Mixed anodic oxides for forming-free memristors revealed by combinatorial screening of hafnium-tantalum system

A combinatorial screening approach has been designed for identification of Hf-Ta anodic memristors with enhanced properties. Three distinct compositional zones with similar characteristics have been identified, two of which are relevant for memristive applications. In a Hf-poor zone, (< 50 at.% Hf), the memristive behavior is similar to that of anodic Ta2O5. A second compositional zone (Hf content between 50 and 70 at.%) produced anodic memristors which dramatically differ. Devices in this region resemble forming-free unipolar behavior and show resistive states ratios of more than 8 orders of magnitude while retaining and enduring more than one million switching cycles. Photoelectron spectroscopy with soft and hard X-rays has been applied for understanding the chemistry of these special anodic structures and enrichment of Hf species close to the surface of anodic oxides has been evidenced. Unlike bipolar memristors from the first zone, the newly identified unipolar memristors show an in-depth homogeneous local chemical state of Hf and lower electronic polarizabilities for both O and Hf. High-resolution transmission electron microscopy reveals HfO2 crystallites embedded into an amorphous oxide matrix. These entities formed during the anodization process are considered to be responsible for the coexistence of threshold and non-volatile memristive characteristics.