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.     

Properties of CuIn<Subscript>3</Subscript>Se<Subscript>5</Subscript> crystals and In/CuIn<Subscript>3</Subscript>Se<Subscript>5</Subscript> structures

Using the method of planar crystallization from the melt with deviations from the stoichiometric composition, p-CuIn₃Se₅ single crystals are grown. The electrical properties of the homogeneous crystals are studied. It is found that the resistivity of the p-CuIn₃Se₅ crystals depends on the excess Se content in the melt. It is established that the voltaic photosensitivity of the In/CuIn₃Se₅ structures is enhanced with an increasing excess of Se content in the melt. The energy spectrum and the character of interband transitions in the CuIn₃Se₅ crystals are discussed. It is concluded that the CuIn₃Se₅ ternary compound can be used in high efficiency photoelectric converters of solar radiation.     

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.     

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.     

Development and photoelectric properties of In/<Emphasis Type="Italic">p</Emphasis>-Ag<Subscript>3</Subscript>AsS<Subscript>3</Subscript> surface-barrier structures

Homogeneous p-Ag₃AsS₃ bulk single crystals with rhombic structure have been grown by planar crystallization from melts with atomic composition corresponding to this ternary compound. Photosensitive surface-barrier structures based on the interface between the surface of these crystals and thin films of pure indium are fabricated for the first time. The photosensitivity of fabricated structures is studied in natural and linearly polarized light. Photosensitivity spectra of In/p-Ag₃AsS₃ structures are measured for the first time and used to determine the nature and energy of interband transitions in p-Ag₃AsS₃ crystals. The phenomenon of natural photopleochroism is studied for surface-barrier structures grown on oriented p-Ag₃AsS₃ single crystals. It is concluded that Ag₃AsS₃ single crystals can be used in photoconverters of natural and linearly polarized light.     

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.     

Influence of impurities on the thermoelectric properties of layered anisotropic PbBi<Subscript>4</Subscript>Te<Subscript>7</Subscript> compound: Experiment and calculations

The kinetic coefficients of high-quality single crystals of ternary layered n-PbBi₄Te₇ compounds have been measured in the temperature range of 77–400 K. These crystals, doped with electroactive Cd and Ag impurities, were grown by Czochralski pulling with melt supply through a floating crucible. A significant anisotropy of the thermoelectric properties is found. The means of incorporation of electroactive impurities into the ternary compound lattice is established. The experimental values of the Nernst—Ettingshausen coefficient have been analyzed together with the Seebeck, Hall, and conductivity data. The features of transport phenomena in PbBi₄Te₇ can be explained within the single-band model of nonparabolic energy spectrum and mixed mechanism of electron scattering from acoustic phonons and the Coulomb potential of impurities. It is suggested that acoustic phonon scattering is dominant along the cleavage plane, whereas the impurity scattering dominates along the trigonal axis.     

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.     

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.     

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.     

Transmittance spectra of the CuGa<Subscript>3</Subscript>Se<Subscript>5</Subscript> ternary compound near the fundamental absorption edge

The CuGa₃Se₅ ternary compound films are produced by laser deposition at the substrate temperatures 480 and 580 K. The composition and structure of the films are studied. It is shown that, similarly to the corresponding crystals, the CuGa₃Se₅ films crystallize into the imperfect chalcopyrite structure. The transmittance spectra near the fundamental absorption edge are used to establish the energies and nature of optical transitions. The energies of crystal-field splitting (Δ_cr) and spin-orbit splitting (Δ_SO of the valence band of the CuGa₃Se₅ ternary compound are calculated in the context of the Hopfield quasi-cubic model.     

Fabrication and properties of photosensitive structures based on ZnIn<Subscript>2</Subscript>S<Subscript>4</Subscript> single crystals

Photosensitive structures based on single crystals of the ZnIn₂S₄ ternary compound were fabricated and studied for the first time. The optoelectronic properties of this compound and corresponding structures were analyzed using the results of measurements of the optical-absorption spectra of ZnIn₂S₄ crystals, steady-state current-voltage characteristics, and photosensitivity of the structures at T=300 K. It is concluded that surface-barrier structures and heterojunctions based on ZnIn₂S₄ can be used as wide-band photodetectors of natural optical radiation.     

Physical properties of CuxAg1−x In5S8 single crystals and related surface-barrier structures

The electrical properties and photoluminescence spectra of single crystals of the ternary compounds CuIn₅S₈, AgIn₅S₈ and their solid solutions have been investigated. We have determined the type of conductivity, the mobility, charge carrier concentrations and energies of the radiative transitions in these materials. We have fabricated surface-barrier structures from these single crystals and measured the voltaic photosensitivity. Fiz. Tekh. Poluprovodn. 32, 1043–1046 (September 1998)     

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.     

Phase transitions in thin Ge<Subscript>2</Subscript>Sb<Subscript>2</Subscript>Te<Subscript>5</Subscript> chalcogenide films according to Raman spectroscopy data

Data on the Raman spectra of thin Ge₂Sb₂Te₅ chalcogenide semiconductor films are reported. The study is performed with the purpose of determining the temperatures of phase transitions initiated by laser radiation.     

Features of reflection spectra of single crystals of Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript>-Sb<Subscript>2</Subscript>Te<Subscript>3</Subscript> solid solutions in the region of plasma effects

Reflectance spectra of single crystals of Bi₂Te₃-Sb₂Te₃ solid solutions containing 0, 10, 25, 40, 50, 60, 65, 70, 80, 90, 99.5, and 100 mol % of Sb₂Te₃ have been studied in the range of 400–4000 cm⁻¹ at the temperature T = 291 K and with orientation of the vector of the electric-field strength E perpendicular to the trigonal axis of the crystal C ₃ (E ⊥ C ₃). The shape of the spectra is characteristic of plasma reflection; the spectra include the features in the range 1250–3000 cm⁻¹ corresponding to the optical band gap E _{g opt}. The features become more pronounced as the content of Bi₂Te₃ is increased to 80 mol % in the composition of the Bi₂Te₃-Sb₂Te₃ solid solution. A further increase in the content of Sb₂Te₃ is accompanied by discontinuities in the functional dependences of the parameters characterizing the plasma oscillations of free charge carriers on the solid-solution composition and also by a sharp increase in E _{g opt}.     

Photosensitive structure on CdGa<Subscript>2</Subscript>S<Subscript>4</Subscript> single crystals

Single crystals of the CdGa₂S₄ ternary compound were grown either from melt or by chemical-vapor deposition. The crystal-lattice parameters and some physical properties of homogeneous crystals having defect chalcopyrite structure with the point symmetry group \(I\bar 4(S_4^2 )\) are determined. A number of photosensitive structures—Schottky barriers, heterostructures, photoelectrochemical cells, and natural-protein-CdGa₂S₄ barriers—were formed for the first time on the basis of the single crystals under investigation. The photoelectric properties of the structures obtained were studied using natural and linearly polarized light at T=300 K. The main parameters of these structures are determined, and it is concluded that they can be used in photodetectors.     

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.     

Photosensitivity of structures based on I-IIIn-VIm ternary compounds containing ordered vacancies

Homogeneous crystals of CuIn₃Se₅, CuGa₃Se₅, and CuGa₅Se₈ ternary compounds were grown, and their physical and chemical properties were investigated. Photosensitive structures were fabricated for the first time on the basis of these compounds, and the spectral dependence of the relative quantum photoconversion efficiency was measured. The bandgap of these compounds was also estimated, and it was shown that direct interband transitions are typical in them. It was found that the content and chemical nature of atoms forming an elementary cell in a I-III_n-VI_m ternary compound control the relevant band gap.     

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).