bluuebunny/crossref_metadata_2025 huggingface dataset:
Influence of growth rate on the epitaxial orientation and crystalline quality of CeO2 thin films grown on Al2O3(0001)
Authors: Nandasiri M. I. , Nachimuthu P. , Varga T. , Shutthanandan V. , Jiang W. , Kuchibhatla Satyanarayana V. N. T. , Thevuthasan S. , Seal S. , Kayani A.
Published: January 2011

Growth rate-induced epitaxial orientations and crystalline quality of CeO2 thin films grown on Al2O3(0001) by oxygen plasma-assisted molecular beam epitaxy were studied using in situ and ex situ characterization techniques. CeO2 grows as three-dimensional (3D) islands and two-dimensional layers at growth rates of 1–7 Å/min and ≥9 Å/min, respectively. The formation of epitaxial CeO2(100) and CeO2(111) thin films occurs at growth rates of 1 Å/min and ≥9 Å/min, respectively. Glancing-incidence x-ray diffraction measurements have shown that the films grown at intermediate growth rates (2–7 Å/min) consist of polycrystalline CeO2 along with CeO2(100). The thin film grown at 1 Å/min exhibits six in-plane domains, characteristic of well-aligned CeO2(100) crystallites. The content of the poorly aligned CeO2(100) crystallites increases with increasing growth rate from 2 to 7 Å/min, and three out of six in-plane domains gradually decrease and eventually disappear, as confirmed by XRD pole figures. At growth rates ≥9 Å/min, CeO2(111) film with single in-plane domain was identified. The formation of CeO2(100) 3D islands at growth rates of 1–7 Å/min is a kinetically driven process unlike at growth rates ≥9 Å/min which result in an energetically and thermodynamically more stable CeO2(111) surface.
Stress-induced traps in multilayered structures
Authors: Ciurea Magdalena Lidia , Lazanu Sorina , Stavarache Ionel , Lepadatu Ana-Maria , Iancu Vladimir , Mitroi Mihai Razvan , Nigmatullin Raoul Rashid , Baleanu Cristina Mihaela
Published: January 2011

The trap parameters of defects in Si/CaF2 multilayered structures were determined from the analysis of optical charging spectroscopy measurements. Two kinds of maxima were observed. Some of them were rather broad, corresponding to “normal” traps, while the others, very sharp, were attributed to stress-induced traps. A procedure of optimal linear smoothing the noisy experimental data has been developed and applied. This procedure is based on finding the minimal value of the relative error with respect to the value of the smoothing window. In order to obtain a better accuracy for the description of the trapping-detrapping process, a Gaussian temperature dependence of the capture cross-sections characterizing the stress-induced traps was introduced. Both the normal and the stress-induced traps have been characterized, including some previously considered as only noise features.
The ideal dimensions of a Halbach cylinder of finite length
Authors: Bjørk R.
Published: January 2011

In this paper the smallest or optimal dimensions of a Halbach cylinder of a finite length for a given sample volume and desired flux density are determined using numerical modeling and parameter variation. A sample volume that is centered in and shaped as the Halbach cylinder bore but with a possible shorter length is considered. The external radius and the length of the Halbach cylinder with the smallest possible dimensions are found as a function of a desired internal radius, length of the sample volume and mean flux density. It is shown that the optimal ratio between the outer and inner radius of the Halbach cylinder does not depend on the length of the sample volume. Finally, the efficiency of a finite length Halbach cylinder is considered and compared with the case of a cylinder of infinite length. The most efficient dimensions for a Halbach cylinder are found and it is shown that the efficiency increases slowly with the length of the cylinder.
Spectroscopic ellipsometry study on the dielectric function of bulk Ti2AlN, Ti2AlC, Nb2AlC, (Ti0.5,Nb0.5)2AlC, and Ti3GeC2 MAX-phases
Authors: Mendoza-Galván A. , Rybka M. , Järrendahl K. , Arwin H. , Magnuson M. , Hultman L. , Barsoum M. W.
Published: January 2011

The averaged complex dielectric function ε=(2ε⊥+ε∥)/3 of polycrystalline Ti2AlN, Ti2AlC, Nb2AlC, (Ti0.5,Nb0.5)2AlC, and Ti3GeC2 was determined by spectroscopic ellipsometry covering the mid infrared to the ultraviolet spectral range. The dielectric functions ε⊥ and ε∥ correspond to the perpendicular and parallel dielectric tensor components relative to the crystallographic c-axis of these hexagonal compounds. The optical response is represented by a dispersion model with Drude–Lorentz and critical point contributions. In the low energy range the electrical resistivity is obtained from the Drude term and ranges from 0.48 μΩ m for Ti3GeC2 to 1.59 μΩ m for (Ti0.5,Nb0.5)2AlC. Furthermore, several compositional dependent interband electronic transitions can be identified. For the most important ones, Im(ε) shows maxima at: 0.78, 1.23, 2.04, 2.48, and 3.78 eV for Ti2AlN; 0.38, 1.8, 2.6, and 3.64 eV for Ti2AlC; 0.3, 0.92, and 2.8 eV in Nb2AlC; 0.45, 0.98, and 2.58 eV in (Ti0.5,Nb0.5)2AlC; and 0.8, 1.85, 2.25, and 3.02 eV in Ti3GeC2.
Open-system electronic dynamics and thermalized electronic structure
Authors: Chapman Craig T. , Liang Wenkel , Li Xiaosong
Published: January 2011

We propose and implement a novel computational method for simulating open-system electronic dynamics and obtaining thermalized electronic structures within an open quantum system framework. The system–bath interaction equation of motion is derived and modeled from the local harmonic oscillator description for electronic density change. The nonequilibrium electronic dynamics in a thermal bath is simulated using first-order kinetics. The resultant electronic densities are temperature-dependent and can take characteristics of the ground and excited states. We present results of calculations performed on H\documentclass[12pt]{minimal}\begin{document}$_2$\end{document}2 and 1,3-butadiene performed at the Hartree–Fock level of theory using a minimal Slater-type orbital basis set.
Monomer density profiles for polymer chains in confined geometries: Massive field theory approach
Authors: Usatenko Z.
Published: January 2011

Taking into account the well known correspondence between the field theoretical \documentclass[12pt]{minimal}\begin{document}$\phi ^4$\end{document}φ4 O(n)-vector model in the limit n → 0 and the behavior of long flexible polymer chains in a good solvent, the universal density–force relation is analyzed and the corresponding universal amplitude ratio \documentclass[12pt]{minimal}\begin{document}$B_{\rm real}$\end{document}B real is obtained using the massive field theory approach in fixed space dimensions d < 4. The monomer density profiles of ideal chains and real polymer chains with excluded volume interaction in a good solvent between two parallel repulsive walls, one repulsive and one inert wall, are obtained in the framework of the massive field theory approach up to one-loop order. Besides, the monomer density profiles for the dilute polymer solution confined in semi-infinite space containing mesoscopic spherical particle of big radius are calculated. The obtained results are in qualitative agreement with previous theoretical investigations and with the results of Monte Carlo simulations.
The effect of stress migration on electromigration in dual damascene copper interconnects
Authors: Heryanto A. , Pey K. L. , Lim Y. K. , Liu W. , Raghavan N. , Wei J. , Gan C. L. , Lim M. K. , Tan J. B.
Published: January 2011

Interconnects in very large scale integration (VLSI) chips are susceptible to failure due to stress migration (SM) and electromigration (EM). At use condition, these two failure mechanisms play a collective role in causing interconnect failure. We present a study on SM and EM interaction in lower (MX structure) and upper metal (MX+1 structure) of dual-damascene Cu/low-κ interconnects. It is found that both mechanisms are dependent; statistical analysis shows that EM failure time is affected by the presence of residual stress induced by SM. This effect was more severe in the lower metal, where the EM median-time-to-failure (t50) for the majority of samples could be degraded by 30%–60%. For the upper metal of Cu interconnects, the t50 is degraded by about 10%. The reliability implication of the residual stress in copper interconnects on the EM is further investigated with various failure analysis techniques and three-dimensional finite element simulation. It is proposed that SM can influence EM when there is significant amount of vacancy accumulation due to SM in the cathode area which accelerates EM nucleation time. In the case of the MX structure, our experimental results show that SM and EM interaction occurs exactly below the via at the MX cathode side, leading to abrupt failures. On the other hand, in MX+1 structure, vacancies are likely to accumulate at the edge of upper metal lead during SM test, thus accelerating the failure during subsequent EM test. A failure mechanism model for stress evolution and void formation is proposed to provide insight into the interaction between these two failure mechanisms.
Projection effect on extraction of Si/SiO2 interface roughness from high-resolution transmission electron microscopy measurements
Authors: Zhao Yi
Published: January 2011

In this study, we carry out a theoretical investigation of the impact of transmission electron microscopy (TEM) sample thickness, which we varied from 2 to 140 nm, on the Si/SiO2 interface roughness that is directly extracted from a TEM image and is used for further calculation of the roughness scattering limited mobility (μsr) of Si metal-oxide-semiconductor (MOS) field-effect transistors [Zhao et al. IEEE Trans. Electron Devices 57, 2055 (2010)]. We found that the values of the roughness height (Δ) and the correlation length (Λ) of the interface, as extracted from the TEM image, are underestimated, especially for the region in which the TEM sample thicknesses are large. Furthermore, we confirmed that the distribution function of the projected one-dimensional SiO2/Si interface is identical to that of the original two-dimensional interface. These results may help us to gain a more quantitative understanding of the roughness scattering limited mobility of advanced MOS devices by performing TEM measurements.
Studies on structures, electronic and magnetic properties of TM-doped InnSbn (n=7–12,14,16) clusters (TM=Mn, Fe, and Co)
Authors: Ding Jian-Ning , Yuan Ning-Yi , Li Chang-Lin , Wang Xiu-Qin , Chen Guang-Gui , Chen Xiao-Shuang , Lu Wei
Published: January 2011

We study the geometric, electronic, and magnetic properties of TM monodoped and bidoped (TM=Mn,Fe,Co) InnSbn (n=7–12,14,16) clusters using the first principles of density functional theory. For monodoping, three doping modes (substitutional, endohedral, and exohedral doping) are considered. The calculations suggest that TM-doped InnSbn (n=7–12,14,16) clusters are magnetic except Co-doped In7Sb7 and Co-doped In14Sb14. The lowest energy configurations of TM2In16Sb16 clusters are ferromagnetic (FM) with the TM atoms at the nearest neighbor position and the coupling between the TM atoms is mainly governed by the FM interaction via TM–TM direct interaction. The hybridization between Mn 3d state and the Sb 5p state is also favorable to the stability of FM state.
Dielectric dispersion of BaSrTiO3 thin film from centimeter to submillimeter wavelengths
Authors: Houzet Gregory , Blary Karine , Lepilliet Sylvie , Lippens Didier , Burgnies Ludovic , Vélu Gabriel , Carru Jean-Claude , Nguéma Edwin , Mounaix Patrick
Published: January 2011

The dielectric dispersion of ferroelectric BaxSr1−xTiO3 (BST) thin film in a paraelectric phase was characterized from centimeter to submillimeter wavelengths. To this aim, interdigitated capacitors were patterned on a micrometer scale onto a BST layer with a barium concentration of 0.5 and were subsequently integrated by using a coplanar waveguide technology. The retrieval of the complex permittivity of BST was performed by vectorial scattering parameter measurements up to 190 GHz for various controlling dc field up to 300 kV/cm. At higher frequency, submillimeter wavelength measurements were performed by time domain spectroscopy under free space condition. On this basis, the dispersion of the real part of the permittivity along with the loss tangent are retrieved in agreement with a distributed interaction of ac-field with soft phonons vibration modes, and overlapping between dipole polarization and ionic polarization is observed, around 700 GHz. It is also shown that dipole polarization can be attributed to the presence of small polar nanoregions in the BST film which act as in ferroelectric material with diffuse phase transition.
Specific heat of shape memory alloys with soft elastic moduli
Authors: Chernenko V. A. , Homenko D. V. , L’vov V. A. , Barandiaran J. M.
Published: January 2011

The crystal lattice of shape memory alloys is unstable toward shears along the certain crystallographic directions. This gives rise to a martensitic transformation accompanied by an anomalous reduction in the shear elastic moduli in a wide proximity to the transformation temperature. In the present paper, we report a considerable influence of the reduction in the elastic moduli on the temperature dependence of heat capacity of shape memory alloys. The Debye formula for the heat capacity has been modified to take into account the abnormally low values of the shear elastic moduli. The modified formula was used to achieve a good fit of the experimental values for a number of Ni–Mn–Ga ferromagnetic shape memory alloys. The fitting procedure allows the separation of the electron and phonon contributions to the heat capacity of these alloys. As a result, the Debye temperatures, the elastic moduli, and the average velocities of sound waves were evaluated.
Impact of I-V behavior and estimated temperature rise on surface and tip modification of the nanocontact between a highly doped silicon scanning probe microscope tip and gold surface under ambient conditions
Authors: Parkhi Anjali , Gross Todd S.
Published: January 2011

We studied the evolution of the current voltage behavior of a doped Si tip on 1-octadecanethiol coated Au film on mica to understand the impact of current on material modification for negative tip biases up to 10 V. The resistance is non linear ranging from a 550 kΩ to essentially infinite resistance at tip bias of 250 mV to as low as 6–10 kΩ for tip biases of 10 V. The measured resistance is shown to be primarily due to the contact resistance. The tip radius increases with repeated exposure to 10 V tip bias. The oxide layer on the tip initially decreases but then increases with repeated exposure to 10 V tip bias. We performed heat transfer analysis of the tip-substrate interface to show that the measured powers are sufficient to raise the temperature at the interface to as high as 200–1100 °C which is sufficient to cause rapid tip oxidation of bare Si tip in an ambient environment and can cause diffusion or melting of organic or inorganic thin coatings from coated Si tips [Liu and Miller, J. Phys. Chem. C 111, 10758 (2007) and Liu and Miller, Nanotechnology 20, 055303 (2009)]. We observed 5–20 nm high bumps on the gold surface for negative tip bias >5.5 V. The bumps frequently disappeared with repetitive scanning leaving a 2–10 nm deep pit behind and the bumps created in high humidity (>60%) were more easily removed.
Correlation between nanoparticle and plasma parameters with particle growth in dusty plasmas
Authors: Chai Kil Byoung , Seon C. R. , Chung C. W. , Yoon N. S. , Choe Wonho
Published: January 2011

Since plasma parameters are altered by dust particles, studying how plasma parameters are related to dust particle growth is an important research issue in dusty plasma. In this paper, the correlation between plasma parameters (electron temperature and ion flux) and particle parameters (particle radius and density) is investigated in silane plasma both experimentally using a floating probe and theoretically by solving balance equations including an additional electron and ion loss to the dust. The results reveal that while the ion flux shows two peak values in the early discharge phase and at the end of coagulation phase, the electron temperature shows a sudden increase in the coagulation step and a gradual decrease in the molecular accretion step. Moreover, the calculated results with the secondary electron emission taken into account produce the best fit with the experimental results. Thus the study confirms that the secondary electron emission plays a crucial role in the coagulation of the dust particles.
Three-dimensional mapping of stresses in plastically deformed diamond using micro-Raman and photoluminescence spectroscopy
Authors: Erasmus R. M. , Daniel R. D. , Comins J. D.
Published: January 2011

The extreme mechanical properties of diamond have made it the material of choice for many industrial applications, ranging from cutting and grinding to wire-drawing dies. A detailed knowledge of its mechanical properties, also at high temperature, is thus of importance. Micro-Raman and photoluminescence (PL) spectroscopy were used to map the three-dimensional (3D) stress distribution surrounding a plastic impression made in a synthetic, type Ib single crystal diamond. The impression was created on a (100) face of the crystal with an Si3N4 impressor at 1400 °C using the so-called soft impressor technique. The diamond Raman peak was mapped at room temperature at the surface and at fixed intervals of 10 μm below the surface using a motorised X-Y stage. The depth (Z)-resolution was limited to 10 μm by means of a confocal pinhole. Using data from the Raman peak position, a 3D map of the stress contours surrounding the impression was generated, while the Raman width data yielded a map of the plastic deformation volume. The surface stress map shows a cross-shaped rosette pattern that corresponds very closely with micrographs imaging the pile-up on the surface due to dislocation movement. The “arms” of the pattern are in compression (∼1.5 GPa), while the center of the impression is in tension (∼1 GPa). The deformation map shows a radially symmetric area of deformation centered on the impression, with the maximum degree of deformation at the center. The stress contours compare favorably with the resolved shear stress contours calculated for diamond. PL intensity maps of the zero phonon line (ZPL) associated with the [N–V]− defect center at 1.945 eV provide images of the extent of vacancy formation and movement during the impression process. Data concerning the position and width of the ZPL correspond well with the Raman results.
Atomic layer deposited Ta2O5 gate insulation for enhancing breakdown voltage of AlN/GaN high electron mobility transistors
Authors: Deen D. A. , Storm D. F. , Bass R. , Meyer D. J. , Katzer D. S. , Binari S. C. , Lacis J. W. , Gougousi T.
Published: January 2011

AlN/GaN heterostructures with a 3.5 nm AlN cap have been grown by molecular beam epitaxy followed by a 6 nm thick atomic layer deposited Ta2O5 film. Transistors fabricated with 150 nm length gates showed drain current density of 1.37 A/mm, transconductance of 315 mS/mm, and sustained drain-source biases up to 96 V while in the off-state before destructive breakdown as a result of the Ta2O5 gate insulator. Terman’s method has been modified for the multijunction capacitor and allowed the measurement of interface state density (∼1013 cm−2 eV−1). Small-signal frequency performance of 75 and 115 GHz was obtained for ft and fmax, respectively.
Superconductivity in oxygen-annealed FeTe1−xSx single crystal
Authors: Mizuguchi Yoshikazu , Deguchi Keita , Kawasaki Yasuna , Ozaki Toshinori , Nagao Masanori , Tsuda Shunsuke , Yamaguchi Takahide , Takano Yoshihiko
Published: January 2011

We investigated the S-doping-driven phase transition from antiferromagnetic to superconducting in FeTe1−xSx single crystals. The partial substitution of Te by S suppresses antiferromagnetism in Fe-square lattice. Superconductivity is induced by oxygen annealing for only FeTe1−xSx in which the long-range magnetic ordering is suppressed. To realize superconductivity in FeTe1−xSx, both S concentration enough to suppress antiferromagnetism and oxygen annealing are required. Anisotropy of superconductivity in oxygen-annealed FeTe0.886S0.114 was estimated to be 1.17.
Dielectric and ferroelectric behaviors of BaTiO3–(K1/4Bi3/4)(Mg1/4Ti3/4)O3 ceramics
Authors: Lu Bin , Wang Xiaoli , Shi Jing
Published: January 2011

Perovskite ferroelectric ceramics of (1−x)BaTiO3–x(K1/4Bi3/4)(Mg1/4Ti3/4)O3 [(1−x)BT–xKBMT, x=0.06, 0.1, and 0.14] were synthesized via solid-state reaction method. Dielectric constant maximum εm of BT-KBMT ceramics declines with KBMT content. ε(T) peak temperature Tm (402 K) is close to Tc (403 K) of the parent BT for composition 0.94BT-0.06KBMT, and decreases to 382 K for 0.9BT-0.1KBMT. Dielectric behaviors of the two compositions exhibit features of ferroelectrics with diffuse phase transition. A high-field linear polarization behavior was observed in the two compositions around temperature range of the ε(T) peak summit. ε(T) peak of 0.86BT-0.14KBMT is saddle-backed. The high turning temperature Tm1 fixes around 384 K, and is the Burns temperature TB of the composition. The low turning point Tm2 is 326 K at 100 Hz and highly frequency dependent. The dielectric anomaly of 0.86BT-0.14KBMT around Tm2 is associated with diffuse transitions from ferroelectric macrodomains to microdomains, polar nanoregions and paraelectric states.
Inner surface enhanced femtosecond second harmonic generation in thin ZnO crystal tubes
Authors: Zheng C. C. , Xu S. J. , Ning J. Q. , Zhang S. F. , Wang J. Y. , Che C. M. , Hao J. H.
Published: January 2011

At room temperature, efficient second harmonic generation (SHG) emission is observed in thin ZnO crystal hollow rods (tubes) with diameter ∼0.2 mm under the excitation of femtosecond laser from 700 to 860 nm. Power and polarization dependence of the SHG signal on the primary excitation beam is also measured. The multiple total reflections between the outer and inner surfaces of the sample are analyzed to be responsible for the efficient SHG. Ninefold amplification of SHG signal in the tube structure is estimated.
Semiconductor nanotube in strong electrostatic field
Authors: Harutyunyan V. A.
Published: January 2011

The states of interacting electron-hole pair in semiconductor nanotube in the presence of strong lateral homogeneous electric field are considered theoretically. It is shown in single-particle approximation that along with the size-quantization of charge carriers’ motion by the radial direction the external strong field leads to the additional (field) localization of particles by the angular variable also. The corresponding additional energy spectrum has an equidistance character. At the same time the strong external field polarizes the electron-hole pair and traps them on the opposite ends of tube’s diameter. Consequently, the excitonic complex with transversal dimensions of the order of the system’s diameter is formed in a nanotube. By using the variation approach, the binding energies and wave functions of first two states of such field excitonlike complex in the tube are also calculated. The electro-optical transitions in the tube without and with the account of electron-hole interaction are considered. It is shown that along with the dependence on the geometric dimensions of the sample and the intensity of the external field, the presence of strong electric field leads also to an explicit dependence of the intensity and the threshold frequencies of absorption on the effective mass of carriers. The resulting field dependence of the optical characteristics of the system can serve as a basis for the direct experimental observation of such excitonlike complexes in semiconductor nanotubes.
A nonequilibrium Green’s function study of thermoelectric properties in single-walled carbon nanotubes
Authors: Jiang Jin-Wu , Wang Jian-Sheng , Li Baowen
Published: January 2011

The phonon and electron transport in single-walled carbon nanotubes (SWCNT) are investigated using the nonequilibrium Green’s function approach. In zigzag SWCNT (n,0) with mod(n,3)≠0, the thermal conductance is mainly attributed to the phonon transport, while the electron only has few percentage contribution. The maximum value of the figure of merit (ZT) is about 0.2 in this type of SWCNT. The ZT is considerably larger in narrower SWCNT because of enhanced Seebeck coefficient. ZT is smaller in the armchair SWCNT, where Seebeck coefficient is small due to zero band gap. It is found that the cluster isotopic doping can reduce the phonon thermal conductance obviously and enhance the value of ZT. The uniaxial elongation and compress strain depresses phonons in whole frequency region, leading to the reduction in the phonon thermal conductance in whole temperature range. Interestingly, the elongation strain can affect the phonon transport more seriously than the compress strain, because the high frequency G mode is completely filtered out under elongation strain ϵ>0.05. The strain also has important effect on the subband edges of the electron band structure by smoothing the steps in the electron transmission function. The ZT is decreased by strain as the reduction in the electronic conductance overcomes the reduction in the thermal conductance.
Probing phase transitions and stability of organic semiconductor single crystals by dielectric investigation
Authors: Sassella A. , Braga D. , Campione M. , Ciabattoni T. , Moret M. , Parravicini J. , Parravicini G. B.
Published: January 2011

The characterization of organic crystalline semiconductors in terms of possible phase transitions with temperature may be very important for the general knowledge of the material but also in view of application in devices: such properties may indeed cause variations in the macroscopic behavior of the material, especially relevant at the operation temperatures of few tens of degrees. Here, phase transitions in α-quaterthiophene single crystals are detected and studied by means of dielectric investigation, a powerful tool to go deeper in this matter. After describing the relative dielectric constant and the ac conductivity of the different solid phases, found to display either an insulating or a semiconducting character, quantitative information is provided on the relative stability of the different phases the active material may transform into, by giving an estimate of their entropy content.
Shock equation of state of multi-constituent epoxy-metal particulate composites
Authors: Jordan Jennifer L. , Herbold Eric B. , Sutherland Gerrit , Fraser Andrew , Borg John , Richards D. Wayne
Published: January 2011

The shock properties of epoxy-based particulate composites have been extensively studied in the literature. Generally, these materials only have a single particulate phase; typically alumina. This paper presents equation of state experiments conducted on five epoxy-based particulate composites. The shock stress and shock velocity states were measured for five different composites: two epoxy-aluminum two-phase composites, with various amounts of aluminum, and three epoxy-aluminum-(metal) composites, where the metal constituent was either copper, nickel, or tungsten. The impact velocities ranged from 300 to 960 m/s. Numerical simulations of the experiments of epoxy-Al are compared with mesoscale simulations of epoxy-Al2O3 composites to investigate the effect of the soft versus hard particulate; additionally, an epoxy-Al–W simulation was conducted to investigate the material properties of the second phase on shock response of these materials. In these epoxy-based particulate composites, the slope of the shock velocity-particle velocity curve appears to depend on the epoxy binder. It is shown that the addition of only 10 vol % of a second, denser metallic phase significantly affects the shock response in these composites.
Numerical investigation of the nanosecond opening-mechanism of drift step recovery diodes
Authors: Yang Xiudong , Li Yongdong , Wang Hongguang , Li Zhongjie , Ding Zhenjie
Published: January 2011

A circuit-fluid coupled model is proposed for describing the dynamics of the electron-hole plasma and the evolution of the electric field in the drift step recovery diodes (DSRDs). The model takes account of the real doping profile of the p+-p-n+ type silicon DSRD and the elementary processes such as the impact ionization, Shockley-Read-Hall recombination and Auger recombination. The current opening process of the DSRD is simulated with the proposed model and then the nanosecond opening-mechanism is investigated. The opening process is found to appear at the base region of the DSRD, due to the high density electron-hole plasma formed at the forward pumping period. According to the opening rate, the opening process can be divided into two stages: a slow stage near the p+-p boundary and an abrupt one at p-n junction. Moreover, simulation results show that the p+-p-n+ type DSRD with a lower base tends to generate pulses with shorter rise time and higher peak voltage.
Magnetic characterization of diluted magnetic semiconductor thin films
Authors: Gu Jian-Jun , Liu Li-Hu , Qi Yun-Kai , Xu Qin , Zhang Hai-Feng , Sun Hui-Yuan
Published: January 2011

In studies of weak magnetism in diluted magnetic semiconductor thin films, there is often a significant difficulty in subtracting the background signal arising from the substrate. In the present work, an improved magnetic correction method is proposed. For Al doped ZnO and Fe doped TiO2 films, the magnetic moment and coercivity of the samples were corrected by the improved method, and the maximum fitting error due to the glass substrate was calculated. The accuracy and rationale of the improved method are discussed and compared with the traditional method. The results show that room temperature ferromagnetism is observed in Fe doped TiO2 thin films. The ferromagnetism is strongly correlated with the substitution of Ti by Fe in the TiO2 lattice, which results in a change in the crystal structure and the quality of the crystallization of the TiO2 films. The changes are responsible for altering ferromagnetism in the films.
Communication: A dramatic transition from nonferromagnet to ferromagnet in finite fused-azulene chain
Authors: Qu Zexing , Zhang Shushu , Liu Chungen , Malrieu Jean-Paul
Published: January 2011

One-dimensional fused-azulene oligomers (n = 2 − 6) are studied with the effective valence bond as well as density functional theory methods. A nonferromagnetic (closed-shell singlet) to ferromagnetic (triplet) ground state transformation is witnessed with increasing length of oligomers. The computational results are interpreted in terms of spin coupling between the unpaired electrons of two nonbonding molecular orbitals localized, respectively, on the top and bottom chains of the oligomers. The present study provides a theoretical suggestion for understanding the ferromagnetic spin polarizations that has been observed very recently in graphene nanoribbons.
In situ vacuum measurement of the thickness dependence of electron mobility in naphthalenetetracarboxylic diimide-based field-effect transistors
Authors: Liu Shun-Wei , Lee Chih-Chien , Wen Je-Min , Chen Chin-Ti
Published: January 2011

We present an in situ electrical measurement for the characteristics of n-channel organic field-effect transistors (OFETs) based on N,N′-dipentadecafluorooctyl-1,4,5,8-naphtalene tetracarboxylic diimide (NTCDI-C8F15) organic semiconductors. The field-effect mobilities have been estimated as a function of the number of monolayers (MLs). The electron mobility (μe) of NTCDI-C8F15 OFET as thin as 2 ML has been determined. μe increases rapidly with increasing film thickness, although it reaches saturation thickness (d0) around 3.5 ML. Atomic force microscopy confirms island mode growth mechanism of NTCDI-C8F15 with near upright position stacking on SiO2 substrate and it is attributed to the fluorophobic effect of the material.
An electrospun fiber phototransistor by the conjugated polymer poly[2-methoxy-5-(2’-ethylhexyloxy)-1,4-phenylene-vinylene]
Authors: Tu Deyu , Pagliara Stefano , Cingolani Roberto , Pisignano Dario
Published: January 2011

We investigate the photoresponse of field-effect transistors based on conjugated polymer electrospun fibers. The electrical performances of single fiber transistors are controlled by modulating the channel conductivity under white light illumination. We demonstrate a photoresponsivity up to 100 mA/W for a 500-nm channel width fiber phototransistor illuminated by an intensity of 9.6 mW/cm2. Studying the photoresponse switching cycles evidences that the photocurrent relaxation time can be reduced down to about 40 s by increasing the fiber surface-to-volume ratio.
Strain modulated band gap of edge passivated armchair graphene nanoribbons
Authors: Peng Xihong , Velasquez Selina
Published: January 2011

First principles calculations were performed to study strain effects on band gap of armchair graphene nanoribbons (AGNRs) with different edge passivation, including H, O, and OH group. The band gap of the H-passivated AGNRs shows a nearly periodic zigzag variation under strain. For O and OH passivation, the zigzag patterns are significantly shifted by a modified quantum confinement due to the edges. In addition, the band gap of the O-passivated AGNRs experiences a direct-to-indirect transition with sufficient tensile strain (∼5%). The indirect gap reduces to zero with further increased strain.
Anisotropic scattering of surface state electrons at a point defect on Bi(111)
Authors: Cottin M. C. , Bobisch C. A. , Schaffert J. , Jnawali G. , Sonntag A. , Bihlmayer G. , Möller R.
Published: January 2011

Scanning tunneling microscopy was applied to study the lateral variation of the local density of electronic states on the Bi(111) surface in the vicinity of a point defect. At an energy close to the Fermi level a characteristic pattern with a threefold symmetry is found. The pattern can be attributed to the scattering between two electronic surface states which are split by spin orbit coupling. The observation is well described by the superposition of three monochromatic waves. The phase of the waves relative to the center of the defect leads to a reduction to a threefold symmetry.
Embedded voids approach for low defect density in epitaxial GaN films
Authors: Frajtag P. , El-Masry N. A. , Nepal N. , Bedair S. M.
Published: January 2011

We have developed a technique for defect reduction in GaN epitaxial films grown on sapphire substrates. This technique relies on the generation of high densities of embedded microvoids (∼108/cm2), a few microns long and less than a micron in diameter. These voids are located near the sapphire substrate, where high densities of dislocations are present. Network of embedded voids offer free surfaces that act as dislocation sinks or termination sites for the dislocations generated at the GaN/sapphire interface. Both transmission electron and atomic force microscopy results confirm the uniform reduction of the dislocation density by two orders of magnitude.
The effects of device geometry on the negative bias temperature instability of Hf-In-Zn-O thin film transistors under light illumination
Authors: Kim Jeong Hwan , Kim Un Ki , Chung Yoon Jang , Jung Ji Sim , Ra Sang Ho , Jung Hyung Suk , Hwang Cheol Seong , Jeong Jae Kyeong , Lee Sang Yoon
Published: January 2011

The negative bias illumination temperature stress instability of amorphous Hf-In-Zn-O thin film transistors with different dimensions was evaluated. The threshold voltage (Vth) shift increased in devices with shorter channel lengths but showed almost no association with the channel width. This behavior was attributed to the diffusion and drift of the photogenerated holes at the channel/dielectric interface from regions near the drain to those near the source, which were due to the simultaneous presence of gate and drain biases. The Vth near the source, which shows the largest shift and hence has the highest local value, governs the overall Vth.
Experimental observation of Fano resonance in a single whispering-gallery microresonator
Authors: Li Bei-Bei , Xiao Yun-Feng , Zou Chang-Ling , Liu Yong-Chun , Jiang Xue-Feng , Chen You-Ling , Li Yan , Gong Qihuang
Published: January 2011

We experimentally observe Fano resonance in a single silica toroidal microresonator, in which two whispering-gallery modes (WGMs) are excited simultaneously through a fiber taper. By adjusting the fiber-cavity coupling strength and the polarization of incident light, the Fano-like resonance line shape can be engineered and further convert to the electromagnetically induced transparency (EIT) like line shape. Our theoretical analysis reveals that both the Fano and EIT resonances originate from an indirect-coupling of two originally orthogonal WGMs, which is mediated by the common fiber taper waveguide. The sharp Fano line shape holds great potential in optical switching and sensitivity-enhanced biochemical sensing.
Band gap parameters of one-dimensional bicomponent nanostructured magnonic crystals
Authors: Lin C. S. , Lim H. S. , Wang Z. K. , Ng S. C. , Kuok M. H.
Published: January 2011

We investigate theoretically the lowest-frequency spin waves in one-dimensional periodic arrays of alternating cobalt and permalloy nanostripes to analyze their recently measured first band gap parameters. Based on insights from numerical calculations, we formulate simple equations relating center frequencies and widths of the first band gaps of these magnonic crystals to their geometric and magnetic parameters. Analytical calculations based on these equations accord well with both the measured and the numerically generated size-dependences of the band gap parameters. Our approach permits the easy and convenient evaluation of the band gap parameters of bicomponent magnonic crystals comprising one-dimensional arrays of nanostripes of alternating magnetic materials.
Stability of polarization in organic ferroelectric metal-insulator-semiconductor structures
Authors: Kalbitz R. , Frübing P. , Gerhard R. , Taylor D. M.
Published: January 2011

Dielectric measurements have been carried out on all-organic metal-insulator-semiconductor structures with the ferroelectric polymer poly(vinylidenefluoride-trifluoroethylene) as the gate insulator. It is shown that the polarization states remain stable after poling with accumulation and depletion voltage. However, negative charge trapped at the semiconductor-insulator interface during the depletion cycle masks the negative shift in flatband voltage expected during the sweep to accumulation voltages.
Electromagnetic nonreciprocity and gyrotropy of graphene
Authors: Sounas Dimitrios L. , Caloz Christophe
Published: January 2011

The authors study the transmission properties of magnetically biased graphene via the general anisotropic conductivity tensor, which accounts for both the diagonal and the Hall conductivities. Appreciable gyrotropic and electromagnetic nonreciprocal (time reversal asymmetry) properties are observed at subterahertz frequencies, which result in an extremely broadband nonreciprocal polarization rotation phenomenon.
Transient molecular dynamics simulations of liquid viscosity for nonpolar and polar fluids
Authors: Thomas Jason C. , Rowley Richard L.
Published: January 2011

A transient molecular dynamics (TMD) method for obtaining fluid viscosity is extended to multisite, force-field models of both nonpolar and polar liquids. The method overlays a sinusoidal velocity profile over the peculiar particle velocities and then records the transient decay of the velocity profile. The viscosity is obtained by regression of the solution of the momentum equation with an appropriate constitutive equation and initial and boundary conditions corresponding to those used in the simulation. The transient velocity decays observed appeared to include both relaxation and retardation effects. The Jeffreys viscoelastic model was found to model accurately the transient responses obtained for multisite models for n-butane, isobutane, n-hexane, water, methanol, and 1-hexanol. TMD viscosities obtained for saturated liquids over a wide range of densities agreed well for the polar fluids, both with nonequilibrium molecular dynamics (NEMD) results using the same force-field models and with correlations based on experimental data. Viscosities obtained for the nonpolar fluids agreed well with the experimental and NEMD results at low to moderate densities, but underpredicted experimental values at higher densities where shear-thinning effects and viscous heating may impact the TMD simulations.
Grazing-incidence x-ray diffraction from GaN epitaxial layers with threading dislocations
Authors: Barchuk M. , Holý V. , Miljević B. , Krause B. , Baumbach T.
Published: January 2011

We used high-resolution x-ray reciprocal space mapping in noncoplanar grazing-incidence geometry for the determination of the density of edge threading dislocations in c-oriented GaN epitaxial layers and we compared the measured intensity distributions with results of numerical Monte Carlo simulations of diffuse scattering. We demonstrated that a combination of diffraction data taken in coplanar symmetric and noncoplanar grazing-incidence geometries makes it possible to obtain the densities of screw and edge threading dislocations.
Magnetism and carrier modulation in (Ga,Mn)As/organic-dye hybrid devices
Authors: Herrera Diez L. , Konuma M. , Placidi E. , Arciprete F. , Rushforth A. W. , Campion R. P. , Gallagher B. L. , Honolka J. , Kern K.
Published: January 2011

We present the manipulation of magnetic and electrical properties of (Ga,Mn)As via the adsorption of dye-molecules as a step toward the realization of light-controlled magnetic-semiconductor/dye hybrid devices. A significant lowering of the Curie temperature with a corresponding increase in electrical resistance and a higher coercive field is found for the (Ga,Mn)As/fluorescein system with respect to (Ga,Mn)As. Upon exposure to visible light a shift in Curie temperature toward higher values as well as a reduction of the electrical resistance and the coercive field can be achieved. This points toward a hole quenching effect at the molecule-(Ga,Mn)As interface which is susceptible to light exposure.
Charged basal stacking fault scattering in nitride semiconductors
Authors: Konar Aniruddha , Fang Tian , Sun Nan , Jena Debdeep
Published: January 2011

A theory of charge transport in semiconductors showing built-in polarization (polar) is developed in the presence of basal stacking faults. The theory is based on quantum tunneling in conjunction with the semiclassical description of diffusive charge transport. It is shown that the presence of basal stacking faults leads to anisotropy in carrier transport. The theory is applied to carrier transport in gallium nitride films consisting of a large number basal stacking faults, and the result is compared with experimental data.
Single-layer metallicity and interface magnetism of epitaxial graphene on SiC(0001¯)
Authors: Deretzis I. , La Magna A.
Published: January 2011

We perform density functional theory calculations for the determination of the structural and electronic properties of epitaxial graphene on 4H–SiC(0001¯). Using commensurate supercells that minimize nonphysical stresses we show that in contrast with Si-face epitaxial films, the first graphene layer that forms on the C-face of SiC is purely metallic with its π-bands partially preserved. Typical free-standing characteristics are fully recovered with a second graphene layer. We moreover discuss on the magnetic properties of the interface and the absence of Fermi-level pinning effects that could allow for a plausible device operation starting from the off-state.
Enhanced figure of merit of a porous thin film of bismuth antimony telluride
Authors: Kashiwagi Makoto , Hirata Shuzo , Harada Kentaro , Zheng Yanqiong , Miyazaki Koji , Yahiro Masayuki , Adachi Chihaya
Published: January 2011

A porous thin film of Bi0.4Te3Sb1.6 with an enhanced figure of merit of 1.8 at room temperature was fabricated by flash evaporation on an alumina substrate containing hexagonally arranged nanopores with an average diameter of 20 nm, separated by an average distance of 50 nm. The thermal conductivity was significantly reduced compared with standard Bi0.4Te3Sb1.6 films to 0.25 W/(m⋅K) with no major decrease in either the electrical conductivity (398 S/cm) or the Seebeck coefficient (198 μV/K). The reduction in thermal conductivity was rationalized using a model for the full distribution of the phonon mean free path in the film.
Lure-assisted visual census: a new method for quantifying fish abundance, behaviour, and predation risk in shallow coastal habitats
Authors: Kruschel Claudia , Schultz Stewart
Published: January 2011

Understanding the causes of variation in faunal abundance and diversity across shallow coastal habitats is a fundamental goal of marine ecology. Field methods for inferring a habitat effect on population density and predation risk are informative only if method biases are equal across habitats and species. We hypothesised that observation of fixed lures has a species by bias interaction if sampled species have different modes of predation, and that these biases are overcome by use of moving lures. We tested this hypothesis by observation of fixed and moving lures within seagrass and bare sediment in the Novigrad Sea, Croatian Adriatic. Both methods showed that ambush predators peaked in seagrass, wait–chasers peaked over bare sediment, and move–chasers were abundant in both. Stationary lures underestimated wait–chase and wait–ambush predators relative to moving lures, whereas moving lures did not underestimate the density of predators. These results indicate that stationary lures can underestimate both fish abundance and predation risk in the presence of waiting predators, and that if waiting predators are more abundant in structured habitat, then stationary lures will underestimate the predation risk within such habitats. Use of moving lures may be preferable for comparing habitats differing in structural complexity and frequency of predation modes.
Locomotory activity and depth distribution of adult great barracuda (Sphyraena barracuda) in Bahamian coastal habitats determined using acceleration and pressure biotelemetry transmitters
Authors: O'Toole A. C. , Murchie K. J. , Pullen C. , Hanson K. C. , Suski C. D. , Danylchuk A. J. , Cooke S. J.
Published: January 2011

Documenting free-swimming fish in their natural environment using acoustic transmitters equipped with acceleration and pressure sensors may contribute to knowledge of locomotory behaviour for a variety of aquatic species. Previously, collection of acceleration data has been limited to archival loggers, necessitating retrieval of the devices; however, recent advances in biotelemetry have allowed for acceleration data to be transmitted to a remote receiver. To illustrate the application of this technology, relative locomotory activity and depth utilisation of adult great barracuda (Sphyraena barracuda) were monitored across habitat types and diel periods using acoustic transmitters equipped with tri-axial acceleration and pressure sensors within an acoustic telemetry array (n = 53 receivers) deployed in The Bahamas. Although there were no differences in acceleration or depth use across habitats or diel periods, there was evidence of movement into shelf habitat during mid-day where they occupied depths >10 m. Given both the method of calculating the accelerometer output, and that the transmitters were unable to store and transmit large quantities of data, we suggest choosing transmitter settings with a short average delay and high transmission frequency to optimise data quality and resolution. This paper represents one of the first reports of the use of telemetered acceleration values from free-swimming fish.