Theoretical investigation of the conduction and valence band offsets of GaAs1−x Nx/GaAs1−yNy heterointerfaces

Theoretical investigations of the conduction band offset (CBO) and valence band offset (VBO) of the relaxed and pseudo-morphically strained GaAs_1−xN_x/GaAs_1−yN_y heterointerfaces at various nitrogen concentrations (x and y) within the range 0-0.05 and along the [0 0 1] direction are performed by means of the model-solid theory combined with the empirical pseudopotential method under the virtual crystal approximation that takes into account the effects of the compositional disorder. It has been found that for y < x, the CBO and VBO have negative and positive signs, respectively, whereas the reverse is seen when y > x. The band gap of the GaAs_1−xN_x over layer falls completely inside the band gap of the substrate GaAs_1−yN_y and thus the alignment is of type I (straddling) for y < x. When y > x, the alignment remains of type I but in this case it is the band gap of the substrate GaAs_1−yN_y which is fully inside the band gap of the GaAs_1−xN_x over layer. Besides the CBO, the VBO and the relaxed/strained band gap of two particular cases: GaAs_1−xN_x/GaAs and GaAs_1−xN_x/GaAs_0.98N_0.02 heterointerfaces have been determined.     

Bulk Sn1−xMnxO2 magnetic semiconductors without room-temperature ferromagnetism

Polycrystalline Sn_1−xMn_xO₂ (0≤x≤0.05) diluted magnetic semiconductors were prepared by solid-state reaction method and their structural and magnetic properties had been investigated systematically. The three Mn-doped samples (x=0.01, 0.03, 0.05) undergo paramagnetic to ferromagnetic phase transitions upon cooling, but their Curie temperatures are far lower than room temperature. The magnetization cannot be attributed to any identified impurity phase. It is also found that the magnetization increases with increasing Mn doping, while the ratio of the Mn ions contributing to ferromagnetic ordering to the total Mn ions decreases.     

Effect of Cr on the magnetic properties of evaporated CoxCr1−x/Si(1 0 0) and CoxCr1−x/glass thin films

Series of Co_xCr_1−x thin films have been evaporated under vacuum onto Si(1 0 0) and glass substrates. Thickness ranges from 17 to 220 nm, and x from 0.80 to 0.88. Alternating gradient field magnetometer (AGFM) measurements provided saturation magnetization values ranging from 220 to 1200 emu/cm³. Values of squareness exceeding 0.8 have been measured. Coercive field may reach values up to 700 Oe, depending on the percentage of chromium, as well as the substrate nature and the direction of the applied magnetic field. The saturation magnetization value decreases as the Cr content increases. In order to study their dynamical magnetic properties, Brillouin Light Scattering (BLS) measurements have been performed on these samples. Stiffness constant value and anisotropy magnetic field were adjusted to fit the experimental BLS spectra. These results are analyzed and correlated.     

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).     

Stress induced preferred orientation and phase transition for ternary WCxNy thin films

We deposit ternary WC_xN_y thin films on Si (1 0 0) substrates at 500 °C using direct current (DC) reactive magnetron sputtering in a mixture of CH₄/N₂/Ar discharge, and explore the effects of substrate bias (V_b) on the intrinsic stress, preferred orientation and phase transition for the obtained films by virtue of X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and selective area electron diffraction (SAED). We find that with increasing the absolute value of V_b up to 200 V the carbon (x) and nitrogen (y) atom concentrations of WC_xN_y films keep almost constant with the values of 0.75 and 0.25, respectively. The XPS and SAED results, combined with the density-functional theory (DFT) calculations on the electronic structure of WC_0.75N_0.25, show our obtained WC_xN_y films are single-phase of carbonitrides. Furthermore, we find that the compressive stress sharply increases with increasing the absolute value of V_b, which leads to a pronounced change in the preferred orientation and phase structure for the film, in which a phase transition from cubic β-WC_xN_y to hexagonal α-WC_xN_y occurs as V_b is in the range of −40 to −120 V. In order to reveal the relationship between the stress and phase transition as well as preferred orientation, the DFT calculations are used to obtain the elastic constants for β-WC_xN_y and α-WC_xN_y. The calculated results show that the preferred orientation is dependent on the competition between strain energy and surface energy, and the phase transition can be attributed to a decrease in the strain energy.     

The magnetic behavior of La1−xTbxMn2Si2 (0≤x≤0.3) in weak magnetic fields

The structure and magnetic properties of La_1−xTb_xMn₂Si₂ (0≤x≤0.3) were studied by X-ray powder diffraction and DC magnetization measurements. All the compounds crystallize in ThCr₂Si₂-type structure. Substitution of Tb for La led to a linear decrease in the lattice constants and the unit-cell volume. A ferromagnetic phase for x≤0.15, and an antiferromagnetic phase for x=0.3 have been observed at about room temperature, whereas the compounds with x=0.2 and 0.25 exhibit a magnetic phase transition from ferromagnetism to antiferromagnetism.     

First-principles study of single-walled armchair Cx(BN)y nanotubes

The structural and electronic properties of the armchair C_x(BN)_y nanotubes are studied using the density functional theory with a generalized gradient approximation. The results show that the properties of the C_x(BN)_y nanotubes are intermediate between those of carbon nanotubes and BN nanotubes, and also adjustable by their radius, ratio of carbon component, and configurations.     

Preparation of (CdO)1−x(PbO)x and (CdS)1−x(PbS)x thin films by spray pyrolysis technique and their characterization

Mixed thin films of (CdO)_1−x(PbO)_x and (CdS)_1−x(PbS)_x (x=0.25) were prepared on glass substrates by spray pyrolysis technique for various substrate temperatures 300, 320 and 340 °C. Structural and optical properties were studied. XRD studies reveal the formation of mixed films. The substrate temperature of 340 °C seems to be critical for the formation of CdO-PbO mixed films. It is observed that (CdS)_1−x(PbS)_x mixed films were formed at all the three substrate temperatures. The direct band gap value of (CdO)_1−x(PbO)_x and (CdS)_1−x(PbS)_x mixed films is about 2.6 and 2.37 eV, respectively.     

Luminescent properties of HTP AgGd1−xW2O8:Eux and AgGd1−x(W1−yMoy)2O8:Eux phosphor for white LED

Intense red phosphors, AgGd_1−xEu_x(W_1−yMo_y)₂O₈ (x=0.0-1.0, y=0.0-1.0), have been synthesized through traditional solid-state reaction and characterized by X-ray diffraction (XRD) and photoluminescence (PL). XRD results reveal that AgGd_1−xEu_xW₂O₈ synthesized at 1000 °C has a tetragonal crystal structure, which is named as high temperature phase (HTP) AgGdW₂O₈. All phosphors compositions with Eu³⁺ show red and green emission on excitation either in the charge-transfer or Eu³⁺ levels. Analysis of the emission spectra with different Eu³⁺ concentrations reveal that the optimum dopant concentration for Eu³⁺ is x=0.6 in the HTP AgGd_1−xEu_xW₂O₈ (x=0.0-1.0). Studies on the AgGd_0.4Eu_0.6(W_1−yMo_y)₂O₈ (y=0.0-1.0) and AgGd_1−xEu_x(W_0.7Mo_0.3)₂O₈ (x=0.0-1.0) show that the emission intensity is maximum for compositions with y=0.3 and x=0.5, respectively, and a decrease in emission intensity is observed for higher y or x values. The Mo⁶⁺ and Eu³⁺ co-doped AgGd(WO₄)₂ phosphors show higher emission intensity in comparison with the singly Eu³⁺-doped AgGd(WO₄)₂ in UV region. The intense emission of the tungstate/molybdate phosphors under 394 and 465 nm excitations, respectively, suggests that these materials are promising candidates as red-emitting phosphors for near-UV/blue GaN-based white LED for white light generation.     

Low temperature specific heat of bi-layered manganites La2−2xSr1+2xMn2O7 (x=0.3 and 0.5)

The specific heat (C) of bi-layered manganites La_2−2xSr_1+2xMn₂O₇ (x=0.3 and 0.5) is investigated for the ground state of low temperature excitations. A T^3/2 dependent term in the low temperature specific heat (LTSH) is identified at zero magnetic field and suppressed by magnetic fields for x=0.3 sample, which is consistent with a ferromagnetic metallic ground state. For x=0.5 sample, a T² term is observed and is consistent with a two-dimensional (2D) antiferromagnetic insulator. However, it is almost independent of magnetic field within the range of measured temperature (0.6-10 K) and magnetic field (6 T).     

Doping strategies for increased performance in BiFeO3

Investigation of crystal structure, dielectric, magnetic and local ferroelectric properties of the diamagnetically substituted Bi_1−xA_xFeO_3−x/2 (A=Ca, Sr, Pb, Ba; x=0.2, 0.3) polycrystalline samples has been carried out. It has been shown that the heterovalent A²⁺ substitution result in the formation of oxygen vacancies in the host lattice. The solid solutions have been found to possess a rhombohedrally distorted perovskite structure described by the space group R3c. Piezoresponse force microscopy has revealed signs of existence of the ferroelectric polarization in the samples at room temperature. Magnetization measurements have shown that the magnetic state of these compounds is determined by the ionic radius of the substituting elements. A-site substitution with the biggest ionic radius ions has been found to suppress the spiral spin structure of BiFeO₃ giving rise to the appearance of room-temperature weak ferromagnetism.     

Structural and optical properties of a-Si1−xCx:H films synthesized by dc magnetron sputtering technique

Hydrogenated amorphous SiC films (a-Si_1−xC_x:H) were prepared by dc magnetron sputtering technique on p-type Si(1 0 0) and corning 9075 substrates at low temperature, by using 32 sprigs of silicon carbide (6H-SiC). The deposited a-Si_1−xC_x:H film was realized under a mixture of argon and hydrogen gases. The a-Si_1−xC_x:H films have been investigated by scanning electronic microscopy equipped with an EDS system (SEM-EDS), X-ray diffraction (XRD), secondary ions mass spectrometry (SIMS), Fourier transform infrared spectroscopy (FTIR), UV-vis-IR spectrophotometry, and photoluminescence (PL). XRD results showed that the deposited film was amorphous with a structure as a-Si_0.80C_0.20:H corresponding to 20 at.% carbon. The photoluminescence response of the samples was observed in the visible range at room temperature with two peaks centred at 463 nm (2.68 eV) and 542 nm (2.29 eV). In addition, the dependence of photoluminescence behaviour on film thickness for a certain carbon composition in hydrogenated amorphous SiC films (a-Si_1−xC_x:H) has been investigated.     

Atomistic simulation for the phase stability, site preference and thermal expansion of YFe12−xTx (T=Ti, V, Cr, Mn, Zr, Nb, Mo, W)

The effect of the ternary element on the structural properties of YFe_12−xT_x (T=Ti, V, Cr, Mn, Zr, Nb, Mo, W) has been studied by using interatomic pair potentials based on ab initio method and lattice inversion equations. Calculated results show that adding ternary element T makes the crystal cohesive energy of YFe_12−xT_x decrease markedly, which indicates that T helps stabilize the 1:12 phases. The ternary elements T prefer to occupy the 8i sites in these compounds. The calculated results are found in good agreement with the values deduced from experiments. Furthermore, we have calculated the thermal expansion of YFe₁₁V compounds with the ThMn₁₂ structure. The method utilized in the present investigation offers a rather easy and direct way to study the structural properties of YFe_12−xT_x.     

Structure and superconductivity in FexCu1−xBa2YCu2O7+y superconductors synthesized by high pressure

Comprehensive studies of X-ray diffraction, oxygen content, superconductivity and Mössbauer effect have been made on Fe_xCu_1−xBa₂YCu₂O_7+y superconductors (0.00≤x≤0.70) synthesized by ambient (AM) and high pressure (HP). Results indicate that all the HP-samples have tetragonal structure, smaller lattice parameter c and unit-cell volume than the AM-samples. The studies of oxygen content, and Mössbauer spectroscopy indicate that the HP-samples have higher oxygen content, carrier concentration and average valence of Fe than the AM-samples. Moreover, for the HP-samples more Fe atoms located in CuO_x chains have fivefold-oxygen coordination. These are important reasons for the enhancement of T_c in the HP-samples.     

Magnetic properties of ErxY1−xF3 solid solutions

The Er_xY_1−xF₃ (x=0.1, 0.2, 0.7, 0.9, 1) solid solutions were synthesized and characterized by X-ray powder diffraction and magnetic measurements. The crystal structure refinements done by the Rietveld profile method show that no significant change of the structure parameters with the erbium concentration occurs. On the basis of DC susceptibility measurements in the 2-300 K range the lowest four crystal field levels have been determined, giving the ground level magnetic moment value of 6.7 μ_B. Results of M(H) studies point to the presence of complex exchange interactions between erbium ions.     

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.     

Structural studies of evaporated CoxCr1−x/Si (1 0 0) and CoxCr1−x/glass thin films

Series of Co_xCr_1−x thin films have been evaporated under vacuum onto Si (1 0 0) and glass substrates. Chemical composition and interface properties have been studied by modelling Rutherford backscattering spectra (RBS) using SIMNRA programme. Thickness ranges from 17 to 220 nm, and x from 0.80 to 0.88. Simulation of the energy spectra shows an interdiffusion profile in the thickest films, but no diffusion is seen in thinner ones. Microscopic characterizations of the films are done with X-ray diffraction (XRD) measurements. All the samples are polycrystalline, with an hcp structure and show a 〈0 0 0 1〉 preferred orientation. Atomic force microscopies (AFM) reveal very smooth film surfaces.     

Observation of magnetoelectric behavior at room temperature in Pb(FexTi1−x)O3

The co-existence of ferroelectric and ferromagnetic properties at room temperature is very rarely observed. We have been successful in converting ferroelectric PbTiO₃ into a magnetoelectric material by partly substituting Fe at the Ti site. The Pb(Fe_xTi_1−x)O₃ system exhibits ferroelectric and ferromagnetic ordering at room temperature. Even more remarkably, our results demonstrate a coupling between the two order parameters. Hence it could be a futuristic material to provide cost effective and simple path for designing novel electromagnetic devices.     

Superconductivity and magnetism of La2Cu1−xZnxO4+δ chemically oxidized by NaClO

The structural, superconducting and magnetic properties of La₂Cu_1−xZn_xO_4+δ (0≤x≤0.1) chemically oxidized by NaClO at room temperature were studied. All the samples before and after oxidation are single phase with orthorhombic structure, as indicated by their powder X-ray diffraction analysis. The iodometric titration results indicate that Zn-substituted La₂Cu_1−xZn_xO₄ is more favorable for the insertion of the excess oxygen, as compared to the Zn-free La₂CuO₄. The T_c suppression rate resulting from Zn substitution in La₂Cu_1−xZn_xO_4+δ is −12.4 K/%. The effective magnetic moment induced by the non-magnetic Zn ion is the order of one Bohr magneton, which decreases with increasing the Zn concentrations in the range examined. The latter two results are qualitatively well consistent with those obtained in La_2−xSr_xCu_1−yZn_yO₄ with the Sr optimal doping. This reveals that the non-magnetic Zn ions play the same role in both of the La₂Cu_1−xZn_xO_4+δ with the excess oxygen content of about 0.1 and the La_2−xSr_xCu_1−yZn_yO₄ with the Sr optimal doping.     

Refractive index and dielectric constants of GaxIn1−xP: Disorder effect

A pseudopotential scheme, which incorporates compositional disorder as an effective potential, is used so as to calculate the optical properties of Ga_xIn_1−xP ternary alloys in the zinc-blende structure. Generally, the agreement between our results and the existing data in the literature is reasonable. The composition dependence of the studied features showed a non-monotonic behaviour for most studied features. Reasons for these findings are discussed. Moreover, the effect of the compositional disorder on the features of interest has been examined and found to be important for the calculation of optical properties of Ga_xIn_1−xP.