bluuebunny/crossref_metadata_2025 huggingface dataset:
Cluster coarsening in zinc oxide thin films by postgrowth annealing
Authors: Tan S. T. , Sun X. W. , Zhang X. H. , Chua S. J. , Chen B. J. , Teo C. C.
Published: August 2006

Postgrowth annealing was carried out on ZnO thin films grown by metal-organic chemical-vapor deposition. It was found from the scanning electron microscopy and atomic force microscopy measurements that the morphology of the thin films changed drastically after annealing. The as-grown thin films consist of fine nanoscale-sized sheets with random orientation. Upon annealing at 800°C, the ZnO nanosheets changed to three-dimensional nanoneedles. The different types of the mass transport mechanisms are discussed and correlated with the experimental results. A coarsening kinetics developed by Lifshitz and Slyozov [J. Phys. Chem. Solids 19, 35 (1961)] and Wagner [Z. Elektrochem. 65, 581 (1961)] was used to estimate the activation energy of the coarsening process. The activation energy of the Ostwald ripening in ZnO films was estimated in the first attempt, and the value is at around 1.33eV. Hall effect and photoluminescence measurements were carried out to investigate the effect of coarsening on electrical and optical properties of the ZnO thin films.
Effective properties of split-ring resonator metamaterials using measured scattering parameters: Effect of gap orientation
Authors: Varadan Vasundara V. , Tellakula Anilkumar R.
Published: August 2006

This paper presents experimental S-parameter data (amplitude and phase) in the 8–26GHz range for split-ring resonator (SRR) metamaterial samples exhibiting electric (E) and/or magnetic (H) resonances at 10.4GHz depending on the orientation of the gaps of the SRRs with respect to the E and H fields. Use of a thru, reflect, line (TRL) calibrated focused plane wave, free space microwave system permits inversion of the complex permittivity, permeability, and refractive index of planar metamaterial samples from measurement of the reflected and transmitted S parameters of the system. Samples were prepared by patterning concentric SRRs on a dielectric substrate. Three different orientations of the SRR have been used in the experimental study; (i) SRRparallel—gaps in the split rings parallel to the incident E field to realize a dielectric resonance; (ii) SRRrandom—randomly oriented gaps to realize simultaneous dielectric and magnetic resonances; and (iii) SRRperpendicular—gap oriented perpendicular to the E-field to realize a strong magnetic resonance. The experimentally extracted material properties show very good agreement with numerical simulations of effective properties giving rise to a negative magnetic permeability. A second resonance at 23GHz clearly establishes a passband between 10.4 and 23GHz for all samples. The extraction of effective properties from measured S parameters is a useful development in metamaterials research and the study of random orientations of gaps with respect to the E field is an interesting design for metamaterials that may lead to negative index behavior with further optimization.
Networks of channels for self-healing composite materials
Authors: Bejan A. , Lorente S. , Wang K.-M.
Published: August 2006

This is a fundamental study of how to vascularize a self-healing composite material so that healing fluid reaches all the crack sites that may occur randomly through the material. The network of channels is built into the material and is filled with pressurized healing fluid. When a crack forms, the pressure drops at the crack site and fluid flows from the network into the crack. The objective is to discover the network configuration that is capable of delivering fluid to all the cracks the fastest. The crack site dimension and the total volume of the channels are fixed. It is argued that the network must be configured as a grid and not as a tree. Two classes of grids are considered and optimized: (i) grids with one channel diameter and regular polygonal loops (square, triangle, hexagon) and (ii) grids with two channel sizes. The best architecture of type (i) is the grid with triangular loops. The best architecture of type (ii) has a particular (optimal) ratio of diameters that departs from 1 as the crack length scale becomes smaller than the global scale of the vascularized structure from which the crack draws its healing fluid. The optimization of the ratio of channel diameters cuts in half the time of fluid delivery to the crack.
Cryogenic small-signal conversion with relaxation oscillations in Josephson junctions
Authors: Furlan Miha
Published: August 2006

Broadband detection of small electronic signals from cryogenic devices, with the option of simple implementation for multiplexing, is often a highly desired, although nontrivial task. We investigate and demonstrate a small-signal analog-to-frequency conversion system based on relaxation oscillations in a single Josephson junction. Operation and stability conditions are derived, with special emphasis on noise analysis, showing the dominant noise sources to originate from fluctuation processes in the junction. At optimum conditions the circuit is found to deliver excellent noise performance over a broad dynamic range. Our simple models apply within the regime of classical Josephson junction and circuit dynamics, which we confirm by experimental results. A discussion on possible applications includes a measurement of the response to a cryogenic radiation detector.
Dielectric relaxation induced by a space charge in poled glasses for nonlinear optics
Authors: Dussauze M. , Bidault O. , Fargin E. , Maglione M. , Rodriguez V.
Published: August 2006

Unpoled and efficient nonlinear optical (NLO) poled sodium niobium borophosphate glasses have been studied by means of dielectric spectroscopy. The thermal poling process induces a dielectric relaxation that gradually vanishes on heating for T≳500K. The data are interpreted by a two-layer (Maxwell-Wagner) model, consisting of a bulk and a Na+ poor surface layer with lower conductivity. Our results suggest that the ion transport is due to hopping mechanism implying at least Na+. All the dielectric results are in full agreement with previous optical analyses. This is an example of poled glasses where an efficient NLO response is directly correlated to the conductivity of the material enhanced by a high alkali loading.
Analysis of the chemistry in CH4∕O2 plasmas by means of absorption spectroscopy and a simple numerical model
Authors: Möller I. , Serdyuchenko A. , Soltwisch H.
Published: August 2006

The chemistry in CH4∕O2 rf discharges has been investigated by comparing molecular concentrations measured by tunable diode laser absorption spectroscopy with a hypothetical plasma composition calculated by a simple chemical rate equation system. This comparison allowed us to investigate the reaction paths of molecules, which undergo long reaction chains, and to gather information on the high-energy tail of the electron energy distribution function (EEDF). The procedure has been applied to a capacitively and two inductively coupled plasma sources with strongly varied external parameters (p=10–100Pa, Q=2–60SCCM) and for a wide range of electron densities (ne=1014–1017m−3). In mixtures of CH4 and O2 the main production and depletion channels of two stable products, CO and CO2, have been obtained, and their relative importance under modified operational conditions has been studied. Furthermore, the analysis of the molecular densities of CH4, CH3, C2H2, and C2H6 dependent on the input power in an inductively coupled discharge with pure CH4 as source gas allowed us to determine a range of EEDFs, which are compatible with the measured concentrations.
Study of electrical frequency response through the metal-insulator transition in highly conducting polymers
Authors: Zhou Li-Ping , Li Zhen-Ya
Published: August 2006

Based on a random resistor network (RRN), we study the electrical frequency response, including the unusual frequency dependent conductivity and multiple zero crossing of the dielectric function of highly conducting polymers. The electronic transport properties can be studied by modeling the system as a chain-linked granular network, with metallic regions randomly distributed in an amorphous host. Taking into account the contributions of coherent and incoherent transport channels, and the percolation effect of conducting polymers in our RRN model, we can well explain the negative value of ε and the low-energy structure of conducting polymers. The calculated ε(ω) and σ(ω) reproduce the main features of recent broadband optical experiments.
Reaction of thin Ni films with Ge: Phase formation and texture
Authors: Gaudet S. , Detavernier C. , Lavoie C. , Desjardins P.
Published: August 2006

The solid-state reaction between a 30-nm-thick Ni film and Ge substrates was investigated using in situ x-ray diffraction, diffuse light scattering, and four-point probe electrical measurements. Our results reveal that Ni5Ge3 and NiGe appear consecutively on Ge(111) while they grow simultaneously on amorphous Ge(α-Ge) and Ge(001). Furthermore, phase formation temperatures depend strongly on the nature of the substrate being the lowest on α-Ge and the highest on Ge(111). X-ray pole figure measurements of the NiGe phase obtained from the reaction with an amorphous substrate indicate a completely random texture while several epitaxial and axiotaxial texture components are observed on both Ge(001) and Ge(111). The texturing for the NiGe film on Ge(111), which showed a sequential phase formation, is an order of magnitude more pronounced than for the film on Ge(001) which showed a simultaneous growth.
Variation of structural, electrical, and optical properties of Zn1−xMgxO thin films
Authors: Kim Jae Won , Kang Hong Seong , Kim Jong Hoon , Lee Sang Yeol , Lee Jung-Kun , Nastasi Michael
Published: August 2006

Zn 1 − x Mg x O thin films on (001) sapphire substrates were deposited using pulsed laser deposition. As the substrate temperature increased, the Mg content in the Zn1−xMgxO thin films increased and the photoluminescence (PL) peak position of the Zn1−xMgxO thin films shifted from 370to356nm, indicating a band gap expansion. Variations of the structural, electrical, and optical properties of Zn1−xMgO thin films have been observed and analyzed by x-ray diffraction, Hall measurements, and PL measurements.
Pulsed low-pressure wire discharge
Authors: Makarov Maxime , Loumani Youssef , Kozyrev Andrei
Published: August 2006

A pulsed low-pressure wire discharge was studied experimentally. One can clearly distinguish at least three very different phases in the discharge development. During the breakdown lag the ionization takes place principally near the wire anode. The cathode secondary emission under ionic bombardment begins to play a (minor) role throughout the low-current non-steady-state phase since the cathode sheath is not yet formed. Once the sheaths are formed, the hollow-cathode discharge develops. The stability of a hollow-cathode discharge with an immersed small-size wire anode was analyzed. It is shown that the ratio of electrode surfaces Scathode∕Sanode necessary for uniform discharge formation depends on secondary emission yield, the nature of gas, and anode geometry. If the stability condition is violated, a double anode sheath appears and discharge becomes unstable.
Structural origin of the relaxor-to-normal ferroelectric transition in Pb(Mg1∕3Nb2∕3O3)–xPbTiO3
Authors: Cao Hu , Li Jiefang , Viehland D.
Published: August 2006

A relaxor-to-normal, or micro-to-macrodomain, transition in Pb(Mg1∕3Nb2∕3O3)–xPbTiO3, has been found under electric fields applied along [001] for x⩾0.24. It is shown that an intermediate tetragonal ferroelectric phase is important to the sharpness of this transition, as observed in its dielectric response.
Ferroelectric and conductivity behavior of multilayered PbZr0.52Ti0.48O3∕Pb(Mg1∕3Ta2∕3)0.7Ti0.3O3∕PbZr0.52Ti0.48O3 thin films
Authors: Li Fang , Zhou Zhaohui , Wang John
Published: August 2006

Ferroelectric and impedance behavior of sandwich-structured PbZr0.52Ti0.48O3∕Pb(Mg1∕3Ta2∕3)0.7Ti0.3O3∕PbZr0.52Ti0.48O3 thin films was studied as a function of temperature (23–300°C) and frequency (0.1–104Hz). A change in the controlling mechanism of the electrical behavior from grain interior to grain boundary occurred in the temperature range studied. A low-frequency dielectric relaxation was observed in the temperature range of 200–300°C, the activation energy of which was calculated to be 0.90eV. This suggests that oxygen vacancies are the most likely charge carriers at high temperatures. The change in fatigue behavior of the sandwich-structured thin film with temperature can be accounted for by the increased mobility of oxygen vacancies at elevated temperatures. Frequency dependent conductivities were analyzed with an augmented Jonscher relation. The activation energies for dc conductivity and hopping frequency were calculated to be 0.90 and 0.89eV, respectively.
Spin-dependent transport in diluted-magnetic-semiconductor/semiconductor quantum wires
Authors: Xu Wen , Guo Yong
Published: August 2006

Spin-polarized transport properties have been investigated in diluted-magnetic-semiconductor/semiconductor quantum wires. We stress the effects introduced by the structural configuration and geometric parameters as well as the external magnetic field. It is found that the symmetric quantum wire shows quite different spin-dependent transport characteristics from the asymmetric one. It is also found that the spin-up component of the conductance can be considerably suppressed when the magnetic field is large enough, and thus nearly 100% spin polarization can be obtained in such kind of quantum wire. Moreover, one can effectively modulate the spin-dependent conductance in the quantum wire structure by varying the geometric parameters and the Fermi energy.
Magnetic domain pinning in an anisotropy-engineered GdTbFe thin film
Authors: Konings Stan , Miguel Jorge , Goedkoop Jeroen , Camarero Julio , Vogel Jan
Published: August 2006

Focused ion beam irradiation was used to reduce locally the perpendicular anisotropy of magnetic thin films in rectangular lattices of 50nm sized dots. The effect of the anisotropy patterns, differing in ion fluence and interdot spacing, on the magnetization reversal process was determined in q space with x-ray resonant magnetic scattering and in real space with magnetic force microscopy. At remanence only a slight alignment of the irregularly shaped domains is observed. In perpendicular magnetic fields, however, the high field bubble domains display a pronounced localization on the dots, showing that this form of local anisotropy reduction is a highly efficient way of domain positioning.
A method to calculate tunneling leakage currents in silicon inversion layers
Authors: Lujan Guilherme S. , Sorée Bart , Magnus Wim , De Meyer Kristin
Published: August 2006

This paper proposes a quantum mechanical model for the calculation of tunneling leakage currents in a metal-oxide-semiconductor structure. The model incorporates both variational calculus and the transfer matrix method to compute the subband energies and the lifetimes of the inversion layer states. The use of variational calculus simplifies the subband energy calculation due to the analytical form of the wave functions, which offers an attractive perspective towards the calculation of the electron mobility in the channel. The model can be extended to high-k dielectrics with several layers. Good agreement between experimental data and simulation results is obtained for metal gate capacitors.
Deterministic shape control in plasma-aided nanotip assembly
Authors: Tam E. , Levchenko I. , Ostrikov K.
Published: August 2006

The possibility of deterministic plasma-assisted reshaping of capped cylindrical seed nanotips by manipulating the plasma parameter-dependent sheath width is shown. Multiscale hybrid gas phase/solid surface numerical experiments reveal that under the wide-sheath conditions the nanotips widen at the base and when the sheath is narrow, they sharpen up. By combining the wide- and narrow-sheath stages in a single process, it turns out possible to synthesize wide-base nanotips with long- and narrow-apex spikes, ideal for electron microemitter applications. This plasma-based approach is generic and can be applied to a larger number of multipurpose nanoassemblies.
Statistical thermodynamics of bond torsional modes: Tests of separable, almost-separable, and improved Pitzer–Gwinn approximations
Authors: Ellingson Benjamin A. , Lynch Vanessa Audette , Mielke Steven L. , Truhlar Donald G.
Published: August 2006

Practical approximation schemes for calculating partition functions of torsional modes are tested against accurate quantum mechanical results for H2O2 and six isotopically substituted hydrogen peroxides. The schemes are classified on the basis of the type and amount of information that is required. First, approximate one-dimensional hindered-rotator partition functions are benchmarked against exact one-dimensional torsion results obtained by eigenvalue summation. The approximate one-dimensional methods tested in this stage include schemes that only require the equilibrium geometries and frequencies, schemes that also require the barrier heights of internal rotation, and schemes that require the whole one-dimensional torsional potential. Then, three classes of approximate full-dimensional vibrational-rotational partition functions are calculated and are compared with the accurate full-dimensional path integral partition functions. These three classes are (1) separable approximations combining harmonic oscillator-rigid rotator models with the one-dimensional torsion schemes, (2) almost-separable approximations in which the nonseparable zero-point energy is used to correct the separable approximations, and (3) improved nonseparable Pitzer–Gwinn-type methods in which approaches of type 1 are used as reference methods in the Pitzer–Gwinn approach. The effectiveness of these methods for the calculation of isotope effects is also studied. Based on the results of these studies, the best schemes of each type are recommended for further use on systems where a corresponding amount of information is available.
Effect of particle size on the magnetic properties of core-shell structured nanoparticles
Authors: Ceylan Abdullah , Baker C. C. , Hasanain S. K. , Ismat Shah S.
Published: August 2006

Effect of particle size on exchange bias observed in Fe∕Fe oxide core/shell structured nanoparticles was investigated. Inert gas condensation was utilized for the synthesis of samples. Two sets of different particle size samples were prepared and the structural and magnetic properties were probed. It was found that the small particles show superparamagnetic behavior and exhibit high exchange bias field, 1574±25Oe at 5K, when field cooled in the presence of 2T magnetic field. Structural analyses of the particles in correlation with the magnetic measurements show that the smaller particle size (6nm Fe core, 1.5nm Fe oxide shell) favors amorphous oxide shell structure, and this in turn causes high magnetocrystalline anisotropy and enhanced exchange bias. Furthermore, we have also observed a vertical shift of the hysteresis loop related to the pinned spins at the ferromagnetic/antiferromagnetic (AFM) interface of the small particles. Decreased core size, high AFM anisotropy, and pinned spins observed from the small size particles support the domain wall model of the exchange bias.
Composite state control and magnetic properties of Co and Si cluster assemblies prepared with double-glow-discharge sources
Authors: Katoh Ryoji , Hihara Takehiko , Peng Dong-Liang , Sumiyama Kenji
Published: August 2006

Using a double-glow-discharge-cluster-source system, in which one glow discharge was a dc mode and the other an rf discharge mode, Co and Si clusters were independently produced and simultaneously deposited on a substrate. When a separation plate was inserted between two glow-discharge chambers, a mixture of Co and Si clusters was obtained: small Co clusters were distributed at random, while the Si clusters were aggregated to form large secondary particles. Without inserting the separation plate, on the other hand, core-shell clusters were obtained: a Co core was surrounded by small Si crystallites. The magnetization measurement indicated that the magnetic coercive force of Co∕Si core-shell cluster assemblies was much smaller than that of Co cluster assemblies in which Co clusters were covered with antiferromagnetic CoO shells, indicating that the Si shell prevented Co cluster surfaces from their oxidation. Therefore, the present double-cluster-source system is useful in fabricating various sorts of cluster composites, which cannot be prepared by conventional coevaporation or precipitation methods.
Vacancy-impurity complexes in polycrystalline Si used as gate electrodes of HfSiON-based metal-oxide-semiconductors probed using monoenergetic positron beams
Authors: Uedono A. , Ikeuchi K. , Otsuka T. , Yamabe K. , Eguchi K. , Takayanagi M. , Ishibashi S. , Ohdaira T. , Muramatsu M. , Suzuki R.
Published: August 2006

Vacancy-impurity complexes in polycrystalline Si (poly-Si) used as a gate electrode of the metal-oxide-semiconductor field-effect transistor (MOSFET) were probed using monoenergetic positron beams. Doppler broadening spectra of the annihilation radiation and the positron lifetimes were measured for poly-Si(150nm)∕HfSiON(5nm)∕Si. In addition, first principles calculations were used to identify impurities coupled with defects in the poly-Si film. The major defect species in the poly-Si film was identified as vacancy-type defects; their mean open volume was close to that of a divacancy. Vacancy-oxygen complexes were observed in the P-implanted poly-Si film after annealing (at 1000°C); the number of oxygen atoms coupled with each defect was estimated to be one or two. For the B-implanted poly-Si film, however, the formation of the complexes was suppressed, suggesting the formation of oxygen clusters or precipitate. We discuss the interaction between such oxygen-related defects and boron in terms of the electric properties of p-channel MOSFETs.
In-guide pump and probe characterization of photoinduced absorption in hydrogenated amorphous silicon thin films
Authors: Della Corte Francesco Giuseppe , Gagliardi Massimo , Nigro Maria Arcangela , Summonte Caterina
Published: August 2006

The in-guide pump and probe analysis illustrated in this paper is useful for the characterisation of photoinduced phenomena in thin films that are already part of an optoelectronic device. Measurements were carried on a-Si:H∕ZnO waveguiding multilayers designed for the telecommunication wavelength of 1.55μm. Because of the waveguiding geometry, the probe beam propagates along the film under test, a configuration that allows two advantages. First, the underlying layers do not affect the acquisition, and therefore the use of a transparent substrate is not necessary. Second, a longer region is available for the interaction between the pump and probe beams with respect to normal incidence probe beam setups, a characteristic conferring a high sensitivity to the technique. An analysis of the experimental data is proposed to extract information on the material, independent, e.g., of the device geometry. As an example, we illustrate an analysis of the photoinduced absorption by starting from measures directly performed on the a-Si:H core of an operating waveguide.
Determination of interstitial oxygen concentration in germanium by infrared absorption
Authors: Litvinov V. V. , Svensson B. G. , Murin L. I. , Lindström J. L. , Markevich V. P. , Peaker A. R.
Published: August 2006

The intensities of infrared absorption due to the asymmetric stretching vibrations of interstitial oxygen atoms in Ge crystals enriched with O16 and O18 isotopes have been compared with oxygen concentrations determined by means of secondary ion mass spectrometry. For Ge samples with oxygen content less than 5×1017cm−3, a good correlation has been found between the values of oxygen concentration and the values of the absorption coefficient at the maximum of the Oi16 related absorption band at 855.6cm−1 with a proportionality coefficient CO=1.05×1017cm−2.
Microstructure-dependent dielectric properties of TbMnO3 in Au∕TbMnO3∕YBa2Cu3O7−x capacitors
Authors: Cui Yimin , Cai Wei , Li Yuan , Qian JianQiang , Xu Ping , Wang Rongming , Yao Junen , Zhang Liuwan
Published: August 2006

Au ∕ Tb Mn O 3 ( TMO ) ∕ Y Ba 2 Cu 3 O 7 − x ( YBCO ) capacitors were fabricated on LaAlO3 (LAO) and SrTiO3 (STO) substrates under identical conditions. Dielectric properties were measured in the temperature range from 20to350K. High dielectric constants and single dielectric transition peaks were found in both TMO films. The dielectric constants of TMO film on STO substrate were larger than those of TMO film on LAO substrate; correspondingly, the transition temperature of TMO film on STO substrate was higher than that of TMO film on LAO substrate. Although x-ray diffraction showed both TMO films had the same epitaxial orientation, atom force microscopy revealed that the films had different surface morphologies, which originated from the YBCO layers on different substrates. The good surface morphology of YBCO layer and the element diffusion between TMO and YBCO layers may be suggested to be responsible for the significant improvement in the dielectric properties of TMO thin film.
Simulated admittance spectroscopy measurements of high concentration deep level defects in CdTe thin-film solar cells
Authors: Seymour Fred H. , Kaydanov Victor , Ohno Tim R.
Published: August 2006

Computer simulations of thin-film p-CdTe∕n-CdS solar cells using the SCAPS software program are examined to offer explanations for a number of experimentally observed admittance spectroscopy results. We show that for a high concentration of deep level defects, the magnitude of the admittance spectroscopy signal which is an indicator of defect concentration is also affected by the cell thickness. For CdTe cells thinner than 3μm, the signal for defects within 0.25eV of the band edge can be weak and may not be detected at all. We also show that Fermi level pinning resulting from high concentrations of deep level defects can distort the measured activation energy and apparent capture cross section. Finally, we show that decreasing capacitance values with increasing temperature can be caused by the interaction between the CdTe cell back contact Schottky barrier, a defect concentration gradient adjacent to the back contact, and a small shallow acceptor concentration relative to the defect concentration.
Solution-processed nickel tetrabenzoporphyrin thin-film transistors
Authors: Shea Patrick B. , Kanicki Jerzy , Pattison Lisa R. , Petroff Pierre , Kawano Manami , Yamada Hiroko , Ono Noboru
Published: August 2006

We describe nickel tetrabenzoporphyrin (NiTBP) as a solution-processible organic semiconductor. Whereas porphyrins in an unmodified state are typically planar and insoluble, a precursor synthetic route (NiCP) was used to deposit thin films via solution. Amorphous, insulating thin films of NiCP were deposited, and thermally converted to polycrystalline, semiconducting NiTBP. Films were studied using optical absorption and microscopy, atomic force microscopy, and x-ray diffraction. Highly concentrated NiCP was shown to form large, needle-shaped crystals drop-cast from solution. NiTBP thin-film field-effect transistors fabricated from spun-cast films demonstrated charge-carrier field-effect mobilities on the order of 0.1 and 0.2cm2∕Vs and accumulation threshold voltages of −19 and −13, in the linear and saturation regimes, respectively.
Perpendicular hot electron transport in the spin-valve photodiode
Authors: Huang Biqin , Appelbaum Ian
Published: August 2006

The spin-valve photodiode is a ferromagnetic metal multilayer/n-type semiconductor Schottky device operated by photoexciting hot electrons in the metal and causing internal photoemission (IPE) into the semiconductor. Simple IPE theory predicts that the magnitude of the spin-valve effect (modulation of the photocurrent) should monotonically increase as a metallic capping layer thickness increases. Experimentally, however, we observe a nonmonotonic behavior with cap layer thickness, where the magnetocurrent reaches an optimum value and then decreases. The disagreement between this experimental result and the previous theoretical model is discussed, leading to an alternative interpretation of transport including reflection from the air-metal interface. Calculations with this model are consistent with the observed phenomena.
Laser ablation of a turbid medium: Modeling and experimental results
Authors: Brygo F. , Semerok A. , Weulersse J.-M. , Thro P.-Y. , Oltra R.
Published: August 2006

Q -switched Nd:YAG laser ablation of a turbid medium (paint) is studied. The optical properties (absorption coefficient, scattering coefficient, and its anisotropy) of a paint are determined with a multiple scattering model (three-flux model), and from measurements of reflection-transmission of light through thin layers. The energy deposition profiles are calculated at wavelengths of 532nm and 1.064μm. They are different from those described by a Lambert-Beer law. In particular, the energy deposition of the laser beam is not maximum on the surface but at some depth inside the medium. The ablated rate was measured for the two wavelengths and compared with the energy deposition profile predicted by the model. This allows us to understand the evolution of the ablated depth with the wavelength: the more the scattering coefficient is higher, the more the ablated depth and the threshold fluence of ablation decrease.
Damage and recovery in boron doped silicon on insulator layers after high energy Si+ implantation
Authors: Ferri M. , Solmi S. , Nobili D. , Armigliato A.
Published: August 2006

The effects of 2MeV Si+ implantation on silicon-on-insulator layers uniformly doped with B at concentrations 1.0 and 1.8×1020cm−3, and the kinetics of damage recovery were investigated by carrier density, mobility measurements, and transmission electron microscopy (TEM) observations. High energy implantation reduces the hole density by about 98%; the mobility is also reduced at an extent which increases with B concentration. Isochronal and isothermal annealings show that recovery of the hole density takes place in three stages: the first stage (α) is accompanied by a mobility decrease and is followed by the second stage (β) where mobility increases attaining values close to the ones of the reference undamaged samples. Mobility keeps nearly constant in the third recovery stage (γ), which takes place above 800°C. As a characterizing feature the mobility values for each B concentration only depend on the hole density, irrespective of the thermal history of the samples. Experiments and TEM observations allowed us to distinguish defect recovery from SiB3 precipitation, which can take place at temperatures higher than 700°C. Recovery stages are discussed, and it is concluded that dissolution of B rich clusters in stage (α) modifies the concentration, or the charge state, of the defects responsible of the second (β) stage. These defects are identified as boron interstitial clusters in consideration of their mobility behavior and of the activation energy Eβ for their recovery process, which results to be 3±0.2eV.
Optimization of aperiodic dielectric structures
Authors: Seliger Philip , Mahvash Mohammad , Wang Chunming , Levi A. F. J.
Published: August 2006

The electromagnetic scattering properties of identical parallel dielectric cylinders configured to closely match a desired response are investigated. Spatial arrangements obtained using a computationally efficient gradient-based optimizer and finite difference forward solver are aperiodic and nonintuitive. We find good agreement between calculations and experiments performed at 37.5GHz and suggest that finite-sized aperiodic configurations may provide access to functionality inaccessible by conventional periodic, photonic crystal inspired, designs.
Electron microscopy study of Ti-doped sodium aluminum hydride prepared by mechanical milling NaH∕Al with Ti powder
Authors: Kang Xiang-Dong , Wang Ping , Cheng Hui-Ming
Published: August 2006

Quite recently, we have demonstrated that mechanical milling NaH∕Al mixture with metallic Ti powder resulted in the formation of catalytically active Ti hydride. In the present study, the morphology and the distribution state of the catalytically active Ti hydride and the parent phases of the as-prepared and cycled samples were examined by electron microscopy in combination with energy dispersive x-ray analysis. It was observed that the de-∕hydriding cycles resulted in a substantial change on the particle size and grain size of Al. In particular, a specific correlation between the distribution states of Al and Ti hydride was developed after de-∕hydriding cycles. In addition, a variation of milling atmosphere during the doping process was found to lead to a varied distribution state of Ti hydride phase. Understanding of these structural characteristics, especially in combination with the previously observed hydrogen storage performance of the materials, has provided insight into the catalytic mechanism and reaction processes of metallic Ti-doped NaAlH4.
Growth of high crystalline quality semi-insulating GaN layers for high electron mobility transistor applications
Authors: Yu Hongbo , Caliskan Deniz , Ozbay Ekmel
Published: August 2006

Semi-insulating character (sheet resistivity of 3.26×1011Ω∕sq) of thick GaN layers was developed for AlGaN∕GaN high electron mobility transistor (HEMT) applications on an AlN buffer layer. Electrical and structural properties were characterized by a dark current-voltage transmission line model, x-ray diffraction, and atomic force microscope measurements. The experimental results showed that compared to semi-insulating GaN grown on low temperature GaN nucleation, the crystal quality as well as surface morphology were remarkably improved. It was ascribed to the utilization of a high quality insulating AlN buffer layer and the GaN initial coalescence growth mode. Moreover, the significant increase of electron mobility in a HEMT structure suggests that this is a very promising method to obtain high performance AlGaN∕GaN HEMT structures on sapphire substrates.
Anomalous behavior of silver doped indium sulfide thin films
Authors: Mathew Meril , Jayakrishnan R. , Ratheesh Kumar P. M. , Sudha Kartha C. , Vijayakumar K. P. , Kashiwaba Y. , Abe T.
Published: August 2006

The effect of doping spray pyrolyzed thin films of In2S3 with silver is discussed. It was observed that silver diffused into In2S3 films in as deposited condition itself. Depth profile using x-ray photoelectron spectroscopy clearly showed diffusion of silver into In2S3 layer without any annealing. X-ray analysis revealed significant enhancement in crystallinity and grain size up to an optimum percentage of doping concentration. This optimum value showed dependence on thickness and atomic ratio of indium and sulfur in the film. Band gap decreased up to the optimum value of doping and thereafter it increased. Electrical studies showed a drastic decrease in resistivity from 1.2×103to0.06Ωcm due to doping. A sample having optimum doping was found to be more photosensitive and low resistive when compared with a pristine sample. Improvement in crystallinity, conductivity, and photosensitivity due to doping of spray pyrolyzed In2S3 films with Ag helped to attain efficiency of 9.5% for Ag∕In2S3∕CuInS2∕ITO (indium tin oxide) solar cell.
Magnetic circular dichroism of ferromagnetic Co2+-doped ZnO
Authors: Kittilstved Kevin R. , Zhao Jialong , Liu William K. , Bryan J. Daniel , Schwartz Dana A. , Gamelin Daniel R.
Published: August 2006

Cobalt-doped ZnO (Co2+:ZnO) films were studied by magnetic circular dichroism (MCD) spectroscopy. A broad 300K ferromagnetic MCD signal was observed between 1.4 and 4.0eV after exposure of paramagnetic Co2+:ZnO films to zinc metal vapor, attributed to low-energy photoionization transitions originating from a spin-split donor impurity band in ferromagnetic n-type Co2+:ZnO.
Growth of ultrahigh-density quantum-confined germanium dots on SiO2 thin films
Authors: Berbezier I. , Karmous A. , Ronda A. , Sgarlata A. , Balzarotti A. , Castrucci P. , Scarselli M. , De Crescenzi M.
Published: August 2006

The spontaneous formation of nanometric and highly dense (∼3×1012cm−2) Ge droplets on thin SiO2 film on Si(001) is investigated by scanning tunneling microscopy and spectroscopy. Ge dots have been grown by depositing Ge on the clean SiO2 surface at room temperature and then annealing the sample at 500°C. Ge dots appear to be free of germanium oxides and characterized by a flat surface with the onset of {113} faceting. I-V curves show that they have an energy gap of approximately 1.8eV, well above that of bulk Ge. Fabrication of nanometer-sized, highly dense pure Ge droplets is very promising for nanoelectronics applications.
Experimental evidence for dislocation-related gettering in metamorphic InP∕InGaAs high electron mobility transistor (HEMT) structures on GaAs substrate
Authors: Liu Yuwei , Wang Hong
Published: August 2006

The thermal stability of metamorphic InP∕InGaAs high electron mobility transistor (HEMT) structures on GaAs substrate subjected to rapid thermal annealing (RTA) has been systematically investigated in the temperature range of 350–700°C. Based on the characterization of the electrical and optical properties, we observed that the metamorphic structures, in which the HEMT structures are grown on a metamorphic stain-relief buffer, exhibit better thermal stability when compared to a lattice-matched structure (control) at annealing temperatures higher than 550°C. An improved photoluminescence (PL) intensity in the temperature range of 550–650°C for metamorphic samples was observed. This behavior is absent in lattice-matched control structures. The better thermal stability and enhanced PL intensity at high RTA temperatures observed in metamorphic structures can be explained as a consequence of the possible dislocation-related gettering effect due to the presence of the dislocation network (gettering sink) located immediately beneath the device zone. Our experimental observation may partially ease the potential concerns about the thermal stability of metamorphic HEMTs.
Investigation of the charge transport through disordered organic molecular heterojunctions
Authors: Houili H. , Tutiš E. , Batistić I. , Zuppiroli L.
Published: August 2006

We present a three-dimensional multiparticle Monte Carlo (3DMPMC) simulation of hopping transport in disordered organic molecular media. We used this approach in order to study the charge transport across an energetically disordered organic molecular heterojunction which is known to strongly influence the characteristics of the multilayer devices based on thin organic films. The role of the energetic disorder and its spatial correlations, which govern the transport in the bulk, are examined here for the bilayer homopolar system where the heterojunction represents the bottleneck for the transport. We study the effects of disorder on both sides of the heterojunction, including the effects of the spatial correlation within each material and among the layers. The 3DMPMC approach allowed us to correctly tackle the effects of the Coulomb interaction among carriers in the region where the charge accumulation in the device is particularly important and the Coulomb interaction most pronounced. The Coulomb interaction enhances the current by increasing the electric field at the heterojunction as well as by affecting the thermalization of the carriers in front of the barrier. In order to build a rather comprehensive picture of the hopping transport over the homopolar heterojunction, we supplemented the MC simulations by a master equation (ME) calculation.
Infiltration and defiltration of an electrolyte solution in nanopores
Authors: Surani Falgun B. , Qiao Yu
Published: August 2006

The motion of confined liquid in a nanoporous silica is controlled by external loadings and modified using calcium chloride. The addition of calcium chloride results in a moderate increase in infiltration pressure and an order-of-magnitude increase in system recoverability, which, especially for the smallest nanopores, is contradictory to the predictions of classic interface/surface theories. This should be attributed to the confinement effect of nanopore walls.
Impedance spectroscopy study of dye-sensitized solar cells with undoped spiro-OMeTAD as hole conductor
Authors: Fabregat-Santiago Francisco , Bisquert Juan , Palomares Emilio , Haque Saif A. , Durrant James R.
Published: August 2006

The photoelectrical properties of a solid state dye-sensitized (DSSC) solar cell with 2,2’,7,7’- tetrakis(N,N-di-p-methoxyphenilamine)-9,9’-spirobifluorene (Spiro-OMeTAD) as transparent hole conductor have been studied in comparison with liquid electrolyte DSSC. Measurements of photopotential decay, transient spectroscopy, and impedance spectroscopy show that the lower electron lifetime determines the lower efficiency of the solid state DSSC with respect to the liquid electrolyte cell. Due to the high transport resistance of pure spiro-OMeTAD the cell response is transport limited at the more positive potentials studied, but recombination dominates at the potentials near the maximum efficiency peak and more negative potentials. Low frequency capacities presented by the solid state cells are much smaller than those of respective liquid cell versions.
Diamond cone arrays with controlled morphologies formed by self-organized selective ions sputtering
Authors: Wang Q. , Gu C. Z. , Xu Z. , Li J. J. , Wang Z. L. , Bai X. D. , Cui Z.
Published: August 2006

Controlled preparation of nanoscale materials and the underlying mechanisms are essential issues nowadays. Here, we report a significant subtractive formation process of large-area diamond conical nanostructure arrays using a hot filament chemical vapor deposition (HFCVD) system with negative biasing of the substrates, and the etching effect of energetic ions on the formation of diamond cone arrays with controlled morphology has been studied in detail. It shows that methylic ions dominantly contribute to diamond cone formation based on a neutral-ion charge exchange collision model. The self-organized selective sputtering process of as-formed hillock bottoms on a roughened surface by low energetic ions plays a key role for the formation and development of diamond cones. The cone morphologies under various experimental parameters are systematically studied, and they nicely confirm and supplement the as-established cone formation mechanism.
Swift heavy ion irradiation-induced modifications in structural, magnetic and electrical transport properties of epitaxial magnetite thin films
Authors: Kumar Ravi , Khan M. Wasi , Srivastava J. P. , Arora S. K. , Sofin R. G. S. , Choudhary R. J. , Shvets I. V.
Published: August 2006

The effect of swift heavy ion (SHI) irradiation (190MeVAg) on structural, electrical transport and magnetic properties of epitaxial magnetite (Fe3O4) thin films (thickness ∼70nm) grown on MgO⟨100⟩ oriented substrate have been investigated. The x-ray diffraction shows that at low fluence values up to 5×1011ions∕cm2, the strain in the films is relaxed, whereas, at higher fluence range 1×1012–1×1013ions∕cm2, the epitaxial relationship with the substrate is lost along with a phase transformation from magnetite to more oxidized magnetite phase (i.e., maghemite). The Verwey transition temperature measured by electrical transport is found to increase from 109 to 117K with the low fluence SHI irradiation, which is related to the irradiation induced strain relaxation and structural modifications. At higher fluences the system did not show Verwey transition and the resistance is also increased. The similar results were obtained by magnetization studies. The observed magnetization at 1T field is increased at low fluence suggesting the reduction of areas with frustrated exchange interactions associated with the cationic arrangement at the anti phase boundaries. At higher fluences it decreases monotonically, indicating the emergence of other phases. The observed modifications are explained on the basis of structural strain and disorder induced by swift heavy ions, which lead to modification of the interionic Coulomb potential at octahedral sublattices and bandwidth in this system.
Magnetoelastic analysis and tensile testing of a soft ferromagnetic strip with a single-edge crack
Authors: Shindo Yasuhide , Komatsu Tetsu , Narita Fumio , Horiguchi Katsumi
Published: August 2006

This article describes the results of an analytical and experimental study of the effect of magnetic fields on the stress intensity factors in a soft ferromagnetic strip under uniaxial tension. The linear magneto-elastic problem for a soft ferromagnetic strip with a single-edge crack is analyzed. Fourier transform techniques are used to formulate the mixed boundary value problem as a singular integral equation. The stress intensity factors are obtained for several values of material and geometrical parameters, and magnetic field. Static experiments are also conducted on nickel–iron soft magnetic materials with a single-edge cracked plate specimen geometry in the bore of a superconducting magnet at room temperature. A strain gauge method is employed in experiments to determine the stress intensity factor. A comparison of the stress intensity factor is made between theory and experiment, and the agreement is good for the magnetic field considered.
Reversible electroresistance at the Ag∕La0.67Sr0.33MnO3 interface
Authors: Xie Y. W. , Sun J. R. , Wang D. J. , Liang S. , Shen B. G.
Published: August 2006

We report the observation of reversible electroresistance in a metallic film La0.67Sr0.33MnO3 (LSMO). The transport behavior of the Ag/LSMO system is found to be sensitive to external electric field, and a switching of the resistance between two definite resistive states can be induced by electric pulses of different polarity. The current-voltage relation, measured by tuning bias voltage, is significantly nonlinear, asymmetric against field polarity, and hysteretic in the field increase/decrease processes. This relation can be well described by a simple equation I=σV+kVn with n being a number between 1.8 and 2.4. It is found that the rough surface of the LSMO film favors the electroresistance effects.
Initiation of re-entry in an excitable medium: Structural investigation of cardiac tissue using a genetic algorithm
Authors: Scarle S. , Clayton R. H.
Published: August 2006

The detailed mechanisms by which re-entry and ventricular fibrillation are initiated in the heart remain poorly understood because they are difficult to investigate experimentally. We have used a simplified excitable media computational model of action potential propagation to systematically study how re-entry can be produced by diffuse regions of inexcitable tissue. Patterns of excitable and inexcitable tissue were generated using a genetic algorithm. The inexcitable tissue was modeled in two ways: (i) diffusive, electrically connected but inexcitable tissue, or (ii) zero-flux, areas of tissue electrically disconnected in the same way as zero-flux boundary conditions. We were able to evolve patterns of diffuse inexcitable tissue that favored re-entry, but no single structure or pattern emerged. Diffusive inexcitable regions were inherently less arrhythmogenic than zero-flux inexcitable ones.