Analysis of Threshold Voltage Drift Under Electrical Stress Accumulation in Ovonic Threshold Switching Chalcogenide Films

A 3D crossbar array has received attention due to its simplified two-terminal structure, making it suitable for use in embedded conventional and emerging memories for brain-inspired computing applications. Emerging memories are generally used with selector devices as 1-selector 1-resistor (1S1R) to suppress leakage current from half-selected cells. Chalcogenide-based Ovonic Threshold Switching (OTS) selector is one of the candidates to address this leakage current issue, ensuring high selectivity, high endurance, and other requirements. For reliable operation, the threshold voltage (Vth) of OTS devices should be stable. However, Vth tends to fluctuate with repeated on/off switching cycles but can recover over some relaxation time. The cause of Vth drift under electrical stress accumulation remains unclear, due to the difficulty that confines the switching region inside the amorphous chalcogenide OTS thin films. Here, Atomic Force Microscopy (AFM) provides a powerful method to directly characterize the switching behavior induced in the local region. Thanks to the unique advantages of AFM, changes in the conductivity of OTS thin film can be characterized along with Vth drift. Furthermore, the slice and scan analysis with a diamond-coated tip allows 3D tomography of the nanoscale region after degradation (without recovery).<br/>In this research, variations in conductivity within the local region of Te-based chalcogenide OTS films are investigated throughout the expected lifetime of OTS material, from the pristine state to the permanent degradation resulting in a fixed at the low-resistance state (LRS) by using Conductive AFM. A gradual drift in Vth, along with the increase of conductive region within the local switching region is generally observed after repeated on/off switching cycles. Given sufficient relaxation time, these highly conductive regions tend to diminish but can remain permanently degraded once a certain level of Vth drift is reached. The results can help us to understand why Vth drifts gradually under electrical stress accumulation by directly observing the physical changes in OTS thin films.<br/><br/><b>Acknowledgments</b><br/>This research was supported by Samsung Electronics Co., Ltd. (Project No. IO2102021-08356-01), the BK21 FOUR (Fostering Outstanding Universities for Research) funded by the Ministry of Education (MOE) of Korea and National Research Foundation (NRF) of Korea.