Effect of substitution of Ga and In atoms by Yb on physical properties of TlGa(In)S2 layered crystals

Full text : Equilibrium in the TlGaS2-TlYbS2 system was investigated. According to the results of differentialthermal nalysis in the system of TlGaS2-TlYbS2 with the relationship of components 1:1 the compound of Tl2GaYbS4 is formed with the congruent melting at a temperature of 1380 K. On the basis of TlGaS2, solid solutions up to 8 mol percent of TlYbS2 are formed at room temperature. Nonvariant eutectic point relates to the composition of (TlGaS2)0.84(TlYbS2)0.16 and the temperature of 1085 K. TlGaS2 single crystals, in which Ga atoms are replaced by rare-earth element Yb in quantities of 0.1 - 1 percent, are grown by Bridgeman method. Over a wide range of temperatures (300 - 900 K) the electro-physical properties of the ternary compound of TlYbS2 are investigated. It has been found that the prevailing mechanism of current carriers scattering is the scattering on the lattice acoustic phonons. Effective masses, carriers state densities, mobility proportions, temperature coefficients of the forbidden gap are determined. From studies of the heat transport phenomena at temperatures of 77-650 K in the above-mentioned single crystals it has been found that the three-phonon processes are the basic mechanisms of phonon scattering. Parameters of crystal lattice and space symmetry group for several compositions of the given system were determined. Temperature dependence of electrical conductivity and Hall coefficient of (TlInS2)1-x(TlYbS2) solid solutions were studied. Change of carrier mobility with temperature corresponds to scattering on acoustic vibration of the lattice. Thermal properties of crystals were also studied and it was determined that measured thermal conductivity is caused by lattice contribution, since its electron component, determined by Wiedemann-Franz ratio, is negligible. It is determined that the optical absorption edge of single crystals is formed by direct exciton and is shifted towards long wavelength side as compared to TlInS2 (at 77K this shift is around 50meV for TlIn0,995Yb0,005S2). Coefficient of temperature shift of exciton peak is also decreased, in 77-200K interval it is equal to -5,1*10-4eV/K for TlInS2 and to -2,0*10-4eV/K for TlIn0,995Yb0,005S2. If we assume that exciton binding energy varies only slightly, then partial substitution of In atoms with rare earth element Yb considerably reduces width of the band gap of TlInS2 single crystal