Computation of the electronic structure and direct-gap absorption spectra in Ge-rich Si<Subscript>1−x</Subscript> Ge<Subscript>x</Subscript>/Ge/Si<Subscript>1−x</Subscript>Ge<Subscript>x</Subscript> type-I quantum wells

This paper presents the results of conduction band discontinuities calculation for strained/relaxed Si_1?x Ge _x /Si_1?y Ge _y heterointerfaces in Γ _15C , Γ _2′C and L upper bands minima, as well as the room-temperature strained (vs. relaxed) band gaps deduced from the classical model-solid theory. Based upon the obtained data, we propose a type-I W-like Si_1?y Ge _y /Si_1?x Ge _x /Ge/Si_1?x Ge _x /Si_1?y Ge _y quantum wells heterostructure optimized in terms of compositions and thicknesses. Electronic states and wave functions are found by solving Schrödinger equation without and under applied bias voltage. An accurate investigation of the optical properties of this heterostructure is done by calculating the energies of the interband transitions and their oscillator strengths. Moreover, a detailed computation of the bias-voltage evolution of the absorption spectra is presented. These calculations prove the existence of type-I band alignment at Γ _2′C point in compressively strained Ge quantum wells grown on relaxed Ge-rich Si_1?y Ge _y buffers. The strong absorption coefficient (> 8 × 10³ cm^-1) and the large Stark effect (0.1 eV @ 2 V) of the Γ _2′C transitions thresholds open up perspectives for application of these heterostructures for near-infrared optical modulators.     

Phase transitions in nonequilibrium electron-hole systems of Si/SiGe/Si nanoheterostructures

The exciton condensation in a Si_1?x Ge _x solid solution layer of Si/Si_1?x Ge _x /Si heterostructures with the formation of electron-hole liquid has been investigated by low-temperature photoluminescence spectroscopy. In the temperature range above the critical temperature of the transition from an exciton gas to electron-hole liquid, a Mott transition from an exciton gas to electron-hole plasma has been found and investigated.     

Substitution effects on the bulk and surface properties of (Li,Ni)Mn<Subscript>2</Subscript>O<Subscript>4</Subscript>

Manganese oxides of spinel structure, LiMn₂O₄, Li_1-x Ni _x Mn₂O₄ (0.25 ≤ x≤ 0.75), and NiMn₂O₄, were studied by EDS, XRD, SEM, magnetic (M-H, M-T), and XPS measurements. The samples were synthesized by an ultrasound-assisted sol-gel method. EDS analysis showed good agreement with the formulations of the oxides. XRD and Rietveld refinement of X-ray data indicate that all samples crystallize in the Fd3m space group characteristic of the cubic spinel structure. The a-cell parameter ranges from a = 8.2276 Å (x = 0) to a = 8.3980 Å (x = 1). SEM results showed particle agglomerates ranging in size from 2.3 μm (x = 0) down to 0.8 μm (x = 1). Hysteresis magnetization vs. applied field curves in the 5–300K range was recorded. ZFC-FC measurements indicate the presence of two magnetic paramagnetic-ferrimagnetic transitions. The experimental Curie constant was found to vary from 5 to 7.1 cm³ K mol^?1 for the range of compositions studied (0 ≤ x ≤ 1). XPS studies of these oxides revealed the presence of Ni²⁺, Mn³⁺, and Mn⁴⁺. The experimental Ni/Mn atomic ratios obtained by XPS were in good agreement with the nominal values. A linear relationship of the average oxidation state of Mn with Ni content was observed. The oxide’s cation distributions as a function of Ni content from x = 0 ?Li⁺[Mn³⁺Mn⁴⁺]O₄ to x = 1 \( {\mathrm{Ni}}_{0.35}^{2+}{\mathrm{Mn}}_{0.65}^{3+}\left[{\mathrm{Ni}}_{0.65}^{2+}\right.\left.{\mathrm{Mn}}_{1.35}^{3+}\right]{\mathrm{O}}_4 \) were proposed.     

Electrospun Dy-doped SrFe<Subscript>12</Subscript>O<Subscript>19</Subscript> nanofibers: microstructure and magnetic properties

SrDy _x Fe_12?x O₁₉ (x ≤ 0.08) nanofibers have been synthesized by the electrospinning method followed by calcinations process. The partial substitution of rare earth ions Dy³⁺ (10.5 μ _B of magnetic moments) mainly occupying 12k sublattice sites in the SrFe₁₂O₁₉ crystal structure is investigated and discussed in this work. An enhanced coercivity of 7155 Oe has been obtained when the doped content reached to 0.08 at a relative low calcination temperature of 800 °C. As a result, we believe the synthesized SrDy _x Fe_12?x O₁₉ nanofibers can potentially be useful in high-density recording media as well as permanent magnets.     

Effect of the composition of the solid solution on the high-temperature microhardness of SiGe heteroepitaxial layers grown on Ge and Si substrates

The effect of the composition of epitaxial layers (ELs) of the Si_xGe_1?x solid solution grown on Ge and Si substrates on their microhardness and the length of dislocation rosettes forming around indentations is studied at a homologous temperature 0.5T_melt for each composition. For the Si_xGe_1?x/Ge (0≤x<0.15) and Si_xGe_1?x/Si (0.85<x≤1) ELs, the dependences of the microhardness and the length of dislocation rosettes on the solid-solution composition are nonmonotonic. The nonmonotonic change in the plasticity of the ELs is most likely caused by hardening of the solid solutions in a certain composition range due to their spinodal decomposition with the formation of clusters and disperse precipitates.     

Analysis of the gain and luminescence properties of Si/Si<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>G<Subscript>x</Subscript>: Er/Si heterostructures produced by sublimation molecular-beam epitaxy in a gas phase

Si/Si_1?xGe_x: Er/Si structures grown by sublimation molecular-beam epitaxy (SMBE) in a gas phase are studied. These structures are considered possible structures for realizing a Si/Er-based laser. It is shown that SMBE in a gas phase can be applied to create effective light-emitting structures that generate an intense luminescence signal at a wavelength of 1.54 μm. The structures and chemical compositions of the Si/Si_1?xG_x: Er/Si structures, whose parameters are close to those calculated for creating laser-type structures, are examined, and their photoluminescence (PL) spectra and kinetics are studied at 4.2 and 77 K. It is shown that the fraction of Er³⁺ optically active centers in the Si_1?xG_x: Er layers thus grown reaches ～10% of the total erbium-impurity concentration. The optical gains in the active Si_1?xG_x: Er layers at x = 0.1 and 0.02 are estimated to be ～0.03 and ～0.2 cm^?1, respectively. The gain in structures of this type can be significantly increased via the intentional formation of isolated Er³⁺ optically active centers whose PL spectra have a characteristic fine structure.     

The electronic and optical properties of InGaN-based solar cells alloys: First-principles investigations via mBJLDA approach

First-principles calculations of the electronic and optical properties of the bulkIn _x Ga_1 ? x N alloys aresimulated within the framework of full-potential linearized augmented plane-wave (FP-LAPW)method. To this end, a sufficiently adequate approach, namely modified Becke-Johnson(mBJLDA) exchange correlation potential is employed for calculating the energy band gapand optical absorption of InGaN-based solar cells systems. The quantities such as theenergy gap, density of states, imaginary part of dielectric function, refractive index andabsorption coefficient are determined for the bulkIn _x Ga_1?x N alloys, in thecomposition range from x = 0 to x = 1. It is found thatthe indium composition robustly controls the variation of band gap. From the examinationof the density of states and optical absorption ofIn _x Ga_1?x N ternary alloys,the energy gaps are significantly reduced for largest In concentration. The computed bandgaps vary nonlinearly with the composition x. It is also surmised thatthe significant variation in the band gaps elaborated via the experimental crystallinegrowth process, is originated by altering the In composition. Interestingly, it isworthwhile to perform InGaN solar cells alloys with improved efficiencies, because oftheir entire energy gap variation from 0.7 to 3.3 eV.     

Si<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Ge<Subscript>x</Subscript>/Si(001) relaxed buffer layers grown by chemical vapor deposition at atmospheric pressure

Relaxed step-graded buffer layers of Si_1?xGe_x/Si(001) heterostructures with a low density of threading dislocations are grown through chemical vapor deposition at atmospheric pressure. The surface of the Si_1?xGe_x/Si(001) (x ～ 25%) buffer layers is subjected to chemical mechanical polishing. As a result, the surface roughness of the layers is decreased to values comparable to the surface roughness of the Si(001) initial substrates. It is demonstrated that Si_1?xGe_x/Si(001) buffer layers with a low density of threading dislocations and a small surface roughness can be used as artificial substrates for growing SiGe/Si heterostructures of different types through molecular-beam epitaxy.     

Influence of Si substitution on structural,electronic and magnetic properties of Fe<Subscript>2</Subscript>MnGa Heusler compound

We investigate the electronic and magnetic properties of Fe₂MnGa_1?x Si _x alloy (x = 0, 0.25, 0.5, 0.75, and 1) using first-principles density functional theory within the generalized gradient approximation method. The lattice constant decreases linearly whereas bulk modulus increases with increasing Si content. The total magnetic moment varies linearly with increasing Si content, which follows the Slater-Pauling rule. Electronic band structure calculations indicate that the Fe₂MnGa_1?x Si _x exhibits half-metallic character for all the concentrations studied and the spin polarization and the spin-down band gap both increase with the Si content. Based on the magnetic properties calculations, the Heisenberg exchange coupling parameters give Fe-Mn ferromagnetic coupling and Mn-Mn antiferromagnetic coupling. The T _C first decreases and then increases with Si content, which is in well agreement with the experimental results.     

X-ray reflectivity and photoelectron spectroscopy of superlattices with silicon nanocrystals

The structural properties and features of the chemical composition of SiO _x N _y /SiO₂, SiO _x N _y /Si₃N₄, and SiN _x /Si₃N₄ multilayer thin films with ultrathin (1–1.5 nm) barrier SiO₂ or Si₃N₄ layers are studied. The films have been prepared by plasma chemical vapor deposition and have been annealed at a temperature of 1150°С for the formation of silicon nanocrystals in the SiO _x N _y or SiN _x silicon-rich layers with a nominal thickness of 5 nm. The period of superlattices in the studied samples has been estimated by X-ray reflectivity. The phase composition of superlattices has been studied by X-ray electron spectroscopy using the decomposition of photoelectron spectra of the Si 2p, N 1s, and O 1s levels into components corresponding to different charge states of atoms.     

Effect of a Low-Temperature Si Underlayer on the Photoluminescence of Misfit Dislocations in Si(001)Si<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Ge<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> Heterostructures with Ge-Rich Alloy Layers

The effect a low-temperature Si underlayer has on the photoluminescence of misfit dislocations in Si(001)Si_{1 – x}Ge _x heterostructures with high Ge contents in a SiGe alloy layer is investigated. It is found that the introduction of this layer prevents the formation of dislocations in the alloy layer.     

Solid state synthesis of nonstoichiometric Bi<Subscript>2</Subscript>WO<Subscript>6</Subscript>/Bi<Subscript>2</Subscript>O<Subscript>3</Subscript> composites as visible-light photocatalyst

Nonstoichiometric Bi₂WO₆ photocatalyst with the composition of Bi_2?+?x WO_6?+?1.5x (?0.25 ≤ x ≤ 1) wa synthesized by a facile solid state reaction method. The products were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and UV-vis absorption spectrum. The Bi_2.5WO_6.75 photocatalyst showed excellent visible-light-driven photocatalytic performance; nearly 100 % of RhB (10 ppm, pH?=?3?~?4) was decomposed within 25 min, which demonstrated that nonstoichiometric semiconductors could be an efficient visible-light-driven photocatalyst.     

Intersubband cyclotron resonance of holes in strained Ge/GeSi(111) heterostructures with germanium wide quantum wells and cyclotron resonance of 1<Emphasis Type="Italic">L</Emphasis> electrons in GeSi layers

The submillimeter (?ω=0.5–5 meV) magnetoabsorption spectra of strained Ge/Ge_1?xSi_x(111) multilayer heterostructures with thick Ge layers (d_Ge=300–850 Å, d_GeSi≈200 Å, x≈0.1) are investigated at T=4.2 K upon band-gap optical excitation. It is revealed that the absorption spectra contain cyclotron resonance lines of 1L electrons localized in GeSi solid solution layers (unlike the previously studied structures with thin Ge layers as quantum wells for 3L electrons). The absorption spectra of the samples with thick Ge layers (d_Ge=800–850 Å) exhibit cyclotron resonance lines of holes due to transitions from the lower Landau levels in the first quantum-well subband to the Landau levels belonging to the third and fifth higher subbands.     

Reciprocal Time Relation of Noncolliding Brownian Motion with Drift

We consider an N-particle system of noncolliding Brownian motion starting from x ₁≤x ₂≤…≤x _N with drift coefficients ν _j , 1≤j≤N satisfying ν ₁≤ν ₂≤…≤ν _N . When all of the initial points are degenerated to be zero, x _j =0, 1≤j≤N, the equivalence is proved between a dilatation with factor 1/t of this drifted process and the noncolliding Brownian motion starting from ν ₁≤ν ₂≤…≤ν _N without drift observed at reciprocal time 1/t, for arbitrary t>0. Using this reciprocal time relation, we study the determinantal property of the noncolliding Brownian motion with drift having finite and infinite numbers of particles.     

Synthesis and characterization of Ce-doped Sm<Subscript>2</Subscript>CuO<Subscript>4 + <Emphasis Type="Italic">δ</Emphasis></Subscript> cathode for IT-SOFC applications

Single-phase solid solutions of Sm_2 ? x Ce _x CuO_4 + δ (0.05 ≤ x ≤ 0.20) with tetragonal structure are synthesized using acetate combustion followed by sintering at 1373 K for 10 h. X-ray powder diffraction and transmission electron microscopy studies confirmed the formation of solid solution in a single phase. Maximum conductivity (σ = 96.0 ± 0.5 S cm^?1 at 973 K) giving composition Sm_1.90Ce_0.10CuO₄ offers the minimum activation energy (E _a = 0.32 ± 0.004 eV) among all prepared compositions. Lowest cathode polarization resistance (R _p = 3.92 ± 0.07 Ω cm² at 973 K) and activation energy (E _a = 1.12 ± 0.03 eV) values across the measured temperature range are obtained for Sm_1.90Ce_0.10CuO₄. The impedance data fitted well to the Gerischer model indicates that a chemical-electrochemical-chemical-type reaction occurred at the mixed electronic-ionic conducting cathode.     

Enhanced Conductivity and High Thermal Stability of W-Doped SnO<Subscript>2</Subscript> Based on First-Principle Calculations

Tungsten (W)-doped SnO₂ is investigated by first-principle calculations, with a view to understand the effect of doping on the lattice structure, thermal stability, conductivity, and optical transparency. Due to the slight difference in ionic radius as well as high thermal and chemical compatibility between the native element and the heterogeneous dopant, the doped system changes a little with different deviations in the lattice constant from Vegard’s law, and good thermal stability is observed as the doping level reaches x = 0.125 in Sn_1-x W _x O₂ compounds. Nevertheless, the large disparities in electron configuration and electronegativity between W and Sn atoms will dramatically modify the electronic structure and charge distribution of W-doped SnO₂, leading to a remarkable enhancement of conductivity, electron excitation in the low energy region, and the consequent optical properties, while the visible transparency of Sn_1-x W _x O₂ is still preserved. Particularly, it is found that the optimal photoelectric properties of W-doped SnO₂ may be achieved at x = 0.03. These observations are consistent with the experimental results available on the structural, thermal, electronic, and optical properties of Sn_1-x W _x O₂, thus presenting a practical way of tailoring the physical behaviors of SnO₂ through the doping technique.     

A Minkowski Type Trace Inequality and Strong Subadditivity of Quantum Entropy II: Convexity and Concavity

We revisit and prove some convexity inequalities for trace functions conjectured in this paper’s antecedent. The main functional considered is

$ \Phi_{p,q} (A_1,\, A_2, \ldots, A_m) = \left({\rm Tr}\left[\left( \, {\sum\limits_{j=1}^m A_j^p } \, \right) ^{q/p} \right] \right)^{1/q} $

for m positive definite operators A _j . In our earlier paper, we only considered the case q = 1 and proved the concavity of Φ _p,1 for 0 < p ≤ 1 and the convexity for p = 2. We conjectured the convexity of Φ _p,1 for 1 < p < 2. Here we not only settle the unresolved case of joint convexity for 1 ≤ p ≤ 2, we are also able to include the parameter q ≥ 1 and still retain the convexity. Among other things this leads to a definition of an L ^q (L ^p ) norm for operators when 1 ≤ p ≤ 2 and a Minkowski inequality for operators on a tensor product of three Hilbert spaces – which leads to another proof of strong subadditivity of entropy. We also prove convexity/concavity properties of some other, related functionals.

     

Ferrimagnetic nanoparticles for self-controlled magnetic hyperthermia

Based on the Heisenberg model including single-site uniaxial anisotropy and using aGreen’s function technique we studied the influence of size and composition effects on theCurie temperature T _C , saturationmagnetization M _S and coercivityH _C of spherical nanoparticles with astructural formulaM e _1?x Zn _x Fe₂O₄,Me = Ni, Cu, Co, Mn. It is shown that for x = 0.4–0.5and d = 10–20 nm these nanoparticles have aT _C  = 315 K and are suitable for aself-controlled magnetic hyperthermia.     

Phase transformations and ordering effects in nanocrystalline carbon-containing silicon films

The phase composition, surface morphology, and crystal structure of carbon-containing Si layers obtained on silicon surface by chemical conversion are discussed. The effect of structural ordering in the grains formed in the growth figures on an epitaxial surface has been studied for the 3C-SiC/Si(111) system. The possibility of Si_{1 ? x} C _x → 3C-SiC phase transitions in a layer upon structure annealing during growth is considered.     

Representation of type I heterostructure junctionless tunnel field effect transistor for high-performance logic application

In this paper, a gate-all-around junctionless tunnel field effect transistor (JLTFET) based on heterostructure of compound and group III–V semiconductors is introduced and simulated. In order to blend the high tunneling efficiency of narrow band gap material JLTFETs and the high electron mobility of III–V JLTFETs, a type I heterostructure junctionless TFET adopting Ge–Al _x Ga_1?x As–Ge system has been optimized by numerical simulation in terms of aluminum (Al) composition. To improve device performance, we considered a nanowire structure, and it was illustrated that high-performance logic technology can be achieved by the proposed device. The optimal Al composition founded to be around 20 % (x = 0.2). The numerical simulation results demonstrate that the proposed device has low leakage current I _OFF of ~1.9 × 10^?17, I _ON of 4 µA/µm, I _ON/I _OFF current ratio of 1.7 × 10¹¹ and subthreshold swing SS of 12.6 mV/decade at the 40 nm gate length and temperature of 300 K.