. To illustrate the predictions of the approach, the effect of various distributions in the grain boundary conductivity and the grain size, as well as the grain shape and porosity, are considered. The calculations show that such variations lead to depression of the boundary arc and deviations of the impedance spectra from the well-known brick-layer model with an idealized microstructure consisting of identical grains and grain boundaries.
Oxygen precipitation in nitrogen-doped Czochralski-grown silicon crystals
Authors: Nakai Katsuhiko , Inoue Yoshiharu , Yokota Hideki , Ikari Atsushi , Takahashi Jun , Tachikawa Akiyoshi , Kitahara Kouichi , Ohta Yasumitsu , Ohashi Wataru
Published: July 2002

Oxygen precipitate behavior of nitrogen-doped Czochralski-grown silicon (CZ-Si) crystals is investigated. It is found that nitrogen doping enhances oxygen precipitation after heat treatment. The oxygen precipitate volume density in nitrogen-doped crystals after heat treatment does not change regardless of the heat treatment temperature, while the oxygen precipitate volume density of crystals which are not nitrogen doped decreases as the heat-treatment temperature increases. The characteristics of precipitation behavior in nitrogen-doped CZ-Si crystals are due to the “grown-in” oxygen precipitates, which already exist in an as-grown state with a high volume density. The oxygen precipitation growth of nitrogen-doped crystals is found to be an oxygen diffusion limited process, the same as in the case of the oxygen precipitation growth of crystals which are not nitrogen doped. The formation mechanism of grown-in oxygen precipitates will also be discussed in this article.
Investigation of the structure, stability, and ionization dynamics of zirconium oxide clusters
Authors: Foltin M. , Stueber G. J. , Bernstein E. R.
Published: July 2002

The growth dynamics, stabilities, and structures of small zirconium oxide clusters (ZrnOm) are studied by covariance mapping time-of-flight mass spectrometry and density functional theory calculations. The zirconium oxide clusters are produced by laser ablation of zirconium metal into a helium gas flow seeded with up to 7% O2. The neutral (ZrnOm) cluster distribution is examined at high and low ionization laser intensities. At high ionization laser intensities (∼107 W/cm2) the observed mass spectra consist entirely of fragmented, nonstoichiometric clusters of the type [(ZrO2)n−1ZrO]+, while in case of lower laser intensities (∼0.2×107 W/cm2), cluster fragmentation is strongly reduced and predominantly stoichiometric clusters (ZrO2)n+ appear. Under such gentle conditions, (ZrO2)5+ is found to be much more abundant than its neighboring clusters (ZrO2)n+, n=1,2,4,6,7,8. The unusually high signal intensity of the Zr5O10+ ion is found to be due to the high stability of the (ZrO2)5 neutral cluster. Density functional theory calculations show a number of different conceivable isomer structures for this cluster and reveal the most likely growth pattern that involves the sequential uptake of ZrO2 units by a (ZrO2)4 cluster to yield (ZrO2)5 and (ZrO2)6. Based on a series of different density functional theory and Hartree–Fock theory calculations, and on kinetic modeling of the experimental results, isomer structures, growth mechanisms, and stability patterns for the neutral cluster distribution can be suggested. The (ZrO2)5 structure most stable at temperatures less than 3000 K is essentially a tetragonal pyramid with five zirconium atoms at the vertices, whereas an octahedral structure is the main building block of (ZrO2)6. Modeling of the covariance matrix over a wide range of ionization laser intensities suggests that (ZrO2)n neutral clusters absorb two photons of 193 nm radiation to ionize and then, for high laser intensity, the ion absorbs more photons to fragment.
Shear force scanning near-field optical microscope based on a piezoelectric bimorph cantilever
Authors: Shang G. Y. , Wang C. , Wu J. , Bai C. L. , Lei F. H.
Published: July 2002

In this article we present a simple, nonoptical shear force detection scheme to control probe–sample distance for a scanning near-field optical microscope (SNOM). Shear force detection is realized by attaching a tapered optical fiber probe to a piezoelectric bimorph cantilever in which one piezo layer generates a maximum piezo voltage when the cantilever is excited at resonance by the other piezo layer. The amplitude of the piezo voltage will decrease as the probe approaches a sample's surface due to probe–sample interacting shear force. Keeping the piezo voltage constant provides a very sensitive method by which to control probe–sample distance. Based on the shear force detection scheme, a shear force SNOM system has been built, operating in transmission collection mode. Shear force topographic and optical images have been taken using uncoated optical fiber probes fabricated by a chemical etching technique. The results suggest that the system is very reliable, repeatable, and easy to use.
Edge and screw dislocations as nonradiative centers in InGaN/GaN quantum well luminescence
Authors: Cherns D. , Henley S. J. , Ponce F. A.
Published: July 2002

Transmission electron microscopy (TEM) and scanning electron microscope cathodoluminescence (CL) have been used to determine the influence of edge and screw dislocations on the light emitting properties of InxGa1−xN quantum wells. TEM is used to locate and identify the nature of dislocations. CL on the same samples is used to determine the spatial variation of the luminescence. A direct correlation of CL maps with TEM has been established, showing that threading edge dislocations act as nonradiative recombination centers with an associated minority carrier diffusion length of 200 nm. Threading dislocations of screw and mixed type were found to be associated with surface pits which were also nonradiative in the quantum well (QW) emission, but owing to the absence of QW growth on the pit facets. The contributions of edge and screw/mixed dislocations to the reduction of the QW emission are quantified, and the wider significance of these results is discussed.
Meyer–Neldel rule for dark current in charge-coupled devices
Authors: Widenhorn Ralf , Mündermann Lars , Rest Armin , Bodegom Erik
Published: July 2002

We present the results of a systematic study of the dark current in each pixel of a charged-coupled device chip. It was found that the Arrhenius plot, at temperatures between 222 and 291 K, deviated from a linear behavior in the form of continuous bending. However, as a first approximation, the dark current, D, can be expressed as: D=D0 exp(−ΔE/kT), where ΔE is the activation energy, k is Boltzmann’s constant, and T the absolute temperature. It was found that ΔE and the exponential prefactor D0 follow the Meyer–Neldel rule (MNR) for all of the more than 222,000 investigated pixels. The isokinetic temperature, T0, for the process was found as 294 K. However, measurements at 313 K did not show the predicted inversion in the dark current. It was found that the dark current for different pixels merged at temperatures higher than T0. A model is presented which explains the nonlinearity and the merging of the dark current for different pixels with increasing temperature. Possible implications of this finding regarding the MNR are discussed.
Growth of InxGa1−xAs quantum dots by metal–organic chemical vapor deposition on Si substrates and in GaAs-based lasers
Authors: Kazi Zaman Iqbal , Egawa Takashi , Umeno Masayoshi , Jimbo Takashi
Published: July 2002

The growth conditions of low-dimensional dot structures of strained InxGa1−xAs on Si substrates using the Stranski–Krastanov growth mode by metal–organic chemical vapor deposition are optimized. Atomic force microscopy measurement has been performed to characterize the dot structures. The dot density and their size are found to be strongly dependent on the substrate temperature, In content, and V/III ratio. The optimized growth condition was further used to fabricate quantum dot-like laser diodes on Si. The characteristics of the laser diode with an InxGa1−xAs quantum dot-like active region are analyzed.
Multicomponent mixture of charged hard-sphere chain molecules in the polymer mean-spherical approximation
Authors: Kalyuzhnyi Yu. V. , Cummings P. T.
Published: July 2002

The analytical solution of the recently proposed ideal chain polymer mean-spherical approximation [Kalyuzhnyi, Mol. Phys. 94, 735 (1998)] is presented for the multicomponent mixture of charged hard-sphere linear chain flexible molecules. The solution applies to any mixture of chain molecules with arbitrary distribution of the charge and size of the beads along the molecular backbone. Closed form analytical expressions for the internal energy, Helmholtz free energy, chemical potentials, and pressure are derived. By way of illustration thermodynamical properties of several versions of the fluid of charged chain molecules of different length, including the molecules with uniform, diblock, and alternating distribution of the charge, are studied. Theoretical predictions are in reasonable agreement with available computer simulation predictions. We also present the liquid–gas phase diagrams for systems with diblock and alternating distribution of the charge.
Spectral diffusion in liquids with fluctuating solvent responses: Dynamical heterogeneity and rate exchange
Authors: Richert Ranko
Published: July 2002

A recent theory for the time dependence of inhomogeneous line shapes is extended to account for fluctuations of the solvent response times τ in both space (heterogeneity) and time (rate exchange). Different simulation techniques are outlined for solving the Ornstein–Uhlenbeck type spectral diffusion in the situation expected for supercooled liquids. For intrinsically exponential solvent dynamics, slow rate exchange (or static heterogeneity) has to be assumed in order to reproduce measurements of the Stokes-shift correlation function C(t) and inhomogeneous linewidth σ(t) simultaneously.
Efficient, narrow linewidth excitonic emission at room temperature from GaAs/AlGaAs V-groove quantum wire light-emitting diodes
Authors: Weman H. , Dupertuis M.-A. , Martinet E. , Rudra A. , Kapon E.
Published: July 2002

We report on efficient, narrow linewidth exciton recombination in GaAs/AlGaAs V-groove quantum wire light-emitting diodes at room temperature. The high efficiency is due to a selective carrier injection mechanism resulting in an estimated internal quantum efficiency of ∼20% with an electroluminescence (EL) linewidth as narrow as 15 meV. The thermal broadening contribution to the linewidth is 6 meV due to exciton scattering with optical phonons. An analysis of the EL peak shift in a magnetic field points out the typical superlinear behavior of the excitonic binding energy for a quantum wire.
Generalized relativistic effective core potential and relativistic coupled cluster calculation of the spectroscopic constants for the HgH molecule and its cation
Authors: Mosyagin Nikolai S. , Titov Anatoly V. , Eliav Ephraim , Kaldor Uzi
Published: July 2002

Generalized relativistic effective core potential (GRECP) calculations of spectroscopic constants of the HgH molecule ground and low excited states and the HgH+ cation ground state are carried out, with correlation included by the Fock-space relativistic coupled cluster (RCC) method. Basis set superposition errors (BSSE) are estimated and discussed. It is demonstrated that connected triple excitations of the 13 outermost electrons are necessary to obtain accurate results for mercury hydride. Spectroscopic constants derived from potential curves which include these terms are in very good agreement with experiment, with errors of a few mbohr in Re, tens of wave numbers in excitation energies and vibrational frequencies, and proportionately for other properties. Comparison with previous calculations is also presented.
Dissociation of grain boundary dislocations in SrBi2Ta2O9 ferroelectric thin films
Authors: Zhu Xinhua , Zhu Jianmin , Zhou Shunhua , Li Qi , Liu Zhiguo , Ming Naiben
Published: July 2002

In this work, the dissociation of grain boundary dislocations (GBDs) is reported in SrBi2Ta2O9 (SBT) ferroelectric thin films with c-axis orientation grown by pulsed-laser deposition on Pt/TiO2/SiO2/Si(100) substrates. Small-angle (8.2°) [001] tilt grain boundaries with a boundary plane close to the (110) plane exhibit partial GBDs separated by stacking faults. The dissociated grain-boundary structures have twice the number of GBDs and interdislocation core channel width smaller than that Frank’s geometrical rule predicts. At the equilibrium, the repulsive elastic force between partial dislocations is balanced by an attractive force produced by the formation of a stacking fault between the partials. Based on this, the stacking fault energy is evaluated to be 0.27–0.29 J/m2. The relationship between the leakage current of SBT films and dissociation of GBDs is also discussed.
All-epitaxial fabrication of thick, orientation-patterned GaAs films for nonlinear optical frequency conversion
Authors: Eyres L. A. , Tourreau P. J. , Pinguet T. J. , Ebert C. B. , Harris J. S. , Fejer M. M. , Becouarn L. , Gerard B. , Lallier E.
Published: July 2002

Orientation-patterned GaAs (OPGaAs) films of 200 μm thickness have been grown by hydride vapor phase epitaxy (HVPE) on an orientation-patterned template fabricated by molecular beam epitaxy (MBE). Fabrication of the templates utilized only MBE and chemical etching, taking advantage of GaAs/Ge/GaAs heteroepitaxy to control the crystal orientation of the top GaAs film relative to the substrate. Antiphase domain boundaries were observed to propagate vertically under HVPE growth conditions so that the domain duty cycle was preserved through the thick GaAs for all domain periods attempted. Quasiphase-matched frequency doubling of a CO2 laser was demonstrated with the beam confocally focused through a 4.6 mm long OPGaAs film.
Quantitative interpretation of magnetic force microscopy images from soft patterned elements
Authors: Garcı́a J. M. , Thiaville A. , Miltat J. , Kirk K. J. , Chapman J. N. , Alouges F.
Published: July 2002

By combining a finite element tip model and numerical simulations of the tip–sample interaction, it is shown that magnetic force microscopy images of patterned soft elements may be quantitatively compared to experiments, even when performed at low lift heights, while preserving physically realistic tip characteristics. The analysis framework relies on variational principles. Assuming magnetically hard tips, the model is both exact and numerically more accurate than hitherto achieved.