The effect of external stress on the electrical characteristics of AlGaN/GaN HEMTs

The electrical characteristics of AlGaN/GaN high electron mobility transistors under the application of uniform in-plane tensile and compressive stress were measured. The results demonstrate the change of the drain–source I_ds–V_ds characteristics as a function of the external stress. The output current at V_ds = 10 V increases linearly with the stress with the slope about 3 × 10⁻⁶ A MPa⁻¹. It is associated with the piezoelectric effect and kink effect. Moreover, the magnitude of the kink effect is found to be affected by the stress. It displays a linear changing trend with the slope of 3.3 × 10⁻⁴ mS MPa⁻¹ within the stress level. The energy band structure is suggested to be responsible for the dependence of the kink effect on the stress.     

Preparation and physical properties of the layered niobate Cu0.5Nb3O8: Application to photocatalytic hydrogen evolution

The new layered niobate Cu_0.5Nb₃O₈ is synthesized by soft chemistry in aqueous electrolyte via Cu²⁺→H⁺ exchange between copper nitrate and HNb₃O₈·H₂O. The characterization of the exchanged product is made by means of thermal gravimetry, chemical analysis, X-ray diffraction and IR spectroscopy. Thermal analysis shows a conversion to anhydrous compound above 500 °C. The oxide displays a semiconductor like behavior; the thermal variation of the conductivity shows that d electrons are strongly localized and the conduction is thermally activated with activation energy of 0.13 eV. The temperature dependence of the thermopower is indicative of an extrinsic conductivity; the electrons are dominant carriers in conformity with an anodic photocurrent. Indeed, the Mott–Schottky plot confirms n-type conduction from which a flat band potential of −0.82 V_SCE, an electronic density of 8.72×10¹⁹ m⁻³ and a depletion width of 4.4 nm are determined. The upper valence band, located at ~5.8 eV below vacuum is made up predominantly of Cu²⁺: 3d with a small admixture of O²⁻: 2p orbitals whereas the conduction band consists of empty Nb⁵⁺: 5s level. The energy band diagram shows the feasibility of the oxide for the photocatalytic hydrogen production upon visible light (29 mW cm⁻²) with a rate evolution of 0.31 mL g⁻¹ min⁻¹.     

Electrochemical growth of tin(II) oxide films: Application in photocatalytic degradation of methylene blue

We have investigated the semiconducting and photoelectrochemical properties of SnO films grown potentiostatically on tin substrate. The oxide is characterized by X-ray diffraction, scanning electron microscopy and Raman spectroscopy. The anodic process corresponds to the formation of SnO·nH₂O pre-passive layer that is removed upon increasing potential due to surface etching at the metal/oxide interface. SnO films deposited for long durations (>50 mn) are uniform and well adhered; they thicken up to ~50 nm by diffusion-controlled process and the growth follows a direct logarithmic law. The thickness is determined by coulometry and the X-ray diffraction indicates the tetragonal SnO phase (SG: P4/mmm) with a crystallite size of 32 nm. The Mott–Schottky plot is characteristic of n type conductivity with an electrons density of 5.72×10¹⁸ cm⁻³, a flat band potential of −0.09 V_SCE and a depletion width of ~10 nm. The valence band, located at 5.91 eV below, vacuum is made up of hybridized O²⁻:2p Sn²⁺:5s while the conduction band (4.45 eV) derives from Sn²⁺:5p orbital. The electrochemical impedance spectroscopy (EIS) measured in the range (10⁻²–10⁵ Hz) shows the contribution of the bulk and grain boundaries. The energy band diagram predicts the photodegradation of methylene blue on SnO films. 67% of the initial concentration (10 mg L⁻¹) disappears after 3 h of exposure to visible light (9 mW cm⁻²) with a quantum yield of 0.072.     

Synthesis and semiconducting properties of Na2MnPO4F. Application to degradation of Rhodamine B under UV-light

Na₂MnPO₄F is synthesized by hydrothermal route at 453 K and the physical properties and photo-electrochemical characterizations are reported. The compound crystallizes in a monoclinic system (SG: P 2₁/n) with the lattice constants: a=13.7132 Å, b=5.3461 Å, c=13.7079 Å, β=119.97°. The UV–visible spectroscopy shows an indirect optical transition at 2.68 eV; a further direct transition occurs at 3.70 eV, due to the charge transfer O²⁻: 2p → Mn²⁺: e_g. The thermal variation of the electrical conductivity is characteristic of a semiconducting behavior with activation energy of 39 meV and an electron mobility (µ_318 K=5.56×10⁻⁴ cm² V⁻¹ s⁻¹), thermally activated. The flat band potential (+0.47 V_SCE) indicates that the valence band derives mainly from O²⁻: 2p orbital with a small admixture of F⁻ character while the conduction band is made up of Mn²⁺: t_2g orbital. The electrochemical impedance spectroscopy shows the contribution of both the bulk and grains boundaries. The photocatalytic performance of Na₂MnPO₄F for the degradation of Rhodamine B (RhB) is demonstrated on the basis of the energy diagram. 88% of the initial concentration is degraded under UV light and the oxidation follows a first order kinetic with a rate constant of 0.516 h⁻¹. Neither adsorption nor photolysis is observed. The photoactivity results from the electron transition from the hybridized band (O²⁻, F⁻) to the Mn²⁺: e_g orbital, occurring in the UV region. The catalyst was subjected to three successive photocatalytic cycles, thus proving its long term stability.     

Defect characterization of Tl4GaIn3Se2S6 layered single crystals by photoluminescence

Photoluminescence (PL) spectra of Tl₄GaIn₃Se₂S₆ layered crystals grown by the Bridgman method have been studied in the energy region of 2.02–2.35 eV and in the temperature range of 16–45 K. A broad PL band centered at 2.20 eV was observed at T=16 K. Variations of emission band has been studied as a function of excitation laser intensity in the 0.1 to 149.9 mW cm⁻² range. Radiative transitions from shallow donor level located at 10 meV below the bottom of conduction band to moderately deep acceptor level located at 180 meV above the top of the valence band were suggested to be responsible for the observed PL band. An energy level diagram showing transitions in the band gap of the crystal was plotted taking into account the results of present work and previously reported paper on thermally stimulated current measurements carried out below room temperature. Analysis of the transmission and reflection measurements performed in the wavelength range of 400–1030 nm at room temperature revealed the presence of indirect transitions with 2.22 eV band gap energy.     

Highly nonlinear and highly birefringent dispersion compensating photonic crystal fiber

This paper presents an optimum design for highly birefringent hybrid photonic crystal fiber (HyPCF) based on a modified structure for broadband compensation covering the S, C, and L-communication bands i.e. wavelength ranging from 1460 to 1625 nm. The finite element method (FEM) with perfectly matched layer (PML) circular boundary is used to investigate the guiding property. It is demonstrated that it is possible to obtain broadband large negative dispersion, and dispersion coefficient varies from −388.72 to −723.1 ps nm⁻¹ km⁻¹ over S, C and L-bands with relative dispersion slope (RDS) matched to that of single mode fiber (SMF) of about 0.0036 nm⁻¹ at 1550 nm. According to simulation, a five-ring dispersion compensating hybrid cladding photonic crystal fiber (DC-HyPCF) is designed that simultaneously offers birefringence of order 3.79 × 10⁻², nonlinear coefficient of 40.1 W⁻¹ km⁻¹ at 1550 nm wavelength. In addition to this, effective area, residual dispersion, and confinement loss of the proposed DC-HyPCF are also reported and discussed.     

Effects of oxygen content on the structural,optical, and electrical properties of NiO films fabricated by radio-frequency magnetron sputtering

Nickel oxide (NiO) films were deposited on Corning glass substrate with variable (0–100%) oxygen content by radio-frequency sputtering. Effects of different oxygen content on the structural, optical, and electrical properties of NiO films were studied. X-ray diffraction showed that the NiO film deposited on substrate with 0% oxygen content resulted in a random polycrystalline structure and small grain size. The introduction of oxygen gas leaded to a (200) preferential orientation and larger grain size. The transmittance decreases with oxygen content due to the increase of oxygen interstitials in NiO films. The 0%-O₂ deposited NiO film has a tensile strain and a small band gap. Upon introducing 33%-O₂ content, the NiO film exhibits a compressive strain, increasing the bandgap. However, the compressive strain is released and gradually turns into tensile strain, which leads to the narrowing of bandgap with the increase of oxygen content. Hall measurement shows the obtained NiO is p-type and the resistivity decreases from 4.3 × 10⁵ Ω-cm to 5.02 Ω-cm with increasing oxygen content from 0% to 100%. The carrier concentration increases from 6.3 × 10¹⁴ cm⁻³ to 4.6 × 10¹⁸ cm⁻³ and the mobility decreases from 26 cm²/V-s to 0.26 cm²/V-s for the NiO films deposited with oxygen content increasing from 50% to 100%. X-ray photoelectron spectroscopy showed that the Ni⁺³/Ni⁺² ratio is the origin of p-type NiO and the ratio increases from 1.32 to 2.63 by increasing the oxygen content from 0% to 100%, which caused more defects, oxygen interstitials and nickel vacancies.     

Proposal for highly birefringent broadband dispersion compensating octagonal photonic crystal fiber

In this paper, we propose and demonstrate a highly birefringent photonic crystal fiber based on a modified octagonal structure for broadband dispersion compensation covering the S, C, and L-communication bands i.e. wavelength ranging from 1460 to 1625 nm. It is shown theoretically that it is possible to obtain negative dispersion coefficient of about −400 to −725 ps/(nm km) over S and L-bands and a relative dispersion slope (RDS) close to that of single mode fiber (SMF) of about 0.0036 nm⁻¹. According to simulation, birefringence of the order 1.81 × 10⁻² is obtained at 1.55 μm wavelength. Moreover, effective area, residual dispersion, effective dispersion, confinement loss, and nonlinear coefficient of the proposed modified octagonal photonic crystal fiber (M-OPCF) are also reported and discussed.     

Measurement of stresses in Cu and Si around through-silicon via by synchrotron X-ray microdiffraction for 3-dimensional integrated circuits

Through-silicon via (TSV) has been used for 3-dimentional integrated circuits. Mechanical stresses in Cu and Si around the TSV were measured using synchrotron X-ray microdiffraction. The hydrostatic stress in Cu TSV went from high tensile of 234 MPa in the as-fabricated state, to ?196 MPa (compressive) during thermal annealing (in situ measurement), to 167 MPa in the post-annealed state. Due to this stress, the keep-away distance in Si was determined to be about 17 μm. Our results suggest that Cu stress may lead to reliability as well as integration issues, while Si stress may lead to device performance concerns.     

Ultra-flattened chromatic dispersion and highly nonlinear photonic crystal fibers with ultralow confinement loss employing hybrid cladding

In this paper, two new types of dispersion-flattened photonic crystal fibers (DF-PCFs) with highly nonlinear and ultralow confinement loss are proposed. These new PCF structures adopt hybrid cladding with different air-holes diameters, pitches and air-holes arranged fashions. In order to analyze the proposed PCFs, the full-vector finite element method with anisotropic perfectly matched layers has been used. Results show that the ultra-flattened dispersion of 0.931 ps/(nm km) (DF-PCF1) and 1.533 ps/(nm km) (DF-PCF2) can be achieved in the wavelength range from 1.3 to 1.6 μm with confinement losses lower than 0.001 dB/km in the same wavelength range. Meanwhile, the nonlinear coefficients of our proposed PCFs are greater than 23.83 W⁻¹ km⁻¹ (DF-PCF1) and 29.65 W⁻¹ km⁻¹ (DF-PCF2) at the wavelength of 1.55 μm, and two near-zero dispersion values of 0.328 ps/(nm km) (DF-PCF1) and −0.015 ps/(nm km) (DF-PCF2) can also be obtained at the same wavelength. Furthermore, the influence of manufacturing imperfections of parameters on dispersion and nonlinearity is discussed to verify the robustness of our design.     

Characteristics of pulse plated copper indium telluride films

Copper Indium Telluride films were deposited for the first time by the pulse electrodeposition technique at different duty cycles in the range of 6–50% at room temperature and at a constant potential of −0.66 V(SCE). The films exhibited single phase copper indium telluride. The grain size increased with increase of duty cycle. Optical band gap of the films varied in the range of 0.98–1.02 eV. Atomic force microscopy studies indicated that the grain size and surface roughness vary from 15 nm to 30 nm and 1.0 nm to 1.5 nm, respectively with increase of duty cycle. Capacitance voltage measurements indicated the films to exhibit n-type behavior. The flat band potential was −0.76 V(SCE) and carrier density was in the range of 10¹⁶ cm⁻³.     

Mechanical stress field assisted charge de-trapping in carbon doped oxides

Leakage current and dielectric breakdown effects are conventionally studied under electrical fields alone, with little regard for mechanical stresses. In this letter, we demonstrate that mechanical stress can influence the reliability of dielectrics even at lower field strengths. We applied tensile stress (up to 8 MPa) to a 33% porous, 504 nm thick carbon doped oxide thin film and measured the leakage current at constant electrical fields (up to 2.5 MV/cm). The observed increase in leakage current at relatively low electric fields suggests that mechanical stress assists in trap/defect mediated conduction by reducing the energy barrier potential to de-trap charges in the dielectric.     

Substrate temperature effect on structural,optical and electrical properties of vacuum evaporated SnO2 thin films

Tin oxide (SnO₂) thin films were deposited on glass substrates by thermal evaporation at different substrate temperatures. Increasing substrate temperature (T_s) from 250 to 450 °C reduced resistivity of SnO₂ thin films from 18×10⁻⁴ to 4×10⁻⁴▒ Ω ▒cm. Further increase of temperature up to 550 °C had no effect on the resistivity. For films prepared at 450 °C, high transparency (91.5%) over the visible wavelength region of spectrum was obtained. Refractive index and porosity of the layers were also calculated. A direct band gap at different substrate temperatures is in the range of 3.55−3.77 eV. X-ray diffraction (XRD) results suggested that all films were amorphous in structure at lower substrate temperatures, while crystalline SnO₂ films were obtained at higher temperatures. Scanning electron microscopy images showed that the grain size and crystallinity of films depend on the substrate temperature. SnO₂ films prepared at 550 °C have a very smooth surface with an RMS roughness of 0.38 nm.     

Effect of substrate temperature on structural,morphological, optical and electrical properties of MnIn2S4 thin films prepared by nebulizer spray pyrolysis technique

Manganese indium sulphide (MnIn₂S₄) thin films were deposited using an aqueous solution of MnCl₂, InCl₃ and (NH₂)₂CS in the molar ratio 1:2:4 by simple chemical spray pyrolysis technique. The thin film substrates were annealed in the temperature range between 250 and 350 °C to study their various physical properties. The structural properties as studied by X-ray diffraction showed that MnIn₂S₄ thin films have cubic spinel structure. The formation of cube and needle shaped grains was clearly observed from FE-SEM analysis. The energy dispersive spectrum (EDS) predicts the presence of Mn, In and S in the synthesized thin film. From the optical studies, it is analyzed that the maximum absorption co-efficient is in the order between 10⁴ and 10⁵ cm⁻¹ and the maximum transmittance (75%) was noted in the visible and infrared regions. It is noted that, the band gap energy decreases (from 3.20 to 2.77 eV) with an increase of substrate temperature (from 250 to 350 °C). The observations from photoluminescence studies confirm the emission of blue, green, yellow and red bands which corresponds to the wavelength range 370–680 nm. Moreover, from the electrical studies, it is observed that, as the substrate temperature increases the conductivity also increases in the range 0.29–0.41×10⁻⁴ Ω⁻¹ m⁻¹. This confirms the highly semiconducting nature of the film. The thickness of the films was also measured and the values ranged between 537 nm (250 °C) to 483 nm (350 °C). This indicates that, as the substrate temperature increases, the thickness of the film decreases. From the present study, it is reported that the MnIn₂S₄ thin films are polycrystalline in nature and can be used as a suitable ternary semiconductor material for photovoltaic applications.     

Preparation and characterization of the semiconductor CuMnO2 by sol-gel route

Nano crystallites of the crednerite CuMnO₂ are prepared by sol–gel method with two-step annealing process. The powder heated at 450 °C under air flow shows a mixture of CuO, Mn₂O₃ and Cu_xMn_3−xO₄. However, when calcined at 900 °C under N₂ atmosphere, the crednerite CuMnO₂ with a monoclinic structure (space group: C2/m) is obtained. The Raman spectrum shows a single peak at 679 cm⁻¹ assigned to A_1g mode whereas the infrared analysis confirms the linearity of CuO₂³⁻ units. The optical transition at 1.70 eV, determined from the diffuse reflectance is attributed to the inter-band d-d transition of Cu⁺ ion. The oxide exhibits semiconducting properties with an activation energy of 0.21 eV. The photo-electrochemical measurement shows p-type conduction due to O²⁻ insertion in the two dimensional lattice. The flat band potential (+0.12 V_SCE), indicates a cationic character of both valence and conduction bands deriving from Cu⁺: 3d orbital.     

Tailoring chromatic dispersion in chalcogenide–tellurite microstructured optical fiber

We report fabrication of a highly nonlinear hybrid microstructured optical fiber composed of chalcogenide glass core and tellurite glass cladding. The flattened chromatic dispersion can be achieved in such an optical fiber with near zero dispersion wavelength at telecommunication wavelengths λ = 1.35–1.7 μm, which cannot be achieved in chalcogenide glass optical fibers due to their high refractive index, i.e. n > 2.1. We demonstrate a hybrid 4-air hole chalcogenide–tellurite optical fiber (Δn = 0.25) with flattened chromatic dispersion around λ = 1.55 μm. In optimized 12-air hole optical fiber composed of the same glasses, the chromatic dispersion values were achieved between −20 and 32 ps/nm/km in a broad wavelength range of 1.5–3.8 μm providing the fiber with extremely high nonlinear coefficient 86,000 km⁻¹W⁻¹. Hybrid chalcogenide/tellurite fibers pumped with the near infrared lasers give good promise for broadband optical amplification, wavelength conversion, and supercontinuum generation in the near- to mid-infrared region.     

Compact conduction band model for transition-metal dichalcogenide alloys

Monolayer transition metal dichalcogenide (TMD) alloys, such as Mo_1 − xW_xS₂, owing to the unique electronic properties of the atomically thin two-dimensional layered structure, can be made into high performance metal–oxide–semiconductor field-effect transistors. The compact conduction band model of effective mass approximation (EMA) with the second nonparabolic correction is proposed for monolayer Mo_1 − xW_xS₂. The three band tight-binding (TB) method is used for calculating the band structure for monolayer TMD alloys such as Mo_1 − xW_xS₂, and a compact conduction band model is precisely developed to fit the band structures of TMD alloys calculated with tight-binding methods for the calculation of electron mobility. The impact of alloys on electron mobility of monolayer Mo_1 − xW_xS₂ is discussed in this study.     

A highly nonlinear photonic quasi-crystal fiber with low confinement loss and flattened dispersion

We proposed a novel photonic quasi-crystal fiber with near-zero flattened dispersion, highly nonlinear coefficient, and low confinement loss by using the dual concentric core structure. By optimizing the structure parameter, the proposed photonic quasi-crystal fiber can achieve a nonlinear coefficient larger than 33 W⁻¹ km⁻¹ and near-zero flatten dispersion of 0 ± 3.4 ps/nm/km with a near-zero dispersion slope of 8.5 × 10⁻³ ps/nm²/km at the wavelength of 1550 nm. Near-zero flattened dispersion and low confinement loss in the ultralow order of 10⁻⁷ dB/m are simultaneously obtained in the wavelength range from 1373 to 1627 nm. Furthermore, two zero dispersion wavelengths can be achieved in a wide wavelength ranger from 1373 to 1725 nm. From the point of view of practical fabrication, the influence of deviation of each air hole diameter within 3% of imperfections on dispersion, nonlinearity, and is discussed to verify the robustness of our design.     

Influence of Mn doping on structural,optical and electrical properties of CdO thin films prepared by cost effective spray pyrolysis method

Transparent and conducting cadmium oxide (CdO) and manganese doped CdO (Mn: CdO) thin films were deposited using a low cost spray pyrolysis method on the glass substrate at 300 °C. For Mn doping, various concentrations of manganese acetate (1–3 wt%) was used in the spraying precursor solution. The structural, electrical and optical properties of CdO and Mn: CdO films were investigated using X-ray diffraction (XRD), scanning electron microscope (SEM), atomic force microscope (AFM), UV–vis and Hall measurement. X-ray diffraction study reveals that the CdO and Mn: CdO films are possessing cubic crystal structures. SEM and AFM studies reveal that the grain size and roughness of the films are increased with increasing Mn doping concentration. Optical transmittance spectra of the CdO film decreases with increasing doping concentration of manganese. The optical band gap of the films decreases from 2.42 eV to 2.08 eV with increasing concentration of manganese. A minimum resistivity of 1.11×10⁻³ Ω cm and maximum mobility of 20.77 cm² V⁻¹ s⁻¹ is achieved for 1 wt% of manganese doping.     

Charge trapping in ultrathin Gd2O3 high-k dielectric

Charge trapping in ultrathin high-k Gd₂O₃ dielectric leading to appearance of hysteresis in C-V curves is studied by capacitance-voltage and current-voltage techniques. It was shown that the large leakage current at a negative gate voltage causes the generation of the positive charge in the dielectric layer, resulting in the respective shift of the C-V curve. The capture cross-section of the hole traps is around 2 × 10⁻²⁰ cm². The distribution of the interface states was measured by conductance technique showing the concentration up to 7.5 × 10¹² eV⁻¹ cm⁻² near the valence band edge.