Correlation of the optical properties with ε(T)-dependence at ferroelectric phase transition in TlInS2 doped with different impurities

Full text : Impurity doping of semiconductors with ferroelectric phase transition (FPT) leads to the temperature broadening of this transition and shifting of dielectric anomaly (DA) maximum in ε (T) - dependence to lower temperatures. Usually broadening and shifting are correlated. The more broadening the higher is shifting. The reason of DA shifting is a result of contribution of the energy of carriers to the crystal free energy, which according to Ginzburg-Landau theory of FPT shifts Tc only to lower temperatures for both electrons and holes. This phenomenon is investigated in this report by comparing of the forms of the room temperature photoconductivity (PC) and absorption (α ) spectra around the forbidden gap region (Ηω ≤Εg) with FPT ε(T) - dependence for samples doped with different impurities. As a result of these investigations it was established that deep level impurities which have no peculiarities in TlInS2 band edge PC and α - spectrums do not lead to any broadening or shifting in FPT ε (T) - dependence. Rare earth elements Yb, Sm, Ln and other lanthanides form deep levels and do not change FPT. In contrary, when doping creates shallow impurity (SI) states, which broad band edge α - spectrums and create PC lines at Ηω < Ε , then FPT broadening and shifting in ε (T) -dependence of samples takes place. At high doping levels in TlInS2 some impurities Fe, Mn, B, Cr and C besides of SI states create deep level states as well. This is seen from comparing of absorption and PC at different doping concentrations. DA of ε (T) dependence at FPT accompanies by simultaneous conductivity σ (T) -anomaly as for doped as well as for samples with residual SI. This fact proves that at DA temperatures carriers frozen on charged SI are thrown out utterly into free bands due to SI ionization energy ΕiDA = Εi / ε2 (Τ) decrease as a result of SI Coulomb potential screening at DA. Consequently ε (T) -broadening at FPT in samples doped with SI arises as a result of increase of damping factor γ ≅ τ-1 due to ionized SI scattering (τ ≅ Ni-1) and changing of polarization of crystal cells at FPT by randomly distributed SI. Ionized SI potential fluctuations besides of known effects (broadening of absorption and PC edges, hopping conductivity and so on) lead to fluctuations of refractive index η. The visibility of interference spectra (IS) decreases dramatically in samples doped with SI. Model calculation of IS with fluctuations of η testifies this. So doping only with SI in TlInS2 crystals leads to FPT broadening and shifting of DA to lower temperatures.