Redox-based metal oxide resistive switching devices, often referred to as memristors, offer new perspectives towards replacement of classic transistor based memories. Despite the promises of fast non-volatile ns-switching and high package density there is a lack of studies in which the electronic band gap and carrier types are systematically varied for an oxide solid solution in a memristor. In this study we tackle this by doing an extrinsic doping investigation on the oxide system Sr(Ti_x,Fe_1-x)O₃ for which the p-type electronic conduction and oxygen vacancy mobility and concentration is strongly altered as band gap of the oxide by almost 1.5 eV. Memory arrays of the model system Pt|Sr(Ti_x,Fe_1-x)O₃|Pt cross-bar structures were prepared via sputtering, pulsed laser deposition and microfabrication with respect to oxide doping in the range of 0<x<1. Here, we discuss carrier contributions for the restive switch`s kinetics and switching mechanism dependent on extrinsic doping level of the thin film structures.

First, we classically analyze switching kinetics via cyclic voltammetry and impedance.^1,2 In these experiments hysteretic bipolar I-V profiles were recorded; a decrease of resistance together with an oppressed resistive switching is observed with increasing doping level 25%. Additionally, we propose chronoamperometry measurements¹ to analyze diffusion kinetics of resistive switching oxide materials to the field to understand equilibrium vs. non equilibrium transport processes relative to extrinsic doping concentration applied in the oxide of the resistive switching device. In contrast to state-of-the art cyclovoltametry and pulsed experiments in which the material remains in a metastable state this method allows analyzing the resistive switch in thermodynamical equilibrium giving therefore additional insights into its switching behavior. Through the newly proposed “Memristor-based Cottrell analysis” we determine bias-dependent diffusion constants of 3x10^-15 for 6 MV/m for SrTiO₃ at ambient.¹ These complementary measurement methods demonstrate an extended strategy to analyze redox-based metal oxide resistive switching kinetics.

Secondly, we study the influence of humidification effects on the devices performances are discussed.³ We find that Pt|SrTiO_3‑δ|Pt will strongly modify the overall resistance states by up to 4 orders of magnitude as well as the device`s current-voltage profile shape, number of crossings and switching capability with the moisture level exposure, see figure 1. Comparison of the same humidity shows that this process is fully reversible.³ We attribute this behavior to the changed Schottky barrier by adsorbed surface water molecules and its interplay with the charge transfer of oxygen anionic-electronic charge carriers in the oxide affecting the memristance itself. From a fundamental perspective, these results show that moisture cannot be neglected in the development of anionic-electronic resistive switches as hydroxyl interaction seems to be crucial to the basic property of memristance. This study shows how by systematic doping of the metal oxide the resistive switching properties can be actively tuned to increase its performance.

References:

1. F. Messerschmitt, M. Kubicek, S. Schweiger, J.L.M. Rupp, Adv. Funct. Mater. 24, 47, 7448, 2014.
2. M. Kubicek, R. Schmitt, F. Messerschmitt, J.L.M. Rupp ACS Nano, 9, 11, 10737, 2015.
3. F. Messerschmitt, M. Kubicek, J.L.M. Rupp, Adv. Funct. Mater. 25, 32, 5117, 2015.

 Fig.1: This figure shows five consecutive I-V profiles for Pt|SrTiO_3-δ|Pt bits under three different humidity levels: Blue lines show increasing voltage branch and red lines decreasing, respectively. Depending on the humidity level the conductivity and the I-V profile shape changes drastically and without humidity no resistive switching is observable.²