Composition and lattice mismatch dependent dielectric constants and optical phonon modes of InAs1−x−ySbxPy quaternary alloys

Based on the pseudopotential formalism under the virtual crystal approximation, the dielectric and lattice vibration properties of zinc-blende InAs_1−x−ySb_xP_y quaternary system under conditions of lattice matching and lattice mismatching to InAs substrates have been investigated. Generally, a good agreement is noticed between our results and the available experimental and theoretical data reported in the literature. The variation of all features of interest versus either the composition parameter x or the lattice mismatch percentage is found to be monotonic and almost linear. The present study provides more opportunities to get diverse high-frequency and static dielectric constants, longitudinal and transversal optical phonon modes and phonon frequency splitting by a proper choice of the composition parameters x and y (0 ⩽ x ⩽ 0.30, 0 ⩽ y ⩽ 0.69) and/or the lattice mismatch percentage.     

Investigation of defect-related optical properties in AlxInyGa1−x−yN quaternary alloys with different Al/In ratios

Al_xIn_yGa_1?x?yN quaternary alloys with different ratios of Al/In were grown by metal-organic chemical vapor deposition on GaN/Al₂O₃ substrates. The structural and emission properties of the as-grown samples were investigated, respectively, by high-resolution X-ray diffraction and photoluminescence (PL) measurements. The PL emission character is related to the two prominent quenching bands, which have been determined to be located at around 1.1 eV and 1.7 eV above the valence band, respectively, by the method of optical quenching of photoconductivity. PL emission is most intense when the Al/In ratio is 7.5 for the Al_xIn_yGa_1?x?yN layer. In addition, a stronger quenching phenomenon with Al/In ratio of 5.0 in Al_xIn_yGa_1?x?yN is observed in accordance with a reduction of the intensity of Al_xIn_yGa_1?x?yN-related emission peak.     

The effect of SixNy interlayer on the quality of GaN epitaxial layers grown on Si(1 1 1) substrates by MOCVD

In the present paper, the effects of nitridation on the quality of GaN epitaxial films grown on Si(1 1 1) substrates by metal–organic chemical vapor phase deposition (MOCVD) are discussed. A series of GaN layers were grown on Si(1 1 1) under various conditions and characterized by Nomarski microscopy (NM), atomic force microscopy (AFM), high resolution X-ray diffraction (HRXRD), and room temperature (RT) photoluminescence (PL) measurements. Firstly, we optimized LT-AlN/HT-AlN/Si(1 1 1) templates and graded AlGaN intermediate layers thicknesses. In order to prevent stress relaxation, step-graded AlGaN layers were introduced along with a crack-free GaN layer of thickness exceeding 2.2 μm. Secondly, the effect of in situ substrate nitridation and the insertion of an Si_xN_y intermediate layer on the GaN crystalline quality was investigated. Our measurements show that the nitridation position greatly influences the surface morphology and PL and XRD spectra of GaN grown atop the Si_xN_y layer. The X-ray diffraction and PL measurements results confirmed that the single-crystalline wurtzite GaN was successfully grown in samples A (without Si_xN_y layer) and B (with Si_xN_y layer on Si(1 1 1)). The resulting GaN film surfaces were flat, mirror-like, and crack-free. The full-width-at-half maximum (FWHM) of the X-ray rocking curve for (0 0 0 2) diffraction from the GaN epilayer of the sample B in ω-scan was 492 arcsec. The PL spectrum at room temperature showed that the GaN epilayer had a light emission at a wavelength of 365 nm with a FWHM of 6.6 nm (33.2 meV). In sample B, the insertion of a Si_xN_y intermediate layer significantly improved the optical and structural properties. In sample C (with Si_xN_y layer on Al_0.11Ga_0.89N interlayer). The in situ depositing of the, however, we did not obtain any improvements in the optical or structural properties.     

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.     

A magnetic study of HoMn6Sn6−xGax single crystals (0.14⩽x⩽1.89)

Single crystals of hexagonal HfFe₆Ge₆-type HoMn₆Sn_6−xGa_x compounds (0.14⩽x⩽1.89) have been obtained by a flux method and studied by magnetisation measurements. All the compounds order ferrimagnetically (308⩽T_c⩽386 K) with moments lying in the (0 0 1) plane and undergo a moment reorientation transition at lower temperatures (156⩽T_SR⩽195 K). At 5 K, the moments are aligned along an intermediate direction (44⩽φ_c⩽50°). These results are discussed and compared with the neutron diffraction results related to the isotypic TmMn₆Sn_6−xGa_x and TbMn₆Sn_6−xGa_x series where a change of the easy direction is observed with increasing gallium contents.     

Structural and electronic properties of zincblende phase of Tl x Ga1−x As y P1−y quaternary alloys: First-principles study

Using the first-principles band-structure method, we have calculated the structural and electronic properties of zincblende TlAs, TlP, GaAs and GaP compounds and their new semiconductor Tl _x Ga_1?x As _y P_1?y quaternary alloys. Structural properties of these semiconductors are obtained with the Perdew and Wang local-density approximation. The lattice constants of Tl _x Ga_1?x As, Tl _x Ga_1?x P ternary and Tl _x Ga_1?x As _y P_1?y quaternary alloys were composed by Vegard’s law. Our investigation on the effect of the doping (Thallium and Arsenic) on lattice constants and band gap shows a non-linear dependence for Tl _x Ga_1?x As _y P_1?y quaternary alloys. The band gap of Tl _x Ga_1?x As _y P_1?y , E _g (x, y) concerned by the compositions x and y. To our awareness, there is no theoretical survey on Tl _x Ga_1?x As _y P_1?y quaternary alloys and needs experimental verification.     

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.     

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.     

Ternary effects on the optical interface phonons in onion-like GaN/AlxGa1 − xN quantum dots

The interface optical phonons and its ternary effects in onion-like quantum dots are studied by using dielectric continuum model and the modified random-element isodisplacement model. The dispersion relations, the electron–phonon interactions and ternary effects on the interface optical phonons are calculated in the GaN/Al_xGa_1 − xN onion-like quantum dots. The results show that aluminium concentration has important influence on the interface optical phonons and electron–phonon interactions in GaN/Al_xGa_1 − xN onion-like quantum dots. The frequencies of interface optical phonons and electron–phonon coupling strengths change linearly with increase of aluminium concentration in high frequency range, and do not change linearly with increasing aluminium concentration in low frequency range.     

Exciton binding energies in GaN/AlxGa1 − xN pseudomorphic quantum wells

Interband transitions of pseudomorphic GaN/Al_xGa_1 − xN quantum wells are analysed theoretically with respect to the piezoelectric field utilizing a 6  ×  6 Rashba–Sheka–Pikus (RSP) Hamiltonian. Band structure modifications due to the built-in Stark effect explain a shift of the emission peak in GaN/Al_0.15Ga_0.85N of up to 400 meV. Quantum well exciton binding energies are calculated by the variational method and are discussed in terms of spatial separation of electrons and holes by the built-in electric field, as well as the interaction between valence subbands.     

Band structure of a cylindrical GaAs/AlxGa1 − xAs superwire

We investigate the existence of a band structure in GaAs/Al_xGa_1 − xsuperlattices with cylindrical symmetry, namely GaAs/Al_xGa_1 − xAs cylindrical superwires. These systems consists of a large number of concentric GaAs and Al_xGa_1 − xAs alternate cylindrical shells around a central GaAs cylindrical wire. Despite the radial configuration (that breaks the translational symmetry) and the electron confinement in the central three-dimensional well, a band structure can emerge depending on the number and thickness of the cylindrical shells.     

Determination of the critical indium composition corresponding to the metal–insulator transition in InxGa1−xN (0.06 ⩽ x ⩽ 0.135) layers

The low-temperature conductivity of In_xGa_1−xN alloys (0.06 ⩽ x ⩽ 0.135) is analyzed as a function of indium composition (x). Although our In_xGa_1−xN alloys were on the metallic side of the metal–insulator transition, neither the Kubo-Greenwood nor Born approach were able to describe the transport properties of the In_xGa_1−xN alloys. In addition, all of the In_xGa_1−xN alloys took place below the Ioeffe–Regel regime with their low conductivities. The observed behavior is discussed in the framework of the scaling theory. With decreasing indium composition, a decrease in thermal activation energy is observed. For the metal–insulator transition, the critical indium composition is obtained as x_c = 0.0543 for In_xGa_1−xN alloys.     

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.     

Correlation between Meyer–Neldel rule and phase separation in Se98−xZn2Inx chalcogenide glasses

The present work reports the observation of Meyer–Neldel rule for the thermally activated crystallization of glassy Se_98−xZn₂In_x (0 ⩽ x ⩽ 10) alloys. We have observed a strong co-relation between the pre-exponential factor K₀ of rate constant K(T) of crystallization and activation energy of crystallization E_c in the present case. This indicates the presence of compensation effect for the non-isothermal crystallization process in the present glassy system, which is explained in terms of phase separation of the present alloys due to flaw bonds of these amorphous solids.     

Band overlap via chemical pressure control in double perovskite (Sr2−xCax)FeMoO6 (0 ⩽ x ⩽ 2.0) with TMR effect

The chemical pressure control in (Sr_2−xCa_x)FeMoO₆ (0 ⩽ x ⩽ 2.0) with double perovskite structure has been investigated systematically. We have performed first-principles total energy and electronic structure calculations for x = 0 and x = 2.0. The increasing Ca content in (Sr_2−xCa_x)FeMoO₆ samples increases the magnetic moment close to the theoretical value due to reduction of Fe/Mo anti-site disorder. An increasing Ca content results in increasing (Fe²⁺ + Mo⁶⁺)/(Fe³⁺ + Mo⁵⁺) band overlap rather than bandwidth changes. This is explained from simple ionic size arguments and is supported by X-ray absorption near edge structure (XANES) spectra and band structure calculations.     

The effect of In/N ratio on the optical quality and lasing threshold in GaxIn1 − xAs1 − yNy/GaAs laser structures

We present a review of published work concerning the effect of In and N compositions on the operation wavelength, optical quality and lasing threshold in Ga_xIn_1 − xAs_1 − yN_y/GaAs QW and double heterostructure lasers. We show that the emission wavelength in the range between 1.0 and 1.4 μ m can be obtained for a wide range of In and/or N concentrations. However, in most Fabry–Perot lasers and vertical cavity surface emitting lasers (VCSELs) reported in the literature, the threshold current density plotted as a function of the relative In/N composition (R =  (1  − x) / y) indicate a broad minima for 40  < R <  70, suggesting an optimum relative composition. We also present the results of our studies concerning the optical quality of Ga_xIn_1 − xAs_1 − yN_y/GaAs single quantum wells for R =  15. We show that the optical quality of GaInAsN can be improved while achieving a red shift in the PL spectra. This is unlike the results obtained by rapid thermal annealing or conventional annealing, which are widely employed as post-growth treatment techniques, where any increase in the PL intensity is almost always accompanied by an undesired blue shift.     

Structural,optical and electrical properties of undoped and Si-doped AlxGa1−xN thin films on Si (1 1 1) substrate grown by PA-MBE

We have studied the structural properties of undoped and Si-doped Al_xGa_1?xN/GaN/AlN on Si (1 1 1) substrate prepared by plasma-assisted molecular beam epitaxy (PA-MBE) using high-resolution X-ray diffraction (HR-XRD) and atomic force microscopy (AFM). In comparison with undoped AlGaN, the roughness and dislocation density on the surface of the AlGaN layer decrease with Si doping. Full width half maximum (FWHM) of the undoped and Si-doped samples were equal to 0.69° and 0.52°, respectively. This indicates that the Si doping improves the crystalline quality of the Al_xGa_1?xN layer compared with the undoped one. Raman scattering measurement reveals that the optical phonon modes of A₁(LO) and E₂(H) of the AlGaN show a one-mode and two-modes behavior, respectively. The Fourier-transform infrared reflectance (FTIR) investigation confirms the one-mode (two-mode) behavior of the LO (TO) phonon in our samples. This is in good agreement with Raman measurement. Finally, the barrier height (Φ_B) of undoped and Si-doped Al_xGa_1?xN samples was found to be 0.86 and 0.74 eV, respectively.     

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.     

Theoretical study of optoelectronic properties of GaAs1−xBix alloys using valence band anticrossing model

The (12 × 12) and (14 × 14) valence band anticrossing (V-BAC) models were applied to calculate the electronic band structure of GaAs_1−xBi_x dilute alloys along Δ-, Λ- and Σ-directions at room temperature. A comparative study based on these models was performed in terms of energy levels, optical transitions, spin–orbit splitting and effective mass. We found a significant reduction of the band-gap energy E_g by roughly 81 meV/%Bi accompanied by an increase in the spin–orbit splitting Δ_so+ by about 56 meV/%Bi. Furthermore, Δ_so+ does come into resonance with E_g at ∼12%Bi for resonance energy equal to 0.73 eV. An excellent agreement has occurred between the (14 × 14) V-BAC model predictions and experimental results reported in the literature. In addition, we have investigated the Bi composition and k-directions dependence of the effective mass at Γ point. A slight increase of the holes effective mass with x can affect the holes transport properties of GaAsBi. The intrinsic carrier density increases with both x and the temperature T, but it remains below 10¹⁰ cm⁻³ for x ⩽ 5% and T ⩽ 300 K.