Defect-Engineered Composite Hf-Ta Anodic Memristors for Reram Applications
Authors: Mardare Andrei Ionut , Zrinski Ivana , Minenkov Alexey , Hassel Achim Walter 
Published: July 2022

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