Electronic and Optical Modeling of Solar Cell Compounds CuGaSe<Subscript>2</Subscript> and CuInSe<Subscript>2</Subscript>

We present dielectric-function-related optical properties such as absorption coefficient, refractive index, and reflectivity of the semiconducting chalcopyrites CuGaSe₂ and CuInSe₂. The optical properties were calculated in the framework of density functional theory (DFT) using linear combination of atomic orbitals (LCAO) and full-potential linearized augmented plane wave (FP-LAPW) methods. The calculated spectral dependence of complex dielectric functions is interpreted in terms of interband transitions within energy bands of both chalcopyrites; for example, the lowest energy peak in the e₂ (w) varepsilon_{2} (omega ) spectra for CuGaSe₂ corresponds to interband transitions from Ga/Se-4p → Ga-4s while that for CuInSe₂ emerges as due to transition between Se-4p → In-5s bands. The calculated dielectric constant, e₁ (0) varepsilon_{1} (0) , for CuInSe₂ is higher than that of CuGaSe₂. The electronic structure of both compounds is reasonably interpreted by the LCAO (DFT) method. The optical properties computed using the FP-LAPW model (with scissor correction) are close to the spectroscopic ellipsometry data available in the literature.     

Ultra-Low Loss Microwave Dielectric Ceramic Li<Subscript>2</Subscript>Mg<Subscript>2</Subscript>TiO<Subscript>5</Subscript> and Low-Temperature Firing Via B<Subscript>2</Subscript>O<Subscript>3</Subscript> Addition

Li₂Mg₂TiO₅, a rock-salt structured ceramic fabricated by a solid-state sintering technique, was characterized at the microwave frequency band. As a result, a microwave dielectric permittivity (ε_r) of 13.4, a quality factor of 95,000 GHz (at 11.3 GHz), and a temperature coefficient of resonance frequency (τ_f) of ? 32.5 ppm/°C have been obtained at 1320°C. Li₂Mg₂TiO₅ ceramics have low permittivity, a broad processing temperature region, and a low loss, making them potential applications in millimeter-wave devices. Furthermore, B₂O₃ addition efficiently lowered the sintering temperature of Li₂Mg₂TiO₅ to 900°C, which opens up their possible applications in low-temperature co-fired ceramics (LTCC) technology.     

Optical properties of imperfect In<Subscript>2</Subscript>Se<Subscript>3</Subscript>

Spectra of complete sets of optical functions for α-and β-In₂Se₃ in the range of 0–20 eV were calculated using experimental reflection spectra and the Kramers-Kronig relation. Special features in the spectra of optical functions for both In₂Se₃ phases were analyzed. The spectra of both permittivity and characteristic electron energy losses were decomposed into elementary transverse and longitudinal components using the combined Argand diagrams. The main parameters of the electron transitions for these components were determined. The structure of the components was compared with the structure of the expected spectrum of interband transitions.     

Optical properties of In<Subscript>2</Subscript>Se<Subscript>3</Subscript> thin films

In₂Se₃ films are produced by ion-beam evaporation at substrate temperatures of 313 and 623 K. As the target, In₂Se₃ single crystals grown by the vertical Bridgman method are used. The composition and structure of the crystals and films are determined by the X-ray spectral analysis and X-ray diffraction techniques, respectively. It is established that the crystals and films crystallize with the formation of a hexagonal structure. The band gap and refractive index of the In₂Se₃ films are determined from the transmittance and reflectance spectra. It is found that, as the substrate temperature is increased, the band gap increases.     

Optical Properties of K<Subscript>2</Subscript>O-Li<Subscript>2</Subscript>O-WO<Subscript>3</Subscript>-B<Subscript>2</Subscript>O<Subscript>3</Subscript> Glasses: Evidence of Mixed Alkali Effect

Glass with compositions xK₂O-(30 ? x)Li₂O-10WO₃-60B₂O₃ for 0 ≤ x ≤ 30 mol.% have been prepared using the normal melt quenching technique. The optical reflection and absorption spectra were recorded at room temperature in the wavelength range 300–800 nm. From the absorption edge studies, the values of the optical band gap (E _opt) and Urbach energy (ΔE) have been evaluated. The values of E _opt and ΔE vary non-linearly with composition parameter, showing the mixed alkali effect. The dispersion of the refractive index is discussed in terms of the single oscillator Wemple Di-Domenico model.     

Optical and dielectric characteristics of the rare-earth metal oxide Lu<Subscript>2</Subscript>O<Subscript>3</Subscript>

The characteristics of the Lu₂O₃ oxide and their variations controlled by compositional defects are studied. The defects are anion vacancies produced on partial reduction of the oxide. Such defects exhibit features typical of quantum objects and have a profound effect on the optical transmittance spectrum, the character of conduction (insulator or semiconductor properties) and the order of magnitude of the permittivity ɛ (capable of varying from 11.2 to 125). The structural features of vacancies in the oxides are considered, and the effect of vacancies on the polarization, conductivity, and lattice vibrations is studied. The studies are carried out in the temperature range 200–900 K, the wavelength range 0.03–50 μm, and the current frequency range 10²–10⁵ Hz. The rare-earth metal oxides attract interest for applications in microelectronics due to their high permittivity (several times higher than the permittivity of SiO₂) and, hence, the prospects for use of these oxides instead of SiO₂.     

Magnetic susceptibility of Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript>-Sb<Subscript>2</Subscript>Te<Subscript>3</Subscript> alloys

The magnetic susceptibility of Czochralski-grown single crystals of Bi₂Te₃-Sb₂Te₃ alloys containing 0, 10, 25, 40, 50, 60, 65, 70, 80, 90, 99.5, or 100 mol % Sb₂Te₃ has been investigated. The magnetic susceptibility of these crystals was determined at the temperature T = 291 K and the magnetic field H oriented parallel (χ_‖) and perpendicularly (χ_⊥) to the trigonal crystallographic axis C ₃. A complicated concentration dependence of the anisotropy of magnetic susceptibility χ_‖/χ_⊥ has been revealed. The crystals with the free carrier concentration p ≈ 5 × 10¹⁹ cm^?3 do not exhibit anisotropy of magnetic susceptibility. The transition to the isotropic magnetic state occurs for the compositions characterized by a significantly increased (from 200 to 300 meV) optical bandgap.     

Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript>-Sb<Subscript>2</Subscript>Te<Subscript>3</Subscript> Superlattices Grown by Nanoalloying

In this work, Bi₂Te₃-Sb₂Te₃ superlattices were prepared by the nanoalloying approach. Very thin layers of Bi, Sb, and Te were deposited on cold substrates, rebuilding the crystal structure of V₂VI₃ compounds. Nanoalloyed super- lattices consisting of alternating Bi₂Te₃ and Sb₂Te₃ layers were grown with a thickness of 9 nm for the individual layers. The as-grown layers were annealed under different conditions to optimize the thermoelectric parameters. The obtained layers were investigated in their as-grown and annealed states using x-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive x-ray (EDX) spectroscopy, transmission electron microscopy (TEM), and electrical measurements. A lower limit of the elemental layer thickness was found to have c-orientation. Pure nanoalloyed Sb₂Te₃ layers were p-type as expected; however, it was impossible to synthesize p-type Bi₂Te₃ layers. Hence the Bi₂Te₃-Sb₂Te₃ superlattices consisting of alternating n- and p-type layers showed poor thermoelectric properties.     

Nanotribological Characteristics of Cu<Subscript>6</Subscript>Sn<Subscript>5</Subscript>, Cu<Subscript>3</Subscript>Sn,and Ni<Subscript>3</Subscript>Sn<Subscript>4</Subscript> Intermetallic Compounds

Nanotribological characteristics, including the coefficient of friction, wear coefficient, and wear resistance, of Cu₆Sn₅, Cu₃Sn, and Ni₃Sn₄ intermetallic compounds developed by the annealing of Sn–Cu or Sn–Ni diffusion couples were investigated in this work. The scratch test conditions combined a constant normal load of 10 mN, 20 mN, or 30 mN and a scratch rate of 0.1 μm/s, 1 μm/s, or 10 μm/s. Experimental results indicated that, as the normal load increases, the pile-up grows taller and the scratch deepens, leading to a greater coefficient of friction and wear coefficient, and reduced wear resistance. Moreover, the scratch rate does not have a significant effect on the nanotribological characteristics except for those of Cu₆Sn₅ and Cu₃Sn under a normal load of 10 mN. Though the hardness of Cu₆Sn₅, Cu₃Sn, and Ni₃Sn₄ is similar, Ni₃Sn₄ appears to be more prone to wear damage.     

Density-Functional Study of the Electronic Structure and Optical Properties of Transparent Conducting Oxides In<Subscript>4</Subscript>Sn<Subscript>3</Subscript>O<Subscript>12</Subscript> and In<Subscript>4</Subscript>Ge<Subscript>3</Subscript>O<Subscript>12</Subscript>

The electronic structure and optical properties of In₄Sn₃O₁₂ and In₄Ge₃O₁₂ are studied by the projector-augmented-wave method based on the density-functional theory within the generalized gradient approximation. The cation ordering of the two compounds is explored by means of first-principles calculations. It is found that the valence-band maximum of the materials is determined by the d states of metal elements and O-2p states; the conduction-band minimum is occupied by an admixture of the O-2p states, In-5s states, and Sn-5s or Ge-4s states, respectively. The two compounds are direct-bandgap semiconductors. The low intensity of the absorption coefficient, reflectivity, and loss function shows that they are good transparent conducting oxides.     

Influence of Element Substitution on Corrosion Behavior of Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript>-Based Compounds

Atmospheric water may condense on the surface of Bi₂Te₃-based compounds constituting the Peltier module, depending on the operating environment used. In the stage of disposal, Bi₂Te₃-based compounds may come into contact with water in waste disposal sites. There are very few publications about the influence of condensed water on Peltier modules. Bi₂Te₃-Sb₂Te₃ or Bi₂Te₃-Bi₂Se₃ pseudo binary system compounds are used as p-type material or n-type material, respectively. The lattice distortion will be induced in the crystal of Bi₂Te₃-based compounds by element substitution due to the reduction in their thermal conductivity. However, the influence of element substitution on the corrosion behavior of Bi₂Te₃-based compounds remains unclear. In this study, the influence of element substitution on the corrosion behavior of Bi₂Te₃-based compounds with practical compositions has been investigated. Bi_0.5Sb_1.5Te₃ or Bi₂Te_2.85Se_0.15 was prepared by the vertical Bridgman method. The electrochemical properties at room temperature were evaluated by cyclic voltammetry in a standard three-electrode cell. The working electrolyte was a naturally aerated 0.6 or 3.0 mass% NaCl solution. From the tendency for corrosion potential for all the samples, the corrosion sensitivity of ternary compounds was slightly higher than that of binary compounds. From the trend of current density, it was found that Bi_0.5Sb_1.5Te₃ had a corrosion resistance intermediate between Bi₂Te₃ and Sb₂Te₃. On the other hand, corrosion resistance was affected despite a small amount of Se substitution, and the corrosion resistance of Bi₂Te_2.85Se_0.15 was close to or lower than that of Bi₂Se₃. From the observation results of the corrosion products, the trends of morphology and composition of corrosion products for Bi_0.5Sb_1.5Te₃ or Bi₂Te_2.85Se_0.15 were consistent with those of Sb₂Te₃ or Bi₂Se₃, respectively. From the results of x-ray photoelectron spectroscopy for the electrolyte after testing, the possibility that a corrosion product diffuses to the environment including the salt was suggested in Bi_0.5Sb_1.5Te₃. However, the amount of dissolved corrosion product was very low, and the chemical stability of the corrosion product was not changed or improved by element substitution.     

Conduction Mechanisms in Multiferroic Multilayer BaTiO<Subscript>3</Subscript>/NiFe<Subscript>2</Subscript>O<Subscript>4</Subscript>/BaTiO<Subscript>3</Subscript> Memristors

Memristive devices and materials are extensively studied as they offer diverse properties and applications in digital, analog and bio-inspired circuits. In this paper, we present an important class of memristors, multiferroic memristors, which are composed of multiferroic multilayer BaTiO₃/NiFe₂O₄/BaTiO₃ thin films, fabricated by a spin-coating deposition technique on platinized Si wafers. This cost-effective device shows symmetric and reproducible current–voltage characteristics for the actuating voltage amplitude of ±10 V. The origin of the conduction mechanism was investigated by measuring the electrical response in different voltage and temperature conditions. The results indicate the existence of two mechanisms: thermionic emission and Fowler–Nordheim tunnelling, which alternate with actuating voltage amplitude and operating temperature.     

Electrodeposition and Thermoelectric Characteristics of Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript> and Sb<Subscript>2</Subscript>Te<Subscript>3</Subscript> Films for Thermopile Sensor Applications

A thermopile sensor was processed on a glass substrate by electrodeposition of n-type bismuth telluride (Bi-Te) and p-type antimony telluride (Sb-Te) films. The n-type Bi-Te film electrodeposited at −50 mV in a 50 mM electrolyte with a Bi/(Bi + Te) mole ratio of 0.5 exhibited a Seebeck coefficient of −51.6 μV/K and a power factor of 7.1 × 10⁻⁴ W/K² · m. The p-type Sb-Te film electroplated at 20 mV in a 70 mM solution with an Sb/(Sb + Te) mole ratio of 0.9 exhibited a Seebeck coefficient of 52.1 μV/K and a power factor of 1.7 × 10⁻⁴ W/K² · m. A thermopile sensor composed of 196 pairs of the p-type Sb-Te and the n-type Bi-Te thin-film legs exhibited sensitivity of 7.3 mV/K.     

Electrical Transport Properties of Filled CoSb<Subscript>3</Subscript> Skutterudites: A Theoretical Study

We present an investigation of electronic structures and electrical transport properties of some filled CoSb₃ skutterudites by combining ab initio projected augmented plane-wave calculations and Boltzmann transport theory with electron group velocity evaluated by the momentum matrix method. The systems are studied in a 2 × 2 × 2 supercell of Co₄Sb₁₂ to reveal the effects induced by different filler atoms and their filling fractions. The temperature dependences of the Seebeck coefficient and power factor are studied, and they are in good agreement with experimental data. Our results reveal an optimal filling fraction for n-type filled CoSb₃ skutterudites and related compounds for achieving the highest power factor values.     

Comparative studies of p-type InP layers formed by Zn<Subscript>3</Subscript>As<Subscript>2</Subscript> and Zn<Subscript>3</Subscript>P<Subscript>2</Subscript> diffusion

The Zn₃As₂ and Zn₃P₂ were used as Zn-diffusion sources to form a p-region in undoped-InP wafers. The p-type InP formed by Zn diffusion from a Zn₃P₂ source has higher transmittance over the testing-spectrum range 1,000–1,700 nm versus Zn diffusion from a Zn₃As₂ source. In the case of a p-type region formed from a Zn₃As₂ source, x-ray photoelectron spectroscopy (XPS) showed As atoms were reduced from the oxide state and formed an InAs composition, which introduces more absorption loss.     

Effect of Vacancy Distribution on the Thermal Conductivity of Ga<Subscript>2</Subscript>Te<Subscript>3</Subscript> and Ga<Subscript>2</Subscript>Se<Subscript>3</Subscript>

Our group has focused attention on Ga₂Te₃ as a natural nanostructured thermoelectric material. Ga₂Te₃ has basically a zincblende structure, but one-third of the Ga sites are structural vacancies due to the valence mismatch between Ga and Te. It has been confirmed that (1) vacancies in Ga₂Te₃ exist as two-dimensional (2D) vacancy planes, and (2) Ga₂Te₃ exhibits an unexpectedly low thermal conductivity (κ), most likely due to highly effective phonon scattering by the 2D vacancy planes. However, the effect of the size and periodicity of the 2D vacancy planes on κ has been unclear. In addition, it has also been unclear whether only the 2D vacancy planes reduce κ or if point-type vacancies can also reduce κ. In the present study, we tried to prepare Ga₂Te₃ and Ga₂Se₃ with various vacancy distributions by controlling annealing conditions. The atomic structures of the samples were characterized by means of transmission electron microscopy, and κ was evaluated from the thermal diffusivity measured by the laser flash method. The effects of vacancy distributions on κ of Ga₂Te₃ and Ga₂Se₃ are discussed.     

First-Principles Study of Structural,Electronic, Optical,and Thermal Properties of BeSiSb<Subscript>2</Subscript> and MgSiSb<Subscript>2</Subscript>

Structural, electronic, optical, and thermal properties of ternary II–IV–V₂ (BeSiSb₂ and MgSiSb₂) chalcopyrite semiconductors have been calculated using the full-potential linearized augmented plane wave scheme?in the generalized gradient approximation. The optimized equilibrium structural parameters (a, c, and u) are in good agreement with theoretical results obtained using other methods. The band structure and density of states reveal that BeSiSb₂ has an indirect (Γ–Z) bandgap of about 0.61 eV, whereas MgSiSb₂ has a direct (Γ–Γ) bandgap of 0.80 eV. The dielectric function, refractive index, and extinction coefficient were calculated to investigate the optical properties, revealing that BeSiSb₂ and MgSiSb₂ present very weak birefringence. The temperature dependence of the volume, bulk modulus, Debye temperature, and heat capacities (C _v and C _p) was predicted using the quasiharmonic Debye model at different pressures. Significant differences in properties are observed at high pressure and high temperature. We predict that, at 300 K and 0 GPa, the heat capacity at constant volume C _v, heat capacity at constant pressure C _P, Debye temperature θ _D, and Grüneisen parameter γ will be about 94.91 J/mol K, 98.52 J/mol K, 301.30 K, and 2.11 for BeSiSb₂ and about 96.08 J/mol K, 100.47 J/mol K, 261.38 K, and 2.20 for MgSiSb₂, respectively.     

Optical and photoelectrical properties of AgCd<Subscript>2</Subscript>GaS<Subscript>4</Subscript> single-crystal compounds

The poorly known AgCd₂GaS₄ single-crystal compounds that crystallize in a rhombic structure (space group Pmn2₁) are studied. Deviations from the stoichiometric composition of the samples and random occupation of the cation sublattice sites by Ag and Ga ions result in violation of long-range order in the atomic arrangement and make the AgCd₂GaS₄ compounds structurally closer to disordered systems. In this case, it is found that the fundamental optical-absorption edge is smeared and shifted to longer wavelengths, and is adequately described by the Urbach rule. In addition, a broadening of the spectral peaks of photoconductivity and luminescence is observed. The concentration of charged point defects responsible for the smearing of the absorption edge is calculated. It is found to be 1.2×10²⁰ cm^?3. The AgCd₂GaS₄ single crystals are photosensitive semiconductors. From the position of the absorption edge, the optical band gap of the compound is estimated (E _g0 = 2.28 eV at T = 297 K). The photoluminescence spectra of the AgCd₂GaS₄ single crystals are similar to the spectra of defect-containing CdS single crystals; for the AgCd₂GaS₄ crystals, the emission peaks are shifted to longer wavelengths with respect to the peaks for CdS crystals by Δλ = 0.06?0.1 μm. From the analysis of the experimental data, some conclusions on the nature of photoactive centers in AgCd₂GaS₄ compounds are drawn.     

Influence of Group IV-Te Alloying on Nanocomposite Structure and Thermoelectric Properties of Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript> Compounds

Thermoelectric compounds based on doped bismuth telluride and its alloys have recently attracted increasing interest. Due to their structural features they show increased values of the thermoelectric figure of merit (ZT). A promising approach to improve the thermoelectric properties is to manufacture nanocomposite materials exhibiting lower thermal conductivities and higher ZT. The ZT value of compounds can be shifted reasonably to higher values (>1) by alloying with IV-Te materials and adequate preparation methods to form stable nanocomposites. The influence of PbTe and Sn on the thermoelectric properties is studied as a function of concentration and preparation methods. Melt spinning and spark plasma sintering were applied to form nanocomposite materials that were mechanically and thermodynamically stable for applications in thermoelectric devices. The structural properties are discussed based on analysis by transmission electron microscopy and x-ray diffraction.     

Effects of Cooling Rate on Thermoelectric Properties of <Emphasis Type="Italic">n</Emphasis>-Type Bi<Subscript>2</Subscript>(Se<Subscript>0.4</Subscript>Te<Subscript>0.6</Subscript>)<Subscript>3</Subscript> Compounds

A series of Bi₂(Se_0.4Te_0.6)₃ compounds were synthesized by a rapid route of melt spinning (MS) combined with a subsequent spark plasma sintering (SPS) process. Measurements of the Seebeck coefficient, electrical conductivity, and thermal conductivity were performed over the temperature range from 300 K to 520 K. The measurement results showed that the cooling rate of melt spinning had a significant impact on the transport properties of electrons and phonons, effectively enhancing the thermoelectric properties of the compounds. The maximum ZT value reached 0.93 at 460 K for the sample prepared with the highest cooling rate, and infrared spectrum measurement results showed that the compound with lower tellurium content, Bi₂(Se_0.4Te_0.6)₃, possesses a larger optical forbidden gap (E _g) compared with the traditional n-type zone-melted material with formula Bi₂(Se_0.07Te_0.93)₃. Our work provides a new approach to develop low-tellurium-bearing Bi₂Te₃-based compounds with good thermoelectric performance.