bluuebunny/crossref_metadata_2025 huggingface dataset:
Kinetic model for plasma-based ion implantation of a short, cylindrical tube with auxiliary electrode
Authors: Sheridan T. E. , Kwok T. K. , Chu P. K.
Published: July 2002

Plasma-based ion implantation of the inner surface of a short, cylindrical tube is modeled using a two-dimensional particle-in-cell simulation. An auxiliary electrode, here a coaxial anode, is used to increase the ion impact energy. Initially, ions inside the tube impact the inner surface at approximately normal angles. At later times, ions enter the tube from the exterior plasma and impact predominantly near its center at glancing angles. Ions are found to cross the midplane of the tube and in some cases to pass completely through the tube, in contrast to the predictions of the “collisionless” fluid model. The total incident dose is greatest around the center of the tube, and least at its ends.
Photoluminescence and electronic density of states in a-C:H films
Authors: Giorgis F. , Giuliani F. , Pirri C. F. , Tagliaferro A. , Tresso E.
Published: July 2002

The density of states in the energy region near Fermi level for hydrogenated amorphous carbon thin films is presented. The different types of states are identified in their origin and the problem of their detection is considered. It is shown that only some of these states are accessible to detection by electron spin resonance. A quantitative correlation between their density and the quantum efficiency of the room temperature photoluminescence process is achieved. Such correlation applies to films having a wide range of physical properties deposited by different techniques.
High efficiency blue–green electroluminescence and scanning tunneling microscopy studies of porous silicon
Authors: Kuznetsov V. A. , Andrienko I. , Haneman D.
Published: July 2002

Schottky barrier porous silicon diodes have been fabricated showing electroluminescence peaking at 500 nm, with an internal efficiency for blue–green emission of about 0.1%. The structures, on low-resistivity n-type silicon, operate in reverse bias. Scanning tip light emission measurements show a peak emission at 630 nm, closer to that of photoluminescence from the identical surface at 700 nm than that of the electroluminescence. The latter is concluded to arise from nonquantum effects, at the metal interface. The threshold for visible light emission is at 0.2 mA/cm2, and for infrared light is an order of 10 higher. The lifetime in air is short when unencapsulated, but longer in vacuum.
Strong visible photoluminescence from Ge/porous Si structure
Authors: Gao Ting , Tong Song , Zheng Xiangqin , Wu Xinglong , Wang Liming , Bao Ximao
Published: July 2002

Strong visible photoluminescence was observed on Ge/porous silicon structure prepared by pulsed laser deposition of Ge on porous Si. The photoluminescence spectrum shows three subbands peaked at 400, 620, and 720 nm, respectively. The 400 nm peak was ascribed to GeO color centers, and the 720 nm peak to porous Si. The 620 nm peak is much stronger than the light emitting from the host porous Si. Photoluminescence and transmission electronic microscopy analyses suggest that the strong 620 nm luminescence is the result of the joint function of Ge and porous Si in the transition layer of Ge/porous Si structure. Plausible mechanisms for the 620 nm photoluminescence are discussed.
Conduction of heat in inhomogeneous solids
Authors: Dramićanin M. D. , Ristovski Z. D. , Djoković V. , Galović S.
Published: July 2002

In this letter we present a method for calculation of linear heat flow in inhomogeneous solids. The method is based on the evaluation of transfer matrices for each layer in a multilayered structure from the Laplace transformation of the partial differential equation of heat conduction. The multilayered structure is then described by a matrix obtained as a chain of products of individual layer transfer matrices and corresponding boundary thermal resistivity matrices. The analytic expression for the nth power of the multilayered transfer matrix is found, describing a periodic multilayered structure composed of n equal multilayered structures. The application of the presented method for calculation of photothermal signals is also shown. Dispersion relation for the thermal waves in inhomogeneous solids is obtained from the matrix elements of the transfer matrix. Finally, from the dispersion relation explicit expressions for the effective values of thermal diffusivity and conductivity of both the discontinuously and continuously inhomogeneous solids are evaluated.
Mechanism of low temperature C54 TiSi2 formation bypassing C49 TiSi2: Effect of Si microstructure and Mo impurities on the Ti–Si reaction path
Authors: Kittl J. A. , Gribelyuk M. A. , Samavedam S. B.
Published: July 2002

X-ray diffraction, high resolution transmission electron microscopy, and resistivity measurements were used to demonstrate a modification of the Ti/Si reaction path consisting of direct nucleation followed by diffusion limited growth of low resistivity C54 TiSi2 without nucleation of high resistivity C49 TiSi2, for the reaction of Ti with Mo doped polycrystalline or Mo doped amorphous Si by rapid thermal processing at 650 °C. We also report the mechanism of early C54 nucleation. We demonstrate that MoSi2 and an unidentified silicide phase lattice matched to C54 TiSi2, with spacings of 4.15 and 2.26 Å, nucleate along the Ti/Si interface at early stages of the reaction and act as templates on which C54 TiSi2 nucleates and grows epitaxially. In contrast, the conventional phase sequence, nucleation and growth of C49 TiSi2 preceding nucleation of C54 TiSi2, was observed for the Ti/Mo doped single crystal (100) Si reaction and for all samples without Mo.
Superconductor-normal metal-superconductor Josephson junctions with Ti interlayer
Authors: Fritzsch L. , Schubert M. , Wende G. , Meyer H.-G.
Published: July 2002

Single nonhysteretic superconductor-metal-superconductor Josephson junctions consisting of Nb-Ti-Nb were prepared using the well-known anodization process for the isolation of the junctions. The 4×4 μm2 junctions show overdamped Josephson current-voltage characteristics at 4.2 K. The characteristic voltage IcRn is about 8.0 μV at a critical current density jc of about 29 kA/cm2. Under the influence of 10.5 GHz microwave irradiation the constant-voltage steps of n=0 and ±1 are observed with current amplitudes of about 3 mA.
Observation of CuPt-A type atomic ordering in AlxIn1−xAs alloys
Authors: Suzuki Tohru , Ichihashi Toshinari , Nakayama Tatsuo
Published: July 2002

CuPt-A type atomic ordering, which has been reported for only Al0.5In0.5P and Ga0.5In0.5P, was observed in Al-rich AlxIn1−xAs grown on (001) InP substrates by solid-source molecular-beam epitaxy; only the triple-period-A type ordering in the [111]A directions has been previously reported for this alloy system grown under a similar growth condition. While the triple-period-A type ordering was observed in Al0.5In0.5As grown with a (2×3) surface reconstruction, the CuPt-A type ordering was observed in Al-rich AlxIn1−xAs when the surface showed a (1×2) surface reconstruction during growth. This observation provides strong support for the previous inference that the (1×2) surface reconstruction gives rise to CuPt-A type ordering.
Spatial ordering of islands grown on patterned surfaces
Authors: Lee Choongseop , Barabási Albert-László
Published: July 2002

We demonstrate that growth on a sample patterned with an ordered defect array can lead to islands with rather narrow size distribution. However, improvement in the size distribution is achieved only if the growth conditions (flux and temperature) have optimal values, determined by the patterning length scale. Since the scanning tunelling and the atomic force microscopes are capable of inducing surface perturbations that act as potential preferential nucleation sites, our work demonstrates that nanoscale surface patterning can improve the ordering of platelets and self-assembled quantum dots.
A fast-response and short-wavelength nonlinear optical chromophore for a photorefractive composite
Authors: Chen Zhijian , Wang Feng , Yao Cong , Huang Zhiwen , Gong Qihuang , Chen Yiwang , Chen Huiying
Published: July 2002

A nonlinear optical molecule, β,β-diacetyl-4-methoxylstyrene (DAMST), was synthesized and used as the electro-optic chromophore in a photorefractive polymeric composite. Its absorption peak lies at 320 nm and shows no absorption at wavelengths longer than 400 nm. In a 100 μm thickness film of DAMST:poly(N-vinylcarbazole):2,4,7-trinitro-9-fluorenone with a weight ratio of 59:40:1, two-beam coupling gain was measured as high as 32 cm−1 at a wavelength of 543 nm. A response time as short as 17 ms was estimated at an applied electric field of 84 V/μm with a writing beam intensity of 1 W/cm2.
Improved hydrogenated amorphous silicon thin films by photon assisted microwave electron cyclotron resonance chemical vapor deposition
Authors: Song Y. J. , Anderson W. A.
Published: July 2002

The conventional microwave electron cyclotron resonance chemical vapor deposition process has been modified by illuminating the substrate with a tungsten halogen lamp beam of intensity 0.1–1 W/cm2 during the deposition of hydrogenated amorphous silicon films. Compared to the conventional process without photon assist, this technique provides improved photoconductivity, carrier lifetime, and solar cell efficiency, as well as reduced light-induced degradation. The efficiency of undoped hydrogenated amorphous silicon/p-type crystalline silicon solar cells fabricated by this process is 10%, without an antireflective coating, and has degraded to only 9.8% after 3 weeks continuous exposure to light.
Highly sensitive magnetometers based on YBa2Cu3O7 Josephson junction arrays
Authors: Krey S. , Brügmann O. , Schilling M.
Published: July 2002

The dependence of the critical current of a Josephson junction on the magnetic flux in the junction area can be used for the sensitive detection of external magnetic fields. In contrast to superconducting quantum interference devices, also the measurement of absolute magnetic fields is possible. To increase the transfer function ∂V/∂B, we use serial arrays of up to 105 Josephson junctions between flux-concentrating areas on 24° SrTiO3 bicrystal substrates. We investigate the scaling properties of the critical current Ic, the normal state resistance Rn and the flux density noise SB in dependence on the number of Josephson junctions in the serial array. By the use of a magnetic field modulation scheme, the 1/f noise from critical current fluctuations in the junctions can be suppressed. At 77 K, we achieve a white noise level of SB=1.2 pT/Hz for a 105-junction array.
Carrier-density dependence of the photoluminescence lifetimes in ZnCdSe/ZnSSe quantum wells at room temperature
Authors: Jordan C. , Donegan J. F. , Hegarty J. , Roycroft B. J. , Taniguchi S. , Hino T. , Kato E. , Noguchi N. , Ishibashi A.
Published: July 2002

Photoluminescence lifetimes have been measured at room temperature as a function of carrier density in ZnCdSe/ZnSSe quantum wells. We show that, at low carrier density (5×109–5×1010 cm−2), nonradiative recombination dominates, while radiative recombination becomes more dominant as the carrier density is increased from 5×1010 to 5×1011 cm−2. Above ∼5×1011 cm−2, band filling effects are shown to produce a saturation of the lifetimes. A simple rate equation model approach can be used to describe the carrier density dependence of the photoluminescence decay data obtained on a wide range of samples. A representative band-to-band recombination coefficient of 8×10−4 cm2 s−1 and a Shockley–Read–Hall rate of 7.3×107 s−1 were determined for one of the better samples studied. We believe that the excellent quality of our samples has allowed for the radiative recombination coefficient to be characterized accurately at room temperature.
Ge-related faceting and segregation during the growth of metastable (GaAs)1−x(Ge2)x alloy layers by metal–organic vapor-phase epitaxy
Authors: Norman A. G. , Olson J. M. , Geisz J. F. , Moutinho H. R. , Mason A. , Al-Jassim M. M. , Vernon S. M.
Published: July 2002

( GaAs ) 1−x ( Ge 2 ) x alloy layers, 0<x<0.22, have been grown by metal–organic vapor-phase epitaxy on vicinal (001) GaAs substrates. Transmission electron microscopy revealed pronounced phase separation in these layers, resulting in regions of GaAs-rich zinc-blende and Ge-rich diamond cubic material that appears to lead to substantial band-gap narrowing. For x=0.1 layers, the phase-separated microstructure consisted of intersecting sheets of Ge-rich material on {115}B planes surrounding cells of GaAs-rich material, with little evidence of antiphase boundaries. Atomic force microscopy revealed {115}B surface faceting associated with the phase separation.
Blue and infrared up-conversion in Tm3+-doped fluorozirconate fiber pumped at 1.06, 1.117, and 1.18 μm
Authors: Mejía E. B. , Starodumov A. N. , Barmenkov Yu. O.
Published: July 2002

We report up-conversion in a fluorozirconate Tm3+-doped fiber pumped by the first and second Stokes components of a Raman-shifted Nd3+:YAG laser. Relative fluorescence in the ultraviolet, blue, red, and near-infrared regions is analyzed as a function of the intensity ratio between the Stokes components. It is demonstrated that side fluorescence in the blue at 480 nm strongly dominates over other transitions when pumping with the two Stokes components. This suggests that high-efficiency blue up-conversion lasing is possible by using this pumping scheme with double-clad, Nd3+- or Yb3+-doped fiber laser pumps.
Light-emitting diodes with 31% external quantum efficiency by outcoupling of lateral waveguide modes
Authors: Windisch R. , Heremans P. , Knobloch A. , Kiesel P. , Döhler G. H. , Dutta B. , Borghs G.
Published: July 2002

The external quantum efficiency of light-emitting diodes (LEDs) is usually limited by total internal reflection at the semiconductor–air interface. This problem can be overcome by a combination of light scattering at a textured top surface and reflection on a backside mirror. With this design, we achieve 22% external quantum efficiency. One of the main loss mechanisms in such nonresonant cavity (NRC) light-emitting diodes is coupling into an internal waveguide. Texturing the surface of this waveguide allows the partial extraction of the confined light. In this way, we demonstrate an increase in the external quantum efficiency of NRC-LEDs to 31%.
Surface morphology in InAs/GaAs(111)A heteroepitaxy: Experimental measurements and computer simulations
Authors: Nosho Brett Z. , Zepeda-Ruiz Luis A. , Pelzel Rodney I. , Weinberg W. Henry , Maroudas Dimitrios
Published: July 2002

The surface morphology of InAs films grown on GaAs(111)A has been studied by scanning tunneling microscopy. The vertical surface displacement on the InAs films has been found to depend on the underlying GaAs buffer layer thickness: specifically, thin GaAs layers are observed to behave mechanically similar to compliant substrates. Atomistic simulations within a valence force field model have been used to compare quantitatively how the InAs surface morphology depends on film thickness and the underlying GaAs layer thickness. The experimental and theoretical results are in excellent agreement over a range of film thicknesses where the misfit dislocation network at the semicoherent InAs/GaAs interface is fully developed.
Metalorganic chemical vapor deposition selective growth and characterization of InGaN quantum dots
Authors: Wang J. , Nozaki M. , Lachab M. , Ishikawa Y. , Qhalid Fareed R. S. , Wang T. , Hao M. , Sakai S.
Published: July 2002

InGaN quantum dots (QDs) have been formed by metalorganic chemical vapor deposition selective growth on Si-patterned GaN/sapphire substrates. The QDs were fabricated on GaN plinths grown within nanoscale circular windows opened in the Si mask by both focused ion-beam irradiation and photoassisted wet chemical etching. Using this process, the epitaxial growth of GaN plinths could be achieved on an underlying GaN layer spared from process-induced damage. The optical properties of the InGaN QDs have been investigated by low-temperature cathodoluminescence measurements. Compared to the band-gap emission from a reference sample of InGaN/GaN multiple quantum wells, the evident blueshift of the emission peak from the InGaN QDs was demonstrated. This result suggests that the carriers in the InGaN QDs were three-dimensionally confined.
Spin-polarized quasiparticle tunnel injection in a YBa2Cu3Oy/Au/Co junction
Authors: Lee Kiejin , Wang Wan , Iguchi Ienari , Friedman Barry , Ishibashi Takayuki , Sato Katsuaki
Published: July 2002

We report the strong suppression of YBa2Cu3Oy (YBCO) supercurrent by injection of spin-polarized quasiparticles (QP) using a cobalt ferromagnetic injector. The injection of spin-polarized QP generates a substantially larger nonequilibrium population as compared with that of an unpolarized injection current. The observed current gain depends on the thickness of Au interlayer (dAu) and is directly related to the nonequilibrium magnetization due to spin relaxation effects. For dAu=15 nm, the tunnel characteristic a YBCO/Au/Co junction exhibited a zero bias conductance peak, which may be interpreted by spin scattering processes at a ferromagnetic/d-wave superconductor junction.
Temperature dependence of the reversal mechanism in spin-valve films
Authors: Portier X. , Petford-Long A. K. , Anthony T. C. , Brug J. A.
Published: July 2002

The thermal behavior of spin valves (SVs) and exchange-coupled films has been investigated by in situ experiments in a Lorentz microscope. In situ magnetizing combined with in situ heating experiments have been performed during observation of the magnetization reversal of SV materials. A clear demonstration of the thermal effect on the reversal of the free layer is shown as well as a decrease of the exchange-bias pinning with increasing temperature. This latter effect results in the switching of the pinned layer near the blocking temperature.
Photoluminescence and interfacial heat transfer in gallium arsenide
Authors: Liu Chang Sheng , Kauffman John F.
Published: July 2002

Laser heating of irradiated samples will occur whenever the sample does not make thermal contact to a heat sink. We have explored laser heating of GaAs samples under modest excitation powers by observing photoluminescence intensity versus time. The observed photoluminescence diminishes exponentially in time following illumination, and the initial photoluminescence intensity is recovered after illumination ceases with the same time constant as observed in the degradation. A model based on linear interfacial heat transfer predicts exponential photoluminescence degradation, and the effect is parameterized in terms of an interfacial heat transfer constant. A variety of experimental results are consistent with predictions of the model.
Time-resolved microphotoluminescence of epitaxial laterally overgrown GaN
Authors: Holst J. , Kaschner A. , Hoffmann A. , Fischer P. , Bertram F. , Riemann T. , Christen J. , Hiramatsu K. , Shibata T. , Sawaki N.
Published: July 2002

Epitaxial laterally overgrown GaN (ELOG) structures are microscopically characterized using spatially resolved microphotoluminescence (micro-PL) and time-dependent spectroscopy. To understand the influence of the different lateral growth mechanisms on the peak position and the temporal behavior of the transition lines, we correlated the different micro-PL emission spectra with results of spatially resolved time-dependent spectroscopy experiments.
GeSi films with reduced dislocation density grown by molecular-beam epitaxy on compliant substrates based on porous silicon
Authors: Romanov S. I. , Mashanov V. I. , Sokolov L. V. , Gutakovskii A. , Pchelyakov O. P.
Published: July 2002

To grow high-quality heteroepitaxial layers, we propose a compliant silicon substrate consisting of a thin epitaxial silicon film on a high-density porous layer as a membrane and an expansive low-density porous layer as a mechanical damper which shields the overlying layers from the massive wafer. GeSi films over the critical thickness have been grown by molecular-beam epitaxy on these substrates. Transmission electron microscopy analysis shows that Ge0.2Si0.8 films have no dislocations owing to just elastic strain relaxation whereas plastic flow in the pseudomorphic films that are being grown on conventional Si substrates occurs with generation of dislocations in a regular manner. The experimental data on porous silicon structure are presented in some detail and are briefly discussed in connection with substrate compliance.
Photorefractive index gratings in SnO2:SiO2 optical fibers
Authors: Brambilla G. , Pruneri V. , Reekie L.
Published: July 2002

Permanent photorefractive gratings with considerable refractive index modulations (∼3×10−4) were written in silica optical fibers doped with very low levels of SnO2(∼0.15 mol %) using ultraviolet (UV) laser radiation at 248 nm. The photoinduced refractive index change does not show any sign of saturation even for long exposure times (i.e., total fluence >20 kJ/cm2). A comparison with GeO2:SiO2 fibers shows that under similar UV intensities saturation takes place for shorter exposure time and comparable refractive index changes are obtainable for GeO2 concentrations ∼10 mol %.
Near-surface InAs/GaAs quantum dots with sharp electronic shells
Authors: Fafard S.
Published: July 2002

The interaction between zero-dimensional states and surface states is studied using near-surface quantum dot (QD) ensembles with well-defined electronic shells. The inhomogeneous broadening of self-assembled InAs/GaAs QDs increases from ∼30 to more than ∼46 meV as the distance of the QDs from the surface is changed from 100 to 5.0 nm. Simultaneously, a decrease of the radiative recombination intensity by ∼3 orders of magnitude, and a red-shift of ∼65 meV are observed. For QDs capped with less than ∼10 nm, remarkable charge transfers between the QD and surface states lead to optical memory effects lasting over time-scales of several minutes.
Improved flux pinning and anomalous magnetization peak in heavily overdoped Bi2−xPbxSr2CaCu2−yCryO8+δ single crystals
Authors: Sun Y. P. , Song W. H. , Zhao B. , Du J. J. , Wen H. H. , Zhao Z. X. , Ku H. C.
Published: July 2002

Flux pinning and anomalous magnetization peak of heavily overdoped Bi2−xPbxSr2CaCu2−yCryO8+δ [Cr-doping (Bi, Pb)-2212] single crystals are investigated by means of magnetization measurements with the magnetic field parallel to the c axis. Compared with undoped Bi2Sr2CaCu2O8+δ (Bi-2212) and Pb-doping Bi2−xPbxSr2CaCu2O8+δ [(Bi, Pb)-2212] single crystals, the irreversibility line of Cr-doping (Bi, Pb)-2212 single crystals is pronouncedly shifted to high temperatures. The anomalous magnetization peak H2p with maximum peak field near 8000 G is observed. The H2p exhibits strong temperature dependence from 0.19 to 0.93Tc. The temperature dependence of H2p can be explained according to vortex decoupling theory. The improved flux pinning may originate from reduced anisotropy due to Pb doping and a sufficient number of effective pinning centers due to the random Cr substitution on the Cu site.
Time- and frequency-resolved spontaneous emission: Theory and application to the NO2 X̃ 2A′/Ã 2A′ conical intersection
Authors: Santoro Fabrizio , Petrongolo Carlo , Lami Alessandro
Published: July 2002

We present a theoretical scheme for the calculation of time- and frequency-resolved spontaneous emission spectra of nonstationary states prepared by a laser pulse, considering explicitly the effect of the frequency filter and the time gate of the measurement instrument. Our scheme treats in a perturbative manner the matter-radiation interaction taking into account the states radiative lifetimes, and utilize the eigenstates of the molecular Hamiltonian up to the maximum excitation energy. We study the fluorescence of a nonstationary state of NO2 created by a Gaussian pulse mainly on the Ã 2A′ excited adiabatic potential, following an absorption from the ground adiabatic electronic state X̃ 2A′. We analyze the X̃ 2A′/Ã 2A′ conical intersection effects on the spectra and dynamics in a 2A1(ground)/2B2 (excited) diabatic electronic representation. We have pointed out that the wave packet emits more strongly at times corresponding to partial recurrences, i.e., when it returns to the region of space where it was initially, and that the whole spectrum is red-shifted. The nonadiabatic interactions between the electronic states bring the wave packet from the bright 2B2 state to the quasi-dark 2A1 one, and thus they quench the oscillations of the total emitted energy. Moreover, they cause the broadening of the part of the wave packet that remains on the upper diabatic surface, and this results in a further quenching of the emission. On the contrary, the nonadiabatic interactions have a negligible effects on the times at which the emission peaks occur. The striking effect of the duration of the interval in which the time-gate is opened on the time- and frequency-resolved emission is investigated and discussed.
Optical gate action of a molecular thin film probed with femtosecond near-field optical microscopy
Authors: Kawashima H. , Furuki M. , Tatsuura S. , Tian M. , Sato Y. , Pu L. S. , Tani T.
Published: July 2002

We have combined a near-field scanning optical microscope with a two-color time-resolved pump–probe measurement system. The sample was a molecular thin film that revealed an excitonic resonance and also had a characteristic domain structure. The measurement system has a noise-equivalent transmittance change as small as 5.0×10−5 for a probe pulse with an intensity of 30 nW, which allows us to detect an optical gate action of a single domain. The results suggest that the film composition is uniform over a distance of several microns while it may vary on a greater scale.
Second-harmonic scanning optical microscopy of poled silica waveguides
Authors: Pedersen Kjeld , Bozhevolnyi Sergey I. , Arentoft Jesper , Kristensen Martin , Laurent-Lund Christian
Published: July 2002

Second-harmonic scanning optical microscopy (SHSOM) is performed on electric-field poled silica-based waveguides. Two operation modes of SHSOM are considered. Oblique transmission reflection and normal reflection modes are used to image the spatial distribution of nonlinear susceptibilities in the sample surface plane and in depth. It is shown that the spatial resolution in normal reflection mode can be better than 1 μm for second-harmonic images. A simple qualitative description of this operation mode is suggested and found to be in good agreement with the results obtained. Advantages and limitations of the two operation modes when used for SHSOM studies of poled silica-based waveguides are discussed. The influence of surface defects on the resulting second-harmonic images is also considered.
Absolute W value measurements for 5.9 keV x rays in Ne–Xe mixtures at atmospheric pressures
Authors: Vinagre F. L. R. , Conde C. A. N.
Published: July 2002

Experimental absolute W values are presented in Ne–Xe mixtures for 5.9 keV x rays with a standard uncertainty lower than 1%. The measured W values range from 23.9 to 22.1 eV for mixtures in the 10% Xe–90% Ne to 80% Xe–20% Ne concentration range, respectively. The measurements were performed using a recent technique based on the measurement of the primary ionization charge produced by a certain number of x-ray photons absorbed in the gas. We experimentally investigated the dependence of the W value in Ne–Xe mixtures with the Ne or Xe concentration, showing the influence of Penning effects. The discrepancies between our W values and data obtained with other methods for W-value determination are discussed. Along with the experimental measurements, an analytical model for the W value in binary mixtures was implemented, taking into consideration the most important energy transfer mechanisms which influence the ionization yield formation, namely the Penning interactions. The experimental and the analytical model results are compared and discussed.
Luminescence properties of GaN and Al0.14Ga0.86N/GaN superlattice doped with europium
Authors: Lozykowski H. J. , Jadwisienczak W. M. , Han J. , Brown I. G.
Published: July 2002

We report the observation of visible photoluminescence and cathodoluminescence of Eu3+ ions implanted in GaN and Al0.14Ga0.86N/GaN superlattice. The sharp characteristic emission lines corresponding to Eu3+ intra-4f6-shell transitions are resolved and observed over the temperature range of 7–330 K. The luminescence shows dominant transitions D50→7F1,2,3 and weaker D50→7F4,5,6 and D51→7F1. The luminescence emission is very weakly temperature dependent. The intensity of Eu3+ emission from Al0.14Ga0.86N/GaN superlattice annealed in N2 is ∼58% stronger than from Eu3+ in the GaN layer. The Al0.14Ga0.86N/GaN superlattice and GaN epilayers may be suitable as a material for visible optoelectronic devices.
A falling head permeameter for the measurement of the hydraulic conductivity of granular solids
Authors: Wilson M. A. , Hoff W. D. , Brown R. J. E. , Carter M. A.
Published: July 2002

A falling head permeameter is described in which pairs of infrared emitters and detectors on a sight tube are used to measure the flow rate associated with the passage of water through a granular solid under the action of a diminishing pressure head. An equation relating pressure head to elapsed time is derived from which permeability may be calculated. In order to verify the accuracy and sensitivity of the instrument, permeability measurements carried out on a graded quartz sand are compared to those obtained by the more conventional constant head measurement. Excellent agreement is obtained between the permeability values obtained using both measurement methods. Experimental results are also reported for the measurement of the permeability of a range of sieved sand fractions. The falling head permeameter described here is particularly suitable for the measurement of the hydraulic conductivity of granular solids such as sands and soils through which a high flow rate may be expected.
Thin-film-induced index change and channel waveguiding in epitaxial GaN films
Authors: Kim Euisong , Lee Byounghee , Nahhas Ahmed , Kim Hong Koo
Published: July 2002

We report on optical waveguiding in the channel region of an epitaxial GaN film defined by a SiN cladding layer of a stripe window pattern. We carried out numerical analyses on the various possible effects that might contribute to the overcompensation of the negative loading effect of a SiN cladding window. This includes the photoelastic, piezoelectric, and electro-optic effects in GaN induced by a SiN window layer. The analysis result suggests that the observed phenomenon can be ascribed to a combination of both the photoelastic and electro-optic effects, and especially that the spontaneous polarization field in undoped GaN with a low background carrier concentration might play an important role in forming a channel waveguide in the window region.
Formation and annihilation of H-point defect complexes in quenched Si doped with C
Authors: Fukata Naoki , Suezawa Masashi
Published: July 2002

We investigated the formation and annihilation of H-point defect complexes formed in C-doped Si by heating at high temperatures followed by quenching in hydrogen gas. Specimens of C-doped Si were sealed in quartz capsules together with hydrogen (H) gas, at pressure 0.8–1.5 atm at high temperature, and were heated at high temperature for 1 h followed by quenching in water. We measured their optical absorption spectra at about 7 K with an Fourier transform infrared spectrometer. We observed several optical absorption peaks due to H-point defect complexes. The optical absorption peaks observed at 2192 and 2203 cm−1 were assigned to the Si–H stretching mode of three hydrogen atoms bound to a vacancy (VH3 defect). The formation of the VH4 defect is due to the reaction between H and the VH3 defect. From isothermal annealing experiments, the activation energy for the dissociation of the VH4 defect was determined to be about 2.5 eV.
Field emission from cobalt-containing amorphous carbon composite films heat-treated in an acetylene ambient
Authors: Li Y. J. , Lau S. P. , Tay B. K. , Sun Z. , Chen G. Y. , Chen J. S. , Ding X. Z. , Shi X.
Published: July 2002

Cobalt-containing amorphous carbon composite films have been prepared by the filtered cathodic arc technique using a cobalt-containing graphite target at room temperature. After heat treatment at 550 °C in a mixture of acetylene and nitrogen gases, the field emission properties were significantly improved. A threshold electric field of 1.7 V/μm and an emission site density of 105/cm2 were obtained without conditioning. The composite films, which can be deposited with a high rate at room temperature and require a relatively low temperature heat-treatment process to enhance electron emission, are promising for practical applications in field emission display.
The microstructure of submicrometer wide planar-reactive ion etched versus trench-damascene AlCu lines
Authors: Rodbell K. P. , Gignac L. M. , Hurd J. L. , Filippi R. , Wang Yun-Yu , Clevenger L. A. , Iggulden R. C. , Schnabel R. F. , Weber S.
Published: July 2002

The microstructure was measured for AlCu lines, formed using either a traditional planar metal subtractive etch process or a newly developed hot AlCu-trench-damascene process. It was found that 0.35 μm wide damascene AlCu lines formed a large grained bamboo microstructure with little or no Al (111) texture. The local crystallographic texture was measured in a scanning electron microscope using electron backscatter pattern analysis often referred to as backscatter Kikuchi diffraction. Damascene structures consisted of AlCu films deposited at greater than 400 °C onto Ti or Ti/TiN into preformed amorphous SiO2 trenches, 0.3–5.0 μm wide by 0.4 μm deep, followed by aluminum chemical mechanical polishing to remove the metal overburden. Standard planar metal control samples consisted of blanket Al or AlCu films deposited onto either an amorphous SiO2 substrate or onto SiO2/Ti/TiN substrates, followed by subtractive etching to define 0.45–10 μm wide lines as well as large (e.g., 10×10 μm2) pads. The planar metal samples exhibited either little change or a slight strengthening of their (111) fiber texture with decreasing line width; this was in sharp contrast to the damascene films in which a marked weakening in the (111) fiber texture with decreasing line width was found. In addition a trimodal (111) texture distribution developed in trenches where TiAl3 intermetallic formed. The role of intermetallic formation (TiAl3), elevated (>400 °C) AlCu deposition temperature, large bamboo grain size, local AlCu crystallographic texture and differences in sidewall coverage between subtractive etched and trench-damascene processed AlCu on film microstructure are examined.
Lattice effects of doping the Mn sites in La0.5Ca0.5MnO3
Authors: Li Run-Wei , Sun Ji-Rong , Wang Zhi-Hong , Shen Bao-Gen
Published: July 2002

The structure, magnetic, and transport properties of La0.5Ca0.5Mn1−xMxO3 (0⩽x⩽0.1) with M=Ge and Sn have been investigated systematically. A significant lattice expansion is observed in the Sn-doped samples, but not in the Ge-doped ones. At the same time, a substitution of Mn by Ge has little effect on Curie temperature (TC), but increases the metal-insulator transition temperature (TP). In contrast, the TC decreases sharply for Sn doping, while the TP shows an increase for x<0.02 and a decrease for x>0.02. Compared with Ge doping, Sn doping is more effective in destroying the charge-ordered state. The difference in local lattice distortions due to the Ge and Sn doping is believed to be responsible for the present observations.
Phase-integral formulas for quantal matrix elements
Authors: Fröman Per Olof
Published: July 2002

Simple phase-integral formulas, not involving wave functions, are derived for quantal matrix elements associated with bound states of a quantal particle in a smooth single-well potential. In these formulas one uses an arbitrary order of the phase-integral approximation generated from an unspecified base function.
Effect of Si doping of metalorganic chemical vapor deposition-GaN templates on the defect arrangement in hydride vapor phase epitaxy-GaN overgrown layers
Authors: Paskova T. , Valcheva E. , Birch J. , Tungasmita S. , Persson P. O. Å. , Beccard R. , Heuken M. , Monemar B.
Published: July 2002

Two different types of dislocation arrangements have been observed in hydride vapor-phase epitaxial GaN films grown on sapphire substrates using both undoped and Si-doped GaN templates grown by metalorganic chemical vapor deposition: (i) predominantly straight threading dislocations parallel to the [0001] direction in the layer grown on an undoped template, and (ii) a network of interacting dislocations of edge, screw, and mixed character in the layer grown on a Si-doped template. The two types of defect distribution result in essentially different surface morphologies, respectively: (i) low-angle grain boundaries formed by pure edge dislocations around spiral grown hillocks, and (ii) smooth surface intersected by randomly distributed dislocations. The Si doping of the GaN templates was found to enhance defect interaction in the templates and to enable a reduction of the dislocation density in the overgrown thick GaN films, although it does not lead to an improvement of the overall structural properties of the material.
Deep level transient capacitance measurements of GaSb self-assembled quantum dots
Authors: Magno R. , Bennett Brian R. , Glaser E. R.
Published: July 2002

Deep level transient spectroscopy (DLTS) measurements have been made on GaAs n+p diodes containing GaSb self-assembled quantum dots and control junctions without dots. The self-assembled dots were formed by molecular beam epitaxy using the Stranski–Krastanov growth mode. The dots are located in the depletion region on the p side of the junction where they act as a potential well that may capture and emit holes. Spectra recorded for temperatures between 77 and 440 K reveal several peaks in diodes containing dots. A control sample with a GaSb wetting layer was found to contain a single broad high temperature peak that is similar to a line found in the GaSb quantum dot samples. No lines were found in the spectra of a control sample prepared without GaSb. DLTS profiling procedures indicate that one of the peaks is due to a quantum-confined energy level associated with the GaSb dots while the others are due to defects in the GaAs around the dots. The peak identified as a quantum-confined energy level shifts to higher temperatures and its intensity decreases on increasing the reverse bias. The activation energy for the quantum-confined level increases from 400 meV when measured at a low reverse bias to 550 meV for a large reverse bias. Lines with activation energies of 400, 640, and 840 meV are associated with defects in the GaAs based on the bias dependence of their peak positions and amplitudes.
Modeling and simulation of bipolar junction transistors using the theories of thermodynamics
Authors: Maa Yann Jiun , Abdel-Motaleb Ibrahim M.
Published: July 2002

The bipolar junction transistor is modeled using the thermodynamic theories. The main reason for using the thermodynamic theories is to ensure that the law of conservation of energy is maintained. Violating such a fundamental law is expected to result in errors in simulating the device performance. Although using some empirical parameters can reduce the severity of such deviations, such a process requires some guessing that cannot be guaranteed to be accurate in all cases. Using the thermodynamic theories, the transistor I–V relationships were derived and the device characteristics were simulated. The results show that the proposed model can predict the device performance without using empirical fitting parameters.
The onset of superconductivity in semi-infinite strips
Authors: Almog Y.
Published: July 2002

The existence of a superconducting branch bifurcating from the normal state is proved in semi-infinite strips. It is proved that the critical magnetic field at which bifurcation takes place, or the onset field, for a semi-infinite strip is greater than the onset field for an infinite strip with the same width. In addition we find the loci of the vortices far away from the corners and show convergence of the bifurcating modes in long rectangles to those in the semi-infinite strip with the same width.
Epitaxial growth of SrBi2Nb2O9 on (110) SrTiO3 and the establishment of a lower bound on the spontaneous polarization of SrBi2Nb2O9
Authors: Lettieri J. , Zurbuchen M. A. , Jia Y. , Schlom D. G. , Streiffer S. K. , Hawley M. E.
Published: July 2002

Epitaxial SrBi2Nb2O9 thin films have been grown on (110) SrTiO3 substrates by pulsed laser deposition. Four-circle x-ray diffraction and transmission electron microscopy reveal nearly phase pure epitaxial films with the c axis of the films at 45° with respect to the substrate normal. Electrical characterization is presented for films grown on epitaxial SrRuO3 electrodes. The low-field relative permittivity was 235, the remanent polarization was 11.4 μC/cm2, and the dielectric loss was 3.0% for 0.3-μm-thick films. From the remanent polarization and an understanding of the epitaxial geometry, a lower bound of 22.8 μC/cm2 was determined for the spontaneous polarization of SrBi2Nb2O9.
Methane isotopomers adsorbed on KBr(100): An exploration of adsorbate orientation and rotation
Authors: Davis Kent A. , Ewing George E.
Published: July 2002

The adsorption of CH4, CH2D2, and CHD3 on the single crystal (100) surfaces of KBr in the temperature range 5–42 K was studied using polarized infrared spectroscopy. The spectra of CH4 at 42 K show two features in the ν3 C–H stretching region: the higher frequency feature with a transition dipole tilted 18° away from the surface normal and the lower frequency component at 85° from the surface normal. These data are consistent with a slightly tilted tripod for CH4 on KBr(100). With decreasing temperature a splitting occurs in the lower frequency component. A similar splitting is seen in the ν4 C–H stretch mode of CHD3 where the three features are assigned to a tilted H-up tripod and two types of tilted H-down tripods. The CH2D2 spectra are also consistent with the tripod assignment. Selection rules indicate that observation of free-rotation is allowed as vibrator transitions but none were seen. The temperature dependent splitting observed in the CH4 spectrum is consistent with hindered rotation that is quenched at low temperatures to allow detection of molecules with transition dipoles in slightly different environments.
Adsorbate aggregation and relaxation of low-frequency vibrations
Authors: Pykhtin M. V. , Rappe Andrew M. , Lewis Steven P.
Published: July 2002

We present a study of resonant vibrational coupling between adsorbates and an elastic substrate at low macroscopic coverages. In the first part of the paper we consider the situation in which adsorbates form aggregates with high local coverage. Based upon our previously published theory, we derive formulas describing the damping rate of adsorbate vibrations for two cases of such aggregation: (i) adsorbates attached to step edges and (ii) adsorbates forming two-dimensional islands. We have shown that damping is governed by local coverage. Particularly, for a wide range of resonant frequencies, the damping rate of adsorbates forming well-separated islands is described by the damping rate formula for a periodic overlayer with the coverage equal to the local coverage in the island. The second part of the paper is devoted to facilitating the evaluation of damping rates for a disordered overlayer. The formula describing the damping rate involves the parameter β, which is related to the local density of phonon states at the substrate surface and does not allow a closed-form representation. For substrates of isotropic and cubic symmetries, we have developed a good analytical approximation to this parameter. For a vast majority of cubic substrates the difference between the analytical approximation and numerical calculation does not exceed 4%.
Central projective quaternionic representations
Authors: Scolarici G. , Solombrino L.
Published: July 2002

We obtain all the irreducible not similar central quaternionic projective representations of a finite group G from the knowledge of the vector, complex irreducible representations of suitable extensions of G, and classify them according to a generalized Frobenius–Schur classification. Finally, a definition of equivalence of such representations is proposed.
A new superconformal mechanics
Authors: Deotto E. , Furlan G. , Gozzi E.
Published: July 2002

In this paper we propose a new supersymmetric extension of conformal mechanics. The Grassmannian variables that we introduce are the basis of the forms and of the vector fields built over the symplectic space of the original system. Our supersymmetric Hamiltonian itself turns out to have a clear geometrical meaning being the Lie derivative of the Hamiltonian flow of conformal mechanics. Using superfields we derive a constraint which gives the exact solution of the supersymmetric system in a way analogous to the constraint in configuration space which solved the original nonsupersymmetric model. Besides the supersymmetric extension of the original Hamiltonian, we also provide the extension of the other conformal generators present in the original system. These extensions also have a supersymmetric character being the square of some Grassmannian charge. We build the whole superalgebra of these charges and analyze their closure. The representation of the even part of this superalgebra on the odd part turns out to be integer and not spinorial in character.
Commuting self-adjoint extensions of symmetric operators defined from the partial derivatives
Authors: Jorgensen Palle E. T. , Pedersen Steen
Published: July 2002

We consider the problem of finding commuting self-adjoint extensions of the partial derivatives {(1/i)(∂/∂xj):j=1,…,d} with domain Cc∞(Ω) where the self-adjointness is defined relative to L2(Ω), and Ω is a given open subset of Rd. The measure on Ω is Lebesgue measure on Rd restricted to Ω. The problem originates with Segal and Fuglede, and is difficult in general. In this paper, we provide a representation-theoretic answer in the special case when Ω=I×Ω2 and I is an open interval. We then apply the results to the case when Ω is a d cube, Id, and we describe possible subsets Λ⊂Rd such that {eλ|Id:λ∈Λ} is an orthonormal basis in L2(Id).
Three-terminal bistable twisted nematic liquid crystal displays
Authors: Guo Jian-Xin , Meng Zhi-Guo , Wong Man , Kwok Hoi-Sing
Published: July 2002

A three-terminal bistable twisted nematic liquid crystal display has been demonstrated. This display makes use of a combination of strong in-plane electric fields and vertical electric field for switching between the φ and the φ+π twist states. The lifetimes of the two bistable twist states are infinite, which is a significant improvement over conventional bistable twisted nematic displays.
Photoionization of [(η-C6H6)2Cr] with the explicit continuum B-spline density-functional method
Authors: Stener M. , Fronzoni G. , Furlan S. , Decleva P.
Published: July 2002

The cross section and asymmetry parameter profiles of (C6H6)2Cr have been calculated with a method based on the density-functional theory with an explicit treatment of the continuum wave function, with a single center basis set of B-spline functions and with an exchange-correlation potential with the correct Coulomb asymptotic behavior. The method has proven efficient from the outer valence states up to the metal and carbon core. The results are in good agreement with the available experimental data, and suggest that important features, not yet measured, should be present in the high-energy range of the valence and core profiles.