Processes to achieve high spatial resolution ion implantation induced quantum well intermixing in GaAs/AlGaAs superlattices have been developed. Ion implantation has been carried out using various doses of 4 MeV As2+ ion beam, followed by rapid thermal annealing at various temperatures for 60 s. Low temperature photoluminescence measurements reveal a blue-shift up to 90 nm in the energy band-gap. Propagation losses have been characterized in the intermixed waveguides, and losses as low as 0.55 cm−1 have been observed for 0.5 × 1013 cm−2 implantation dose which gives a blue-shift of 68 nm when annealed at 775 °C. The spatial resolution of ~1.2 μm has been observed at the depth of 2 μm inside the epitaxial structure.

xPb(Yb1/2Nb1/2)O3-(1 − x)Pb(Zr0.36Ti0.64)O3 (xPYN-(1 − x)PZT) piezoelectric ceramics were prepared by the conventional ceramic processing via a B-site oxide mixing route. The synthesized xPYN-(1 − x)PZT ceramics exhibit majority of perovskite structure with slight content of impurity, which exhibit typical tetragonal structure with slight orthorhombic distortion depending on compositions. All the xPYN-(1 − x)PZT ceramics exhibit high Curie temperature (TC/Tm), higher than 380 °C, and their dielectric behavior above TC/Tm can be fitted well by the Curie-Weiss law. The xPYN-(1 − x)PZT ceramics exhibit large resistivity, and excellent ferroelectric and piezoelectric properties, which provide promising for the high-power and high-temperature piezoelectric applications. However, electric energy density of the xPYN-(1 − x)PZT ceramics is small due to their nearly rectangular shape of polarization-electric field (P-E) hysteresis loop and early electric displacement saturation, which is not suitable for high energy and power storage applications.

A theoretical study of transconductance characteristics (gm − Vgs profile) of AlGaN/GaN high electron mobility transistors (HEMTs) with a graded AlGaN layer is given in this paper. The calculations were made using a self-consistent solution of the Schrödinger-Poisson equations and an AlGaN/GaN HEMTs numerical device model. Transconductance characteristics of the devices are discussed while the thickness and Al composition of the graded AlGaN layer are optimized. It is found that graded AlGaN layer structure can tailor device’s gm − Vgs profile by improving polar optical phonon mobility and interface roughness mobility. Good agreement is obtained between the theoretical calculations and experimental measurements over the full range of applied gate bias.

The development of power electronics in the field of transportations (automotive, aeronautics) requires the use of power semiconductor devices providing protection and diagnostic functions. In the case of series protections power semiconductor devices which provide protection may operate in shortcircuit and act as a current limiting device. This mode of operations is very constraining due to the large dissipation of power. In these particular conditions of operation, electro-thermal models of power semiconductor devices are of key importance in order to optimize their thermal design and increase their reliability. The development of such an electro-thermal model for power MOSFET transistors based on the coupling between two computation softwares (Matlab and Cast3M) is described in this paper. The 2D electro-thermal model is able to predict (i) the temperature distribution on chip surface well as in the volume under short-circuit operations, (ii) the effect of the temperature on the distribution of the current flowing within the die and (iii) the effects of the ageing of the metallization layer on the current density and the temperature. In this paper, the electrical and thermal models are described as well as the implemented coupling scheme.

The quantum confined Stark effect (QCSE) has been investigated in detail in GaAs/AlGaAs and InAs/GaAs/AlGaAs HEMTs structures grown by molecular beam epitaxy on (1 0 0) oriented GaAs substrates. A power dependent photoluminescence (PL) study allowed to highlight the QCSE caused by the internal electric field in the GaAs channel then a piezoelectric field reinforce the red-shift. Increasing the Si-δ-doping leads to; first a red-shift then a blue shift at high excitation power due to the stabilize quantum well structures tendency and the saturation phenomenon. The HEMT band structures and the theoretical activated electron densities at 10 K have been studied using the finite difference method and the simultaneously solve of the Schrödinger and the Poisson equations written within the Hartree approximation.

A broadband omnidirectional absorber which is realized by heterostructures containing a collision plasma layer and a zero − n̅ mirror is theoretically investigated. A collision plasma layer and an appropriate dielectric layer are put on the top of the PC. It is shown to absorb roughly 70% of all available electromagnetic wave in a relative omnidirectional absorption band width 244 MHz. The absorption band edge of the PC is influenced by the range of the reflection band gap. Meanwhile, the absorption range for the transverse magnetic (TM) wave decreases at large incident angle. Compared with some previous designs, our proposed structure has a relative flatter total absorption spectrum over a broad microwave frequency range and using a zero − n̅ gap as a mirror is insensitive to the incident angle. This kind of heterostructure offers additional opportunities to design novel optoelectronic devices.

An experimental methodology compliant with industrial constraints was deployed to uncover the origin of soft breakdown events in large planar silicon-based NMOS capacitors. Complementary advanced failure analysis techniques were advantageously employed to localize, isolate and observe structural defects at nanoscale. After an accurate localization of the failing area by optical beam-induced resistance change (OBIRCH), focused ion beam (FIB) technique enabled preparing thin specimens adequate for transmission electron microscopy (TEM). Characterization of the gate oxide microstructure was performed by highresolution TEM imaging and energy-filtered spectroscopy. A dedicated experimental protocol relying on iterative FIB thinning and TEM observation enabled improving the quality of electron imaging of defects at atom scale. In that way, the gate oxide integrity was evaluated and an electrical stress-induced silicon epitaxy was detected concomitantly to soft breakdown events appearing during constant voltage stress. The growth of silicon hillocks enables consuming a part of the breakdown energy and may prevent the soft breakdown event to evolve towards a hard breakdown that is catastrophic for device functionality.

Recent studies have shown that graphene paper (GP) is a promising candidate for novel applications in energy storage systems such as electrical batteries and supercapacitors. In this study, a low-cost and free-standing GP was produced by graphene oxide paper (GOP) in the thermal reduction method which heated GOP slowly to 500 °C in a special fixture. In the process of reduction, oxygen functional groups were decomposed and graphene oxide (GO) sheets were reduced to grapene sheets. Moreover, the smooth paper-like structure was maintained by the protection of the special fixture during the heat treatment. The GP paper showed good electrochemical property. The electrical conductivity of the GP reached 182 S cm−1 after the decomposition of oxygen functional groups. The GP as binder-free anodes of lithium ion batteries afforded a capacity of 576 mAh g−1 with good cycling stability.

In this paper, terminal capacitances of a normally-on SiCED-JFET are measured, analyzed and simulated. All these capacitances are measured using an auto-balanced (guarded) capacitance test-bench that leads to the standard 3-terminal model capacitors CGS, CDS and CGD. This test bench is developed to measure each capacitance individually, without any mutual influence. 2D finite-element simulations are used to show that the capacitance CGD cannot be modeled by a standard planar junction model. This is due to the influence of two dimensional effects around the buried layer P+. A new analytical model of CGD is proposed. A good agreement is obtained between simulations and measurements of the different capacitances.

ZnO thin films were deposited by reactive cathodic radio-frequency (RF) sputtering from a pure Zn target in a gas mixture of 30% O2 and 70% Ar and at different RF powers. The structural properties of the as-deposited thin films were studied by X-ray diffraction (XRD). The optical properties (especially the refractive index, absorption coefficient and optical band gap) were investigated by optical transmission measurements in the ultraviolet-visible-near Infrared wavelength range. The XRD patterns showed that the as-deposited ZnO thin films are polycrystalline. The crystallite size varied with RF power reaching a maximum at 200 W. These results were correlated with X-ray refectometry measurements which revealed a minimum in the film density at 200 W. The deposition rate of these films varied from 2.53 to 5.27 nm/min depending on the RF-power, with a maximum at 200 W. On the other hand, the optical band gap Eg was quasi-constant (about 3.28 eV) when the RF power was increased from 100 to 300 W.

In this paper porous silicon (PS) has been prepared by electrochemical etching technique and then ZnO thin film deposition on PS by spray pyrolysis method, the study of AFM show improve the structural stability of the PS substrate with crystalline growth of ZnO thin film. PL spectra explained a blue-shifting in PS layer come from oxidation the surface of PS after coating with ZnO film, Raman measurement show quantum confinement in PS layers with decreasing in variation mode of ZnO film, and the J-V characteristic show increasing in resistivity of Al/ZnO/PS/c-Si/Al due to increasing in depletion layer junction compere with PS layer.

In this work, PANI:PVDF composites films were prepared with different PANI contents (p = 1, 2, 3, 4 and 5%). The resulting films were dried at various temperatures such as 30, 90 and 120 °C. The alternating current mechanisms and dielectric relaxation and of PANI:PVDF films were studied using complex impedance spectroscopy over a wide range of temperature (303–453 K) and a frequency range (1 kHz to 1 MHz). We found that the ac conductivity in PANI:PVDF composite is governed by correlated barrier hopping (CBH) model. In dielectric loss modulus study, two relaxation processes were identified. The first peak was associated to Maxwell Wagner-Sillas (MWS) relaxation whereas the second one which obtained at higher frequency was attributed to the αc relaxation. For PANI:PVDF film which dried at 30 °C, the MWS relaxation appears only at higher temperature. The temperature dependence of αc relaxation was suitably fitted according to Vogel Flucher Temman model whereas MWS relaxation follows Arrhenius type behavior. The effect of drying temperature on microstructure and phase crystallization of PVDF in the composites was carried out using atomic force microscopy (AFM) and Fourier transform infrared (FTIR) spectroscopy. These results were used to find a reasonable correlation between microstructure and electrical properties.

The electronic transport properties of Ga2As2 cluster, which is sandwiched between two semiinfinite Au (1 0 0)-3 × 3 pyramical-shaped electrodes with the Ga-Ga axis of the cluster parallel to the transport direction and the As-As axis of the cluster parallel to the transport direction, respectively, is investigated with a combination of density functional theory and the non-equilibrium Green’s function method. We have simulated the nanoscale junctions breaking process and found that the conductance of cluster decreases then increases when the contact is pulled apart in two configurations. We analyzed the difference of conductance from transmission spectra and projected density of states, and calculated the I-V characteristics of devices in this two configurations when dz = 2.0 Å. The I-V curves display a linear characteristics in the voltage range of 0 ∼ 2.2 V. The negative differential resistance appears within a small range of voltage in the junctions with As-As axis of the cluster parallel to the transport direction when bias is larger than 2.2 V.

In this work, the correlation between BEMA model and FTIR analysis was employed to investigate the chemical composition of silicon oxynitride (SiOxNy) films deposited by low pressure chemical vapour deposition (LPCVD) technique at temperature of 850 °C from nitrous oxide N2O, ammonia NH3 and dichlorosilane SiH2Cl2. Different stoichiometries were obtained for different values of relative gas flow ratio NH3/N2O while keeping the SiH2Cl2 flow constant. The optical properties were studied using spectroscopic ellipsometry. Apart from films thickness, their refractive index as well as their SiO2 and Si3N4 volume fractions were deduced using the Bruggeman effective medium approximation (BEMA) model. It was found that the refractive index increases from 1.45 to 1.60 with increasing nitrogen incorporation. The Fourier Transform Infrared spectroscopy was carried out to study the evolution of chemical bonding of LPCVD SiOxNy films. In order to improve the qualitative analysis of their Si-N and Si-O vibrational modes, a correlation between Fourier transform infrared and spectroscopic ellipsometry measurements was established. A shift of infrared peaks position with the increase of relative gas flow ratio is observed. Furthermore, the calculated areas of absorption bands were used to estimate the SiOxNy stoichiometry. This quantitative analysis was proved with an adequate method in the literature. We found a decrease of x values from 1.94 to 1.26 and an increase of y from 0.04 to 0.49, when the NH3/N2O gas flow ratio increases. This behavior was explained by the increase of nitrogen content as well as the decrease of oxygen content in the SiOxNy film.

In this paper, carrier dynamics in N,N′-diphenyl-N,N′bis(1,1′-biphenyl)-4,4′-diamine (NPB) was studied using impedance spectroscopy (IS) and particle swarm optimization algorithm (PSO). We applied PSO to fit the frequency dependence of impedance spectroscopy of NPB, and achieved the charge-carrier transit time and the dispersive parameters of NPB, and then obtained carrier mobility. The impacts of the dispersive degree on the impedance had been analyzed. Though PSO, the three unknown parameters, charge-carrier transit time τdc and dispersive degree M, α in the admittance model were achieved simultaneously. The results verified the reliability of this method. Furthermore, we have presented the advantages of PSO compared with the traditional nonlinear least squares algorithm. In our limited knowledge, this paper begins the work to study materials in the deep level of algorithm

The properties of electronic transport in graphene superlattices which consist of symmetrical period potentials have been studied. It is found that such structures possess an unusual tunneling state inside the original forbidden gaps. Furthermore, the position of tunneling mode is highly dependent on the lattice constant and potential voltage but insensitive to period number, and a theoretical explanation for this phenomenon is proposed. Finally it is revealed that the electronic conductance is greatly enhanced and the Fano factor is strongly suppressed near the energy of the tunneling state. The characteristics of electron transport over symmetrical graphene superlattices may facilitate the development of many graphene-based electronics.

Nanocrystalline Ni0.7−yZn0.3CayFe2O4 (0.0 ≤ y ≤ 0.7) has been obtained by soft coprecipitation method at different calcination temperatures. The influence of Ca2+ content and heat treatment on microstructure and magnetic properties are investigated using X-ray diffraction (XRD), thermal analysis, scanning electron microscope (SEM), atomic force microscope (AFM) and infrared (IR) spectroscopy. XRD analysis reveals single phase of spinel structure in ferrite with y = 0.2. An endothermic peak in DSC curve is appeared around 700 °C for all compositions due to collapse of the defective surface layer covers the grains. IR spectra for precursors or calcined ferrites shows the existence of bands characteristic for cubic spinel phase. The higher values for specific saturation magnetization (Ms) was obtained for ferrites calcined at 600 °C. Paramagnetic ordered particles was dominated when Ca2+ content increases more than 0.3. On the other hand, the coercivity of the present ferrites is strongly depends on Ca2+ content than calcination temperature.

In this article, a simple method was proposed for determining the thermal effusivity of defects using pulsed thermography. This method is complementary to other methods such as those that implement periodic thermal stresses for identification of the thermal properties of materials, or which use specific photothermal analysis or extended. Theoretically, this technique is based on solving the heat equation in the case of an isotropic solid heated by a delta Dirac pulse. The application conditions of this new technique have been studied by numerical simulations. A finite element modeling has been performed in the case of a steel sample which may contain geometry defects filled in by air, resin, oil, wax or water. The analysis of surface temperature distribution of the infected samples by the proposed method allowed to determine the thermal effusivity of defects. The experimental measurements have confirmed the importance and simplicity of this method in the characterization of defects using pulsed thermography. The results show the possibility of approaching, at an acceptable error, the defect thermal effusivity.

Lithium ion battery cathode material LiNi0.8Co0.15Al0.05O2 cathode has successfully prepared by co-precipitation. CeO2 surface modification has improved LiNi0.80Co0.15Al0.05O2 electrochemical performance use sol-gel method and subsequent heat treatment at 600 °C for 5 h. Different to other conventional coating material, CeO2 coating layer can not only inhibit the reaction of the electrode and the electrolyte, but also can reduce the impedance of electron transfer due to its high conductivity, and inhibit the production of Ni2+ because of its high oxidation. The surface-modified and pristine LiNi0.80Co0.15Al0.05O2 powders are characterized by XRD, SEM, TEM, XPS, CV and DSC. When CeO2 coating is 0.02% (mole ratio), contrast to pristine NCA, the CeO2-coated NCA cathode exhibits no decrease in its initial specific capacity of 184 mAh g −1 (at 0.2 C) and excellent capacity retention (86% of its initial capacity at 1 C) between 2.75 and 4.3 V after 100 cycles. The results indicate that the CeO2 surface treatment should be an effective way to improve cycle properties due to CeO2 inhibit the electrodes and the electrolyte side effects.

This paper analyzes surface waves (SW) supported by a biaxial metamaterial layer placed between two semi-infinite isotropic media. The metamaterial fabricated by periodic alternating semiconductor and dielectric layers and subjected to an external magnetic field. The magnetic field is applied parallel to the layers. We consider such structure in the long-wave limit so that we apply the effective medium theory to describe the electrodynamic properties. The necessary conditions of SW existence were found in terms of anisotropy of the structure. The dependences of penetration depth and attenuation constant (AC) of SW on the external magnetic field and thicknesses of the layers were examined analytically and numerically.