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This paper reports the changes in morphology and topographic roughness on the surface of a pristine TlGaS e 2 layered crystal caused by an external electric field applied perpendicular to the layer plane at room temperature. These electric-field-induced surface reconstructions and modifications in the TlGaS e 2 sample were monitored through x-ray reflectivity, x-ray diffraction, and atomic force microscopy techniques. Two distinct electric-field-induced surface responses have been observed: the shifting of the XRD peaks to the higher Bragg angles and the variations in the XRR curves depending on the strength and polarity of the applied external electric fields. AFM results show that the applied electric field leads to a reproducible transformation of the surface roughness of the TlGaS e 2 single crystal from smooth to disheveled, with well-defined depth protrusions. The relaxation time of these surface topological configurations induced under an applied dc electric field was found to be much longer than a few days. The electrostatic capacitive behavior of this two-dimensional semiconducting material is believed to be caused by lattice distortions and the formation of inner stresses (strains) during electric field poling, as well as a drop in the unit-cell characteristics of TlGaSe2. The current–voltage (I–V) measurements show a pronounced nonlinear relationship for a previously poled sample. This nonlinearity is attributed to the field-effect-induced capacitance in TlGaS e 2. Electromigration of intrinsic defects such as Se-anion vacancies, which are already present in the crystal lattice structure of virgin TlGaS e 2 , may diffuse into the sample surface from a bulk (or vice versa) during electric field applications. Finally, employing DFT simulations, we present that the Se-anion vacancy model may be beneficial because changes in the charge state of metal ions positioned around selenium vacancies could be expected. The slightly asymmetric capacitance with respect to the polarity of the bias potential applied to the top surface of TlGaS e 2 is justified by our theoretical calculations. [ABSTRACT FROM AUTHOR]

Herein, the thermoelectric performance of TlGaSe2 ternary layered dichalcogenides is evaluated by applying ab initio density functional theory calculations combined with Boltzmann's transport equation. A novel approach to design the intrinsic structural defects via Se‐anion vacancies in unit cell has been developed. Two kinds of Se‐vacancy defects in host TlGaSe2 crystal lattice are engineered: the single vacancy defect induced intrinsically in the unit cell (1×1×1) and in the supercell lattice (1×1×4). It is found that the electrical transport properties and thermoelectric efficiency of this semiconductor could be significantly altered by introducing Se‐vacancy states into crystalline structure. In addition, simulation shows that inclusion of Se‐vacancy defects significantly improves the thermoelectric efficiency as well as the thermoelectric power factor and figure of merit (ZT) values of this compound. Additionally, the thermoelectric performance of TlGaSe2 is estimated by means of the electronic fitness function calculations in the valence and conduction edges. The results demonstrate that TlGaSe2 with introduced Se‐vacancies may be a perspective material for thermoelectric applications. [ABSTRACT FROM AUTHOR]

Lanthanum-doped high quality TlInS2 (TlInS2:La) ferroelectric-semiconductor was characterized by photo-induced current transient spectroscopy (PICTS). Different impurity centers are resolved and identified. Analyses of the experimental data were performed in order to determine the characteristic parameters of the extrinsic and intrinsic defects. The energies and capturing cross section of deep traps were obtained by using the heating rate method. The observed changes in the Thermally Stimulated Depolarization Currents (TSDC) near the phase transition points in TlInS2:La ferroelectric-semiconductor are interpreted as a result of self-polarization of the crystal due to the internal electric field caused by charged defects. The TSDC spectra show the depolarization peaks, which are attributed to defects of dipolar origin. These peaks provide important information on the defect structure and localized energy states in TlInS2:La. Thermal treatments of TlInS2:La under an external electric field, which was applied at different temperatures, allowed us to identify a peak in TSDC which was originated from La-dopant. It was established that deep energy level trap BTE43, which are active at low temperature (T≤156 K) and have activation energy 0.29 eV and the capture cross section 2.2×10-14 cm2, corresponds to the La dopant. According to the PICTS results, the deep level trap center B5 is activated in the temperature region of incommensurate (IC) phases of TlInS2:La, having the giant static dielectric constant due to the structural disorders. From the PICTS simulation results for B5, native deep level trap having an activation energy of 0.3 eV and the capture cross section of 1.8×10-16cm2 were established. A substantial amount of residual space charges is trapped by the deep level localized energy states of B5 in IC-phase. While the external electric field is applied, permanent dipoles, which are originated from the charged B5 deep level defects, are aligned in the direction of the applied electric field and the equilibrium polarization can be reached in a relatively short time. When the polarization field is maintained, while cooling the temperature of sample to a sufficiently low degrees, the relaxation times of the aligned dipoles drastically increases. Practically, frozen internal electric field or electrets states remain inside the TlInS2:La when the applied bias field is switched off. The influence of deep level defects on TSDC spectra of TlInS2:La has been revealed for the first time. [ABSTRACT FROM AUTHOR]

The strong enhancement, by several orders of magnitude, of the excitonic peak within the photoconductivity spectrum of TlGaSe2 semiconductor was observed. The samples were polarized in external dc electric field, which was applied prior to the measurements. Due to the accumulation of charges near the surface, an internal electric field was formed. Electron-hole pairs that were created after the absorption of light are fallen in and then separated by the built-in electric field, which prevents radiative recombination process. [ABSTRACT FROM AUTHOR]

We report on the characteristics of metal-p-type high resistance TlGaSe2 semiconductor junction barrier fabricated by deposition of indium and gold metals. The electrical properties of /TlGaSe2/ semiconductor contacts were monitored as a function of temperature in the range of ∼80–300 K using current–voltage () and capacitance–voltage () measurements. Device characteristics of /TlGaSe2/ showed rectification properties. From forward bias characteristics it was revealed that the increase of current is slower than the predictions by Schottky barrier theory. The rectification properties of /TlGaSe2/ semiconductor device were simulated by the Mott barrier model where the presence of an undoped layer on the semiconductor surface is assumed, and measurements and computational simulation agreed on the validity of this model. measurements showed that at higher temperatures /TlGaSe2/ barrier showed a capacitance from accumulation toward depletion mode, whereas at low temperatures the barrier capacitance degradation was found. The observed of /TlGaSe2/ does not significantly change over the entire bias range 30 V, confirming that the /TlGaSe2/ is fully depleted, thus the structures are Mott barrier. First-principles electronic band diagram calculations showed that introduction of -atom vacancies in the various sites of TlGaSe2 unit cell greatly affects the electronic band structure of this semiconductor in an increasing manner in band gaps with respect to stoichiometric compound. Consequently, the - vacancies localized inside the thin surface layer of TlGaSe2 are responsible for high average surface electrical resistivity of the material and could be proposed as a physical basis for the existence of native insulator layer on TlGaSe2 surface. Thus we propose a model for TlGaSe2 crystal which is comprised from an electrically conducting bulk semiconductor sandwiched between two high resistivity thin surface sheets. We suggest that the selenium vacancy defects on the surface of TlGaSe2 layered semiconductor are also responsible for the memristive behavior of this compound. [ABSTRACT FROM AUTHOR]

The results from the investigation of optical absorption in TlGaSe2 layered bulk semiconductor in the wavelength range of 400–900 nm between the temperatures of 19 K and 300 K are presented and discussed. In the results, the layered semiconductor TlGaSe2 is interpreted as a direct band gap semiconductor with a band gap of about 2.04–2.14 eV at 300 K. Additionally, the optical transmission spectrum of TlGaSe2 exhibited some peculiarities, which were attributed to indirect allowed interband transitions. It has been explained that the optical absorption edge of TlGaSe2 can be characterized by the presence of an Urbach-like tail as well as a high-energy Tauc absorption region. The Tauc slope and Tauc energy gap were extracted from the absorption coefficient data by using the Tauc procedure. The results revealed that the 'amorphized' structure of TlGaSe2 which is prominent in near conduction/valance band energy states, cannot be ignored for the understanding of the optical transition mechanisms in this semiconductor. [ABSTRACT FROM AUTHOR]

The structural, electrical, and thermal properties of the ferroelectric-semiconductor TlGaSe2 have been investigated after the thermal annealing of crystals inside the incommensurate (INC) phase for a few hours at the annealing temperature (Tann). It is found that all outlined physical parameters of TlGaSe2 are significantly modified after the annealing process. Besides the well-known memory effect, thermal annealing within the INC phase leads to previously unknown anomalies of the different physical properties of TlGaSe2 crystals at temperatures quite far from the INC-phase. To explain the experimental results it is supposed that annealing leads to the formation of a new polarized state that influences most of the physical parameters of the crystals. This outstanding feature of the annealing effect in TlGaSe2 crystals allows us to reveal some new phase transitions that take place in TlGaSe2 crystals in the 140-180 K temperature range. [ABSTRACT FROM AUTHOR]

The electrical properties of TlGaSe2 ferroelectric-semiconductor with layer crystalline structure in the direction parallel to the layers plane is studied by impedance spectroscopy in the frequency range 30 Hz-15 MHz. The impedance spectra were analyzed together with earlier results for perpendicular direction. Low frequency dispersion due to the interfacial polarization from Maxwell-Wagner type space charge effect has been observed. It was established that anomalous rise of relaxation time in the 145-190 K temperature range far from the known phase transitions temperatures is characteristic only for the polarization perpendicular to the layers plane. The model of a specific phase transition taking place in 145-190 K temperature region assuming carriers accumulation was proposed. Charge disproportination is the main characteristic feature of this phase transition which leads to the charge density modulation in one crystallographic direction perpendicular to the layers plane of the crystal. [ABSTRACT FROM AUTHOR]

We conducted comparison of the original experimental data of the temperature dependences of thermal expansion in crystals with layered crystalline structure. It is shown that in most crystals with layered structure (graphite, boron nitride, GaSe, GaS, and InSe) the effect of negative thermal expansion can be explained by the specific character of the phonon spectra. It was shown, that in contrast to other crystals with layered structure, negative thermal expansion in the layers' plane of TlGaSe2 is the result of negative area compressibility. We demonstrate that the thermal expansion of TlGaSe2 crystals can be controlled by illumination, external electric field, and thermal annealing. The nature of observed effects and a special mechanism of the negative area compressibility in TlGaSe2 crystals are discussed. [ABSTRACT FROM AUTHOR]

The effect of annealing within the incommensurate phase on the dielectric function [variant_greek_epsilon] 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. [ABSTRACT FROM AUTHOR]

The effect of annealing within the incommensurate phase on the dielectric function [variant_greek_epsilon] 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. [ABSTRACT FROM AUTHOR]

The effect of electric field on the thermal expansion of the TlGaSe2 ferroelectric-semiconductor with a layer crystalline structure has been investigated. A strong transformation of negative linear expansion coefficient in the layer plane has been observed as a result of the applied electric field. It was concluded that internal electric fields created by local polarized states in the ferroelectric- semiconductor are responsible for a negative thermal expansion and its behavior under the electric field. Predominantly, the electrostriction effect is the driving mechanism of the lattice deformation of TlGaSe2 crystals at low temperatures. [ABSTRACT FROM AUTHOR]