bluuebunny/crossref_metadata_2025 huggingface dataset:
Evidence of enhanced epitaxial crystallization at low temperature by inelastic electronic scattering of mega-electron-volt heavy-ion-beam irradiation
Authors: Nakata Jyoji
Published: July 2002

The inelastic electronic scattering of 84Kr at energies of 0.5–5 MeV and 131,132Xe ions at energies of 1–5 MeV is directly confirmed to play a role in the ion-beam-induced epitaxial crystallization (IBIEC) of amorphous Si layers on crystalline Si substrates at 310–450 °C. The crystallization rate per unit vacancy (normalized crystallization rate) created by the elastic nuclear scattering of the incident ion beam at the amorphous–crystalline (a/c) interface is increased 40%–50% by increasing the inelastic electronic scattering three- to fourfold while maintaining the same elastic nuclear scattering conditions at the a/c interface. This normalized crystallization rate also increases with the inelastic electronic scattering at the a/c interface. However, the percentage of increase falls to about 20% as the absolute value of the inelastic electronic energy loss at the a/c interface decreases to below about 20 keV/ion/(20 nm) or as the number of vacancies created at the a/c interface increases to more than 250/ion/(20 nm). At 310–450 °C the activation energy for IBIEC induced by Kr or Xe irradiation is 0.5–0.6 eV. This value slightly larger than the 0.2–0.3 eV reported for temperatures below 300 °C reflects the thermally activated solid-phase epitaxy. The mechanism of this enhanced crystallization by inelastic electronic scattering is discussed qualitatively.
Analysis of high-field hole transport characteristics in Si1−xGex alloys with a bond orbital band structure
Authors: Liou Tsyr-Shyang , Wang Tahui , Chang Chun-Yen
Published: July 2002

We present an analysis of high-field hole transport in strained Si1−xGex alloys using a Monte Carlo technique. A bond orbital model is employed to calculate the valence-band structure in the simulation so that the transport behavior of high-energy holes can be described accurately. The model combines the k⋅p and the tight binding methods and contains no fitting parameters. The spin–orbit interaction and lattice-mismatch-induced biaxial compressive strain are included in the model. The steady-state hole drift velocity and the impact ionization rate are calculated as a function of an electric field up to 500 kV/cm. Good agreement between experiment and simulation is obtained.
Spallation of thin elastic coatings from elastic substrates by laser induced pressure pulses
Authors: Lev L. C. , Argon A. S.
Published: July 2002

The laser pulse induced spallation technique has been used in recent years to successfully measure the tensile strength of interfaces between coatings of micron thickness and substrates in nondispersive and nondissipative bimaterial systems of interest in composites with strong but brittle components. In previously reported measurements a one-dimensional wave propagation solution was used to evaluate the measurements. In the present article the results of a three-dimensional analytic solution are presented, which demonstrate the range of validity of a one-dimensional analysis, that remains convenient for the evaluation of experimental results. The present development demonstrates that all the interface strength measurements reported earlier by V. Gupta et al. [J. Mech. Phys. Solids 40, 141 (1992)], obtained by the previous one-dimensional analysis remain unaffected.
Interaction of deep levels and potential fluctuations in scattering and recombination phenomena in semi-insulating GaAs
Authors: Kažukauskas V. , Storasta J. , Vaitkus J.-V.
Published: July 2002

The complex influence of recombination centers and potential fluctuations of the band gap on the scattering and recombination phenomena in n-type semiinsulating liquid- encapsulated-Czochralski-grown GaAs were investigated by using the transient photoconductivity and photo-Hall effects. The inhomogeneities cause a hyperbolic decrease of nonequilibrium carrier concentration and the saturation of Hall mobility, while the exponential parts of the decay appear due to the recharge of deep levels. The mean recombination barrier heights of potential fluctuations were evaluated. We propose a complex ‘‘island’’ model of scattering and recombination centers, consisting of defect clusters and their associations around dislocations, surrounded by potential barriers. At low light intensities and at the temperatures below 330 K they are insulating for majority charge carriers, thus reducing an effective crystal volume and causing percolation transport effects. At the temperature higher than 330–360 K the main barrier of the island can be recharged or screened by nonequilibrium carriers and its fine barrier structure appears as an effective scatterer, causing a sharp decrease of the nonequilibrium Hall mobility. It was demonstrated that although doping with Sb reduce dislocation density, it can intensify the effect of smaller defects on transport phenomena.
Highly selective etching for polysilicon and etch-induced damage to gate oxide with halogen-bearing electron-cyclotron-resonance plasma
Authors: Chang Kow-Ming , Yeh Ta-Hsun , Deng I-Chung , Lin Horng-Chih
Published: July 2002

The investigations of polysilicon etching with three halogen-bearing plasmas (SF6, Cl2, and HBr) in an electron-cyclotron-resonance reactor have been made. We examine the etching characteristics which include etching rate, anisotropy, and selectivity based on the discussions of the chemical and electrical properties of the F, Cl, and Br radicals. It was found that the degree of anisotropy strongly depends on the deposition of a silicon halide film on sidewall and on the spontaneous etching properties of three halogen plasma mixtures. The selectivity to oxide is related to the amount of silicon halides that are produced during etching poly-Si and the electrically polarized level of these products when they adsorb on the oxide surface. We find that the selectivity on oxide is approximately infinite for the HBr system and over 90 for the Cl2 system. In addition, the effects of oxygen addition, microwave power, and rf power are also investigated. It was found that the maximum etching rate and selectivity of SF6, Cl2, and HBr plasmas occurred at concentrations of added oxygen at about 7.5%, 5%, and 3% total flow rates, respectively. The suitable range for the microwave power applied is from 250 to 300 W and the most respectable value of applied rf power is about 35 W. Concurrently, the local surface charging causing damage to the gate oxide during the overetching period is studied. It is observed that low device yield, large flatband voltage Vfb shift, and high interface-state density Dit are correlated with longer overetch and high antenna ratio structure of the device. A more obvious degradation is observed in the SF6 gaseous system than in Cl2 and HBr ones. Finally, the influence of magnetic coil current on device damage is also addressed.
Aging effect of Co/C soft x-ray multilayer mirrors
Authors: Jiang E. Y. , Bai H. L. , Tian R. Y. , Wang C. D.
Published: July 2002

The effect of aging on Co/C multilayer x-ray mirrors is presented. One significant result is the enhancement of the reflectivity at grazing incidence with time for the Co/C multilayers with C-on-top stored in air or argon and Co-on-top stored in argon. This can be interpreted in terms of a Co–C phase-separation at Co–C interfaces due to the positive enthalpy of Co–C mixing. Results also show that oxidation of the surface of Co-on-top Co/C multilayers plays an important role in the decrease of reflectivity. The oxidation can be prevented by storing the multilayers in an oxygen-free atmosphere or by depositing amorphous carbon as the top layer. The reflectivity of tarnished multilayers can be restored by removing the oxides by wet chemical methods.
Energy-transfer rate in Coulomb coupled quantum wires
Authors: Tanatar B.
Published: July 2002

We study the energy transfer rate for electrons in two parallel quantum wires due to interwire Coulomb interactions. The energy transfer rate between the wires (similar to the Coulomb drag effect in which momentum transfer rate is measured) is calculated as a function of temperature for several wire separation distances. We employ the full wave vector and frequency dependent random-phase approximation at finite temperature to describe the effective interwire Coulomb interaction. We find that the energy transfer rate at intermediate temperatures (i.e., T∼0.3EF) is dominated by the collective modes (plasmons) of the system. Nonlinear effects on the energy transfer rate is also explored.
Effect of grain size and dislocation density on the performance of thin film polycrystalline silicon solar cells
Authors: Imaizumi Mitsuru , Ito Tadashi , Yamaguchi Masafumi , Kaneko Kyojiro
Published: July 2002

Three kinds of important properties of the solar cell were calculated: short-circuit current density, open-circuit voltage, and conversion efficiency. Two equations which show the relationship between the minority-carrier diffusion length and the grain size or the etch pit density were used for the calculation. The dependence of the properties on the cell thickness were estimated as a function of grain size and etch-pit density. The effect of the internal reflectance with varying minority-carrier diffusion length was also examined. The results show that thin film polycrystalline silicon solar cells have the potential to attain an efficiency of 17% even at a film thickness of 2 μm if the grain size is bigger than 10 μm and the etch-pit density of less than 1×106 cm−2. The principal requirement is to achieve efficient light trapping.
Emissivity of liquid germanium in visible and near infrared region
Authors: Takasuka Eiryo , Tokizaki Eiji , Terashima Kazutaka , Kimura Shigeyuki
Published: July 2002

The normal emissivity of liquid germanium was determined from the direct measurement of the thermal radiation from the liquid surface and a blackbody. The monochromatic emissivity in the visible region and the average emissivity in the near infrared region were measured to investigate the wavelength dependence. The emissivity of the liquid has little dependence on the wavelength and is 0.217±0.002 for the visible region (600–900 nm) and is 0.200±0.005 for the near infrared region at the melting point. The emissivity has a weak dependence on the temperature from 910 to 1160 °C. The weak dependence agrees well with a dielectric response of the free electrons.
Polarization and dipole moments of Co-doped potassium sodium strontium barium niobate crystals
Authors: Xia H. R. , Wang C. J. , Yu H. , Chen H. C. , Wang M.
Published: July 2002

Single crystals of potassium sodium strontium barium niobate (KNSBN) and cobalt-modified KNSBN were prepared using the Czochralski technique. The ferroelectric hysteresis loops and the infrared reflectivity spectra were collected. Compared with the undoped KNSBN crystals, the cobalt-modified crystals have stable hysteresis loops, whose spontaneous polarization is about 0.17 C/m2 and coercive field strength is about 670 V/mm, but those of the undoped KNSBN crystals are about 0.04 C/m2 and 530 V/mm, respectively. The measured infrared reflectivities vary with the orientations of the dipole moments owing to the Co doping. The c axis becomes the most stable orientation of the dipole moments, and the polarization can be locked and does not recede when the cobalt-modified crystals are polarized into a single domain.
Investigation of the x-ray storage phosphors Cs2NaYF6:Pr3+ or Ce3+
Authors: Pawlik Th. , Spaeth J.-M.
Published: July 2002

We present Cs2NaYF6:Ce3+ or Pr3+ as new x-ray storage phosphors. The radiation damage centers upon x irradiation were investigated using magneto-optical and magnetic resonance spectroscopy. F centers are the photostimulable electron trap centers. For the activator Pr3+ it could be shown that upon room temperature x irradiation only F centers and Pr4+ hole trap centers are involved in the information storage and readout process. There are indications that the process is the same for Ce3+-doped Cs2NaYF6.
Finite laser beam size effects in thermal wave interferometry
Authors: Fabbri L. , Cernuschi F.
Published: July 2002

A theoretical and experimental analysis of the finite laser beam size effects in thermal wave interferometry has been reported. The limits of the heat transfer one-dimensional approximation, that is usually employed in this measuring method, have been discussed. This study has been performed by considering the actual three-dimensional temperature distribution induced by gaussian laser heating. Measurements on yttria partially stabilized zirconia coatings plasma-sprayed onto two different metallic substrates were carried out for different values of the heating beam dimension.
Radiative recombination in CuInSe2 thin films
Authors: Zott S. , Leo K. , Ruckh M. , Schock H.-W.
Published: July 2002

We perform a detailed analysis of the radiative recombination processes in CuInSe2 thin films grown by multisource physical vapor deposition. The photoluminescence and photoluminescence excitation spectra are investigated as a function of stoichiometry, temperature and excitation intensity. Using samples with a large composition gradient, we are able to obtain a coherent picture of the optical transitions in the films. The broad–band photoluminescence spectrum typical for In–rich films breaks into a number of well–defined emission lines in Cu–rich CuInSe2. At low temperatures, emission peaks due to free–exciton, bound–exciton, and free–to–bound recombination are identified in Cu–rich films. The spectra of In–rich films tend to be dominated by donor–acceptor transitions. From the optical spectra, exciton ionization energies and the temperature dependence of the band gap are determined. The observed optical transitions are related to intrinsic defects.
Scattering of electrons at threading dislocations in GaN
Authors: Weimann Nils G. , Eastman Lester F. , Doppalapudi Dharanipal , Ng Hock M. , Moustakas Theodore D.
Published: July 2002

A model to explain the observed low transverse mobility in GaN by scattering of electrons at charged dislocation lines is proposed. Filled traps along threading dislocation lines act as Coulomb scattering centers. The statistics of trap occupancy at different doping levels are investigated. The theoretical transverse mobility from Coulomb scattering at charged traps is compared to experimental data. Due to the repulsive potential around the charged dislocation lines, electron transport parallel to the dislocations is unaffected by the scattering at charged dislocation lines.
Optical transitions and frequency upconversion emission of Er3+ ions in Ga2S3–GeS2–La2S3 glasses
Authors: Higuchi H. , Takahashi M. , Kawamoto Y. , Kadono K. , Ohtsuki T. , Peyghambarian N. , Kitamura N.
Published: July 2002

Spectroscopic properties of Er3+ in Ga2S3–GeS2–La2S3 glasses are investigated on the basis of the spontaneous emission probabilities calculated by the use of the Judd–Ofelt theory and lifetime data. Frequency upconversion spectra are also measured with excitation at 800 and 980 nm. The spontaneous emission probabilities are much larger than those of the other glass systems such as oxides and fluorides. This is mainly attributed to the large refractive indices of the sulfide glasses. The quantum efficiencies of I11/24 and F9/24 levels are comparable or larger than the typical values of Er3+ in a fluoride glass. This is a consequence of the large spontaneous emission probabilities and low phonon energies of the sulfide glasses. However, the quantum efficiency of the S3/24 level is less than one-half of the value in the fluoride glass. The nonradiative transition probability of the S3/24 level is much larger than the value which is evaluated from the multiphonon relaxation theory. This suggests the presence of the other nonradiative relaxation process in addition to the multiphonon relaxation. This process is considered to be the energy transfer from the excited Er3+ to the band-gap excitation of the matrix glass. In upconversion spectra, the 497 nm emission from the F7/24 level is observed in addition to the red (665 nm) and the green (533 and 552 nm) emission bands.
Thermodynamics and kinetics of the Mg65Cu25Y10 bulk metallic glass forming liquid
Authors: Busch R. , Liu W. , Johnson W. L.
Published: July 2002

The thermodynamics and kinetics of the bulk metallic glass forming Mg65Cu25Y10 liquid were investigated using differential scanning calorimetry and three-point beam bending. The experiments lead to the determination of the thermodynamic functions as well as the viscosity of the supercooled liquid. The viscosity shows a temperature dependence, which is consistent with that of a strong glass similar to Zr–Ti–Cu–Ni–Be bulk metallic glasses or sodium silicate glasses. This contrasts with more fragile conventional metallic glass formers or pure metals. The relatively weak temperature dependence of the thermodynamic functions of the supercooled liquid is related to these sluggish kinetics in the supercooled liquid. Entropy, viscosity, and kinetic glass transition are compared in the frameworks of the fragility concept and the Adam–Gibbs theory. Strong liquid behavior retards the formation of crystals kinetically and thermodynamically.
Synthesis and characterization of SiC:H ultrafine powder generated in an argon–silane–methane low-pressure radio-frequency discharge
Authors: Vivet F. , Bouchoule A. , Boufendi L.
Published: July 2002

The peculiarity of dusty plasma reactors offers a convenient way to obtain processed particles at submicronic levels, with successive layers of different materials grown by using pulsed gas flows, and different plasma chemistries in succession. This concept is applied to the synthesis of silicon carbide (SiC) particles. In this paper two significant situations are reported showing that particles can be synthesized with different properties by varying the process parameters (gas-flow handling, radio-frequency power level). These properties include broad or narrow size dispersion, almost crystalline or amorphous structure, and widely varying Si/C stoichiometry. Monosized particles with high specific surfaces have been obtained by a two-step growth process by using limited radio-frequency power.
Hydrogenation and disproportionation of Sm2Fe17−xGax at high hydrogen pressures
Authors: Kubis M. , Müller K.-H. , Schutz L. , Gutfleisch O. , Harris I. R.
Published: July 2002

In order to apply the hydrogenation-disproportionation-desorption-recombination process for the preparation of highly coercive Sm2Fe15Ga2Cy, which exhibits excellent magnetic properties together with a high thermal stability, the disproportionation of Sm2Fe17−xGax was investigated. A systematic study of the hydrogen absorption behavior of Sm2Fe17−xGax (x=0, 0.5, 1, and 2) at hydrogen pressures between 0.5 and 8 bar by means of hydrogen differential thermal analysis (HDTA) showed that increased hydrogen pressures promote the disproportionation of the stabilized 2:17 phase. X-ray diffraction investigations of the HDTA samples showed a decreasing content of the 2:17 phase for increasing hydrogen pressures. It was possible to achieve a nearly full disproportionation even for the most stable compound Sm2Fe15Ga2 by applying a pressure of 8 bar. Microstructural changes within the disproportionated mixture as a result of the applied pressure have been documented in detail by high resolution scanning electron microscopy.
Temperature-dependent magnetic anisotropy in Ni nanowires
Authors: Jorritsma J. , Mydosh J. A.
Published: July 2002

Magnetic properties of Ni nanowire arrays, prepared by oblique evaporation of Ni onto V-groove InP substrates, were investigated between 5 and 300 K using magnetoresistance and SQUID magnetization measurements. The results show that as-prepared wires, which range from 70–130 nm in width, have an easy axis of magnetization parallel to the wire axis at room temperature, but transverse to the wire axis at low temperature. The crossover of the easy axis direction from transverse to parallel as a function of temperature is more pronounced for the narrower wires. We interpret our results in terms of a competition between a temperature-dependent magnetic anisotropy (K⊥), which tends to align the magnetization transverse to the wire axis, and the shape anisotropy of the wires which tends to orient it along the wire axis. Several mechanisms are proposed (e.g., oblique evaporation, stress, and surface oxidation) from which K⊥ could originate. Based upon the stress values deduced from K⊥, and the thermal expansion mismatch between Ni and InP, the stress mechanism appears to dominate.
Predictions of metal droplet formation in gas metal arc welding. II.
Authors: Haidar J.
Published: July 2002

Predictions of metal droplet formation in gas metal arc welding (GMAW) are made for mild steel wires with argon as a shielding gas, using a generalized theory for predictions of arc and electrode properties in arc welding. The theory is a unified treatment of the arc and the electrodes and includes a detailed analysis of the anode sheath region, together with a free surface treatment for the molten metal at the tip of the welding wire. Results of calculations made for a mild steel wire of 0.12 and 0.16 cm diameters are in good agreement with experimental measurements of droplet diameter and droplet detachment frequency at currents between 150 and 330 A. The results indicate that the anode sheath region and the effects due to variation of surface tension with temperature have an important influence on the dynamics of droplet growth at the tip of the wire in GMAW. Predictions are made for the transition from the globular to the spray mode of metal transfer, in agreement with experimental observations in arc welding.
Multiple microdischarge dynamics in dielectric barrier discharges
Authors: Xu Xudong “Peter” , Kushner Mark J.
Published: July 2002

Dielectric barrier discharges (DBDs) are pulsed atmospheric pressure devices in which the plasma forms as an array of microdischarges having diameters expanding from 10 to 100’s μm and area densities of 10 to 100’s cm−2. The microdischarges are <10’s ns in duration and are terminated by charging of the dielectric barrier which removes voltage from the gap. If microdischarges are spaced sufficiently close together they may interact during their expansion. In this article, we discuss results from a two-dimensional plasma hydrodynamics model for microdischarge development in DBDs with the goal of investigating the interaction between closely spaced microdischarges. We find that the efficiency of ionization is only moderately affected by microdischarges which expand into physical contact. The residual charge left on the dielectric following a current pulse can, however, significantly impact the spatial extent of the subsequent microdischarges. During expansion the underlying dielectric charges to progressively larger radii as the microdischarge expands. This leads to voltage collapse in the center of the microdischarge prior to the outer radius. In attaching gas mixtures larger rates of attachment relative to ionization at the lower values of the electric field/number density produce cores which are highly electronegative, surrounded by shells of higher electron density.
High-dose helium-implanted single-crystal silicon: Annealing behavior
Authors: Tonini R. , Corni F. , Frabboni S. , Ottaviani G. , Cerofolini G. F.
Published: July 2002

The modifications induced in single-crystal silicon by implanted helium have been investigated by ion beam techniques. The damage has been detected by 2 MeV He+4 backscattering in channeling conditions and the helium in-depth distribution by 7 and 8 MeV N++15 elastic recoil scattering. The samples prepared by implanting 2×1016 cm−2 helium ions at 20 keV in silicon wafers held either at 77 K (LNT sample) or at 300 K (RT sample) have been heat treated for 2 h in the 100–800 °C temperature range. In the as-implanted LNT sample the damage maximum is at 130±20 nm and shifts in-depth to 180±10 nm after annealing at 200 °C, in the as-implanted RT sample, the damage maximum is already located at 180±10 nm. In the 250–500 °C temperature range, the LNT and RT samples follow the same annealing path with only slight differences in the temperature values; in both cases, the dechanneling signal increases and reaches a maximum value of nonregistered silicon atoms of 2.2–2.5×1022 at/cm3. In the same temperature range, the helium signal becomes narrower, builds up in a region centered on 220±20 nm and no appreciable loss of helium can be detected. The growth of the damage is consistent with the creation of cracks and a etherogenous distribution of bubbles filled with high pressure helium which stress the lattice; for the channeling Rutherford backscattering technique, their action is similar to silicon interstitials. At temperatures above 500 °C, helium is released from the samples; this process is associated with a decrease of the damage and the formation and increase in size of voids. At 900 °C empty voids with a diameter around 20 nm are found.
Numerical study of diffusion coefficient measurements with sinusoidal varying accelerations
Authors: Matsumoto Satoshi , Yoda Shinichi
Published: July 2002

The effect of periodic gravitational acceleration on diffusion coefficient measurements was analyzed in terms of material properties in the specimen such as viscosity, density, and specimen configuration. We found that convection is induced in microgravity with sinusoidal varying accelerations because a concentration difference exists even under isothermal conditions. g-jitter with lower frequency produced large error in the accuracy of the diffusion coefficient measurement. The errors increased abruptly with a decrease in the frequency below 0.1 Hz. The error increased monotonously with a decrease in the density ratio of elements in diffusion materials. In order to obtain optimized experimental conditions for experiment planning, the predicted error due to gravitational disturbances was plotted as a function of material properties such as the viscosity, the density ratio, and the specimen configuration.
Impact ionization rate calculations in wide band gap semiconductors
Authors: Harrison D. , Abram R. A. , Brand S.
Published: July 2002

An algorithm for calculating impact ionization rates in the semiclassical Fermi’s Golden Rule approximation which is efficient close to threshold is presented. Electron and hole initiated rates are calculated for three semiconductors with particular band structure characteristics, as are the distributions of the generated carriers. Simple analytic expressions of the form R=A(E−E0)P are fitted to the calculated rates. The role of the matrix elements in influencing the distribution of final states is investigated. In the direct gap materials, they act to significantly enhance the low-q transitions, while in the indirect gap case they have a lesser effect on the distribution. Results for GaAs obtained here and by several other workers are compared and possible causes of the discrepancies examined, including differences in band structure and approximations made in evaluation of the matrix element. It is found that these differences do not influence the rate sufficiently to account for the wider variation between authors, and so it is concluded that differences in the implementation of the rate integration algorithm are the main cause.
Geometry dependence of nonlinear effects in high temperature superconducting transmission lines at microwave frequencies
Authors: Booth James C. , Beall J. A. , Rudman D. A. , Vale L. R. , Ono R. H.
Published: July 2002

We investigate the nature of low-power nonlinear effects in high-temperature superconducting microwave devices by measuring third harmonic generation at 76 K in coplanar waveguide transmission lines of different geometries fabricated from YBa2Cu3O7−δ thin films. The measured power in the third-harmonic signal changes systematically with film thickness, center conductor linewidth, and line length. We analyze these results using a simple model for a transmission line with a nonlinear inductance arising from a current-dependent superconducting penetration depth. This analysis describes quantitatively the observed differences in harmonic generation for transmission lines of different dimensions, and yields a single geometry-independent parameter (the nonlinear scaling current density J0) to quantify the observed nonlinear behavior. For the thin film samples studied here J0=3.0×107 A/cm2 at 76 K for all geometries investigated. These results provide the means to establish a lower limit for the expected nonlinear response of superconducting components of arbitrary geometry at microwave frequencies.
Positive and negative persistent photoconductivities in semimetallic AlxGa1−xSb/InAs quantum wells
Authors: Wang W. C. , Tsai L. C. , Fan J. C. , Chen Y. F. , Lo Ikai
Published: July 2002

We report a detailed investigation of the “positive” persistent photoconductivity (PPC) and “negative” persistent photoconductivity (NPPC) in semimetallic AlxGa1−xSb/InAs quantum wells. The studies of the NPPC and PPC effects have been performed under various conditions, such as different photon energy of excitation, different temperature, different Al composition x, and different well width. It is found that all the previously proposed mechanisms fail to explain several of our observations. We suggest that the NPPC and PPC effects are produced principally by two competing processes. At a high temperature, the photoconduction is dominated by the photogenerated electrons in the InAs well, in which the relaxation of the excess electrons is prohibited by an energy barrier due to the trapping of photoexcited holes by deep defects in the InAs well. As a result, the PPC is observed. At low temperature, electrons in the InAs layer are photoexcited into the local potential minima induced by compositional fluctuations at the AlGaSb and InAs interface, the number of electrons in the InAs well decreases, thus the NPPC occurs.
Properties of atomic layer deposited (Ta1−xNbx)2O5 solid solution films and Ta2O5–Nb2O5 nanolaminates
Authors: Kukli Kaupo , Ritala Mikko , Leskelä Markku
Published: July 2002

( Ta 1−x Nb x ) 2 O 5 solid solution films and Ta2O5–Nb2O5 nanolaminates have been grown in an atomic layer deposition process at 300 and 325 °C. TaCl5 or Ta(OC2H5)5 and Nb(OC2H5)5 have been used as metal precursors while H2O has been applied as the oxygen source. Application of Ta(OC2H5)5 resulted in amorphous films with considerably better thickness uniformity than that characteristic of TaCl5-based process. Application of TaCl5 resulted in crystallized films. The high-field leakage current in (Ta1−xNbx)2O5 solid solution films with x=0.02–0.07 decreases by two to three orders of magnitude when compared to that of the nondoped Ta2O5. The permittivity of Ta2O5 films was 25 while the permittivity of amorphous or partially crystallized solid solution films and nanolaminates increased up to 33.
Calculation of the contribution to grain boundary diffusion in ionic systems that arises from enhanced defect concentrations adjacent to the boundary
Authors: Chung Yong-Chae , Kim Chang Kyung , Wuensch Bernhardt J.
Published: July 2002

The enhancement of the concentration of a defect in the space-charge region near a grain boundary in an intrinsic ionic system was used to specify the change in the corresponding diffusion coefficient as a function of distance, x, normal to the boundary. This zone of enhanced and continuously varying diffusivity serves as a vehicle for enhanced transport along the boundary in addition to the expected contribution arising from the different structure at the boundary core. A two-dimensional diffusion equation was established for concentration c(x,y,t) in the space-charged region, solved numerically using the Crank–Nicolson finite-difference method, and integrated normal to the boundary to provide the average concentration gradient c̄(y,t) along the boundary—the solute distribution usually measured by experiment. The present gradients were analyzed in the same fashion as experimental data using a recent solution to the conventional model for the grain–boundary diffusion problem in which the interface is treated as a thin slab of half-width a and enhanced diffusivity D′ imbedded in a material with bulk diffusivity D. The analyses provided effective values for the conventional diffusion parameter β=[(D′/D)−1]⌊a/Dt⌋ that describe the influence of space charge on enhanced diffusion along the boundary. The calculations were repeated for a number of values of the parameter Zieφ(∞)/kT, where Zie is the effective charge of the defect and φ(∞) is bulk electric potential far from the boundary. In the finite-difference calculations the value of the “annealing time,” t, that appears in β was set equal to the product of the square of the Debye length, δ2, divided by the bulk diffusion coefficient, D. Given a value of φ(∞), the present results thus permit an estimation of the effect of space charge on preferential diffusion along a grain boundary in a material in terms of β. If the Debye length is known for the material or can be measured, the value of β can readily be converted to an effective value of the conventional grain boundary diffusion product aD′/D.
Fabrication and nanostructure of oriented FePt particles
Authors: Bian Bo , Laughlin David E. , Sato Kazuhisa , Hirotsu Yoshihiko
Published: July 2002

Thin films of oriented tetragonal FePt particles separated by amorphous alumina have been fabricated by electron beam evaporation. The ordering of the FePt particles without coarsening can be tailored by annealing conditions. The value of coercivity of the annealed film reached as high as 4.4 kOe. The perpendicular magnetic coercivity of the annealed film was slightly larger than in-plane coercivity. Some of the tetragonal FePt particles were found to have {111} twins and stacking faults. From our high-resolution electron microscopy observations, it was determined that central region of the ordered FePt particles tended to have c-axis perpendicular to the film plane.
Amplification and diffusion of spontaneous emission in strongly scattering medium
Authors: Totsuka Kouki , van Soest Gijs , Ito Tetsu , Lagendijk Ad , Tomita Makoto
Published: July 2002

We experimentally examined the threshold of the spectral collapse in dye embedded in a strongly scattering medium as a function of the excitation beam diameter and the transport mean free path in order to find a condition to minimize the threshold. We found a critical transport mean free path, below which the threshold pulse energy is almost independent of the mean free path. Experimental observations are well explained by a theoretical model based on the rate and diffusion equations, which shows that the luminescence spectra are also dependent on the position in the medium.
Influence of polarization orientation on the effective properties of piezoelectric composites
Authors: Nan Ce-Wen , Weng G. J.
Published: July 2002

The effective properties of 0–3- and 1–3-type piezoelectric composites of lead zirconate titanate and vinylidene fluoride–trifluoroethylene with different polarization status in both phases are calculated using an effective-medium theory. The effects of volume fraction and polarization orientation on the effective behavior are presented in detail. The theory gives results in reasonable agreement with recent experimental ones. The theoretical predictions demonstrate the interesting behavior of the composites and provide a general guideline for optimizing the microstructural scale of the composites for piezoelectric transducers and pyroelectric sensors.
Dynamics of intramolecular vibrational-energy redistribution (IVR). III. Role of molecular rotations
Authors: Felker Peter M. , Zewail Ahmed H.
Published: July 2002

Experimental results on jet-cooled anthracene pertaining to the role of rotations in IVR processes are presented. For theoretical comparison, we consider the effects of molecular rotational level structure on the beat-modulated decays that arise as manifestations of IVR. It is shown theoretically for anharmonic coupling that small differences in rotational constants between coupled vibrational states give rise to decays, the beat envelopes of which decay faster than the unmodulated portions of the decays. These envelope decay rates are shown to be rotational temperature dependent. The experimental results reveal behavior entirely consistent with the theoretical expectations. The results also show that although rotational effects are present in experimental decays, they are not so marked as to wash out the manifestations of vibrational coherence.
Diatomics-in-molecules models for H2O and H2O−. I. Valence bond diatomic fragment matrices
Authors: Polák R. , Paidarová I. , Kuntz P. J.
Published: July 2002

Valence bond wave functions for the ground states OH(X 2Π) and OH−(X 2Σ+) were analyzed in order to arrive at DIM models for those states of H2O and H2O− correlating with the ground states of the separated atoms. The models are meant to provide a framework for studying the negative ion reaction O−+H2 → OH−+H and to test the utility of DIM in describing at a simple level the bonding in the water molecule. Projection analysis of the ab initio wave functions shows that in addition to the VB structures arising from the coupling of ground state atoms, ionic structures are important, especially for H2O, but that structures with excited states of H can be neglected. Within the space of the DIM model basis functions, diatomic fragment matrices were computed by the VB method to produce library files for eight state manifolds of OH−, six manifolds of OH, two manifolds of OH−−, and four manifolds of H−2. These will be useful as input for future DIM calculations on molecules containing O–H bonds.
NH(A 3Π) rotational population generated by ArF laser photolysis of NH3
Authors: Hofzumahaus A. , Stuhl F.
Published: July 2002

The energy distribution of the NH(A) radicals generated by two photons in the ArF laser (193.3 nm) photolysis of NH3 was studied in detail using high spectral resolution. The different spin components were found to be statistically populated. For the likely photodissociation process NH3+2hν(ArF)→NH(A, v′=0, N′)+H+H, the rotational population matches the prior distribution for fragmentation into two rather than three fragments indicating that both atomic fragments are not equivalent with respect to the dissociation process. It therefore seems to be likely that first an excited NH2 intermediate is formed which subsequently is photolyzed to yield NH(A). From energy conservation, the upper limit for the heat of formation of NH(X 3Σ−) is deduced to be ΔH0f0 (NH)≤367 kJ mol−1. The quantum yield for the generation of NH(A) was estimated to be φ[NH(A)]≊5×10−4. About 5.2% of the excited radicals were found to be produced in v′=1. In addition, a process involving the absorption of at least three photons was observed. About 1% of the excited radicals are formed this way with a rotational temperature of 2500 K for v′=0. These radicals are generated with such a short delay that reactions of ions appear to be involved in their formation.
Photofragmentation of (CO2)+n: Simple energy dependence for the number of neutrals ejected, 13<n<26
Authors: Alexander M. L. , Johnson M. A. , Lineberger W. C.
Published: July 2002

Ionic photofragmentation patterns of (CO2)+n, 2<n<26, are determined at 1064, 650, and 532 nm, and are found to be remarkably simple. The average number of neutral CO2 molecules lost increases smoothly with cluster size, approaching a constant value that depends linearly on the photon energy. A model for involving sequential loss of CO2 monomers is indicated, and the average CO2-cluster bond energy is found to be constant at 4.9 kcal/mol for n>13.
ESR spectra of Ag3(2A1) in an N2 matrix
Authors: Kernisant K. , Thompson G. A. , Lindsay D. M.
Published: July 2002

ESR spectra assigned to Ag3 molecules have been produced by codepositing atomic silver with excess nitrogen at temperatures close to 4.2 K. The spectra are characterized by an axially symmetric spin Hamiltonian having g∥ =1.9933(3) and g⊥ =1.9558(3) and, for 107Ag3, A∥(1) =310.8(6) G with A⊥(1) =310.1(6) G and A∥(2) =76.0(2) G with A⊥(2) =72.6(2) G for the apical(1) and basal(2) nuclei, respectively. The axial symmetry of the spectra is believed to imply that the trimer is rotating about one axis. There is no evidence for a pseudorotating trimer spectrum. The isotropic spin populations are ρ5s(1) =0.51 and ρ5s(2) =0.12 implying an acute angled geometry with ground state symmetry 2A1. This is in sharp contrast to the obtuse angled isomer (2B2 ground state) found for Ag3 in a C6D6 matrix. For Ag3(2A1) there is little p character on the apical atom but a 10%–15% p hybridization on each of the two basal nuclei.
Theoretical studies on excited states of Ne2. I. MRD-CI potential energy curves
Authors: Grein Friedrich , Peyerimhoff Sigrid D. , Buenker Robert J.
Published: July 2002

Using a 5s3p contracted Gaussian basis with d-polarization functions and 3s, 4s, 3p, and 4p diffuse functions, potential energy curves for excited states of Ne2 dissociating to Ne+Ne* (3s,3p,4s) were calculated by the MRD-CI method. Five singlet and triplet states each of Σ+g Σ+u, Πg, and Πu type, and one singlet and triplet state of Δg and Δu type were considered. The features of the potential curves are discussed and compared with those obtained by previous workers.
Multiple collision rotational rainbows: Theory and experiment for Xe–CO2
Authors: Buck U. , Otten D. , Schinke R. , Poppe D.
Published: July 2002

In a crossed molecular beam experiment differential energy loss spectra have been measured for Xe+CO2 collisions at energies of 0.2, 0.58, 1.0, and 1.6 eV. Nearly the complete angular range from 40° to 180° in the center-of-mass system was covered. At large deflection angles and small energy transfers (Δ E/E≂0.1) the spectra exhibit a large intensity peak which cannot be explained by usual rotational rainbow theory. Quantum and classical calculations in the centrifugal sudden approximation demonstrate that this effect is a multiple collision rotational rainbow. In the first collision the kinetic energy is nearly completely transferred to rotational energy of the CO2 molecule. Since the heavy Xe atom leaves the interaction region very slowly, a second collision occurs and the rotational motion is deaccelerated. The classical excitation function J (γi), which relates the final angular momentum with the orientation angle of the molecule, has three extrema, two of which give rise to the multiple collision rotational rainbow. Various test calculations show that the effect depends strongly on the reduced mass, on the anisotropy and, in contrast to the normal rotational rainbow, also on the slope of the repulsive part of the interaction potential. Exact three-dimensional classical trajectory calculations at E=1.0 eV based on a realistic model potential agreed satisfactorily with the experimental results.
Influence of basis set on the calculated properties of (H3N–HCl)
Authors: Latajka Zdzisl/aw , Scheiner Steve
Published: July 2002

The structure of (H3N–HCl) is investigated by ab initio calculations using a number of different basis sets ranging from minimal to split valence. The effects of including a diffuse sp shell and d orbitals on Cl are considered as well. The geometries of the complex and the isolated subunits are fully optimized. Minimal basis sets (STO-3G, STO-6G, and MINI-1) lead to an overestimate of the interaction between the subunits. Addition of d functions produces only a marginal improvement. The 3-21G, 3-21+G, MIDI-1, and LP-31G split-valence sets erroneously predict an ion pair (H3NH+⋅⋅⋅−Cl) in the equilibrium structure, a conclusion which is reversed by polarization of each basis. On the other hand, both the ion pair and (H3N⋅⋅⋅HCl) complexes are identified as minima in the 4-31G potential. When this basis set is augmented with d functions, agreement with previous calculations involving large basis sets is quite good.
FT-IR spectra of vacuum deposited clathrate hydrates of oxirane H2S, THF, and ethane
Authors: Richardson Hugh H. , Wooldridge Paul J. , Devlin J. Paul
Published: July 2002

An ability to prepare clathrate hydrates using low temperature–high vacuum techniques, originally demonstrated for the hydrate of oxirane (Bertie and Devlin), has been extended to include the structure I hydrate of H2S, the mixed structure I hydrate of oxirane and ethane, as well as the structure II simple hydrate of THF and the double hydrates of THF with oxirane and H2S. The crystalline clathrate films (∼6 μ) have been formed either by annealing amorphous host–guest deposits at ∼130 K, epitaxial growth at 110 K (oxirane and mixed ethane–oxirane), or direct deposition at 150 K (THF and its double clathrates). Use of the epitaxial approach at ∼100 K has permitted the formation of the oxirane clathrate hydrate containing intact isolated D2O molecules. This has permitted the FT–IR observation of the ν3–ν1 doublet in the O–D stretching region (2455 and 2380 cm−1 at 100 K) with the values, after correction for Fermi resonance, suggesting a splitting from intramolecular coupling of ∼56 cm−1 (2455 vs 2399 cm−1), which compares closely with the 52 cm−1 deduced for cubic ice. Spectra for the structure I hydrates of oxirane and H2S contain absorption bands produced by guest molecules confined to both small and large clathrate cages. Use of the structure II double hydrates has permitted the firm identification of the structure I infrared bands with oxirane and H2S molecules in cages of one size or the other. Thus, the weaker ν3 and (ν11,ν14) bands of oxirane at 1281 and 1152 cm−1 have been assigned to molecules in the small cages since only these oxirane features remain in the structure II double hydrate with THF. In this case the smaller oxirane molecules occupy the small cages while the THF molecules enter the larger cages, exclusively. In a similar manner, the H–S stretching vibrations of H2S in the structure I small cages have been assigned to a band complex near 2610 cm−1, some 50 cm−1 above the band system for H2S in the large structure I clathrate cages.

Such a result suggests that the net H2S perturbation, relative to the gas phase, is greater for the large than for the small cages and may be interpreted as evidence for a ‘‘double well’’ large cage potential or as further evidence that the cage model of Pimentel and Charles for guest-molecule stretching modes is valid.
A new upper limit to the vibrational predissociation lifetime of ethylene dimer excited at 950 cm−1
Authors: Mitchell Annette , McAuliffe Michael J. , Giese Clayton F. , Gentry W. Ronald
Published: July 2002

A direct measurement in the time domain has been performed of ethylene dimer population decay following single-quantum excitation of the ν7 monomer vibrational mode. Nonresonant multiphoton ionization at 355 nm was used to probe the dimer population as a function of time following excitation with a pulsed CO2 laser. The measured signal decay was found to be determined by the experimental time resolution function, implying that the predissociation lifetime of the dimer is less than about 10 ns.
Algebraic resonance dynamics of the normal/local transition from experimental spectra of ABA triatomics
Authors: Kellman Michael E.
Published: July 2002

An algebraic transformation is used to demonstrate the exact equivalence of the local and normal mode Hamiltonians for coupled anharmonic stretches. This SU(2) model is then interpreted semiclassically to extract quantitative information about nonlinear resonances in ABA triatomics from the Darling–Dennison spectral fit. A ‘‘glossary’’ is presented which makes it very easy to translate between the SU(2) language and standard spectroscopic terminology. In spectra predicted from the Darling–Dennison fit, transitions from a normal to local mode level pattern in molecules such as O3 are easily interpreted semiclassically in terms of trajectories in action/angle space and dynamical barriers. Although the local and normal algebraic Hamiltonians are equivalent for spectral fitting, local modes have the desirable property that they admit a simple representation in the coordinate picture. Local modes such as Morse oscillators therefore are the preferred physical starting point for stretching vibrations of general ABA triatomics.