On the origin of in-gap states in amorphous Ge$_2$Sb$_2$Te$_5$

TL;DR

This study investigates the atomic origins of in-gap electronic states in amorphous Ge$_2$Sb$_2$Te$_5$, revealing that wrong bonds and specific atomic configurations are key contributors, and that structural relaxations can reduce these states, affecting device resistance.

Contribution

The paper combines machine learning potentials and density functional theory to identify structural motifs responsible for in-gap states and their evolution during glass aging in Ge$_2$Sb$_2$Te$_5$.

Findings

In-gap states are mainly localized on wrong bonds and specific atomic configurations.

Structural relaxations during aging deplete in-gap states and increase resistance.

Insights support strategies to mitigate resistance drift in phase change memory devices.

Abstract

The localized states in the band gap of amorphous phase change alloys like Ge$_2$Sb$_2$Te$_5$ control the electrical conduction via the Poole-Frenkel mechanism. Understanding the origin of in-gap states and their evolution in time during aging of the glass is therefore important for the control of the resistance drift in phase change memory devices. Here, we use a machine learning interatomic potential to generate several models of Ge$_2$Sb$_2$Te$_5$ whose electronic structure is then analyzed within density functional theory with a hybrid functional. A detailed statistical analysis of the structural motifs on which the in-gap states are localized, reveals that the vast majority of in-gap states involve wrong bonds (homopolar or Ge-Sb bonds) often accompanied by Ge in tetrahedral configurations or overcoordinated Ge and Sb atoms. Metadynamics simulations mimicking glass aging support the picture that structural relaxations lead to the depletion of in-gap states and then to an increase of resistance. The simulations thus provide important insights for the mitigation of the resistance drift in phase change memory devices.