Investigation of dc hopping conduction in TlGaS2 and TlInS2 single crystals

It is established that variable-range hopping conduction takes place between states localized near the Fermi level in layered TlGaS₂ and TlInS₂ single crystals both along and across their natural layers in a constant electric field at T⩽200 K. The densities of states near the Fermi level and the hopping distances at different temperature are estimated. The occurrence of activationless hopping conduction is established in TlGaS₂ and TlInS₂ single crystals in the temperature range 110–150 K. Fiz. Tverd. Tela (St. Petersburg) 40, 612–615 (April 1998)     

Low-temperature X-ray studies of TlInS2, TlGaS2, and TlGaSe2 single crystals

The results of X-ray diffraction studies of the unit-cell parameters and thermal-expansion coefficients of TlInS₂, TlGaS₂, and TlGaSe₂ crystals in the temperature range 100–300 K are described. It is shown that the unit-cell parameters of all the studied crystals gradually increase with increasing temperature. The temperature dependences of these parameters exhibit anomalies in the form of bends and kinks at temperatures corresponding to phase transitions in the crystals. The thermal-expansion coefficients along the [001] crystallographic direction of the crystals under study are determined. It is found that their values slightly change with increasing temperature.     

Characteristics of traps in TlInS2 single crystals

Characteristics of charge traps in TlInS₂ single crystals are investigated by the use of thermally stimulated current (TSC) technique. The TSC spectra of the sample from 80 K to 300 K are recorded at a constant heating rate. The spectra reveal that there are several trapping levels associated with the complex structure of overlapping peaks. The experimental results indicate that the traps in TlInS₂ associated with the spectra in the measuring range of temperature obey the monomolecular (first order) kinetics. Thus, the spectra are resolved into first order shaped peaks by the use of computerized best fit procedure. The trapping parameters; such as the energy depth, temperature dependent frequency factor and capture cross section, together with concentrations of the corresponding six discrete levels are computed. These centers all having low capture cross sections with strong temperature dependence are found to be at the energies of 0.11 eV, 0.22 eV, 0.25 eV, 0.26 eV, 0.29 eV and 0.30 eV with high concentrations of 6.6 × 10¹⁶, 2.0 × 10¹⁷, 3.3 × 10¹⁷, 9.6 × 10¹⁶, 2.3 × 10¹⁷ and 4.0 × 10¹⁷ cm^?3, respectively.     

Effect of ionizing radiation on the dielectric characteristics of TlInSe<Subscript>2</Subscript> and TlGaTe<Subscript>2</Subscript> single crystals

The temperature dependences of the electrical conductivity and the permittivity of TlInSe₂ and TlGaTe₂ crystals unirradiated and irradiated with 4-MeV electrons at a doze of 10¹⁶ cm⁻² have been investigated. It has been established that electron irradiation leads to a decrease in the electrical conductivity σ and the permittivity ɛ over the entire temperature range under study (90–320 K). It has been revealed that the TlInSe₂ and TlGaTe₂ single crystals undergo a sequence of phase transitions characteristic of crystals of this type, which manifest themselves as anomalies in the temperature dependences σ = f(T) and ɛ = f(T). Electron irradiation at a doze of 10¹⁶ cm⁻² does not affect the phase transition temperatures of the crystals under investigation.     

Optical and photoelectric properties of TlInS2 layered single crystals

Single crystals of the layered compound TlInS₂ were grown by direct synthesis of their constituents. The spectral and optical parameters have been determined using spectrophotometric measurements of transmittance and reflectance in the wavelength range 200–2500 nm. Absorption spectra of thin layers of TlInS₂ crystals are used to study the energy gap and the interband transitions of the compound in the energy region 2–2.4 eV. The dispersion curve of the refractive index shows an anomalous dispersion in the absorption region and a normal one in the transmitted region. The direct and indirect band gaps were determined to be 2.34 and 2.258 eV, respectively. Photoconductivity measurements at room temperature resolve the structure that can be identified with the optical transition.     

Optical properties of thallium indium disulfide (TlInS2) single crystals

Transmission, photoluminescence, and reflectance spectra of TlInS2 single crystals grown by the Bridgman–Stockbarger method were measured at 4.2 K near the fundamental absorption edge. Narrow lines at ~2.5535 and ~2.5694 eV were observed in the transmission spectrum and assigned to ground and excited free-exciton states, respectively. The free-exciton binding energy and band-gap energy Eg were found to be ~21.2 meV and ~2.5747 eV, respectively. A recombination mechanism was proposed for the TlInS2 near-band-edge and deep luminescence.     

Effect of temperature and uniform compression on local-center luminescence in TlGaS2 layered crystals

Changes in the local-center luminescence spectrum initiated by structural defects in the TlGaS₂ crystal are studied in the temperature range from 2 to 77 K and under a hydrostatic pressure up to 35 kbar. The effect of hydrostatic compression can be explained by the relative slipping of the two elementary layers comprising the layer stack, which results in a lowering of the symmetry of the cavities occupied by the thallium atoms.     

Magnetodielectric effects in Co-implanted TlInS2 and TlGaSe2 crystals

The results of investigations of dielectric and magnetodielectric properties of ternary layered TlInS₂ and TlGaSe₂ ferroelectric crystals implanted with 40 keV Co⁺ ions at the fluency of 1.0 × 10¹⁷ ion/cm² are presented. The temperature dependences of the dielectric susceptibility of Co-implanted samples showed that the formation of metal nanoparticulate composite layer in the near-surface irradiated region as a result of high-fluency Co implantation causes considerable shifts of well-known successive structural phase transition points to high temperatures in heating regime. It has been revealed that the application of the magnetic field in the direction perpendicular to implanted surface results in shifting of the phase transition points to low temperature region. The observed peculiarities are considered as magnetocapacitance (magnetodielectric) effects, which appeared as a result of magnetoelectric lock-in interaction between domains of ferroelectric and ferromagnetic substances of the composite structure.     

Magnetic properties of Co implanted TlInS2 and TlGaSe2 crystals

The results of investigations of magnetic properties of ternary layered TlInS₂ and TlGaSe₂ ferroelectric crystals implanted with 40 keV Co⁺ ions at the fluency of 1.0×10¹⁷ ion cm^?2 are presented. It has been revealed that high-fluence implantation with Co ions results in metal nanoparticle formation in the near-surface irradiated region. The calculations of Co concentration profiles and SEM studies show that the metal nanoparticles are located under the surface at the depth of about 20 nm, and they originate the irregular-shaped bumps on the surface. The Co-implanted samples exhibited superparamagnetic behaviour at high temperatures and ferromagnetic state at temperatures lower than T_b, where T_b is a “blocking temperature” of superparamagnetic nanoparticles. It has been suggested that the observed phenomena can be discussed on the basis of strong magnetic dipolar interaction between Co nanoparticles inside the granular composite film formed as a result of implantation.     

Recombination radiation in p-ZnSnP2 single crystals

We have carried out complex investigations into the recombination radiation of p-ZnSnP₂ single crystals produced by methods involving recrystallization from nonstoichiometric melts with cooling at a controlled rate (type-I crystals) and under steady-state conditions (type-II crystals). We discuss the effect of crystal-growth conditions on the spectral contour and the position of the band maxima for recombination radiation. Type-II crystals exhibit edge exhibition and it is demonstrated that the shift of the long-wave bands in type-I crystals is controlled by the rate of cooling. The nature of the observed radiation transitions is discussed.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 43–48, April, 1986.     

Effect of the dielectric background on dispersion characteristics of metallo-dielectric photonic crystals

We theoretically study the effect of the dielectric background in two-dimensional metallo-dielectric photonic crystals. The metallo-dielectric photonic crystal consists of a square lattice of circular metallic rods embedded into a dielectric background. We calculate the photonic band structure by means of the plane wave method and the frequency-dependent finite-difference time-domain method. The transfer matrix method is used to obtain the reflectivity characteristics. Results show that the band structures shift toward lower frequencies and become flatter when the background dielectric constant increases. In addition, degeneracy can be broken and new gaps can be created in function of the dielectric background. We also found that the relative band gap width Δω/ω_g grows with increasing background dielectric constant and widths as large as 42.3% and 13.8% for the second and third band gaps can be achieved for ε_b = 9. We have investigated the origin of the new gap in these structures by studying the electric-field distribution at the band edges for the first five modes.     

Pinning effect on microwave dielectric properties and soft mode in TlInS2 and TlGaSe2 ferroelectrics

This paper presents the results of an investigation of microwave dielectric dispersion in the proper semiconductive ferroelectrics TIInS₂ and TIGaSe₂ with an incommensurate structure modulation. In these crystals there is a strongly overdamped soft ferroelectric mode, whose frequency in the vicinity of the phase transitions drops to the millimetre wave region and causes dielectric microwave dispersion plus a high contribution to the static dielectric permittivity. Within the incommensurate phase crystal defects, such as impurities, cause pinning of the soft mode. Because of this pinning effect the phason frequency increases. Pinning also changes the dynamical dielectric properties and the contribution of the phason and amplitudon to the static permittivity.     

Effect of ionizing irradiation on the mechanism of current passage in TlInSe<Subscript>2</Subscript> single crystals

The temperature dependence of the electrical conductivity and current-voltage characteristics of γ-irradiated TlInSe₂ single crystals with an electrical resistivity of ∼10⁸ Ω cm have been investigated. It has been established that the anomalies of the conductivity observed in weak electric fields and at low dozes of irradiation are related to the decomposition of neutral complexes containing an interstitial cation atom. In strong electric fields, a thermal-field ionization of traps occurs. The main mechanism of radiation defect formation is the formation of complexes [V _In⁻In _i ⁺], [V _Se⁻Se _i ⁻], and others with the structural defects characteristic of unirradiated crystals. The activation energy, trap concentrations, and the potential well shape near the traps have been determined.     

Effect of K+ ion on the dielectric properties of metallo organic L-alanine cadmium chloride single crystals

The single crystals of pure and Potassium doped L-alanine cadmium chloride (LACC), a metallo organic nonlinear optical material is grown by a slow evaporation technique. The grown crystals were confirmed by single crystal, powder XRD analyses and atomic absorption studies. Dielectric measurements were carried out for different frequencies at different temperatures. The dielectric constant decreases due to the introduction of large ionic radius K⁺ ion in the pure LACC crystal. The low dielectric constant and dielectric loss suggest that it can be used as inter-metal dielectric material.     

Infrared radiation of zinc selenide single crystals

Long-wave photoluminescence (PL) spectra of both as-grown and Au-doped n-ZnSe single crystals are studied in the temperature range from 81 to 300 K. A narrow band of infrared (IR) radiation centered at 878 nm (1.411 eV) manifests itself in the low-temperature PL spectrum. It is established that this band intensity first increases and then decreases with increasing concentration of doping impurity. With increasing excitation radiation intensity, spectral position of the IR PL band is unchanged and its intensity increases under the linear law. With increasing excitation radiation wavelength, the IR PL band intensity increases, it becomes narrower and shifts towards long wavelengths. It is shown that the observed IR radiation is caused by recombination of free electrons with holes localized on associative acceptors in the ZnSe:Zn:Au crystals or in the undoped crystals.