In-Gap States and Band-Like Transport in Memristors

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, as happens in redox-based memristive devices. Here, an external electric field is applied to an initially insulating oxide layer, thereby inducing a nonvolatile switching between a low-resistance state (LRS) and a high-resistance state (HRS). Switching occurs through the creation and annihilation of the so-called conductive filaments, which are generated at the nanoscale by the assembly of donor-type point defects, namely oxygen vacancies or cation interstitials. By using PEEM, we could directly map of the electronic structure of such a filament in an epitaxial SrTiO_3–x-based memristive device. To gain access to the defect states in the band gap, we employed soft X-ray resonant

photoelectron spectroscopy (RESPES), which allows the element-specific measurement of valence and defect levels through resonant emission at the Ti L absorption edge. These measurements unambiguously demonstrate that oxygen vacancies result in in-gap states. As shown by theory, these in-gap states are situated below the Fermi level. Even if they do not contribute to the current, they impact the shape of the conduction band.

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In-Gap States and Band-Like Transport in Memristive Devices; C. Bäumer, C. Funck, A. Locatelli, T.O. Menteş, F. Genuzio, T. Heisig, F. Hensling, N. Raab, C. Schneider, S. Menzel, R. Waser, R. Dittman; Nano Lett. Nano Lett. 19(1), 54–60 (2019); doi: 10.1021/acs.nanolett.8b03023.