Electrical conductivity and optical properties of tellurium-rich Ge-Sb-Te films

The Ge-Sb-Te based (GST) films are intensively studied because they are suitable for reversible phase-change storage media. Ge₂Sb₂Te₅ films are most frequently used for memory applications while Te enriched GST films are also promising but not well studied. In this work Te enriched GST films with two different compositions and various thicknesses (120 - 750 nm) were prepared by thermal evaporation of previously synthesized Ge₁₅Sb₅Te₈₀ and Ge₁₅Sb₁₅Te₇₀ glasses. Optical transmission and electrical conductivity measurements in sandwich and planar contact configuration were carried out. Temperature measurements of the planar electrical conductivity were performed in the range 77 - 300 K. Values of around 0.7 eV have been determined for the optical band gap of both compositions; the film refractive index is in the range 3.8 - 4.5. The sandwich conductivity is in the interval (2.5 - 5)×10⁻⁴ (Ω.cm)⁻¹ while the planar conductivity is around 10⁴ times greater. The obtained results are discussed in terms of existence of an amorphous volume part as well as a crystalline surface layer in the films