TlInS2 Single Crystals Research Articles

Influence of electron irradiation on the band gap and microhardness of TlInS2, TlInSSe and TlIn0.99Cr0.01S2 single crystals

ABSTRACT This work investigated the effect of partial replacement of sulphur atoms with selenium atoms (TlInSSe) and indium atoms with chromium atoms (TlIn0.99Cr0.01S2) on the band gap and microhardness of TlInS2 single crystals irradiated with electrons with energy of 2 MeV and a fluence of up to 1.5 × 1017 el/cm2. The band gap from the measured absorption spectra was determined using the Tauc method. It has been established that this irradiation leads to a decrease in the band gap in TlInS2, TlInSSe and TlIn0.99Cr0.01S2 crystals. It has been shown that after electron irradiation the microhardness of these crystals increased due to an increase in the size of crystallites on their surface.

Ab initio calculations of electronic properties, frequency dispersion of dielectric coefficients and the edge of the optical absorption of TlInS-=SUB=-2-=/SUB=-<Sn> single crystals

Calculations of the band structure and density of states for supercells of TlInS2 and TlInS2<Sn> semiconductor crystals with monoclinic singony within the density functional theory (DFT) are presented. In DFT calculations, the Coulomb repulsion (the Hubbard parameter (U)) was taken into account in order to correctly describe the band gap (Eg) of crystals. It is shown that the maximum of the valence band and the minimum of the conduction band of TlInS2 are located at the center (point ) of the Brillouin zone, which points to the direct energy of the band gap of the TlInS2 and TlInS2<Sn> crystals. Thrue results of calculating the band structure and features of the distribution of the density of electronic states in TlInS2 and TlInS2<Sn> are discussed. TlInS2 single crystals doped with 0.1 mol.% tin (TlInS2<0.1 mol.% Sn>) were synthesized and then grown by the Bridgman--Stockbarger method. The frequency dispersion of the dielectric coefficients and conductivity of a TlInS2 and TlInS2<0.1 mol.% Sn> single crystals in the frequency range f=5·104-3.5·107 Hz is studied. It is shown that relaxation losses occur in TlInS2<Sn>. A hopping mechanism of ac charge transfer in TlInS2<Sn> has been established. In TlInS2<Sn>, the parameters of localized states, such as the density of states near the Fermi level and their energy spread, the average hopping time and distance, and the concentration of deep traps, are estimated. The optical properties of TlInS2 and TlInS2<0.1 mol.% Sn> single crystals have been studied. The values of Eg for direct optical transitions in TlInS2 and TlInS2<0.1 mol.% Sn> crystals were obtained from the optical absorption spectra. It has been shown that the introduction of 0.1 mol.% Sn, which replaces indium atoms, into TlInS2 reduces the value of Eg, for example, at 150 K from 2.539(TlInS2) to 2.486 eV (TlInS2<0.1 mol.% Sn>). From optical measurements, the average temperature coefficient of the band gap d Eg/d T=-7·10-4 eV/K for TlInS2<Sn> was calculated. The decrease in the band gap of the TlInS2Sn> single crystal with respect to TlInS2 is 16 meV at 300 K and 53 meV at 150 K. Keywords: supercell, semiconductor TlInS2, tin doping, monoclinic syngony, density functional theory, electronic structure, single crystals, dielectric properties, optical absorption, charge transfer, parameters of localized states.

Electronic irradiation of TlInSxSe2−x (x = 1): Morphology, structure and raman scattering

It is shown that the replacement of a part of sulfur atoms with selenium atoms in a TlInS2 single crystal stimulates the formation of a single-phase state with a monoclinic structure (space group [Formula: see text]/[Formula: see text] in TlInS[Formula: see text]Se[Formula: see text] ([Formula: see text]). Irradiation with 2 MeV electrons and a fluence of [Formula: see text] electron/cm2 of powder TlInS[Formula: see text]Se[Formula: see text] ([Formula: see text]) leads to an increase in the crystallite size from 56.5 nm to 65 nm, which is most likely associated with a decrease in the interface. The difference between the surface morphology of the synthesized TlInS[Formula: see text]Se[Formula: see text] ([Formula: see text]) single crystal and the surface morphology of the TlInS2 single crystal is established, which consists in a decrease in the height and width of the roughness in TlInS[Formula: see text]Se[Formula: see text] ([Formula: see text]). Irradiation of a TlInS[Formula: see text]Se[Formula: see text] ([Formula: see text]) single crystal with electrons with a fluence of [Formula: see text] electron/cm2 does not lead to a change in the height of the tubercle on its surface, and the average value of its width increases more than ten-fold. The identity of the peaks in the Raman spectra of the TlInS[Formula: see text]Se[Formula: see text] ([Formula: see text]) single crystal before and after its irradiation with electrons with an energy of 2 MeV and upto a fluence of [Formula: see text] electron/cm2, along with the absence of a shift of the peaks, indicates the radiation resistance of the TlInS[Formula: see text]Se[Formula: see text] ([Formula: see text]) single crystal.

Study of the influence of electronic radiation on the surface, structure and Raman spectrum of a TlInS2 single crystal

The effect of electrons on the surface, Raman spectra, structure and phase transition in a TlInS2 single crystal was studied. Irradiation of TlInS2 with electrons with an energy of 2 MeV and a fluency of 2 × 1017 electron/cm2 leads to an increase in the height of the bump and its width on the sample surface. Radiation treatment of TlInS2 with electrons indicates the appearance of new peaks at 204 cm−1, 224 cm−1 in the Raman spectra and stimulates the formation of a single-phase state with a hexagonal structure (space group P63/mmc).

Type of Optical Transitions at the Fundamental Absorption Edge in TlGaSe2 and TlInS2 Crystals Subjected to γ-Radiation

The effect of γ-radiation on the optical properties of layered TlGaSe2 and TlInS2 crystals has been studied within a wavelength range of 400–1100 nm at 300 K. By means of analysis of optical absorption spectra, the energies of direct and indirect optical interbend transitions before and after γ-irradiation have been determined. It has been shown that the energies of direct and indirect nonforbidden optical transitions grow with accumulation of γ-radiation dose within 0–25 Мrad in TlGaSe2 and TlInS2 single crystals from Egd = 2.06 eV and Egi = 1.90 eV at D = 0 Мrad to Egd = 2.11 eV and Egi = 1.98 eV at D = 25 Мrad for TlGaSe2 crystals and from Egd = 2.32 eV and Egi = 2.27 eV at D = 0 Мrad to Egd = 2.35 eV and Egi = 2.32 eV at D = 25 Мrad for TlInS2 crystals. A decrease in the transmission coefficient at doses from 0 to 5 Mrad with a further increase in the transmission coefficient at a radiation dose of D = 25 Мrad is observed.

Тип оптических переходов на краю фундаментального поглощения кристаллов TlGaSe-=SUB=-2-=/SUB=- и TInS-=SUB=-2-=/SUB=-, подвергнутых γ-облучению

The effect of gamma irradiation on the optical properties of layered TlGaSe2 and TlInS2 crystals in the wavelength range of 400–1,100 nm at 300 K is studied. From the analysis of optical absorption spectra, the energies of direct and indirect optical interband transitions before and after gamma irradiation are determined. It is shown that as the gamma-irradiation dose is accumulated in the range of 0–25 Mrad by TlGaSe2 and TlInS2 single crystals, the energies of direct and indirect allowed optical transitions increase from Egd = 2.06 eV and Egi = 1.90 eV at D = 0 Mrad to Egd = 2.11 eV and Egi = 1.98 eV at D = 25 Mrad for TlGaSe2 and Egd = 2.32 eV crystals and Egi = 2.27 eV at D = 0 Mrad to Egd = 2.35 and Egi = 2.32 eV at D = 25 Mrad for TlInS2 crystals. A decrease in the transmittance at doses from 0 to 5 Mrad and a further increase in the transmittance at the radiation dose D = 25 Mrad are observed.

Defect characterization in neodymium doped thallium indium disulfide crystals by thermoluminescence measurements

Characteristics of defect centers in neodymium doped TlInS2 single crystals have been investigated in virtue of thermoluminescence measurements carried out at low temperatures (10–300K) with various heating rates between 0.4 and 1.2Ks−1. One glow peak was detected with peak maximum temperature of 26K at a rate of 0.4Ks−1. The observed glow peak was analyzed using three points and heating rate methods. The analysis results revealed the presence of one trap level with activation energy of 14meV. Three points method showed that mixed order of kinetic dominates the trapping level. Shift of peak maximum temperature to higher values and decrease in TL intensity were observed as the heating rate was increased progressively. Distribution of traps was demonstrated using an experimental method based on illumination temperature varying between 10 and 14K.

Ferroelasticity of TlInS2 crystal

The studies of dielectric and optical properties of TlInS2 single crystal were carried out in the temperature interval 150–300K and in the hydrostatic pressure range up to 700MPa. The presence of the structure of ferroelastic domains at high pressures enables us to claim that the high-pressure phase in the TlInS2 crystal is a ferroelastic one. The transition to this phase with increasing pressure is achieved via a sequence of two first-order phase transitions with an intermediate phase. A number of features are revealed in the polycritical region in the range of pressures 550–650MPa and temperatures 180–220K. Thus, the (p,T) phase diagram of the TlInS2 crystal in the pressure range p>550MPa is redetermined.

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.

Multiphonon absorption processes in layered structured TlGaS2, TlInS2 and TlGaSe2 single crystals

The infrared transmittance and Raman scattering spectra in TlGaS2, TlInS2 and TlGaSe2 layered single crystals grown by Bridgman method were studied in the frequency ranges of 400–1500 and 10–400cm−1, respectively. Three, three and five bands observed at room temperature in IR transmittance spectra of TlGaS2, TlInS2 and TlGaSe2, respectively, were interpreted in terms of multiphonon absorption processes.

Effect of ionizing radiation on the dielectric characteristics of TlInS2 and TlGaS2 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.

Temperature oscillating optical second-harmonic generation in TlGaSe2 and TlInS2 single crystals

Second-harmonic (SH) generation effects have been investigated in TlGaSe2 and TlInS2 layered crystals. Pronounced periodical changes of the SH signal with temperature are discovered. The observed effect is explained within the phase synchronism condition, which changes with temperature in the course of thermal expansion. A major role of the high value of dispersion for the refractive indexes of the investigated crystals in the observed phenomenon is proposed.

Optical properties of thallium indium disulfide (TlInS2) single crystals

2, TlInSe2, TlInTe2, TlGaS2, TlGaSe2, and TlGaTe2 have unusual structural, optical, photoelectric, ferroelectric, and other properties and are of definite interest for semiconducting opto- and microelectronics [6–8]. The optical properties of TlInS2 near the fundamental absorption edge were studied using various methods such as transmission, luminescence, reflectance, photoconductivity, etc. [9–17]. Based on these studies, a direct-band energy structure was established and the exciton binding energy, band-gap width, and other optical characteristics of the material were determined. However, experimental optical spectroscopic data for semiconducting TlInS2 were in most instances contradictory. This indicated that further research was required in order to refine known fundamental optical parameters and to determine the unknown ones. In particular, the value of the exciton binding energy, an important parameter that defines the band-gap width, varied over wide ranges from 12.3 meV [12] to 33 meV [18]. This prevented the band-gap width Eg of TlInS2 from being accurately determined and the optical properties of dichalcogenide semiconducting compounds from being analyzed with a unified approach. Moreover, the nature of the energy-band structure of TlInS2 in the vicinity of direct allowed transitions was in need of refinement. Herein transmission and reflectance of light, photoluminescence (PL), and luminescence spectra were used to obtain new data on the optical characteristics of semiconducting TlInS2. These data may be useful for process monitoring during the production of single crystals with the maximum degree of structural perfection. Experimental. Layered TlInS2 single crystals were grown by the Bridgman–Stockbarger method [15]. Transmission was measured in thin (~2–40 μm) plane-parallel plates prepared by splitting larger samples along cleavage planes. It should be noted that the absorption coefficient of TlInS2 near the fundamental absorption edge is α ~ 5⋅10 3 cm –1 [12] and α ~ 1⋅10 3 cm –1 [18]. Cleaved thin plates (~5 × 5 mm) were yellow with a mirror-smooth surface that coincided with the crystallographic (001) plane. Non-equilibrium charge carriers in the crystals were excited during PL measurements by radiation from a He–Cd laser (λex = 325 nm, power up to 10 mW). In several instances, PL

Charged Defects as an Origin of the Memory Effect in Incommensurate Phase of TlInS2 Ferroelectric–Semiconductors

The effects of annealing within the incommensurate phase and doping with the lanthanum impurity on the dielectric function ε of the TlInS2 single crystals have been investigated. It is shown that illumination and application of external electric field transform the ε(T) dependences for both the annealed and doped samples in the same manner. The inference is made that the correlation between observed effects is conditioned by the internal electric fields induced either by activation (polarization) of native defects during the annealing procedure or after the doping the crystals with active La centers.

Photoinduced Current Transient Spectroscopy of TlInS2 Layered Crystals Doped with Er, B, and Tb Impurities

Photoinduced current transient spectroscopy (PICTS) has been utilized to study electrically active defects in TlInS2 single crystals doped with Er, B, and Tb. Eight traps with activation energies lying in between 0.21 to 0.55 eV have been detected. The origin of these traps is discussed.

Activated impurity states in the incommensurate phase of ferroelectric semiconductor TlInS2

The effect of annealing within the incommensurate phase on the dielectric function ε of the TlInS2 single crystals has been investigated. It is shown that the effect of annealing is very close to the effect of doping by electrically active impurity La. The inference is made that the correlation between observed effects in annealed and doped crystals is conditioned by the internal electric fields induced by the activation (polarization) of native defects during the annealing procedure. The investigations of the second harmonic generation in undoped TlInS2 crystal and the pyrocurrent in TlInS2:La confirms the proposed model.

Effect of gamma irradiation on the dielectric properties and electrical conductivity of the TlInS2 single crystal

The effect of gamma irradiation on the dielectric properties and ac conductivity of a TlInS2 single crystal with a layered structure has been investigated in the frequency range from 5 × 104 to 3.5 × 107Hz. It has been shown that gamma irradiation of the TlInS2 single crystal with a dose of 104–2.25 × 106 rad leads to a considerable increase in the dielectric loss tangent tanδ, the real part ɛ′ and imaginary part ɛ″ of the complex permittivity, and the ac conductivity σac across the layers. It has been established that, for all gamma irradiation doses, the TlInS2 single crystal is characterized by the dielectric loss due to electrical conduction up to a frequency of 107 Hz and by the relaxation loss at a higher frequency. Irradiation of the TlInS2 single crystal results in an increase in the dispersion of tan δ, ɛ′, and ɛ″. It has been demonstrated that, as the gamma irradiation dose is accumulated in the TlInS2 single crystal, the density of localized states near the Fermi level NF increases (from 5.2 × 1018 to 1.9 × 1019 eV−1 cm−3).

XPS of incommensurate and electronic structure of thallium chalcogenides

The paper presents the X-ray photoelectron spectra (XPS) of the valence band (VB) and of the principal core levels (CL) for the TlInS2 and TlInSe2 single crystals. The XPS were measured with monochromatized Al Kα radiation in the energy range of 0 -- 1400 eV at different temperatures. The resolved splitting of XPS in the incommensurate phase of the ferroelectric semiconductor TlInS2 single crystal was obtained. The VB is located from 0.6 to 10 eV below the Fermi level. Experimental energies of the VB and CL are compared with the results of quantum mechanical ab initio calculations of the molecular model of the TlInSe2 crystal. The electronic structure of the VB and CL is described theoretically by quantum-mechanical Hartree-Fock calculations. The surface and bulk atoms influence the shape of the VB and CL, which is crystallographic plane dependent. The chemical shifts in the both crystals for the Tl, In, S and Se states are obtained.

EPR study of the structural phase transitions in Fe3+ doped TlInS2

AbstractThe results of low temperature EPR investigations of Fe doped TlInS2 single crystals in the temperature range 5–300 K are presented. The EPR signal due to Fe3+ centers located at the centers of InS4 tetrahedrons has been observed. It has been established that the resonance lines exhibit a slight shift and remarkable splitting at the temperatures lower than 200 K, which is attributed to the effect of the ferroelectric phase transition. The rotational patterns of the resonance fields reveal a doubling of the number of centers at low temperatures. The preliminary simulation indicates that the doubling is related to the rotation of the principal axis system of the centers, while the magnitude of fine structure splitting is nearly the same. ( © 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)