Magnetic properties and surface morphology of layered In2Se3 crystals intercalated with cobalt

The magnetic properties of layered Co _x In₂Se₃ crystals electrochemically intercalated with cobalt in an external magnetic field and without a magnetic field and the morphology of the van der Waals surfaces of layers of these crystals have been investigated. It has been found that the ferromagnetic ordering at room temperature is observed only for Co _x In₂Se₃ crystals intercalated in an external magnetic field. These crystals are nanocomposite materials that consist of a layered matrix and arrays of nanorings and nanowires formed from Co nanocrystals on the van der Waals surfaces of the In₂Se₃ layers. Cobalt nanocrystals in Co _x In₂Se₃ crystals have a pyramidal equilibrium shape, which is characteristic of the face-centered cubic crystal structure, and their geometrical sizes are of the order of a few nanometers. The specific features of self-organization of cobalt magnetic nanostructures on the van der Waals surfaces of layered semiconductor crystals during their electrolytic intercalation in a magnetic field and the magnetic properties of these structures have been considered.     

Auger electron spectroscopy studies of In4Se3 layered crystals

In₄Se₃ pure and silver intercalated layered semiconductor crystals’ surfaces have been studied with application of Auger electron spectroscopy (AES). It was found that peculiarities of their adsorptial activity appear as the result of atomic and electron-energetic structure of the crystals’ cleavage surfaces. Silver intercalation (2.5–3 at.%) of In₄Se₃ leads to a rise in the CO sticking coefficient and simultaneous appearance of adsorption activity relative to oxygen. It was established that In₄Se₃ crystal heating leads to active interaction of cleavage surfaces with oxygen and its chemisorption.     

High pressure γ-to-β phase transition in bulk and nanocrystalline In2Se3

Pressure-dependent phase transitions of In₂Se₃ bulk powders and nanowire samples were studied at room temperature using synchrotron X-ray diffraction and a diamond anvil cell. γ-In₂Se₃, metastable under ambient conditions, transforms into to the stable high pressure β phase between 2.8 and 3.2?GPa in bulk powder samples and at slightly higher pressures, between 3.2 and 3.7?GPa, in nanowire samples. While the γ phase bulk modulus is similar to that of the β phase, the decrease due to pressure in the unit cell parameter ratio, c/a, is less than half the decrease seen in the β phase. First-principles calculations show that γ-In₂Se₃ has a higher energy and unit-cell volume than β-In₂Se₃, consistent with the experimental observations.     

Optical parameters of In–Se and In–Se–Te thin amorphous films prepared by pulsed laser deposition

The thin films of materials based on In–Se are under study for their applicability in photovoltaic devices, solid-state batteries and phase-change memories.The amorphous thin films of In₂Se_3−xTe_x (x=0–1.5) and InSe were prepared by pulsed laser deposition method (PLD) using a KrF excimer laser beam (λ=248 nm, 0.5 J cm⁻²) from polycrystalline bulk targets. The compositions of films verified by energy-dispersive X-ray analysis (EDX) were close to the compositions of targets. The surfaces of PLD films containing small amount of droplets were viewed by optical and scanning electron microscopy (SEM).The optical properties (transmittance and reflectance spectra, spectral dependence of index of refraction, optical gap, single-oscillator energy, dispersion energy, dielectric constant) of the films were determined.The values of index of refraction increased with increasing substitution of Te for Se in In₂Se₃ films, the values of the optical gap decreased with increasing substitution of Te for Se in In₂Se₃ films.     

Synthesis and properties of In2(Se1-xTex)3 thin films: a new semiconductor compound

In₂(Se_1-xTe_x)₃ polycrystalline films were prepared by a dual-source thermal evaporation technique. The depositions onto glass and SnO₂-coated glass substrates were carried out in a vacuum chamber and followed by an annealing in neutral ambient (Ar or N₂). The structural, morphological and compositional studies of the films were made by X-ray diffraction, energy-dispersive X-ray analysis, X-ray photoelectron spectroscopy, scanning electron microscopy, Raman scattering and optical transmission. Optimum conditions are investigated for the formation of the ternary compound In₂(Se_1-xTe_x)₃ in order to tune the band gap by changing the Te concentration. The film properties as a function of Te amount are discussed. It is shown that single-phase, textured and homogeneous layers of In₂(Se_1-xTe_x)₃ can be grown with x≤0.2 at optimal deposition and heat treatment conditions. For x≅0.17 these films showed an energy band gap of about 1.45 eV and an electrical conductivity at room temperature six orders of magnitude higher than that of the binary γ-In₂Se₃ thin films. Received: 9 July 1999 / Accepted: 25 November 1999 / Published online: 13 July 2000     

Photoelectric Phenomena in Au3In5Se9 and Au3Ga5Se9 Compounds

The anisotropy of photoconductivity in Au₃In₅Se₉ and Au₃Ga₅Se₉ crystals grown by the Bridgman method have been investigated as a function of temperature in the temperature range of 100 - 420 K. It is shown that the crystals have a wide range of spectral sensitivity of 0.9 - 1.8 eV. The width of the band gaps and their temperature coefficient are determined. The life time of the current carriers are determined at different levels of excitations.     

Investigation of the optical properties of In2Se3xTe3(1−x) single crystals

In [1] it was shown that when In₂Te₃ interacts with In₂Se₃ in samples which are mainly indium telluride, substitutional solid solutions having the zinc blende structure are formed in the concentration range 0–40 mol.% In₂Se₃ (0 x 0.4). The solubility of the low temperature hexagonal modification-In₂Se₃ did not exceed 2 mol.%; formation of solid solutions having the medium temperature hexagonal modification of-In₂Se₃ was observed for the interval 0.9 x 0.95. These published data [2, 3] are concerned with electrical properties of the crystals. In this paper the results of an investigation of the optical properties of In₂Se_3xTe_3(1–x) single crystals for temperatures between 300 and 77°K are presented. Crystals suitable for optical measurements were obtained by a modified Bridgman method, which included vibarational mixing. All the samples were n-type. The resistivity varied from 10⁶ to 10⁴ ohm · cm in the course of the transition from-In₂Te₃ to the solid solutions (0 x 0.4).Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 65–68, May, 1972.     

Temperature changes in the photoluminescence and the intrinsic absorption edge of the (Ga0.1In0.9)2Se3 crystal

The photoluminescence and the intrinsic absorption edge of the (Ga_0.1In_0.9)₂Se₃ uniaxial crystal are investigated in the temperature range 77–300 K. Exciton and impurity luminescence bands are revealed at low temperatures and the Urbach tail of the absorption edge is found in the temperature range under study. The temperature dependences of the spectral position and the half-width of the luminescence bands, as well as the optical pseudogap and the energy width of the absorption edge, are studied. The mechanisms of radiative recombination and light absorption and the processes of lattice disordering in the (Ga_0.1In_0.9)₂Se₃ crystal are discussed.     

The heterostructure and electrical properties of Sb2Se3/Bi2Se3 grown by molecular beam epitaxy

We report the growth and characterization of Sb₂Se₃/Bi₂Se₃ bilayer films fabricated by molecular beam epitaxy. High quality heterostructures are obtained as evidenced from the X-ray diffraction (XRD), atomic force microscopy and high-resolution transmission electron microscopic (HRTEM) analysis. Interestingly, Sb₂Se₃ grows as a (120) hexacrystal film in orthorhombic phase on rhombohedral Bi₂Se₃ (0001) plane, as verified by the out-of-plane and in-plane XRD scans. The cross-sectional TEM studies indicate a sharp interface between Sb₂Se₃ and Bi₂Se₃, which is important for the protection of surface states Bi₂Se₃. The ultraviolet photoelectron spectroscopy indicates that the Fermi level located 0.95 eV above the valence band maximum in Sb₂Se₃. The insulating nature of Sb₂Se₃ is confirmed by the nonlinear current-voltage curve via the vertical junction electrical measurement. By four point probe measurements, we confirm the charge transfer effect from Sb₂Se₃ into Bi₂Se₃, and such effect can be reduced in the Sb₂Se₃/(Bi_0.7Sb_0.3)₂Se₃ bilayer. This work opens a new avenue for the synthesis of multilayers consisting of topological insulators and ordinary insulator, which is important for harvesting of the multiple surface states for advanced electronic and spintronic applications.     

Photoelectric Properties of the In/CuIn3Se5 and In/CuGa3Se5 Structures

Using the method of oriented crystallization of a melt (a vertical variant), we grew homogeneous single crystals of the ternary compounds CuIn₃Se₅ and CuGa₃Se₅. Photosensitive structures based on them were created for the first time, the spectral dependences of the relative quantum efficiency of the phototransformation of structures were investigated, and the energy gap width of the ternary compounds was determined. It is established that the CuIn₃Se₅ and CuGa₃Se₅ compounds are materials with direct interband transitions. We show that the structures created can be used as wideband photoconverters of natural radiation.     

Synthesis and properties of InxMo6Se8-yIy indium insertion compounds

We present a study of the properties of the series In_xMo₆Se_8-yI_y (y=0.5, 1.0, 2.0) having the hexagonal-rhombohedral Mo₆Se₈ structure type. The materials were prepared by a low temperature (525 °C) insertion technique. For y=0.5, only the new superconducting phase In_.5Mo₆Se_7.5I_.5 was found. For y=1.0 and y=2.0, the In_xMo₆Se_8-yI_y phases represent the first Chevrel phases which are solid solutions with respect to indium insertion. In_xMo₆Se₇I (0 < × < 1) presents a phase diagram which indicates phenomena associated with an In⁺¹-In⁺³ valence change as x increases. In_xMo₆Se₆I₂ (0 < × < 0.66) is a well-behaved solid solution of In³⁺ in Mo₆Se₆I₂. For both y=1.0 and y=2.0, an increase in In³⁺ concentration increases the rhombohedral angle into the 94°– 95° range and the hexagonal unit cell volumes into the range of 940 Å ³ at maximum indium concentrations. At higher temperatures, the indium abstracts iodide from the cluster units which then condense to form known phases with larger clusters. Even though Mo₆X₈ phases are stabilized by partial halogen substitution, attempts to synthesize niobium Chevrel-type phases stabilized by halogen-chalcogen substitution were unsucessful.     

Carrier transport in Bi2Se3 topological insulator slab

Electron transport in Bi₂Se₃ topological insulator slabs is investigated in the thermal activation regime (>50 K) both in the absence (ballistic) and presence of weak and strong acoustic phonon scattering using the non-equilibrium Green function approach. Resistance of the slab is simulated as a function of temperature for a range of slab thicknesses and effective doping in order to gain a handle on how various factors interact and compete to determine the overall resistance of the slab. If the Bi₂Se₃ slab is biased at the Dirac point, resistance is found to display an insulating trend even for strong electron–phonon coupling strength. However, when the Fermi-level lies close to the bulk conduction band (heavy electron doping), phonon scattering can dominate and result in a metallic behavior, although the insulating trend is retained in the limit of ballistic transport. Depending on values of the operating parameters, the temperature dependence of the slab is found to exhibit a remarkably complex behavior, which ranges from insulating to metallic, and includes cases where the resistance exhibits a local maximum, much like the contradictory behaviors seen experimentally in various experiments.     

Trapping centers and their distribution in Tl2In2Se3S layered single crystals

Thermally stimulated current (TSC) measurements have been carried out on Tl₂In₂Se₃S layered single crystals in the temperature range of 10–175 K. The TSC spectra reveal the presence of two peaks (A and B). The electronic traps’ distributions have been analyzed by different light illumination temperature techniques. It was revealed that the obtained traps’ distribution can be described as an exponential one. The variations of one order of magnitude in the traps’ density for every 30 meV (A peak) and 59 meV (B peak) were estimated. Moreover, the mean activation energy, attempt-to-escape frequency, capture cross section and concentration of the traps were determined.     

Effect of post annealing on the characteristics of In2S3 buffer layer grown by chemical bath deposition on a CIGS substrate

In₂S₃ as an alternative Cd-free buffer in Cu(In,Ga)Se₂ (CIGS) solar cells was deposited on CIGS substrate by a chemical bath deposition and characterized after post annealing to optimize film properties for CIGS solar cells. A uniform and pinhole-free In₂S₃ film was deposited on a CIGS substrate by H₂O₂ treatment prior to chemical bath deposition. The In₂S₃ layer was an amorphous state due to the co-existence of In–S, In–O, and In–OH bonds. Annealing at 200 °C induced copper diffusion from CIGS into In₂S₃ layer and lowered the band gap from 3.3 to 1.9 eV, leading to phase change from amorphous state to crystalline state. The conduction band alignment at the In₂S₃/CIGS interface can be controlled by the post annealing. The shunt current through In₂S₃ film was prevented down to the thickness of 30 nm and a 1.15 eV shallow defect was eliminated by the annealing. The results indicated that post annealing in air is a critical to fabricate CIGS solar cells with a sub-30 nm CBD-In₂S₃ buffer layer.     

Cr adsorption induced magnetism in Bi2Se3 film by proximity effects

Based on first-principles calculations within density functional theory, we studied the effects of Cr adsorption on the electronic and magnetic properties of Bi₂Se₃ topological insulators employing spin–orbit coupling (SOC) self-consistently. Cr atom induces a spin-polarization with total net magnetic moments of 2.157 μ_B (spin up). There is a p-d hybridization between the Cr 3d states and the nearest neighbor Se 4p states. A peak of density of states appears at Fermi level. The electronic structures change and the energy levels split near the Fermi level. No gap opening has been found at the Dirac point of the surface state from the bottom surface.     

Lattice dynamics of crystalline In4Se3

Calculations of the phonon spectrum of crystalline In₄Se₃ in a model of central-pair interactions with neglect of the long-range forces are presented. The model developed contains five unknown parameters, which are determined from experimental values of the elastic moduli without consideration of the internal displacement of the sublattices. The phonon spectrum obtained contains a large number of low-frequency modes, which deform the acoustic branches. Some common features are discovered in the dispersion curves of the electron and phonon spectra. Fiz. Tverd. Tela (St. Petersburg) 40, 2103–2108 (November 1998)     

Theoretical study of influencing factors on the dispersion of bulk band-gap edges and the surface states in topological insulators Bi2Te3 and Bi2Se3

The dispersion of the band-gap edge states in bulk topological insulators Bi₂Te₃ and Bi₂Se₃ is considered within density functional theory. The dependences of this dispersion both on the approximation used for an exchange-correlation functional at fixed unit cell parameters and atomic positions and on these parameters and positions that are obtained upon structural relaxation performed using a certain approximated functional are analyzed. The relative position of the Dirac point of topologically protected surface states and the valence band maximum in the surface electronic structure of the topological insulators is discussed.     

Synthesis and Growth of New Compounds Au3In5Se9 and Au3Ga5Se9

The phase analysis was performed and a character of interaction in AuInSe₂ - In₂Se₃ and AuGaSe₂ -Ga₂Se₃ quasi-binary systems were considered. The technology of synthesis and growth of single crystals of new semiconducting compounds Au₃In₅Se₉ and Au₃Ga₅Se₉ were developed. The materials for ohmic contacts were chosen and the electrical, optical and thermal properties of obtained compounds have been investigated. The forbidden band gap, its temperature coefficient, the type of the optical transitions and heat capacity of Au₃In₅Se₉ and Au₃Ga₅Se₉ compounds were determined. The charge of entropy and enthalpy has been estimated by numerical integration.     

Observation of π Berry phase in quantum oscillations of three‐dimensional Fermi surface in topological insulator Bi2Se3

We report the quantum transport studies on Bi₂Se₃ single crystal with bulk carrier concentration of ～10¹⁹ cm^–3. The Bi₂Se₃ crystal exhibits metallic character, and at low temperatures, the field dependence of resistivity shows clear Shubnikov–de Haas (SdH) oscillations above 6 T. The analysis of these oscillations through Lifshitz–Kosevich theory reveals a non‐trivial π Berry phase coming from three‐dimensional (3D) Fermi surface, which is a strong signature of Dirac fermions with three‐dimensional dispersion. The large Dingle temperature and non zero slope of Williamson–Hall plot suggest the presence of enhanced local strain field in our system which possibly transforms the regions of topological insulator to 3D Dirac fermion metal state. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Optical properties of Tl2In2Se3S layered single crystals

The optical properties of Tl₂In₂Se₃S layered single crystals have been analyzed using transmission and reflection measurements in the wavelength region between 500 and 1100 nm. The optical indirect transitions with a band gap energy of 1.96 eV and direct transitions with a band gap energy of 2.16 eV were determined from analysis of absorption data at room temperature. Dispersion of the refractive index is discussed in terms of the Wemple–DiDomenico single-effective-oscillator model. The refractive index dispersion parameters – oscillator energy, dispersion energy, oscillator strength and zero-frequency refractive index – were found to be 4.67 eV, 45.35 eV, 1.38 × 10¹⁴ m ^{? 2} and 3.27, respectively. Transmission measurements were also performed in the temperature range 10–300 K. As a result of temperature-dependent transmission measurements, the rate of change in the indirect band gap with temperature, i.e. γ = ?5.6 × 10^?4 eV/K, and the absolute zero value of the band gap energy, E _gi(0) = 2.09 eV, were obtained.