In this work we report on the characteristics of (Ni/Au)/AlGaN/GaN/SiC Schottky barrier diode (SBD). A variety of electrical techniques such as capacitance-voltage (C-V) and deep-level transient spectroscopy (DLTS) measurements were used to characterize the diodes. We observed an hysteresis phenomenon on the C-V characteristics in the Schottky diode. The parasitic effect can be attributed to the presence of traps in the heterostructure. Deep defects analysis was performed by deep-level transient spectroscopy (DLTS). One hole trap have been detected with an activation energy and a capture cross-section of 0.75 eV and 1.093 × 10−11 cm2. The localization and the identification of this trap have occurred and a correlation between the defect and the hysteresis phenomenon has been discussed. At high temperatures, the DLTS signal sometimes becomes negative, likely due to an artificial surface-state effect.

TlInS2 single crystals were grown by using Bridgman-Stockbauer technique. Measurements of DC conductivity were carried out in parallel (σ//) and perpendicular (σ⊥) directions to the c-axis over a temperature range from 303 to 463 K. The anisotropic behaviour of the electrical conductivity was also detected. AC conductivity and dielectric measurements were studied as a function of both frequency (102–106 Hz) and temperature (297–375 K). The frequency dependence of the AC conductivity revealed that σac(ω) obeys the universal law: σac(ω) = Aωs. The mechanism of the ac charge transport across the layers of TlInS2 single crystals was referred to the hopping over localized states near the Fermi level in the frequency range >3.5 × 103 Hz. The temperature dependence of σac(ω) for TlInS2 showed that σac is thermally activated process. Both of ϵ1 and ϵ2 decrease by increasing frequency and increase by increasing temperature. Some parameters were calculated as: the density of localized states near the Fermi level NF = 1.5 × 1020 eV-1 cm-3, the average time of charge carrier hoping between localized states τ = 3.79 μs and the average hopping distance R = 6.07 nm.

Using a Plane-Wave Pseudo-Potential (PWPP) method, total energy and band structure calculations for M2TaN3, ε-TaN and MTa2N3 (M = a transition metal, TM) compounds have been performed in order to understand their structural stability and electronic properties. To do that, we first focus on ε-TaN compounds. The exchange correlation is treated using the Generalized Gradient Approximation (GGA). The Virtual Crystal Approximation (VCA) is then used to examine the structural stability when substituting Ti, Zr or Hf to a Ta atom either in a cell corner [c(M)(Ta2N3)] or inside the cell [in(M2)(TaN3)]. Actually, substitution of Ta by a Ti, Zr or Hf atom at a corner site does slightly change the corresponding lattice constant. Also we calculate ground-state quantities such as elastic constants, shear moduli, Young’s modulus and bulk modulus as well as Poisson’s ratio. The corresponding results for band structures and densities of states are shown as well. As far as we know our work is a pioneer attempt to determine elastic, mechanic and electronic properties for M2TaN3 and MTa2N3 (M = TM) compounds.

In this work, we investigated the hot-electron dynamics of AlGaN/GaN HEMT structures grown by MOCVD on sapphire and SiC substrates at 80 K. High-speed current-voltage measurements and Hall measurements over the temperature range 27–300 K were used to study hot-electron dynamics. At low fields, drift velocity increases linearly, but deviates from the linearity toward high electric fields. Drift velocities are deduced as approximately 6.55 × 106 and 6.60 × 106 cm/s at an electric field of around E ~ 25 kV/cm for samples grown on sapphire and SiC, respectively. To obtain the electron temperature as a function of the applied electric field and power loss as a function of the electron temperature, we used the so-called mobility comparison method with power balance equations. Although their low field carrier transport properties are similar as observed from Hall measurements, hot carrier energy dissipation differs for samples grown on sapphire and SiC substrates. We found that LO-phonon lifetimes are 0.50 ps and 0.32 ps for sapphire and SiC substrates, respectively. A long hot-phonon lifetime results in large non- equilibrium hot phonons. Non-equilibrium hot phonons slow energy relaxation and increase the momentum relaxation. The effective energy relaxation times at high fields are 24 and 65 ps for samples grown on sapphire and SiC substrates, respectively. They increase as the electron temperature decreases.

This is a theoretical study of the InGaN back-barrier on the properties of the Al03Ga0.7N/AlN/GaN/InGaN/GaN HEMT structure by self-consistently solving coupled Schrödinger and Poisson equations. Our calculation shows that by increasing the indium composition, the conduction band of the GaN buffer layer is raised and the confinement of 2DEG is improved. However, the additional quantum well formed by InGaN becomes deeper, inducing and confining more electrons in it. Another conductive channel is formed which may impair the device performance. With the increasing InGaN thickness, the well depth remains the same and the conduction band of GaN buffer layer rises, enhancing the confinement of the 2DEG without inducing more electrons in the well. The 2DEG sheet density decreases slightly with the indium composition and the physical mechanism is discussed. Low indium composition and thick InGaN back-barrier layer are beneficial to mitigate the short-channel effects, especially for high-frequency devices.

We document the magnetotransport properties of multicomponent La0.6Pb0.4Mn0.8Ru0.2O3 oxide thin films deposited using Pulsed Electron Deposition. This is a model composition in this series, which shows adequate hole transport aided by the presence of both Mn4+ and Ru5+ ions with similar t2gand eg parentage. The optimized metal to insulator transition temperature (Tmit) of the film has been found to be ~298 K and the magnetoresistance (—MR%) ratio of about —65% near the Tmit was observed for the La0.6Pb0.4Mn0.8Ru0.2O3 thin films. High-Resolution Transmission Electron Microscopy (HRTEM) studies reveal a good epitaxial growth along the (0 0 1) direction on LaAlO3 (LAO) substrate, with the width of the rocking curves (ω scans) <0.6°.

Electroabsorption spectroscopy of well-identified index-defined semiconducting carbon nanotubes is reported. The measurement of high definition electroabsorption spectra allows direct indexation with unique nanotube chirality. Results show that at least for a limited range of diameters, electroabsorption is directly proportional to the exciton binding energy of nanotubes. Electroabsorption is a powerful technique which directly probes into carbon nanotube excitonic states, and may become a useful tool for in situ study of excitons in future nanotube-based photonic devices such as electroabsorption modulators.

Chemically sprayed aluminum-doped ZnO (1% Al) and undoped ZnO thin films were deposited on glass substrates at T = 420 °C. The thin films are characterized by X-ray diffraction and scanning electron microscopy. The characterization results show that all the compounds are wurtzite with hexagonal structure (0 0 2). They are well crystallized and the grain size is (e = 0.1 μm) for undoped ZnO and (e = 0.13 μm) for Al-doped ZnO. The prepared Al-doped ZnO showed good gas responses to ethanol for two concentrations 200 ppm and 400 ppm. Short response time is given by Al-doped ZnO comparing with undoped ZnO films, its estimate at (~10 s) and recovery time (~15 s) to 200 ppm at T = 220 °C.

This paper introduces theoretic and experimental analyses of short-duration pulse propagation through a negative group delay (NGD) circuit. The basic analysis method of this electronic circuit operating in baseband and microwave frequencies is investigated. Then, its electrical fundamental characteristics vis-à-vis transient signals are developed. To validate the theoretic concept, planar hybrid devices with one- and two-stage NGD cells were designed, simulated, fabricated and tested. Transient analyses with ultra-wide band (UWB) pulse signals with different widths are realized. Then, experimental results in good agreement with the theoretical predictions were observed. Consequently, group delay going down under −2.5 ns is evidenced in baseband frequency up to 63 MHz with one-stage NGD cell. In time-domain, a Gaussian pulse in advance of about t0 = −1.5 ns or 20% of its half-height time-width was measured. This corresponds to a negative group velocity of about vg = L/t0 = −0.13c (L is the physical length of the tested device and c is light speed in the vacuum). More significant NGD value over 100-MHz bandwidth is stated with two-stage NGD cells. This results in a Gaussian pulse peak advance of about −5 ns (raising a group velocity of about vg = −0.12c) or 31% of its half-height time-width. Finally, some potential applications based on the NGD function are discussed.

In0.18 Ga0.82As/GaAs quantum well sample is prepared by molecular beam epitaxy. The integrated photoluminescence dependence on the excitation power intensity is studied. The critical exciton temperature is found to be around 210 K. The high critical exciton temperature is due to the increased in-plane confinement potential. To understand the photoluminescence behavior in this sample an assumption of the existence of trapping centers that has quantum dot-like effect is introduced. These trapping centers are due to In-atom segregation during growth, a multi-peak Gaussian fitting showed additional broad peak in the high energy side of the photoluminescence spectrum which is attributed to the segregated In-atoms. The segregated In-atoms cause additional confinement to be added to the system, and hence excitons survive longer with temperature. The results show that electron hole pairs in the studied quantum well sample are weakly correlated in the near room temperature region.

We have experimentally investigated the unwinding process of the ferroelectric Sm-C* phase of a chiral AFLC smectic liquid crystal compound 12OF1M7 using the electro-optic properties, microscopic observations and constant current method. We found that this process occurs via two steps through an intermediate phase. Furthermore, we have established the (E-T) phase diagram of the studied compound by combining the experimental results.

The temperature dependence of the I-V characteristics on Au/Ni-HEMT Schottky contacts was measured and analyzed. Large deviations from the thermionic emission and thermionic-field emission model were observed in the I-V-T characteristics. The thin surface barrier model only fits the measured curves in the high bias region, but deviates drastically in the low bias region. Using a revised thin surface barrier model, the calculated curves match well with the measured curves. It is also found that tunneling emission model is the dominant current transport mechanism at low temperature, yet thermionic-field emission model is the dominant current transport mechanism at high temperature.

Co2+ doped ZnO nanoparticles (NPs) using PEG as a capping agent were prepared by colloidal wet-chemical method. The structure, morphology and characteristics of as-prepared samples were investigated. X-ray diffraction patterns studies revealed wurtzite crystal phase. STM-TEM micrographs show a spherical shape and nearly well distribution with an average particle size of ~15–20 nm. UV-VIS spectra show the presence of exciton peak at 349 nm which can be effectively tuned versus cobalt doping and PEG concentration. PL studies were done under the excitation of 347 nm, which exhibited a UV (~386 nm) and visible (blue-orange) emission peak because of free-exciton recombination and oxygen vacancy.

Pentacene field-effect transistors (FETs), fabricated on strontium titanate without self-assembled monolayer, showed mobility as high as 1.9 cm2/V s at operating voltage as low as −3 V. The mobility of the FET was significantly increased to attain 3.3 cm2/V s simply by storage in a desiccator for 1032 h. Based on careful examination of the time dependence of the mobility for the FET devices, it was indicated that the mobility significantly increased in initial stage to several thousand hours and then monotonously decreased beyond then regardless of the initial value of the mobility.

Polycrystalline Cd thin films were evaporated in a vacuum onto glass substrates at Cd source temperature of 770 K. The as-deposited Cd films were subjected to a gradual heating at the rate of 5 K/min, up to a temperature of 650 K and were then maintained at this temperature for 5 min. Then, the respective samples were cooled down to room temperature at the same rate. The obtained CdO thin films were UV irradiated for 2 h (150 W mercury lamp, 3.18–3.65 eV). By means of XRD, AFM and XPS techniques, the structural characteristics of the typical obtained CdO samples before and after UV treatment were investigated. The obtained results indicate that the UV treatment induces a recrystallization process: changes in sample morphology, surface roughness and crystallite size and orientation. XRD and XPS studies evidenced an improvement in crystalline structure and stoichiometry. UV irradiated sample shows photo-catalytic properties.

By engineering the interface between the intermediate photoactive layer and the cathode aluminum (Al) electrode, through the introduction of ultra-thin layers of various materials, in a standard bulk heterojunction (BHJ) polymer solar cell (PSC) fabricated of regioregular poly(3-hexylthiophene) (rr-P3HT) and phenyl-C61-butyric acid methyl ester (PCBM), the power conversion efficiency (PCE) has been effectively improved. The devices fabricated using individual single interlayer of bathocuproine (BCP), lithium fluoride (LiF) and Buckminster fullerene C60 have shown optimal efficiencies of ~2.40%, ~3.21% and ~1.92% respectively. Further improvement of the photovoltaic efficiency was achieved by introducing a composite bilayer made of LiF in combination with BCP as well as with C60 at the BHJ/cathode interface. The best results were obtained by interposing a 9 nm of C60 interlayer in combination with a 0.9 nm thick LiF layer, with the PCE of the PV cells being effectively increased up to 3.94% which represents an improvement of 23% as compared to the standard device with LiF interlayer alone. The photocurrent density (Jsc) versus voltage (V) characteristic curves shows that the increase of the efficiency is essentially due to an increase in Jsc. Moreover, all the sets of devices fabricated using various interlayers over a certain range of thickness exhibit an optimum PCE that is inversely proportional to the series resistance of the PV cells. We presume that interposing a C60/LiF layer at the interface could repair the poor contact at the electron acceptor/cathode interface and improve the charge career extraction from the BHJ.

Magnetic Resonance Imaging (MRI) that provides superior soft tissue contrast is commonly used for diagnosis of many diseases. However specificity of MRI cancer diagnosis can be further increased by application of target contrast agents comprising superparamagnetic nanoparticles (NPs) synthesised with biological objects, which deliver the contrast to the specific cancer cells. These superparamagnetic NPs shorten T2 relaxation time thus change contrast to noise ratio for tumor tissues. Therefore the impact of Fe3O4 size and silica coating for Fe3O4/silica core/shell superparamagnetic nanoparticles (NPs) on T2 relaxation time was studied at 9.4 T. The magnetic resonance imaging (MRI) studies were performed using homogenous agar solution of NPs. Naked Fe3O4NPs with a mean core diameter of 10.0 ± 1.3 (mean ± SD), 15.0 ± 2.5 and 20.0 ± 0.9 nm were analyzed. Silica coated Fe3O4 NPs with core size of 10.0 ± 1.3 nm and the shell thickness of 16.7 ± 1.8, 25.3 ± 2.7 and 33.9 ± 4.0 nm were also investigated. The T2 values of agar solutions with different NPs were calculated using a single slice multi echo method and single exponential fitting of the echo train. The measurements showed linear correlation between T2 and Fe3O4 core diameter as well as shell thickness. Silica coating, while improving functionalization and potentially reducing toxicity of NPs, decreases the impact of the magnetic core on T2, thus decreasing MRI contrast efficacy.

ZnO nanorods arrays were prepared on soda lime glass substrate by pulsed laser deposition method. Hexagonal rod-like ZnO rods were obtained under different conditions. Well-defined ZnO nanorods arrays were selected among different samples having various morphologies and sizes already studied by X-ray diffraction (XRD) and atomic force microscopy (AFM). Here, we report on the contact angle measurement (CAM) of one of these samples. A systematic change of the surface wettability is observed in W-doped ZnO nanostructures. The water contact angle (WCA) of a 1 wt.% of WO3 target content was found to be the transition doping level from hydrophilic surface to a hydrophobic surface. We attributed the transition in surface wettability of the film with the doping to incorporation increase of tungsten into the film. Such characteristic surface wettability can play a key role in the adhesion of various layers on W-ZnO nanorods arrays for optoelectronic device applications.

Nanostructure thin films and sandwich devices of palladium phthalocyanine (PdPc) and chloro-aluminum-phthalocyanine (ClAlPc) were prepared by thermal evaporation technique. Optical and struc- tural properties of nanostructure thin films were investigated by XRD, SEM and optical absorption. The SEM images demonstrated PdPc (40–60 nm) and ClAlPc (30–50 nm) nanostructures. XRD pat- terns showed that thin films are in α-phase at room temperature. Also, optical bandgap energy of thin films was calculated by optical absorption spectra. Heterojunction (Au|PdPc|ClAlPc|Al) and single layer (Au|PdPc|Al and Au|ClAlPc|Al) devices were fabricated. Electrical measurements demonstrated the semi-conducting and photo-conducting behavior of thin films. After that, devices were exposed to different concentrations of O2 at 300 K and 350 K and conductivity of thin films was increased on exposure to O2 . Heterojunction devices were more sensitive than other thin films and had better response and reversibility in comparison with single layer devices at 350 K. Finally, 10% O2 was mixed with different percentages of relative humidity and all results showed that the conductivity of thin films is reduced on exposure to O2 mixed to humidity.

We report the effect of nitrogen and boron incorporation in the structure of spherical nanodiamond particles using ab initio density-functional theory. We changed the site of boron and nitrogen impurity from the center to the surface of the particles and calculated structural, electronic, cohesive and surface charge properties of each configuration. By moving the impurity from the center toward the surface, the internal strain of N and B doped nanodiamond drops. Unlike nitrogen, doping of boron causes lower broken bonds around impurity in the central region and also replacement of boron in the surface releases the strain of nanoparticle. We explain this observation by calculating the variation of the surface charge or polarity of the doped particles versus the site of dopant during the movement of impurities toward the surface which reduces the polarity in contrast to behavior of nitrogen doped particle. By moving the boron atom from the center toward the surface of the particle, p-type conduction occurs whereas nitrogen impurity does not create an n-type conduction. Results suggest that boron is a more effective dopant than nitrogen provided that it has not been placed in the central region of nanoparticle.