Bi-Doped Single-Crystalline (001) Epitaxial TiO₂ Anatase Nanostructures for Resistive Random Access Memory Applications

Resistive switching memory devices are an emerging class of nonvolatile memories and have shown that outstanding device performance originates from the migration of oxygen vacancies. Research efforts are made to enhance the resistive switching behavior of metal oxide nanostructures. This work reports resistive switching properties of bismuth doped single-crystalline epitaxial anatase TiO2 nanostructures fabricated on (001) oriented Nb:SrTiO3 substrates. These nanostructures are fabricated with a Bi4Ti3O12 precursor using a “phase separation and evaporation” approach. Resistive switching measurements, performed by conducting atomic force microscopy on the as-fabricated TiO2 nanostructures, reveal a stable ON/OFF ratio (∼5 × 104) (irrespective of the spatial position of the tip) at a read voltage of −0.4 V. Energy dispersive X-ray spectroscopy mapping by scanning transmission electron microscopy confirms presence of trapped bismuth (Bi) near the interface for these as-grown nanostructures. Annealing the nanostructures under high vacuum (10–7 Torr) and temperature (900 °C), i.e., conditions that promote Bi loss and evaporation, results in a marked drop in the ON/OFF ratio to ∼2 × 102. As none of the other morphological or structural parameters change after annealing, it reveals the key role played by the trapped Bi. We propose that these residual Bi ions in the vicinity of the interface act as charge trapping/detrapping sites for carriers under the applied electric field, which enhances the resistance switching behavior. This work demonstrates that purposely introducing defects or dopants during oxide heteroepitaxy can be a very potent method to realize emergent electronic properties in metal oxide nanostructures for the next generation resistive switching application.