bluuebunny/crossref_metadata_2025 huggingface dataset:
Electrical conduction properties of sputter-grown (Ba, Sr)TiO3 thin films having IrO2 electrodes
Authors: Shin Ju Cheol , Hwang Cheol Seong , Kim Hyeong Joon
Published: July 2002

The electrical conduction behavior of sputter-grown (Ba, Sr)TiO3(BST) thin films having IrO2 electrodes were studied under the assumption of a fully accumulated film. In the film-thickness range studied (40–80 nm), the dielectric constants and their electric field dependencies were found to be independent of the film thickness. Contrary to similar BST films grown on Pt electrodes, the leakage current density decreased with increasing film thickness at a given field. The phenomena were explained from the electric-field depression effect inside the film due to the accumulated negative space charges in the films. The leakage current conductions were controlled by the Poole–Frenkel mechanism in the low field whereas it changed to thermionic field emission in the high-field region. The dielectric constant obtained from the Poole–Frenkel fitting was approximately 300, which was in a qualitative agreement with the value obtained from the low-frequency capacitance measurement. The calculated interfacial potential-barrier height at zero volt and effective mass of electrons were 1.03 eV and 0.06m0, respectively, from the thermionic field emission fittings.
Evidence of localization effects in InGaN single-quantum-well ultraviolet light-emitting diodes
Authors: Chichibu S. F. , Wada K. , Müllhäuser J. , Brandt O. , Ploog K. H. , Mizutani T. , Setoguchi A. , Nakai R. , Sugiyama M. , Nakanishi H. , Korii K. , Deguchi T. , Sota T. , Nakamura S.
Published: July 2002

The importance of doping or alloying with In for obtaining high external quantum efficiency was shown for GaN-based single-quantum-well (SQW) structures in terms of localization effects due to quantum-disk (Q-disk [M. Sugawara, Phys. Rev. B 51, 10743 (1995)])-size potential minima in the QW plane. The ultraviolet light-emitting diode with lightly In-alloyed InGaN SQW exhibited an electroluminescence peak from the band-tail states. Monochromatic cathodoluminescence mapping images of In0.03Ga0.97N SQW indicated the presence of Q-disk-size effective bandgap variation. Furthermore, cubic InGaN QW which does not suffer from the piezoelectric field normal to the QW plane, also exhibited a broad band-tail.
Field effect enhanced signal-to-noise ratio in chemically deposited PbS thin films on Si3N4/n-Si substrates
Authors: Pintilie L. , Pentia E. , Pintilie I. , Botila T.
Published: July 2002

p-type lead sulphide (PbS) thin films were chemically deposited onto Si3N4/n-Si substrates. A three-contact pseudo-metal–oxide–semiconductor structure was designed in order to investigate the possibility of enhancing the signal-to-noise ratio in the PbS film by field effect. It was proved that on the entire sensitivity domain of PbS (1–3 μm at room temperature), the magnitude of the photoconductive signal generated by the PbS film depends on the polarity and value of the gate voltage. It is shown that the signal-to-noise ratio can be enhanced in this way with by at least 25% compared with the case when the gate electrode is in air (standard configuration for photoconductive measurements).
Efficiency peaks in the transient electroluminescence of multilayer organic light-emitting devices
Authors: Savvate’ev V. , Friedl J. , Zou L. , Oldham W. J. , Shinar J.
Published: July 2002

It is shown that when multilayer organic light-emitting devices (OLEDs) containing hole (h+) and electron (e−) transporting layers (HTLs and ETLs, respectively) are biased with microsecond to millisecond voltage pulses higher than a threshold value Vth, the electroluminescence (EL) intensity increases dramatically to a peak value which then relaxes to the lower dc value; the relaxation time decreases strongly with increasing pulse amplitude. Since the current waveforms are essentially rectangular, the transient EL is proportional to the external quantum efficiency η. The value of Vth coincides with the bias for maximum dc efficiency typically observed when η is monitored vs V. This relation and the apparent absence of the transient peak in single-layer OLEDs suggest that it is due either to internal field redistribution processes in the ETL and HTL or to space charges, e.g., trapped polarons which accumulate at the HTL/ETL interface, and quench the emitting singlet excitons. It is concluded that highly efficient OLED operation may be achieved at high brightness by pulsed bias at an optimized duty cycle.
InGaP/InGaAsN/GaAs NpN double-heterojunction bipolar transistor
Authors: Chang P. C. , Baca A. G. , Li N. Y. , Xie X. M. , Hou H. Q. , Armour E.
Published: July 2002

We have demonstrated a functional NpN double-heterojunction bipolar transistor (DHBT) using InGaAsN for the base layer. The InGaP/In0.03Ga0.97As0.99N0.01/GaAs DHBT has a low VON of 0.81 V, which is 0.13 V lower than in a InGaP/GaAs heterojunction bipolar transistor (HBT). The lower turn-on voltage is attributed to the smaller band gap (1.20 eV) of metalorganic chemical vapor deposition-grown In0.03Ga0.97As0.99N0.01 base layer. GaAs is used for the collector; thus the breakdown voltage (BVCEO) is 10 V, consistent with the BVCEO of InGaP/GaAs HBTs of comparable collector thickness and doping level. To alleviate the current blocking phenomenon caused by the larger conduction band discontinuity between InGaAsN and GaAs, a graded InGaAs layer with δ doping is inserted at the base–collector junction. The improved device has a peak current gain of seven with ideal current–voltage characteristics.
Diffuse x-ray rods and scattering from point defect clusters in ion implanted silicon
Authors: Beck U. , Metzger T. H. , Peisl J. , Patel J. R.
Published: July 2002

We have studied the early stages of annealing in boron implanted silicon. In a grazing incidence diffuse scattering investigation of implantation-induced defects, we have observed narrow diffuse rods of intensity along 〈111〉 directions. These diffuse streaks arise from stacking faults formed during annealing in the 1000 °C range. From the width of the diffuse streak the average size of the stacking fault is 71 nm in diameter. These intensity rods are distinct from the point defect or point defect cluster scattering in the tails of the Bragg peak (Huang scattering). From the q dependence of the scattered intensity in the Huang scattering region we find clear evidence for defect clusters with an average effective size of 4 nm, remarkably independent of the annealing temperature. These observations are discussed in the context of the enhanced diffusion of implanted boron over its bulk value referred to as transient enhanced diffusion.
Thermally induced local gain suppression in vertical-cavity surface-emitting lasers
Authors: Degen C. , Fischer I. , Elsäßer W.
Published: July 2002

Joule heating is one of the dominant mechanisms determining the transverse mode formation in vertical-cavity surface-emitting lasers at high injection currents. We give experimental evidence that in this operation regime, strong heating results in local gain suppression in the center of the laser, which overbalances the confining effect of thermal lensing, and thus favors the formation of high order modes. From our investigations of small aperture devices, we conclude that efficient heat removal is crucial for achieving single mode emission at high injection currents.
Do Pb1 centers have levels in the Si band gap? Spin-dependent recombination study of the Pb1 “hyperfine spectrum”
Authors: Mishima Tetsuya D. , Lenahan Patrick M. , Weber Werner
Published: July 2002

The electronic properties of the (001) Si/SiO2 Pb1 defect are the subject of considerable controversy. We present spin-dependent recombination results which indicate most strongly that the Pb1 centers have levels in the Si band gap. Our results suggest that the Pb1 correlation energy is smaller than that of the more widely studied (001) Si/SiO2 defect called Pb0.
Metal–semiconductor–metal GaN ultraviolet photodetectors on Si(111)
Authors: Zhao Z. M. , Jiang R. L. , Chen P. , Xi D. J. , Luo Z. Y. , Zhang R. , Shen B. , Chen Z. Z. , Zheng Y. D.
Published: July 2002

GaN metal–semiconductor–metal photoconductive detectors have been fabricated on Si(111) substrates. The GaN epitaxial layers were grown on Si substrates by means of metalorganic chemical-vapor deposition. These detectors exhibited a sharp cutoff at the wavelength of 363 nm and a high responsivity at a wavelength from 360 to 250 nm. A maximum responsivity of 6.9 A/W was achieved at 357 nm with a 5 V bias. The relationship between the responsivity and the bias voltage was measured. The responsivity saturated when the bias voltage reached 5 V. The response time of 4.8 ms was determined by the measurements of photocurrent versus modulation frequency.
Engineering coercivity in epitaxially grown (110) films of DyFe2–YFe2 superlattices
Authors: Sawicki M. , Bowden G. J. , de Groot P. A. J. , Rainford B. D. , Beaujour J. M. L. , Ward R. C. C. , Wells M. R.
Published: July 2002

Molecular beam epitaxial methods have been used to grow single crystal Laves phase DyFe2–YFe2 superlattice samples with a (110) growth direction. It is shown that it is possible, in principle, to engineer a desired coercivity between the limits KDyFe2⩽K⩽∞. This can be achieved by adjusting the relative thickness of the individual DyFe2 and YFe2 layers, in multilayer films This novel feature is illustrated, using the superlattice films [x Å DyFe2/(100-x) Å YFe2]×40, with x=80, 60, 50, and 45. It is found that the measured coercivity is in semiquantitative agreement with a simple theoretical expression, for the nucleation fields in both bilayer and multilayer compounds. However, in practice, exchange spring penetration into the DyFe2 layers can set a limit to the maximum coercivity that can be achieved.
Effect of the dissolved hydrogen on the densification of synthetic silica glass under ArF excimer laser irradiation
Authors: Shimbo Masaru , Nakajima Toshio , Tsuji Naoki , Zhongha Zhou , Yamaguch Hirosi , Kakuno Tsutomu , Obara Takashi
Published: July 2002

The degree of densification-induced birefringence under ArF excimer laser irradiation decreases with increasing dissolved hydrogen concentration. The hydrogen concentration at the irradiation site was decreased after irradiation. Good correlation between the residual hydrogen and the degree of birefringence suggests that the birefringence change is due to the change in volume caused by the decrease in hydrogen. As with spontaneous decay, the rate at which hydrogen was released was proportional to the remaining hydrogen.
Electronic energy levels of Er3+ in ZnInGaS4:Er3+ single crystal and its site symmetry
Authors: Choe Sung-Hyu , Park Hong-Lee , Kim Wha-Tek
Published: July 2002

ZnInGaS 4 and ZnInGaS4:Er3+ single crystals were grown using the CTR technique. The single crystals grown have a layered crystal structure. These compounds have both a direct and an indirect energy gap. Eight distinctive emission peaks of the ZnInGaS4:Er3+ single crystal were observed at 10 K. These photoluminescence peaks were attributed to the radioactive decay among the 4f split electron energy levels of the Er3+ ions that occupy the C2v site symmetry of the ZnInGaS4 single crystal host lattice.
All-optical technique for measuring thermal properties of materials at static high pressure
Authors: Pangilinan G. I. , Ladouceur H. D. , Russell T. P.
Published: July 2002

The development and implementation of an all-optical technique for measuring thermal transport properties of materials at high pressure in a gem anvil cell are reported. Thermal transport properties are determined by propagating a thermal wave in a material subjected to high pressures, and measuring the temperature as a function of time using an optical sensor embedded downstream in the material. Optical beams are used to deposit energy and to measure the sensor temperature and replace the resistive heat source and the thermocouples of previous methods. This overcomes the problems introduced with pressure-induced resistance changes and the spatial limitations inherent in previous high-pressure experimentation. Consistent with the heat conduction equation, the material’s specific heat, thermal conductivity, and thermal diffusivity (κ) determine the sensor’s temperature rise and its temporal profile. The all-optical technique described focuses on room-temperature thermal properties but can easily be applied to a wide temperature range (77–600 K). Measurements of thermal transport properties at pressure up to 2.0 GPa are reported, although extension to much higher pressures are feasible. The thermal properties of NaCl, a commonly used material for high-pressure experiments are measured and shown to be consistent with those obtained using the traditional methods.
Density functional study of intramolecular ferromagnetic interaction through m-phenylene coupling unit (I): UBLYP, UB3LYP, and UHF calculations
Authors: Mitani Masaki , Mori Hiroki , Takano Yu , Yamaki Daisuke , Yoshioka Yasunori , Yamaguchi Kizashi
Published: July 2002

Polyradicals comprised of m-phenylene-bridged organic radicals are well known as building blocks of organic ferromagnets, in which radical groups are connected with each other at the meta position in the benzene ring, and the parallel-spin configurations between radical sites are more stabilized than the antiparallel ones. Topological rules for spin alignments enable us to design organic high-spin dendrimers and polymers with the ferromagnetic ground states by linking various radical species through an m-phenylene unit. However, no systematic ab initio treatment of such spin dendrimers and magnetic polymers has been reported until now, though experimental studies on these materials have been performed extensively in the past ten years. As a first step to examine the possibilities of ferromagnetic dendrimers and polymers constructed of m-phenylene units with organic radicals, we report density functional and molecular orbital calculations of six m-phenylene biradical units with radical substituents and polycarbenes linked with an m-phenylene-type network. The relative stability between the spin states and spin density population are estimated by BLYP or B3LYP and Hartree–Fock calculations in order to clarify their utility for constructions of large spin denderimers and periodic magnetic polymers, which are final targets in this series of papers. It is shown that neutral polyradicals with an m-phenylene bridge are predicted as high-spin ground-state molecules by the computations, while m-phenylene-bridged ion-radical species formed by doping may have the low-spin ground states if zwitterionic configurations play significant roles to stabilize low-spin states. Ab initio computations also show an important role of conformations of polyradicals for stabilization of their high-spin states. The computational results are applied to molecular design of high-spin dendrimers and polymers. Implications of them are also discussed in relation to recent experimental results for high-spin organic molecules.
On the absolutely continuous spectrum of Stark Hamiltonians
Authors: Sahbani Jaouad
Published: July 2002

We study the spectral properties of the Schrödinger operator with a constant electric field perturbed by a bounded potential. It is shown that if the derivative of the potential in the direction of the electric field is smaller at infinity than the electric field, then the spectrum of the corresponding Stark operator is purely absolutely continuous. In one dimension, the absolute continuity of the spectrum is implied by just the boundedness of the derivative of the potential. The sharpness of our criterion for higher dimensions is illustrated by constructing smooth potentials with bounded partial derivatives for which the corresponding Stark operators have a dense point spectrum.
Role of hydrogen on field emission from chemical vapor deposited diamond and nanocrystalline diamond powder
Authors: Cui J. B. , Stammler M. , Ristein J. , Ley L.
Published: July 2002

The field emission properties of diamond-graphite composites were investigated as a function of composition both for oxidized and hydrogen covered diamond. The composites consist of mixtures of nanocrystalline diamond and graphite particles. In this way their composition could be varied at will while the field enhancement factor of the individual crystallites remained unchanged. The measurements prove that graphite is the phase responsible for low threshold field emission. The apparent emission threshold is strongly influenced by the conductivity of the composites. Hydrogenation has two beneficial effects. It provides a conducting path to the emission sites via the hydrogen induced surface conductivity of diamond. It also lowers the effective emission threshold of graphite in contact with diamond that exhibits negative electron affinity after hydrogenation. The latter effect was experimentally verified by photoelectron yield spectroscopy.
Nonlinear cross talk between gratings recorded in BaTiO3 by mutually incoherent beam pairs
Authors: Limeres J. , Carrascosa M. , Petersen P. M. , Andersen P. E.
Published: July 2002

The nonlinear cross talk between two photorefractive gratings recorded in a BaTiO3 crystal has been investigated. Besides the photorefractive response described by the material equations, our theoretical analysis also includes the effect of beam coupling through the coupled-wave equations. We show that beam coupling has a strong influence on the nonlinear cross talk in BaTiO3. Depending on the initial conditions, it may lead to an increase or decrease of the nonlinear grating cross talk. Moreover, our experimental results provide experimental evidence of nonlinear cross talk in BaTiO3. Significant cross talk values, up to ∼20%, have been found experimentally. Furthermore, the experimental data are in good agreement with the theoretical predictions. The results have relevant implications in practical situations such as sequential recording of holograms for holographic storage.
Prediction of amorphous phase stability in metallic alloys
Authors: Shao G.
Published: July 2002

The stabilization of the amorphous solid with respect to the high temperature liquid phase is modeled thermodynamically as a second order phase transformation. The model proposed in this work incorporates the thermodynamic description of the high temperature liquid phase, the glass transition temperature, and the maximum entropy of amorphization in an explicit formalism. The predicted heat of formation of the amorphous solid in the Cu–Zr and Ni–Zr systems agrees well with experimentally determined data.
Hydrogen dynamics in SiO2 triggered by electronic excitations
Authors: Yokozawa Ayumi , Miyamoto Yoshiyuki
Published: July 2002

Ab initio approaches have been used to study microscopic mechanisms of the dielectric degradation of SiO2 induced by electronic excitation. In this article, we focus on the possibility of H dissociation from H-terminated O vacancies in SiO2 induced by Si–H σ→σ* excitation. To take the finite lifetime of this excitation into account, real-time electron dynamics were treated by solving the time-dependent Schrödinger equation coupled with Newton’s equations for ions. We found that the decay-time constant of the Si–H σ→σ* excitation is on the order of 10 fs, which is too short to cause direct H dissociation. Therefore, not only the electronic excitation, but also thermal assistance and/or transport of the excited electron from SiO2 to the Si substrate appears responsible for the Si–H bond breaking and subsequent SiO2 degradation.
Amorphous silicon junction field-effect transistor for digital and analog applications
Authors: Caputo D. , de Cesare G. , Kellezi V. , Palma F.
Published: July 2002

A hydrogenated amorphous silicon field-effect transistor suitable for digital and analog applications is described. The device consists of a p+-i-n− junction, with the drain and source contacts connected to the n− layer and the control gate electrode to the p+ layer. As in the corresponding crystalline junction field-effect transistor, the channel resistivity is modulated by the gate voltage. The selection of the thickness of the intrinsic layer and the doping and thickness of the n layer is critical for correct operation of the device. A device with a well defined triode and saturation region was manufactured. A transconductance of 1.5×10−6 A/V at VDS=25 V was achieved. This value is comparable to that of state-of-the-art thin-film transistors.
Photodissociation spectroscopy of Ag4+(N2)m, m=0–4
Authors: Schooss Detlef , Gilb Stefan , Kaller Julian , Kappes Manfred M. , Furche Filipp , Köhn Andreas , May Klaus , Ahlrichs Reinhart
Published: July 2002

We have determined photodissociation spectra of Ag4+(N2)m=0–4 in the photon energy range from 2.1 to 3.75 eV. The cluster complexes were generated by laser vaporization and their absolute photodissociation cross sections were measured in depletion using a modified reflectron time-of-flight mass spectrometer. The band spectra show significant nonmonotonic variations with increasing N2 coverage. The experimental data are contrasted with quantum chemical calculations of ground and electronically excited states using density functional as well as post-Hartree–Fock methods. Experiment and theory are in qualitative agreement allowing tentative assignment of the depletion spectra and insight into the relevant bonding interactions.
Second-harmonic imaging of semiconductor quantum dots
Authors: Erland John , Bozhevolnyi Sergey I. , Pedersen Kjeld , Jensen Jacob R. , Hvam Jo/rn M.
Published: July 2002

Resonant second-harmonic generation is observed at room temperature in reflection from self-assembled InAlGaAs quantum dots grown on a GaAs (001) substrate. The detected second-harmonic signal peaks at a pump wavelength of ∼885 nm corresponding to the quantum-dot photoluminescence maximum. In addition, the second-harmonic spectrum exhibits another smaller but well-pronounced peak at 765 nm not found in the linear experiments. We attribute this peak to the generation of second-harmonic radiation in the AlGaAs spacer layer enhanced by the local symmetry at the quantum-dot interface. We further observe that second-harmonic images of the quantum-dot surface structure show wavelength-dependent spatial variations. Imaging at different wavelength is used to demonstrate second-harmonic generation from the semiconductor quantum dots.
Superheating field for the Ginzburg–Landau equations in the case of a large bounded interval
Authors: Bolley Catherine , Foucher Françoise , Helffer Bernard
Published: July 2002

We study the asymptotic behavior of the local superheating field for a film of width 2d in the regime κ small, κd large, where κ is the Ginzburg–Landau parameter. This gives a mathematical justification for the introduction of the semi-infinite model as a good approximation for this regime.
Medium effects on the spectroscopy and intramolecular energy redistribution of C60 in cryogenic matrices
Authors: Chergui M.
Published: July 2002

We review some of our works on the absorption, excitation and emission spectra of C60 embedded in rare gas matrices while stressing the role of the environment. The gas phase resonant two-photon ionization spectrum of C60 is reanalysed in the light of our previous work and the energy of the lowest three excited singlet states is determined with precision. In matrices, the visible absorption bands are red-shifted by an amount ranging from ∼30 cm−1 in Ne matrices to ∼330 cm−1 in Xe matrices. The observed reversal of state ordering of the lowest two singlets states (T1g and T2g) between Ne and Ar matrices (in emission) and in Ne matrices, between the absorption and emission spectra, is attributed to different Stokes shifts of the T1g and T2g states and to the small energy splitting (∼50 cm−1) between them. Finally, a detailed picture of the intramolecular energy redistribution processes is obtained thanks to a combination of picosecond fluorescence experiments and femtosecond pump–probe transient absorption experiments. The intramolecular relaxation processes among the pure electronic levels of the lowest three singlet states are found to be strongly medium dependent. Medium effects are manifest even on the very short time scale of the internal conversion in the singlet vibronic manifold.
Highly stable dye-sensitized solid-state solar cell with the semiconductor 4CuBr 3S(C4H9)2 as the hole collector
Authors: Tennakone K. , Senadeera G. K. R. , De Silva D. B. R. A. , Kottegoda I. R. M.
Published: July 2002

Construction of a dye-sensitized solid-state solar cell with the semiconductor 4CuBr 3S(C4H9)2 as the hole collector is reported. The cell is unusually stable compared to dye-sensitized solid state cells reported previously and delivers a short-circuit photocurrent and an open-circuit voltage of ∼4.3 mA cm−2 and 400 mV respectively, at 1.5 air mass, 1000 W m−2 sunlight.
Time scales and pathways for kinetic energy relaxation in solvated proteins: Application to carbonmonoxy myoglobin
Authors: Sagnella Diane E. , Straub John E. , Thirumalai D.
Published: July 2002

Using a novel method for determining the coordinate friction for systems that possess bounded diffusion, the rates of kinetic energy partitioning for various elements of solvated carboxy-myoglobin were calculated. Energy redistribution within the heme group and solvent is found to be rapid compared with energy redistribution within the protein. Within the protein, charged residues exhibit much more rapid dispersal than neutral residues. The results suggest that a possible doorway for energy release from the photolyzed heme involves the interaction of its isopropionate groups with the neighboring solvent molecules. The results are analyzed as a function of atom type, protein residue and residue group (charged, polar, aliphatic, and aromatic) leading to general observations relating to the inherent inhomogeneity in the spatially dependent relaxation rate of the solvated protein. The computational results are used to analyze a variety of estimates of the internal friction, viscosity or damping invoked to interpret experimental measures of protein dynamics. The concluding discussion includes speculations on the origin of internal viscosity in proteins.
Efficiency of encounter-controlled reaction between diffusing reactants in a finite lattice
Authors: Kozak John J. , Nicolis C. , Nicolis G.
Published: July 2002

Numerically-exact values of the mean walk length before first encounter for reactants diffusing on a lattice are calculated by generalizing a Markovian method introduced previously. The approach developed is illustrated by presenting results for two random walkers diffusing simultaneously on a square-planar lattice; the results obtained are compared with those calculated assuming one of the reactant partners is stationary. In almost all cases, it is found that the former process is the more efficient. Analytic arguments are presented and Monte Carlo studies are carried out to support the results obtained.
Boron penetration in p+ polycrystalline-Si/Al2O3/Si metal–oxide–semiconductor system
Authors: Park Dae-Gyu , Cho Heung-Jae , Yeo In-Seok , Roh Jae-Sung , Hwang Jeong-Mo
Published: July 2002

We report a flat band voltage instability of a p+ polycrystalline-Si (poly-Si)/Al2O3/n-Si metal–oxide–semiconductor (MOS) system due to boron penetration. The flat band voltage shift of the p+ poly-Si/Al2O3/n-Si MOS capacitor determined by capacitance–voltage measurement was ∼1.54 V, corresponding to a p-type dopant level of 8.8×1012 B ions/cm2 as the activation temperature increased from 800 to 850 °C. Noticeable boron diffusion into the n-type Si channel was also observed by secondary ion mass spectroscopy with activation annealing above 850 °C. Incorporation of an ultrathin (∼5 Å) silicon oxynitride interlayer between Al2O3 and Si was effective in blocking B penetration, reducing the flat band shift to ∼90 mV.
Transient effects in ultra shallow depth profiling of silicon by secondary ion mass spectrometry
Authors: van der Heide P. A. W. , Lim M. S. , Perry S. S. , Rabalais J. W.
Published: July 2002

The significant and often unpredictable variations, or transient effects, observed in the secondary ion intensities of O± and Si± during the initial stages of depth profiling with Cs+ have been studied. These were found to be primarily due to two competing effects: (a) the steady accumulation of Cs in the substrate as a function of sputtering time and (b) the varying oxygen content from the native oxide as a function of depth. These effects prevail over depths approximated by ∼2Rnorm, where Rnorm is the primary ion range normal to the surface. The Cs+ induced effects are consistent with a work function controlled resonance charge transfer process. A method for controlling these effects, namely the prior evaporation of Cs and use of an O2 leak during analysis is described. Doped (As and Sb) and undoped Si wafers with ∼0.9 nm thick native oxides were analyzed using 0.75 and 1 keV Cs+ beams incident at 60°. The more intense polyatomic AsSi− and SbSi− emissions did not exhibit these effects, although other relatively minor intensity fluctuations were still noted over the first ∼0.5 nm.
An image plate chamber for x-ray diffraction experiments in Debye–Scherrer geometry
Authors: Stachs Oliver , Gerber Thomas , Petkov Valeri
Published: July 2002

A new design for implementation of x-ray diffraction experiments is proposed. The exploitation of the imaging plate technique opens new possibilities to carry out diffraction experiments. The detector chamber in Debye–Scherrer geometry equipped with an image plate takes advantage of this technology such as high resolution, high sensitivity, linearity in intensity, and linearity over the detector area. The proof of this measurement principle is demonstrated by presentation of the x-ray diffraction pattern of corundum (crystalline), nickel (crystalline), and sodium borate (amorphous).
The cation-vacancy ordering transition in dehydrated Na6 sodalite
Authors: Campbell Branton J. , Delgado J. Miguel , Cheetham Anthony K. , Iversen Bo B. , Blake Nick P. , Shannon Scott R. , Latturner Susan , Stucky Galen D.
Published: July 2002

Variable-temperature synchrotron x-ray powder diffraction data from dehydrated Na6[Al6Si6O24] reveal a structural phase transition that involves both the commensurate ordering of the extra-framework Na cations and a one-dimensional incommensurate modulation of the framework. Peak splittings and superlattice reflections implicate an orthorhombically-distorted volume-doubled supercell at room temperature (a=12.9432 Å, b=12.8403 Å, and c=9.1372 Å) ∼(√a×√a×a). These data also included additional superlattice peaks associated with an incommensurate long-period modulation with τ=(3/2 1/2 1)/8.9. All unique orderings of the 12 Na cations among the 16 available sites of the commensurate supercell are enumerated. A unique solution is identified that minimizes the structural energy and accounts for the observed superlattice peaks. Rietveld analysis reveals a significant Pauling “partial-collapse” tilt angle in the cation-ordered phase, that appears to decrease with increasing temperature.
An ab initio study of (H3B←NH3)−—a dipole-bound anion supported by the dative charge-transfer bond in the neutral host
Authors: Barrios Robyn , Skurski Piotr , Rak Janusz , Gutowski Maciej
Published: July 2002

The possibility of electron binding to the complex (H3BNH3) was studied at the coupled cluster level of theory with single, double, and noniterative triple excitations. The staggered conformation (minimum), with a dipole moment of 5.3 Debye, binds an electron by 984 cm−1, whereas the eclipsed conformer (saddle point) possesses a larger dipole moment (5.5 Debye) and binds an electron by 1014 cm−1. The neutral parent of the (H3BNH3)− anion involves a dative bond that is responsible for a significant polarization of the neutral species and generates a significant dipole moment.
An intramolecular theory of the mass-independent isotope effect for ozone. II. Numerical implementation at low pressures using a loose transition state
Authors: Hathorn B. C. , Marcus R. A.
Published: July 2002

A theory is described for the variation in the rate constants for formation of different ozone isotopomers from oxygen atoms and molecules at low pressures. The theory is implemented using a simplified description which treats the transition state as loose. The two principal features of the theory are a phase space partitioning of the transition states of the two exit channels after formation of the energetic molecule and a small (ca. 15%) decrease in the effective density of states, ρ [a “non-Rice–Ramsperger–Kassel–Marcus (RRKM) effect”], for the symmetric ozone isotopomers [B. C. Hathorn and R. A. Marcus, J. Chem. Phys. 111, 4087 (1999)]. This decrease is in addition to the usual statistical factor of 2 for symmetric molecules. Experimentally, the scrambled systems show a “mass-independent” effect for the enrichments δ (for trace) and E (for heavily) enriched systems, but the ratios of the individual isotopomeric rate constants for unscrambled systems show a strongly mass-dependent behavior. The contrasting behavior of scrambled and unscrambled systems is described theoretically using a “phase space” partitioning factor. In scrambled systems an energetic asymmetric ozone isotopomer is accessed from both entrance channels and, as shown in paper I, the partitioning factor becomes unity throughout. In unscrambled systems, access to an asymmetric ozone is only from one entrance channel, and differences in zero-point energies and other properties, such as the centrifugal potential, determine the relative contributions (the partitioning factors) of the two exit channels to the lifetime of the resulting energetic ozone molecule. They are responsible for the large differences in individual recombination rate constants at low pressures. While the decrease in ρ for symmetric systems is attributed to a small non-RRKM effect η, these calculated results are independent of the exact origin of the decrease. The calculated “mass-independent” enrichments, δ and E, in scrambled systems are relatively insensitive to the transition state (TS), because of the absence of the partitioning factor in their case (for a fixed non-RRKM η). They are compared with the data at room temperature. Calculated results for the ratios of individual isotopomeric rate constants for the strongly mass-independent behavior for unscrambled systems are quite sensitive to the nature of the TS because of the partitioning effect. The current data are available only at room temperature but the loose TS is valid only at low temperatures. Accordingly, the results calculated for the latter at 140 K represent a prediction, for any given η. At present, a comparison of the 140 K results can be made only with room temperature data. They show the same trends as, and are in fortuitous agreement, with the data. Work is in progress on a description appropriate for room temperature.
Intermolecular potential-energy surface for the Ar–SH(2Πi) complex studied by Fourier-transform microwave spectroscopy
Authors: Sumiyoshi Yoshihiro , Endo Yasuki , Ohshima Yasuhiro
Published: July 2002

Rotational spectra of the Ar–SH(2Πi) radical complex and its deuterated species have been studied using a Fourier-transform microwave spectrometer. The complexes are produced in a supersonic free jet using a pulsed discharge of H2S or D2S diluted in Ar. R-branch transitions in the lower spin component (Ω=3/2) for the linear Πi2 radical were observed for J″=1.5–7.5 in the 8–26 GHz region, in which the parity doublings and hyperfine splittings associated with the H/D nuclei have been observed. Effective rotational constants for Ar–SH and Ar–SD are determined to be 1569.660(3) and 1567.723(4) MHz, respectively, using an effective Π2 Hamiltonian including hyperfine terms. An effective centrifugal distortion constant, D, has been determined to be negative. A two-dimensional intermolecular potential energy surface for the Ar–SH(2Πi) complex has been derived from a least-squares fitting of the observed rotational transitions, where several parameters are constrained to the values from an ab initio calculation at the RCCSD(T)/aug-cc-pVQZ level. The average potential turned out to be fairly isotropic with two shallow minima corresponding to the linear Ar⋯SH and Ar⋯HS configurations, among which the former is 7.2 cm−1 more stable than the latter. The determined equilibrium distance between Ar and the SH center-of-mass is 3.791 Å at the Ar⋯SH global minimum configuration. The vibrational ground state is located above the barrier of only about 20 cm−1 between the two minima, and its wave function is widely spread along the bending coordinate. The negative sign of the effective D constant is well explained by the enhancement of the probability at the linear Ar⋯SH configuration by the centrifugal force, which results in a decrease of the vibrationally averaged Ar–SH intermolecular distance.
Raman and absorption spectrum of mass-selected lutetium dimers in argon matrices
Authors: Fang Li , Chen Xiaoyu , Shen Xiaole , Lombardi John R.
Published: July 2002

We report on the absorption, resonance Raman, and Raman excitation profile spectra of mass selected lutetium dimers in argon matrices. Two broad absorption regions were found between 415 and 545 nm with peaks at 418, 433, 452, 472, 505, 540 nm, and between 635 and 660 nm with peaks at 640 and 652 nm. Resonance Raman spectra were obtained by excitation into one of these regions with the visible light of an Ar ion laser between 458 and 514 nm. Two progressions were assigned to lutetium dimers. We interpret them to represent two distinct electronic states: A ground-state X and an excited state A. For the ground (X) state, we obtain ωe=121.6±0.8 cm−1 with ωexe=0.16±0.10 cm−1, leading to a spectroscopic dissociation energy of 2.9±1.8 eV, and force constant ke=0.76±0.01 mdyne/Å. The lowest excited electronic state (A) has an origin at 210.7 cm−1 and almost identical vibrational parameters. Comparison of spectroscopic properties of lanthanide dimers is discussed, and evidence is presented that lutetium should occupy the position normally given to lanthanum in the periodic table.
The degree of dissociation of ionic surfactant shells within a W/O microdroplet
Authors: Tsao Heng-Kwong , Sheng Yu-Jane , Lu C.-Y. David
Published: July 2002

The degree of dissociation of ionic surfactant shells within a W/O microdroplet is investigated on the basis of the Poisson–Boltzmann equation and locally ideal mixing assumption. The degree of dissociation is the result of the competition between the entropy associated with counterions in the aqueous core and mixing of surfactant ions with associated surfactants on the shell, and the electrostatic energy. The degree of dissociation is found to depend on the geometry and is in the order: Two planar surfaces>cylinder>sphere of the same size. Moreover, for the same geometry, the degree of dissociation increases with the size of the droplet. The free energy change associated with a droplet due to dissociation is obtained. The effect of the solubilization of a neutral particle on the degree of dissociation is also studied. The solubilization is found to hinder the dissociation.
A quantum electrodynamical theory of three-center energy transfer for upconversion and downconversion in rare earth doped materials
Authors: Andrews David L. , Jenkins Robert D.
Published: July 2002

Three-center energy transfer affords the basic mechanism for a variety of multiphoton processes identified within materials doped with rare earths. Addressing the theory using quantum electrodynamics, general results are obtained for systems in which the fundamental photophysics engages three ions. Distinct cooperative and accretive mechanistic pathways are identified and the theory is formulated to elicit their role and features in energy transfer phenomena of pooling upconversion, sensitization, and downconversion or quantum cutting. It is shown that although the two mechanisms play significant roles in pooling and cutting, only the accretive mechanism is responsible for sensitization processes. Both mechanisms are shown to invoke Raman selection rules, which govern transitions of the mediator ions in the accretive mechanisms and transitions of the acceptor ions in the cooperative mechanisms. The local, microscopic level results are used to gauge the lattice response, encompassing concentration and structural effects. Attention is drawn to a general implication of implementing a multipolar description for the optical properties of doped solid-state ionic materials.
Laser ablation and column formation in silicon under oxygen-rich atmospheres
Authors: Pedraza A. J. , Fowlkes J. D. , Lowndes D. H.
Published: July 2002

The microstructure formed at the surface of silicon by cumulative pulsed-laser irradiation in oxygen-rich atmospheres consists of an array of microcolumns surrounded by microcanyons and microholes. Formation of SiOx at the exposed surface of silicon is most likely responsible for the occurrence of etching/ablation that causes the continuous deepening of canyons and holes. The growth mechanism of columns that is supported by the experimental evidence presented here is a process in which the columns are fed at their tips by the silicon-rich ablation plasma produced during pulsed-laser irradiation.
Relaxation in charge-transfer systems with very large tunnel splitting: A semiclassical stochastic approach
Authors: Casado-Pascual J. , Denk C. , Morillo M. , Cukier R. I.
Published: July 2002

Electron transfer in strongly coupled systems, appropriate to mixed-valence compounds, is studied to explore the competition between electronic coherence and dissipation. A set of stochastic equations is derived for a spin-boson Hamiltonian with large tunneling coupling matrix element (adiabatic regime) and strong system-bath-coupling. The bath dynamics is treated classically while the quantum character of the system is maintained. The bath dynamics is affected by the system dynamics, the effect being included by a mean-field description, valid for the adiabatic regime. Numerical solutions of the stochastic equations are presented and compared with exact quantum mechanical results. The numerical implementation of the method is straightforward and the long-time behavior of the system can be accessed. Analytic equilibrium solutions for the adiabatic regime are obtained, and we find good agreement between the long-time solution of the stochastic equations and these equilibrium solutions. We examine the dependence of the electronic population on the initial preparation of the bath and find that the proportion between oscillation (coherence) and decay (dissipation) is quite sensitive to this initial condition.
Photoexcited spin-glass state in (Mg, Fe){Mg,Fe,Ti}O4 spinel ferrite films
Authors: Muraoka Yuji , Tabata Hitoshi , Kawai Tomoji
Published: July 2002

Spin-glass states above room temperature have been found in (Mg, Fe){Mg,Fe,Ti}2O4 spinel ferrite thin films formed on Al2O3 (0001) and SrTiO3 (111) substrates. The films show a long-time relaxation of the magnetization in zero-field-cooled operation and a frequency dependence of the cusp temperature in ac susceptibility measurement, both of which are typical characteristics of a spin glass. This high-temperature spin-glass state has been achieved by the fine tuning of spin states through the control of composition, random oxygen deficiencies, and the stress induced by the film/substrate lattice mismatch, all of which give rise to effects that enhance the exchange interaction of spins in the ferrite films. We have demonstrated the change of magnetic state by means of light irradiation from spin glass to a ferrimagnet over a wide range of temperatures below 290 K. The direct photoexcitation of spins with photon energy in the visible-light region (1.7–3.2 eV), which corresponds to the spin excitation energy of crystal field for Fe ions located in both tetrahedral and octahedral sites, is effective for realizing photoinduced magnetization.
Soliton asymptotics of nondecaying solutions of the modified Kadomtsev–Petviashvili–I equation
Authors: Anders Igor , Boutet de Monvel Anne
Published: July 2002

We prove the existence of nondecaying real solutions of the mKP-I equation, vanishing for x→+∞, and we obtain asymptotic formulas as t→∞ in the form of an infinite series of asymptotic solitons with curved lines of constant phase and varying amplitude and width.
Convergence of the exponential Lie series
Authors: Moan P. C. , Oteo J. A.
Published: July 2002

Improved results on convergence of the Magnus expansion are reported. Consequences are discussed for exponential perturbation theory, construction of numerical methods, control theory or, in general, for problems where time-ordered integrals are involved.
Near-field fluorescence imaging with 32 nm resolution based on microfabricated cantilevered probes
Authors: Eckert Rolf , Freyland J. Moritz , Gersen Henkjan , Heinzelmann Harry , Schürmann Gregor , Noell Wilfried , Staufer Urs , de Rooij Nico F.
Published: July 2002

High-resolution near-field optical imaging with microfabricated probes is demonstrated. The probes are made from solid quartz tips fabricated at the end of silicon cantilevers and covered with a 60-nm-thick aluminum film. Transmission electron micrographs indicate a continuous aluminum layer at the tip apex. A specially designed instrument combines the advantages of near-field optical and beam-deflection force microscopy. Near-field optical data of latex bead projection patterns in transmission and of single fluorophores have been obtained in constant-height imaging mode. An artifact-free optical resolution of 31.7±3.6 nm has been deduced from full width at half maximum values of single molecule images.
Time delayed electroluminescence overshoot in single layer polymer light-emitting diodes due to electrode luminescence quenching
Authors: Lupton J. M. , Nikitenko V. R. , Samuel I. D. W. , Bässler H.
Published: July 2002

We investigate the transient response of single layer polymer light-emitting diodes (LEDs) based on poly[2-methoxy, 5-(2′ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV). An electroluminescence overshoot is observed between 1 and 2 μs after turning off the voltage pulse, depending on temperature. We attribute the delay between overshoot and voltage turnoff to detrapping of majority charge carriers at the polymer/cathode interface. Due to the luminescence quenching zone of the metal the luminescence is found to decrease rapidly after the voltage is switched off, but then rise again to up to a third of the steady state value. The overshoot is found to be independent of the applied bias, but is strongly influenced by temperature and pulse length, indicating a trapping process in interfacial traps is responsible for the effect. The overshoot area corresponds to the charge located on interfacial traps near the cathode and scales as a power law with the pulse length. Our findings demonstrate the presence of extrinsic traps in MEH-PPV devices and highlight the importance of distinguishing between the effects of extrinsic and intrinsic traps in polymer LEDs. Spatial correlations of electrons and holes due to their mutual Coulombic field also appear to be important.