Local electronic transport across probe/ionic conductor interface in scanning probe microscopy

Spatially-Resolved Study of the Electronic Transport and Resistive Switching in Polycrystalline Bismuth Ferrite

Ferroelectric materials attract much attention for applications in resistive memory devices due to the large current difference between insulating and conductive states and the ability of carefully controlling electronic transport via the polarization set-up. Bismuth ferrite films are of special interest due to the combination of high spontaneous polarization and antiferromagnetism, implying the possibility to provide multiple physical mechanisms for data storage and operations. Macroscopic conductivity measurements are often hampered to unambiguously characterize the electric transport, because of the strong influence of the diverse material microstructure. Here, we studied the electronic transport and resistive switching phenomena in polycrystalline bismuth ferrite using advanced conductive atomic force microscopy (CAFM) at different temperatures and electric fields. The new approach to the CAFM spectroscopy and corresponding data analysis are proposed, which allow deep insight into the material band structure at high lateral resolution. Contrary to many studies via macroscopic methods, postulating electromigration of the oxygen vacancies, we demonstrate resistive switching in bismuth ferrite to be caused by the pure electronic processes of trapping/releasing electrons and injection of the electrons by the scanning probe microscopy tip. The electronic transport was shown to be comprehensively described by the combination of the space charge limited current model, while a Schottky barrier at the interface is less important due to the presence of the built-in subsurface charge.

Characterization of Vegard strain related to exceptionally fast Cu-chemical diffusion in Cu[Formula: see text]Mo[Formula: see text]S[Formula: see text] by an advanced electrochemical strain microscopy method

Electrochemical strain microscopy (ESM) has been developed with the aim of measuring Vegard strains in mixed ionic-electronic conductors (MIECs), such as electrode materials for Li-ion batteries, caused by local changes in the chemical composition. In this technique, a voltage-biased AFM tip is used in contact resonance mode. However, extracting quantitative strain information from ESM experiments is highly challenging due to the complexity of the signal generation process. In particular, electrostatic interactions between tip and sample contribute significantly to the measured ESM signals, and the separation of Vegard strain-induced signal contributions from electrostatically induced signal contributions is by no means a trivial task. Recently, we have published a compensation method for eliminating frequency-independent electrostatic contributions in ESM measurements. Here, we demonstrate the potential of this method for detecting Vegard strain in MIECs by choosing Cu[Formula: see text]Mo[Formula: see text]S[Formula: see text] as a model-type MIEC with an exceptionally high Cu chemical diffusion coefficient. Even for this material, Vegard strains are only measurable around and above room-temperature and with proper elimination of electrostatics. The analyis of the measured Vegards strains gives strong indication that due to a high charge transfer resistance at the tip/interface, the local Cu concentration variations are much smaller than predicted by the local Nernst equation. This suggests that charge transfer resistances have to be analyzed in more detail in future ESM studies.