Moisture effect on the diffusion of Cu ions in Cu/Ta(2)O(5)/Pt and Cu/SiO(2)/Pt resistance switches: a first-principles study

Cu/Ta(2)O(5)/Pt and Cu/SiO(2)/Pt are two of the most promising resistance switches. From experimental observations, it is speculated that the presence of H(2)O in the amorphous Ta(2)O(5) and SiO(2) (a-Ta(2)O(5) and a-SiO(2)) 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-Ta(2)O(5) nanopore and in the one covered with H(2)O–together with those in a-SiO(2) nanopore. Our work reveals that Ta and Si atoms on the sidewalls of bare a-Ta(2)O(5) and a-SiO(2) nanopores are in the unsaturated (TaO(5)) and saturated (SiO(4)) forms, respectively. Consequently, H(2)O molecules are adsorbed on the nanopore sidewall strongly in the case of a-Ta(2)O(5,) and weakly in a-SiO(2), by forming O-Ta and H∙∙∙O bonds, respectively. This can explain the experimental observation that the desorption of H(2)O occurs only at high temperatures for a-Ta(2)O(5) films, while it is observed for a-SiO(2) even when the temperature is low. The calculated diffusion barrier of Cu ions in a-Ta(2)O(5) nanopores covered with H(2)O is about 0.43 eV, which is much lower than that without H(2)O (~1.40 eV). In view of the similar chemical environments of O and the adsorbed Cu ions in a-SiO(2) and a-Ta(2)O(5) nanopores, it is expected that the diffusion of Cu ions in a-SiO(2) nanopore without H(2)O is much more difficult than with H(2)O. This could be attributed to the strong and weak adsorption of Cu ions on the sidewall in the absence and presence of H(2)O, respectively, for both, a-Ta(2)O(5) and a-SiO(2). Our investigation provides a full atomic picture to understand the moisture effect on the diffusion of Cu ions in Cu/a-Ta(2)O(5)/Pt and Cu/a-SiO(2)/Pt resistance switches.