Rauf A. Suleymanov

Deformation effects in electronic spectra of the layered semiconductors TlGaS2, TlGaSe2 and TlInS2

The deformation effects in electronic spectra of the ternary layered semiconductors TlGaS2, TlGaSe2 and TlInS2 are considered. It is shown that the influence of hydrostatic pressure, thermal expansion and variation of composition in solid solutions on the band gap of the crystals investigated can be described in the framework of one common model of deformation potentials. This model appears to be close to that of layered semiconductors of the A3B6 group, attesting to the fact that the main principles of formation of band structure in these two groups of layered crystals are the same.

The effect of impurities on the phase transitions in the ferroelectric semiconductors TlInS2 and TlGaSe2

The temperature dependences of the dielectric constants of the ferroelectric semiconductors TlInS2 and TlGaSe2 have been studied following their annealing within the incommensurate phase. Unusual memory effects accompanied by both a remarkable inflection of the temperature dependence curves in the incommensurate phase and various shifts of the incommensurate (Ti) and commensurate (Tc) phase transition temperatures have been revealed in both crystals. The observed effects are explained on the basis of a defect density wave model taking into account the interaction of modulation waves with charge carriers localized at impurity states. The thermally activated population of these states during the heating or cooling processes is responsible for the changes of the phase transition temperatures.

Memory effect in ferroelectric-semiconductor with incommensurate phase TlGaSe2

Abstract The paper is devoted to the investigations of the non-equilibrium properties of the incommensurate (INC) phase of the improper ferroelectric-semiconductor TlGaSe2. The influence of the prehistory of the heat treating of the crystal, i.e. annealing at a fixed, stabilized temperature in the region of INC-phase on the dielectric constant (e) in the vicinity of the phase transition (PT)-INC phase (the commensurate (C) ferroelectric phase) was studied. The peculiar case of the memory effect leading to the temperature range change of the INC-phase existence is observed for the first time in TlGaSe2.

Negative thermal expansion due to negative area compressibility in TlGaSe2 semiconductor with layered crystalline structure

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.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.

The effect of thermal annealing on impurity states in ferroelectric-semiconductor TlGaSe2 within the incommensurate phase

The effect of thermal annealing on dark conductivity and thermally stimulated conductivity in ferroelectric-semiconductor TlGaSe2 within the incommensurate phase is investigated. It is shown that both types of conductivities are drastically changed after annealing of the crystal for some hours within the incommensurate phase. The results obtained lead to the conclusion that the main effect of annealing is a large decrease of impurity capture cross sections leading to the dramatic increasing of relaxation times.

Dielectric spectroscopy and nonequilibrium phase transitions in TlGaSe2 layered crystals

The real and imaginary parts of the dielectric function of TlGaSe2 single crystals are investigated in the frequency range 30 Hz–13 MHz and at temperatures 80 K–300 K. A low frequency dispersion due to the interfacial polarization within the crystals is observed. The temperature dependence of relaxation time is obtained in a wide temperature region. A well- distinguished anomalous rise of relaxation time has been established in the 145 K–190 K temperature range for the first time far from the traditional phase transitions known in TlGaSe2 crystals. It is shown that the observed behaviour of relaxation time is due to instabilities in the electronic subsystem revealing themselves as a non-equilibrium phase transition. A close relationship between the observed anomaly and anomalous behaviour of the various physical parameters of the investigated crystals is proposed.

Effect of external fields on the memory effect of the incommensurate phase in the ferroelectric-semiconductor TlGaSe2

The effect of external fields (dc electric field, light illumination) on the memory effect of the incommensurate phase in the ferroelectric-semiconductor TlGaSe2 is studied using the measured dielectric constant. The results obtained are discussed. It is shown for the first time that the effect of external fields on the anomaly related to the memory effect in TlGaSe2 can be reduced to the following universal empirical rule: when a sample is held for many hours at a constant temperature T0 in the temperature range of the incommensurate phase in a dc electric field, the deflection amplitude in the low-temperature part of the anomaly in the temperature dependence of the relative change in the dielectric constant Δɛ/ɛ increases (the deflection in the high-temperature part of the Δɛ/ɛ anomaly disappears) as compared to this segment in the dependence obtained during isothermal annealing of this sample at the same temperature without an electric field. The crystal remembers its thermal history at a temperature that is several kelvins higher than T0. Light illumination increases the deflection amplitude in the high-temperature part of the Δɛ/ɛ(T) anomaly and shifts the temperature at which the crystal remembers its thermal history toward lower temperatures with respect to T0.

Memory effect and new polarized state in the incommensurate phase of TlGaSe2 ferroelectric – semiconductor

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.

Photodielectric effect in TlInS2 activated by the La impurity

The influence of light on the low-frequency dielectric properties of the TlInS2 layered crystal is investigated for the first time. It is established that the illumination of the crystal during measurements leads to a substantial change in the behavior of the temperature dependence of the permittivity ɛ of the TlInS2 compound doped with the lanthanum impurity in the range of the existence of the incommensurate phase. The temperature dependences of the permittivity ε of the TlInS2 compound doped with the lanthanum impurity after preliminary cooling of the crystal in constant electric fields with different strengths are studied for the first time. The inference is made that the experimentally observed photodielectric effect is associated with the localization of charge carriers at defect levels in the band gap of the crystal with the formation of local polarized states.

Anomalies in the electrophysical, thermal, and elastic properties of layered ferroelectric semiconductor TlGaSe2: Instability in the electronic subsystem

The temperature dependences of the permittivity, dark current, and current-voltage characteristics of the layered ferroelectric semiconductor TlGaSe2 were measured over the range 77–300 K. The ε(T) curve measured perpendicular to the layers at low frequencies is discovered for the first time to exhibit an anomalous dip with clearly defined boundaries at ∼150 and ∼200 K. Electrical instability in the form of low-frequency oscillations of current in current-voltage characteristics of a sample in the same temperature range are also observed experimentally for the first time. It is shown that this instability occurs only if an electric field is applied to a sample using potential-controlling contactless electrodes in the form of thin mica spacer layers. The nature of the instability and its influence on various physical properties of the layered TlGaSe2 crystal are discussed.