Resistive Switching of Individual Dislocations in Insulating Perovskites -- A Potential Route Towards Nanoscale Non-Volatile Memories.

Electrically controlled resistive switching effects have been reported for a broad variety of binary and multinary oxides in recent years. In particular, titanates, zirconates, and manganites have been in the focus of the studies. In many cases, the mechanism of the switching and the geometrical extension of the phenomenon (filaments vs. bulk) are still under discussion. In this work, we present evidence for a redox-based switching mechanism and we indicate a potential route towards highly scalable non-volatile memories based on this switching effect. The challenge our work is to utilize resistive switching mechanism with the aim to construct \textit{active} electronic elements on a real nanoscale level, here by reversibly switching the electrical properties of individual dislocations by electrical stimuli. We demonstrate that standard undoped SrTiO$_{3}$ single crystals, utilized as a model system, exhibit a switching behavior along filaments based on dislocations, mediated by oxygen transport. For this, we employed a three-step procedure: the crystals were, at first, annealed at elevated temperatures under reducing conditions, then exposed to 200mbar O$_{2}$ pressure at room temperature, and finally subjected to an electric field under ultrahigh vacuum (electroformation). This treatment induced in a metal-insulator (SrTiO$_{3})$-metal (MIM) system a transition to metallic state. A hysteretic behavior appears after dynamical polarization of the MIM structure at the maximum electroforming currents. The shape of the I/V curve has the typical signature for bi-stable switching known for these types of perovskites. The positive temperature dependence of the resistance of the low- (LRS) and the high-resistance (HRS) state clearly identifies both states to be metallic in character. The inhomogeneity of the electrical transport becomes directly evident from a simple optical inspection and the conductivity maps as measured by LC-AFM of a planar structure. One can trace the formation of the filaments, emerging from the cathode and propagating towards the anode during the electroformation process. These filaments are well-oriented along the $<$100$>$-axis of the crystal and show a discrete and granular substructure on the nano-scale. The similarity in lateral distribution of exit points (spots) of conducting nano-filaments with respect to the distribution of etch pits suggests that the electrical transport along dislocations determines the micro- and meso-scopic electrical transport phenomena. Our results suggest that a dedicated contact arrangement is required to handle the filamentary conduction in a practical way by using macroscopic electrodes. At the same time, it emphasizes the need to control the relevant processes on the level of individual dislocations. With LC-AFM it is possible to specifically address single dislocations crossing the surface with adequate spatial resolution and use the conducting cantilever as the nano-electrode through galvanic point contact. We succeeded to initiate the local electroformation process for a single dislocation by applying a dc bias to the tip of the cantilever. Such nano-prepared dislocations reveal bi-stable switching behavior between a linear and a non-linear $I/V$-characteristics. The dynamic range of the electrical resistance covers at least 3 to 4 orders of magnitude at read-out voltages of 0.1 V. In order to develop a microscopic model for the filament, we performed first-principles calculations of extended, linear defects in SrTiO$_{3}$. Our analyse of electronic structure for extended defects with TiO enrichment establish that already subtle changes in O-content are sufficient to modulate the electronic properties and provide the necessary self-doping capability with a reversible transition between non-metallic and metallic behaviour. We propose a model for the resistive switching in SrTiO$_{3}$ based on the modulation of the electrical properties through electrical stimuli in a small segment of an orthogonal network of dislocations. Switching in our case corresponds then to an electrochemical ``closing'' or ``opening'' of the single dislocation in the uppermost portion of the network. Our results show that the switching behaviour in single-crystalline SrTiO$_{3}$ is an inherent property of the material and can be easily activated by external stimuli. Due to the availability of dislocation densities up to 10$^{12}$ cm$^{-2}$ in single crystals and thin film, one can even envisage to approach the Tbit regime, as long as the dislocations can be successfully arranged into registered superstructures. In summary, evidence is given that the electrical conductance of individual dislocations in a prototype perovskite, SrTiO$_{3}$, can be switched between a low and a high conducting state by the application of an electrical field. We demonstrate on the basis of \textit{ab initio} calculations and measurements with a scanning probe microscope SPM that the modulation of the electrical properties is related to the induced change in oxygen stoichiometry and the self-doping capability with a local insulator- metal transition along the core of the dislocations. A model is presented based on a three-dimensional network of such a filamentary structure to analyze the bi-stable resistive switching in the macroscopic metal-insulator-metal (MIM) structure. Our results show that electrically addressing individual dislocations in single crystals as well as epitaxial thin films provides a dynamic range for switching between low and high conducting states which covers several orders of magnitude in resistance and can be of technological interest for the application in Tbit non-volatile memory devices..