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

A new class of ternary semiconductor compounds has been proposed and synthesized. ZnFe₂S₄ single crystals, which belong to this class, have been grown for the first time; and their structural, electrical, and optical properties have been investigated. The first photosensitive structures have been fabricated, and their photoelectric characteristics have been studied. A conclusion was made that heterostructures and surface-barrier structures based on ZnFe₂S₄ single crystals are promising for practical applications.     

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.     

Photosensitive structures based on ZnIn<Subscript>2</Subscript>Se<Subscript>4</Subscript> single crystals

Photosensitive structures of surface-barrier and homojunction types have been fabricated for the first time on the basis of ZnIn₂Se₄ single crystals. The spectral dependence of the quantum efficiency of photoconversion has been studied and discussed. It is concluded that the structures are promising for commercial applications.     

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.     

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.     

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.     

Photosensitive structures based on HgGa<Subscript>2</Subscript>S<Subscript>4</Subscript> single crystals: Preparation and properties

Photosensitive structures based on n-HgGa₂S₄ single crystals were prepared and investigated. It was concluded that HgGa₂S₄ crystals are promising for the fabrication of photodetectors of natural and linearly polarized light in the short-wavelength spectral region.     

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.     

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.     

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.     

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).     

Photosensitive structures based on ZnP<Subscript>2</Subscript> single crystals of monoclinic and tetragonal modifications: Fabrication and properties

The method of gas-phase resublimation is used to grow the single-crystals of monoclinic and tetragonal modifications whose composition are identical and corresponds to the ZnP₂ stoichiometry. The crystal-lattice parameters are determined and natural facets of the crystals of both modifications are identified. The obtained single crystals were used to fabricate for the first time the Schottky barriers and welded point structures; the latter exhibited rectification and the photovoltaic effect. On the basis of the first studies of photosensitivity of obtained structures subjected to the natural and linearly polarized radiation, the character of interband transitions is suggested, the values of the band gap are determined, and the influence of the position ordering of atoms on the structures’ properties is observed. The phenomenon of natural photopleochroism observed in the structures based on oriented ZnP₂ single crystals were studied. It is concluded that it is possible to use the zinc diphosphide in photoconversion of the intensity and polarization of optical radiation.     

Photosensitive structures based on CuIn<Subscript>5</Subscript>Te<Subscript>8</Subscript> single crystals: Development and properties

A new ternary compound is synthesized for the first time, and bulk CuIn₅Te₈ single crystals are grown by directed crystallization of near-stoichiometric melt. It is established from X-ray diffraction patterns of grown crystals that they exhibit the structure of imperfect chalcopyrite with parameters of the unit cell of CuIn₅Te₈, which were close to those known for the CuInTe₂ ternary compound with the composition index n = 0. First, photosensitive structures are fabricated based on CuIn₅Te₈ crystals, and photosensitivity spectra are obtained for them; it is shown that it is possible to achieve broadband photosensitivity under illumination of the barrier side of these crystals. From the analysis of photosensitivity spectra, the character of band-to-band transitions and corresponding energies of these transitions in CuIn₅Te₈ are determined. This opens up prospects to use this new semiconductor in photoconverters of solar radiation.     

Photovoltaic effects in CdV<Subscript>2</Subscript>S<Subscript>4</Subscript> single crystals and structures based on them

Single crystals of the CdV₂S₄ ternary compound are grown, and their crystal structure, electrical properties, and optical absorption are studied. The substitution of vanadium for Group III element in A^IIB ₂ ^III C ₄ ^VI compounds results in the formation of crystals of n-type conduction with an electron density of ～10¹⁸ cm^?3 and a Hall mobility U_n≈150 cm²/(V s) at T=300 K, which is limited by scattering on lattice vibrations. Rectifying photosensitive structures based on CdV₂S₄ single crystals are fabricated for the first time, their photoelectric properties are studied, and a conclusion is made on their applicability in the design of wide-spectral-range photodetectors of unpolarized light.     

Optoelectronic effects in semi-insulating CdTe single crystals and structures based on them

The authors describe a gas-transport reaction method they recently developed using the compounds NH₄Cl (Br, I) as transport agents. Using this method, they were able to grow semiinsulating cadmium telluride single crystals with carrier concentrations p=10⁸–10¹⁰ cm⁻³ at T=300 K. These crystals were used to fabricate In-CdTe surface-barrier structures with peak voltaic photosensitivities of ∼10⁵ V/W. Their investigations of the emission properties of homogeneous crystals at T=77K and distinctive features of their photosensitivity spectra revealed that these material characteristics derive from the use of Cl, Br, and I as dopants. By illuminating their In-CdTe structures with linearly polarized light at oblique incidence, they generated induced photopleochroism, which was measured and used to determine the refraction index of the material, which is found to be n=2.8. The paper concludes with a discussion of how these structures can be used as photodetectors of natural and linearly polarized light. Fiz. Tekh. Poluprovodn. 33, 553–558 (May 1999)     

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.     

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.     

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.     

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.     

Features of the charge-transport mechanism in layered Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript> single crystals doped with chlorine and terbium

The temperature dependences (T = 5−300 K) of the resistivity in the plane of layers and in the direction perpendicular to the layers, as well as the Hall effect and the magnetoresistance (H < 80 kOe, T = 0.5−4.2 K) in Bi₂Te₃ single crystals doped with chlorine and terbium, are investigated. It is shown that the doping of Bi₂Te₃ with terbium atoms results in p-type conductivity and in increasing hole concentration. The doping of Bi₂Te₃ with chlorine atoms modifies also the character of its conductivity instead of changing only the type from p to n. In the temperature dependence of the resistivity in the direction perpendicular to layers, a portion arises with the activation conductivity caused by the hopping between localized states. The charge-transport mechanism in Bi₂Te₃ single crystals doped with chlorine is proposed.