Polaron-Induced Midgap States in Ovonic Threshold Switching Material for 3D Phase-Change Memory Applications

Ovonic threshold switching (OTS)-based selector is essential for suppressing sneak-path current in 3D-crossbar phase-change memory. Although midgap (defect) states of OTS amorphous materials are crucial for the switching process, their origin and evolution remain insufficiently understood, and elucidating their governing mechanism is essential for resolving debates on OTS behavior. Here, first-principles calculations investigate the electron-hole-pair excitation during the switching-on process in three representative OTS materials─GeSe, GeS, and SiGeAsTe. The results reveal that excited carriers (electron-hole-pairs) induce polaron formation through strong electron-phonon coupling. Polarons introduce midgap states that could promote the switching-on process through positive feedback. Further analyses demonstrate that the polarons and midgap states are mainly related to the conduction band. This is attributed to the unique metavalent bonding or hyperbonding effects in chalcogenide OTS materials. These findings clarify the formation of midgap states from excited-state and polaron perspectives, and provide guidance for designing improved OTS materials for 3D phase-change memory.