Speakers

​​​​Multicomponent concentrated solid solutions combining different transition metal oxides have recently been considered beneficial for resistive switching random access memory devices (RRAM)   applications ^[1]. Their unique compositional complexity arising from elevated configuration entropy and thus high levels of chemical and structural disorder has been anticipated to be a possible pathway to improve certain significant drawbacks, that still hinder the widespread adoption of RRAM devices as the next generation of non-volatile memories. ​​Particullarly for RRAM devices that rely on filamentary type of switching mostly in conventional single cation-based layers the mechanism of resistance changes due to stochastically localized formation and migration processes of oxygen vacancies still bring about challenges to improve a pronounced variability and limited reliability. This applies despite their scalability, fast switching, low power operation and compatiblity with CMOS technology, just to mention a few strong benefits of RRAM.

The present study reports about the tailored fabrication of ceramic targets for RF-sputtering of restively switching oxide thin films of various compositions within the senary material system ZrO₂-HfO₂-Nb₂O₅-Ta₂O₅-MoO₆-WO₆, all individual components of this high-order system being relevant to resistively switching. Manufactured targets were consolidated by Spark-Plasma-Sintering of powders that were either synthesized via a solid-state route or by chemical precipitation from liquid precursors, to enhance compositional homogeneity.​ Thin films have been characterized analytically (XPS, AFM, XRD, XRR …) to ensure a predictable and reliable growth of amorphous multi-cation oxide thin films. The thin films were patterned using photolithography and reactive ion beam etching to fabricate microscale memristors (2 x 2 mm²) featuring platinum (Pt) bottom and top electrodes, multi-cation oxide thin films, and an ohmic electrode (OE) consisting of tantalum (Ta), titanium (Ti), or tungsten (W). ReRAM devices were optimized regarding thin film thickness of the multi-cation oxide, the ohmic electrode material choice and respective thickness, the size of the microscale device area. ​​

​Advantages of the ​NbTaO_x-based devices compared to reference TaO_x-based devices include a faster SET speed, lower SET and RESET voltages, higher Off/On ratio between the low-resistance state and the high-resistance state, as well as an excellent cycling endurance of over 10⁹ cycles. For the (ZrHfNbTaMoW)O_x-based devices, an even higher Off/On ratio was achieved, as well as a lower forming voltage, high cycling endurance of at least 10⁸ cycles, and promising retention times.

[1] M. Ahn & J.D. Phillips et al: Adv. Electron. Mater. 7 (2021) 2001258