Resistive Switching Mechanism of Memristors Based on High Entropy Oxide

High entropy oxides (HEOs), characterized by remarkable structural stability, excellent electrochemical properties, and modifiable characteristics, have gained increasing prominence in the field of energy storage. Furthermore, transition metal oxides (TMOs) have been widely used as dielectric materials of resistive random access memory (RRAM) due to their stable structures and variable valence states of transition metals. However, the switching mechanism of TM-HEO-based RRAM remains inadequately understood.
In this work, an epitaxial TM-HEO with the composition (Ca, Ti, Sr, Ta, Nb, O) is grown on the conductive substrate, and Au metal is deposited as the top electrode. A structural transformation between monoclinic and cubic perovskite is observed by the analysis of transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM), and the corresponding Fast-Fourier-Transform Diffraction pattern (FFT-DP). In the pristine state, the TM-HEO crystallizes in the monoclinic perovskite structure. After resistive switching operation, certain regions of the monoclinic phase are transformed into cubic perovskite structure, analogous to CaTiO₃. For a further study of the microstructural evolution, we conduct Electron Energy Loss Spectroscopy (EELS) to demonstrate the valence state changes. EELS analysis revealed that only specific elements undergo valence state changes, while some elements contribute to the stabilization of the crystal structure. In addition, this study compares two different bottom electrodes. It was found that using Nb:STO as the bottom electrode is more stable than using SRO/STO. The device also exhibits low set voltage (~2 V), long retention time (10⁴s), and environmental stability, which suggests that HEO is a promising RRAM material.