Hierarchical structure and phase transition of (GeTe)_n(Sb ₂Te₃)_m used for phase-change memory

(GeTe)n(Sb2 Te3)m is the material which has been extensively studied for phase-change memory applications. However, the local structure and the phase-transition mechanism of this material are not precisely known yet. We propose here, based on the recent extended x-ray absorption fine structure spectroscopy, stibnite-like Sb2 Te3 units and chainlike GeTe units as primary building blocks of the material. Then we further propose a three-dimensional model of secondary building blocks composed of a small number of primary building blocks. The phase change between the crystalline and the amorphous structure is shown to occur via reorientations of the secondary building blocks, establishing a simple reversible transformation process that can explain the high endurance and the fast switching time of the material. In the amorphous phase, the relaxed structure obtained from the ab initio total-energy minimization calculations satisfies the experimentally observed 8-N rule for the atomic coordination number.