Том 53, № 13 (2019)

Compositional variations in GaAs based ternary alloys have exhibited wide range alterations in electronic properties. In the present paper, first-principles study of GaAs_xSb_1 – x ternary alloys have been presented and discussed. Density functional theory (DFT) computation based on the full-potential (linearized) augmented plane-wave (FP-LAPW) method has been utilized to calculate the Density of States (DOS) and the band structure of ternary alloys GaAs_xSb_1 – x (x = 0, 0.25, 0.50, 0.75, 1). The calculations were performed using the exchange-correlation energy functional from Perdew, Burke, and Ernzerhof, a generalized-gradient approximation (GGA-PBE) and Becke–Johnson exchange potential with local-density approximation (BJLDA) available within the framework of WIEN2k code. As compared to PBE, the results obtained from BJLDA are in close agreement with other experimental works. The DOS results show a reduction in bandgap as the Sb fraction is increased in GaAs_xSb_1 – x ternary alloys. The bandgap obtained by PBE and BJLDA are found to deviate from Vegard’s law, i.e., it doesn’t vary linearly with composition. However, the bandgap obtained by BJLD is found to closely match Vegard’s law when the bowing parameter is considered.

In this study, the effect of deposition time on physical properties of tin monosulphide films capped by polyvinyl alcohol was investigated. Chemical bath deposition technique was used to deposit Polyvinyl alcohol capped SnS films on cleaned glass substrates and the deposition was carried out at four different times varying from 45 to 90 min. The X-ray diffraction study revealed that the deposited layers were polycrystalline in nature with (040) as the preferred plane. Single phase SnS was observed in the layers grown at a deposition time of 90 min while the other layers had secondary phases of Sn and S. The XRD data was also used to determine various parameters such as crystallite size, dislocation density, lattice strain, stacking faults and inter planar spacing. Raman measurements exhibited same structural phases, consistent with the XRD observations. The surface morphology of the layers was initially uneven and become uniform at 90 min deposition time. Fourier transform infrared spectra, confirmed the presence of SnS and PVA in the films. Optical studies revealed high optical absorption coefficient for all the films with a shift in optical band gap value compared with the bulk value.

The purpose of this research is to explore the properties of CoSe nanostructured thin films on glass substrates prepared by a chemical solution deposition method. Special attention is given to study the effects of precursor concentrations (Cobalt(II) acetate, Ammonia and Sodium Selenosulphite), fixing the Co:Se concentration ratio, on the grainy structure and optical properties of the resulting thin films. The structural, chemical composition and morphological characteristics were investigated by X-ray diffraction (XRD), energy dispersive X-ray microanalysis (EDX) and scanning electron microscopy (SEM), respectively. The optical properties were determined by UV–visible spectroscopy determining the optical band gap energy (E_g). According to these results, the average particle diameter was decreased from 119 to 42 nm as the precursor salt concentration was decreased from 0.3 to 0.1 M, being the bigger nanoparticles more spherical. With the reduction of the particle size, a blue shift in the absorption peaks of the UV–visible spectra and an increment of their optical band gap energy (from 1.8 to 3.6 eV) occurred due to confinement effects.

A series of Mg_xZn_{1 –x}O and Zn_xMg_{1 –x}O (mol %) were synthesized by the sol–gel method. All the synthesized nanoparticles were investigated as catalyst in the one-pot, three-component Biginelli condensation reaction of benzaldehyde, ethyl acetoacetate and urea under heterogeneous conditions. Findings revealed Mg_0.007Zn_0.093O (Zn7) as the best in the mentioned condensation reaction. Zn7 was characterized by FT-IR, SEM, CHN, and XRD. In addition, linear and nonlinear optical properties of Mg_xZn_{1 –x}O and Zn_xMg_{1 –x}O thin films and their colloidal solutions were studied. Using an exact numerical method, some photo-physical properties of the prepared films such as dispersion curve, absorption spectrum, optical permittivity and optical band gap were attained and compared. Finally, using z-scan method at low laser irradiation, thermo-optical effect has also studied and a nonlinear refractive index due to this effect was attained. Doping effect on the optical properties was also investigated physically. It was found in both the particles groups, increasing the doping percent reduces the band gap and it suggests new optoelectronic materials with a spectrum of optical properties.

A charge control based analytical model is followed to study the impact of donor-layer doping and gate-length on microwave frequency performance of AlGaN/GaN/AlGaN double heterostructure high electron mobility transistor (DH-HEMT). DH-HEMT is observed to be more sensitive to gate-length and doping variation as compared to single heterostructure high electron mobility transistor (SH-HEMT). The effect of gate-length and doping on various performance parameters, i.e., transconductance, drain conductance, cut-off frequency and maximum oscillation frequency has been analysed. The results so obtained are compared with simulation results and are found to be in good agreement.

In the present paper, analytical modeling of surface potential and drain current for hetero-dielectric double gate tunnel FET (HDG-TFET) has been done. The two dimensional (2D) Poisson’s equation has been solved with suitable boundary conditions by using the method of parabolic approximation to find the surface potential along the channel length. Subsequently, using the electric field expression derived from surface potential has been used to calculate the drain current expression. The proposed analytical models have been verified and compared with the results obtained from the ATLAS^TM simulator. The effect of varying gate voltage, drain voltage, doping concentration, length and thickness of dielectric on surface potential of the device has been calculated and analyzed in detail. Additionally, a detailed study of the influence of various parameters on the RF and analog properties of HDG-TFET like drain current (Id), transconductance to drain current ratio (g_m/I_d), the cut-off frequency (f_T) and the maximum frequency of oscillation (f_max) are evaluated using Y- and H-parameters obtained from ATLAS^TM, a 2D device simulator.

ZnO nanorods were grown on bare or SiO₂-coated Si wafers by the hydrothermal method. The ZnO nanorods were annealed at 200, 400, and 600°C, respectively. The structural, optical, and electrical property variation of the ZnO nanorods with the annealing temperature was investigated by X-ray diffraction, field-emission scanning electron microscopy, photoluminescence, and current–voltage measurements. For the ZnO nanorods, compressive strain was detected, which decreased with annealing. Moreover, annealing at 600°C led to nanorod agglomeration. The ZnO nanorods annealed at 400°C exhibited the highest crystallinity.

Most of the energy produced is lost, mainly as waste heat. Thermoelectricity, which can directly convert heat into electricity, is seen with huge potential to recover part of such heat. However, to be widely used, it is necessary to be based on cheap, available, non-toxic, stable and easy-to-produce good thermoelectric materials. Here we present an overview of the potential of Cu–S based synthetic minerals for thermoelectric applications. In particular, we focus on tetrahedrites, which are world spread minerals with Cu₁₀M₂Sb₄Si₃ (M = Cu, Mn, Fe, Co, Ni, Zn) general formula that show high potential for thermoelectrics. An overview of their properties are presented, with emphasis on those relevant for thermoelectric applications.

Thermoelectric material development typically aims at maximizing produced electrical power and efficiency of energy conversion, even though sometimes, this means adding expensive or toxic materials. An alternative is to use highly available and low toxic silicides. In fact, magnesium silicide and magnesium stannide have low densities (1.99 and 3.49 g/cm³, respectively), and exhibit good thermoelectric properties with their thermoelectric figure of merit zT > 1 for n-type and near 0.6 for p-type Mg₂Si–Mg₂Sn solid solutions in the range of 723–773 K. These properties turn the materials into logical candidates for light-weight and efficient thermoelectric generators (TEG). The research on their mechanical properties is however lagging behind and little effort has been put into understanding them. In this work we study the effect of the composition over the Mg₂Si–Mg₂Sn solid solution series on hardness and fracture toughness values. Hardness ranges from 2.44–5.56 GPa whereas fracture toughness values are in a tighter range (0.64–0.88 MPa m^1/2). However, the highest value does not belong to binary Mg₂Si but a composition within the solid solution that exhibits secondary phase nanostructuring.

The dependences of the electrical resistance on temperature (2–400 K) and magnetic field (0–140 kOe) of high-purity polycrystalline ytterbium were studied. Large magnetoresistance in the temperature range below 150 K (in low temperature hcp phase) was observed.

The surface states of Dirac fermions in p-Bi₂Te₃, p-Bi_{2 –x}Sb_xTe_{3 –y}Se_y, and p-Bi_{2 –x–y}Sn_xGe_yTe₃ thermoelectrics were studied and its topological characteristics were obtained by scanning tunneling techniques in dependence on compositions. It was shown that the thermoelectric power factor and the material parameter enhance with the shift of the Dirac point to the top of the valence band with the increasing of atomic substitution in these thermoelectrics. A growth contribution of the surface states is determined by an increase of the Fermi velocity for large atomic substitutions of Bi at x > 1.5 and small substitutions in the Te sublattice (y = 0.06). In compositions with smaller substitutions at x = 1–1.3 and y = 0.06–0.09, similar effect of the surface states is determined due to raising the surface concentration of charge carriers.