bluuebunny/crossref_metadata_2025 huggingface dataset:
Enhanced internal quantum efficiency and light extraction efficiency from textured GaN∕AlGaN quantum wells grown by molecular beam epitaxy
Authors: Cabalu J. S. , Thomidis C. , Moustakas T. D. , Riyopoulos S. , Zhou Lin , Smith David J.
Published: April 2006

Ga N ∕ Al 0.2 Ga 0.8 N multiple quantum wells (MQWs) were grown by molecular beam epitaxy on randomly textured and atomically smooth (0001) GaN templates. Smooth and textured GaN templates were deposited on (0001) sapphire substrates by varying the III∕V ratio and the substrate temperature during growth by the hydride vapor-phase epitaxy method. We find that the MQWs replicate the texture of the GaN template, which was found to have a Gaussian distribution. The peak photoluminescence intensity from the textured MQWs is always higher than from the smooth MQWs and for GaN (7nm)∕Al0.2Ga0.8N (8nm) MQWs, it is 700 times higher than that from similarly produced MQWs on smooth GaN templates. This result is attributed partly to the enhancement in light extraction efficiency and partly to the enhancement in internal quantum efficiency. The origin of the increase in internal quantum efficiency is partly due to the reduction of the quantum-confined Stark effect, since the polarization vector intersects the quantum well (QW) planes at angles smaller than 90°, and partly due to the charge redistribution in the QWs caused by the polarization component parallel to the planes of the QWs.
Convergent beam electron diffraction investigation of strain induced by Ti self-aligned silicides in shallow trench Si isolation structures
Authors: Armigliato Aldo , Spessot Alessio , Balboni Roberto , Benedetti Alessandro , Carnevale Gianpietro , Frabboni Stefano , Mastracchio Gianfranco , Pavia Giuseppe
Published: April 2006

The deformation induced onto silicon by the formation of Ti self-aligned silicides (salicides) in shallow trench isolation structures has been investigated by the convergent beam electron diffraction technique (CBED) in the transmission electron microscope (TEM). The splitting of the high order Laue zone (HOLZ) lines in the CBED patterns taken in TEM cross sections close to the salicide/silicon interface has been explained assuming that the salicide grains induce a local bending of the lattice planes of the underlying matrix. This bending, which affects in opposite sense the silicon areas below adjacent grains, decreases with the distance from the interface, eventually vanishing at a depth of 300–400nm. The proposed strain field has been implemented into a fully dynamical simulation of the CBED patterns and has proved to be able to reproduce both the asymmetry of the HOLZ line splitting and the associated subsidiary fringes. This model is confirmed by the shift of a Bragg contour observed in large angle CBED patterns, taken in a cross section cut along a perpendicular direction. The whole experimental results cannot be explained by just a strain relaxation of the TEM cross section, induced by the salicide film onto the underlying silicon.
Effects of x-ray irradiation on polycrystalline silicon, thin-film transistors
Authors: Li Yixin , Antonuk Larry E. , El-Mohri Youcef , Zhao Qihua , Du Hong , Sawant Amit , Wang Yi
Published: April 2006

The effects of x-ray irradiation on the transfer and noise characteristics of excimer-laser-annealed polycrystalline silicon (poly-Si) thin-film transistors (TFTs) have been examined at dose levels up to 1000Gy. Parameters including mobility, threshold voltage, subthreshold swing, and leakage current, as well as flicker and thermal noise coefficients, were determined as a function of dose. In addition, the physical mechanisms of the observed changes in these parameters are analyzed in terms of radiation-generated charge in the gate oxide, at the Si–SiO2 interface, and at the grain boundaries. The results of the studies indicate that poly-Si TFTs exhibit sufficient radiation tolerance for the use in active-matrix flat-panel imagers for most medical x-ray applications.
CoMnSb: A magnetocaloric material with a large low-field magnetic entropy change at intermediate temperature
Authors: Li Shandong , Liu Meimei , Huang Zhigao , Xu Feng , Zou Wenqin , Zhang Fengming , Du Youwei
Published: April 2006

A large magnetocaloric effect (MCE) has been achieved in a rare-earth-free CoMnSb alloy at low magnetic field and intermediate temperature. It is interesting to note that a relatively large magnetic entropy change ∣ΔSM∣ of 2.06J∕kgK with ΔH=0.9T was obtained around 471K. Arrott plots at various temperatures reveal that a second order magnetic transformation occurs near the Curie temperature. The origin of the large ΔSM under the conditions of intermediate temperature and low field is attributed to the magnetic transition from ferromagnetic to paramagnetic. These results suggest that CoMnSb alloy is a good candidate for large MCE when being utilized in intermediate temperature refrigeration systems for some special cases. This investigation demonstrates that some transition metal alloys with large magnetic moments may exhibit large MCE, which extends the region for exploring useful MCE materials.
High concentration in-diffusion of phosphorus in Si from a spray-on source
Authors: Bentzen A. , Holt A. , Christensen J. S. , Svensson B. G.
Published: April 2006

High concentration in-diffusion of phosphorus in both Czochralski grown and solar grade multicrystalline Si from a spray-on liquid source has been studied by secondary ion mass spectrometry and electrochemical capacitance-voltage profiling. By extraction of the concentration dependent effective diffusivity employing the Boltzmann-Matano analysis, we adapt an integrated diffusion model based on a previous work by Uematsu [J. Appl. Phys. 82, 2228 (1997)], in order to gain insight into the mechanisms governing such in-diffusions. We find that in the tail region of the profiles, diffusion is mediated by interaction with Si self-interstitials, whereas a vacancy mechanism via doubly negative vacancies dominates in the higher concentration region towards the surface, in correspondence with a previous analysis by Fair and Tsai [J. Electrochem. Soc. 124, 1107 (1977)]. Moreover, we find that both the vacancy and interstitial mechanisms can be described by an Arrhenius behavior, exhibiting apparent activation energies of 5.2±0.3 and 2.1±0.1eV, respectively. The results form the basis for a simplified diffusion simulation, allowing simulation and subsequent optimization of phosphorus diffused emitters commonly employed in Si solar cells.
Local structure of LiNi0.5Mn0.5O2 cathode material probed by in situ x-ray absorption spectroscopy
Authors: Deb Aniruddha , Bergmann Uwe , Cramer Stephen P. , Cairns Elton J.
Published: April 2006

The layered nickel manganese oxide of LiNi0.5Mn0.5O2 is a technologically important and promising cathode material for lithium-ion batteries. A study of the structural perturbation, charge compensation mechanism, and the valence of the constituent transition metal elements (Ni and Mn) during electrochemical cycling was performed using x-ray absorption spectroscopy (XAS) employing an in situ technique with an electrochemical cell designed for long term x-ray experiments. We present the changes in the oxidation state, bond distance, and coordination number of the Ni and Mn absorbers as a function of the state of charge of the material during electrochemical cycling at moderate rate through a typical Li-ion battery operating voltage range (2.9–4.7V). The oxidation states of the transition metals in LiNi0.5Mn0.5O2 are Mn4+ and Ni2+. The x-ray absorption near edge spectra reveal that on delithiation of Li1−xNi0.5Mn0.5O2 the Mn is electrochemically inactive and remains at Mn4+ whereas the Ni is oxidized from Ni2+ to almost Ni4+ through an intermediate stage of Ni3+. A quantitative picture of the Ni valence during the charging process was obtained employing Faraday’s law calculations in combination with the XAS results. The Faraday’s law calculation indicated that at the end of the charge Ni is at Ni3.91+ in close agreement with the XAS results where Ni is found to be at Ni3.88+ at the end of the charge. Analysis of the extended x-ray absorption fine structure shows that during cycling the material retains its crystallographic symmetry and good structural short-range order which should lead to stable cycling.
Cyclone-anticyclone asymmetry of large-scale wakes in the laboratory
Authors: Perret G. , Stegner A. , Farge M. , Pichon T.
Published: April 2006

We performed an experimental study of large-scale wakes in a rotating shallow-water layer. Standard particle image velocimetry was used to measure the horizontal velocity field, while a laser-induced fluorescence technique was used to measure the geopotential deviation (i.e., the interface deviation). According to these measurements, we were able to quantify the dynamics in a wide region of parameter space beyond the quasi-geostrophic regime. For obstacles larger than the deformation radius and with small Rossby numbers, a significant asymmetry occurs in the wake between cyclonic and anticyclonic vortices. These parameters correspond to a frontal geostrophic regime with the relative interface deviation being larger than 0.1–0.2. In this case, anticyclones remain coherent and circular, whereas cyclones tend to be elongated and distorted. More surprisingly, for some extreme cases, coherent cyclones do not emerge at all, and only an anticyclonic vortex street appears several diameters behind the obstacle. The transition from a quasi-geostrophic to a frontal geostrophic regime is characterized by a strong increase in the Strouhal number, which can reach a value up to 0.6. Hence, we found that a large-scale wake could differ strongly from the classical Karman street when the relative geopotential deviation becomes larger than the Rossby number.
Exact quantum dynamics study of the O++H2(v=,j=)→OH++H ion-molecule reaction and comparison with quasiclassical trajectory calculations
Authors: Martínez Rodrigo , Lucas Josep M. , Giménez Xavier , Aguilar Antonio , González Miguel
Published: April 2006

The close-coupling hyperspherical (CCH) exact quantum method was used to study the title barrierless reaction up to a collision energy (ET) of 0.75eV, and the results compared with quasiclassical trajectory (QCT) calculations to determine the importance of quantum effects. The CCH integral cross section decreased with ET and, although the QCT results were in general quite similar to the CCH ones, they presented a significant deviation from the CCH data within the 0.2–0.6eV collision energy range, where the QCT method did not correctly describe the reaction probability. A very good accord between both methods was obtained for the OH+ vibrational distribution, where no inversion of population was found. For the OH+ rotational distributions, the agreement between the CCH and QCT results was not as good as in the vibrational case, but it was satisfactory in many conditions. The kk′ angular distribution showed a preferential forward character, and the CCH method produced higher forward peaks than the QCT one. All the results were interpreted considering the potential energy surface and plots of a representative sampling of reactive trajectories.
Vibration of a double-walled carbon nanotube aroused by nonlinear intertube van der Waals forces
Authors: Xu K. Y. , Guo X. N. , Ru C. Q.
Published: April 2006

Vibration of a double-walled carbon nanotube aroused by nonlinear interlayer van der Waals (vdW) forces is studied. The interlayer vdW forces as a nonlinear function are described by the interlayer spacing. The inner and outer carbon nanotubes are modeled as two individual elastic beams. Detailed results are demonstrated for double-walled carbon nanotubes (DWCNTs) with an aspect ratios of 10 and 20, based on the simply supported, fixed, or free end conditions, respectively. Harmonic balance method is used to analyze the relation between the amplitudes of deflection and the frequencies of coaxial and noncoaxial free vibrations. Our results indicate that the nonlinear factors of vdW forces have little effect on the coaxial free vibration, and that the deflection amplitudes increase rapidly with the increasing frequency, which are almost the same with those of the linear free vibration. On the other hand, the nonlinear factors of vdW forces have a great effect on noncoaxial free vibration. The relation between the deflection amplitudes and the frequencies shows nonlinear trend, which indicates that the aspect ratio and end condition almost have no affect on the noncoaxial amplitudes of DWCNTs.
The effect of a background shear current on large amplitude internal solitary waves
Authors: Choi Wooyoung
Published: April 2006

Large amplitude internal waves interacting with a linear shear current in a system of two layers of different densities are studied using a set of nonlinear evolution equations derived under the long wave approximation without the smallness assumption on the wave amplitude. For the case of uniform vorticity, solitary wave solutions are obtained under the Boussinesq assumption for a small density jump, and the explicit relationship between the wave speed and the wave amplitude is found. It is shown that a linear shear current modifies not only the wave speed, but also the wave profile drastically. For the case of negative vorticity, when compared with the irrotational case, a solitary wave of depression traveling in the positive x direction is found to be smaller, wider, and slower, while the opposite is true when traveling in the negative x direction. In particular, when the amplitude of the solitary wave propagating in the negative x direction is greater than the critical value, a stationary recirculating eddy appears at the wave crest.
Observation of persistent photoconductivity in 2H-MoSe2 layered semiconductors
Authors: Lee Y. C. , Shen J. L. , Chen K. W. , Lee W. Z. , Hu S. Y. , Tiong K. K. , Huang Y. S.
Published: April 2006

We report the observation of persistent photoconductivity (PPC) effect in 2H-MoSe2 layered semiconductors. The decay behavior of PPC can be well described by a stretch-exponential function. Experimental results indicate that the lattice relaxation of DX-like impurity is responsible for PPC in MoSe2. In addition, the small capture barrier created by lattice relaxation and the temperature-dependent resistance measurement exhibit that the magnitude of PPC effect in MoSe2 is small, consistent with the PPC behavior in indirect gap materials.
Formation of self-organized anisotropic polymer structures by confined dynamic instability
Authors: Kwon S. Joon
Published: April 2006

We report on the formation of self-organized anisotropic structures of the polymer thin films induced by the wave interaction-driven dynamic instability confined by corrugated elastomeric molds. In the initial stage of the morphological instability, self-organized anisotropic structures such as a regularly undulated surface wave pattern formed on the ridge followed by the formation of a highly ordered array of droplets on the wall and a surface wave on the groove. The self-organization in the initial stage is dominated by the formation of the surface wave on the ridge accompanying a period of half the first harmonic wavelength of the mold corrugation. In the final stage of the instability, undulated polymer structure on the ridge is transformed into a periodic array of droplets with the first harmonic wavelength of the mold corrugation, while the undulation in the polymer layer on the groove is nearly sustained. The self-organized anisotropic polymer structures can provide simpler methods of fabricating complicated three-dimensional polymeric structures.
Electrical properties of as-grown molecular beam epitaxy high-k gate dielectrics deposited on silicon
Authors: Goldenblum A. , Pintilie I. , Buda M. , Popa A. , Lisca M. , Botila T. , Teodorescu V. , Dimoulas A. , Vellianitis G.
Published: April 2006

The carrier transport mechanisms in as-grown LaAlO3 and La2Hf2O7 high-k insulator layers deposited on n- and p-Si were deduced from temperature dependent C-V and I-V characteristics correlated with photoelectric measurements. Large, parallel shifts in the high frequency C-V curves are explained by the presence of a large density of interface states and an approximate analytical formula relating the density of states to the C-V shift is deduced. The space charge limited current is explained by the existence of impurity channels situated energetically near the conduction or valence band of silicon.
Photoluminescence study of potential fluctuations in thin layers of Cu(In0.75Ga0.25)(SySe1−y)2
Authors: Schumacher S. A. , Botha J. R. , Alberts V.
Published: April 2006

Thin layers of the pentenary material Cu(In,Ga)(S,Se)2 (CIGSSe) are studied using photoluminescence (PL) spectroscopy. The layers were produced by a two-stage deposition process, which produces thin chalcopyrite films with a high degree of compositional uniformity throughout the thickness of the layer. It is shown that potential fluctuations, caused by compensation and high concentrations of defects, dominate the PL behavior of the investigated layers. This is inferred from a number of PL studies such as variable temperature and excitation power measurements. It is deduced that the average amplitude of fluctuations increases with increasing sulfur mole fraction S∕(Se+S). Thus, sulfur incorporation increases the degree of compensation (and hence the charged defect concentration). From the strong asymmetry and the observed weak shift of the low energy tails in the PL spectra with increasing sulfur content, it is also concluded that a significant fraction of defect states associated with the quaternary CIGSSe is still present in the pentenary layers.
The high temperature thermoelectric performances of Zr0.5Hf0.5Ni0.8Pd0.2Sn0.99Sb0.01 alloy with nanophase inclusions
Authors: Chen L. D. , Huang X. Y. , Zhou M. , Shi X. , Zhang W. B.
Published: April 2006

The Zr0.5Hf0.5Ni0.8Pd0.2Sn0.99Sb0.01∕ZrO2 nanocomposites have been prepared by spark plasma sintering technique. The thermoelectric properties were measured in the temperature range of 300–900K. Thermal conductivity for the nanocomposites is considerably reduced, because the nanophase scatters the thermal phonons which transport most of the heat in the ZrNiSn-based alloy. Seebeck coefficient for the nanocomposites is enhanced due to the potential barrier scattering. The dimensionless figure of merit ZT is improved, with ZT≈0.75 at 800K for the samples containing 9vol% ZrO2 nanoinclusions. The results obtained suggest that considerable improvements in the thermoelectric figure of merit may be obtainable by inducing appropriate nanoinclusions into the matrix.
The mobility spatial distribution function: Turn-on dynamics of polymer photocells
Authors: Rappaport Noam , Solomesch Olga , Tessler Nir
Published: April 2006

To better understand the transport in thin film devices and the role of the excitation density we have measured the photocurrent dynamic response of a photocell upon the switch on of light excitation. Unlike the standard time of flight methods that utilize a pulse excitation we employ a step function that is more compatible with real device operating conditions. The fact that the steady state of the step function is the cw operation of the device allows us to examine the role of charge density and use device analysis tools. To explain the broadened features measured we introduce a physical scheme by which the dispersive nature is due to spatial inhomogeneity, and the semiconducting layer is considered to be comprised of a distribution of parallel pathways (on the nanometer scale) each having a different, but well defined, mobility value. This insight allows us to extract, in a unique way, a mobility distribution function. The mean values of this distribution function are in good agreement with those deduced by previous measurements made on the same sample in which the mobility was determined from the excitation power dependence of the photocurrent quantum efficiency.
Thermal stability of in-grown vacancy defects in GaN grown by hydride vapor phase epitaxy
Authors: Tuomisto F. , Saarinen K. , Paskova T. , Monemar B. , Bockowski M. , Suski T.
Published: April 2006

We have used positron annihilation spectroscopy to study the thermal behavior of different native vacancy defects typical of freestanding GaN grown by hydride vapor phase epitaxy under high pressure annealing at different annealing temperatures. The results show that the VGa-ON pairs dissociate and the Ga vacancies anneal out from the bulk of the material at temperatures 1500–1700K. A binding energy of Eb=1.6(4)eV can be determined for the pair. Thermal formation of Ga vacancies is observed at the annealing temperatures above 1700K, indicating that Ga vacancies are created thermally at the high growth temperature, but their ability to form complexes such as VGa-ON determines the fraction of vacancy defects surviving the cooling down. The formation energy of the isolated Ga vacancy is experimentally determined.
Effect of La substitution on structural and electrical properties of BiFeO3 thin film
Authors: Das S. R. , Bhattacharya P. , Choudhary R. N. P. , Katiyar R. S.
Published: April 2006

The effect of La substitution on the structural and electrical properties of multiferroic BiFeO3 thin films grown on Pt∕TiO2∕SiO2∕Si substrates by pulsed laser deposition has been reported. X-ray diffraction data confirmed the substitutions of La into the Bi site with the elimination of all of the secondary phases. The dielectric constant of the films was systematically increased from 165 to ∼350 and the films showed excellent dielectric loss behavior. We observed a gradual increase in the remnant polarization (2Pr) with lanthanum substitution obtaining a maximum value of ∼42μC∕cm2 at 20mol% La incorporation. The leakage current behavior at room temperature of the films was studied and it was found that the leakage current decreased from 10−4to10−7A∕cm2 for La-substituted films at a field strength of 50kV∕cm. The reduction of dc leakage current of La-substituted films is explained on the basis of relative phase stability and improved microstructure of the material.
How to extend hard sphere density functional approximation to nonuniform nonhard sphere fluids: Applicable to both subcritical and supercritical temperature regions
Authors: Zhou Shiqi
Published: April 2006

A methodology for the formulation of density functional approximation (DFA) for nonuniform nonhard sphere fluids is proposed by following the spirit of a partitioned density functional approximation [Zhou, Phys. Rev. E 68, 061201 (2003)] and mapping the hard core part onto an effective hard sphere whose high order part of the functional perturbation expansion is treated by existing hard sphere DFAs. The resultant density functional theory (DFT) formalism only needs a second order direct correlation function and pressure of the corresponding coexistence bulk fluid as inputs and therefore can be applicable to both supercritical and subcritical temperature cases. As an example, an adjustable parameter-free version of a recently proposed Lagrangian theorem-based DFA is imported into the present methodology; the resultant DFA is applied to Lennard-Jones fluid under the influence of external fields due to a single hard wall, two hard walls separated by a small distance, a large hard sphere, and a spherical cavity with a hard wall. By comparing theoretical predictions with previous simulation data and those recently supplied for coexistence bulk fluid situated at “dangerous” regions, it was found that the present DFA can predict subtle structure change of the density profile and therefore is the most accurate among all existing DFT approaches. A detailed discussion is given as to why so excellent DFA for nonhard sphere fluids can be drawn forth from the present methodology and how the present methodology differs from previous ones. The methodology can be universal, i.e., it can be combined with any other hard sphere DFAs to construct DFA for other nonhard sphere fluids with a repulsive core.
Optical characterization of ZnSe epilayers and ZnCdSe∕ZnSe quantum wells grown on Ge∕Ge0.95Si0.05∕Ge0.9Si0.1∕Si virtual substrates
Authors: Ku J. T. , Kuo M. C. , Shen J. L. , Chiu K. C. , Yang T. H. , Luo G. L. , Chang C. Y. , Lin Y. C. , Fu C. P. , Chuu D. S. , Chia C. H. , Chou W. C.
Published: April 2006

Several approaches have been employed to grow high-quality ZnSe epilayers on Ge∕Ge0.95Si0.05∕Ge0.9Si0.1∕Si virtual substrates. The ZnSe epilayers were characterized by photoluminescence spectroscopy. Migration enhanced epitaxy and inserting an in situ thermal annealing ZnSe buffer layer effectively reduced the intensity of deep level emissions from the ZnSe epilayer grown on a 6°-tilted Ge∕Ge0.95Si0.05∕Ge0.9Si0.1∕Si virtual substrate. Optimized conditions for growing high-quality ZnSe were used to deposit ZnCdSe∕ZnSe multiple quantum wells on Ge∕Ge0.95Si0.05∕Ge0.9Si0.1∕Si virtual substrates. Photoluminescence spectroscopy revealed quantum-confinement effect in the ZnCdSe multiple quantum wells. The evolution of the exciton emission peak energy and the linewidth as a function of temperature indicate a low density of localized sites in the sample with a well width of 1nm. In the high-temperature regime, the thermal quenching of the excitonic emission intensity from ZnCdSe quantum well structures was governed by the thermal activation of carriers from quantum-well-confined states into barrier states.
Characteristics of single metallic nanowire growth via a field-emission induced process
Authors: Oon C. H. , Khong S. H. , Boothroyd C. B. , Thong J. T. L.
Published: April 2006

The growth and characteristics of metallic nanowires formed by field emission in the presence of organometallic precursors are studied. At low growth currents, single nanowires can be formed, which allows a systematic study of the growth characteristics, and wire morphology, structure, and composition. The major role of metal ion deposition in forming the metallic core is demonstrated experimentally, while the formation of the carbonaceous overcoat results from the deposition of neutral atoms from the precursor dissociation process. Transmission electron microscope analysis of tungsten nanowires shows that the core is polycrystalline, with columnar grains dominating the microstructure for thin wires, while larger diameter nanowires are straddled by multiple grains with a wider range of sizes. The axial and radial growth rates of tungsten nanowires as a function of growth current were studied and can be accounted for by assuming a situation in which the rate of ion formation just ahead of the growing tip is supply-rate limited. At higher growth currents, forking and branching phenomena were found to be increasingly probable, and hence, a key to the growth of single, well-defined nanowires is to keep the growth current low. Thermal decomposition of the precursor can also contribute to nanowire growth, and evidence for this mechanism was found in the cases of precursors where autocatalytic decomposition is known to result in metal deposition at relatively low temperatures.
Two-channel electrical conduction in air-stable monodispersed Mo6S3I6 nanowire sheets
Authors: Kutnjak Zdravko , Vrbanić Daniel , Pejovnik Stane , Mihailović Dragan
Published: April 2006

The quasistatic and frequency-dependent electrical conductivities below 1MHz were measured in the temperature range of 4–330K for air-stable monodispersed bundles of Mo6S3I6 nanowires pressed in the form of a sheet. The orientation of the bundles of nanowires within the sheet was approximately isotropic. The observed weak nonlinearity of the current-voltage (I-V) curves could be attributed to the electric-field-induced increase in the number of charge carriers. The temperature dependence of the quasistatic electrical conductivity shows a crossover from a three-dimensional variable-range-hopping mechanism, which dominates at higher temperatures, to a fluctuation-induced tunneling conduction below 50K. In fact, the electrical conductivity becomes almost temperature independent below 50K. The frequency dependence of the electrical conductivity obtained in the frequency range of 10−3–106Hz reveals that this crossover in the temperature dependence is a consequence of the crossover between two parallel conductivity channels. The first channel, which is governed by the three-dimensional variable-range-hopping mechanism, dominates at higher temperatures, while the second channel, governed by the tunneling mechanism, takes over at lower temperatures.
Temperature-dependent photoluminescence from elemental sulfur species on ZnS nanobelts
Authors: Ye Changhui , Fang Xiaosheng , Wang Ming , Zhang Lide
Published: April 2006

We report the experimental study of the temperature-dependent photoluminescence from elemental sulfur species on zinc sulfide nanobelts. The green band emission shows an anomalous blueshift-redshift transition and the half-width undergoes a narrowing-broadening transition with decreasing temperature. We observed that the thermal quenching of the integrated intensity of the green band was suppressed. The anomalous behavior can be attributed to strong carrier localization in the unsaturated dangling orbitals of the elemental sulfur. In addition, the blue band in the undoped ZnS nanobelts is also related to the surface sulfur species that is different from the previously reported self-activation mechanism.
Monte Carlo simulation of localization dynamics of excitons in ZnO and CdZnO quantum well structures
Authors: Makino T. , Saito K. , Ohtomo A. , Kawasaki M. , Senger R. T. , Bajaj K. K.
Published: April 2006

Localization dynamics of excitons was studied for ZnO∕MgZnO and CdZnO∕MgZnO quantum wells (QWs). The experimental photoluminescence (PL) and absorption data were compared with the results of Monte Carlo simulation in which the excitonic hopping was modeled. The temperature-dependent PL linewidth and Stokes shift were found to be in a reasonable agreement with the hopping model, with accounting for an additional inhomogeneous broadening. The density of localized states used in the simulation for the CdZnO QW was consistent with the absorption spectrum.
Instability analysis of a coaxial jet under a radial electric field in the nonequipotential case
Authors: Li Fang , Yin Xie-Yuan , Yin Xie-Zhen
Published: April 2006

The linear temporal stability analysis of a coaxial jet model under the radial electric field is carried out on the assumption that all free charges are “frozen” on the outer interface of the conducting liquid (i.e., the nonequipotential case) in the present paper. The analytical dimensionless dispersion relation is derived and solved numerically. Three unstable modes, namely the paravaricose, parasinuous, and transitional modes, are found in the Rayleigh regime and they change into the other two unstable modes, namely mode 1 and mode 2, in the wind induced and atomization regimes if the dimensionless electrostatic force or Weber number is sufficiently amplified. The numerical results show that the dimensionless electrostatic force as well as the Weber number inhibits the jet instability in the long wavelength region while enhances it in the short wavelength region. The relative dielectric constant of the outer liquid alters the behaviors of the nonequipotential case significantly, whereas the electrical permittivity of the inner liquid does not affect the jet instability much. Besides, the relatively small diameter ratio may make the inner and outer interfaces decouple from each other. And the critical diameter ratio is calculated crudely. The differences between the equipotential and nonequipotential cases are clarified step by step.
Optimum folding pathways of proteins: Their determination and properties
Authors: Güner Uğur , Arkun Yaman , Erman Burak
Published: April 2006

We develop a dynamic optimization technique for determining optimum folding pathways of proteins starting from different initial configurations. A coarse-grained Go model is used. Forces acting on each bead are (i) the friction force, (ii) forces from bond length constraints, (iii) excluded volume constraints, and (iv) attractive forces between residue pairs that are in contact in the native state. An objective function is defined as the total attractive energy between nonbonded residues, which are neighbors in the native state. The objective function is minimized over all feasible paths, satisfying bond length and excluded volume constraints. The optimization problem is nonconvex and contains a large number of constraints. An augmented Lagrangian method with a penalty barrier function was used to solve the problem. The method is applied to a 36-residue protein, chicken villin headpiece. Sequences of events during folding of the protein are determined for various pathways and analyzed. The relative time scales are compared and scaled according to experimentally measured events. Formation times of the helices, turn, and the loop agree with experimental data. We obtain the overall folding time of the protein in the range of 600ns–1.2μs that is smaller than the experimental result of 4–5μs, showing that the optimal folding times that we obtain may be possible lower bounds. Time dependent variables during folding and energies associated with short- and long-range interactions between secondary structures are analyzed in modal space using Karhunen-Loeve expansion.
A mixing model for turbulent flows based on parameterized scalar profiles
Authors: Meyer D. W. , Jenny P.
Published: April 2006

In this paper the closure of molecular mixing in turbulent reactive flows is addressed in the context of probability density function methods. It is safe to say that the lack of a general and accurate mixing model is a major source for uncertainties in turbulent combustion simulation. Here, we propose a model based on constructing statistical distributions of one-dimensional scalar profiles, i.e., fluid particles are associated to parameterized scalar profiles (PSP). Opposed to previous approaches, the PSP model results in a simple formulation and is able to produce very accurate results at low computational cost. For validation, a two-scalar mixing problem in homogeneous isotropic turbulence was used. The accuracy of the PSP model was demonstrated by comparison with direct numerical simulation data and confirms that the intrinsic physical assumptions are justified.
Time-resolved photoluminescence of barium titanate ultrafine powders
Authors: Lu S. G. , Xu Z. K. , Chen Haydn , Mak C. L. , Wong K. H. , Li K. F. , Cheah K. W.
Published: April 2006

Time-resolved photoluminescence (PL) near 540nm, taken from barium titanate (BaTiO3) ultrafine powders of ∼80, 90, and 110nm in size, was measured at 11K. Two-exponential functions were found to fit the decay curves well, and the decay lifetimes were composed of one short, about 10ns, and one long, about 200ns, time constants, respectively. Results also indicate that the PL peaks show a small blueshift with decreasing BaTiO3 particle size. The decay process consists of the recombination of self-trapped excitons (long lifetime decay) and of the electrons in the surface states with the holes in the valence band (short lifetime decay). The short lifetime component makes comparable contribution to the decay intensity of PL with the long lifetime one.
Radio frequency glow discharge source with integrated voltage and current probes used for evaluation of discharge parameters
Authors: Wilken L. , Hoffmann V. , Wetzig K.
Published: April 2006

A radio frequency (rf) Grimm-type glow discharge source for the chemical analysis of solid samples, with integrated voltage and current probes, was developed. All elements of a plasma equivalent circuit are determined from the measured current-voltage characteristics. The procedure is based on the independent evaluation of the ion current and electron current region. The physical meaning of the parameters is investigated by comparisons with measurements from dc glow discharges. We found that the reduced rf current of the powered electrode is comparable to the reduced current in dc discharges. A formula is developed that corrects the reduced current due to gas heating. The sheath thickness at the powered rf electrode is evaluated and is between 75 and 1100μm. The voltage of the bulk plasma is in the range 2–15V, and the resistance is between 30 and 400Ω. The bulk plasma consumes about 3% of the total power, and the reduced voltage is comparable to the reduced electrical field in the positive column of direct current discharges. The sheath voltage at the grounded electrode is in the range 25–100V, the capacities are between 10 and 400pF, and the resistances are in the range 100Ω–5000Ω. We also found invariants for the evaluated sheath parameters.
Effect of ambient pressure on laser ablation and plume expansion dynamics: A numerical simulation
Authors: Chen Zhaoyang , Bleiner Davide , Bogaerts Annemie
Published: April 2006

A comprehensive numerical model is applied to the study of the effect of ambient pressure in laser ablation, more specifically on the copper target heating, melting and vaporization, and the resulting plume expansion in the helium gas, as well as on plasma formation in the plume. Under the laser pulse condition investigated [5ns full width at half maximum (FWHM) and 109W∕cm2 peak irradiance], the calculated results show that the characteristics of the surface temperature and the evaporation depth are very similar even when the ambient pressure varies greatly. The influence of the ambient pressure on the fraction of absorbed laser energy is also small. The maximum ablated material vapor density in the plume is influenced slightly by the different pressures. Before 40ns, the maximum plume temperature for various ambient pressures is in the order of a few 104K. However, the effect of ambient pressure on the plume length is quite large. A specific calculation for a Gaussian-shaped laser pulse with 6ns FWHM and 2.76×109W∕cm2 peak irradiance is made. The calculated evaporation depth agrees well with the experimental data. Therefore, the model can be useful to predict trends in target and plume (plasma) characteristics, which are difficult to obtain experimentally for various ambient pressures.
Spectroscopic properties of Pr3+ ions in biaxial LaB3O6 crystal
Authors: Xiong Feibing , Lin Xiuqin , Luo Zundu , Tan Qiguang , Ma En , Huang Yidong
Published: April 2006

La B 3 O 6 single crystal doped with Pr3+ ions has been grown by the Czochralski method. The angular relationship between the principal axes of the optical indicatrix and the crystallographic axes in the crystal was determined. The polarized absorption spectra, polarized fluorescence spectra, and fluorescence decay curves of the crystal were measured at room temperature. The modified Judd-Ofelt theories, extended to the anisotropic crystal, have been applied to analyze the spectra. The spectroscopic parameters, including the Judd-Ofelt intensity parameters Ωt,q(t=2,4,6), spontaneous emission probabilities, fluorescence branching ratios, radiative lifetimes, fluorescence quantum efficiencies, and stimulated emission cross sections were calculated and analyzed. The promising luminescent and upper laser level of the Pr3+:LaB3O6 crystal is determined.
Short-time dynamics through conical intersections in macrosystems. I. Theory: Effective-mode formulation
Authors: Gindensperger Etienne , Burghardt Irene , Cederbaum Lorenz S.
Published: April 2006

The short-time dynamics through a conical intersection of a macrosystem comprising a large number of nuclear degrees of freedom (modes) is investigated. The macrosystem is decomposed into a “system” part carrying a limited number of modes, and an “environment” part. An orthogonal transformation in the environment’s space is introduced, as a result of which a subset of three effective modes can be identified which couple directly to the electronic subsystem. Together with the system’s modes, these govern the short-time dynamics of the overall macrosystem. The remaining environmental modes couple, in turn, to the effective modes and become relevant at longer times. In this paper, we present the derivation of the effective Hamiltonian, first introduced by Cederbaum et al. [Phys. Rev. Lett. 94, 113003 (2005)], and analyze its properties in some detail. Several special cases and topological aspects are discussed.
Short-time dynamics through conical intersections in macrosystems. II. Applications
Authors: Gindensperger Etienne , Burghardt Irene , Cederbaum Lorenz S.
Published: April 2006

We present several numerical applications based upon the effective-mode formulation for the short-time dynamics through conical intersections in macrosystems, as detailed in the preceding paper and first proposed by Cederbaum et al. [Phys. Rev. Lett. 94, 113003 (2005)]. The macrosystem, containing a vast number of nuclear degrees of freedom (modes), is decomposed into a system part and an environment part. Only three effective environmental modes are needed—together with the system’s modes—to accurately calculate the low resolution spectra and the short-time dynamics of the entire macrosystem. For the systems discussed here, results are compared to those of a full quantum wave-packet propagation. Some rules are extracted to provide general tendencies; these rules allow one to understand and predict the dynamical properties in more general situations where the exact quantum dynamics of the macrosystem is out of reach.
Model for dynamics of inhomogeneous and bulk fluids
Authors: Krishnan S. H. , Ayappa K. G.
Published: April 2006

An accurate model for the density of states (DOS) for strongly inhomogeneous and bulk fluids has been proposed based on γ distributions. The contribution to the density of states from the collective dynamics is modeled as an incomplete γ distribution and the high frequency region is obtained from the solution of the memory equation using a sech memory kernel. Using only the frequency moments as input, the model parameters for the collective dynamics are obtained by matching moments of the resulting distribution. The model results in an analytical expression for the self-diffusivity of the fluid. We present results for soft sphere fluids confined in slit-shaped pores as well as bulk soft sphere liquids. Comparisons of the DOS, velocity autocorrelation functions, and memory kernels with molecular dynamics simulations reveal that the model predicts features in the DOS over the entire frequency range and is able to capture changes in the DOS as a function of fluid density and temperature. As a result the predicted VACFs, memory kernels, and self-diffusivities are accurately predicted over a wide range of conditions. Since the frequency moments for bulk liquids can be obtained from pair correlation functions, our method provides a direct route from fluid structure to dynamics. For fluids confined in slit-shaped pores, where the frequency moments are obtained from molecular dynamics simulations, the predicted self-diffusivities capture the resulting oscillations due to variations in the solvation pressure, and in the case of smooth walled pores, the predictions are superior to those obtained using kinetic theory.
Ferromagnetic Gd-implanted ZnO single crystals
Authors: Potzger K. , Zhou Shengqiang , Eichhorn F. , Helm M. , Skorupa W. , Mücklich A. , Fassbender J. , Herrmannsdörfer T. , Bianchi A.
Published: April 2006

In order to introduce ferromagnetic properties, ZnO single crystals have been implanted with Gd ions at 180keV ion energy and two different fluences. Magnetization reversal hysteresis loops have been recorded for as-implanted as well as annealed samples using a superconducting quantum interference device. It was found that for a fluence of 5×1015Gd+∕cm2, postimplantation annealing leads to an increase of the saturation moment up to 1.8μB∕Gd at 300K. Structural investigations revealed no secondary phase formation.
Intrinsic exciton transitions in high-quality ZnO thin films grown by plasma-enhanced molecular-beam epitaxy on sapphire substrates
Authors: Zhang X. Q. , Yao Z. G. , Huang S. H. , Suemune Ikuo , Kumano H.
Published: April 2006

High-quality ZnO thin films have been grown by plasma-enhanced molecular-beam epitaxy on sapphire substrates. Free-exciton absorption and exciton-LO phonon absorption peaks are observed in the films at room temperature, indicating that the exciton states are stable even at room temperature. Three excitonic transitions associated with valence bands A, B, and C are clearly revealed in the reflectance spectrum measured at low temperatures. This result indicates that the ZnO thin films have a perfect wurtzite crystal structure. Biexciton emission is observed in the photoluminescence spectra at low temperatures, from which the biexciton binding energy is estimated to be 14.5meV, in good agreement with previous results. Exciton-LO (Ex-LO) and exciton-2LO (Ex-2LO) photon emission peaks are observed at low temperature. The energy difference between the Ex-LO and Ex-2LO bands is about 72.5meV, which coincides with previously reported values of the LO phonon energy for ZnO thin films.
Strain effects in ZnO thin films and nanoparticles
Authors: Hur Tae-Bong , Hwang Yoon-Hwae , Kim Hyung-Kook , Lee Ik Jae
Published: April 2006

We grew Stranski-Krastanow-type ZnO thin film and Volmer-Weber-type self-assembled ZnO nanocrystals using magnetron sputtering methods. The evolution of surface roughness and strain effects in thin ZnO films on Al2O3(0001) substrate and ZnO nanocrystals on Pt(111) surface studied by synchrotron x-ray scattering. The well-aligned two-dimensional (2D) planar layer dominated in layer-by-layer growth at the highly strained initial growth stage in the thin films. As the film thickness increased, the discrete nucleations on the 2D planar layer continuously grew until the ZnO film reached the strain relaxed steady-state regime. The accumulated strain energy in the thin film grown at low temperature slowly relaxed while the strain energy in the high temperature system rapidly relaxed. When the three-dimensional islands on the 2D surface of thin ZnO film grown at the low and high temperatures were quickly developed by strain relaxation, the critical exponent β were roughly 0.693 and 1.579, respectively. The thickness-dependent strain of Volmer-Weber-type ZnO nanocrystals relaxed slowly, compared to that of Stranski-Krastanow-type thin film.
Influence of bulk bias on negative bias temperature instability of p-channel metal-oxide-semiconductor field-effect transistors with ultrathin SiON gate dielectrics
Authors: Zhu Shiyang , Nakajima Anri , Ohashi Takuo , Miyake Hideharu
Published: April 2006

Bulk (well) bias effects (grounded, positively biased, and floating) on both static and dynamic negative bias temperature instability of p-channel metal-oxide-semiconductor field-effect transistors with ultrathin SiON gate dielectrics were systematically investigated. The device degradation under both static and dynamic negative bias temperature (NBT) stresses with relatively large gate voltage (Vg) is significantly enhanced by a positive bulk bias (Vb). Moreover, the device degradation under bipolar pulsed bias temperature (BT) stress is dramatically enhanced by floating the bulk electrode. Both phenomena can be attributed to an additional degradation related to hot hole injection. The holes are energized by an electrical field of the induced depletion region between channel and bulk provided by the positive Vb or, in the case of bipolar pulsed BT stress with the bulk electrode floating, by the transient depletion region below the channel induced by the p-n junction between source (drain) and bulk upon the gate voltage Vg being switched from positive to negative with a transition time less than about 0.2–100ms.