Photosensitive structures based on In<Subscript>2</Subscript>S<Subscript>3</Subscript> crystals

Crystals of the compound In₂S₃ were grown by planar crystallization of the melt. The composition, structure, and electrical characteristics of the crystals obtained were determined. Photosensitive structures based on the grown In₂S₃ crystals were fabricated for the first time; spectral dependences of photoconversion quantum efficiency for H₂O/In₂S₃ cells were measured. The features of the band-to-band absorption are discussed; energies of the direct and indirect optical transitions for In₂S₃ crystals are estimated. It is stated that In₂S₃ crystals can be used in wide-range (1.5–3.5 eV) photoconverters of nonpolarized radiation (in particular, in solar cells).     

Schottky barriers based on <Emphasis Type="Italic">n</Emphasis>-In<Subscript>2</Subscript>S<Subscript>3</Subscript> films obtained by laser-induced evaporation

The method of evaporation of starting targets with subsequent deposition onto glass substrates at temperatures of 480–720 K is used to grow homogeneous thin (0.6–1.5 μm) n-In₂S₃ films on which the In/n-In₂S₃ Schottky barriers were formed for the first time; evaporation was induced by a pulsed laser. The temperature dependence of resistivity of the films with the n-type conductivity was studied and the activation energy of donor centers in these films was determined. Spectral dependences of the photoconversion quantum efficiency η(?ω) for the barriers obtained were studied. An analysis of the spectral dependences η(?ω) made it possible to identify the type of band-to-band transitions and estimate the band gap in the In₂S₃ films. It is concluded that the thin In₂S₃ films can be used in broadband photoconverters of optical radiation.     

The influence of solution-precursors on optical properties of In<Subscript>2</Subscript>S<Subscript>3</Subscript> films deposited by successive ionic layer adsorption and reaction

The method of ion lamination from aqueous solutions of various compositions is used to form In₂S₃ films on the surface of TiO₂ and porous anodic Al₂O₃. It is established that the composition of the cationic and anionic solution-precursors affects the films’ optical band gap, which varies in the range 1.97–2.65 eV. This is accounted for in the context of variation in the content of oxygen in In₂S₃. The observed dependences of optical properties on the value of the pH for anionic precursors and on the nature of the anion in cationic precursors are in good agreement with calculated conditions for chemical equilibrium in the studied systems. The dependence of the efficiency of photoelectric conversion in solar cells with ultrathin absorbing layers on the content of oxygen in the In₂S₃ films obtained by ionic lamination is demonstrated.     

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.     

On the band gap in (In2S3) x (CuIn5S8)1 ? x alloy single crystals

For In₂S₃, CuIn₅S₈ and (In₂S₃) _x (CuIn₅S₈)_{1 ? x} alloy single crystals grown by the Bridgman method (vertical variant), the transmittance spectra in the region of the fundamental absorption-band edge are studied at 80 and 295 K. From the spectra, the band gaps of the In₂S₃ and CuIn₅S₈ compounds and the alloys based on them are determined, and the dependences of the band gap on the composition parameter x of the alloys are established. It is shown that, at 80 and 295 K, the band gap nonlinearly varies with x and the variation is described by a quadratic function.     

Temperature dependence of the band gap of FeIn<Subscript>2</Subscript>S<Subscript>4</Subscript> single crystals

Single crystals of the FeIn₂S₄ ternary compound are grown by the Bridgman method. The composition and structure of the crystals are established. The transmittance spectra of the crystals in the region of the fundamental absorption edge are studied in the temperature range of T = 20–300 K. The band gap E _g and its temperature dependence E _g (T) are determined from the transmittance spectra. It is shown that the shape of the dependence E _g (T) is typical of complex compounds.     

Thermal expansion of CuIn<Subscript>5</Subscript>S<Subscript>8</Subscript> single crystals and the temperature dependence of their band gap

Single crystals of the CuIn₅S₈ ternary compound are grown by planar crystallization of the melt (the vertical Bridgman method). The composition and structure of the crystals are established. The specific expansion is measured by the dilatometric technique, and the coefficients of thermal expansion are calculated. From the data, the Debye temperatures (Θ_D) and the root-mean-square dynamic displacements of atoms \(\left( {\sqrt {\bar u^2 } } \right)\) in the CuIn₅S₈ compound are calculated. From the transmittance spectra recorded in the region of the fundamental absorption edge in the temperature range 20 to 300 K, the band gap is determined and its temperature dependence is constructed.     

Microstructure and optical properties of In<Subscript>2</Subscript>S<Subscript>3</Subscript> films produced by thermal evaporation

The data on the influence of the microstructure of In₂S₃ films produced by thermal evaporation upon their optical properties in relation to the film’s thickness are reported. The atomic force spectroscopy data for the layers that are produced in identical technological conditions, but exhibit different spectral positions of the optical absorption edge are presented and discussed. Variations in the optical band gap from 2.0 to 3.6 eV under variations in the thickness of the In₂S₃ films from 800–450 nm to 50–30 nm are observed. The variations are interpreted as a result of variations in the content of grains, specific in dimensions and microstructure.     

Optical properties of CuIn<Subscript>5</Subscript>Se<Subscript>8</Subscript> single crystals

Single crystals of the CuIn₅Se₈ ternary compound are grown by the Bridgman-Stockbarger method (with the vertical layout of the procedure). The composition and structure of the crystals are determined. The spectra of transmittance and photoluminescence are studied in the temperature range from 10 to 300 K. The transmittance spectra and the photoluminescence spectra are used to determine, correspondingly, the band gap and the energy of donor-acceptor transitions in the CuIn₅Se₈ crystals. The temperature dependences of these parameters are obtained.     

Mn<Subscript>0.1</Subscript>Ag<Subscript>0.9</Subscript>In<Subscript>4.7</Subscript>S<Subscript>7.6</Subscript> single crystals: Crystal structure,band gap,and thermal expansion

Mn_0.1Ag_0.9In_4.7S_7.6 single crystals are for the first time grown by the Bridgman method (vertical variant). The single crystals crystallize in the cubic spinel structure. The band gap of the single crystals is determined from the transmittance spectra in the region of the fundamental absorption edge at temperatures of T = 295 and 80 K. Thermal expansion is studied by the dilatometric method in the temperature range 80–500 K. The coefficients of thermal expansion, the Debye temperatures, and the rms (root mean square) dynamic displacements of atoms are calculated.     

Synthesis and Characterization of In<Subscript>2</Subscript>S<Subscript>3</Subscript> Thin Films Deposited by Chemical Bath Deposition on Polyethylene Naphthalate Substrates

Indium sulfide (In₂S₃) thin films were deposited on polyethylene naphthalate (PEN) by chemical bath deposition (CBD). The materials were characterized by ultraviolet (UV)–visible spectroscopy, x-ray photoelectron spectroscopy (XPS), energy-dispersive x-ray spectroscopy (EDX), scanning electron microscopy (SEM), and x-ray diffraction (XRD) to investigate the influence of the polymeric substrate on the resulting thin In₂S₃. The films showed polycrystalline (cubic and tetragonal) structure. A reduction of the ordering of the polymeric chains at the surface of the PEN was also observed, demonstrated by the appearance of two infrared bands at 1094 cm⁻¹ and 1266 cm⁻¹. Presence of oxygen during the early stages of In₂S₃ growth was also identified. We propose a reaction mechanism for both the equilibrium and nucleation stages. These results demonstrate that In₂S₃ can be deposited at room temperature on a flexible substrate.     

Investigation of the Dielectric Permittivity and Electrical Conductivity of Ce<Subscript>2</Subscript>S<Subscript>3</Subscript>

The rare-earth semiconductor β-Ce₂S₃ compound samples were synthesized and their dielectric permittivity and electrical conductivity were measured in the temperature range 90–400 K. The energy-band structure has been determined. It is shown that the long-known large electrical parameter spread of semiconductor compounds close in composition to Ce₂S₃ is explained by the structure of impurity donor levels formed by cerium atoms and ions with different ionization degrees.     

Effect of gadolinium impurity on transmittance and reflectance of Hg<Subscript>3</Subscript>In<Subscript>2</Subscript>Te<Subscript>6</Subscript> crystals

The results of measuring the optical transmission and reflection spectra in the transparency region of Hg₃In₂Te₆:Gd semiconducting crystals are reported. It is shown that, in the entire wavelength range under investigation (2–25 µm), doping with Gd results in an increase in the continuous structureless absorption by impurities and defects, which introduce a quasi-continuous spectrum of localized states within the band gap. A decrease in transmittivity is accompanied by a decrease in the wavelength-independent reflectivity. Variation in the refractive index is attributed to changes in the bonding and electronic polarization of Hg₃In₂Te₆:Gd crystals. It is ascertained that polarization constants depend linearly on the strength of internal electric fields which exist in the vicinity of impurity defects.     

Cadmium-free thin-film Cu(In,Ga)Se<Subscript>2</Subscript>/(In<Subscript>2</Subscript>S<Subscript>3</Subscript>) heterophotoelements: Fabrication and properties

The method of heat treatment of metallic Cu-In-Ga layers in the N₂ inert atmosphere in the presence of selenium and sulfur vapors was used to grow homogeneous films of Cu(In,Ga)(S,Se)₂ alloys onto which the CdS or In₂S₃ films were deposited and, on the basis of these structures, the thin-film glass/Mo/p-Cu(In,Ga) (S,Se)₂/n-(In₂S₃,CdS)/n-ZnO/Ni-Al photoelements were fabricated. The mechanisms of charge transport and the processes of photosensitivity in the obtained structures subjected to irradiation with natural and linearly polarized light are discussed. The broadband photosensitivity of thin-film heterophotoelements and the induced photopleochroism were detected; these findings indicate that there is an interference-related blooming of the structures obtained. It is concluded that it is possible to use ecologically safe cadmium-free thin-film heterostructures as high-efficiency photoconverters of solar radiation.     

Effect of rare-earth impurities on the photoluminescence of Ge<Subscript>2</Subscript>S<Subscript>3</Subscript> glass

The photoluminescence properties of Ge₂S₃ glass doped with rare-earth elements La, Ce, Gd have been studied in the temperature range of 4.2–300 K. It has been shown that these impurities do not form new levels responsible for emission. The variation in the principal parameters of the excitation and emission spectra with the concentration and nature of impurities has been estimated.     

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.     

Superionic conductivity and switching effect with memory in TlInSe<Subscript>2</Subscript> and TlInTe<Subscript>2</Subscript> crystals

The temperature dependences of the conductivity σ(T) and the switching and memory effects in one-dimensional TlInSe₂ and TlInTe₂ single crystals have been studied. A specific feature is found in the dependence σ(T) above 333 K, which is related to the transition of crystals to the state with superionic conductivity. It is suggested that the ion conductivity is caused by the diffusion of Tl⁺ ions over vacancies in the thallium sublattice between (In³⁺Te₂²⁻)⁻ and (In³⁺Se₂²⁻)⁻ nanochains (nanorods). S-type switching and memory effects are revealed in TlInSe₂ and TlInTe₂ crystals, as well as voltage oscillations in the range of negative differential resistance. It is suggested that the switching effect and voltage oscillations are related to the transition of crystals to the superionic state, which is accompanied by “melting” of the Tl sublattice. The effect of electric-field-induced transition of TlInSe₂ and TlInTe₂ crystals to the superionic state is found.     

Influence of the Thermal Conditions of Fabrication and Treatment on the Optical Properties of In<Subscript>2</Subscript>O<Subscript>3</Subscript> Films

In₂O₃ films on Al₂O₃ (012) substrates are fabricated by dc magnetron sputtering at various temperatures (20–600°C). The effect of annealing and the substrate temperature on the film properties are studied by the ellipsometric method and the optical transmission method. Refractive-index profiles are constructed and band gaps for direct and indirect transitions are found. It is established that annealing leads to densification of the film material and unifies the refractive index. Annealing also decreases and unifies the energies of band-to-band transitions, which can be explained by lowering the influence of barriers in annealed films. However, the band gap for direct transitions varies greater than for indirect transitions. This fact can be associated with the mechanism of indirect transitions, notably, the participation of phonons facilitates interband transitions even if they are hindered by extra barriers caused by grain boundaries. The latter can be indirect evidence of the actuality of indirect transitions in indium oxide.     

Predicted Thermoelectric Properties of the Layered XBi<Subscript>4</Subscript>S<Subscript>7</Subscript> (X = Mn,Fe) Based Materials: First Principles Calculations

We report a theoretical investigation of electronic structures, optical and thermoelectric properties of two ternary-layered chalcogenides, MnBi₄S₇ and FeBi₄S₇ , by combining the first principles density functional calculations and semi-local Boltzmann transport theory. The calculated electronic band structure have demonstrated that both compounds exhibit indirect band gaps. The optical transitions are explored via the dielectric function (real and imaginary parts) along with other related optical constants including refractive index, reflectivity, and energy loss spectrum. These chalcogenides have exhibited interesting thermoelectric properties such as Seebeck’s coefficient, electrical and thermal conductivity, and power factor as function of temperatures.     

Electrical properties of In<Subscript>2</Subscript>Se<Subscript>3</Subscript> single crystals and photosensitivity of Al/In<Subscript>2</Subscript>Se<Subscript>3</Subscript> Schottky barriers

In₂Se₃ single crystals ∼40 mm long and 14 mm in diameter were grown by the Bridgman method. The composition of grown single crystals and their crystal structure were determined. The conductivity (σ) and Hall constant (R) of grown single crystals were measured and the first Schottky barriers Al/n-In₂Se₃ were fabricated. Rectification and photovoltaic effect were detected in the new structures. Based on the study of the photosensitivity spectra of Al/n-In₂Se₃ structures, the nature of the interband transitions and band gap of In₂Se₃ crystals were determined. It was concluded that the new structures can be applied to develop broadband photoconverters of optical radiation.