The currently progressively growing interest of a materials design paradigm based on extensive solid solution formation with deliberately enhanced configuration entropy represents a promising approach for the development of novel resistively switching memristor oxides, especially, filamentary systems. Since switching events here represent stochastically and unequally localized processes in space the homogeneous but random distribution of cations in a highly strained lattice structure favours a reduced device variability. At the same time anharmonic phonon dispersion is expected to reduce thermal conductivity facilitating the growth of formed filaments, once they are nucleated, because heat dissipation is reduced.

The unary transition metal oxides ZrO₂/HfO₂/Nb₂O₅/Ta₂O₅/MoO₃/WO₃ are all well known to show resistively switching behaviour with distinct characteristics. A recent study demonstrated that an overall improved electronic switching performance can be obtained in thin films composed of an equimolar mixture of these simple oxides with high configuration entropy deposited by Atomic Layer Deposition. However, a detailed exploration on the fundamental physical mechanisms for this outstanding behaviour of the senary system ZrO₂-HfO₂-Nb₂O₅-Ta₂O₅-MoO₃-WO₃, has not been reported yet, even not for binary solid solutions or other subsystems.

The present study presents aspects of optimizing target fabrication and thin film deposition for the three binary solid solution subsystems (i)ZrO₂–HfO₂ (ii) Ta₂O₅–Nb₂O₅ and (iii) WO₃–MoO₃ as well as for the senary compositions containing all oxides components in equimolar proportions. The general purpose was to explore the mechanism behind the high-entropy system thermal and electronic behaviours, and the influence factors of microstructure of materials, thin-film qualities, and resistive switching properties.

Powders were synthesized by three different routes: (i) solid-state reaction (MO), (ii) Pechini-method (PE), and (iii) mechanochemical method (MC). All these powders were comprehensively characterized regarding chemical composition (ICP-OES), morphology and microstructure (SEM&EDX), thermal stability (DTA&TGA), crystallography (XRD), vibrational modes (Raman), surface condition (AFM), and sintering characteristics (Dilatometry). Consolidation of targets measuring 2 inch in diameter and several mm in thickness was conducted using Spark Plasma Sintering (SPS), which for their part were equally characterized analytically and structurally before thin film deposition (RF-sputtering). Reactive sputtering was used for thin film preparation which were structured into devices (Lithography&Etching) before measuring their resistive switching response by current-voltage measurements.

Results show that the two binary systems ZrHfO₄ and NbTaO₅ have excellent homogeneity for the MO- and PE-synthesis process: ZrO₂ and HfO₂ form continuous solid solution series and Nb₂O₅ is completely dissolved in the orthothombic Ta₂O₅ through substitution of Ta⁵⁺ by Nb⁵⁺. The senary oxides MO essentially forms three different phases, whereas the PE-senary powder after calcination at 700°C revealing a new crystallographic phase probably of high configuration entropy structure, that is totally not corresponding to any presented solid solutions. Therefore, the Pechini process might be better for targets preparation. Both binary and senary materials can be fully densified to targets (1300°C, 50MPa, 10min) via SPS. The effect of different sputtering conditions (RF-power, deposition time, O₂ partial pressure) on the thin-film quality of binary and senary materials, as well as the thin-film device resistive response characteristics curve (I-V curve) are presented in this paper.