Nanoscopic Observation of Structural Changes After First-Firing Behavior in W/GeTe/W Ovonic Threshold Switches Using AFM Tip-Based Local Biasing.

An Ovonic threshold switch (OTS) is a promising selector device for three-dimensional crossbar memory arrays due to its simple two-terminal architecture and unique electrical properties. Despite extensive studies, the switching mechanisms of OTS devices under an electric field (EF) and thermal driving forces remain ambiguous. In this work, a local biasing approach using atomic force microscopy is employed to spatially confine the switching region in Te-rich Ge-Te-based OTS devices. EF simulations are used to evaluate stable switching in two types of local biasing systems. Following local switching, transmission electron microscopy reveals nanocrystalline formation at the interface between the top electrode and the switching layer, providing insights into structural changes and the first-firing (FF) phenomenon. This interfacial crystallization enhances local conductivity and reduces the effective thickness of the amorphous layer from 32 to 24 nm, lowering the voltage required for subsequent threshold switching to 1.27 V relative to the 1.72 V for the initial FF behavior. The study demonstrates the feasibility of directly observing switching at the nanometer scale in targeted regions, offering a clear interpretation of the underlying mechanism of OTS switching and highlighting the critical role of the crystalline/amorphous interface in facilitating localized electron transport.