Fabrication and photosensitivity of heterojunctions based on CuIn<Subscript>3</Subscript>Se<Subscript>5</Subscript> crystals

Heterojunctions based on p-CuIn₃Se₅ crystals are fabricated by magnetron sputtering of an n-ZnO:Al target and by putting naturally cleaved n-GaSe thin wafers onto polished surfaces of p-CuIn₃Se₅ wafers. The current-voltage characteristics and mechanisms of current flow in the diodes under study are analyzed. The photovoltaic effect revealed in the fabricated structures is discussed. It is shown that the fabricated photosensitive heterojunctions are promising for the development of selective analyzers of linearly polarized radiation.     

Fabrication and photoelectric properties of oxide/CuIn<Subscript>5</Subscript>Se<Subscript>8</Subscript> heterojunctions

Single crystals of the n-CuIn₅Se₈ compound of hexagonal modification have been grown by direct crystallization from melt. On the basis of the experimental study of its thermal interaction with air oxygen, a method for fabricating new oxide/n-CuIn₅Se₈ heterojunctions is proposed. Electrical and photoelectric properties of the structures obtained have been investigated. It is shown that the interaction of n-CuIn₅Se₈ of hexagonal modification with air oxygen makes it possible to obtain heterojunctions with high photosensitivity. The new technology can be used in the design of broadband optical radiation converters based on n-CuIn₅Se₈ crystals.     

Anisotropy of the thermal expansion of CuIn<Subscript>5</Subscript>Se<Subscript>8</Subscript> single crystals in two structural modifications

Thermal expansion in the temperature range 80–700 K is studied for two (trigonal and hexagonal) structural modifications of CuIn₅Se₈ single crystals grown by planar crystallization of the melt. From the data, the thermal-expansion coefficients are calculated for both modifications. It is established that, in the temperature range under study, the thermal expansion of both modifications is anisotropic. For the trigonal modification, the thermal-expansion coefficient in the direction of the c axis (α_c) is larger than that in the direction of the orthogonal a axis (α_a). For the hexagonal modification of the CuIn₅Se₈ crystal, the thermal-expansion coefficient in the direction of the c axis exhibits anomalous behavior: as the temperature is increased, the coefficient α_c increases, after which it decreases to negative values, reaches a minimum, and then increases further. Such behavior of the coefficient α_c is associated with the phase transformation of the hexagonal modification of the CuIn₅Se₈ compound into the trigonal modification.     

Photoconversion in <Emphasis Type="Italic">n</Emphasis>-ZnO:Al/Pd<Emphasis Type="Italic">Pc/p</Emphasis>-CuIn<Subscript>3</Subscript>Se<Subscript>5</Subscript> structures

n-ZnO:Al/PdPc/p-CuIn₃Se₅ photosensitive structures have been proposed and fabricated for the first time by vacuum sublimation of palladium phthalocyanine on the surface of wafers of the ternary semiconductor compound CuIn₃Se₅ and by magnetron sputtering of n-ZnO:Al films on the surface of palladium phthalocyanine films. The current-voltage characteristics and spectra of the photoconversion quantum efficiency of the obtained structures are investigated. It is shown that these structures can be used as multiband white-light converters.     

Effect of thallium doping on the mobility of electrons in Bi<Subscript>2</Subscript>Se<Subscript>3</Subscript> and holes in Sb<Subscript>2</Subscript>Te<Subscript>3</Subscript>

The Shubnikov–de Haas effect and the Hall effect in n-Bi_2–xTl_xSe₃ (x = 0, 0.01, 0.02, 0.04) and p-Sb_2–xTl_xTe₃ (x = 0, 0.005, 0.015, 0.05) single crystals are studied. The carrier mobilities and their changes upon Tl doping are calculated by the Fourier spectra of oscillations. It is found shown that Tl doping decreases the electron concentration in n-Bi_2–xTl_xSe₃ and increases the electron mobility. In p-Sb_2–xTl_xTe₃, both the hole concentration and mobility decrease upon Tl doping. The change in the crystal defect concentration, which leads to these effects, is discussed.     

Study of single crystals of the CuIn<Subscript>3</Subscript>Se<Subscript>5</Subscript> ternary compound

Single crystals of the CuIn₃Se₅ ternary compound are grown from the melt by the Bridgman-Stockbarger method. The composition, structure, and electrical properties of the crystals are studied. From the transmittance spectra in the region of the optical absorption edge, the band gap is determined in the temperature range from 10 to 300 K. Using the dilatometry method, the relative elongation is measured for crystals oriented parallel and orthogonal to their principal axis, and the coefficients of thermal expansion are calculated.     

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.     

Photoelectric properties of In/In<Subscript>2</Subscript>Se<Subscript>3</Subscript> structures

The hexagonal modification of In₂Se₃ single crystal is grown by planar crystallization from nearly stoichiometric melt and by the vapor-phase method. For the first time, the Schottky barriers In/n-In₂Se₃, which are photosensitive in a wide incident-photon energy range of 1–3.8 eV at 300 K, are obtained. The nature of the interband photoactive absorption is studied. The energy-barrier height and interband optical-transition energy are estimated. It is concluded that the grown crystals can be used in broadband optical-radiation converters.     

Photoelectric phenomena in the Cu (Al, In)/p-CuIn3Se5 Schottky barriers

Structures are formed on the p-CuIn₃Se₅ crystals and photoelectric phenomena in the Cu/p-CuIn₃Se₅, Al/p-CuIn₃Se₅, and In/p-CuIn₃Se₅ Schottky barriers are studied. The spectra of quantum efficiency for photoconversion in new structures were obtained for the first time. The characteristics of the interband transitions are discussed, and the CuIn₃Se₅ band gap is determined. It is concluded that CuIn₃Se₅ crystals can be used in the fabrication of high-efficiency broadband photoconverters of optical radiation.     

AC Electrical Conductivity of FeIn<Subscript>2</Subscript>Se<Subscript>4</Subscript> Single Crystals

The results obtained in a study of the frequency and temperature dependences of the ac electrical conductivity of FeIn₂Se₄ single crystals are presented. It is found that the law σ ~ f ^S (0.1 ≤ S ≤ 1.0) is obeyed for electrical conductivity in the 295–375 K temperature range at frequencies of 2 × 10⁴–10⁶ Hz. It shown that the frequency dependence of the conductivity in an FeIn₂Se₄ single crystal can be accounted for in terms of the multiplet model, and, consequently, the conductivity in these single crystals is characterized by the band-hopping mechanism.     

Special features of charge transport in PbGa<Subscript>2</Subscript>Se<Subscript>4</Subscript> crystals

The current-voltage (I-V) characteristics of PbGa₂Se₄ single crystals grown by the Bridgman-Stockbarger method with a resistivity of 10¹⁰–10¹² Ω cm were measured. The value of the majority carrier mobility μ=14 cm² V^?1 s^?1, calculated by the differential method of analysis of I-V characteristics, makes it possible to evaluate a number of parameters: the carrier concentration at the cathode (n_c0=2.48 cm^?3), the width of the contact barrier d_c=5.4×10^?8 cm, the cathode transparency D _c ^* =10^?5–10^?4 eV, and the quasi-Fermi level E_F=0.38 eV. It is found that a high electric field provides the charge transport through PbGa₂Se₄ crystals in accordance with the Pool-Frenkel effect. The value of the dielectric constant calculated from the Frenkel factor is found to be equal to 8.4.     

Photoluminescence and composition of amorphous As<Subscript>2</Subscript>Se<Subscript>3</Subscript> films modified with Er(<Emphasis Type="Italic">thd</Emphasis>)<Subscript>3</Subscript> complex compound

The photoluminescence and composition of amorphous As₂Se₃ films modified with an Er(thd)₃ complex compound have been studied. A band centered at 1.54 μm, characteristic of photoluminescence from Er embedded in amorphous matrices, has been revealed at room temperature. The composition of thin amorphous As₂Se₃ films modified with an Er(thd)₃ complex compound has been examined by methods of nuclear microanalysis: Rutherford backscattering and nuclear resonant reactions. Dependences of the concentrations of Er ions, oxygen, and carbon on the growth conditions of the films are obtained. It is shown that the Er concentration in a thin film varies nonlinearly as the relative concentration of the starting complex compound increases. In addition, the increase in the Er content of a film is accompanied by a simultaneous rise in the content of such light elements as oxygen and carbon. Comparative analysis of the nuclear microanalysis data and IR spectra demonstrates that, in modification of As₂Se₃ with the Er(thd)₃ complex compound by the given method, the nearest environment of Er in the complex compound is partly preserved.     

High-Vacuum Evaporation of <Emphasis Type="Italic">n</Emphasis>-CuIn<Subscript>3</Subscript>Se<Subscript>5</Subscript> Photoabsorber Films for Hybrid PV Structures

Thin films of Cu-In-Se (CISe) photoabsorber with an overall composition of CuIn₃Se₅ were deposited onto glass/indium tin oxide (ITO) substrates from a polycrystalline bulk CuIn₃Se₅ source using the high-vacuum evaporation technique. Thermal conditions for the substrates during the evaporation process and the subsequent annealing in vacuum were selected to prepare polycrystalline n-CuIn₃Se₅ photoabsorber layers for use in hybrid photovoltaic structures based on an inorganic photoabsorber and conductive polymer functional layers. The CISe layers were deposited at a substrate temperature of 200°C and were annealed at temperatures from 300°C to 500°C in vacuum. Part of the as-deposited CISe was annealed twice, in argon and in vacuum at 500°C. These layers exhibited high photosensitivity and photoconductivity when illuminated with white light at an intensity of 100 mW/cm². The results showed that the chalcopyrite structure of the prepared CISe photoabsorber films adhered well to the glass/ITO substrate. The average value of charge carrier concentration and the profile of charge carrier concentration in the annealed CISe photoabsorber layer were calculated using impedance spectroscopy.     

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.     

Ab initio calculations of the electron spectrum and density of states of TlFeS<Subscript>2</Subscript> and TlFeSe<Subscript>2</Subscript> crystals

The results of ab initio calculations of the electron spectrum of TlFeS₂ and TlFeSe₂ crystals in the antiferromagnetic phase are reported. Calculations are carried out in the context of the density functional theory. The origin of the bands of s, p, and d electron states of Tl, Fe, S, and Se atoms is studied. It is established that, in the antiferromagnetic phase, the crystals possess semiconductor properties. The band gaps are found to be 0.05 and 0.34 eV for TlFeS₂ and TlFeSe₂ crystals, respectively.     

A comparative study on the electronic and optical properties of Sb<Subscript>2</Subscript>Se<Subscript>3</Subscript> thin film

The thin film of Sb₂Se₃ was deposited by thermal evaporation method and the film was annealed in N₂ flow in a three zone furnace at a temperature of 290°С for 30 min. The structural properties were characterized by scanning electron microscopy (SEM), transmission electron microscopy (ТЕМ), X-ray diffraction (XRD) and Raman spectroscopy, respectively. It is seen that the as-deposited film is amorphous and the annealed film is polycrystalline in nature. The surface of Sb₂Se₃ film is oxidized with a thickness of 1.15 nm investigated by X-ray photolecetron spectroscopy (XPS) measurement. Spectroscopic ellipsometry (SE) and UV–vis spectroscopy measurements were carried out to study the optical properties of Sb₂Se₃ film. In addition, the first principles calculations were applied to study the electronic and optical properties of Sb₂Se₃. From the theoretical calculation it is seen that Sb₂Se₃ is intrinsically an indirect band gap semiconductor. Importantly, the experimental band gap is in good agreement with the theoretical band gap. Furthermore, the experimental values of n, k, ε₁, and ε₂ are much closer to the theoretical results. However, the obtained large dielectric constants and refractive index values suggest that exciton binding energy in Sb₂Se₃ should be relatively small and an antireflective coating is recommended to enhance the light absorption of Sb₂Se₃ for thin film solar cells application.     

Fabrication and photoelectric properties of the ZnO-Cu(In,Ga)Se<Subscript>2</Subscript> heterojunctions

Thin-film n-ZnO:Al/p-Cu(In,Ga)Se₂ heterojunctions are fabricated by magnetron sputtering of an ZnO target, leading to a deposition of Cu(In,Ga)Se₂ films on the surface. The photoelectric properties of the fabricated heterojunctions are studied under exposure to natural and linearly polarized light. It is concluded that the resulting cadmium-free environmentally safe heterostructures can be used as high-efficiency broad-band photoconverters of natural and linearly polarized light.     

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.     

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.     

Effect of copper doping on kinetic phenomena in <Emphasis Type="Italic">n</Emphasis>-Bi<Subscript>2</Subscript>Te<Subscript>2.85</Subscript>Se<Subscript>0.15</Subscript>

In single crystals of copper-doped and undoped Bi₂Te_2.85Se_0.15 solid solutions with an electron concentration close to 1 × 10¹⁹ cm^?3, the temperature dependences are investigated for the Hall (R ₁₂₃, R ₃₂₁) and Seebeck (S ₁₁) kinetic coefficients, the electrical-conductivity (σ ₁₁), Nernst-Ettingshausen (Q ₁₂₃), and thermalconductivity (k ₁₁) coefficients in the temperature range of 77–400 K. The absence of noticeable anomalies in the temperature dependences of the kinetic coefficients makes it possible to use the one-band model when analyzing the experimental results. Within the framework of the one-band model, the effective mass of density of states (m _d ≈ 0.8m ₀), the energy gap (ε_g ≈ 0.2 eV), and the effective scattering parameter (r _eff ≈ 0.2) are estimated. The obtained value of the parameter r _eff is indicative of the mixed electron-scattering mechanism with the dominant scattering by acoustic phonons. Data on the thermal conductivity and the lattice resistivity obtained by subtracting the electron contribution according to the Wiedemann-Franz law are presented.