Structure, electrical, optical and thermal properties of Ge4Sb4Tex (x = 8, 9 and 10) thin films

The crystalline samples of Ge4Sb4Te10, Ge4Sb4Te9, and Ge4Sb4Te8 were prepared and their amorphous semiconducting thin films obtained by flash evaporation. Their sheet resistance decreased slowly with temperature up to 147–160 °C with activation energy of electrical conductivity ΔE = 0.40–0.44 eV. Above these temperatures, the sheet resistance drops abruptly by several orders due to crystallization. The drop of resistivity proceeds in two steps. Two steps of phase change were also found on curves of DSC and on the temperature dependence of index of refraction. It pays for slow heating rates, crystallization induced by short (≈30 ns) laser pulses proceeds probably in one step only for all studied samples (as it follows from X-ray diffraction), not only for Ge2Sb2Te5 in which a single phase formation was confirmed. The crystallization temperatures are increasing slightly with decreasing Te content in the series Ge4Sb4Te10–Ge4Sb4Te9–Ge4Sb4Te8 from 147 to 160 °C. The X-ray diffractograms revealed that in laser crystallized samples can be found only cubic modification of Ge2Sb2Te5 type (a = 0.6 nm), while the samples annealed (230 °C, 2 h) or annealed after the crystallization with laser pulse, contain also small amounts of hexagonal phase.