Effect of Gadolinium Doping on the Electrical and Optical Properties of TlInS2 Single Crystals

Abstract The influence of low concentrations of gadolinium (Gd) on the dielectric and optical characteristics of layered ferroelectric semiconductor TlInS2 is analyzed. A series of TlIn1 − xGdxS2 single crystals (x = 0, 0.001, and 0.005) was synthesized and grown by directional solidification (Bridgman–Stockbarger) from pre-synthesized polycrystals. Phase/structure identification was performed by X-ray diffraction. Dielectric measurements were carried out at room temperature over 20 Hz–1 MHz to determine the complex dielectric permittivity ε*(f) and the AC conductivity σac(f). Optical absorption was measured at 100–300 K, and the direct optical band gap was determined using the Tauc method. The results show that Gd doping leads to a redshift of the TlInS2 band gap by approximately 70 meV at 100 K for x = 0.005 and increases the real and imaginary parts of ε*(f) at a fixed frequency, with the effect being most pronounced in the low-frequency region. Doping also increases σac and modifies the frequency power-law behavior (σac ∼ f s). At high frequencies, a regime with s ≈ 0.8–typical of hopping transport via localized states near the Fermi level–is observed. In addition, the optical band gap decreases relative to undoped TlInS2. Within the hopping-conductivity model, we estimated localized-state parameters (density of states NF, average hopping time τ, hopping distance R, and energy spread ΔE), finding an approximately threefold increase in NF at x = 0.005 and about fivefold decrease in ΔE compared with the undoped sample. These results are attributed to the simultaneous enhancement of interfacial/space-charge polarization at low frequencies and an increased density of defect- and impurity-induced localized states in the band gap.