The creation of point defects in matter can profoundly affect the physical and chemical properties of materials. If appropriately controlled, these modifications can be exploited in applications promising advanced and novel functionalities. Redox-based memristive devices – one of the most attractive emerging memory technologies – provide one of the most striking examples for the potential exploitation of defects. Applying an external electric field to an initially insulating oxide layer is known to induce a non-volatile, voltage-history dependent switching between a low resistance state and a high resistance state, also named memristive device. This switching occurs through the creation and annihilation of the so-called conductive filaments, which are generated at the nanoscale by assembly of donor-type point defects such as oxygen vacancies.
To date, the exact relationship between concentration and nanoscale distribution of defects within the filament on the one hand and the electronic transport properties of the devices on the other hand is still elusive. Due to limitations in sensitivity or spatial resolution of most characterization methods, the electronic structure of conductive filaments has not yet been characterized in detail. However, this knowledge is crucially needed as input for the development of electronic transport models with high predictive power.
Image: (a) Ti3+ map based on the Ti 3p3/2 spectrum. (b) Ti 2p 3/2 spectra for the filament and the surrounding. (c) Spatial map of the in-gap state distribution. (d) Valence band spectrum extracted from the filament at a photon energy of 463.3 eV with a fit of the valence band maximum and the in-gap states (red lines). (e) Band diagram of the device calculated based on the position of the in-gap states. The blue line shows the conduction band and the dashed green lines shows position of the defect states obtained by PEEM in respect to the conduction band