Unveiling defect motifs in amorphous GeSe using machine learning interatomic potentials

TL;DR

This study employs advanced machine learning interatomic potentials, particularly graph neural networks, to identify and analyze defect motifs in amorphous GeSe, revealing their structural origins and electronic implications relevant to threshold switching.

Contribution

It introduces a highly accurate GNN-based interatomic potential for amorphous GeSe and uncovers specific defect motifs linked to electronic states, advancing understanding of defect-driven properties.

Findings

Identified two defect motifs: aligned Ge chains and overcoordinated Ge chains.

Correlated defect electronic levels with structural features like bond angles and Peierls distortion.

Demonstrated the importance of medium-range order and higher-order interactions in modeling amorphous GeSe.

Abstract

Ovonic threshold switching (OTS) selectors play a critical role in non-volatile memory devices because of their nonlinear electrical behavior and polarity-dependent threshold voltages. However, the atomic-scale origins of the defect states responsible for these properties are not yet fully understood. In this study, we use molecular dynamics simulations accelerated by machine-learning interatomic potentials to investigate defects in amorphous GeSe. We begin by benchmarking several potential architectures-including descriptor-based models and graph neural network (GNN) models-and show that faithfully representing amorphous GeSe requires capturing higher-order interactions (at least four-body correlations) and medium-range structural order. We find that GNN architectures with multiple interaction layers successfully capture these correlations and structural motifs, preventing the spurious defects that less expressive models introduce. With our optimized GNN potential, we examine twenty independent 960-atom amorphous GeSe structures and identify two distinct defect motifs: aligned Ge chains, which give rise to defect states near the conduction band, and overcoordinated Ge chains, which produce defect states near the valence band. We further correlate these electronic defect levels with specific structural features-namely, the average alignment of bond angles in the aligned chains and the degree of local Peierls distortion around overcoordinated Ge atoms. These findings provide a theoretical framework for interpreting experimental observations and deepen our understanding of defect-driven OTS phenomena in amorphous GeSe.