Metal-semiconductor transition in the supercooled liquid phase of the Ge2Sb2Te5 and GeTe compounds

The ${\mathrm{Ge}}_{2}{\mathrm{Sb}}_{2}{\mathrm{Te}}_{5}$ and GeTe compounds are of interest for applications in phase change memories. In the reset process of the memory the crystal is rapidly brought above the melting temperature ${T}_{m}$ by Joule heating and then the liquid phase rapidly cools down leading to the formation of the amorphous phase. Since the liquid above ${T}_{m}$ is metallic and the amorphous phase is semiconducting a semiconductor-to-metal transition occurs in the supercooled liquid. Based on density functional simulations, we estimated the metal-semiconductor transition temperature ${T}_{\text{M-SC}}$ by monitoring the opening of a band gap in the supercooled liquid phase. Due to previous evidence on the importance of the van der Waals (vdW) interaction in describing the liquid phase of these materials, we used both the revised Vydrov-van Voorhis functional which includes vdW nonlocal interactions and the Perdew-Burke-Ernzerhof functional without vdW corrections. The estimated ${T}_{\text{M-SC}}$ is about 100--150 K higher with the former than with the latter framework for both compounds. By including vdW interactions the estimated ${T}_{\text{M-SC}}$ is closer to ${T}_{m}$ than to the glass transition temperature for both systems. The analysis of the structural properties as a function of temperature suggests a correlation between the metal-semiconductor transition and a Peierls distortion. However, the data support more a continuous structural transformation than the presence of a first order liquid-liquid phase change associated the metal-semiconductor transition.