Anodic Oxides As Electrolytes for Resistive Switching Devices
Authors: Zaffora Andrea , Di Quarto Francesco , Valov Ilia , Santamaria Monica 
Published: February 2020

Redox-based resistive switching memories (ReRAMs) are one of most promising candidates to be next generation nonvolatile memories. They meet all requirements for green IT e.g. low power consumption, high write, erase and read speed and can also be used for beyond von Neumann computing, alternative logic and neuromorphic operations. ReRAM devices have a simple structure, high scalability, data retention, endurance and can satisfy high-density integration criterion [1]. These systems are usually divided in two categories, VCM (Valence Change Memories) and ECM (Electrochemical Metallization cells), depending on the mechanism underlying the device operation [2]. The cell structure consists of a solid electrolyte, e.g. a metal oxide or a layered oxides structure, sandwiched between two electrodes with metallic behavior (metals, TiN or conducting transparent oxides). Despite the oxide films have macroscopic dielectric (high-k) properties, deposited as nanoscale thin films they behave as solid electrolytes [3]. Electrolyte for such devices are usually prepared by physical (e.g. sputtering, pulsed laser deposition) or chemical (atomic layer deposition, chemical vapor deposition) techniques. An alternative way to produce the bottom electrode/oxide junction is the anodizing, a low-cost and low temperature technique that allows to electrochemically prepare oxides on the surface of valve metals or valve metals alloys such as Ta, Ti, Nb, Al, Hf and so on [4]. It is possible to tailor oxides features, such as thickness, morphology, structure and composition by adjusting process parameters (e.g. growth rate, anodizing electrolyte, formation voltage). Barrier-type anodic films seem to be suitable solid electrolytes for ReRAM devices since they are smooth, uniform in thickness and composition and show perfect adhesion to the metallic substrate.
In this work we want to present several valve metal/anodic oxide systems prepared by anodizing of metal thin films (namely Hf, Nb, Ta, Al and their alloys). Barrier-type anodic films were grown up to different formation voltages, in order to get oxides with different thickness and properties. An investigation based on electrochemical impedance and photoelectrochemical measurements was carried out to get information on the solid-state properties of the anodic oxides (i.e. band gap, flat band potential and dielectric constant) as a function of the formation conditions.
Furthermore, we fabricated ReRAM-type devices (metal/oxide/metal junctions) by depositing Pt top electrodes on the anodic films surfaces. Electrical characterization was performed to check whether these devices exhibit resistive switching and, in addition, they are suitable for the use in redox-based memories. In particular, cyclic voltammetries, dc I-V sweeps and pulse measurements were carried out to study the performances of the devices and to establish the kind of mechanism underlying the device operation [5]. The experimental findings (I-V sweeps stability, data retention and endurance) prove that anodic oxides can be considered promising electrolytes for ReRAM devices.
[1] Lee, M.-J.; Lee, C. B.; Lee, D.; Lee, S. R.; Chang, M.; Hur, J. H.; Kim, Y.-B.; Kim, C.-J.; Seo, D. H.; Seo, S.; Chung, U.-I.; Yoo, I.-K.; Kim, K. Nat. Mater. 10 (2011) 625-630.
[2] Valov, I. ChemElectroChem 1 (2014) 26-36.
[3] Valov, I.; Lu, W. D. Nanoscale 8 (2016) 13828-13837.
[4] Zaffora, A.; Di Franco, F.; Santamaria, M.; Habazaki, H.; Di Quarto, F. Electrochim. Acta 180 (2015) 666–678.
[5] Lübben, M.; Karakolis, P.; Ioannou-Sougleridis, V.; Normand, P.; Dimitrakis, P.; Valov, I. Adv. Mater. 27 (2015) 6202–6207.