Moisture effect on the diffusion of Cu ions in Cu/Ta₂O₅/Pt and Cu/SiO₂/Pt resistance switches: a first-principles study

Cu/Ta₂O₅/Pt and Cu/SiO₂/Pt are two of the most promising resistance switches. From experimental observations, it is speculated that the presence of H₂O in the amorphous Ta₂O₅ and SiO₂ (a-Ta₂O₅ and a-SiO₂) facilitates the rate-limiting step during the switching process. This rate-limiting step is essentially the diffusion of Cu ions along the nanopores of the amorphous. To better understand this behavior and obtain a detailed examination of the atomic structures, a first-principles simulation was conducted. In addition, we investigate the diffusion behaviors of Cu ions in bare a-Ta₂O₅ nanopore and in the one covered with H₂O-together with those in a-SiO₂ nanopore. Our work reveals that Ta and Si atoms on the sidewalls of bare a-Ta₂O₅ and a-SiO₂ nanopores are in the unsaturated (TaO₅) and saturated (SiO₄) forms, respectively. Consequently, H₂O molecules are adsorbed on the nanopore sidewall strongly in the case of a-Ta₂O_5, and weakly in a-SiO₂, by forming O-Ta and H∙∙∙O bonds, respectively. This can explain the experimental observation that the desorption of H₂O occurs only at high temperatures for a-Ta₂O₅ films, while it is observed for a-SiO₂ even when the temperature is low. The calculated diffusion barrier of Cu ions in a-Ta₂O₅ nanopores covered with H₂O is about 0.43 eV, which is much lower than that without H₂O (~1.40 eV). In view of the similar chemical environments of O and the adsorbed Cu ions in a-SiO₂ and a-Ta₂O₅ nanopores, it is expected that the diffusion of Cu ions in a-SiO₂ nanopore without H₂O is much more difficult than with H₂O. This could be attributed to the strong and weak adsorption of Cu ions on the sidewall in the absence and presence of H₂O, respectively, for both, a-Ta₂O₅ and a-SiO₂. Our investigation provides a full atomic picture to understand the moisture effect on the diffusion of Cu ions in Cu/a-Ta₂O₅/Pt and Cu/a-SiO₂/Pt resistance switches.