Understanding the Cycling-Dependent Threshold Voltage Instability in Selector Devices

Recently, two-terminal selector devices based on chalcogenide materials have attracted much attention towards next-generation high-density memory cell arrays. In this work, the reliability of the selector devices has been studied in collaboration with imec, the world-leading R&D center in electronics technologies. The ON/OFF switching mechanism of the devices is considered as follows: the formation/break-up of a conducting path induced by applying/removing an electric field (Fig.1).

In the devices used in this work, it is founded that the threshold voltage (V_th) relaxation, that is, V_th drift towards larger values with increasing the wait time during OFF state (relaxation time, t_relax), becomes faster after ON/OFF switching cycling (Fig.2). In other words, the relaxation rate becomes larger after cycling. Even if the same relaxation time passes, V_th values vary depending on the cycling stress applied to the devices. This V_th instability should have a significant impact on the reliability of array operations.

To address this issue, the mechanism behind the accelerated relaxation has been deeply investigated. The detailed electrical characterization and modeling revealed that Joule heating generated during cycling causes this phenomenon. The cycling-induced accelerated relaxation is suppressed in a material which has high thermal stability (Fig.3). This result further confirms that the accelerated relaxation is caused by thermal energy.

These results contribute to reliable array operations.