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 Ge50Se50 is related to GeGe bonds, whereas in Se‐rich Ge30Se70 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 GeGe bonds in favor of more exothermic GeSe. 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 GeGe bonds (or “chains”/clusters thereof) in Ge‐rich GexSe1–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.
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More From: physica status solidi (RRL) – Rapid Research Letters
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