Davydov splitting and rigid-layer modes in InS crystal

Polarized infrared reflection spectra of InS layer crystal have been measured. Both components of the Davydov doublets caused by the splitting of intralayer vibrations due to the interlayer interactions are active in i.r. spectra. Doublet frequencies have been used to calculate the rigid-layer mode frequencies which are correlated with the experimental ones. It has been established that in InS crystal the frequencies of rigid-layer modes are much higher than those of ordinary layer crystals. Atomic displacements have been found at i.r.-active normal modes.     

Thermally Stimulated Current Observation of Trapping Centers in Undoped GaSe Layered Single Crystals

Undoped p‐GaSe layered single crystals were grown using Bridgman technique. Thermally stimulated current measurements in the temperature range of 10‐300 K were performed at a heating rate of 0.18 K/s. The analysis of the data revealed three trap levels at 0.02 , 0.10 and 0.26 eV. The calculation for these traps yielded 8.8 × 10^‐27, 1.9 × 10^‐25, and 3.2 × 10^‐21 cm² for capture cross sections and 3.2 × 10¹⁴, 1.1 × 10¹⁶, and 1.2 × 10¹⁶ cm^‐3 for the concentrations, respectively.     

Photoluminescence Spectra of GaS0.75Se0.25 Layered Single Crystals

Photoluminescence (PL) spectra of GaS_0.75Se_0.25 layered single crystals have been studied in the wavelength region of 500‐850 nm and in the temperature range of 10‐200 K. Two PL bands centered at 527 ( 2.353 eV, A‐band) and 658 nm (1.884 eV, B‐band) were observed at T = 10 K. Variations of both bands have been studied as a function of excitation laser intensity in the range from 8 × 10^‐3 to 10.7 W cm^‐2. These bands are attributed to recombination of charge carriers through donor‐acceptor pairs located in the band gap. Radiative transitions from shallow donor levels located 0.043 and 0.064 eV below the bottom of conduction band to acceptor levels located 0.088 and 0.536 eV above the top of the valence band are suggested to be responsible for the observed A‐ and B‐bands in the PL spectra, respectively.     

Study of trapping and recombination centres in Tl2InGaTe4 chain crystals by dark electrical conductivity and photoconductivity measurements

Dark electrical conductivity and photoconductivity of Tl₂InGaTe₄ single crystals have been measured and analyzed in the temperature region 100–300 K. The dark electrical conductivity measurements revealed an intrinsic- or extrinsic-type of conductivity above or below 210 K, respectively. From intrinsic conductivity data analysis, the energy band gap of Tl₂InGaTe₄ crystals was determined as 0.85 eV. In the extrinsic region, the dark conductivity arises from a donor energy level located at 0.30 eV below the conduction band. The photocurrent increases with increasing illumination intensity. The recombination mechanism in the crystal changes as temperature decreases due to the effect of exponential trapping centres. Two trapping and/or recombination centres located at 89 and 27 meV were determined from the temperature dependence of the photocurrent, which decreased or increased with increasing temperature in the regions above or below 180 K, respectively.     

Optical properties of TlGaxIn1-xSe2-layered mixed crystals (0.5 ≤ x ≤ 1) by spectroscopic ellipsometry,transmission, and reflection measurements

The layered semiconducting TlGa_xIn_1-xSe₂-mixed crystals (0.5?≤?x?≤?1) were studied for the first time by spectroscopic ellipsometry measurements in the 1.2–6.2?eV spectral range at room temperature. The spectral dependence of the components of the complex dielectric function, refractive index, and extinction coefficient were revealed using an optical model. The interband transition energies in the studied samples were found from the analysis of the second-energy derivative spectra of the complex dielectric function. The effect of the isomorphic cation substitution (indium for gallium) on critical point energies in TlGa_xIn_1-xSe₂ crystals was established. Moreover, the absorption edge of TlGa_xIn_1-xSe₂ crystals have been studied through the transmission and reflection measurements in the wavelength range of 500–1100?nm. The analysis of absorption data revealed the presence of both optical indirect and direct transitions. It was found that the energy band gaps decrease with the increase of indium content in the studied crystals.     

Spectroscopic ellipsometry study of above-band gap optical constants of layered structured TlGaSe2, TlGaS2 and TlInS2 single crystals

Spectroscopic ellipsometry measurements on TlGaSe₂, TlGaS₂ and TlInS₂ layered crystals were carried out on the layer-plane (0 0 1) surfaces, which are perpendicular to the optic axis c^?, in the 1.2–6.2 eV spectral range at room temperature. The real and imaginary parts of the pseudodielectric function as well as pseudorefractive index and pseudoextinction coefficient were found as a result of analysis of ellipsometric data. The structures of critical points in the above-band gap energy range have been characterized from the second derivative spectra of the pseudodielectric function. The analysis revealed four, five and three interband transition structures with critical point energies 2.75, 3.13, 3.72 and 4.45 eV (TlGaSe₂), 3.03, 3.24, 3.53, 4.20 and 4.83 eV (TlGaS₂), and 3.50, 3.85 and 4.50 eV (TlInS₂). For TlGaSe₂ crystals, the determined critical point energies were assigned tentatively to interband transitions using the available electronic energy band structure.     

Ellipsometry study of interband transitions in TlGaS2xSe2(1−x) mixed crystals (0≤x≤1)

In this paper, the spectroscopic ellipsometry measurements on TlGaS_2xSe_2(1?x) mixed crystals (0≤x≤1) were carried out on the layer-plane (001) surfaces with light polarization E⊥c^? in the 1.2–6.2 eV spectral range at room temperature. The real and imaginary parts of the dielectric function, refractive index and extinction coefficient were calculated from ellipsometric data using the ambient-substrate optical model. The critical point energies in the above-band gap energy range have been obtained from the second derivative spectra of the dielectric function. Particularly for TlGaSe₂ crystals, the determined critical point energies were assigned tentatively to interband transitions using the available electronic energy band structure. The effect of the isomorphic anion substitution (sulfur for selenium) on critical point energies in TlGaS_2xSe_2(1?x) mixed crystals was established.     

Thermally stimulated current measurements in undoped Ga3InSe4 single crystals

The trap levels in nominally undoped Ga₃InSe₄ crystals were investigated in the temperature range of 10-300 K using the thermally stimulated currents technique. The study of trap levels was accomplished by the measurements of current flowing along the c-axis of the crystal. During the experiments we utilized a constant heating rate of 0.8 K/s. Experimental evidence is found for one hole trapping center in the crystal with activation energy of 62 meV. The analysis of the experimental TSC curve gave reasonable results under the model that assumes slow retrapping. The capture cross-section of the trap was determined as 1.0×10⁻²⁵ cm² with concentration of 1.4×10¹⁷ cm⁻³.     

Trap levels in layered semiconductor Ga2SeS

Trap levels in nominally undoped Ga₂SeS layered crystals have been characterized by thermally stimulated current (TSC) measurements. During the measurements, current was allowed to flow along the c-axis of the crystals in the temperature range of 10-300 K. Two distinct TSC peaks were observed in the spectra, deconvolution of which yielded three peaks. The results are analyzed by curve fitting, peak shape and initial rise methods. They all seem to be in good agreement with each other. The activation energies of three trapping centers in Ga₂SeS are found to be 72, 100 and 150 meV. The capture cross section of these traps are 6.7×10⁻²³, 1.8×10⁻²³ and 2.8×10⁻²² cm² with concentrations of 1.3×10¹², 5.4×10¹² and 4.2×10¹² cm⁻³, respectively.     

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.