Filamentary-based oxide resistive
memory is considered as a disruptive technology for nonvolatile data
storage and reconfigurable logic. Currently accepted models explain
the resistive switching in these devices through the presence/absence
of a conductive filament (CF) that is described as a reversible nanosized
valence-change in an oxide material. During device operation, the
CF cycles billion of times at subnanosecond speed, using few tens
of microamperes as operating current and thus determines the whole
device’s performance. Despite its importance, the CF observation
is hampered by the small filament size and its minimal compositional
difference with the surrounding material. Here we show an experimental
solution to this problem and provide the three-dimensional (3D) characterization
of the CF in a scaled device. For this purpose we have recently developed
a tomography technique which combines the high spatial resolution
of scanning probe microscopy with subnanometer precision in material
removal, leading to a true 3D-probing metrology concept. We locate
and characterize in three-dimensions the nanometric volume of the
conductive filament in state-of-the-art bipolar oxide-based devices.
Our measurements demonstrate that the switching occurs through the
formation of a single conductive filament. The filaments exhibit sizes
below 10 nm and present a constriction near the oxygen-inert electrode.
Finally, different atomic-size contacts are observed as a function
of the programming current, providing evidence for the filament’s
nature as a defects modulated quantum contact.
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