Novel BTlGaN semiconducting materials for infrared opto-electronic devices

BTlGaN quaternary alloys are proposed as new semiconductor materials for infrared opto-electronic applications. The structural and opto-electronic properties of zinc blende B_xTl_yGa_1−x−yN alloys lattice matched to GaN with (0 ⩽ x and y ⩽ 0.187) are studied using density functional theory (DFT) within full-potential linearized augmented plane wave (FP-LAPW) method. The calculated structural parameters such as lattice constant a₀ and bulk modulus B₀ are found to be in good agreement with experimental data using the new form of generalized gradient approximation (GGA-WC). The band gaps of the compounds are also found very close to the experimental results using the recently developed Tran–Blaha-modified Becke–Johnson (TB-mBJ) exchange potential. A quaternary B_xTl_yGa_1−x−yN is expected to be lattice matched to the GaN substrate with concentrations x = 0.125 and y = 0.187 allows to produce high interface layers quality. It has been found that B incorporation into BTlGaN does not significantly affect the band gap, while the addition of dilute Tl content leads to induce a strong reduction of the band gap, which in turn increases the emission wavelengths to the infrared region. The refractivity, reflectivity and absorption coefficient of these alloys were investigated. BTlGaN/GaN is an interesting new material to be used as active layer/barriers in quantum wells suitable for realizing advanced Laser Diodes and Light-Emitting Diodes as new sources of light emitting in the infrared spectrum region.     

Dielectric,optical and structural properties of Bi4V2−xSrxO11−δ (0.05≤x≤0.20)

Composition Bi₄V_2−xSr_xO_11−δ (0.05≤x≤0.20) is synthesized by melt quench technique followed by heat treatment at 800 °C for 12 h. These compounds are characterised by X-ray diffraction, Fourier transform infrared (FTIR) spectroscopy, UV–visible spectroscopy, impedance spectroscopy and scanning electron microscopy. X-ray diffraction patterns of all the samples show γ-phase stabilization at room temperature except x=0.05 heat treated sample. The optical band gap of all the samples is observed in semiconducting range. The lowest and the highest optical band gap is 2.39 eV and 2.57 eV for x=0.10 heat treated and x=0.20 quenched samples, respectively. The highest value of dielectric constant is obtained ~10⁷ with very low dielectric loss for x=0.15 and 0.20 samples at ~350 °C and below 10 Hz. The grain size increases with dopant concentration leads to increase the dielectric constant.     

Effect of doping on cohesive and thermophysical properties of MgB2

The effects of doping Al and Mn on the cohesive and thermophysical properties of MgB₂ have been investigated using a Rigid Ion Model (RIM). The interatomic potential of this model includes contributions from the long-range Coulomb attraction and the short-range overlap repulsion and the van der Waals attraction. This model has been applied to describe the temperature dependence of the specific heat of MgB₂, Mg_1−xAl_xB₂ (x = 0.1–0.9) and Mg_1−xMn_xB₂ (x = 0.01–0.04) in the temperature range 5 K ⩽ T ⩽ 1000 K. The calculated results on cohesive energy (ϕ), Bulk modulus (B_T), molecular force constant (f), Restrahalen frequency (ν₀), Debye temperature (Θ_D) and Gruneisen parameter (γ) are also reported for these materials. Our results on Bulk modulus, Restrahalen frequency and Debye temperature are closer to the available experimental data. The comparison between our calculated and available experimental results on the specific heat at constant volume for MgB₂ and Mg_1−xAl_xB₂ (x = 0.1–0.4), particularly, at lower temperatures has shown almost an excellent agreement. The trend of variation of the specific heat with temperature is more or less similar in pure and doped MgB₂.     

Excitonic magnetic polaron dynamics of MBE grown CdTe/Cd1 − xMnxTe,Cd1 − xMnxTe/Cd1 − vMgyTe single quantum wells in magneti fields

Excitonic lifetimes in Cd_1 − xMn_Ue2Te, Cd_1 − xMg_xTe epilayers and CdTe/Cd_1 − xMn_xTe, Cd_1 − xMn_xTe/Cd_1 − vMg_yTe single quantum wells with different well widths and Mn, Mg compositions are investigated. The excitonic lifetimes are found to reduce drastically by applying external magnetic fields to samples with giant Zeeman splittings. The observed phenomenon is interpreted in terms of the PL decay time contribution from the long-life dark excitons which can convert to excitons for recombinations by a spin-flip process. We attribute the lifetime reduction to the depletion of dark excitons due to their crossing over the exciton energies for dipole allowed transitions in magnetic fields.     

Empirical expression for the composition and temperature dependence of the energy gap in InAlSb

An empirical expression for the energy bandgap as a function of alloy composition x and temperature for In_1−xAl_xSb was reported. The In_1−xAl_xSb epitaxial layers were grown by molecular beam epitaxy (MBE) on InSb(1 0 0) substrate, utilizing a p⁺–p⁺–n–n⁺ structure. High resolution X-ray diffraction was used to characterize the epitaxial layers. The Al composition of 2.8% was obtained by assuming the Bragg’s formula and Vegard’s law. Spectral response measurement of the diodes has been employed to investigate the temperature dependence of the band gap of In_1−xAl_xSb alloys in the range between 77 K and 260 K. The calculated results for energy gap of InAlSb were in good agreement with the available data and our experimental observation.     

Experimental and theoretical studies on bis(glycine) lithium nitrate (BGLiN): A physico-chemical approach

Good quality and bulk size single crystal (size: 20×13×8 mm³) of bis(glycine) lithium nitrate (BGLiN) was grown by a slow evaporation solution technique from the aqueous solutions at constant temperature i.e. 27 °C using synthesized materials. Crystal system and lattice parameters were determined by single crystals as well as powder X-ray diffraction analysis. The lattice parameters of the titled compound are a=10.0223 Å, b=5.0343 Å, c=17.0510 Å, and V=860.312 Å³ and it crystallized in an orthorhombic system with space group Pca2₁ obtained by single crystal XRD. Elemental composition was confirmed by energy dispersive X-ray spectroscopic analysis. Optical absorption spectrum was recorded and various optical parameters such as optical transmission (~60%), and optical band gap (4.998 eV) were calculated. Photoluminescence study shows that the grown crystal is free from major defects. Crystalline perfection of the grown crystal was assessed and found good. Ground state optimized geometry has been obtained by using DFT with 6-31G(d,p) basis set. HOMO and LUMO energy gap was found to be 6.01 eV and dipole moment was 1.65 D.     

Optical parameters of In–Se and In–Se–Te thin amorphous films prepared by pulsed laser deposition

The thin films of materials based on In–Se are under study for their applicability in photovoltaic devices, solid-state batteries and phase-change memories.The amorphous thin films of In₂Se_3−xTe_x (x=0–1.5) and InSe were prepared by pulsed laser deposition method (PLD) using a KrF excimer laser beam (λ=248 nm, 0.5 J cm⁻²) from polycrystalline bulk targets. The compositions of films verified by energy-dispersive X-ray analysis (EDX) were close to the compositions of targets. The surfaces of PLD films containing small amount of droplets were viewed by optical and scanning electron microscopy (SEM).The optical properties (transmittance and reflectance spectra, spectral dependence of index of refraction, optical gap, single-oscillator energy, dispersion energy, dielectric constant) of the films were determined.The values of index of refraction increased with increasing substitution of Te for Se in In₂Se₃ films, the values of the optical gap decreased with increasing substitution of Te for Se in In₂Se₃ films.     

Investigation of the effect of annealing on the magnetic properties of Sn1−xEuxTe single crystals

A study of the effect of annealing on the magnetic properties of single crystals Sn_1−xEu_xTe is reported. The width of the electron paramagnetic resonance line of the crystal is found to decrease upon annealing but its g-value of 1.991 is nearly unaffected. Magnetization results indicate that the pair exchange interaction is weakly antiferromagnetic with a value of −0.67 K for the non-annealed sample and −0.29 K after annealed sample. Susceptibility measurements performed as a function of temperature also indicate the presence of EuTe clusters in the as-grown Sn_1−xEu_xTe crystals. Therefore it was deduced that the Eu²⁺ ions tend to form clusters, particularly pairs, in the as-grown crystal and these clusters disappear after annealing, as the Eu²⁺ ions occupy isolated sites in the SnTe host lattice.     

Electronic band structure calculation of GaNAsBi alloys and effective mass study

Electronic band structures of GaN_xAs_1−x−yBi_y dilute nitrides–bismides have been determined theoretically within the framework of the band anticrossing (BAC) model and k ⋅ p method. We have developed computer codes based on our extended BAC model, denoted (16 × 16), in which the dimension of the used states basis was equal to 16. We have investigated the band gap and the spin orbit splitting as a function of Bi composition for alloys lattice matched to GaAs. We have found that the substitution of As element by N and Bi impurities leads to a significant reduction of band gap energy by roughly 198 meV/%Bi. Meanwhile, spin orbit splitting increases by 56 meV/%Bi regardless N content. There is an excellent agreement between the model predictions and experiment reported in the literature. In addition, alloys compositions and oscillator strengths of transition energies have been calculated for GaNAsBi alloys which represent active zone of temperature insensitive (1.55 μm and 1.3 μm) wavelength laser diodes intended for optical fiber communications. A crossover at about 0.6 eV has occurred between E_g and Δ_so of GaN_.039As_.893Bi_.068. When the quaternary is lattice mismatched to GaAs, resonance energy increases with Bi content if N content decreases. On the other hand, effective mass behavior of carriers at Γ point has been discussed with respect to alloy composition, k-directions and lattice mismatch.     

Synthesis,crystal structure,spectroscopic characterization and optical properties of bis(4-acetylanilinium) tetrachlorocobalt (II)

The chemical preparation, crystal structure, spectroscopic investigations and optical features are given for a novel organic–inorganic hybrid material [C₈H₁₀NO]₂CoCl₄.The compound is crystallized in the orthorhombic space group Cmca, with the following unit cell parameters: a=19.461(2) Å, b=15.523(2) Å, c=13.7436(15) Å, and Z=8. The atomic arrangement shows an alternation of organic and inorganic layers along the b-axis. The cohesion between these entities is performed by N–H…Cl and N–H…O hydrogen bonds and π–π stacking interactions.Infrared and Raman spectra at room temperature are recorded in the 4000−400 and 4000−0 cm⁻¹ frequency regions, respectively and analyzed on the basis of literature data. This study confirms the presence of the organic cation [C₈H₁₀NO]⁺ and of the [CoCl₄]²⁻ anion. UV–vis spectroscopy results showed the indirect transition with band gap energy 2.98 eV.     

A single-crystal study of the electrochemically Li-ion intercalated spinel-type Li4Ti5O12

Single crystals of Li_4 + xTi₅O₁₂ were prepared by means of electrochemical Li-ion intercalation technique using parent Li₄Ti₅O₁₂ single crystals. The obtained Li_4 + xTi₅O₁₂ (x = 1.35) crystallizes in the cubic spinel-related type structure, space group Fd3?m, and lattice parameters of a = 8.346(2) Å and V = 581.3(5) Å³ and Z = 8. The Li-ion intercalated sites were successfully determined to be both the 8a and 16c sites by using the difference Fourier synthesis map. The structure was determined by single-crystal X-ray structure analysis and refined to the conventional value of R = 3.7% for 132 independent observed reflections. The chemical composition has been determined to be Li_5.35Ti₅O₁₂ from the result of site-population refinements. In addition, theoretical electron density distributions and total energy were calculated for three postulated compounds of “Li_4.5Ti_4.5O₁₂” and “Li_4.5 + xTi_4.5O₁₂” with x = 1.5 and 3.0.     

Effects of Ni and Co doping on the physical properties of tetragonal FeSe0.5Te0.5 superconductor

Isoelectronic Tellurium (Te) substitution for Selenium (Se) in the tetragonal phase of FeSe (β-FeSe) increases the superconducting transition temperature (T_c) by applying a negative pressure on the lattice. However, the normal state resistivity increases and shows semi-metallic behavior for samples with higher Te concentration. With increasing Te concentration, the T_c increases and reaches a maximum for FeSe_0.5Te_0.5 and then decreases with further increase of Te. We have investigated the effect of Cobalt (Co) and Nickel (Ni) doping in FeSe_0.5Te_0.5 in the nominal composition range Fe_1?xTM_xSe_0.5Te_0.5 (TM = Co (x = 0.05, 0.1, 0.15, 0.2) and Ni (x = 0.05, 0.1)). Both Co and Ni doping suppress T_c and drives the system to metal–insulator transition. The in-plane (‘a’) and out-of-plane (‘c’) lattice constants decrease with increasing dopant concentration.     

A co-doping approach towards enhanced ionic conductivity in fluorite-based electrolytes

A co-dopant strategy is used to investigate the effect that the elastic strain in the lattice has on the grain ionic conductivity of doped ceria electrolytes. Based on critical dopant ionic radius (r_c), different compositions in the Lu_xNd_yCe_1−x−yO_2−δ (x + y = 0.05, 0.10, 0.15, and 0.20) system are studied. Dopants are added such that the weighted average dopant ionic radius matches r_c for all the compositions. Dense ceramic discs are prepared using conventional solid oxide route and sintering methods. Precise lattice parameter measurements are used to calculate the lattice strain. The ionic conductivity of the samples is measured in the temperature range of 250 °C to 700 °C using two-probe electrochemical impedance spectroscopy technique. The elastic strain present in Lu_xNd_yCe_1−x−yO_2−δ system is found to be negligible when compared to Lu_xCe_1−xO_2−δ (negative) and Nd_xCe_1−xO_2−δ (positive) systems. Grain ionic conductivity of Lu_xNd_yCe_1−x−yO_2−δ (where x + y = 0.05) at 500 °C is observed to be 1.9 × 10^− 3 S/cm which is twice as high as that of Lu_0.05Ce_0.95O_2−δ. These results extend the validity of the r_c concept as a strategy for co-doping ceria electrolytes and open new designing avenues for solid oxide electrolytes with enhanced ionic conductivity.     

Missing superconductivity in BaAlSi with the AlB2 type structure

The solid solutions BaAl_1−xSi_1+x (0 ⩽ x ⩽ 0.5) were prepared. The compound with the stoichiometric composition (x = 0) did not show superconductivity as reported by other investigators, but the solid solutions with x > 0 became superconductors with a transition temperature T_c = 2.8 K. The comparison of the lattice parameters with those of the other isotypic ternary superconductors MAlSi (M = Ca, Sr) suggested that the superconductivity could be related to the lattice parameter within the (AlSi) plane rather than the interlayer spacing. The band structures near the Fermi level of MAlSi (M = Ca, Sr, Ba) were measured using soft X-ray photoelectron spectroscopy, which were in good agreement with the calculated ones, confirming that the contribution of the d orbitals of the alkaline-earth metals were predominant in the conduction bands.     

Synthesis and structural characterization of perovskite type proton conducting BaZr1−xInxO3−δ (0.0 ≤ x ≤ 0.75)

Solid state sintering has been used to prepare the cubic perovskite structured compounds BaZr_1−xIn_xO_3−δ (0.0 ≤ x ≤ 0.75). Analysis of X-ray powder diffraction (XRPD) data reveals that the unit cell parameter, a, increases linearly with an increased Indium concentration. XRPD data was also used to demonstrate the completion of sample hydration, which was reached when the materials showed a set of single-phase Bragg-peaks. Dynamic thermogravimetric analysis (TGA) data showed that approx. 89% of the total number of available oxygen vacancies can be filled in BaZr_1−xIn_xO_3−δ for x = 0.50, and that the maximum water uptake occurs below 300 °C. Rietveld analysis of the room temperature neutron powder diffraction (NPD) data confirmed the average cubic symmetry (space group Pm-3m), and an expansion of the unit cell parameter after the hydration reaction. The strong O–H stretch band, 2500–3500 cm^− 1, in the infrared absorbance spectrum clearly manifests the presence of protons in the hydrated material. Proton conductivity of hydrated BaZr_1−xIn_xO_3−δ, x = 0.75 was investigated during heating and cooling cycles under dry argon atmosphere. The total conductivity during the heating cycle was nearly two orders of magnitude greater than that of cooling cycle at 300 °C, whilst these values were similar at higher temperatures i.e. T > 600 °C.     

The effect of compositional disorder on electronic band structure in GaxIn1 − xAsySb1 − yalloys lattice matched to GaSb

A pseudopotential formalism coupled with the virtual crystal approximation are applied to study the effect of compositional disorder upon electronic band structure of cubic Ga_xIn_1 − xAs_ySb_1 − yquarternary alloys lattice matched to GaSb. The effects of compositional variations are properly included in the calculations. Our theoretical results show that the compositional disorder plays an important role in the determination of the energy band structure of Ga_xIn_1 − xAs_ySb_1 − y/GaSb and that the bowing parameter is dominated by the group V-anion-based sublattice. Moreover, the absorption at the fundamental optical gaps is found to be direct within a whole range of the x composition.     

Electrochemical deposition and characterization of cadmium indium telluride thin films for photovoltaic application

Cd–In–Te films were electro-deposited potentiostatically over SnO₂:F coated glass substrates from aqueous acidic solutions containing CdSO₄, InCl₃ and TeO₂ at 85 °C. Cyclic voltammetric studies were carried out to study the growth process and the potential range for ternary Cd–In–Te thin film deposition. Crystalline structure and surface morphological studies were carried out and correlated with the deposition potential. The deposited film showed cubic structure having a high degree of (1 1 1) orientation. Optical transmission studies showed a direct band structure and the band gap of the deposited film change with the deposition potential. A typical decrease from 1.2 to 1.1 eV in the optical band gap was observed on increasing the cathodic deposition potential from −0.5 to −0.54 V (SCE).     

Effect of Fe doping on the room temperature ferromagnetism in chemically synthesized (In1−xFex)2O3 (0 ⩽ x ⩽ 0.07) magnetic semiconductors

Observation of room temperature ferromagnetism in Fe doped In₂O₃ samples (In_1−xFe_x)₂O₃ (0 ⩽ x ⩽ 0.07) prepared by co-precipitation technique is reported. Lattice parameter obtained from powder X software shows distinct shrinkage of the lattice constant indicating an actual incorporation of Fe ions into the In₂O₃ lattice. X-ray diffraction data measurements show that the entire sample exhibits single phase polycrystalline behavior. SEM micrographs showed the prepared powder was in the range 25–36 nm. SEM EDS mapping showed the presence of Fe and In ions in the Fe doped In₂O₃ sample. The highest remanence magnetization moment (6.624 × 10⁻⁴ emu/g) is reached in the sample with x = 0.03.     

Characteristics of pulse plated CdxZn1−xSe films

Cd_xZn_1−xSe films (0 ⩽ x ⩽ 1) were deposited for the first time by the pulse plating technique at different duty cycles in the range 6–50% at room temperature from an aqueous bath containing zinc sulphate, cadmium sulphate and selenium oxide. To the author’s knowledge this is the first report on pulse plated Cd_xZn_1−xSe films. The deposition potential was −0.9 V (SCE). The as deposited films exhibited cubic structure. Composition of the films was estimated by Energy Dispersive Analysis of X-ray studies. X-ray photoelectron spectroscopy studies indicated the binding energies corresponding to Zn(2p_3/2), Cd(3d_5/2 and 3d_3/2) and Se(3d_5/2 and 3d_3/2). Optical band gap of the films varied from 1.72 to 2.70 eV as the composition varied from CdSe to ZnSe side. Atomic force microscopy studies indicated grain size in the range of 20–150 nm.     

Nanocrystal growth of single-phase Si1−xGex alloys

We present the formation of single-phase Si_1−xGe_x (x=0.2, 0.4, 0.6, and 0.8) alloy nanocrystals dispersed in a SiO₂ matrix. The studied samples were prepared by co-sputtering with excess Si_1−xGe_x in SiO₂ of approximately 33 at%. Upon heat treatment, crystallization of Si_1−xGe_x alloys was examined by using X-ray diffraction and high-resolution transmission electron microscopy measurements. Single structure of face-centered cubic nanocrystals in a space group Fd-3m was concluded. The average nanocrystal size (from 2 nm to 10 nm) and the lattice constant a of the single-phase Si_1−xGe_x nanocrystals were found to increase with the Ge composition parameter x. Density functional theory-generalized gradient approximation calculation showed the replacement of Ge into the Si sites and vice versa.