An optical study of vacuum evaporated amorphous Ge<Subscript>20</Subscript>Te<Subscript>80−<Emphasis Type="Italic">x</Emphasis></Subscript>Bi<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> thin films using transmission and reflection spectra

The present paper reports the effect of Bi addition on the optical behavior (optical band gap and refractive index) of Ge₂₀Te_80?x Bi _x (where x=0, 1.5, 2.5, 5.0) glassy alloys by analyzing the transmission and reflection spectra of their thin films in the 900–2400 nm range. Films are deposited on glass substrate using a thermal evaporation technique under vacuum. Various optical parameters viz. refractive index, extinction coefficient, absorption coefficient, optical band gap, etc. are determined and the effect of Bi incorporation on these parameters is studied. The refractive index has been found to increase with increasing Bi content over the entire spectral range and this behavior is due to the increased polarizability of the larger Bi atomic radius (1.46 Å) compared to Te atomic radius (1.36 Å). Dispersion energy, E _d , average energy gap, E ₀ and static refractive index, n ₀ is calculated using Wemple–DiDomenico model. Optical band gap is estimated using Tauc’s extrapolation and is found to decrease from 0.86 to 0.73 eV with the Bi addition. This behavior of the optical band gap is interpreted in terms of the electronegativity difference of the atoms involved and the cohesive energy of the system.     

Multicomponent <Emphasis Type="Italic">n</Emphasis>-(Bi,Sb)<Subscript>2</Subscript>(Te,Se,S)<Subscript>3</Subscript> solid solutions with different atomic substitutions in the Bi and Te sublattices

The thermoelectric properties of n-Bi_{2 ? x} Sb _x Te_{3 ? y ? z} Se _y S _z solid solutions are studied in the temperature range 300–550 K. It is shown that an increase in the parameter β determining the figure-of-merit Z of the material is observed in compositions with the optimally related effective mass of the density of states m/m ₀, the carrier mobility μ₀, and the lattice thermal conductivity κ _L . Within the temperature range 300–350 K, the parameter β and the figure-of-merit Z are found to increase in solid solutions with substitutions in both bismuth telluride sublattices Bi → Sb and Te → Se, S (x = 0.16, y = z = 0.12) for optimum electron concentrations. An increase in the electron concentration and substitutions of atoms only in the tellurium sublattice bring about an increase in the β parameter and the value of Z at higher temperatures. Within the range 350–450 K, the parameters β and Z are observed to increase in a solid solution with a low content of substituted atoms in the tellurium sublattice Te → Se, S for y = z = 0.09 and, at higher temperatures up to 550 K, in compositions with tellurium substituted by selenium only, with increasing content of substituted atoms.     

Effect of intrinsic defects on the electronic structure of BN nanotubes

The effect of intrinsic defects on the electronic structure of boron-nitrogen nanotubes (5, 5) and (9, 0) is investigated by the method of linearized associated cylindrical waves. Nanotubes with extended defects of substitution N _B of a boron atom by a nitrogen atom and, vice versa, nitrogen by boron BN with an impurity concentration of 1.5 to 5% are considered. It is shown that the presence of such defects significantly affects the band structure of boron-nitrogen nanotubes. A defect band D^π(B, N) is formed in the bandgap, which sharply reduces the width of the gap. The presence of impurities also affects the valence band: the widths of s, sp, and p_π bands change and the gap between s and sp bands is partially filled. These effects may be detected experimentally by, e.g., optical and photoelectron spectroscopy.     

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.     

Effects of nonstoichiometry and ordering on the basic lattice constant of vanadium carbide VC<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript>

The effect of nonstoichiometry and ordering on the lattice constant a _B1 of the basic lattice of vanadium carbide VC _y (0.65 < y < 0.875) is studied. A change in the lattice constant of disordered carbide VC _y at the reduction of the carbon content is considered using the direction of static displacements of atoms near a vacancy. A model for the calculation of the basic lattice constant a _B1 of vanadium carbide is proposed taking into account nonstoichiometry and ordering. It is shown that the ordering of vanadium carbide VC _y with the formation of V₆C₅ and V₈C₇ superstructures results in an increase in the basic lattice constant as compared to disordered carbide.     

New boron barrelenes and tubulenes

The structure of a new class of boron nanostructures—barrelenes and tubulenes—based on a boron atomic lattice constructed by the alternating B-atomic polygons with central atoms and without them has been proposed and their properties have been described. Ab initio density functional calculations have been performed for the energy and electronic structure of the fullerene-barrelene-nanotube series based on the lowest energy fullerene B₈₀. It has been shown that the energy and band gap of a barrelene are lower than the respective quantities of the corresponding fullerene and tend to the respective values for nanotubes in the infinite limit. It has been shown that there are isomers of nanotubes of the same type that are significantly different in symmetry and electronic properties: a semiconductor (C _5v symmetry) and a metal (D _5h symmetry).     

Galvanomagnetic properties of thin films of bismuth and bismuth–antimony alloys on substrates with different thermal expansions

Temperature dependences of the galvanomagnetic properties of films of bismuth and Bi_{100 – x}Sb_x (x ≤ 12) on substrates with different temperature expansion coefficients were studied in the temperature range of 77–300 K. The block films were prepared through thermal deposition, and single-crystal Bi_{100 – x}Sb_x were grown by zone recrystallization under a coating. It was found that the temperature expansion coefficient of a substrate substantially influenced the galvanomagnetic properties of Bi and Bi_{100 – x}Sb_x films. Using the experimental data, the change in the charge-carrier concentration in the Bi and Bi_{100 – x}Sb_x films on different substrates at 77 K was estimated.     

Simulation of the structural,electronic, and magnetic properties of Fe<Subscript>3</Subscript>C<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>B<Subscript>x</Subscript> borocementites

The structural, electronic, and magnetic properties and the enthalpy of formation of iron borocementites Fe₃C_1?x B_x (x= 0, 0.25, 0.50, 0.75, 1.00) are analyzed using ab initio calculations in the framework of the electron density functional theory. It is found that the unit cell parameter a of the orthorhombic lattice increases linearly and the parameters b and c decrease as the boron concentration increases. The density of states at the Fermi level changes only slightly, and the main variations in the band structure occur in the region of the bottom of the valence bands. The magnetic moment of the iron atoms and the total magnetization and stability of the Fe₃C_1?x B_x phases increase linearly with an increase in the boron concentration.     

Low-temperature transport and ferromagnetism in GaAs-based structures with Mn

GaAs structures with implanted Mn and, additionally, with Mg for increasing the hole concentration in the implanted Mn layer are synthesized and investigated. SQUID magnetometer measurements revealed the existence of ferromagnetism in the temperature range 4.2 K ≤ T < 400 K, which is associated with the formation of the Ga_1?x Mn _x As solid solution and MnAs and Ga_1?y Mn _y clusters in the sample as a result of rapid high-temperature annealing. At temperatures from 4.2 to approximately 200 K, the anomalous Hall effect associated with additional magnetization of the sample is observed. As the temperature increases from 4.2 K, the colossal negative magnetoresistance is transformed into a positive magnetoresistance at T ≈ 35 K.     

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.     

Growth of (GaAs)<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>(ZnSe)<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> solid solution films and investigation of their structural and some photoelectric properties

Single-crystal films of the substitutional solid solution (GaAs)_{1 ? x} (ZnSe) _x (0 ≤ x ≤ 0.80) on GaAs substrates have been grown using liquid phase epitaxy. The X-ray diffraction patterns, photoluminescence spectra, and current-voltage characteristics of the n-(GaAs)-p-(GaAs)_{1 ? x} (ZnSe) _x (0 ≤ x ≤ 0.80) heterostructures prepared have been investigated. The lattice parameters of the film a _f = 5.6544 Å and the substrate a _s = 5.6465 Å have been determined, and the profile of the molecular distribution of the solid solution components has been obtained. The photoluminescence spectrum of the (GaAs)_{1 ? x} (ZnSe) _x (0 ≤ x ≤ 0.80) films exhibits a narrow peak (against the background of the broad luminescence band) with a maximum in the luminescence intensity at a photon energy of 2.67 eV due to the presence of Zn-Se bonds in the structure (ZnSe is covalently bonded to the tetrahedral lattice of the GaAs matrix). It has been shown that the direct branch of the current-voltage characteristics of the structures under investigation is described by an exponential dependence I = I ₀exp(qV/ckT) at low voltages (V > 0.3 V) and by a power-law dependence I ～ V ^α with exponents α = 4 at V = 0.4–0.8 V, α = 2 at V = 0.8–1.4 V, and α = 1.5 at V > 2 V. The experimental results have been explained in the framework of the double-injection model for the n-p-p ⁺ structure under the condition that the concentration distribution of nonequilibrium charge carriers has a minimum.     

The influence of imperfection of the crystal lattice on the electrokinetic and magnetic properties of disordered titanium monoxide

The conductivity and magnetic susceptibility of disordered titanium monoxide TiO_y (0.920≤y≤1.262) containing vacancies in titanium and oxygen sublattices are investigated. For TiO_y monoxides with an oxygen content y≤1.069, the temperature dependences of the conductivity are described by the Bloch-Grüneisen function at a Debye temperature ranging from 400 to 480 K and the temperature dependences of the magnetic susceptibility are characterized by the contribution from the Pauli paramagnetism due to conduction electrons. The behavior of the conductivity and magnetic susceptibility of TiO_y monoxides with an oxygen content y≥1.087 is characteristic of narrow-gap semiconductors with nondegenerate charge carriers governed by the Boltzmann statistics. The band gap ΔE between the valence and conduction bands of TiO_y monoxides with y≥1.087 falls in the range 0.06–0.17 eV.     

Effect of chromium and tellurium impurities on the thermoelectric parameters of galena

Concentration dependences of the Seebeck coefficient, resistivity, and thermal conductivity of thermoelectric PbS crystals with chromium (0 < x ≤ 0.01) and tellurium (0 < y ≤ 0.03) impurities are examined in the temperature region of 300–800 K. It is shown that the introduction of chromium increases the number of free electrons in PbCr _x S _1–x crystals and reduces the Seebeck coefficient. However, an increase in the concentration of tellurium in PbCr _x S _1–x–y Te _y alloys raises the Seebeck coefficient while simultaneously reducing the thermal conductivity. As a result, the thermoelectric efficiency of PbCr _x S _1–x–y Te _y crystals increases. The reasons for the observed effects are discussed.     

Electronic energy structure and x-ray spectra of wide-gap AlN and BN crystals and B<Subscript>x</Subscript>Al<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>N solid solutions

The electronic energy structure of 2H and 3C AlN and BN crystals and B_xAl_1?xN solid solutions is calculated on the basis of the local coherent potential method using the cluster version of the MT approximation and the theory of multiple scattering. The features of the electronic structure of 2H-AlN crystals are compared with x-ray K and L absorption and emission spectra of aluminum and nitrogen. An interpretation of these features is given. The concentration dependences of the width of the upper subband of the valence band and the band gap in B_xAl_1?xN solid solutions (x = 0.25, 0.5, 0.75) are investigated. Charge transfer from aluminum to nitrogen atoms is shown to occur and increase with boron doping in both crystallographic modifications.     

The effect of concentration of <Emphasis Type="Bold">H</Emphasis> <Subscript> <Emphasis Type="Bold">2</Emphasis> </Subscript> physisorption on the current–voltage characteristic of armchair BN nanotubes in CNT–BNNT–CNT set

In this research, we have studied physisorption of hydrogen molecules on armchair boron nitride (BN) nanotube (3,3) using density functional methods and its effect on the current–voltage (I–V) characteristic of the nanotube as a function of concentration using Green’s function techniques. The adsorption geometries and energies, charge transfer and electron transport are calculated. It is found that H₂ physisorption can suppress the I–V characteristic of the BN nanotube, but it has no effect on the band gap of the nanotube. As the H₂ concentration increases, under the same applied bias voltage, the current through the BN nanotube first increases and then begins to decline. The current–voltage characteristic indicates that H₂ molecules can be detected by a BN-based sensor.     

Fragmentation of bands and Fermi surfaces in cuprate stripe phases

The band structure and evolution of the Fermi surfaces of stripe phases were studied using the t-t′-U Hubbard model in the mean field approximation. The appearance of quasi-one-dimensional “impurity” subbands caused by the localization of particles on domain walls inside the Hubbard gap is confirmed. Among vertical stripe phases parallel to y bonds, the Y₈ and Y₄ structures with distances l = 8a and 4a between domain walls were found to be stable. Fermi surface segments in antinodal or nodal directions were shown to correspond to an “ impurity” band or the main band related to the entire antiferromagnetic domain region. This is a probable explanation of the difference in the properties of ARPES spectra at different Fermi surface regions observed for La_2?xSr_xCuO₄. It was shown for the Y₈ structure that the topology of the Fermi surface changed and an isotropic pseudogap opened at the point corresponding to a p = 1/8 doping level. Attempts at relating this property to the anomalous suppression of T _c in LSCO at p = 1/8 encountered difficulties. The low dispersion of the impurity band and the wide gap separating it from the lower Hubbard band in diagonal stripe phases formed at p < 0.05 create prerequisites for the existence of the insulating state at nonzero doping.     

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.     

Lattice IR spectroscopy of Zn<Subscript>1 −<Emphasis Type="Italic">x</Emphasis></Subscript>Cd<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Se epitaxial layers grown on a GaAs substrate by molecular-beam epitaxy

Based on the results from comparison of the data on lattice reflection for Zn_{1 ?x} Cd _x Se bulk crystals (x = 0.08, 0.22) and Zn_{1 ?x} Cd _x Se thin layers of the same compositions grown on GaAs, a more adequate interpretation is given to the lattice IR reflection spectra of Zn_{1 ?x} Cd _x Se alloy films (x = 0–0.55) grown on a GaAs substrate by molecular-beam epitaxy. In this composition region, two ZnSe-and CdSe-like lattice modes are observed corresponding to a double-mode type of rearrangement of the vibration spectrum with varying x. A weak (with oscillator strength smaller than 0.25) high-frequency mode (with respect to the dominant modes) at 220 cm^?1 is also observed.     

Electronic band structure and x-ray spectra of boron nitride polytypes

The electronic band structures of boron nitride crystal modifications of the graphite (h-BN), wurtzite (w-BN), and sphalerite (c-BN) types are calculated using the local coherent potential method in the cluster muffin-tin approximation within the framework of the multiple scattering theory. The specific features of the electronic band structure of 2H, 4H, and 3C boron nitride polytypes are compared with those of experimental x-ray photoelectron, x-ray emission, and K x-ray absorption spectra of boron and nitrogen. The features of the experimental x-ray spectra of boron nitride in different crystal modifications are interpreted. It is demonstrated that the short-wavelength peak revealed in the total densities of states (TDOS) in the boron nitride polytypes under consideration can be assigned to the so-called outer collective band formed by 2p electrons of boron and nitrogen atoms. The inference is made that the decrease observed in the band gap when changing over from wurtzite and sphalerite to hexagonal boron nitride is associated with the change in the coordination number of the components, which, in turn, leads to a change in the energy location of the conduction band bottom in the crystal.     

Atomic and Electronic Structures of Metal-Rich Noncentrosymmetric ZrO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>

The atomic and electronic structures of metal-rich noncentrosymmetric zirconium oxide synthesized by the ion beam sputtering of a metallic target in an oxygen atmosphere has been studied by X-ray photoelectron spectroscopy, Raman scattering, spectral ellipsometry, and quantum-chemical simulation. It has been established that ZrO_{x < 2} consists of ZrO₂, metallic Zr, and zirconium suboxides ZrO_y. The stoichiometry parameter of ZrO_y has been estimated. It has been shown that the optical properties of ZrO_{x < 2} are determined by metallic Zr. A model of fluctuation of the width of the band gap and a potential for electrons and holes in ZrO_{x < 2} based on spatial fluctuations of the chemical composition has been proposed.