All-Inorganic Bismuth Halide Perovskite-Like Materials A₃Bi₂I₉ and A₃Bi1.8Na0.2I8.6 (A = Rb and Cs) for Low-Voltage Switching Resistive Memory

As silicon-based metal oxide semiconductor field effect transistors get closer to their scaling limit, the importance of resistive random-access memory devices increases due to their low power consumption, high endurance and retention performance, scalability, and fast switching speed. In the last couple of years, organic-inorganic lead halide perovskites have been used for resistive switching applications, where they outperformed conventional metal oxides in terms of large on/off ratio and low power consumption. However, there were scarce reports on lead-free perovskites for such applications. In this report, we prepared lead-free Au/A₃Bi₂I₉/Pt/Ti/SiO₂/Si (A is either Cs⁺ or Rb⁺) devices and tested their resistive switching characteristics. They showed a forming step prior to repeating switching, low operating voltage (0.09 V for Rb₃Bi₂I₉ and 0.1 V for Cs₃Bi₂I₉), large on/off ratio (>10⁷), relatively high endurance (200 cycles for Rb₃Bi₂I₉ and 400 cycles for Cs₃Bi₂I₉ cycles), and high retention (1000 s). Such low voltage could be explained by grain boundary-modulated ion drift. Difference in endurance was speculated to be due to the difference in the surface roughness of films because Cs₃Bi₂I₉ films are smoother. To get rid of the forming step, 10% of the Bi³⁺ cations were substituted with Na⁺ cations. However, this method only worked on Rb-based structures. This phenomenon was explained by the defect formation energy, which can only be negative in a corner-sharing Rb₃Bi₂I₉ structure compared to a face-sharing octahedral Cs₃Bi₂I₉ structure. As a result, the forming step was removed, and 100 cycles endurance and 1000 s retention performance were obtained. Similarly, the lower endurance is suspected to be due to the poor surface quality of the film.