  Resistive random-access memories, also known as memristors, whose resistance
can be modulated by the electrically driven formation and disruption of
conductive filaments within an insulator, are promising candidates for
neuromorphic applications due to their scalability, low-power operation and
diverse functional behaviours. However, understanding the dynamics of
individual filaments, and the surrounding material, is challenging, owing to
the typically very large cross-sectional areas of test devices relative to the
nanometre scale of individual filaments. In the present work, conductive atomic
force microscopy is used to study the evolution of conductivity at the
nanoscale in a fully CMOS-compatible silicon suboxide thin film. Distinct
filamentary plasticity and background conductivity enhancement are reported,
suggesting that device behaviour might be best described by composite core
(filament) and shell (background conductivity) dynamics. Furthermore, constant
current measurements demonstrate an interplay between filament formation and
rupture, resulting in current-controlled voltage spiking in nanoscale regions,
with an estimated optimal energy consumption of 25 attojoules per spike. This
is very promising for extremely low-power neuromorphic computation and suggests
that the dynamic behaviour observed in larger devices should persist and
improve as dimensions are scaled down.
