bluuebunny/crossref_metadata_2025 huggingface dataset:
Localized picosecond resolution with a near-field microwave/scanning-force microscope
Authors: van der Weide D. W.
Published: July 2002

We modify a scanning-force microscope tip/cantilever with a coaxial metal shield for use as an ultra-small field probe, observing 30 ps waveforms on an integrated circuit at a spatial resolution set by the tip radius, the first direct measurements of electric field at these levels of temporal and spatial resolution. This new ability to acquire topography while maintaining constant tip–sample distance for calibrated measurement or excitation of near-zone picosecond time-resolved electric fields is useful not only for probing advanced circuits but also for localized broadband spectroscopy of condensed matter and biological samples.
The origin of nonlinearity in KTiOPO4
Authors: Xue Dongfeng , Zhang Siyuan
Published: July 2002

The origin of nonlinearity in KTiOPO4 was investigated quantitatively from the chemical bond viewpoint. All constituent chemical bonds in this crystal were considered and their contributions to the total linearity and nonlinearity were quantitatively determined. Calculated results agree satisfactorily with experimental data in both signs and numerical values. These results show us that TiO6 groups and P(1)O4 groups have relatively larger linear contributions and the nonlinearity derives from KOx (x=8,9) groups and P(2)O4 groups.
Effects of interface charges on imprint of epitaxial Bi4Ti3O12 thin films
Authors: Park B. H. , Noh T. W. , Lee J. , Kim C. Y. , Jo W.
Published: July 2002

Using La0.5Sr0.5CoO3 (LSCO) or Pt film as a bottom electrode layer, epitaxial Bi4Ti3O12 (BTO) thin films were grown on MgO(001) substrates by pulsed laser deposition. A symmetric Pt/BTO/Pt capacitor structure shows a surprisingly large asymmetric polarization switching behavior, but a Pt/BTO/LSCO structure has a nearly symmetric P–V hysteresis. The strong asymmetric behavior in the Pt/BTO/Pt was attributed to positive charges resulting from interdiffusion at the bottom BTO/Pt interface. P–V hysteresis studies using numerous top electrode materials and Auger electron spectroscopy depth profile measurement also support formation of interfacial charges. Imprint pulse test shows that such an imprint failure cannot be recovered by applying a dc bias field.
Influence of growth conditions on Al-Ga interdiffusion in low-temperature grown AlGaAs/GaAs multiple quantum wells
Authors: Feng W. , Chen F. , Cheng W. Q. , Huang Q. , Zhou J. M.
Published: July 2002

Low-temperature growth and subsequent rapid thermal anneal were used to intermix Al and Ga atoms in AlGaAs/GaAs multiple quantum wells (QWs). The intermixed samples were characterized by photoluminescence (PL) spectroscopy, and the observed blue shifts in PL energies are interpreted as the result of modification of the QW shape due to the enhanced Al-Ga interdiffusion in the samples. The enhancement of interdiffusion was found to be strongly dependent on the growth and annealing conditions. In addition, the saturation behavior of Al-Ga interdiffusion was also observed.
Characterization of wafer bonded photodetectors fabricated using various annealing temperatures and ambients
Authors: Levine B. F. , Hawkins A. R. , Hiu S. , Tseng B. J. , Reilley J. P. , King C. A. , Gruezke L. A. , Johnson R. W. , Zolnowski D. R. , Bowers J. E.
Published: July 2002

A detailed study of the important role of temperature and gas ambient on the bonding between Si and InGaAs wafers was performed. The heterointerface was characterized by fabricating p-i-n photodetectors and measuring the forward and reverse currents, the capacitance, the absolute quantum efficiency, and the response bandwidth. Clear evidence for a thin tunneling barrier is found for nonoptimum fusing conditions.
Efficient white light-emitting diodes realized with new processable blends of conjugated polymers
Authors: Tasch S. , List E. J. W. , Ekström O. , Graupner W. , Leising G. , Schlichting P. , Rohr U. , Geerts Y. , Scherf U. , Müllen K.
Published: July 2002

An alternative method for producing efficient white light-emitting polymer diodes based on a blend of two polymers is reported. The white light emission is composed of a broad blue emission of laddertype (polyparaphenylene) (m-LPPP) and a red-orange emission of a new polymer, poly(perylene-co-diethynylbenzene) (PPDB). The red-orange electroluminescence emission is promoted by an excitation energy and charge transfer from m-LPPP to the PPDB. A concentration of 0.05% PPDB in the polymer blend is required in order to obtain white light emission. By inserting an insulating material in the blend, so that a maximum external quantum efficiency of 1.2% is obtained.
(YBa
2
Cu
3
O
7−δ
,Au)/SrTiO
3
/LaAlO
3
thin film conductor/ferroelectric coupled microstripline phase shifters for phased array applications
Authors: Van Keuls F. W. , Romanofsky R. R. , Bohman D. Y. , Winters M. D. , Miranda F. A. , Mueller C. H. , Treece R. E. , Rivkin T. V. , Galt D.
Published: July 2002

We report on the design, fabrication, and testing of novel YBa2Cu3O7−δ/SrTiO3/LaAlO3 (YBCO/STO/LAO) and Au/SrTiO3/LaAlO3 (Au/STO/LAO) coupled microstrip line phase shifters (CMPS). These CMPS were tested at frequencies within the Ku and K bands (12–20 GHz), at temperatures from 24 to 77 K, and at dc voltages (Vdc) from zero to 350 V. A relative insertion phase shift (Δφ) of 390° was measured for an eight-element YBCO/STO/LAO CMPS at Vdc=350 V, 16 GHz, and 40 K. At 77 K, a Δφ ∼260° was obtained for the CMPS at the same bias and frequency. Both results correspond to an effective coupling length of 0.33 cm. At both temperatures, the phase shifter exhibits a figure of merit of ∼30°/dB. To our knowledge, these are the best results published so far at these frequencies where miniaturization, insertion loss, and phase delay are key considerations.
Some qualitative features of the Couette flow of monodisperse, smooth, inelastic spherical particles
Authors: Zamankhan Piroz , Mazouchi Ali , Sarkomaa Pertti
Published: July 2002

Computer simulations have been performed to examine the occurrence of power-law correlations for the stresses exerted on the confining walls by the particles in the three dimensional Couette flow of hard, smooth, dissipative spherical particles of uniform size. At high particle concentrations, the wavelet analysis of the wall shear stress has revealed the existence of anomalous, long-ranged temporal correlations. Based on the results obtained, there are indications that the dense Couette flow of monodisperse, smooth, inelastic, spherical particles is a system which may be characterized by continuous distributions of the physical measures of its particles, such as size.
Third-order nonlinear microwave response of YBa2Cu3O7−δ thin films and single crystals
Authors: Hampel G. , Batlogg B. , Krishana K. , Ong N. P. , Prusseit W. , Kinder H. , Anderson A. C.
Published: July 2002

The nonlinear microwave response of superconducting YBa2Cu3O7−δ thin films and crystals was studied quantitatively by measuring the third-order harmonic amplitude E3ω (ω: 35 GHz, jRF⩽3×104 A/cm2). We find E3ω to be dominated by two extrinsic effects: (1) the interaction with flux lines in weak links, leading to an increased nonlinear response at low temperatures, and (2) rf related flux line creation at film edges, yielding a high E3ω-signal close to Tc. Both effects together lead to a minimum in E3ω around 40–60 K, which makes this temperature interval interesting for rf applications. Intrinsic nonlinearities are smaller by orders of magnitude and could not be detected.
Microscopic behavior of silicon in silicon delta-doped layer in GaAs
Authors: Grandidier B. , Stiévenard D. , Nys J. P. , Wallart X.
Published: July 2002

Silicon δ-doped layers in GaAs have been studied using scanning tunneling microscopy and scanning tunneling spectroscopy on cleaved (110) surfaces. The samples were grown using molecular beam epitaxy with two growth temperatures: 480 and 580 °C. The concentration of the δ-doped layer was 0.03 monolayer. We show that, as in the case of bulk doping, the silicon has an amphoteric character: both SiGa and SiAs are observed at 480 °C and donors are formed before acceptors. At 580 °C, the spatial repartition of silicon evidences the segregation of silicon.
Evidence of hydrogen–carbon interactions in plasma hydrogenated carbon-doped n-InP
Authors: Theys B. , Benchimol J. L. , Rao E. V. K. , Chevallier J. , Juhel M.
Published: July 2002

Unlike GaAs which is p type, InP, when doped with the amphoteric C, is known to exhibit n-type conduction but with a low carrier-to-dopant ratio. To learn more about C behavior, we have intentionally introduced atomic H in C-doped n-InP by exposing the samples to a radio-frequency deuterium plasma. We show here that C, unlike other n dopants (S, Sn, and Si), strongly interacts with H in InP. First, the distribution of deliberately introduced H closely follows that of C. Second, for all C dopings studied here, the H concentration is nearly equal to that of C. Finally, and most importantly, the electrical properties of the material are also significantly altered, for instance, the free-electron concentration increases by more than an order of magnitude in certain samples.
Observation of a highly phase-correlated chirped frequency comb output from a frequency-shifted feedback laser
Authors: Nakamura Koichiro , Miyahara Toshiharu , Ito Hiromasa
Published: July 2002

A laser cavity closed via the first-order diffracted light of an intracavity acousto-optic modulator is referred to as a frequency-shifted feedback laser laser. It has been proven that the laser output consists of a chirped frequency comb evenly spaced at a cavity-free spectral range. In this letter a phase correlation between the chirped comb components is discussed. A homodyne experiment with a Mach–Zehnder interferometer was carried out to examine the problem. It was clarified that the comb components are highly correlated in phase.
Effect of crystallinity on the magnetoresistive properties of La0.8MnO3−δ thin films grown by chemical vapor deposition
Authors: Pignard S. , Vincent H. , Sénateur J. P. , Fröhlich K. , Šouc J.
Published: July 2002

We report here a study on the role of crystallinity on the magnetoresistive properties of self-doped La0.8MnO3−δ thin films. Films have been grown by a liquid-source metalorganic chemical vapor deposition technique on two different single crystals: SrTiO3 (001) and Al2O3 (012). Epitaxial films are obtained on strontium titanate and show a high magnetoresistance peak at low fields: maximum magnetoresistive effect in the vicinity of the magnetic transition temperature, and negligible 50 K below. Polycrystalline films are observed on sapphire; they exhibit a lower magnetoresistance of several percent which remains nearly constant over a wide temperature range. Two magnetoresistive regimes can be distinguished in these films: in low fields (up to 5 mT), a strong magnetoresistance is obtained with a sensitivity of 0.28%/mT at 78 K; in higher fields, the magnetoresistance is linear and lower than this obtained in epitaxial films. The role of grain boundaries on the magnetoresistive effect is discussed.
Thermal stability of H2-implanted n- and p-type GaN
Authors: Pearton S. J. , Wilson R. G. , Zavada J. M. , Han J. , Shul R. J.
Published: July 2002

Implantation of H+2 into n- and p-type GaN creates high resistivity material in which the resistance displays activation energies of 0.8 and 0.9 eV, respectively. Annealing at 500 °C restores the initial, preimplanted resistance of the n-GaN, due to removal of the deep trap states created by the ion stopping. By contrast, in p-type GaN annealing at 500 °C produces motion of the implanted deuterium and formation of Mg–H complexes that keep the resistance high. About 20% of the deuterium remains in n-GaN even after annealing at 1200 °C, where it decorates the residual implant damage. In p-type GaN all of the deuterium is evolved from the crystal by 1000 °C.
Normal-incidence intersubband (In, Ga)As/GaAs quantum dot infrared photodetectors
Authors: Pan Dong , Towe Elias , Kennerly Steve
Published: July 2002

We report the device performance of normal-incidence (In, Ga)As/GaAs quantum dot intersubband infrared photodetectors. A primary intersubband transition peak is observed at the wavelength of 13 μm (E0→E1) and a secondary peak at 11 μm (E0→E2). The measured energy spacing in the conduction band of the quantum dots is in good agreement with low temperature photoluminescence measurement and calculations. A peak detectivity of 1×1010 cm Hz1/2/W at 13 μm was achieved at 40 K for these devices.
Wavelength dependence of the magnetic resolution of the magneto-optical near-field scanning tunneling microscope
Authors: Schad R. , Jordan S. M. , Stoelinga M. J. P. , Prins M. W. J. , Groeneveld R. H. M. , van Kempen H. , van Kesteren H. W.
Published: July 2002

A magneto-optical near-field scanning tunneling microscope is used to image the prewritten magnetic domain structure of a Pt/Co multilayer. A semiconducting tip acts as a local photodetector to measure the magnetic circular dichroism signal coming from the magnetic sample. The resolution of the magnetic imaging is given by the photoelectrically active volume of the tip. Reduction of the laser light wavelength resulted in a factor of 4 improvement of the magnetic resolution. Based on a sound and applicable definition we estimate the resolution to be (60±35) nm for a wavelength of 532 nm.
Random domain formation in 0°–360° bistable nematic twist cells
Authors: Bock H.
Published: July 2002

An analogue gray scale effect in bistable twisted nematic displays is presented. The disappearance of twist during the Frederiks transition is sufficiently inhomogeneous to allow the formation of a variable ratio of twisted and untwisted domains by applying a short variable pulse. Thermal fluctuations in the director profile lead to lateral alterations of the unwinding voltage through the cell. The effect offers high contrast, good viewing angle characteristics, and short address times.
Spatially resolved microwave field distribution in YBaCuO disk resonators visualized by laser scanning
Authors: Kaiser T. , Hein M. A. , Müller G. , Perpeet M.
Published: July 2002

The applicability of superconducting films in high-performance microwave devices suffers from inhomogeneous growth and local defects. Therefore we have installed a laser scanning system to visualize with spatial resolution the distribution of the microwave fields in 2-in.-diam YBaCuO disk resonators. The TM310 mode was imaged at a loaded quality factor of 8.3×104 via changes of the resonant frequency at 2.21 GHz and 40 K. The field distribution agreed with a magnetic-wall model, indicating at the contour of the disk fringe fields, but without strong edge enhancement. The data also indicated an inhomogeneous effective microwave penetration depth. The spatial resolution is presently limited to about 2 mm by heat diffusion.
Hotspot mixing: A framework for heterodyne mixing in superconducting hot-electron bolometers
Authors: Floet D. Wilms , Miedema E. , Klapwijk T. M. , Gao J. R.
Published: July 2002

We propose a framework to interpret heterodyne mixing in superconducting hot-electron bolometers. The physical conversion process of the mixer is the result of an electronic hotspot, of which the length, and consequently the resistance, oscillates at the intermediate frequency. On the basis of this concept, we calculate the (un)pumped current–voltage relation, the dc voltage responsivity, and the mixer conversion efficiency.
Carbon-nanotube tips for scanning probe microscopy: Preparation by a controlled process and observation of deoxyribonucleic acid
Authors: Nishijima Hidehiro , Kamo Satsuki , Akita Seiji , Nakayama Yoshikazu , Hohmura Ken I. , Yoshimura Shige H. , Takeyasu Kunio
Published: July 2002

We report a controlled process to make carbon-nanotube tips for scanning probe microscopes. The process consists of three steps: (1) purification and alignment of carbon nanotubes using electrophoresis, (2) transfer of a single aligned nanotube onto a conventional Si tip under the view of a scanning electron microscope, and (3) attachment of the nanotube on the Si tip by carbon deposition. Nanotube tips fabricated using this procedure exhibit strong adhesion and are mechanically robust. Finally, the performance of these tips is demonstrated by imaging the fine structure of twinned deoxyribonucleic acid with tapping-mode atomic force microscopy in air.
Vacancy defects in solid-phase epitaxial grown layers of self-implanted Si
Authors: Xu Jun , Roth E. G. , Holland O. W. , Mills A. P. , Suzuki Ryoichi
Published: July 2002

A method for preparing shallow dopant distributions via solid-phase epitaxial growth (SPEG) following amorphization by low-energy Si self-ion implantation leaves defects that can lead to unwanted dopant impurity diffusion. The double implant method for SPEG [O. W. Holland et al., J. Electron. Mater. 25, 99 (1996)] uses both low- and high-energy Si self-ion implantation to remove most of the interstitials. Nevertheless, we find that measurable crystalline imperfections remain following the SPEG annealing step. Measurements of defect profiles using variable-energy positron spectroscopy show that there are divacancy-impurity complexes in the SPEG layer and V6 and larger vacancy clusters near the SPEG-crystalline interface. These measurements should be useful for modeling the diffusion of dopant atoms and for fine tuning the double implant parameters.
Boron-enhanced diffusion of boron: Physical mechanisms
Authors: Agarwal Aditya , Gossmann H.-J. , Eaglesham D. J.
Published: July 2002

Silicon layers containing B in excess of a few atomic percent create a supersaturation of Si self-interstitials in the underlying Si, resulting in enhanced diffusion of B in the substrate [boron-enhanced diffusion (BED)]. The temperature and time dependence of BED is investigated here. Evaporated boron as well as ultralow energy 0.5 keV B-implanted layers were annealed at temperatures from 1100 to 800 °C for times ranging from 3 to 3000 s. Isochronal 10 s anneals reveal that the BED effect increases with increasing temperature up to 1050 °C and then decreases. In contrast, simulations based on interstitial generation via the kick-out mechanism predict a decreasing dependence leading to the conclusion that the kick-out mechanism is not the dominant source of excess interstitials responsible for BED. The diffusivity enhancements from the combined effects of BED and transient-enhanced diffusion, measured in 2×1015 cm−2, 0.5 keV B-implanted samples, show a similar temperature dependence as seen for evaporated B, except that the maximum enhancement occurs at 1000 °C. The temperature-dependent behavior of BED supports the hypothesis that the source of excess interstitials is the formation of a silicon boride phase in the high-boron-concentration silicon layer.
Optical properties of low-pressure metalorganic vapor phase epitaxy AlxGa1−xN thin-film waveguides by prism coupling technique
Authors: Dogheche E. , Remiens D. , Omnes F.
Published: July 2002

Epitaxial thin films of aluminum gallium nitride (AlxGa1−xN) were grown by low-pressure metalorganic vapor phase epitaxy on c-oriented sapphire substrates. The prism-coupling technique was carried out to determine the optical properties of Al0.17Ga0.93N thin films, i.e., the refractive index, the film thickness, and the optical loss. Optical transmission measurements were additionally used to determine the dispersion of the refractive index. A demonstration of optical waveguiding was successful in an AlGaN/AlN/sapphire planar structure, the optical propagation loss was determined to be around 1.8 dB cm−1 at 632.8 nm. An analysis of optical anisotropy using guided modes with the optical axis oriented normal to the film surface confirmed the uniaxial nature of the layer.
Transient effects of positive oxide charge on stress-induced leakage current in tunnel oxides
Authors: Zous Nian-Kai , Wang Tahui , Yeh Chih-Chich , Tsai C. W. , Huang Chimoon
Published: July 2002

The role of positive oxide charge in excess low-level leakage current in tunnel oxides induced by Fowler/Nordheim stress is investigated. A correlation between stress-induced gate current and substrate current in an n-channel metal-oxide-semiconductor field-effect transistor is observed. Both the gate current and the substrate current exhibit a significant transient effect. The mechanisms of the stress-induced currents and their field dependence are explored. Positive oxide charge tunnel detrapping is found to be the cause of the observed transient behavior in the two currents. The stress-created positive oxide charge can be significantly annealed by substrate hot electron injection.
Reactive molecular-beam epitaxy of GaN layers directly on 6H–SiC(0001)
Authors: Thamm A. , Brandt O. , Takemura Y. , Trampert A. , Ploog K. H.
Published: July 2002

We investigate the quality of GaN layers directly grown on 6H–SiC(0001) substrates by reactive molecular-beam epitaxy. Despite a pure three-dimensional nucleation, step-flow growth is achieved by in situ adjusting conditions such that the (2×2) reconstruction observed during growth is maximized in intensity. The resulting surface morphology exhibits large terraces separated by mono- and multiatomic steps, and is clearly superior to that obtained by plasma-assisted growth. Furthermore, the structural and optical properties of these layers are comparable to those of layers grown by plasma-assisted molecular-beam epitaxy.
Piezoelectric effects on many-body optical gain of zinc-blende and wurtzite GaN/AlGaN quantum-well lasers
Authors: Park Seoung-Hwan , Chuang Shun-Lien , Ahn Doyeol
Published: July 2002

Electronic and optical properties for (001)-, (111)-oriented zinc-blende (ZB), and (0001)-oriented wurtzite (WZ) structures are investigated taking into account the piezoelectric (PZ) effects. Self-consistent (SC) calculation of electronic structure is also considered within Hartree approximation. Using the SC model with the PZ field, we show that the many-body optical gain of (111)-oriented ZB and (0001)-oriented WZ quantum well (QW) lasers is reduced compared to results using the flat-band (FB) model. This is caused by the separation between the electron and hole wave functions in the presence of PZ electric field. However, when the optical gain is plotted as a function of the radiative current density Jrad, it is found that there is no significant difference between the results of the FB and SC models. We believe that this is due to the fact that the peak gain and the radiative recombination current are reduced simultaneously with increasing PZ field. For the same threshold gain level, the ZB and WZ quantum well structures are expected to have nearly the same threshold current density Jth within 10%.
Electrical characteristics of thin-film transistors using field-aided lateral crystallization
Authors: Jun Seung-Ik , Yang Yong-Ho , Lee Jae-Bok , Choi Duck-Kyun
Published: July 2002

A polycrystalline silicon thin-film transistor (TFT) technology, field-aided lateral crystallization (FALC), has been explored. Polycrystalline silicon thin film, as an active layer, was prepared by applying an electric field to amorphous silicon film during Ni metal-induced lateral crystallization (MILC). Compared with the conventional metal-induced lateral crystallization thin-film transistors (MILC TFTs), these field-aided lateral crystallization thin-film transistors (FALC TFTs) show a low off-state leakage current of 1.79×10−11 A at Vg=−10 V and a high on/off current ratio of 8.82×105. Moreover, the threshold voltage is lower and field-effect mobility is higher than those of MILC TFTs. Therefore, the possibility of high-performance and low-temperature (<500 °C) polycrystalline silicon TFTs was demonstrated by using FALC technology.
Optical switching property of a light-induced pinhole in antimony thin film
Authors: Fukaya Toshio , Tominaga Junji , Nakano Takashi , Atoda Nobufumi
Published: July 2002

Optical near-field recording, called a super-resolution near-field structure, records and retrieves small marks beyond the diffraction limit. A thin layer of an antimony (Sb) film, added to the usual phase-change optical disk, is the key material of this technique. Nonlinear optical properties of an Sb film, especially optical switching, were studied in the stationary state using a nanosecond pulse laser. Clear switching was observed under microscopic measurement.
Dielectric loss of SrTiO3 single crystals under direct current bias
Authors: Ang Chen , Bhalla A. S. , Guo Ruyan , Cross L. E.
Published: July 2002

The dielectric behavior of SrTiO3 single crystals under high dc electric field (up to 50 kV/cm) is reported in this letter. The rounded dielectric constant peaks are induced by the application of dc bias, and the corresponding dielectric losses are observed. The results show that dielectric loss under dc bias consists of several components coming from “defects mode” and “induced mode.” The field dependence of these modes is studied and their physical nature is discussed.
Ballistic magnetoresistance in transition-metal nanocontacts: The case of iron
Authors: Garcı́a N. , Muñoz M. , Zhao Y.-W.
Published: July 2002

This letter shows that the ballistic magnetoresistance of Fe at room temperature and low magnetic fields is ten times smaller than for Ni and Co. The results are well explained by theory that provides a global understanding for 3d transition metals because, for Fe, the ratio of majority to minority spins at Fermi level is much smaller than for Ni and Co. The data indicate that conduction is carried out by majority d electrons in the case of Fe, in contrast to what happens for Ni and Co.
Protection of SrBi2Ta2O9 ferroelectric capacitors from hydrogen damage by optimized metallization for memory applications
Authors: Hong Suk-Kyoung , Suh Chung Won , Lee Chang Goo , Lee Seok Won , Kang Eung Youl , Kang Nam Soo , Hwang Cheol Seong , Kwon Oh Seong
Published: July 2002

The degradation behavior of integrated Pt/SrBi2Ta2O9/Pt capacitors by hydrogen impregnation during the intermetal dielectric deposition and passivation is investigated. The hydrogen ions generated as a reaction byproduct from the SiH4-based deposition processes of the dielectric films induce reduction in the remanent polarization (Pr) as well as the imprint behavior of the small size capacitors (2×2 μm2). The degree of degradation is quite dependent on the size of the individual capacitors. The smaller capacitors underwent more serious degradation implying that the hydrogen ions impregnate into the SBT layer mainly along the etched side area of the capacitors not through the top Pt electrode. Metallization adopting TiN/Al/TiN/Ti multilayer is very effective in suppressing the hydrogen impregnation. In particular, the Ti layer appears to block the hydrogen penetration. Therefore, the optimized metallization scheme, wider metal lines than the top electrode area by 1 μm, successfully protects the integrated capacitors from the hydrogen damage. 12 μC/cm2 of 2Pr and 1.1 V of 2Vc (coercive voltage) with an imprinting voltage of 0.16 V were obtained from the passivated 2×2 μm2 array capacitors by the optimized metallization.
Two-dimensional theory of chirality. I. Absolute chirality
Authors: Le Guennec Patrick
Published: July 2002

Chirality is a notion already familiar to undergraduates as the fact that an object is not superposable to its mirror image. Its dichotomous or “yes/no” character, implicit in this definition due to Lord Kelvin, has always been considered self-evident. We prove here and in the companion article that in two dimensions chirality—the very concept of chirality—is a continuous phenomenon in the special case of square-integrable wave functions. This conception of chirality is to Kelvin’s definition what the continuous conception of door opening is to the closed/nonclosed dichotomy; hence it provides a continuous description of the discrete achiral symmetry breaking. This result is first extended to three and higher dimensions, then to the whole nonrelativistic quantum description of matter. Thus molecules are more or less chiral just as doors are more or less open, and molecular chirality changes continuously during chemical reactions. Chirality splits into two complementary forms—absolute and relative chirality. We present here the theory of absolute chirality. More generally, this unexpected and paradoxical breakthrough in symmetry theory is based on a geometrical description of wave functions that should find broad applications in molecular physics and in stereochemistry, where the notion of chirality has an overwhelming importance since long ago.
Structure of Nd2Fe14−xSixBHy measured by neutron diffraction
Authors: Chacon Cyril , Isnard Olivier
Published: July 2002

Synthesis and characterization of Nd2Fe14−xSixBHy compounds are presented for x=1–2 and y=0–4. The samples have been studied with emphasis given to the structural aspects and the magnetic properties. Our present investigation has been performed both at 2 and 300 K by means of high-resolution powder neutron diffraction. We report the evolution of the maximum hydrogen uptake, which decreases when the Si concentration increases. This behavior is discussed in the light of earlier neutron diffraction results concerning the preferential occupancy of some Fe sites by Si atoms and the H location in the prototype, Nd2Fe14B phase. The decrease of hydrogen uptake occurs mainly on two interstitial sites out of the four different interstitial sites involved in the insertion scheme. The saturation magnetization results are compared with the magnetic moments obtained by neutron diffraction analysis. Finally, the effects of Si substitution and/or H insertion on the spin reorientation that occurs at low temperatures have been investigated by means of ac susceptibility measurements.
Intermodulation measurements in Nb superconducting microstrip resonators
Authors: Monaco R. , Andreone A. , Palomba F.
Published: July 2002

The issue of intermodulation distortion in superconducting devices for rf and microwave applications is discussed. The classical frequency conversion theory for nonlinear two-port devices in the small signal limit is shown to apply to microstrip resonators regardless of their geometry and material. Two tone, third order intermodulation measurements allow more sensitive detection of nonlinearity compared to surface impedance measurements, provided a low noise, spurious free experimental setup is adopted. Measurements carried out on high quality Nb meanderline microstrip resonators show that at very low power level nonlinearity is dominated by intrinsic device properties. Film defects start to play a predominant role above a critical power level that strongly depends on the defect’s nature.
Detailed comparison of simulated and measured plasma profiles in the scrape-off layer and edge plasma of DIII-D
Authors: Porter G. D. , Isler R. , Boedo J. , Rognlien T. D.
Published: July 2002

The results of detailed comparisons between experimental measurements of the scrape-off layer and divertor plasmas and simulations using the UEDGE code for a DIII-D discharge [J. Luxon et al., Proceedings of the 11th International Conference on Plasma Physics and Controlled Nuclear Fusion (International Atomic Energy Agency, Vienna, 1986), Vol. I, p. 159] are reported. The simulations focus on understanding the flow of both fuel and impurity particles throughout the edge and scrape-off layer (SOL) plasma. The core impurity content and the core hydrogen ionization rate can be explained by sputtering and recycling in the divertor region alone. The model reproduces most of the detailed experimental measurements. The simulations include the effect of intrinsic impurities, assumed to be carbon originating from sputtering of the plasma facing surfaces. The simulations accurately reproduce the total radiated power, although the spatial profile of radiation is somewhat narrower in the simulation. The measured carbon density on closed field lines is reproduced well with the simulation. Comparison of carbon emission lines indicates the total carbon sputtering yield is a factor of 2 to 4 less than expected, although the total radiated power and core carbon content are insensitive to the sputtering yield. The agreement between simulation and experiment permits more meaningful interpretation of the experimental measurements.
Large-scale behavior of the tokamak density fluctuations
Authors: Zaslavsky G. M. , Edelman M. , Weitzner H. , Carreras B. , McKee G. , Bravenec R. , Fonck R.
Published: July 2002

An analysis of tokamak density fluctuations data permits the determination of two characteristic exponents. The exponents correspond to the powers of a power-law dependence of the distributions of the long-lasting monotonic change (“flight”) of the density and the time length of these changes. Speculation based on these results leads to construction of the fractional kinetic equation for the distribution function of the flights. The asymptotic transport properties of the particle density distribution function are directly connected with the exponents obtained from the density fluctuations data.
Voltage-dependent dielectric breakdown and voltage-controlled negative resistance in anodized Al–Al2O3–Au diodes
Authors: Hickmott T. W.
Published: July 2002

Theories of dielectric breakdown in insulating films normally assume that dielectric breakdown depends on the electric field in the sample; that is, the thicker the film the higher the breakdown voltage. Contrary to theoretical expectations, voltage-dependent dielectric breakdown is observed in Al–Al2O3–Au diodes where Al2O3 is made by anodizing in different electrolytes. The breakdown voltage is ∼4.5 V, independent of Al2O3 thickness and anodizing electrolyte. Voltage-controlled negative resistance (VCNR) develops in the current–voltage (I–V) characteristics of Al–Al2O3–Au diodes after voltage-dependent breakdown. Electron emission into vacuum accompanies the formation of VCNR in the I–V characteristics. Detailed studies of the development of VCNR show that the maximum current, the voltage for maximum current, and the voltage threshold for electron emission depend on the maximum voltage applied to the sample. A large current increase occurs for maximum applied voltage between 5 and 7 V. A fully developed VCNR characteristic has an ohmic contact suggesting that the development of an ohmic contact at a metal–insulator interface initiates breakdown.
Origin of multiple peak photoluminescence in InGaN/GaN multiple quantum wells
Authors: Pozina G. , Bergman J. P. , Monemar B. , Takeuchi T. , Amano H. , Akasaki I.
Published: July 2002

Optical spectroscopy has been performed for a set of In0.12Ga0.88N/GaN multiple quantum wells (MQW) grown by metalorganic vapor phase epitaxy at 820 °C. Time-resolved, temperature- and power-dependent photoluminescence as well as spatially-resolved cathodoluminescence measurements have been applied to elucidate the nature of the recombination mechanisms responsible for the radiative transitions in the samples. The photoluminescence spectra in this set of samples are dominated by strong multiple peak emissions associating with both confined levels of the MQW system (the higher energy band) and with strongly localized states of energies much lower than the QW band gap. We suggest that the photoluminescence originate from (i) the MQW exciton recombination, (ii) excitons localized in the quasidot regions with indium concentrations higher than in the alloy due to segregation processes, and (iii) from localized states in zero-dimensional quantum islands created by surface defects such as pits and V defects. Buried side-wall quantum wells caused by V defects might also influence the photoluminescence spectra.
Low-energy electron scattering cross section for the production of CO within condensed acetone
Authors: Lepage M. , Michaud M. , Sanche L.
Published: July 2002

We report on the low-energy electron induced production of CO within thin solid films of acetone condensed at low temperature on a solid Ar substrate. The CO fragments, which remain trapped within the bulk of the acetone film, are detected in situ via their first electronic state a 3Π using high-resolution electron-energy-loss spectroscopy. The production of CO is studied as a function of the electron energy (2–25 eV), electron dose, and film thickness. The energy dependence of CO production is calibrated in terms of an electron scattering cross section σp. It is characterized by an energy threshold at 8 eV, a strong rise up to about 14 eV, and a broad maximum of σp≈6.8×10−17 cm2 at 16 eV followed by a relatively small and monotonous decrease up to 25 eV. The production of CO is discussed in terms of the formation of several core-excited electron resonances, which may lead directly to the fragmentation of the molecule via dissociative electron attachment or indirectly by decaying into an entirely repulsive part of the corresponding neutral excited state and positive ion states.
Metallic etching by high power Nd:yttrium–aluminum–garnet pulsed laser irradiation
Authors: Torrisi L. , Ciavola G. , Gammino S. , Ando L. , Barnà A. , Laska L. , Krasa J.
Published: July 2002

A Nd:yttrium–aluminum–garnet pulsed laser, with 1064 nm wavelength, 9 ns pulse width, and 0.9 J maximum pulse energy, is employed to irradiate in vacuum different metal targets (Al, Ti, Ni, Cu, Ta, W, Au, and Pb). In order to measure the erosion thresholds, the etching rates, and the chemical yields, a mass quadrupole spectrometer is interfaced to the vacuum chamber. Etching process shows a threshold, which ranges between 0.1 and 1.6 J/cm2 for lead and tungsten, respectively. Etching rates range between 0.3 and 10 μg/pulse for copper and lead, respectively. The irradiation produces chemical yields ranging between 0.04 and 0.6 atoms/100 eV for copper and lead, respectively. A simple theoretical approach is presented to justify obtained results. The objective of collected data concerns the possibility to use ejected atoms, neutral and ionized, in an electron cyclotron resonance ion source, in order to provide high current, multiply charge ion beams.
Study of electron, proton, and swift heavy ion irradiation of n-type germanium using deep level transient spectroscopy
Authors: Colder A. , Levalois M. , Marie P.
Published: July 2002

Time dependence and temperature annealing effects are studied for different n-type germanium irradiated with swift heavy ions (SHI), protons and electrons. Deep level transient spectroscopy measurements show that the main peak located at about 200 K, previously ascribed to the A-center complex, is due to the E-center complex whose energy level is located at Ec−0.41 eV. Moreover, in the case of SHI and protons, this peak includes an other component which could be related to a multivacancy complex defect.
Chain dynamics and conformational transition in cis-polyisoprene: Comparison between melt and subglass state by molecular dynamics simulations
Authors: Fukuda Mitsuhiro , Kikuchi Hiroaki
Published: July 2002

Molecular-dynamics simulations are performed to analyze the local chain dynamics of cis-1,4-polyisoprene at various temperatures. The volumetric glass transition was found at around 247 K. The torsional angle autocorrelation functions (TACF) for the three kinds of backbone chain and the orientation autocorrelation functions (OACF) for bond vectors in skeletal and side chain were analyzed both near the chain ends and in the middle of the chain. Various types of cooperative conformational jumps (and librations) were found in the melt chain. However, the cooperative counter-rotation at second-neighbor single bond pairs, HC–CH2 and CH2–C, became a major correlation as the temperature decreased to the subglass region. Even in the subglass chain at 173 K, more than 30% bond pairs showed cooperative counter-rotation at the second-neighbor (Group A) under the conditions in which about 60% of the bond pairs showed no conformational jump (Group B) during 20 ns molecular dynamics (MD) runs. The TACF decays for the torsions in Group A were highly different from those in Group B. In contrast, the OACF decays for the classified C–CH3 vectors next to Group A showed no difference from that next to Group B. These findings indicated that the cooperative transitions at the second-neighbor were only a short-range correlation on the single bond pairs without changing the reorientation of the two double bond planes next to the single bond pairs.
A new method of calculation of Franck–Condon factors which includes allowance for anharmonicity and the Duschinsky effect: Simulation of the He I photoelectron spectrum of ClO2
Authors: Mok Daniel K. W. , Lee Edmond P. F. , Chau Foo-Tim , Wang DeChao , Dyke John M.
Published: July 2002

A new method of Franck–Condon (FC) factor calculation for nonlinear polyatomics, which includes anharmonicity and Duschinsky rotation, is reported. Watson’s Hamiltonian is employed in this method with multidimensional ab initio potential energy functions. The anharmonic vibrational wave functions are expressed as linear combinations of the products of harmonic oscillator functions. The Duschinsky effect, which arises from the rotation of the normal modes of the two electronic states involved in the electronic transition, is formulated in Cartesian coordinates, as was done previously in an earlier harmonic FC model. This new anharmonic FC method was applied to the simulation of the bands in the He I photoelectron (PE) spectrum of ClO2. For the first band, the harmonic FC model was shown to be inadequate but the anharmonic FC simulation gave a much-improved agreement with the observed spectrum. The experimentally derived geometry of the X̃ 1A1 state of ClO2+ was obtained, for the first time, via the iterative FC analysis procedure {R(Cl–O)=1.414±0.002 Å, ∠O–Cl–O=121.8±0.1°}. The heavily overlapped second PE band of ClO2, corresponding to ionization to five cationic states, was simulated using the anharmonic FC code. The main vibrational features observed in the experimental spectrum were adequately accounted for in the simulated spectrum. The spectral simulation reported here supports one of the two sets of published assignments for this band, which was based on multireference configuration interaction (MRCI) calculations. In addition, with the aid of the simulated envelopes, a set of adiabatic (and vertical) ionization energies to all five cationic states involved in this PE band, more reliable than previously reported, has been derived. This led also to a reanalysis of the photoabsorption spectrum of ClO2.
Rotational effects in the H2+OH→H+H2O reaction rate: Full-dimensional close-coupling results
Authors: Manthe Uwe , Matzkies Frank
Published: July 2002

The dependence of the thermal rate constant of the H2+OH→H+H2O reaction on the rotational motion is investigated. Full-dimensional quantum calculations accurately accounting for the overall rotation, i.e., close-coupling (CC) calculations, are presented. These calculations are based on a flux correlation function approach and employ a rigorously correct statistical sampling scheme for the rotational degrees of freedom and the multi-configurational time-dependent Hartree (MCTDH) approach for the wavefunction propagation. They provide a first strictly correct description of the rate constant of the title reaction on the Schatz–Elgersma potential energy surface. The results are compared to approximate results obtained within the centrifugal sudden or coupled states (CS) approximation and the J-shifting approximation. No significant differences have been found between the accurate results and rate constants obtained within the CS approximation. In contrast, the J-shifting approximation overestimates the accurate results by 38% to 44% for temperatures between 300 K and 700 K. Reasons for the inaccuracy of the J-shifting approximation are discussed in detail.
Temperature and field dependence of the charge injection from metal electrodes into random organic media
Authors: Burin Alexander L. , Ratner Mark A.
Published: July 2002

We consider the average injection rate from a metal electrode into disordered organic media, and its field dependence for a fixed density of carriers in the metal. At sufficiently high field the injection occurs through particular pathways, lowering the energetic barrier between the metal Fermi energy and the molecular levels of the organic layer. Assuming a Gaussian distribution of random energies, we find the dependence of current j on field E j∝exp(βE) in the relevant domain of electric fields. This behavior agrees both with numerical simulations of three-dimensional systems and with experimental data.
Sm–Co–Cu–Ti high-temperature permanent magnets
Authors: Zhou J. , Skomski R. , Chen C. , Hadjipanayis G. C. , Sellmyer D. J.
Published: July 2002

A class of promising permanent-magnet materials with an appreciable high-temperature coercivity of 8.6 kOe at 500 °C is reported. The Sm–Co–Cu–Ti magnets are prepared by arc melting and require a suitable heat treatment. Magnetization measurements as a function of temperature and x-ray diffraction patterns indicate that the samples are two-phase mixtures of 2:17 and 1:5 structures. Depending on heat treatment and composition, some of the magnets exhibit a positive temperature coefficient of coercivity. The promising high-temperature behavior of the coercivity is ascribed to the temperature dependence of the domain-wall energy, which affects the curvature of the walls and the pinning behavior.
Molecular dynamics study of the solid–liquid interface
Authors: Geysermans P. , Gorse D. , Pontikis V.
Published: July 2002

The atomic structure and diffusion at the solid–liquid heterophase interface are investigated by using Molecular Dynamics. The system studied is made of crystalline copper with surface terminations (100) and (111) and liquid aluminum, both modeled via adapted n-body potentials from the literature and cross interactions obtained by fitting the mixing enthalpy of the two species to experimental values. It is shown that at the interface the liquid forms layers with spacing such that the local average density equals that of the bulk liquid. The interfacial liquid is layered whatever the surface orientation is even if the solid is reduced to a single crystalline or amorphous layer, in agreement with density functional theory. Layering is however suppressed at the interface between the liquid and a bulk amorphous solid with a rough surface termination. Surprisingly, diffusion in the interfacial layers proceeds via vacancies, which also accommodate the density misfit between solid (Cu) and liquid (Al). These results are further discussed in the frame of existing experimental and theoretical works.
The construction of a gas aggregation source for the preparation of size-selected nanoscale transition metal clusters
Authors: Baker S. H. , Thornton S. C. , Edmonds K. W. , Maher M. J. , Norris C. , Binns C.
Published: July 2002

The design and operation of a gas aggregation source is described. The source combines the attributes of high-temperature operation (enabling preparation of transition metal clusters), mass selection, ultrahigh vacuum compatibility, and transportability. This makes it ideally suited to in situ studies such as scanning tunneling microscope or synchrotron radiation experiments. Data are presented to illustrate the performance of the source; recent results obtained in synchrotron radiation studies are highlighted.
Shift of the absorption spectrum of organic dyes due to saturation
Authors: Marcano O. A. , Melikechi N. , Verde G.
Published: July 2002

We measure the absorption spectrum of a rhodamine 101 solution in ethanol in the spectral region 555–583 nm as a function of the incident light fluence. We show that increasing the field fluence results in a blueshift of the spectrum and absorption depletion with no spectral broadening. These results are explained in terms of a model that includes the influence of the solvent over the spectrum of the dye molecule.