Evolution of Nb-Ta Anodic Memristors Identified By Combinatorial Screening
Authors: Zrinski Ivana , Mardare Andrei Ionut , Hassel Achim Walter , Minenkov Alexey , Cancellieri Claudia 
Published: July 2022

The conventional memory technology, based on von Neumann architecture, has challenged technical and scaling limitations. The architectures with separated memory and computing units have resulted in the low efficiency of computing systems. Hence, huge scientific attention has been recently drawn into the development of novel systems using the in-memory computing concept. Memristors are foreseen as a type of non-volatile memories that can fulfill such requirements but also as electronic components that can solve sneak path currents problems while consuming low power. The information storage mechanism is based on the resistance change of memristors which is driven by an applied external electric field. With this regard, the memory medium plays a crucial role, being an oxide layer sandwiched between the metallic bottom and top electrode. Valve metals and their oxides have shown promising switching properties when used as elements of metal-insulator-metal memristive (MIM) structures.
The switching mechanism still depends on the nano-dimensional conductive filaments formation inside the oxide. Even though devices based on Hf, Nb, Ti, and Ta demonstrated a long lifetime, high data retention or resistive state ratio, devices are suffering from low lifetimes in the order of several thousand switching cycles, mainly due to the unpredictable size and position of conductive filaments being affected by the movement of cations or O species. Therefore, the development of materials for defect-engineered memristive applications is crucial. This was the main motivation for this study supported by the fact that memristors based on pure valve metals exhibited promising switching behavior. In this work, devices based on Ta, Nb, and their alloys were investigated. Since the goal was to improve the properties of devices based on pure metals, Nb and Ta were mixed and their alloys were used as bottom electrode for anodic memristors. It can be inferred that by mixing Ta and Nb, multifunctional devices can be produced.
High number of Nb-Ta alloys were co-deposited by sputtering onto previously oxidized Si wafers. Afterwards, the samples were anodized for growing an insulating active layer for memristive devices. Onto the so-formed mixed oxides, Pt was deposited as top electrode. The compositional gradient was mapped by a scanning energy-dispersive X-ray spectroscopy (SEDX) system. The total compositional spread of the Nb-Ta library was ranging from Nb - 13 at.%Ta to Nb – 80 at% Ta over three Si wafers. Memristive devices based on Nb or Ta were also fabricated in identical conditions and used as reference samples. By using a combinatorial approach, the investigation of various electrical parameters could be established for more than 300 memristive devices produced per Si wafer. Electrical characterization was performed by a self-developed Gantry robot controlled via Labview software specifically designed for basic tests required for memristive characterization (I-U sweeps, endurance, retention tests). Electrical properties were screened with a compositional resolution of 1 at.%.
The results have shown that memristive devices based on Ta reached a long lifetime and high resistive state ratio, whereas devices based on Nb indicated multi-level switching as well as non-volatile and threshold switching characteristics. The formation of Ta-oxyphosphate, responsible for conductive filaments pining, was confirmed by hard X-ray photoelectron spectroscopy. Similarly, multi-level switching characteristics were explained by the incorporation of electrolyte species in Nb2O5. However, Nb devices did not show reversible switching from unipolar to bipolar mode and vice versa. The improvement regarding reversible switching between unipolar and bipolar mode has been observed for memristive devices based on alloys containing Nb -35 at. % Ta. These memristors have also demonstrated longer endurance and data retention as compared to devices based on pure Nb and Ta. The switching mechanism was investigated by conductive filaments imaging using transmission electron microscopy.
Conclusively, anodic memristors based on Nb and Ta exhibited non-volatile and threshold characteristics which can spread the range of memristive applications. Reversible switching behavior and multi-level switching possibilities are relevant for the development of memristive synapses and neuromorphic systems. The anodic growth of an insulating layer for devices based on Nb, Ta, and Nb/Ta alloys has simplified, speed up and decreased the cost of fabrication processes. The performance of anodic memristors based on metallic thin films containing Nb -35 at. % Ta indicated a substantial improvement. The switching mechanisms could be suggested based on the nanostructures found in oxide revealing the position and size of conductive filaments.
I. Zrinski et al., Appl. Surf. Sci., 548, 149093 (2021) https://doi.org/10.1016/j.apsusc.2021.149093.
I. Zrinski, et al., Appl. Mater. Today, accepted, (2021).
I. Zrinski, et. al, J. Phys. Chem. Lett.,
12, 8917 (2021) https://doi.org/10.1021/acs.jpclett.1c02346

Figure 1