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Various oxides and especially their modifications are in actual scientific focus for memristors fabrication due to their applications in non-volatile memories. Such devices exceed most limits of conventional memory technology. Memristors are recognized as resistive random access memories (ReRAMs), in which the data storage profits from a non-volatile change in the material resistance. The switching between a high resistance state (HRS) and a low resistance state (LRS) depends on the selection of electrodes and active (oxide) layers. This generally impacts the conductive pathways (filaments) formation, which is mediated by oxygen vacancies and/or cations, and their field-activated movement inside the oxide. Anodic oxides of valve metals (Al, Hf, Ta, Ti, Nb, Zr) have shown remarkable performances as memristive elements. Studies on Hf- and Ta-based memristors reported excellent electrical and memory properties, such as multi-level switching, high endurance and data retention. Even though for research purposes the synthesis of such oxide layers is commonly done by atomic layer deposition or sputtering, the electrochemical anodization process should not be neglected. This is a faster, less complex and inexpensive method, with precise composition and oxide thickness control through electrochemical parameters. The value of this approach is clearly emphasized by its continuous industrial implementation in various sectors. Previous works have confirmed that the performance of Hf or Ta anodic memristors can be improved by carefully selecting the anodization electrolyte or other electrochemical parameters1,2. These play a crucial role in positioning and sizing of conductive filaments within the oxide. This approach directly leads to defect-engineered memristors fabrication, which is nowadays a major motivation for investigating devices based on mixed oxides formed in different electrolytes. Predicting the position and shape/thickness of a conducting filament may eventually lead to enhanced device stability and resistive states control. The focus of the current work is on the behavior of anodic memristors based on ultra-thin Hf superimposed on Ta films. The main idea linked to the control of resistive filaments is based on the particularities of the anodization process, when the interface between both oxides is dynamically changing. In situ oxide self-nanostructuring is already known for various superimposed valve metals, including Hf and Ta. Their anodization leads to nanoscale oxide columns (“fingers”) formation, when a metal producing a more resistive oxide is superimposed on a metal producing a less resistive one. This phenomenon is recognized as an electrical version of the Rayleigh-Taylor effect and results from the ionic current preferring the less resistive paths, enhancing the growth of the correspondent oxide. Oxide resistivities and structures, transport numbers, Pilling-Bedworth ratios are all considered as determining factors for the anodization process of such superimposed systems. In the current work, anodization of Hf/Ta system leads to Rayleigh-Taylor effects since HfO2 is the more resistive oxide. The boundary between Hf and Ta oxides may influence the conductive pathways required for the memristive effect, thus being most relevant for fabrication of highly stable and forming-free memristors. Additionally, the use of superimposed films with gradient but complementary thicknesses allows investigating the ideal Hf/Ta ratios for which the best memristive behavior is obtained.3 From this study, one pronounced zone prominent for memristive applications is found for Hf/Ta thickness ratios between 4 and 5. Here, unipolar and bipolar memristors are identified, with remarkable endurance and retention capabilities. The CFs positioning is predefined by the development of Ta2O5 columnar structures grown during the anodization process. It is also possible that few CFs may be found in parallel, according to TEM observations, showing more than one Ta2O5 “finger”. Previous studies on pure Hf anodic memristors have confirmed concurrent competing CFs formation.2 Thus, one could assume that the memristive switching mechanism can be conditioned by the formation of oxides with such structures. The identified Hf/Ta ratio could be an excellent choice for improved memristors fabrication. Controlled O vacancies generation is a critical factor in switching uniformity and reproducibility. Therefore, oxide “fingers” formation is a promising electrochemical approach towards defect-engineered memristors. Further investigation of the composite oxide formation, particularly in Hf/Ta superimposed system, is topical. Until now, such systems were not recognized in the literature for the ReRAM applications. This is highly promising since both memory and electrical characteristics are improved by the forming-free nature of the memristors with CFs mediated by oxide nanostructuring. I. Zrinski et al., Nanomaterials 11, 1 (2021) https://doi.org/10.3390/nano11030666 I. Zrinski et al. Appl. Surf. Sci. 565, 150608 (2021) https://doi.org/10.1016/j.apsusc.2021.150608 I. Zrinski et al., J. Phys. Chem. Lett. 12, 8917 (2021) https://doi.org/10.1021/acs.jpclett.1c02346 Figure 1

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