  Reduction in metal-oxide thin films has been suggested as the key mechanism
responsible for forming conductive nanofilaments within solid-state memory
devices, enabling their resistive switching capacity. The quantitative spatial
identification of such filaments is a daunting task, particularly for
metal-oxides capable of exhibiting multiple phases as in the case of TiOx.
Here, we spatially resolve and chemically characterize distinct TiOx phases in
localized regions of a TiOx-based memristive device by combining full-field
transmission X-ray microscopy with soft X-ray spectroscopic analysis that is
performed on lamella samples. We particularly show that electrically
pre-switched devices in low-resistive states comprise reduced disordered phases
with O/Ti ratios close to Ti2O3 stoichiometry that aggregate in a ~ 100 nm
filamentary region electrically conducting the top and bottom electrodes of the
devices. We have also identified crystalline rutile and orthorhombic-like TiO2
phases in the region adjacent to the filament, suggesting that the temperature
increases locally up to 1000 K, validating the role of Joule heating in
resistive switching. Contrary to previous studies, our approach enables to
simultaneously investigate morphological and chemical changes in a quantitative
manner without incurring difficulties imposed by interpretation of electron
diffraction patterns acquired via conventional electron microscopy techniques.
