Ovonic Threshold‐Switching Ge_<i>x</i>Se_<i>y</i> Chalcogenide Materials: Stoichiometry, Trap Nature, and Material Relaxation from First Principles

Abstract

Density functional theory simulations are used to identify the structural factors that define the material properties of ovonic threshold switches (OTS). They show that the nature of mobility‐gap trap states in amorphous Ge‐rich Ge 50 Se 50 is related to GeGe bonds, whereas in Se‐rich Ge 30 Se 70 the Ge valence‐alternating‐pairs and Se lone‐pairs dominate. To obtain a faithful description of the electronic structure and delocalization of states, it is required to combine hybrid exchange–correlation functionals with large unit‐cell models. The extent of localization of electronic states depends on the applied external electric field. Hence, OTS materials undergo structural changes during electrical cycling of the device, with a decrease in the population of less exothermic GeGe bonds in favor of more exothermic GeSe. This reduces the amount of charge traps, which translates into coordination changes, an increase in mobility‐gap, and subsequently changes in the selector‐device electrical parameters. The threshold voltage drift process can be explained by natural evolution of the nonpreferred GeGe bonds (or “chains”/clusters thereof) in Ge‐rich Ge x Se 1– x . The effect of extrinsic doping is shown for Si and N, which introduce strong covalent bonds into the system, increase both mobility‐gap and crystallization temperature, and decrease the leakage current.