The electronic structures and structural properties of the amorphous Ge$_{2}$Sb$_{2}$Te$_{5}$, a phase change memory material

Ge-Sb-Te compound is one of the most promising materials for phase change random access memory. Recently, Ge$_{2}$Sb$_{2}$Te$_{5}$ has been under intensive researches. However, there exists a critical discrepancy between experimental and theoretical observations. In experiment, the ideal glass following 8-$N$ rule has been observed. There are deviations from 8-$N$ rule for melt-quench structures obtained by molecular dynamics calculations. In this presentation, we compare the melt-quench structure with ideal glass. We theoretically obtained the ideal glass using Si-As-Se compounds with a higher covalency The amorphous structure of Si$_{2}$As$_{2}$Se$_{5}$ is obtained by the melt-quench process and the elements are replaced by Ge-Sb-Te. It is found that the resulting Ge$_{2}$Sb$_{2}$Te$_{5}$ structures satisfy the 8-$N$ rule and all Ge atoms are tetrahedrally coordinated. The total energy of the ideal glass is higher than that of the melt-quench structure, explaining why the ideal glass has not been observed in the MD simulations. The electronic structures are also compared between ideal glass, melt-quench structure, and crystalline phase. It is concluded that the electronic character of the melt-quench structure lies in between those of ideal glass and crystalline phase.