Structural and compositional analysis of InBixAsySb(1−x−y) films grown on GaAs(0 0 1) substrates by liquid phase epitaxy

The growth of epitaxial InBi_xAs_ySb_(1−x−y) layers on highly lattice mis-matched semi-insulating GaAs substrates has been successfully achieved via the traditional liquid phase epitaxy. Orientation and single crystalline nature of the film have been confirmed by X-ray diffraction. Scanning electron micrograph shows abrupt interface at micrometer resolution. Surface composition of Bi(x) and As(y) in the InBi_xAs_ySb_(1−x−y) film was measured using energy dispersive X-ray analysis and found to be 2.5 and 10.5 at.%, respectively, and was further confirmed with X-ray photoelectron spectroscopy. Variation of the composition with depth of the film was studied by removing the layers with low current (20 μA) Ar⁺ etching. It was observed that with successive Ar⁺ etching, In/Sb ratio remained the same, while the As/Sb and Bi/Sb ratios changed slightly with etching time. However after about 5 min etching the As/Sb and Bi/Sb ratios reached constant values. The room temperature band gap of InBi_0.025As_0.105Sb_0.870 was found to be in the range of 0.113–0.120 eV. The measured values of mobility and carrier density at room temperature are 3.1×10⁴ cm² V⁻¹ s⁻¹ and 8.07×10¹⁶ cm⁻³, respectively.     

Viscosity of In100 − xSbx molten alloys in the vicinity of equiatomic composition

The viscosity of In–Sb liquid alloys has been measured in the vicinity of equiatomic concentration (40–60 at.% Sb). The measurements were carried out by a torsional oscillation viscometer. Temperature dependence of viscosity coefficient shows the Arrhenius like behavior with anomalous increase near the melting temperature. It is shown that viscosity isotherms reveal a sharp maximum at equiatomic composition, which becomes smoother with temperature increase.     

Low temperature photoluminescence of GaxIn1−xAs/GaAs strained layer superlattices

The low temperature photoluminescence spectra of several Ga_xIn_1−xAs/GaAs strained layer superlattices have been measured. Excitomic recombination between electrons and holes confined in the ternary layers and conduction band-acceptor transitions have been observed. The excitonic transition energies calculated with a simple model which takes into account both strain and quantization are in good agreement with the measured values provided the additional strain due to the mismatch between the SLS and the buffer layer is taken into account. The hydrogenic acceptor binding energy is smaller in the SLS than in the bulk ternary because the reduction due to the decrease of the hole mass under strain appears to be more important than the two dimensional quantization enhancement in the present samples.     

Crystal structure, electrical and magnetic properties of La1 − xSrxCoO3 − y

The crystal structure and properties of La_{1 − x}Sr_xCoO_{3 − y} with strontium contents ranging from x = 0.1 to x = 0.7 have been studied. The lattice parameters were measured as a function of temperature (4.2–400 K) and the crystal structure was found to change from rhombohedral (at low temperatures and values of x) to cubic. While LaCoO₃ is paramagnetic the oxides in the composition range 0.2 < x < 0.6 are soft ferromagnets. The strontium additions are compensated by the formation of Co⁴⁺ (cobalt ions with one positive effective charge, Co_Co^.) and oxygen vacancies (V_o^..). From the results it is concluded that the relative importance of oxygen vacancies increases with increasing temperature and decreasing oxygen activity. As a result the concentration of electronic charge carriers — and the resultant electrical conductivity — decrease with increasing temperature. The defect structure is discussed and it is concluded that defect associations — probably between oxygen vacancies and strontium ions — and formation of microdomains of perovskite-related phases are important aspects of the overall structure of these perovskite phases.     

Far-infrared spectra of Pb1−xMnxTe alloys

Far-infrared reflectivity spectra of Pb_1−xMn_xTe (0.0001x0.1) single crystals were measured in the 10–250 cm⁻¹ range at room temperature. The analysis of the far-infrared spectra was made by a fitting procedure based on the model of coupled oscillators. In spite of the strong plasmon–LO phonon interaction, we found that the long wavelength optical phonon modes of these mixed crystals showed an intermediate one–two mode behavior.     

Pressure studies of conduction-band N-pair-state mixing in dilute GaAs1−xNx alloys

High-pressure photoluminescence (PL) experiments (at 9 K) are reported for GaAs_1−xN_x/GaAs quantum wells having N compositions (x=0.0025, 0.004) in the dilute regime where the GaAs_1−xN_x alloy conduction band (CB) evolves rapidly by incorporation of N-pair states. Under increasing pressure, the PL spectra exhibit several new N-pair features that derive from CB-resonant states at 1 atm. Two of these features appear strongly at sub-band-gap energies for P29 kbar in the x=0.0025 sample, but are absent for all pressures in the x=0.004 sample. Several competing PL assignments due to bound-exciton recombination at NN_i pairs (i=1–4 is the anion separation) are considered in light of prior findings for N-doped (10¹⁷ cm⁻³) GaAs. The absence of certain PL features in the x=0.004 sample shows that N-pair states mix into the CB-continuum via a selective process, and this selectivity offers an important test for band-structure calculations in dilute GaAs_1−xN_x alloys.     

Luminescence from Si-Si1−xGex/Si1−yCy-Si structures

Near band edge photoluminescence has been obtained from Si_1−yC_y quantum well (QW) and neighboring Si_1−xGe_x/Si_1−yC_y double QW (DQW) structures. Enhanced no-phonon recombination is observed from the DQW structures and it is attributed to a breaking of the k-selection rule in the presence of the heterointerface. The luminescence persists for measurement temperatures up to 30–50 K and the intensity exhibits a quenching behavior with an activation energy equal to 8–20 meV. In electroluminescence only recombination in the Si_1−xGe_x layer has been observed from neighboring Si_1−xGe_x and Si_1−yC_y DQW structures.     

First-principles calculations of the optical band-gaps of ZnxCd1−xO alloys

Using first-principles calculations, we investigated the structural and electronic properties of two binaries: ZnO in wurtzite structure and CdO in wurtzite and rock-salt structures. In addition several compositions with various ordered structures of Zn_xCd_1−xO alloys were studied within the theory of order–disorder transformation. The full potential linearized augmented plane wave method was used and the d orbitals of Zn and Cd were included in the valence bands. In this investigation of alloying ZnO with CdO, the fundamental band-gap of the alloys is shown to be direct and to decrease versus the Cd composition.     

The T-contribution to the electrical resistivity at low temperatures in NiAs-type Ni1−xS1−ySey

The y(1−y)-like maxima in Ni_0.98S_1−ySe_y are found beyond y_c in the y-dependences of the coefficient of the T² contribution to the resistivity and of the residual resistivity. Here, the critical concentration of the antiferromagnetic and less-conductive phase is determined to be y_c = 0.095.     

Nax/2Bi1−x/2MoxV1−xO4 and Bi1−x/3MoxV1−xO4 New Scheelite-related phases

New Scheelite-related solid solutions of the compositions Na_x/2Bi_1−x/2Mo_xV_1−xO₄ (0≤x≤1) and Bi_1−x/3 Mo_xV_1−xO₄(0≤x≤0.2) have been synthesised by the substitution of Na and Mo at the A and B sites respectively of the ABO₄ type ferroelastic BiVO₄. The phases were characterised using chemical analysis, powder X-ray diffraction, scanning electron microscopy, EDAX, and Raman spectroscopy. While almost a continuous solid solution is obtained for the series Na_x/2Bi_1−x/2Mo_xV_1−xO₄, the absence of Na at the A-site results only in a narrow stability region for the other series, Bi_1−x/3 Mo_xV_1−xO₄ where 0≤x≤0.2. Raman spectra of selected samples at room temperature also suggest that vanadium and molybdenum atoms are disordered at the tetrahedral sites.     

Surface chemistry and microstructural analysis of CexZr1−xO2−y model catalyst surfaces

Cerium-zirconium mixed metal oxides are widely used as promoters in automotive emissions control catalyst systems (three-way catalysts). The addition of zirconium in the cubic lattice of ceria improves the redox properties and the thermal stability, thereby increasing the catalyst efficiency and longevity. The surface composition and availability of surface oxygen of model ceria-zirconia catalyst promoters was considered to develop a reference for future catalytic reactivity studies. The microstructure was characterized with X-ray diffraction (XRD) to determine the effect of zirconium substitution on crystalline structure and grain size. Additionally, the Ce/Zr surface atomic ratio and existence of Ce³⁺ defect sites were examined with X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) for samples with different zirconium concentrations. The surface composition of the model systems with respect to cerium and zirconium concentration is representative of the bulk, indicating no appreciable surface species segregation during model catalyst preparation or exposure to ultrahigh vacuum conditions and analysis techniques. Additionally, the concentration of Ce³⁺ defect sites was constant and independent of composition. The quantity of surface oxygen was unaffected by electron bombardment or prolonged exposure to ultrahigh vacuum conditions. Additionally, XRD analysis did not indicate the presence of additional crystalline phases beyond the cubic structure for compositions from 100 to 25 at.% cerium, although additional phases may be present in undetectable quantities. This analysis is an important initial step for determining surface reactions and pathways for the development of efficient and sulfur-tolerant automotive emissions control catalysts.     

Influence of chemical disorder on the magnetic behaviour and phase diagram of the FexNi1−x system

In this paper, we calculate the equilibrium phase diagram and the magnetic moment curve for the Fe_xNi_1−x system and simulate their Mössbauer spectra assuming a binomial distribution to reproduce the chemical disorder in these alloys. We also assume that the high-spin/low-spin transition for a central iron atom is governed by the number of nearest neighbours and next nearest neighbours of the iron atoms. The calculated equilibrium phase diagram and the magnetic moment curve are very close to that presented in the literature and the simulated Mössbauer spectra are in excellent agreement with that of their corresponding phases measured in our lab.     

High-resolution X-ray diffraction analysis of SnTe/Sn1−xEuxTe superlattices grown on (1 1 1) BaF2 substrates

A set of SnTe/Sn_1−xEu_xTe superlattice (SL) samples with increasing nominal Eu content x up to 0.28 was successfully grown on (1 1 1) BaF₂ substrates by molecular beam epitaxy. A complete structural characterization was performed by triple-axis X-ray diffractometry and reciprocal space mapping. The X-ray results showed that, despite the phase separation that normally occurs for unstrained Sn_1−xEu_xTe layers with x0.02, an SL stack with homogeneous individual layers can be formed for SL samples with a nominal Eu content up to 0.16. No SL satellite peak structure could be identified for samples with x values higher than 0.24. The structural parameters of the individual layers that compose the SL were determined using a best-fit simulation procedure which compared the calculated X-ray spectra to the measured (2 2 2) ω/2Θ scans. The strain information used in the simulation was obtained from the reciprocal space maps measured around the (2 2 4) lattice point.     

The structural features of Cu1−xPbx liquid alloys

Structure of Cu_1−xPb_x (x=0, 0.025, 0.05, 0.075, 0.1, 0.125) molten alloys has been studied by means of X-ray diffraction method. Structural parameters obtained from structure factors (SF) and radial distribution functions were analyzed. Partial structure factors were calculated by using Reverse Monte-Carlo method. It is shown that tendency to preferred interaction of atoms of the same kind grows with increasing of lead content.     

Ferromagnetic Zr1−xMnxCo2 laves phase compounds

Structure and magnetic properties of the Zr_1−xMn_xCo_2+δ alloys were studied for 0 x <0.7, δ=0, 0.45. The cubic C15 Laves phase structure shows Mn solubility up to x≈0.4. The other Laves phase with the hexagonal C36 structure found for x0.5 apparently has a small region of Mn solubility in the vicinity of Zr_0.4Mn_0.6Co₂. Though the parent Mn-free compounds are known to be paramagnetic, the Mn-substituted alloys show ferromagnetic behavior with the Curie temperatures up to 625 K and the room-temperature saturation magnetization of about 100 emu/g. The onset of ferromagnetism with the Mn substitution for Zr may be caused by polarization of itinerant 3d electrons, like it was earlier supposed for the off-stoichiometric ZrCo_2+δ. The universal composition dependencies of the intrinsic magnetic properties for different δ can be obtained, if plotted against the amount of zirconium atoms missing in its sublattice. The room-temperature anisotropy with the noticeable anisotropy field of 24 kOe and the 1 1 0 easy magnetization direction laying in a basal plane was found in the hexagonal Zr_0.5Mn_0.5Co₂.     

Structural and vibrational properties of dilute GaNxAs1−x(P1−x)

We report a comprehensive analyzes of the Fourier transform infrared (FTIR) absorption and Raman scattering data on the structural and vibrational properties of dilute ternary GaAs_1−xN_x,[GaP_1−xN_x] (x<0.03) alloys grown on GaAs [GaP] by metal organic chemical vapor deposition (MOCVD) and solid source molecular beam epitaxy (MBE). By using realistic total energy and lattice dynamical calculations, the origin of experimentally observed N-induced vibrational features are characterized. Useful information is obtained about the structural stability, vibrational frequencies, lattice relaxations and compositional disorder in GaNAs (GaNP) alloys. At lower composition (x<0.015) most of the N atoms occupy the As [P] sublattice {N_As[N_P]}—they prefer moving out of their substitutional sites to more energetically favorable locations at higher x. Our results for the N-isotopic shifts of local mode frequencies compare favorably well with the existing FTIR data.     

Electronic and structural properties of ZnxCd1−xSySe1−y alloys lattice matched to GaAs and InP: An EPM study

The electronic, optical and structural properties of Zn_xCd_1−xS_ySe_1−y quaternary alloys lattice matched to GaAs and InP are studied. The electronic band structure and density of states are computed using empirical pseudopotential method. The disorder effects are included via modified virtual crystal approximation. The bandgap computed from band structures are utilized to evaluate refractive indices, dielectric constants and ionicity factors for the alloys. Among structural properties elastic constants and bulk moduli are computed by combining the EPM with Harrison bond orbital model. All possible semiconductors from the ZnCdSSe system are found to have direct bandgap. The lattice matched alloys have larger band gap and more ionic character than the lattice matched compounds.     

Fractional-dimensional polaron corrections in asymmetric GaAs-Ga1−xAlxAs quantum wells

Polaron effects in asymmetric GaAs-Ga_1−xAl_xAs quantum wells (QWs) are investigated within the framework of the fractional-dimensional space approach and by using second-order perturbation theory. A well-width dependence of the polaron corrections with a dip and a peak is obtained for both symmetric and asymmetric QWs. The dip and the peak occur in the case of asymmetric QWs for larger well widths than in the case of symmetric QWs. An enhancement of the contrast between the dip and the peak of the polaron energy shift is found for the case of asymmetric QWs. These results show the convenience of using asymmetric QWs instead of symmetric ones in any experimental attempt of detecting the dip and the peak of the polaron energy shift.     

Magnetic and structural properties of magnetoresistive FexAu100−x alloys produced by mechanical alloying

Fe_xAu_100−x alloys have been produced for the first time by mechanical alloying. X-ray diffractograms show FCC peaks. From the X-ray diffracion peak-widths we estimate the final grain size, which vary with x from 112 nm (for x=15) to 32 nm (for x=30). Lattice parameter decreases with concentration (minimum 0.401 nm at x=30), but above Vegard's law values. Susceptibility measurements show cluster-glass behaviour. Critical temperatures are consistently lower than similar alloys produced by arc melting followed by fast quenching. A magnetic phase diagram is presented. Giant magnetoresistance is present in all samples, with a maximum at x=25. This effect is caused by the dispersion of small iron clusters produced by the mechanical work.