Topological insulating property of Ge-Sb-Te phase change materials

Seung-Hoon Jhi

Phase-change materials like Ge-Sb-Te (GST) compounds are considered the best candidates for next-generation non-volatile memories because of their rapid and reversible cycles between the crystalline and amorphous structures. The mechanism and detailed atomic structure associated with the structural transition of GST compounds have been extensively studied, but the factors responsible for the very fast atomic rearrangement are still unknown. Topological insulators have an energy gap at bulk phase but contain conducting surface states that are protected from external perturbations by time-reversal symmetry. We report a discovery, through first-principles calculations, that the ternary chalcogen compounds commonly used in phase change memory exhibit the topological insulating property and their conducting properties are strongly affected by the surface or surface-like interface states. We show that the conducting properties originate from topological insulating Sb2Te3 layers in GSTs. The interface states are found to be resilient to atomic disorders but sensitive to the uniaxial strains. It is found that Ge migration, which is believed to be responsible for the amorphorization of GSTs, destroys the topological insulating order. We discuss how these topological insulating properties are utilized for developing electronic devices.