The Study Of The Local Structure And Atomic Bonding Of The Phase Change Materials

Phase-change memory?PCM? is considered to be one of the optimal solution for the next generation of nonvolatile storage technology, which has a broad prospect of application and development. The pseudo-binary line between Ge Te and Sb₂Te₃ is one of the most studied phase change materials. In recent decades, people have a relatively clear understanding to the Ge Sb Te crystal structure. However, the amorphous structure and phase transition mechanism is still controversial. A deep understanding of the Ge Sb Te amorphous structure and phase transformation mechanism plays a positive role in promoting the development of the material.In this paper the amorphous structure models of Ge Te and Ge₂Sb₂Te₅ are obtained based on the molecular dynamics simulation using the VASP package. A method of combining cutoff distance with Electron Localization Function?ELF? is proposed, and used to analysis the bond information and the local structure of Ge in Ge Te and Ge₂Sb₂Te₅.The results are as follows:1. The reasonable cutoff distance to analyse the Ge Te amorphous structure is 3.05 ?, and ELF threshold is 0.63; similarly, the cutoff distance and ELF threshold value are 3.2 ? and 0.63 for Ge₂Sb₂Te₅, respectively. Furthermore, we can get more reasonable bonding and structural information for the amorphous structure of Ge Te and Ge₂Sb₂Te₅. In addition, this method can also be applied to other amorphous materials with covalent bonds.2. Basis on the bond information, the amorphous Ge Te model was divided into Ge-polyhedral with Ge-Ge bonds?HBCCs? and Ge-polyhedral with only normal bonds?NBCs?. The coordination number and bond angular distribution analysis of Ge show that the local structure in the HBCCs tend to form tetrahedral and those in NBCs tend to form defective octahedral. The density of states of Ge atoms in the amorphous structure of Ge Te showed that the sp3 hybridization degree of Ge atoms in HBCCs higher than Ge atoms in NBCs, which leads to the tetrahedral Ge in HBCCs and octahedral Ge in NBCs.3. Compared with Ge, the sp3-hybridization of Sb is much weaker. Therefore, the local structure of Sb atoms are dominated by the octahedral configuration. That means the addition if Sb₂Te₃ results in more the octahedral configurations in Ge₂Sb₂Te₅. This may be the reason that lead to high crystallization speed for Ge₂Sb₂Te₅.