Two-stage crystallization process in Ge2Sb2Te5 alloys

The aim of this work is to investigate the amorphous-to-crystalline phase transformation process in Ge₂Sb₂Te₅ ternary alloys. The experiments were carried out using impedance and X-ray diffraction measurements. The results have shown that the crystallization process in an amorphous Ge₂Sb₂Te₅ alloy starts from nuclei, which appear below Tc and which were identified as the crystalline Ge₁Sb₄Te₇. As the temperature increases above Tc these nuclei were transformed into fcc-Ge₂Sb₂Te₅ crystals. In order to establish the mechanism of crystallization for this system, the Brugerman effective medium approximation was implemented to analyze nucleation and growth of the two stage involved (i.e., metastable Ge₁Sb₄Te₇ and stable Ge₂Sb₂Te₅) through impedance measurements. The results of the simulations demonstrate a close agreement with experimental ones showing that phase transformation in Ge₂Sb₂Te₅ is carried in two steps.