Hopping conductivity on relax or ferroelectrics TlInS2 exposed gamma irradiation

Full text: The TlInS2 compound is a crystal in which temperature instabilities of the crystal lattice lead to ferroelectric ordering. It has been established with confidence that the TlInS2 compound of stoichiometric composition is an improper ferroelectric with an incommensurate phase. Our earlier investigations [1-4] revealed that doping of the TlInS2 semiconductor ferroelectric crystal with some impurities leads to a rapid relaxation of the dielectric susceptibility in the temperature range of existence of the incommensurate phase. It was established that this relaxation is caused by the formation of polar nano domains owing to which the ordered phase is preceded by the state of a dipole (or ferroelectric) glass. In turn, the doping atoms, which are responsible for the formation of the relax or state, form attachment levels in the band gap of the semiconductor ferroelectric crystal. In filling these levels, charge carriers turn out to be spatially confined. As a result, electrical conduction is provided by tunnelling charge carriers through potential barriers. It is this mechanism of conduction that was observed in the study of charge transfer in TlInS2 crystals doped with Fe, Mn and Cr; i.e., it was established that, in the range of existence of the incommensurate phase, these crystals exhibit non activated temperature-independent hopping conduction. Thus, we carried out comprehensive investigations into the temperature dependences of the permittivity ε (T) and the electrical conductivity σ (T) of TlInS2 crystals exposed to gamma irradiation. It was established that there exists a correlation between the behavior of relax ors, in which the relax or state is formed as a result of inhomogeneous polarization in nano regions due to the localization of charges at defects, and the electrical conduction, which has a hopping nature and is governed by thermal activation transitions from defects levels. The results obtained in the above investigations made it possible to determine the density of states of defect levels in irradiated TlInS2 crystals, as well as the localization length and the hopping distance. It was demonstrated that varying the electronic subsystem could control the dielectric properties of relax ors. It was assumed that charge transfer in the temperature range of existence of a stable relax or state is provided by tunnelling from electron levels of the quantum dot through the potential barriers generated by the superlattice of the TlInS2 crystal. (author)