Principal energy band gaps of the quaternary alloy AlxGa1−xSbyAs1−y

The correlated function expansion (CFE) interpolation procedure was presented to efficiently estimate principal energy band gaps and lattice constants of the quaternary alloy Al_xGa_1−xSb_yAs_1−y over the entire composition variable space. The lattice matching conditions between x and y for the alloy Al_xGa_1−xSb_yAs_1−y substrated to InAs and GaSb were obtained by optimizing the alloy lattice constant to that of the substrates. The corresponding principal band gaps (E(Γ), E(L), and E(X)) were also calculated along the lattice matching condition on each substrate (InAs and GaSb).     

Microstructural and optical properties of strain compensated InxGa1−xAs/InyAl1−yAs multiple quantum wells

Transmission electron microscopy (TEM) and photocurrent (PC) measurements were carried out to investigate the microstructural and excitonic transitions in In_0.52Ga_0.48As/In_0.55Al_0.45As multiple quantum wells (MQWs). TEM images showed that high-quality 11-period strain-compensated In_0.52Ga_0.48As/In_0.55Al_0.45As MQWs had high-quality heterointerfaces. Based on the TEM results, a possible crystal structure for the In_0.52Ga_0.48As/In_0.55Al_0.45As MQWs is presented, and their strains are compensated. The results for the PC data at 300 K for several applied electric fields showed that several excitonic transitions shifted to longer wavelengths as the applied electric field increased. These results indicate that the strain-compensated In_0.52Ga_0.48As/In_0.55Al_0.45As MQWs hold promise for electroabsorption modulator devices.     

Electronic structure and optical properties of TbNi5−xCux

In this paper we present theoretical investigation of optical conductivity for intermetallic TbNi_5−xCu_x series. Within the framework of LSDA+U calculations, electronic structure for x=0, 1, 2 is calculated and additionally optical conductivity is obtained. Disorder effects of Cu for Ni substitution on a level of LSDA+U densities of states (DOS) are taken into account via averaging over all possible Cu ion positions in the unit cell for given doping level x. Gradual smoothing of optical conductivity structure at 2 eV together with simultaneous intensity growth at 4 eV corresponds to increase of Cu and decrease of Ni content.     

Theoretical studies of structural, elastic, electronic and lattice dynamic properties of AlxYyB1−x−yN quaternary alloys

A theoretical study on the structural, elastic, electronic and lattice dynamic properties of Al_xY_yB_1−x−yN quaternary alloys in zinc-blend phase has been carried out with first-principles methods. Information on the lattice parameter, the lattice matching to available substrates and energy band-gaps is a prerequisite for many practical applications. The dependence of the lattice parameter a, bulk modulus B, elastic constants C₁₁, C₁₂ and C₄₄, band-gaps, optical phonon frequencies (ω_TO and ω_LO), the static and high-frequency dielectric coefficients ε (0) and ε (∞) and the dynamic effective charge Z^? were analyzed for y=0, 0.121, 0.241, 0.362 and 0.483. A significant deviation of the bulk modulus from linear concentration dependence was observed. A set of isotropic elastic parameters and related properties, namely bulk and shear moduli, Young's modulus, Poisson's ratio are numerically estimated in the frame work of the Voigt-Reuss-Hill approximation. The resistance to changes in bond length and lateral expansion in Al_xY_yB_1−x−yN increase with increasing y concentration. We observe that at y concentration about 0.035 and 0.063, Al_xY_yB_1−x−yN changes from brittle to ductile and Γ-X indirect fundamental gap becomes Γ-Γ direct fundamental gap. There is good agreement between our results and the available experimental data for the binary compound AlN, which is a support for those of the quaternary alloys that we report for the first time.     

Photoionization cross-section and binding energy of shallow donor impurities in Ga1−xInxNyAs1−y/GaAs quantum wires

We have investigated the effects of the nitrogen and indium concentrations on the photoionization cross-section and binding energy of shallow donor impurities in Ga_1−xIn_xN_yAs_1−y/GaAs quantum wires. The numerical calculations are performed in the effective mass approximation, using a variational method. We observe that incorporation of small amounts of nitrogen and indium leads to significant changes of the photoionization cross-section and binding energy.     

First-principles calculations of the structural, electronic and optical properties of cubic BxGa1−xAs alloys

Density functional calculations are performed to study the structural, electronic and optical properties of technologically important B_xGa_1−xAs ternary alloys. The calculations are based on the total-energy calculations within the full-potential augmented plane-wave (FP-LAPW) method. For exchange-correlation potential, local density approximation (LDA) and the generalized gradient approximation (GGA) have been used. The structural properties, including lattice constants, bulk modulus and their pressure derivatives, are in very good agreement with the available experimental and theoretical data. The electronic band structure, density of states for the binary compounds and their ternary alloys are given. The dielectric function and the refractive index are also calculated using different models. The obtained results compare very well with previous calculations and experimental measurements.     

Effect of applied bias voltage on corrosion-resistance for TiC1−xNx and Ti1−xNbxC1−yNy coatings

Corrosion-resistance behavior of titanium carbon nitride (Ti-C-N) and titanium niobium carbon nitride (Ti-Nb-C-N) coatings deposited onto Si(1 0 0) and AISI 4140 steel substrates via r.f. magnetron sputtering process was analyzed. The coatings in contact with a solution of sodium chloride at 3.5% were studied by Tafel polarization curves and impedance spectroscopy methods (EIS). Variations of the bias voltage were carried out for each series of deposition to observe the influence of this parameter upon the electrochemical properties of the coatings. The introduction of Nb in the ternary Ti-C-N film was evaluated via X-ray diffraction (XRD) analysis. The structure was characterized by using Raman spectroscopy to identify ternary and quaternary compounds. Surface corrosion processes were characterized using optical microscopy and scanning electron microscopy (SEM). XRD results show conformation of the quaternary phase, change in the strain of the film, and lattice parameter as the effect of the Nb inclusion. The main Raman bands were assigned to interstitial phases and “impurities” of the coatings. Changes in Raman intensities were attributed to the incorporation of niobium in the Ti-C-N structure and possibly to resonance enhancement. Finally, the corrosion data obtained for Ti-C-N were compared with the results of corrosion tests of Ti-Nb-C-N coating. The results obtained showed that the incorporation of niobium to Ti-C-N coatings led to an increase in the corrosion-resistance. On another hand, an increase in the bias voltage led to a decrease in the corrosion-resistance for both Ti-C-N and Ti-Nb-C-N coatings.     

Magnetic and related properties of the solid solution CeCuxGa4−x

Physical properties of polycrystalline samples of CeCu_xGa_4−x (x = 0.2–1.4), crystallizing in the tetragonal BaAl₄-type structure (space group I 4/mmm), were studied by means of X-ray powder diffraction, magnetization, specific heat, electrical resistivity and magnetoresistivity measurements in wide temperature and magnetic fields ranges. The unit-cell volume of the system was found to decrease with increasing x (in total by about 4%) but the magnetic moments of Ce³⁺ ions remain localized in the whole x-range studied. The alloys exhibit ferromagnetic order at low temperatures, which manifests itself as distinct and relatively sharp anomalies in all the temperature characteristics measured. The ordering temperature decreases with increasing the Cu content from 5.5(1) K for x = 0.2 down to 1.35(5) K for x = 1.4, and the electrical transport properties of the system show some features characteristic of Kondo lattices.     

Electrical and optical properties of GexFexSe100−2x chalcogenide thin films

Optical absorption at room temperature and electrical conductivity at temperatures between 283 and 333 K of vacuum evaporated Ge_xFe_xSe_100−2x (0≤x≤15) amorphous thin films have been studied as a function of composition and film thickness. It was found that the optical absorption is due to indirect transition and the energy gap increases with increasing both Ge and Fe content; on the other hand, the width of the band tail exhibits the opposite behavior. The optical band gap E_opt was found to be almost thickness independent. The electrical conductivity show two types of conduction, at higher temperature the conduction is due to extended states, while the conduction at low temperature is due to variable range hopping in the localized states near Fermi level. Increasing Ge and Fe contents were found to decrease the localized state density N(E_F), electrical conductivity and increase the activation energy for conduction, which is nearly thickness independent. Variation of the atomic densities ρ, molar volume V, glass transition temperature T_g cohesive energy C.E and number of constraints N_Co with average coordination number Z was investigated. The relationship between the optical gap and chemical composition is discussed in terms of the cohesive energy C.E, average heat of atomization and coordination numbers.     

Magnetotransport, optical, and electronic subband properties in AlxGa1−xN/AlN/GaN heterostructures

The Shubnikov-de Haas (S-dH) results at 1.5 K for Al_xGa_1−xN/AlN/GaN heterostructures and the fast Fourier transformation data for the S-dH data indicated the occupation by a two-dimensional electron gas (2DEG) of one subband in the GaN active layer. Photoluminescence (PL) spectra showed a broad PL emission about 30 meV below the GaN exciton emission peak at 3.474 eV that could be attributed to recombination between the 2DEG occupying in the AlN/GaN heterointerface and photoexcited holes. A possible subband structure was calculated by a self-consistent method taking into account the spontaneous and piezoelectric polarizations, and one subband was occupied by 2DEG below the Fermi level, which was in reasonable agreement with the S-dH results. These results can help improve understanding of magnetotransport, optical, and electronic subband properties in Al_xGa_1−xAs/AlN/GaN heterostructures.     

Ab initio investigation of phase separation in Ca1−xZnxO alloys

First-principles study of the ground-state properties and the stability of Ca_1−xZn_xO solid solutions is presented using the full-potential linearized augmented plane wave (FP-LAPW) method. We employed the local density approximation (LDA) to the exchange-correlation potential. It is found that the structural parameters, i.e., lattice constants and bulk moduli deviate from the linear function of the composition x. We determined the equation of state of the alloys and showed an increasing compressibility function of composition. The formation energy is viewed as an energetic balance between pure structural constraints and quantum chemical effects. Thus, a phase separation over the whole range of concentration is expected. The origin of the miscibility gap has a chemical nature. Also, the thermodynamic stability of the alloys was investigated.     

AlN, GaN, AlxGa1 − xN nanotubes and GaN/AlxGa1 − xN nanotube heterojunctions

Semiconductor optoelectronic devices based on GaN and on InGaN or AlGaN alloys and superlattices can operate in a wide range of wavelengths, from far infrared to near ultraviolet region. The efficiency of these devices could be enhanced by shrinking the size and increasing the density of the semiconductor components. Nanostructured materials are natural candidates to fulfill these requirements. Here we use the density functional theory to study the electronic and structural properties of (10,0) GaN, AlN, Al_xGa_1 − xN nanotubes and GaN/Al_xGa_1 − xN heterojunctions, 0<x<1. The Al_xGa_1 − xN nanotubes exhibit direct band gaps for the whole range of Al compositions, with band gaps varying from 3.45 to 4.85 eV, and a negative band gap bowing coefficient of −0.14 eV. The GaN/Al_xGa_1 − xN nanotube heterojunctions show a type-I band alignment, with the valence band offsets showing a non-linear dependence with the Al content in the nanotube alloy. The results show the possibility of engineering the band gaps and band offsets of these III-nitrides nanotubes by alloying on the cation sites.     

Photo-excited zero-resistance states in quasi-two-dimensional GaAs / AlxGa1−xAs devices

We illustrate some experimental features of the recently discovered radiation-induced zero-resistance states in the high-mobility GaAs/AlGaAs system, with a special emphasis on the interplay between the radiation-induced changes in the diagonal resistance and the Hall effect. We show that, quantum Hall effects, i.e., quantum Hall plateaus, disappear under photoexcitation, at the minima of the radiation-induced magnetoresistance oscillations.     

Optical and electrical properties of nonstoichiometric a-Ge1−xCx films prepared by magnetron co-sputtering

Amorphous non-hydrogenated germanium carbide (a-Ge_1−xC_x) films have been prepared by magnetron co-sputtering method in a discharge of Ar. The dependence of structural and chemical bonding properties on the Ge/C ratio (R) has been investigated by X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy and Raman spectroscopy. The relationship between the chemical bonding and the optical and electrical properties of the a-Ge_1−xC_x films has also been explored. It has been shown that the refractive index of the films increases from 2.9 to 4.4 and the optical gap decreases from 1.55 to 1.05 eV as R increases from 1.22 to 5.67. Moreover, the conductivity σ increases clearly and the activation energy E_a decreases with the increasing R owing to the reduction of sp³ CGe bonds. The a-Ge_1−xC_x films exhibit refractive index and optical gap values changing with x in a wide range, which may make a-Ge_1−xC_x films good candidates in the fields of protection coatings for IR windows and electronic devices.     

Ferromagnetism and antiferromagnetism in Ni2+x+yMn1−xAl1−y alloys

The magnetic behavior of Ni_2+xMn_1−xAl alloys around the stoichiometric 2:1:1 composition was investigated with several experimental techniques. The results of low-temperature magnetization measurements indicate that a competition mechanism between ferromagnetism and antiferromagnetism is expected in off-stoichiometric alloys. Although the Curie temperature is strongly dependent on the composition, the saturation magnetization has an unsystematic variation for deviations from the stoichiometric Ni₂MnAl alloy. A reentrant-spin-glass behavior is observed below 50 K.     

Sn1−xBixO2 and Sn1−xTaxO2 (0≤x≤0.75): A first-principles study

The structural, elastic, electronic and optical (x=0) properties of doped Sn_1−xBi_xO₂ and Sn_1−xTa_xO₂ (0≤x≤0.75) are studied using the first-principles pseudopotential plane-wave method within the local density approximation. The independent elastic constants C_ij and other elastic parameters of these compounds have been calculated for the first time. The mechanical stability of the compounds with different doping concentrations has also been studied. The electronic band structure and density of states are calculated and the effect of doping on these properties is also analyzed. It is seen that the band gap of the undoped compound narrowed with dopant concentration, which disappeared for x=0.26 for Bi doping and 0.36 for Ta doping. The materials thus become conductive oxides through the change in the electronic properties of the compound for x≤0.75, which may be useful for potential application. The calculated optical properties, e.g. dielectric function, refractive index, absorption spectrum, loss-function, reflectivity and conductivity of the undoped SnO₂ in two polarization directions are compared with both previous calculations and measurements.     

Effect of sulfur substitutions on optical, electrical and structural properties of Ge(SxSe1−x)2 system

Glasses in the system Ge-Se-S were prepared with different Se/S ratios in order to investigate the compositional dependence of selected physical properties. We report the results of a systematic study examining the UV-vis transmission, dc electrical conductivity and X-ray diffraction of the system Ge(S_xSe_1−x)₂ with x=0, 0.1, 0.4 and 1.0 where replacement of S by Se was made. The changes in the optical energy gap, E_g, (from 1.95 to 2.43 eV) and band tail width, E_e, (from 103 to 243 meV) behave contrarily to the change in refractive index, n, (from 2.3 to 2) with the progressive replacement of S by Se. This behavior was discussed and interpreted with the changes in cohesive energy. The analysis of defects in the prepared films was carried out by the examination of activation energies obtained from dc electrical conductivity. The analysis of the X-ray diffraction pattern revealed a remarkable reduction in the intensity of the first and second diffraction peaks with the progressive replacement of S content, which confirms a change in the intermediate range order structure: reorganization of the structural properties.     

The study of optical parameters and DC conductivity of Se100−xHgx thin films

Se_100−xHg_x bulk samples have been prepared by conventional melt quenching technique. The thin films of the material have been prepared on glass substrate using the thermal evaporation technique. The transmission spectra has been studied to measure the optical constants like absorption coefficient (α), extinction coefficient (K), optical band gap (E_g), Urbach energy (E_e). The DC conductivity (σ_dc) of Se_100−xHg_x has been also studied to find the activation energy (ΔE_a)$(\mathrm{\Delta }{E}_a)$. The optical band gap increases and Urbach energy first increases then decreases with increase in Hg concentration. DC conductivity and activation energy increases with increase in Hg concentration. These materials are found suitable for the optical disk materials and in optoelectronic devices due to their high absorption coefficient and dependence of reflectance on composition.     

Magnetic and transport properties of SmCoAsO1−xFx

A series of SmCoAsO_1−xF_x (with x=0, 0.05, 0.1, and 0.2) samples have been prepared by solid state reactions. X-ray powder diffraction proved that all samples can be indexed as a tetragonal ZrCuSiAs-type structure. A clear shrinkage of the lattice constants a and c with increasing F content indicated that F has been doped into the lattice. The magnetic and transport properties of the samples have been investigated. Parent SmCoAsO compound exhibited complicated magnetism including antiferromagnetism, ferromagnetism, and ferrimagnetism. For the fluorine doped samples, the antiferromagnetic Néel temperatures were almost independent of the F content and metamagnetic transitions were observed below antiferromagnetic Néel temperatures. With increasing F content, high temperature (below 142 K) ferrimagnetic state gradually changed to ferromagnetic state. In the resistivity result, metallic conduction in the region of 2-300 K and Fermi liquid behavior at low temperatures were shown in all samples. Transport properties at applied magnetic fields showed anomalies at low temperatures.     

Polar oscillation and dispersion properties of quasi-confined optical phonon modes in a wurtzite GaN/AlxGa1−xN nanowire

Under the dielectric continuum model and Loudon’s uniaxial crystal model, the properties of the quasi-confined (QC) optical phonon dispersions and the electron-QC phonons coupling functions in a cylindrical wurtzite nanowire are deduced via the method of electrostatic potential expanding. Numerical computations on a GaN/Al_0.15Ga_0.85N wurtzite nanowire are performed. Results reveal that, for a definite axial wave number k_z and a certain azimuthal quantum number m, there are infinite branches of QC modes. The frequencies of these QC modes fall into two regions, i.e. a high frequency region and a low frequency region. The dispersion of the QC modes are quite apparant only when k_z and m are small. The lower-order QC modes in the higher frequency region play more important role in the electron-QC phonon interactions. Moreover, for the higher-order QC modes in the high frequency region, the electrostatic potentials “escaping” out of the well-layer material nearly could be ignored.