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International audience; A metasurface consisting of a 2D periodic square lattice of metallic square ring resonators is considered for supporting spoof surface plasmon polariton (SSPP) modes. First, SSPP modes are highlighted by simulated dispersion curves. The dispersion curves are analyzed by considering hybridization of modes in either a patch or an aperture array with frequency shifts. Hybridization of modes is also supported by means of surface current and electric field mappings showing consistent symmetric and anti-symmetric fields. Second, it is shown that a dipole antenna located near the metasurface is able to generate SSPP modes. The transmission spectra between two antennas located at each extremity of the metasurface show transmission enhancements compared with propagation in free space. The transmission enhancements are shown for different locations of the antennas either facing each other or positioned in diagonal. Finally, the transmission enhancement is connected to SSPP modes by comparing the transmission spectra and the measured dispersion curves with each other. Specifically, two transmission windows measured for an embroidered metasurface are related to the cutoff frequency of the two SSPP modes supported by the metasurface as evidenced by the measured dispersion curves.

International audience; We propose the design of three port photonic and plasmonic demultiplexers where filtering toward the two outputs is based on the phenomena of Fano resonances and electromagnetically induced transparency (EIT). We use a Cross-shape resonator in one output and a U-shape resonator composed of two stubs in the other output. We give a theoretical demonstration of the geometrical parameters of both resonators in order to filter one wavelength in one output while leaving the other output unperturbed. These results are confirmed by experimental validation in the radio frequency domain and a numerical simulation in the infrared (IR) domain using plasmonic metal–insulator–metal waveguides. The Cross resonator in the first output can give rise to an EIT resonance, whereas the U-shaped resonator in the second output may exhibit both EIT and Fano resonances depending on the lengths chosen for the stubs. Therefore, different demultiplexing schemes can be proposed such as achieving a Fano resonance in one output and an EIT in the other, or EIT resonances in both outputs. The Fano resonance is obtained by bringing resonance close to transmission zero, whereas the EIT results from the squeezing of resonance between two transmission zeros. When the widths of the resonances tend to zero, they transform to trapped or bound states in the continuum with an infinite lifetime. We show that the crosstalk between the two channels can be reduced to [Formula: see text] dB and the sensitivity can reach 2390.8 nm/RIU, RIU is the refractive index units. Finally, we highlight the performance of our design as a high sensitive filter and sensor in the IR domain. In this work, the analytical calculations and demonstrations are performed by using Green’s function approach, the experimental verifications are realized by means of coaxial cables operating in the radio frequency range and the numerical simulations are obtained using the finite element method via Comsol Multiphysics software.

Despite a high lateral breakdown voltage above 10 kV for large contact distances and a breakdown field of 5 MV cm−1 for short contact distances, an Al0.9Ga0.1N/GaN heterostructure with an 8 nm strained GaN channel grown on an AlN/sapphire template suffers from a low and anisotropic mobility. This work deals with a material study to elucidate this issue. Threading dislocations were observed along the growth direction in transmission electron microscopy pictures and are more in number in the (11−20) plane. Steps were also detected in this plane at the GaN channel interfaces. With the help of device simulations and static characterizations, the deep level transient spectroscopy technique allowed five traps located in the GaN channel to be identified. Most of them are associated with nitrogen- or gallium-vacancy-related defects and are expected to be localized at the interfaces of GaN with the buffer and the barrier. It is likely that these electrically active defects contribute to reduce the mobility in the two-dimensional electron gas. In addition, a link was established between the mobility and the dependence of the quality of the channel interfaces on the crystallographic orientation.

International audience; The development of seismic metamaterials has attracted much research interest in the past decade. Efforts have been made by using experimental and theoretical approaches to isolate buildings and structures susceptible to elastic surface wave damage. However, most seismic metamaterials were designed without considering the viscoelastic effect that widely exists in nature. In this work, we investigate the propagation and attenuation of the Rayleigh and pseudo surface waves (PSWs) in two types of viscoelastic seismic metamaterials, namely, pillared and inclusion-embedded metamaterials, by analyzing the complex band structures and transmission spectra. The complex band structure developed in this work reveals for the first time the existence of PSWs and their propagation properties in inclusion-embedded metamaterials at the surface. These PSW modes are hidden in the traditional ω(k) technique, therefore showing the usefulness of the complex band structure approach. Introducing viscosity to the substrate of both types of seismic metamaterials will enhance the attenuation of both the Rayleigh wave and PSW. For inclusion-embedded metamaterials, the viscoelastic effect in the soft coating layer can have a specific influence only on the PSW. PSWs show advantages to minimize the relative attenuating effect in general. The results in this work will open up great possibilities for designing and optimizing seismic metamaterials in practice.

III-V Impact-ionization (II) metal-oxide-semiconductor FETs (I-MOSFETs) and tunnel FETs (TFETs) are being explored as promising devices for low-power digital applications. To assist the development of these devices from the physical point of view, a Monte Carlo (MC) model which includes impact ionization processes and band-to-band tunneling is presented. The MC simulator reproduces the I-V characteristics of experimental ungated In0.53Ga0.47As 100 nm PIN diodes, in which tunneling emerges for lower applied voltages than impact ionization events, thus being appropriate for TFETs. When the structure is enlarged up to 200 nm, the ON-state is achieved by means of impact ionization processes; however, the necessary applied voltage is higher, with the consequent drawback for low-power applications. In InAs PIN ungated structures, the onset of both impact ionization processes and band-to-band tunneling takes place for similar applied voltages, lower than 1 V; thus they are suitable for the design of low-power I-...

We report on a systematic investigation of the influence of pulsed laser deposition (PLD) parameters upon the microstructure, surface morphology, and magnetic properties of La0.7Sr0.3MnO3 (LSMO) thin films grown on (100) SrTiO3 substrates. The optimization of the physical properties requires a careful exploration of the main parameters such as the oxygen pressure P and the target-to-substrate distance D. We show that there is a strong correlation between both these parameters and an optimal distance (D=L0), which can be calculated from a PD3 scaling law (in accordance with a shock wave model). This particular value L0 corresponds to the distance for which all species are thermalized in the plume. In the D–P diagram, L0 defines two distinct regions for the morphology and the microstructure: (i) when D 1 μm) morphology. These films are perfectly epitaxial on the substrate, single-phase and not relaxed. (ii) When D≫L0, the films are colum...

We report calculations of first-order pressure and second-order pressure gradient fields in the neighborhood of patterned silicon wafers. The patterned wafers consist of a single ridge and two parallel ridges separated by a trench on a planar substrate. The efficacy of megasonic waves for cleaning patterned wafers contaminated with micron to submicron silica particles is discussed by comparing a removal force arising from the second-order pressure gradient to a van der Waals adhesion force. The calculated second-order pressure gradient fields show that acoustic energy may be concentrated in small volumes in the vicinity of pattern features with dimensions significantly smaller than the wavelength of the incident acoustic wave. The angle the incident acoustic wave makes with the planar substrate has a strong impact on the second-order pressure gradient field. Grazing incident waves appear to provide a more efficient way of cleaning inside a trench. Excitation of a trench resonant vibrational mode enhances ...

Multigrained or polycrystalline composite materials have attracted a considerable attention due to their potential applications as advanced materials with outstanding thermal, mechanical, and electromagnetic properties. When the grains' morphology is displayed at the nanoscopic scale, the presence of imperfect interfaces plays a central role in determining the effective transport properties. Therefore, we develop here a self-consistent effective medium theory able to evaluate the influence of real contacts between the different phases of multigrained composite materials. This approach takes into account the classical interface schemes that have been introduced in literature, namely, the low and the high conducting interface models. The theoretical results have been compared with numerical and experimental data concerning the thermal conductivity of (1−x)Si:xGe mixtures and the electrical conductivity of (1−x)Li2O:xB2O3 composites.

International audience; Giant magnetostriction (GMS) reported until now in exchange-coupled multilayers (ECML) present quasi-isotropic magnetostrictive behavior. In the present work it is shown the possibility to induce highly anisotropic GMS at low field with the single domain state in (TbFe/Fe)n multilayers when deposited by ion sputtering under a magnetic polarization field Hd. ECML grown under bias field Hd, applied along the large size of rectangular glass substrates present original magnetic and magnetoelastic (ME) properties associated with strong uniaxial behavior. After saturation by an external field H∥ applied parallel to the easy axis induced by Hd, a single domain state is kept by decreasing H∥ down to zero (Mr/Ms=1) which explains why there is no change of the magnetoelastic coefficient bγ,2 by increasing or decreasing H∥. A different behavior arises along the hard magnetization axis with a strong change of bγ,2 by increasing H∥ up to the saturation. ME anisotropy is the origin of high amplitude flexural and torsional oscillation modes as observed in our ECML

International audience; 350 nm-thick Perovskite PbZr0.54Ti0.46O3 (PZT) thin films were deposited on Al2O3 substrates by sputtering with and without an additional 10-nm-thick TiOx buffer layer. X-ray diffraction patterns showed that in presence of TiOx buffer layer, PZT film was highly oriented along the (111) direction film, whereas the unbuffered, counterpart was polycrystalline. A full wave electromagnetic analysis using a vector finite element method was performed to determine the tunability and the complex permittivity up to 67 GHz. A comparison between the electromagnetic analysis and Cole-Cole relaxation model was proposed. Through an original study of the relaxation time as a function of the electric field, values, such as 2 ps and 0.6 ps, were estimated for EDC = 0 kV/cm and 235 kV/cm, respectively, and in both cases (111)-PZT and polycrystalline-PZT. The distribution of relaxation times is found to be larger for (111)-PZT film, which is probably related to the film microstructure.

Phononic crystals (PC) can be used to control the dispersion properties of acoustic waves, which are essential to direct their propagation. We use a PC-based two-dimensional solid/solid composite to demonstrate experimentally and theoretically the spatial filtering of a monochromatic non-directional wave source and its emission in a surrounding water medium as an ultra-directional beam with narrow angular distribution. The phenomenon relies on square-shaped equifrequency contours (EFC) enabling self-collimation of acoustic waves within the phononic crystal. Additionally, the angular width of collimated beams is controlled via the EFC size-shrinking when increasing frequency.

International audience; Uniform, dense, single-phase, 150 nm thick indium gallium nitride (InGaN) nanostructure (nanorods and nanostripes) arrays have been obtained on gallium nitride templates, by metal organic chemical vapor deposition and nanoscale selective area growth on silicon dioxide patterned masks. The 150 nm thick InGaN nanorods have a perfect hexagonal pyramid shape with relatively homogenous indium concentration up to 22%, which is almost twice as high as in planar InGaN grown in the same condition, and luminesce at 535 nm. InGaN nanostripes feature c-axis oriented InGaN in the core which is covered by InGaN grown along semi-polar facets with higher In content. Transmission electron microscope and sub micron beam X-rays diffraction investigations confirm that both InGaN nanostructures are mostly defect free and monocrystalline. The ability to grow defect-free thick InGaN nanostructures with reduced polarization and high indium incorporation offers a solution to develop high efficiency InGaN-based solar cells.

We provide in this article a comprehensive study of the role of ac cross-talk effects in Kelvin Probe Force Microscopy (KPFM), and their consequences onto KPFM imaging. The dependence of KPFM signals upon internal parameters such as the cantilever excitation frequency and the projection angle of the KPFM feedback loop is reviewed, and compared with an analytical model. We show that ac cross-talks affect the measured KPFM signals as a function of the tip-substrate distance, and thus hamper the measurement of three-dimensional KPFM signals. The influence of ac cross-talks is also demonstrated onto KPFM images, in the form of topography footprints onto KPFM images, especially in the constant distance (lift) imaging mode. Our analysis is applied to unambiguously probe charging effects in tobacco mosaic viruses (TMVs) in ambient air. TMVs are demonstrated to be electrically neutral when deposited on silicon dioxide surfaces, but inhomogeneously negatively charged when deposited on a gold surface. © 2014 AIP Publishing LLC

Nanomaterials composed of a population of particles dispersed in a matrix represent the building block for the next generation of several technologies: energy storage and conversion, thermal management, electronics, and photovoltaics. When interfaces between particles and matrix are imperfect, the size of the particles may strongly influence the effective linear and nonlinear response of the whole system. Here, we study these scale effects mainly focussing on the nonlinear transport behavior of composite structures. The theory is developed, in the framework of the electrical conductivity, for an arbitrary nonlinearity of the constituents; however, explicit results are discussed for Kerr-like nonlinear responses. Two kinds of imperfect interfaces are considered: the T-model and the Π-model, which represent a generalization of the classical schemes largely employed in literature, namely the low and the high conducting interface models. The dependence of the nonlinear effective properties on the size of the dispersed particles is explained through intrinsic length scales governing some universal scaling laws.

TiO2 and TiO2/Nb amorphous thin films were grown on glass substrates by a sol-gel technique (spin coating). Films' surface composition, structure, and morphology were derived from x-ray photoelectron spectroscopy, x-ray diffraction, and atomic force microscopy data. The investigated films showed a smooth surface (roughness values below 5 nm). A separate surface wettability investigation showed that by increasing the Nb amount in pristine titania films results in a decrease of contact angle (CA) values from 40° to nearly 0°, thus, indicating a super-hydrophilic conversion under UV illumination. This conversion rate is greatly enhanced by increasing the Nb content, the surface super-hydrophilic behavior occurring after a couple of minutes in the TiO2/Nb samples, but after 4 h in the pristine titania specimen. The current results are discussed in terms of the optical band gap shift towards higher energies, by increasing the Nb content in the films, a process explained based on small polaron hopping model.

International audience; Luminance and quantum yields of organic light emitting diodes (OLEDs) are generally calculated by using a hypothetic monochromatic emission even though the actual external emission is lower than the internal emission because of internal reflection, and is polychromatic. For organic materials, we present the effects of the total internal reflection on quantum yields. Calculations for the luminance and the efficiency of an ideal pseudomonochromatic source are compared with those for a real polychromatic source. While we demonstrate that the calculation of the yield is practically unchanged whatever hypothesis is used, we do show, however, that by assuming that the source is pseudomonochromatic, the luminances of green and blue OLEDs are overvalued or undervalued by a factor of 4, respectively.

We study theoretically the propagation of elastic waves in two-dimensional composite media composed of a square array of hollow steel cylinders embedded in water using the finite-difference time-domain method. These composite media constitute a class of acoustic band gap materials with narrow pass bands in their transmission stop bands. The frequency at which the pass band occurs is tunable by controlling the inner radius of the tubular steel inclusions. The effect of the tube inner radius on the transmission spectrum is semiquantitatively separable from the effect of the composite periodicity. A linear defect formed of a row of hollow cylinders in an array of filled cylinders produces an elastic waveguide that transmits at the narrow pass band frequency. We show that two of these tunable waveguides with different inner radii can be employed to filter and separate two specific frequencies from a broad band input signal.

Quantum calculations of leakage current through ultrathin dielectric heterostructures are used to propose design criteria for the next generation of metal-oxide-semiconductor devices in a deep submicron technology. By using as input parameters both the dielectric constant and the barrier height of different dielectrics, including at first stage SiO2, Si3N4, TiO2, and Ta2O5, but also such emerging materials as HfO2 and ZrO2, we show that, depending on the voltage range investigated, the hierarchy between those dielectrics is not simply given by the increase of their permittivity. Deeper considerations based on resonant tunneling mechanisms, of prime importance when dielectric heterostructures are used for fabrication purposes, must be taken into account especially for the future low-consumption nanotransistors with operating voltages below 1 V.

International audience; A Monte Carlo method is proposed for the study of in-plane electron transport in narrow gap heterostructures. Special attention is paid to the consequences of the strong nonparabolicity of the conduction band. The electron states are calculated within the framework of envelope function theory, which leads to a Schrödinger equation with an energy-dependent effective mass. This equation is solved in a numerically efficient way by including a standard eigenvalue solver in an iterative method. The mixing between conduction and valence band states is taken into account, at an approximate level, through a “Bloch overlap factor,” defined by analogy with the case of three-dimensional transport. This model was applied to a typical AlSb/InAs single well structure, and realistic results were obtained. The important role played by the Bloch overlap factor is demonstrated. When it is neglected, the mobility is strongly underestimated. A more sophisticated double well structure was also investigated. It is intended to reduce impact ionization, thanks to transfer toward the thinner well. This transfer is found to depend strongly on the potential profile

Ion beam etching of sputtered Pb(Zrx,Ti1-x)O3 (PZT) with x equal to 0.54 thin films grown on Pt/Ti/SiO2/Si substrates has been performed using pure Ar gas. The etch rate dependence on the process parameters (current density, acceleration voltage, gas pressure) has been investigated. The PZT etch rate can reach 600 Å/min with acceleration voltage of 1000 V and current density of 1 mA/cm2. Selectivity ratios between PZT and masks of various natures (photoresist, Pt, Ti) have been evaluated to determine a pertinent material for etching mask. According to our etching conditions, titanium seems to be the best candidate. We evaluated the PZT surface damage by contact mode atomic force microscopy. It appears that the roughness increases after ion bombardment, and that the grain boundary zone is preferentially etched. For some etching parameters, we also observed electrical damage. Carrying out C(V) and hysteresis loops P(E) measurements before and after etching have provided evidence of degradation. We noted a large decrease in permittivity after the etching process irrespective of the current density and acceleration voltage. Ferroelectric damage was illustrated by a large increase in the average coercive field. For each of the electrical properties under study, the same behavior has been observed after etching: the increase of damage was obtained as a function of the current density and acceleration voltage. The evolution of electrical properties when the PZT layer is protected by a metallic mask has also been studied. We observed very slight variations in the electrical properties.

International audience; Electronic properties of the Al0.56In0.44Sb/Ga0.5In0.5Sb heterostructure grown by molecular beam epitaxy are investigated. We measure by means of x-ray photoemission spectroscopy, photoluminescence, and Hall effect measurements the key parameters involved in the achievement of a high speed and very low power consumption field effect transistor: conduction and valence band offsets, energy gaps of channel and barrier materials, electron effective mass, and density of states in the channel. We observe the influence of the quantum well thickness on the electron Hall mobility and sheet carrier density. A type I heterostructure exhibiting a room temperature electron mobility of 25 000 cm2 V-1 s-1 with a sheet carrier density of 1.5×1012 cm-2 is demonstrated for the widest channel. We show that the thickness of the channel influences both Hall density and mobility through a multisubband occupancy. We discuss about the scattering mechanisms limiting the mobility in the second subband.

International audience; This article deals with the influence of dislocations on the conductivity of two dimensional electron gas measured at helium temperature and at high magnetic field. Two kinds of dislocation substructures have been experimentally studied: they correspond either to a family of parallel dislocation segments introduced by plastic bending or to a family of parallel and long misfit dislocations created during the epitaxial growth by the control of the epilayer thickness. It is shown that both types of dislocation substructure lead to a strong anisotropic behavior of the longitudinal conductivity while the quantum Hall effect is not significantly affected by the presence of dislocations. A theoretical treatment of the case of anisotropic potentials is proposed as a generalization of the simpler case, addressed in the literature, of isotropic (short range) potentials. With the help of this analysis, we can explain, in the case of the dislocation segments, why although anisotropic, the normalized conductivity curves obtained in the principal axis are identical and we show that regular Dingle plots may be used to check the extra broadening of the Landau levels due to dislocations. The second kind of dislocation substructure is shown to be equivalent to quasi one dimensional systems issuing from a truncation of the two dimensional electron gas into quite parallel ribbons localized between the dislocation potential barriers, but no ballistic effects could be observed.

International audience; opological physics is emerging as an active area of research, addressing fundamental questions on how geometry, symmetry, and topology affect physical properties, paving the way toward novel technological applications. Originally investigated in quantum systems, these concepts have been thereafter translated across diverse domains including, electromagnetic, plasmonic, elastic, and acoustic waves. Specifically, in elasticity, due to the strong tendency to hybridize of wave modes with different polarization, topological protection is viewed as a revolutionizing approach to design waveguides supporting unique features such as (i) being immune to defects and (ii) suppressing backscattering during the wave propagation phenomenon. These novel features arise as a consequence of their dispersion surface topology. This Tutorial aims to introduce the theoretical, numerical, and experimental frameworks to investigate topological elastic waveguides, discussing the key ideas, first, in the context of discrete systems, and then, in continuous elastic solids. After a comprehensive description of the currently used state of the art scientific techniques, various classes of topological wave phenomena leading to localized waves in elastic architected plates and beams are presented. Implications of the presence of both longitudinal and shear waves in elastic solids are discussed, and the associated challenges, opportunities, and strategies to exploit their interplay highlighted. The symmetry conditions required to induce them are discussed using a number of representative examples. Finally, future research directions of this fledgling field are outlined.

International audience; We investigate the elastic wave dispersion by a phononic metamaterial plate containing low frequency resonator stubs arranged periodically over the plate. We show that this system not only provides stop bands for wavelengths much larger than the periodicity but also displays negative behavior of its effective mass density under the homogenization assumption. A numerical method is used to calculate the plate's effective dynamic mass density as function of the frequency where the metamaterial is considered as homogeneous plate for these large wavelengths. Strong anisotropy of the effective mass density matrix is observed around the resonance frequencies where the gaps are opened. In these regions, we demonstrate that the effective matrix density components take negative values. For each of these components, the negative behavior is studied by taking into account the polarization of the involved resonant modes as well as their associated partial band gaps opened for each specific Lamb symmetry modes. We found that coupling between Lamb waves and resonant modes strongly affects the effective density of the whole plate especially in the coupling frequency regions of the gaps. (C) 2014 AIP Publishing LLC.

Heavily boron doped diamond epilayers with thicknesses ranging from 40 to less than 2 nm and buried between nominally undoped thicker layers have been grown in two different reactors. Two types of [100]-oriented single crystal diamond substrates were used after being characterized by X-ray white beam topography. The chemical composition and thickness of these so-called deltadoped structures have been studied by secondary ion mass spectrometry, transmission electron microscopy, and spectroscopic ellipsometry. Temperature-dependent Hall effect and four probe resistivity measurements have been performed on mesa-patterned Hall bars. The temperature dependence of the hole sheet carrier density and mobility has been investigated over a broad temperature range (6K, 14 pages

In the present article, we report a study on the mechanical behaviour displayed by hydrogen atoms and pores in silicon nitride (SiN) films. A simple three-phase model is proposed to relate the physical properties (stiffness, film stress, mass density, etc.) of hydrogenated nanoporous SiN thin films to the volume fractions of hydrogen and pores. This model is then applied to experimental data extracted from films deposited by plasma enhanced chemical vapour deposition, where hydrogen content, stress, and mass densities range widely from 11% to 30%, −2.8 to 1.5 GPa, and 2.0 to 2.8 g/cm3, respectively. Starting from the conventional plotting of film's Young's modulus against film porosity, we first propose to correct the conventional calculation of porosity volume fraction with the hydrogen content, thus taking into account both hydrogen mass and concentration. The weight of this hydrogen-correction is found to evolve linearly with hydrogen concentration in tensile films (in accordance with a simple "mass correction" of the film density calculation), but a clear discontinuity is observed toward compressive stresses. Then, the effective volume occupied by hydrogen atoms is calculated taking account of the bond type (N-H or Si-H bonds), thus allowing a precise extraction of the hydrogen volume fraction. These calculations applied to tensile films show that both volume fractions of hydrogen and porosity are similar in magnitude and randomly distributed against Young's modulus. However, the expected linear dependence of the Young's modulus is clearly observed when both volume fractions are added. Finally, we show that the stiffer behaviour of compressive films cannot be only explained on the basis of this (hydrogen + porosity) volume fraction. Indeed this stiffness difference relies on a dual mechanical behaviour displayed by hydrogen atoms against the film stress state: while they participate to the stiffness in compressive films, hydrogen atoms mainly behave like pores in tensile films where they do not participate to the film stiffness. © 2014 AIP Publishing LLC

We study both theoretically and experimentally the interaction of surface elastic waves with 2D surface phononic crystal (PnC) on a piezoelectric substrate. A rigorous analysis based on 3D finite element method is conducted to calculate the band structure of the PnC and to analyze the transmission spectrum (module and phase). Interdigital transducers (IDTs) are considered for electrical excitation and detection, and absorbing boundary conditions are used to suppress wave's reflection from the edges. The PnCs are composed of an array of 20 Nickel cylindrical pillars arranged in a square lattice symmetry, and deposited on a LiNbO3 substrate (128°Y cut-X propagating) between two dispersive IDTs. We investigate by means of band diagrams and transmission spectrum the opening band-gaps originating from pillars resonant modes and from Bragg band-gap. The physical parameters that influence and determine their appearance are also discussed. Experimental validation is achieved through electrical measurement of the transmission characteristics, including amplitude and phase.

Green's function can be retrieved from cross-correlation of a diffuse field generated by noise sources. Today, this important result is the fundamental of several passive imaging techniques. The aim of this paper is to establish the suitability of these methods to detect and locate a defect in a reverberant elastic plate. The relations between the noise cross-correlation function over a few number of noise sources and the imaginary part of the Green's function are derived and numerically validated. Then we show through numerical experiments that this technique is exploitable for defect detection and localization in a differential mode, despite a non-perfect estimation of the Green's functions. Finally, a filtering technique based on the singular value decomposition is shown to improve the detection.

In this paper we demonstrate that depending on the surrounding ambient, which may be manifested by different damping conditions, a "bump" in frequency response of Electric Force Microscope (EFM) cantilevers may be observed, either in electrical amplitude or phase signals of AC gradients. This bump is present in air and at room temperature when the cantilever oscillates at fundamental frequency with high excitation amplitude but disappears at increasing temperature. This effect can be explained by a significant temperature decrease of the Q factor of coated cantilevers. At a constant value of AC voltage, the relevant parameter is the ratio of Q factor by cantilever stiffness (of the nth oscillation mode) from which we can predict bump observation. This study highlights significant effects for quantitative EFM studies under different atmospheres and at different temperatures.

This work presents the modeling of high contrast partially electroded resonators by means of a polynomial approach. This method allows easily solving the equations that govern the structure. The boundary, symmetry, and continuity conditions are automatically incorporated into the equations of motion by the use of delta functions for the variables stress (T) and electric displacement (D) and appropriate analytical expression forms for the independent variables, mechanical displacements (u), and electric potential (ϕ). Structure symmetry was used to reduce the number of unknowns. For the zinc oxide (ZnO) resonator in extreme geometrical cases (thin plate and bar cases), a good agreement was obtained between the results of the proposed polynomial approach and those of an analytical approach for both the modal and harmonic analyses. The proposed polynomial approach was used to calculate the 2D resonator electrical admittance (full and partial metallization) near the 1D thickness fundamental mode, and the resu...

The electronic structure and the transport properties of Si nanotubes with outer diameter between 2 and 20 nm are investigated using fully atomistic simulations. A formula is given for the energy of the band edges versus diameter and thickness. The phonon-limited mobility of electrons and holes is calculated and is compared to the case of Si nanowires. It is found that the thickness of the nanotubes is the main parameter controlling the shape of the band structure and the values of the effective masses. Configurations with expected best carrier mobilities are discussed.

This paper reports on numerical and experimental results of acoustic transmission spectra of bead chains with symmetric and asymmetric Fibonacci-like structures. As a matter of comparison, perfect periodic acoustic waveguide structures are also examined. This study shows that Fibonacci structures with mirror symmetry can exhibit localized modes with higher amplitude, due to resonant transmission induced by the presence of dimers inside the 1D structure. A good agreement is observed between the theoretical predictions and the experimental power spectra.

The transport characteristics of low dimensional semiconductors like silicon nano-wires (SiNWs) rarely conform to expectations from geometry and dopant density, exhibiting significant variations as a function of different surface terminations/conditions. The association of these mechanisms with surface states and their exact influence on practical SiNW devices still remains largely unclear. Herein, we report on the influence of surface state charge distributions on SiNW transport characteristics. For this study, p-type SiNW devices with widths of 50, 100, and 2000 nm are fabricated from 25, 50, and 200 nm-thick SOI wafers. A ∼five order difference in effective carrier concentration was observed in the initial SiNWs characteristics, when comparing SiNWs fabricated with and without a thermal oxide. The removal of the surface oxide by a hydrogen fluoride (HF) treatment results in a SiNW conductance drop up to ∼six orders of magnitude. This effect is from a surface depletion of holes in the SiNW induced by positive surface charges deposited as a result of the HF treatment. However, it is observed that this charge density is transient and is dissipated with the re-growth of an oxide layer. In summary, the SiNW conductance is shown to vary by several orders of magnitude, while comparing its characteristics for the three most studied surface conditions: with a native oxide, thermal oxide and HF induced H-terminations. These results emphasize the necessity to interpret the transport characteristics of SiNWs with respect to its surface condition, during future investigations pertaining to the physical properties of SiNWs, like its piezo-resistance. As a sequel, prospects for efficiently sensing an elementary reduction/oxidation chemical process by monitoring the variation of SiNW surface potential, or in practice the SiNW conductance, is demonstrated.

International audience; Ballistic transport in double-barriers resonant tunneling diodes based on GaN is investigated in this work using the non-equilibrium Green's functions formalism. The electron density of states, the electrons concentration, and the current-voltage characteristics are calculated taking into account the internal electric field induced in the AlxGa1−xN/GaN heterostructures. The effect of the geometrical parameters on the evolution of the current resonances characteristics was analyzed qualitatively by varying GaN quantum well width, thicknesses and height of the AlxGa1−xN barriers.

Experimental and theoretical studies of hysteresis loops in piezoresponse force microscopy (PFM) directed on elucidating the influence of top electrode lateral sizes on loop peculiarities were performed for Pt/Pb(Zr, Ti)O3/Pt/TiOx/SiO2/Si-substrate structures. The set of top Pt electrodes (50 nm to 10 μm of lateral size) was deposited on the Pb(ZrxTi1−x)O3 film (x = 0.54, thickness ≈1 μm) by RF magnetron sputtering. Under approaching the top electrode lateral size to the film thickness, the transition-like behaviour of PFM response amplitude and coercive tip voltage was observed. The existence of the critical value of dimensionless electrode size parameter γd/h ≈ 1 (γ is the dielectric anisotropy factor, d is the electrode size parameter, and h is the film thickness) was interpreted in the framework of the model based on Landau-Ginzburg-Devonshire theory combined with the decoupling approximation subject to the nonlinear electric field dependence of the ferroelectric polarization and dielectric permittivity.

International audience; We report on simulations and measurements of the optical absorption of silicon nanowires (NWs) versus their diameter. We first address the simulation of the optical absorption based on two different theoretical methods : the first one, based on the Green function formalism, is useful to calculate the scattering and absorption properties of a single or a finite set of NWs. The second one, based on the Finite Difference Time Domain (FDTD) method is well-adapted to deal with a periodic set of NWs. In both cases, an increase of the onset energy for the absorption is found with increasing diameter. Such effect is experimentally illustrated, when photoconductivity measurements are performed on single tapered Si nanowires connected between a set of several electrodes. An increase of the nanowire diameter reveals a spectral shift of the photocurrent intensity peak towards lower photon energies, that allows to tune the absorption onset from the ultraviolet radiations to the visible light spectrum.

The possibility of control and tuning of the band structures of phononic crystals offered by the introduction of an active magnetoelastic material and the application of an external magnetic field is studied. Two means to obtain large elastic properties variations in magnetoelastic material are considered: Giant magnetostriction and spin reorientation transition effects. A plane wave expansion method is used to calculate the band structures. The magnetoelastic coupling is taken into account through the consideration of an equivalent piezomagnetic material model with elastic, piezomagnetic, and magnetic permeability tensors varying as a function of the amplitude and orientation of the applied magnetic field. Results of contactless tunability of the absolute bandgap are presented for a two-dimensional phononic crystal constituted of Terfenol-D square rod embedded in an epoxy matrix.

The strain relaxation at the initial stages of highly mismatched (11.8%) GaSb grown on a GaP substrate following a Ga-rich surface treatment by molecular beam epitaxy has been investigated. High resolution transmission electron microscopy and moire fringe analysis were used to determine the relaxation state in these GaSb islands in the [110] and [1–10] directions. The measurements revealed an anisotropic strain relaxation in these two directions; there is a higher misfit strain relaxation along the [110] direction where the islands are elongated, which is in agreement with a higher density of misfit dislocations. By combining molecular dynamics simulations and TEM results, the anisotropy in the strain relaxation is shown to be related to the asymmetry in the formation of interface misfit dislocations. The P-core glide set 60° dislocations (α type) and the Ga-core shuffle set Lomer dislocations serve as the primary misfit dislocation which contributes to the strain relaxation in the (1–10) interface, and t...

In this work we report on the characteristics of a (Ni/Au)/AlGaN/GaN/SiC Schottky barrier diode (SBD). A variety of electrical techniques, such as gate current-voltage (I-V), capacitance-voltage (C-V), and deep level transient spectroscopy (DLTS) measurements have been used to characterize the diode. The behavior study of the series resistance, RS, the ideality factor, n, the effective barrier height, Φb, and the leakage current with the temperature have emphasized an inhomogeneity of the barrier height and a tunneling mechanism assisted by traps in the SBD. Hence, C-V measurements successively sweeping up and down the voltage have demonstrate a hysteresis phenomenon which is more pronounced in the temperature range of 240 to 320 K, with a maximum at ∼300 K. This parasitic effect can be attributed to the presence of traps activated at the same range of temperature in the SBD. Using the DLTS technique, we have detected one hole trap having an activation energy and a capture cross-section of 0.75 eV and 1.09 × 10−13cm2, respectively, seems to be responsible for the appearance of the hysteresis phenomenon

A complete study is given in this paper on the structural properties of Ba(Sr,Ti)O3 (BST) thin films which present various preferred orientations: (111) and (001) fiber and epitaxial textures. The films are deposited in situ at 800 °C by sputtering on Si/SiO2/TiOx/Pt substrates and the orientation is controlled by monitoring the concentration of O2 in the reactive plasma or by prior deposition of a very thin TiOx buffer layer between BST films and substrates. The epitaxial films are obtained on (001)-alpha-Al2O3 substrates covered with TiOx buffer layers. In order to analyze finely the preferred orientations, the texture, the microstructural features, and the anisotropy-related quantities such as residual stresses in the films, the conventional Bragg−Brentano θ - 2θ x-ray diffraction diagrams is shown not to be sufficient. So, we systematically used x-ray combined analysis, a recently developed methodology which gives access to precise determination of the structure (cell parameters and space group) of th...

Self-heating effects in a molecular beam epitaxy-grown GaN/AlGaN/GaN structure on a single crystalline diamond is investigated. A transient interferometric method, in combination with a three dimensional model, is used to describe a pulsed operation of a transistor-like heater, and a micro-Raman technique is used in a steady state. Good agreement is found between the techniques. The thermal conductivity of the diamond is found to be 2200 W/m K, and the thermal boundary resistance to the III-N epi-structure is

The initial stages of graphitization on 4H- and 6H-SiC (000-1) under ultrahigh vacuum at temperatures of 1125–1175°C have been studied by atomic force microscopy (AFM), X-ray photoemission spectroscopy and reflected high energy electron diffraction. A progressive coverage of the surface by graphene has been observed depending on the time and temperature of annealing. Graphene growth mainly starts from the step edges, although it sometimes nucleates in the middle of a SiC terrace. Comparison of the topographic and phase AFM images shows that the latter are the most efficient for identifying graphene before complete coverage of the surface., This work was achieved with the financial support of the European Union (FEDER), the ANR (project Xp-Graphene), and the Nord-Pas de Calais Regional Council.

We discuss two points related to the simultaneous existence of phononic and photonic band gaps in a two-dimensional crystal constituted by a square array of holes drilled in a matrix. In a first part, using the case of a sapphire sample in the microwave range, we show that in addition to the phononic gap, an absolute photonic gap may be obtained making use of the high values as well as the anisotropy of the dielectric matrix elements in the microwave regime. In a second part, using the case of silicon in the telecom frequency range, we demonstrate that absolute photonic and phononic gaps may be obtained by making a combination of two crystals having slightly different filling factors. The calculations of the band structures and transmission coefficients were mainly computed using the finite difference time domain method.

Results of multiparticle Monte Carlo simulations of midinfrared quantum cascade lasers structure initially fabricated by Page et al. are presented. The main aim of this paper is to discuss in details how electric current flows through the structure and which subbands are involved in this process. Monte Carlo method allows to predict the electron population inversion between the lasing levels and gives microscopic insight into processes leading to such behavior. Importance of a subband belonging to the laser injector region, with energy slightly below the upper lasing level, is demonstrated. The electron–electron Coulomb interactions influence the shapes of electron distribution functions; the values of average electron energies and effective subbands’ temperatures are calculated.

Piezoelectric nanostructures (islands of dimensions in the lateral size range 50–500 nm) have been fabricated by focused Ga3+ ion beam (FIB) etching on PbZr0.54Ti0.46O3 thin films obtained by magnetron sputtering. The degradation induced by the etching process is investigated through the evolution of electromechanical activity measured by means of local piezoelectric hysteresis loops produced by piezoresponse force microscopy. The analysis of surface potential is performed by kelvin force microscopy and the measurement of current-voltage curves is carried out by conducting atomic force microscopy. Two kinds of structures, namely one based on crystallized films and the other based on amorphous ones, were studied. In this latter case, the amorphous films are postannealed after etching to obtain crystallized structure. For the structures based on the crystallized and then etched films, no piezoelectric signal was registered that evidences a serious degradation of material induced by Ga3+ ion implantation. For the structures based on the films etched in amorphous state and then crystallized, the piezoresponse signal was near to that of the reference films (crystallized and not etched) whatever were the ion dose and the island dimensions. Even for very small lateral size (50 nm), no size effect was observed. The island shapes fabricated by Ga3+ FIB etching process (islands with less than 50 nm lateral size) show a limitation of FIB processing and electron beam lithography seems to be necessary.

The strain relaxation in low mismatched InxAl1-xAs layers has been studied by triple axis x-ray diffraction, transmission electron microscopy, and photoluminescence. Using a two step buffer, a fully relaxed top layer has been grown by adapting the composition and thickness of a first "strained layer." The threading dislocation density in the top layer is below 106/cm2 and strain is relaxed at the substrate/first layer interface by misfit dislocations. This scheme is a promising method to limit the thickness of buffer layers and obtain fully relaxed pseudosubstrates.

We report results from the investigation of the intrinsic high-frequency (HF) behavior of three-terminal junctions based on InGaAs∕InAlAs heterostructures, using a well-calibrated three-dimensional semiclassical ensemble Monte Carlo simulation model. The simulator incorporates a more realistic surface charge model, designed specifically for HF simulations. A full analysis of the dynamics of electron transport in the devices is performed and a prediction of its intrinsic HF performance is presented. Simulation results demonstrate how these devices may be suitable for applications in the terahertz frequency range. Most importantly, we illustrate the important role played by surface charge effects in this frequency regime. The necessity of considering these effects as a key design factor for the development of future nanojunction structures operating in the terahertz regime is therefore discussed.

We report the results of direct measurements and a theoretical investigation of the in-plane effective mass in the two-dimensional electron gas of nominally undoped AlGaN/GaN heterostructures with a different degree of quantum confinement. It is shown that in most cases the conduction band nonparabolicity effect is overestimated and the electron wave-function penetration into the barrier layer should be taken into account. The contribution of the wave-function hybridization is determined to play the dominant role. The band edge effective mass value is deduced to be (0.2 +/- 0.01)m(0). (C) 2009 American Institute of Physics. [DOI: 10.1063/1.3100206]

International audience; In this paper, we present a theoretical study of magnetic properties of MPo (M=Ti, V, Cr, Mn, Fe) compounds in the zinc-blend structure using full-potential linear augmented plane wave method. Our calculation indicates that MnPo and FePo are antiferromagnetic, whereas TiPo, VPo, and CrPo are magnetic. The ferromagnetic phase is enhanced with decreasing lattice constant and increasing atomic number along the sequence TiPo-->VPo-->CrPo. TiPo, VPo, and CrPo are predicted to be half-metal ferromagnetic with magnetic moments of 2.0µB, 3.0µB, and 4.0µB, respectively, which is confirmed from the calculated minority (majority) spin channel band structure showing the semiconducting (metallic) behavior of these compounds. The magnetism comes essentially from the d orbital of Ti, V, and Cr atoms. In addition, we have found that half-metallicity can be maintained even when spin-orbit interaction is included for TiPo and VPo. However spin-orbit effect destroys magnetism in CrPo.

A numerical analysis of negative refraction process is reported using a phononic crystal with an elastic solid matrix. The phononic crystal considered in this study is made of a periodic arrangement of holes in aluminum. Dispersion curves are discussed and conditions for which negative refraction can appear are identified. These conditions are obtained for the transverse waves, whereas the longitudinal waves are evanescent. A calculation is performed with a prism shaped phononic crystal, and it clearly exhibits a negative refraction angle. Several analyses are provided with a view to characterize the wave going out of the phononic crystal. Finally, improvements, with respect to the impedance matching and index tuning, are discussed.