bluuebunny/crossref_metadata_2025 huggingface dataset:
Melting features along the western Ryukyu slab edge (northeast Taiwan): Tomographic evidence
Authors: Lin Jing‐Yi , Hsu Shu‐Kun , Sibuet Jean‐Claude
Published: December 2004

Behind the sedimentary Ryukyu arc lies the Okinawa Trough, whose termination is located at the tip of the Ilan plain (northern Taiwan), just above the Ryukyu slab edge. The present‐day active volcanic front is located 80–100 km above the Ryukyu slab and extends from Japan to Kueishantao Island, an islet situated 10 km offshore the Ilan plain. Between December 1990 and May 1999, 3370 earthquakes recorded in northern Taiwan by 65 seismic land stations were used to determine the three‐dimensional Vp and Vs velocity structures and Vp/Vs ratios. A low Vs but high Vp/Vs sausage‐like body, ∼30 km in diameter, lies within the Eurasian mantle wedge, on top of the western Ryukyu slab extremity, at depths ranging between 20 and 100 km. We suggest that the H2O‐rich component formed by dehydration processes from subducting sediments, oceanic crust, and serpentinized mantle above the Ryukyu slab and along the vertical portion of the slab edge might explain the presence of the sausage‐like body. A low Vs but high Vp/Vs channel rises obliquely from the sausage‐like body at a depth of 40 km in direction of the andesitic Kueishantao Island. We propose that the H2O‐rich component and/or melt rise up from the sausage‐like body and interfere with the Okinawa Trough back arc basin magmas formed in the upper mantle/lower crust. Then magmas propagate upward within the upper brittle crust through veins and/or narrow conduits.
Ba/Ca in benthic foraminifera: Thermocline and middepth circulation in the North Atlantic during the last glaciation
Authors: Hall Jenney M. , Chan L.‐H.
Published: December 2004

This geochemical investigation utilizes Ba/Ca in the benthic foraminifer Cibicides wuellerstorfi from cores taken from the Bahama Banks and the Caribbean Sea to reconstruct changes in basal thermocline ventilation (800–1000 m) and middepth thermohaline circulation (1000–2000 m) in the western North Atlantic during the last glacial period, focusing on the deglacial transition. Previous studies show that an increase in ventilation of the North Atlantic subtropical gyre during the Last Glacial Maximum (LGM) caused a 30–60% decrease in labile nutrients within the thermocline layer. Using foraminiferal Ba/Ca as a proxy of refractory nutrients, increased ventilation during the LGM produced a depletion of less than 20% compared to Holocene values. Following glaciation, the production of Glacial North Atlantic Intermediate Water (GNAIW) shut down owing to the presence of meltwater in the surface ocean, which resulted in a decrease in ventilation, as seen by an enrichment of barium in the basal thermocline. GNAIW was subsequently replaced by barium‐rich southern component water in the middepth western North Atlantic. Foraminiferal Ba/Ca data suggest a 38% contribution from southern component water to a depth as shallow as 1475 m and a 14% contribution at 1123 m during deglaciation.
Transport of colloids in unsaturated porous media: Explaining large‐scale behavior based on pore‐scale mechanisms
Authors: Keller Arturo A. , Sirivithayapakorn Sanya
Published: December 2004

We conducted column‐scale experiments to study the transport of colloids (latex particles and bacteriophage MS2) under water‐unsaturated conditions. The objective was to draw connections between observations at the pore scale and the results obtained from column‐scale experiments. The same system had been previously operated under saturated conditions to determine colloid collision efficiency. Breakthrough of colloids was first evaluated under unsaturated but steady water content conditions, with constant trickling flow. After monitoring the steady breakthrough of the colloids, the column was flushed with water at higher flow rate to increase the water content up to a saturated condition. Colloid breakthrough was monitored during the entire experiment, as water content increased. Colloid removal increases significantly with decreasing initial water saturation, reflecting retention at the air‐water interface and straining in thin‐water films. Colloid breakthrough occurs earlier than a conservative tracer even under unsaturated conditions, although the colloid concentrations are much lower than the tracer. After flushing at similar flow rates, there is increased colloid retention under unsaturated conditions even as the system approaches water saturation, indicating that additional removal is occurring, due possibly to the formation of colloidal clusters. These results can be explained to a great extent by pore‐scale observations of retention and remobilization mechanisms.
Production of higher silanes in radio frequency SiH4 and H2-SiH4 plasmas
Authors: Horvath P. , Rozsa K. , Gallagher A.
Published: December 2004

Stable gases produced in SiH4 and H2-SiH4, capacitively coupled rf discharges operated at typical device-production conditions, were investigated by quadrupole mass spectrometry. The production of disilane and trisilane and the depletion of silane and pressure change were measured during consecutive short discharges. Dissociation rate constants and branching ratios for higher-silane production are obtained from the time-dependent densities. The influence of suspended Si particles on stable-gas production is found to be negligible, by changing the duration of individual discharges and thereby the particle densities and sizes. We have also observed pyrolitic decay of disilane and trisilane at 200°C, although this is negligible under normal reactor conditions.
Band alignment issues related to HfO2∕SiO2∕p-Si gate stacks
Authors: Sayan S. , Emge T. , Garfunkel E. , Zhao Xinyuan , Wielunski L. , Bartynski R. A. , Vanderbilt David , Suehle J. S. , Suzer S. , Banaszak-Holl M.
Published: December 2004

The valence and conduction band densities of states for the HfO2∕SiO2∕Si structure are determined by soft x-ray photoemission and inverse photoemission. First principles calculations are used to help in assigning valence band maxima and conduction band minima. The energies of defect states at the band edges are estimated by comparing the theoretical and experimental results. Determinations of the local surface potentials before and after a forming gas anneal are used to help determine the possible location of the charge in the film.
Impurity free vacancy disordering of InGaAs quantum dots
Authors: Lever P. , Tan H. H. , Jagadish C.
Published: December 2004

The effect of thermal interdiffusion on In(Ga)As∕GaAs quantum dot structures is very significant, due to the large strain and high concentration of indium within the dots. The traditional high temperature annealing conditions used in impurity free vacancy disordering of quantum wells cannot be used for quantum dots, as the dots can be destroyed at these temperatures. However, additional shifts due to capping layers can be achieved at low annealing temperatures. Spin-on-glass, plasma enhanced chemical vapor deposited SiO2, Si3N4, and electron-beam evaporated TiO2 layers are used to both enhance and suppress the interdiffusion in single and stacked quantum dot structures. After annealing at only 750°C the different cappings enable a shift in band gap energy of 100meV to be obtained across the sample.
Second-harmonic generation of green light in periodically poled stoichiometric LiTaO3 doped with MgO
Authors: Bruner Ariel , Eger David , Ruschin Shlomo
Published: December 2004

Periodically poled crystals of stoichiometric lithium tantalate doped with MgO (PP-Mg:SLT) were developed for a high-power optical frequency conversion. Inverted domain structures with 7.7μm period were fabricated in 1-mm-thick crystals by an electric-field poling using patterned metallic electrodes. Frequency-doubling characteristics of these units were investigated with continuous and pulsed Nd:YAG lasers. The nonlinear coefficient, d33, of Mg:SLT derived from these measurements is 13pm∕V, similar to that of the congruent lithium tantalate. A stable generation of 5.5W average power of green light at 532nm was obtained while the photoinduced breakdown of the periodically poled KTiOPO4 was observed under similar experimental conditions. These results demonstrate the advantage of Mg:SLT for high-average-power applications.
Transport properties of electron-beam and photo excited carriers in high-quality single-crystalline chemical-vapor-deposition diamond films
Authors: Teraji T. , Yoshizaki S. , Mitani S. , Watanabe T. , Ito T.
Published: December 2004

We have investigated transport properties of carriers excited in high-quality homoepitaxial diamond (100) films by 5.6eV photons or 15keV electrons. The high-quality single-crystalline diamond films were homoepitaxially grown on type-Ib diamond substrates at a rate of 2.5μm∕h by high-power microwave-plasma chemical-vapor-deposition (MPCVD). In cathodoluminescence (CL) measurements, strong free-exciton recombination emissions were observed at room temperature from the almost whole specimen surface, indicating the grown films have substantially high quality. It is found through an analysis of the visible emission band originating from the type-Ib substrate that decay constants estimated for excited carriers were ∼5μm in the depth direction. This is consistent with the fact that the intensity of spotlike CL images varied with an exponential function of the lateral length. From transient photocurrent measurements using ultrashort pulse laser excitations, decay times τ for the present high-quality diamond were estimated to be 15 and 100ns for electrons and holes, respectively. Charge collection distances at an electric field E of 830V∕cm were deduced to be ∼190μm and over 1.2mm for electrons and holes, respectively. The former may give a high electron drift mobility of μ∼1600cm2∕Vs while the diffusion coefficients estimated for electrons are 55±14cm2∕Vs, which is comparable with or even higher than those of Si. These physical quantities demonstrate high quality of the diamond films grown by means of the high-power MPCVD method.
Impact dynamics of micromachined bidirectional electrothermal vibromotors
Authors: Shih Wen-Pin , Pai Minfan , Lin Yu-Yun , Hui Chung-Yuen , Tien Norman C.
Published: December 2004

The motion of micromachined bidirectional electrothermal vibromotors (BETV) is analyzed using an impact model that takes into account friction and contact, as well as the compliance of the slider and the impact head. This model addresses several important issues on the usage of BETV. These include (a) how to achieve the pull and push mode; (b) avoiding multiple impacts in one actuation cycle; (c) the existence of periodic solutions with the same period as the thermal actuator. Four dimensionless parameters are found to govern the dynamics of the vibromotor. Depending on these parameters, we observed periodic motion, period doubling, and aperiodic solution. A criterion is established to separate push and pull modes. An experiment that includes fabricating and characterizing a BETV is carried out to verify the ability of the model to predict the pull mode.
Measurement of the state of stress in silicon with micro-Raman spectroscopy
Authors: Harris Stephen J. , O’Neill Ann E. , Yang Wen , Gustafson Peter , Boileau James , Weber W. H. , Majumdar Bhaskar , Ghosh Somnath
Published: December 2004

Micro-Raman spectroscopy has been widely used to measure local stresses in silicon and other cubic materials. However, a single (scalar) line position measurement cannot determine the complete stress state unless it has a very simple form such as uniaxial. Previously published micro-Raman strategies designed to determine additional elements of the stress tensor take advantage of the polarization and intensity of the Raman-scattered light, but these strategies have not been validated experimentally. In this work, we test one such stategy [S. Narayanan, S. Kalidindi, and L. Schadler, J. Appl. Phys. 82, 2595 (1997)] for rectangular (110)- and (111)-orientated silicon wafers. The wafers are subjected to a bending stress using a custom-designed apparatus, and the state of (plane) stress is modeled with ABAQUS. The Raman shifts are calculated using previously published values for silicon phonon deformation potentials. The experimentally measured values for σxx, σyy, and τxy at the silicon surface are in good agreement with those calculated with the ABAQUS model.
Design of wide-gap fluoride heterostructures for deep ultraviolet optical devices
Authors: El Ouenzerfi Riadh , Ono Shingo , Quema Alex , Goto Masahiro , Sakai Masahiro , Sarukura Nobuhiko , Nishimatsu Takeshi , Terakubo Noriaki , Mizuseki Hiroshi , Kawazoe Yoshiyuki , Sato Hiroki , Ehrentraut Dirk , Yoshikawa Akira , Fukuda Tsuguo
Published: December 2004

The design of fluoride-based optical devices for deep ultraviolet applications is discussed. Variations in the band-gap energy and band structure with respect to composition are investigated for Li(1−x)KxBa(1−y)MgyF3 perovskites. The band-gap energy, lattice constant, and band structure of perovskitelike fluorides are estimated based on ab initio calculations within the local-density approximation. The lattice-matched double heterostructure with direct band-gap compounds (Li(1−x)KxBa(1−y)MgyF3 on either LiBaF3 or KMgF3 substrates) is promising for fabrication.
High-frequency electron paramagnetic resonance investigation of the Fe3+ impurity center in polycrystalline PbTiO3 in its ferroelectric phase
Authors: Meštrić Hrvoje , Eichel Rüdiger-A. , Dinse K.-P. , Ozarowski Andrew , van Tol Johan , Brunel Louis Claude
Published: December 2004

The intrinsic iron(III) impurity center in polycrystalline lead titanate was investigated by means of high-frequency electron paramagnetic resonance spectroscopy in order to determine the local-environment sensitive fine-structure parameter D. At a spectrometer frequency of 190GHz, a spectral analysis of a powder sample was unambiguously possible. The observed mean value D=+35.28GHz can be rationalized if Fe3+ ions substitute for Ti4+ at the B site of the perovskite ABO3 lattice forming a directly coordinated FeTi′–VO∙∙ defect associate. A consistent fit of the multifrequency data necessitated the use of a distribution of the D values with a variance of about 1GHz. This statistical distribution of values is probably related to more distant defects and vacancies.
Surface hardness increasing of iron alloys by nitrogen-deuterium ion implanting
Authors: Figueroa C. A. , Alvarez F.
Published: December 2004

In situ x-ray photoemission spectroscopy is used to study the deuterium and hydrogen oxygen etching effect in nitrogen-implanted iron alloys. A suitable deuterium-nitrogen mixture can increase the surface original steel hardness up to ∼40%. In similar conditions, hydrogen-nitrogen mixtures improves the hardness by ∼10%. On deuteration, the main change is the reduction of the zero-point energy of the hydrides bond. Due to this, the lower scission energy of hydrogen-metal bonds as compared with deuterium-metal bonds determines the favorable effect of deuterium on the nitriding process.
Back contact and reach-through diode effects in thin-film photovoltaics
Authors: Roussillon Y. , Karpov V. G. , Shvydka Diana , Drayton J. , Compaan A. D.
Published: December 2004

The physics of back contact effects in photovoltaic devices is revisited. We show that the back contact Schottky barrier can act in either back-diode or reach-through diode regimes. This understanding predicts that rare local spots with low back barrier hole transparency and/or weak main junctions can shunt the photocurrent thus decreasing the measured open-circuit voltage and device efficiency. We derive several more specific predictions of our model and verify them experimentally for the case of thin-film CdTe photovoltaics. Our concept has practical implications: a simple recipe leading to an efficient (13%) copper-free CdTe solar cell.
Radiation and injection effects on metal-oxide-semiconductor devices using the gate-controlled-diode technique
Authors: Palumbo Felix , Faigon Adrian N. , Campabadal Francesca
Published: December 2004

Charge trapping and interface states generation by electrical stress and gamma photons (Co60) were studied on n-channel metal-oxide-semiconductor field-effect transistors, using the gate-controlled-diode technique. Creation of states during positive charge trapping and during its recombination with electrons is made evident by the technique. The growth rate of defects and the number of interface states per trapped hole are calculated. While the number of interface states per trapped hole is within the same order of magnitude for both stress mechanisms, the injection case exhibits a turnaround of the interface states density as a function of the trapped charge, which is not seen in the radiation case. The gate-controlled diode is a sensitive technique for interface states detection at low total doses.
Oxygen nonstoichiometry and dielectric evolution of BaTiO3. Part I—improvement of insulation resistance with reoxidation
Authors: Yang G. Y. , Dickey E. C. , Randall C. A. , Barber D. E. , Pinceloup P. , Henderson M. A. , Hill R. A. , Beeson J. J. , Skamser D. J.
Published: December 2004

Impedance spectroscopy, transmission electron microscopy, and electron energy-loss spectroscopy are used to correlate local electrical properties with the microstructure and microchemistry of BaTiO3 in Ni-electrode multilayer ceramic capacitors. High densities of linear defects and some grains with structural modulations are observed in BaTiO3 grains in the as-cofired capacitors. The modulated structure is formed on {111} planes of the BaTiO3. Both types of structural defects are associated with high concentrations of oxygen vacancies. In particular, the oxygen content in the BaTiO3 grains that are in direct contact with the internal Ni electrodes is less uniform with a systematic decrease in oxygen content towards the electrode. In the capacitors that are reoxidized in a higher oxygen partial pressure at lower temperature, the BaTiO3 grains are almost free of linear defects and structural modulations and the oxygen content is homogeneous throughout the BaTiO3 active layers. A concomitant improvement in the total insulation resistance is observed.
Influence of the photodepoling parameters on quasiphase-matched second-harmonic generation and optical parametric fluorescence in polymer channel waveguides
Authors: Martin G. , Alibart O. , Hierle R. , Josse D. , Baldi P. , Zyss J.
Published: December 2004

We report on periodical modulation of the nonlinear susceptibility tensor by optical means in organic channel waveguides and its applications to second-harmonic generation and optical parametric fluorescence. The modulation is achieved by photoisomerization of a typical azo dye, Disperse Red One, through a photolithographic mask in order to transfer the desired periodicity. The choice of the illuminating beam wavelength is shown to be a tradeoff between efficient excitation and penetration depth of the photoisomerization process, the depth to be achieved being directly related to the thickness of the waveguide. For the estimated ideal wavelength, we show experimental evidence of second-harmonic generation and optical parametric flurorescence at a close to degeneracy wavelength of 1586nm. A comparison between the experimental and theoretical expected values is also presented.
Thermal effect on the oxides on Nb(100) studied by synchrotron-radiation x-ray photoelectron spectroscopy
Authors: Ma Qing , Ryan P. , Freeland J. W. , Rosenberg R. A.
Published: December 2004

Synchrotron-radiation glancing-incidence and angle-resolved photoelectron spectroscopy (PES) is used to study the oxides grown in air on a single-crystal Nb(100) surface. Both core-level and valance-band PES are measured for various heat treatments. Glancing-incidence excitation was used to characterize the outer 2-nm Nb2O5 layer without influence from the substrate and to profile through various oxides by varying the incidence angle. Immediately adjacent to this amorphous layer is a layer of NbO2, which contributes to the density of states at 1.2eV below the Fermi level. Thereafter, there exist other oxides that interface with the metal substrate. Annealing at 430K induces a “mild” chemical reaction, which is contained essentially in the oxide layer. Annealing at 550K causes oxygen diffusion into the metal and leaves an oxide layer consisting primarily of Nb2O and a roughened surface, as evidenced by angle-resolved photoemission measurements. Oxide composition alteration caused by annealing at these moderated temperatures is believed to be interface mediated.
Characterization of photonic crystal waveguides based on Fabry-Pérot interference
Authors: Märki Iwan , Salt Martin , Stanley Ross , Staufer Urs , Herzig Hans-Peter
Published: December 2004

We present two methods based on the analysis of Fabry-Pérot interference for a detailed characterization of a 90° corner in a two-dimensional photonic crystal waveguide fabricated in a thin Si membrane. These methods are a means of identifying the critical waveguide elements in the process of improving photonic crystal devices. The effects of the elements forming the photonic crystal waveguide are identified and quantified by means of a stage-by-stage analysis. By Fourier transforming the transmission spectra we observe the amount of light that is back reflected inside the waveguide and based on the fringe contrast of the Fabry-Pérot modulation we calculate the loss contribution of each waveguide element, such as the tapers and the 90° corner.
Magnetic and transport properties of Ag∕Co90Fe10 granular multilayers
Authors: Milano J. , LLois A. M. , Steren L. B. , Butera A. , Barnard J.
Published: December 2004

We report experimental results on the magnetic and transport properties of Ag∕Co90Fe10 multilayers with variable Ag and Co90Fe10 thicknesses. The magnetism samples was characterized by magnetization measurements and ferromagnetic resonance spectroscopy. These measurements show that the magnetic layers are in general composed by single-domain particles and that the shape and the size of the magnetic clusters can be changed by using an applied magnetic field (Hdep) on deposition. The resistivity of these multilayered systems is described by a mixed model that includes current-in-plane and current perpendicular plane transport contributions. The transport model proposed to explain the experimental results emphasizes the role played by the magnetic entities on the scattering processes and strongly depends on the silver layer thickness. An electrical percolative transition is observed for samples grown at Hdep>Hdep* while the magnetic properties of these samples still show a granularlike behavior.
Two-dimensional distributed feedback lasers using a broadband, red polyfluorene gain medium
Authors: Heliotis G. , Xia R. , Bradley D. D. C. , Turnbull G. A. , Samuel I. D. W. , Andrew P. , Barnes W. L.
Published: December 2004

We report the fabrication of widely tuneable (627–702nm) optically pumped two-dimensional distributed feedback polymer lasers that utilize a red-emission fluorene copolymer as the active gain medium. The lasers exhibit efficient, low threshold operation and emit highly directional output beams as a result of the enhanced two-dimensional photonic confinement provided by the employed resonator. Their emission and operating characteristics are described in detail. We demonstrate that the very wide spectral range (Δλ⩾75nm) over which these lasers can be systematically tuned is in very good agreement with theoretical predictions based on a simple waveguide model. In addition, we show that the lasers have long operating lifetimes τ1∕2⩾2×107 pulses and we discuss the impact that degradation has on the laser output characteristics.
Transport in tunneling recombination junctions: A combined computer simulation study
Authors: Vukadinović M. , Smole F. , Topič M. , Schropp R. E. I. , Rubinelli F. A.
Published: December 2004

The implementation of trap-assisted tunneling of charge carriers into numerical simulators ASPIN and D-AMPS is briefly described. Important modeling details are highlighted and compared. In spite of the considerable differences in both approaches, the problems encountered and their solutions are surprisingly similar. Simulation results obtained for several tunneling recombination junctions made of amorphous silicon (a-Si), amorphous silicon carbide (a-SiC), or microcrystalline silicon (μc-Si) are analyzed. Identical conclusions can be drawn using either of the simulators. Realistic performances of a-Si∕a-Si tandem solar cells can be reproduced with simulation programs by assuming that extended-state mobility increases exponentially with the electric field. The same field-enhanced mobilities are needed in single tunneling recombination junctions in order to achieve measured current levels. Temperature dependence of the current-voltage characteristics indicates that the activation energy of enhanced mobilities should be determined. Apparent discrepancies between simulation results and measurements are explained and eliminated making use of Gill’s law. For application in tandem and triple solar cell structures, tunneling recombination junctions made of (μc-Si) are the most promising of all examined structures.
Misfit dislocation formation in the AlGaN∕GaN heterointerface
Authors: Floro J. A. , Follstaedt D. M. , Provencio P. , Hearne S. J. , Lee S. R.
Published: December 2004

Heteroepitaxial growth of AlxGa1−xN alloy films on GaN results in large tensile strain due to the lattice mismatch. During growth, this strain is partially relieved both by crack formation and by the coupled introduction of dense misfit dislocation arrays. Extensive transmission electron microscopy measurements show that the misfit dislocations enter the film by pyramidal glide of half loops on the 1∕3⟨11̱23⟩∕{112̱2} slip system, which is a well-known secondary slip system in hcp metals. Unlike the hcp case, however, where shuffle-type dislocations must be invoked for this slip plane, we show that glide-type dislocations are also possible. Comparisons of measured and theoretical critical thicknesses show that fully strained films can be grown into the metastable regime, which we attribute to limitations on defect nucleation. At advanced stages of relaxation, interfacial multiplication of dislocations dominates the strain relaxation process. This work demonstrates that misfit dislocations are important mechanisms for relaxation of strained III-nitride heterostructures that can contribute significantly to the overall defect density.
Long-range absorption enhancement in organic tandem thin-film solar cells containing silver nanoclusters
Authors: Rand Barry P. , Peumans Peter , Forrest Stephen R.
Published: December 2004

We investigate the optical properties of silver nanoparticles used in tandem ultrathin-film organic photovoltaic cells. Experimental results indicate that the enhancement of an incident optical field persists into an organic dielectric for distances of up to 10nm from the center of an array of approximately 5-nm-diameter nanoparticles. Furthermore, this enhancement exists far from the resonant particle surface-plasmon excitation energy. We propose a model to explain this long-range enhancement and investigate the role that cluster spacing, shape, and an embedding dielectric medium with a complex dielectric constant play in determining plasmon enhancement. This effect is shown to increase the efficiency of tandem organic solar cells, and the implications for further solar cell efficiency improvements are discussed.
Influence of a Si layer intercalated between Si0.75Ge0.25 and Ni on the behavior of the resulting NiSi1−uGeu film
Authors: Seger J. , Jarmar T. , Ericson F. , Smith U. , Hållstedt J. , Zhang Z.-B. , Zhang S.-L.
Published: December 2004

The interaction of Ni films with epitaxially grown Si-capped and not capped Si0.75Ge0.25 layers on Si(100) at 500°C leads to the formation of NiSi1−uGeu films as a bilayer NiSi on NiSi0.75Ge0.25 with a rather clear compositional boundary. In the absence of a Si cap at the surface, NiSi0.75Ge0.25 is formed on NiSi. Epitaxy of NiSi on NiSi0.75Ge0.25, and vice versa, occurs across the compositional boundary. The crystallographic orientation of the NiSi1−uGeu films is strongly affected by the initial layer thicknesses and the layer sequence. Without a Si cap, the NiSi1−uGeu films show an increased fiber texture with increasing Si0.75Ge0.25 thickness. In the presence of a Si cap, on the other hand, the texture collapses into a random orientation already for thin caps. Rapid diffusion of Ge at 500°C results in the presence of some Ge at the NiSi∕Si interface for a NiSi0.75Ge0.25∕NiSi∕Si structure. This diffusion is accompanied by an increased roughness at the NiSi∕Si interface, as compared to the quite flat NiSi∕Si interface in the absence of Ge. For thin Si caps, severe interface roughening with thick NiSi0.75Ge0.25 grains protruding deeply into the remaining Si0.75Ge0.25 is observed.
X-ray diffraction study on residual stress and preferred orientation in thin titanium films subjected to a high ion flux during deposition
Authors: Birkholz M. , Genzel C. , Jung T.
Published: December 2004

The structural properties of thin Ti films were studied by x-ray scattering techniques aiming at an improved understanding of residual stress and preferred orientation in thin metal films when subjected to a high ion flux during deposition. The samples were prepared by gas-flow sputtering and by subjecting the substrate to a midfrequency bias during deposition. Large arrival ratios of ions over deposited atoms, Ji∕Ja, could be realized by this processing. Some hundred nanometers thin Ti layers were characterized by x-ray reflectometry, symmetric θ∕2θ diffraction, pole figure analysis, and residual stress measurements by the sin2ψ and by the scattering vector technique, the latter method enabling a depth-resolved determination of stress fields. Whereas the stress state in an unbiased sample turned out to be tensile accompanied by a dominating (00.l) texture component, the biased samples were found to exhibit an overall compressive stress and a (h0.0) fiber texture. The results for the unbiased sample could be explained by a minimization of the elastic energy density which favors the preferred orientation of crystallographic c axes normal to the substrate plane. The biased samples closely resembled macroscopic Ti workpieces that were subjected to severe plastic deformation as was indicated by (i) the (h0.0) fiber texture along the load direction, (ii) the large compressive in-plane residual stress σ⊥, and (iii) the depth-resolved course of σ⊥(z). It is concluded that a high ion flux onto a growing Ti film has the same effect as a uniaxial mechanical load stress would have.
Angular and wavelength selectivity of parasitic holograms in cerium doped strontium barium niobate
Authors: Ellabban Mostafa A. , Fally Martin , Imlau Mirco , Woike Theo , Rupp Romano A. , Granzow Torsten
Published: December 2004

The angular and wavelength selectivity of noise gratings in doped strontium barium niobate crystals for samples with two different thicknesses are investigated. We found that an offset in the optimum reconstruction angle with respect to the recording geometry occurs as a function of the thickness. This is a consequence of the unidirectional amplification. Thus the scattering intensity for the thinner sample is considerably lowered at the very recording conditions. We analyze the experimental results in terms of the kinematic theory of diffraction.
The relation between phase transformation and onset of thermal degradation in nanoscale CoSi2-polycrystalline silicon structures
Authors: van Dal M. J. H. , Jawarani D. , van Berkum J. G. M. , Kaiser M. , Kittl J. A. , Vrancken C. , de Potter M. , Lauwers A. , Maex K.
Published: December 2004

Insight in the thermal degradation phenomena and the relation to phase transformation is presented for cobalt disilicide (CoSi2) on narrow polysilicon lines (linewidth ranging from 30nmto1μm) using electrical and morphological analysis in the temperature range 650–900°C. When polysilicon lines are scaled laterally to 30nm, an abrupt CoSi2 sheet resistance increase (>1000Ω∕sq) is observed, which is attributed to silicide agglomeration. At localized positions along the 30-nm Co-silicided lines, Si/silicide layer inversion is observed. At low formation temperature (650°C), no agglomeration phenomena occur, but incidentally the transition from CoSi into CoSi2 is delayed on the 30-nm-wide polysilicon structures to such an extent that the CoSi2 growth virtually stops. Only using nitrogen implantation through CoSi and a CoSi2 formation temperature of 900°C, low sheet resistance values are obtained for 35-nm-wide polysilicon lines. Based on the results obtained in the present work, we propose that the abrupt increase in the CoSi2 sheet resistance for the narrow polysilicon lines is a consequence of a decrease in the availability of nucleation sites leading to the reduction of the CoSi∕CoSi2 transformation rate, which, in turn, results in the agglomeration of Co silicide at elevated temperature.
Contact electrification of high-K oxides studied by electrostatic force microscopy
Authors: Lambert J. , Saint-Jean M. , Guthmann C.
Published: December 2004

In order to clarify the mechanisms of charge transfer on insulating surfaces by contact electrification, we performed charge-transfer experiments on high-K oxides using the tip of an electrostatic force microscope. In particular, we investigated the influence of the applied voltage between the tip and the surface and the contact duration on the amount of transferred charges on Al2O3. The electronic motion in the insulating material is analyzed in terms of hopping processes assisted by the electric field created by the tip inside the oxide. We show that this electric field must be described by a three-dimensional model. In this frame, the transfer mechanism is analyzed as an instantaneous wetting of the surface by the charges—the surface being a region of large trap concentration—followed by a progression of the charges inside the oxide.
The Meyer-Neldel rule for diodes in forward bias
Authors: Widenhorn Ralf , Fitzgibbons Michael , Bodegom Erik
Published: December 2004

We analyzed the temperature dependence of the forward current of a silicon diode. Instead of representing the data in the ordinarily used current versus voltage graph, the currents are plotted for different voltages as a function of the inverse temperature. The constant voltage curves can be fitted linearly and the extrapolations of the fits seem to merge to one common focal point. Hence, we demonstrate that a real diode follows the Meyer-Neldel rule (MNR). It is shown that the MNR is due to a shift of the current from ideal-diode to high-injection-diode behavior. We will argue that the merging of the different Arrhenius plots toward one focal point, and hence a MNR, can be the result of various mechanisms. The general requirements to observe a MNR are not very restrictive. It is therefore not surprising that the MNR has been observed in a multitude of systems. The origin that gives rise to the MNR can be manifold and allows for different models to explain its occurrence.
530
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mW
quasi-white-light generation using all-solid-state laser technique
Authors: Li H. X. , Fan Y. X. , Xu P. , Zhu S. N. , Lu P. , Gao Z. D. , Wang H. T. , Zhu Y. Y. , Ming N. B. , He J. L.
Published: December 2004

We report a quasi-white-light laser, which was generated by an all-solid-state laser technique. The approach is to use a cascaded optical superlattice in a LiTaO3 crystal to directly frequency double and triple a diode-pumped, Q-switched dual-wavelength Nd:YVO4 laser. Three primary color lights, red at 671nm, green at 532nm, and blue at 447nm, were simultaneously generated in a proper proportion, forming a quasicontinuous, quasi-white-light laser with a maximum output power of 530mW under a pump power of 18W. From the result, a more effective quasi-white-light output can be anticipated if the superlattice is fabricated in a stoichiometric LiTaO3 crystal and the double resonant technique is further optimized.
Magnetotransport probing of the quality of the heterointerfaces and degree of symmetry of the potential profile of quantum wells in the valence band of the Ge1−xSix/Ge/Ge1−xSix heterosystem
Authors: Yakunin M. V. , Alshanskii G. A. , Arapov Yu. G. , Neverov V. N. , Harus G. I. , Shelushinina N. G. , Kuznetsov O. A. , de Visser A. , Ponomarenko L.
Published: December 2004

It is shown that in a periodic system of p-type Ge1−xSix/Ge/Ge1−xSix quantum wells having a Ge layer more than ∼30 nm wide, the hole gas in each Ge layer is separated into two two-dimensional sublayers concentrated near opposite boundaries of the layer. This follows from the vanishing of the quantum Hall effect plateau and of the corresponding minimum of the longitudinal magnetoresistance for a filling factor ν=1. Here positive magnetoresistance is observed, which is attributed to the presence of two types of holes with different mobilities. A quantitative analysis shows that these are mainly heavy holes having different mobilities in the sublayers that form. The difference of the mobilities indicates that the opposite heterointerfaces of the Ge layers are of different quality. It follows from an analysis of the shape of the quantum Hall effect plateau for ν=2 that the densities of holes in the sublayers formed are close and, consequently, that the profile of the potential wells is close to symmetric.
Parallel magnetotransport in multiple quantum well structures
Authors: Sheregii E. M. , Ploch D. , Marchewka M. , Tomaka G. , Kolek A. , Stadler A. , Mleczko K. , Strupiński W. , Jasik A. , Jakiela R.
Published: December 2004

The results of investigations of parallel magnetotransport in AlGaAs/GaAs and InGaAs/InAlAs/InP multiple quantum-well structures (MQWs) are presented. The MQWs were obtained by metalorganic vapor phase epitaxy with different shapes of the QWs, numbers of QWs, and levels of doping. The magnetotransport measurements were performed in a wide region of temperatures (0.5–300 K) and at high magnetic fields up to 30 T (B is perpendicular to and the current is parallel to the plane of the QW). Three types of observed effects are analyzed: the quantum Hall effect and Shubnikov—de Haas oscillations at low temperatures (0.5–6 K) and magnetophonon resonance at higher temperatures (77–300 K).
Nonmonotonic temperature dependence of the resistivity of p-Ge/Ge1−xSix in the region of the metal–insulator transition
Authors: Arapov Yu. G. , Harus G. I. , Shelushinina N. G. , Yakunin M. V. , Neverov V. N. , Kuznetsov O. A. , Ponomarenko L. , De Visser A.
Published: December 2004

In a two-dimensional (2D) hole system (multilayer p-Ge/Ge1−xSix) heterostructure with conductivity σ≈e2/h at low temperatures (T≈1.5 K) a transition from the insulator phase (dσ/dT>0) to a “metallic” phase (dσ/dT<0) is observed as the temperature is lowered, behavior that is in qualitative agreement with the predictions of the Finkelstein theory. In a magnetic field B perpendicular to the plane of the 2D layer one observes positive magnetoresistance depending only on the ratio B/T. We attribute the positive magnetoresistance effect to the suppression of the triplet channel of Fermi-liquid electron–electron interaction by the magnetic field owing to the strong Zeeman splitting of the hole energy levels.
Molecular epitaxy and the electronic properties of Ge/Si heterosystems with quantum dots
Authors: Dvurechensky A. V. , Nikiforov A. I. , Pchelyakov O. P. , Teys S. A. , Yakimov A. I.
Published: December 2004

We present the results of a study of the processes of preparation and the determination of the electrical and optical characteristics of arrays of Ge nanoislands in Si (artificial “atoms”) with a discrete energy spectrum, which is manifested all the way up to room temperature. We give a brief analysis of the current ideas about the mechanisms of the initial stage of self-formation and ordering of ensembles of nanoclusters in the heteroepitaxy of Ge on Si. The main factors that determine the spectrum of states are the size quantization and Coulomb interaction of the carriers. It is shown that a new factor arising in an array of quantum dots and which distinguishes it from the situation for an isolated quantum dot is the presence of Coulomb correlations between islands. The rate of emission and the cross section for trapping of holes are determined as functions of the energy level depth. The values of the cross sections are several orders of magnitude greater than the known values in Si. The electron transport along the layers of quantum dots is occurs via hopping conduction, the value of which oscillates as the degree of occupation of the occupation of the islands by holes; this can serve as a working principle for electronic data transmission circuits based on quantum dots. It is shown that Ge quantum dots can be used to construct a photodetector tunable over the near- and mid-IR ranges.
Mechanisms of radiative and nonradiative recombination in ZnSe:Cr and ZnSe:Fe
Authors: Godlewski M. , Surma M. , Ivanov V. Yu. , Surkova T. P.
Published: December 2004

Possible applications of ZnSe:Cr in optoelectronics are discussed. It is shown that 2+ to 1+ photo-ionization of chromium results in efficient pumping of Cr2+ intrashell emission and in energy up-conversion from green to blue. A distinct difference in efficiency of the energy up-conversion is observed between chromium- and iron-doped ZnSe samples. This difference we relate to a very efficient Auger mechanism of photoluminescence quenching in Fe-doped samples. We further demonstrate an anticorrelation of the intensities of mid-infrared Cr emission and up-converted blue emission of ZnSe.
Effects of resonance scattering of electrons by donor impurities in semiconductors
Authors: Okulov V. I.
Published: December 2004

A generalized formulation of the Friedel approach is given and a theoretical description of the effects of resonance scattering of conduction electrons by donor impurities in semiconductors is developed. The stabilization of the electron density when the Fermi energy reaches the resonance level and the temperature and concentration dependences of the electron mobility and magnetic susceptibility of the localized resonance states are considered in detail. The limits of applicability of the results are discussed.
Electrophysical properties of semimagnetic solid solutions Hg1−xMnxTe
Authors: Nesmelova I. M. , Ryzhkov V. N. , Ibragimova M. I. , Petukhov V. Yu.
Published: December 2004

A comprehensive study of the electrophysical properties of the semimagnetic ternary solid solution Hg1−xMnxTe, an alternative material to Hg1−xCdxTe, is reported. The charge-carrier scattering, optical, photoelectric, and magnetic properties of the material are investigated. Values are obtained for the effective masses of electrons and holes, the ionization energy of the acceptor level, and the g factor of the charge carriers as functions of the manganese telluride concentration and the densities of electrons and holes at temperatures of 300 and 77 K. It is shown that the methods of radio spectroscopy can be used for diagnostics of the semimagnetic material. Photodiode structures having characteristics close to the values for working in the background-limited regime are obtained by the use of Schottky barriers, diffusion in mercury vapor, and implantation of B+ ions into p-Hg1−xMnxTe samples.
Nonequilibrium processes and ferroelectric phase transition in PbGeTe(Ga) crystals
Authors: Akimov B. A. , Pryadun V. V. , Khokhlov D. R. , Ryabova L. I. , Shtanov V. I. , Slynko E. I.
Published: December 2004

The impedance of single-crystal samples of PbTe(Ga) and Pb1−xGexTe(Ga) (0⩽x⩽0.095) is investigated in the frequency range from 102 to 106 Hz and temperature range 4.2–300 K. The temperature dependence of the capacitance of all the Pb1−xGexTe(Ga) samples studied exhibited two types of features. These are a pronounced peak at a temperature T=Tp, caused by a dielectric anomaly at the ferroelectric phase transition, and a characteristic strong frequency dependence of the rise in capacitance in the temperature region T<100 K. The amplitude of the low-temperature effect decreases monotonically with increasing frequency f, and for f>105 Hz the effect practically vanishes. This behavior of the capacitance at such low frequencies may be associated with charge exchange processes in the impurity subsystem. The experimentally determined value of Tp is substantially higher than the characteristic temperatures for the appearance of long-term relaxation processes, in particular, the delayed photoconductivity. Consequently, the change of the charge states in the impurity subsystem is not accompanied by dielectric anomalies of the crystal lattice as a whole, and the possible restructuring of the lattice is of a local character.
Ultrasonic investigation of a phase transition in ZnSe:Ni
Authors: Gudkov V. V. , Petrov S. B. , Lonchakov A. T. , Zhevstovskikh I. V. , Sokolov V. I. , Gruzdev N. B.
Published: December 2004

Ultrasonic experiments on ZnSe and ZnSe:Ni crystals with impurity concentrations of 5.5×1019 cm−3 have revealed precursor phenomena at temperatures much higher than the temperature of the structural transition (Tc=14.5 K). It is found that the elastic constant C44 softens while C11 and C12 become stiffer as Tc is approached from above; the strains associated with the order parameter are transverse deformations of the ε4 type. A brief symmetry analysis of the possibilities of formation of a tetragonal phase is given. The Jahn–Teller effect and displacement of the charge density on the chemical bonds are discussed as probable causes of the low-temperature phase transition.
Gondwanan mesotherms and cosmopolitan eurytherms: effects of temperature on the development and survival of Australian Chironomidae (Diptera) from tropical and temperate populations
Authors: McKie Brendan G. , Cranston Peter S. , Pearson Richard G.
Published: December 2004

In temperate regions of the northern hemisphere, where stream thermal regimes fluctuate seasonally and predictably, temperature has a role in niche segregation and maintenance of patterns of lotic diversity and distribution, as described by the ‘Thermal Equilibrium Hypothesis’. In Australia, the role of temperature in regulating patterns of diversity and distribution has been obscure, as seasonal variation in stream temperatures can be exceeded by stochastic fluctuation in flow. The thermal responses of five lotic Chironomidae (Diptera) species, contrasting in biogeographic (evolutionary) history, from warm tropical and cool temperate Australian populations, were investigated. All species, including postulated cool-stenotherms, showed broadly eurythermic developmental and morphological responses, and maintained both survivorship and oocyte production at elevated temperatures despite reductions in overall body size. There were subtle differences among species according to biogeographic affinity, with tolerances of Gondwanan species, which were narrower than those of cosmopolitan species, best characterised as ‘mesothermic’, but there was little divergence between populations. These results have implications for the understanding of diversity and distribution of Australian chironomids, and indicate that applicability of the Thermal Equilibrium Hypothesis to Australian lotic faunas may be limited.