Rational engineering of a switching material for an Ovonic threshold switching (OTS) device with mitigated electroforming

Abstract

The Ovonic threshold switch (OTS) has recently attracted renewed interest for its application in neuromorphic computing devices. One of the remaining problems is the electroforming (EF) process, which requires a switching voltage in the pristine state much higher than that in the subsequent state, the origin of which is still unclear. In this work, using a transport model of the OTS, we have investigated the EF phenomenon in OTS devices to find that it results in an increase in the density of trap states and a decrease in the distance from the Fermi energy to the conduction band, implying a change in its local structures. Based on these findings, the switching material, GeSe in this work, is engineered to have the desired properties for mitigating the EF phenomenon. We have found that, as Sn is doped in GeSe, the local structure prefers edge-sharing XSe₂ (X = Ge, Sn) and three-fold bonded orthorhombic structures, while the undoped GeSe prefers corner-sharing GeSe and ethane-like (ETH) Ge₂Se₆ structures. Such a structural change is found to be in line with the change in the density of trap states by the EF process, resulting in a reduction of the forming voltage with increasing Sn concentration. We believe that these findings will provide important information for designing a switching material for a electroforming-free OTS device.