First-principles study of the liquid and amorphous phases of SbTe phase change memory material.

We have investigated the local structure of liquid and amorphous phases of SbTe phase change memory material by the means of density functional theory-molecular dynamics simulations. The models of liquid and amorphous states were generated by quenching from the melt. The results show that the local environment of liquid SbTe is a mixed bonding geometry, where the average coordination numbers (CNs) of Sb and Te atoms are 4.93 and 4.23, respectively. Compared with crystalline state, there are more Sb-Sb bonds (∼53%) and less Sb-Te bonds (∼42%) with the presence of Te-Te bonds (∼5%) in liquid SbTe. Therefore, the formation of homopolar bonds and the breaking of heteropolar bonds are important structural transformations in melt process. For amorphous SbTe, the average CNs of Sb and Te atoms are 4.54 and 3.57, respectively. They are mostly in an octahedral environment, similar to the case in crystalline phase. The fractions of Sb-Sb, Te-Te, and Sb-Te bonds are ∼52%, ∼2%, and ∼46%, respectively. Thus, the increase in the fraction of octahedron accompanied with the decrease in average CN is the major structural changes in quenching process. Furthermore, the octahedral geometry in both the crystalline and amorphous SbTe increases the local structural similarity, facilitating the rapid low-energy crystallization.