Ab initio studies of structural,electronic, optical,elastic and thermal properties of Ag-chalcopyrites (AgAlX2: X=S,Se)

The ab-initio calculations for the structural, electronic, optical, elastic and thermal properties of Ag-chalcopyrites (AgAlX₂: X=S and Se) have been reported using the full potential linearized augmented plane wave (FP-LAPW) method. In this paper, the recently developed Tran–Blaha modified Becke–Johnson potential is used along with the Wu-Cohen generalized gradient approximation (WC-GGA) for the exchange-correlation potential. Results are presented for lattice constants, bulk modulus and its pressure derivative, band structures, dielectric constants and refractive indices. We have also computed the six elastic constants (C₁₁, C₁₂, C₁₃, C₃₃, C₄₄, C₆₆). The thermodynamical properties such as thermal expansion, heat capacity, Debye temperature, entropy, bulk modulus are calculated employing the quasi-harmonic Debye model at different temperatures (0–900 K) and pressures (0–8 GPa) and the silent results are interpreted. Hardness of the materials is calculated for the first time at different temperatures and pressures.     

Indolo[3,2,1-jk]carbazole based planarized CBP derivatives as host materials for PhOLEDs with low efficiency roll-off

Three novel planarized CPB derivatives (ICzCz, ICzPCz, ICzICz) have been synthesized and characterized concerning applications as host materials for PhOLEDs. The incorporation of fully planar indolo[3,2,1-jk]carbazole (ICz) in the CBP scaffold has been systematically investigated, revealing a significant impact on molecular properties, such as improved thermal stability (t_g > 110 °C), high triplet energies (E_T > 2.81 eV) and charge transport properties. Employing the newly developed materials as host materials, efficient green PhOLEDs (CE_max: 60.1 cd A⁻¹, PE_max: 42.1 lm W⁻¹, EQE_max: 15.9%) with a remarkably low efficiency roll-off of 5% at 1000 cd m⁻² as well as blue PhOLEDs (ICzCz) with a high PE of 26.1 lm W⁻¹ have been realized. Hence, the first comprehensive report on the application of ICz as integral building block for electroluminescent materials is presented, establishing this particular structural motive as versatile structural motive in this field.     

The effect of indium additive on the structural relaxation of Se–Sb–Sn semiconducting glasses

Differential scanning calorimetry (DSC) has been used to study the kinetics of structural relaxation in the glass transition region of Se₈₀Sb₁₂Sn_8?yIn_y (y=0, 0.5, and 1) chalcogenide glasses, which were previously annealed at 394 K below the glass transition temperature. Based on the Kovacs–Aklonis–Hutchinson–Ramos model, the dependence of the endothermic peak temperature (T_p) on heating rate has been used to evaluate the relaxation parameter. From the knowledge of this parameter the structural relaxation activation energy (Δh) has been obtained. The shift in T_p with heating rate and annealing time (t_a) has been used to calculate the structural parameter, which controls the contribution of temperature and structure to the relaxation time. Results reveal that Δh decreases with In content due to the replacement of some SnSe_4/2 structural units of strong bond energy by Se₃In₂ of weak bond energy. This reduces the cross-linking energy as well as the overall mean bond energy of the studied glasses as predicted by the chemical bond approach. The values of Δh were used to estimate the fragility index which is found to be in the range 29.7–31.7 indicating that the glasses under investigation were obtained from strong glass-forming liquids. The excess enthalpy (δ_H) has also been calculated, for different annealing times t_a, from the knowledge of the excess specific heat. The values of δ_H were plotted as a function of ln(t_a) and exhibit a linear behavior which indicates that the studied glasses were not completely relaxed.     

Phase separation, effects of biaxial strain, and ordered phase formations in cubic nitride alloys

The thermodynamics as well as the energetics and the structural properties of cubic group-III nitrides alloys have been investigated by combining first-principles total energy calculations and cluster expansion methods. In particular results are shown for the ternary In_xGa_1−xN and the quaternary Al_xGa_yIn_1−x−yN alloys. Phase separation is predicted to occur at growth temperatures, for both fully relaxed alloys. A remarkable influence of an external biaxial strain on the phase separation, with the formation of ordered phase structures has been found for the InGaN alloy. These findings are used to clarify the origin of the light emission process in InGaN-based optoelectronic devices. Results are shown for the composition dependence of the lattice constant and of the energy gap in quaternary Al_xGa_yIn_1−x−yN alloys.     

First-principles study of structural,electronic, elastic and thermal properties of intermetallic ternary compounds (RMn2Si2: R=Ce and Nd)

In this study, the structural, magnetic, electronic, elastic and thermal properties of the ternary intermetallic, RMn₂Si₂ (R=Ce and Nd), compounds are presented. The study is carried out by employing the full-potential (FP) linearized augmented plane wave (LAPW) plus local orbital (lo) approach based on the density functional theory (DFT). To depict the exchange-correlation energy (an important component of total energy calculations), the local-density approximation and the local spin density approximation (LDA/LSDA) are used. Our calculated results for equilibrium lattice parameters are in good agreement with the available experimental measurements. The total energy calculations reveal the strong dependence to the distance between atomic species in these compounds. The analysis of the partial and total densities of states (DOS) of both compounds (CeMn₂Si₂ and NdMn₂Si₂) demonstrates their metallic and magnetic character as well. Whereas the calculated values of Poisson׳s ratio and B/G present their brittle makeup. At the end, using a quasi-harmonic Debye model as implemented in GIBBS code, the thermal properties were calculated.     

First principles calculations of structural,electronic and optical properties of zinc aluminum oxide

A first principles study of structural, electronic and optical properties of zinc aluminum oxide (ZnAl₂O₄) by means of the full potential linear augmented plane wave method is presented. The local density approximation is used for the exchange-correlation potential. A direct band gap of 4.19 eV, in agreement with experiment (E_g=3.9 eV), was determined. ZnAl₂O₄ is transparent in the visible spectral region; the excitonic transition associated with the fundamental band gap is 4.17 eV. The refractive index value is 1.74 in the ultraviolet spectral region.     

Small-signal characteristics of fully-printed high-current flexible all-polymer three-layer-dielectric transistors

All-polymer, semi-transparent, three-layer-dielectric (3L) organic field effect transistors (OFETs) are fabricated on polyethylene terephthalate plastic substrate, using high-throughput printing techniques. Analog small-signal characteristics of the 3L OFET are presented and are compared against the previous version of this technology, which was based on a single-layer dielectric and a metal gate electrode. The 3L transistor withstands 50 V, can continuously drive 50 μA/mm, reaches an excellent intrinsic-gain (A_v0) of 43 dB, an equivalent mobility of 0.85 cm²/V, and a transit frequency (f_T) of 68 kHz, well suited for applications such as driving printed piezoelectric loudspeakers and flexible audio systems. The effects of the relaxor-ferroelectric high-k layer in the 3L stack on the gate capacitance, g_m, and A_v0 are measured in the frequency domain. In addition, it is observed that PEDOT:PSS makes a better interface with polymer dielectric comparing to copper particle ink. Five-hour small- and large-signal bias stress tests are performed. A novel direct A_v0 measurement technique, and an improved transconductance extraction method are also presented.     

A new ultra low-power,universal OTA-C filter in subthreshold region using bulk-drive technique

In this paper, a new ultra low-power universal OTA-C filter which can properly operate in all modes of operation (voltage, current, trans-resistance and trans-conductance) is presented. However, in order to reduce the power consumption effectively, the proposed circuit uses subthreshold transistors which are biased at I_a = 50 nA, I_b = 150 nA. Furthermore, using the bulk-drive technique leads to a reduced power consumption as well as the supply voltage of ±0.3 V. Moreover, the grounded capacitors are used to effectively reduce the parasitic effects. However, the result of sensitivity analysis shows that the proposed circuit has a very low sensitivity to the values of active and passive circuit elements such as: trans-conductance (g_m) and capacitance (C) values. Furthermore, the proposed circuit uses the minimum number of active elements to effectively reduce the power consumption as well as the chip area. Finally, the proposed filter is designed and simulated in HSPICE using 0.18µm CMOS technology parameters, while HSPICE simulation results have very close agreement with theoretical results obtained from MATLAB, which justifies the design accuracy and low-power performance of the proposed universal filter.     

Development of needle-shaped grains from Bi2Sr2CaCu2O8+x with high critical current density suitable for making superconducting wires

We present a grain microstructure for Bi(2212) consisting of only giant needle-shaped grains of around 1.5 mm length and 100 μm diameter. We study the structural and chemical changes suffered by a conventional ceramic Bi(2212) sample in the course of the thermal treatment used to obtain those giant needle-shaped grains. For that, different samples of the same batch were treated with incomplete thermal treatments, and the resulting samples were analysed by using scanning electron microscopy (SEM), optical microscopy, energy dispersed spectroscopy (EDS), inductively coupled plasma (ICP) and X-ray diffraction (XRD). To verify the superconducting nature of the needle-shaped grains, we have performed magnetization, resistivity, and critical current measurements on the original ceramic sample, and on that formed as giant needle-like grains. The critical temperature of these last grains is nearly the same as that of the ceramic sample (T_c∼90 K), which is a high value for the Bi(2212) compound. The critical current density (J_c) of the needle-shaped grains is around 2500 A/cm² at 77 K and in absence of applied magnetic field, a value comparable with that presented for the best wires and thick films. Not only are the shape and the size of these grains very suitable for making superconducting wires, but also the superconducting properties, T_c and J_c, are both high enough to be confident about the possibility of improving the actual Bi(2212) superconducting wires for high current applications.     

First-principles calculations of the structural,electronic and optical properties of In1−xBxAsyP1−y quaternary alloys lattice matched to InP and BeS

The structural, electronic, and optical properties of the cubic In_1−xB_xAs_yP_1−y quaternary alloys lattice matched to InP and BeS have been investigated by using the full-potential linearized augmented plane wave (FP-LAPW) method within the density functional theory (DFT). The generalized gradient approximation (GGA) of Wu and Cohen was used as the exchange correlation potential to calculate the structural and electronic properties. In addition, the alternative GGA proposed by Engel and Vosko and the modified Becke–Johnson potential are utilized to calculate the electronic properties. The computed structural and electronic properties of the binary compounds are in good agreement with the available experimental and theoretical data. For the alloys, non-linear variations of composition x and y with the lattice constant, bulk modulus, direct, indirect band gap, dielectric constant and refractive index are found. All the compounds are direct band gap excluding BP and BAs. The energy band gap of In_1−xB_xAs_yP_1−y quaternary alloys lattice matched to InP and BeS substrates is computed. Finally, the band gap of our materials is less than 3.1 eV. Thus the In_1−xB_xAs_yP_1−y quaternary alloys may possibly be used in visible light devices.     

Tree structures for orthogonal transforms and application to the Hadamard transform

Tree structures for computing orthogonal transforms are introduced. Two cases, delay trees and decimation trees, are investigated. A simple condition, namely the orthogonality of branches with a common root, is shown to be necessary and sufficient for the overall transform to be orthogonal. Main advantages are structural simplicity and a number of operations proportional to N Log₂N.Application of the tree structures to the Walsh-Hadamard Transform (in natural, sequency and dyadic order) is presented. A single module can be multiplexed or used in parallel in order to perform all operations. Such a system is shown to be well suited for hardware implementation.     

Effect of zinc doping and temperature on the properties of sprayed CuInS2 thin films

Copper indium disulfide (CuInS₂) is an efficient absorber material for photovoltaic and solar cell applications. The structural, optical, photoluminescence properties and electrical conductivities could be controlled and modified by suitably doping CuInS₂ thin films with dopants such as Zn, Sn, Bi, Cd, Na, N, O, P and As. In this work Zn (0.01 M) doped CuInS₂ thin films are (Cu/In=1.25) deposited on to glass substrates in the temperature range 300–400 °C. It is observed that the film growth temperature, ion ratio (Cu/In=1.25) and Zn-doping affect structural, optical, photoluminescence and electrical properties of sprayed CuInS₂ thin films. As the XRD patterns depict, Zn-doping facilitates the growth of CuInS₂ thin films along (112) preferred plane and in other characteristic planes. The EDAX results confirm the presence of Cu, In, S and Zn in the films. The optical studies show, about 90% of light transmission occurs in the IR regions; hence Zn-doped CuInS₂ can be used as an IR transmitter. The absorption coefficient (α) in the UV–visible region is found to be in the order of 10⁴–10⁵ cm⁻¹ which is the optimum value for an efficient absorber. The optical band gap energies increase with increase of temperatures (1.66–1.78 eV). SEM photographs reveal crystalline and amorphous nature of the films at various temperature ranges. Photoluminescence study shows that well defined broad Blue and Green band emissions are exhibited by Zn-doped CuInS₂ thin films. All the films present low resistivity (ρ) values and exhibit semiconducting nature. An evolution of p-type to n-type conductivity is obtained in the temperature range 325–350 °C. Hence, Zn species can be used as a donor and acceptor impurity in CuInS₂ thin films to fabricate efficient solar cells, photovoltaic devices and good IR Transmitters.     

Simulated solar cell device of CuGaSe2 by using CdS,ZnS and ZnSe buffer layers

We have performed ab-initio calculations for the structural, electronic, optical, elastic and thermal properties of the copper gallium chalcopyrite (CuGaSe₂). The Full Potential Linearized Augmented Plane Wave (FP-LAPW) method is used to find the equilibrium structural parameters and to compute the full elastic tensors. We have reported electronic and optical properties with the recently developed density functional of Tran and Blaha. Furthermore, optical features such as dielectric functions, refractive indices, extinction coefficient, optical reflectivity, absorption coefficients, optical conductivities, are calculated for photon energies up to 30 eV. The thermodynamical properties such as thermal expansion, heat capacity, Debye temperature, entropy and Grüneisen parameter, bulk modulus and hardness are calculated employing the quasi-harmonic Debye model at different temperatures (0–1200 K) and pressures (0–8 GPa) and the silent results are interpreted. To check the potentiality of CuGaSe₂ as future solar cell material, device modeling and simulation studies have been carried out with a variety of buffer layers over CuGaSe₂ absorption layer. The band diagram and J/V curves are analyzed and device performance parameters i.e. efficiency, open circuit voltage, short circuit current, quantum efficiency are calculated for CdS, ZnS and ZnSe buffer layers. Simulation results for CuGaSe₂ thin layer solar cell show the maximum efficiency (15.8%) with ZnSe as the buffer layer. Most of the investigated parameters are reported for the first time.     

Theoretical study of the structural,elastic and thermodynamic properties of chalcopyrite ZnGeP2

The structural, elastic, and thermodynamic properties of ZnGeP₂ with chalcopyrite structure are investigated using the pseudo-potentials plane wave method based on the density functional theory with the generalized gradient approximation. The lattice parameters (a, c and u) are directly calculated and agree well with previous experimental and theoretical results. The obtained negative formation enthalpy shows that ZnGeP₂ crystal has strong structural stability. We have also calculated the bulk modulus B and the elastic parameters (C₁₁, C₁₂, C₁₃, C₃₃, C₄₄, and C₆₆) which have not been measured yet. The accuracy and reliability of the calculated elastic constants of ZnGeP₂ crystal are discussed. In addition, the pressure and temperature dependencies of the lattice parameters, bulk modulus, Debye temperature, Grüneisen parameter, entropy, volume thermal expansion coefficient, and specific heat capacity are obtained in the ranges of 0–20 GPa and 0–1200 K using the quasi-harmonic Debye model. To our knowledge this is the first quantitative theoretical prediction of the thermodynamic properties for ZnGeP₂ compound and still awaits experimental confirmations.     

Negative resistance and peak velocity in the central (000) valley of III–V semiconductors

The negative resistance in III–V materials such as GaAs at large electric fields is generally recognized as arising from the transfer of electrons from the central (000) valley to higher lying minima in the conduction band. Monte Carlo transport studies show that the negative resistance effect is still present in III–V materials when the valley spacing is increased to large values (> 0.5 eV) and even present when the higher minima are eliminated entirely from the calculations. This negative resistance arises from basic transport properties of the central valley. Studies are presented of the basic negative resistance effect in the central valley of III–V materials as well as studies of Al_1?xIn_xAs (x ～ 0.75) and Ga_1?xIn_xAs (x ～ 0.6) which are two specific materials where the negative resistance effect is due predominantly to the central valley.     

Influence of heat treatment on the structural,optical and electrical properties of Cd20Sn10Se70 thin films

The effect of annealing temperature (T_a) on the structural, optical, and electrical properties of thermally evaporated Cd₂₀Sn₁₀Se₇₀ thin films has been investigated. Differential Thermal Analysis (DTA) was used to determine the glass transition temperature (T_g) of the prepared alloy. X-ray diffraction studies showed that the as-deposited film and the films that were annealed at T_a<T_g are of low crystallinity. On annealing above T_g, these films showed a polycrystalline nature. The surface morphology and microstructure of as-deposited and annealed films have been examined by scanning electron microscopy (SEM) and atomic force microscopy (AFM). Their optical constants were calculated from the transmittance measurements in the range 200–2500 nm. The dispersion of refractive index was analyzed in terms of the single-oscillator Wemple-Di Domenico model. Analysis of the optical absorption data indicates that the optical band gap E_g of these films obeys Tauc׳s relation for the allowed direct transition. The optical band gap E_g as well as the activation energy for the electrical conduction ∆E were found to increase with increase of annealing temperature up to T_g, whereas above T_g there is a remarkable decrease in both E_g and ∆E. The obtained results were interpreted in terms of the Mott-Davis model and amorphous–crystalline transformation.     

AlGaN/GaN self-aligned MODFET with metal oxide gate for millimeter wave application

As promising candidates for future microwave power devices, GaN-based high-electron mobility transistors (HEMTs) have attracted much research interest. An investigation of the operation of AlGaN/GaN n type self-aligned MOSFET with modulation doped GaN channels is presented. Liquid phase deposited (LPD) SiO₂ is used as the insulating material. An analytical model based on modified charge control equations is developed. The investigated critical parameters of the proposed device are the maximum drain current (I_Dmax), the threshold voltage (V_th), the peak DC trans-conductance (g_m), break down voltage (V_br) and unity current gain cut-off frequency (f_T). The typical DC characteristics for a gate length of 1 μm with 100 μm gate width are following: I_max=800 mA/mm, V_break-down=50 V, g_{m_extrinsic}=200 mS/mm, V_pinchoff=−10 V. The analysis and simulation results on the transport characteristics of the MOS gate MODFET structure is compared with the previously measured experimental data. The calculated values of f_T (20-130 GHz) suggest that the operation of the proposed device effectively, has sufficiently high current gain cutoff frequencies over a wide range of drain voltage, which is essential for high-power performance at microwave frequencies. The proposed device offers lower on-state resistance. The results so obtained are in close agreement with the experimental data.     

Novel organic dyes containing N-bridged oligothiophene coplanar cores for dye-sensitized solar cells

Three novel organic dyes adopting fully-fused coplanar heteroarene as the donor moieties end-capped with two cyanoacrylic acids as acceptors and anchoring groups have been synthesized, characterized, and used as the sensitizers for dye-sensitized solar cells (DSSCs). The photophysical and electrochemical properties of the novel dyes and the characteristics of the DSSCs based on the novel organic dyes were investigated. The incorporation of the coplanar cores with electron-donating N-bridges are beneficial for the better intramolecular charge transfer (ICT), giving these new dyes good light-harvesting capability. The LUMO energy levels of these coplanar heteroacene-based dyes are sufficiently high for the efficient electron injection to TiO₂ upon photo-excitation, while the suitable HOMOs allow the regeneration of oxidized dyes with the electrolyte redox (I⁻/I₃⁻). The structural features of the coplanar cores (penta vs. hexa heteroarene) as well as the alkyl substitutions play crucial roles in governing the physical properties and device performance. Among these three novel organic sensitizers, the EHTt dye composed of a fully fused hexa-arene core and less bulky N-alkyl groups caused the DSSC to show the best photovoltaic performance with an open-circuit voltage (V_OC) of 0.58 V, a short-circuit photocurrent density (J_SC) of 13.72 mA/cm², and a fill factor (FF) of 0.69, yielding an overall power conversion efficiency (PCE) of 5.52% under AM 1.5G solar irradiation.     

Structural and frequency-dependent dielectric properties of PVP-SiO2-TMSPM hybrid thin films

In the present study, thin films of PVP-SiO₂-TMSPM (polyvinylpyrrolidone-silicon dioxide- 3-trimethoxysilyl propyl metacrylate) were deposited on p-type Si (111) substrates using spin coating technique. Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), and energy dispersive X-ray spectrometry (EDS) were applied to investigate the chemical bonds and structural properties of the samples. Morphology of the hybrid thin films was studied using atomic force microscopy (AFM) and scanning electron microscopy (SEM) techniques. The frequency dependence of dielectric properties such as dielectric constant (ε^′), dielectric loss (ε″), loss tangent (tan δ) as well as the real component of electric modulus (M′), imaginary component of electric modulus (M″), and AC electrical conductivity (σ_AC) was studied in Al/PVP-SiO₂-TMSPM/PSi used as a metal-polymer-semiconductor (MPS) device. Analysis of dielectric relaxation behavior was performed in the frequency range of 0.1 KHz to 1 MHz. In the frequency range of 1 KHz to 1 MHz, the σ_AC data were varied from 6.35 × 10⁻⁶ to 9.02 × 10⁻⁶ for the sample with 0.15 wt ratios of TMSPM and equivalent values of both PVP and SiO₂. The dielectric, modulus, and AC conductivity analyses were considered the useful factors to detect the effects of the capacitance, ionic conductivity, and dielectric relaxation process.     

Effect of tin content on the electrical and optical properties of sprayed silver sulfide semiconductor thin films

Silver sulfide (Ag₂S) thin films have been deposited on glass substrates by t spray pyrolysis using an aqueous solution which contains silver acetate and thiourea as precursors. The depositions were carried out at a substrate temperature of 250 °C. Structural studies by means of X-ray diffraction show that all tin (Sn)-doped Ag₂S thin films crystallized in a monoclinic space group with noticeable changes in the crystallites' orientation. The discussion of some structural calculated constants has been made with Sn doping in terms of microhardness measurements. Moreover, the optical analysis via the transmittance, reflectance as well as the photocurrent reveals that the direct band gap energy (E_gd) decreases (E_gd varies from 2.34 to 2.16 eV) and the indirect band gap energy (E_gi) increases (E_gi varies from 0.98 to 1.09 eV) slightly as a function of Sn content. Electrical study shows that Sn doping changes the electrical conductivity and proves the thermal activation of electrical conduction.