Combinatorial Screening of Anodic Memristors Based on Hafnium-tantalum Mixed Oxides

Abstract

Among valve metals, Ta and Hf are the most studied high-κ dielectric materials for development of the next-generation non-volatile electronic components, commonly known as memristors. Redox-based resistive switching memories (ReRAMs) and artificial neural networks are applications examples of metal-insulator-metal memory types. An insulating layer, interposed between the bottom and the top conducting electrodes, is the determining factor in memristive performance. From this aspect, oxidized forms of Hf and Ta are promising candidates to be used as solid-state electrolytes, allowing conductive filaments formation (CFs) inside the oxide layer. There is a strongly motivated scientific interest for the implementation of anodic oxides in memristive devices due to its simple, cost-efficient and controllable fabrication. However, properties of anodic memristors based on Hf and Ta are not yet fully investigated and several parameters still have to be examined in detail. Herein, influence of the electrolyte selection on Hf and Ta anodic memristors was studied. Consequently, anodization parameters were optimized and applied for the fabrication of memristors based on Hf-Ta alloys spread across a thin film combinatorial library. Parent metals and their mixtures served as bottom electrodes and, identical to the case of Pt top electrode, they were deposited by sputtering. The total compositional spread of Hf-Ta library was ranging from 5 to 92 at.% Hf and was distributed across three Si wafers. The memristive properties of devices based on mixed anodic oxides were screened with a resolution of 1 at.%, thus offering a comparative investigation of a high number of compositionally different devices. Generally, the ratios between resistance states for Ta and Hf memristors were approximately in the same range, as well as their number of switching voltage levels. Nevertheless, in both cases memory and electrical characteristics were dictated by the incorporation of electrolyte species, which was confirmed by XPS. Therefore, for devices based on pure metals, the memristive behavior varied only as depending on the electrochemical conditions used for device fabrication. However, no electrolyte incorporation was observed in case of devices based on Hf-Ta library. Memristive properties could have been related only to different compositions which were grouped in three zones containing alloys showing similar behaviors. An improvement of resistance states ratios in some zones by orders of magnitude was achieved, as compared to ratios obtained for devices based on pure metals. Additionally, variations between the zones of unipolar and bipolar switching behavior were observed. HRTEM and hard X-ray XPS were done in order to compare the CFs within the mixed oxide layers for memristors from different zones. Clearly, the novelty of this work is the found spectra of different memristive properties for anodic Hf and Ta memristors and anodic memristors based on numerous Hf-Ta alloys. Such approach is offering the possibility of better understanding whether the switching mechanism is based on electrolyte species, oxygen vacancies, metallic species movement or formation of additional vacancies by redox reactions or thermal field for each specific composition or electrochemical parameters. Conclusively, this may be an important step towards defect engineered memristors fabrication, as intrinsically offered by the anodic process of memristive formation.