  The exploration of memristors' behavior at cryogenic temperatures has become
crucial due to the growing interest in quantum computing and cryogenic
electronics. In this context, our study focuses on the characterization at
cryogenic temperatures (4.2 K) of TiO$_\textrm{2-x}$-based memristors
fabricated with a CMOS-compatible etch-back process. We demonstrate a so-called
cryogenic reforming (CR) technique performed at 4.2 K to overcome the
well-known metal-insulator transition (MIT) which limits the analog behavior of
memristors at low temperatures. This cryogenic reforming process was found to
be reproducible and led to a durable suppression of the MIT. This process
allowed to reduce by approximately 20% the voltages required to perform DC
resistive switching at 4.2 K. Additionally, conduction mechanism studies of
memristors before and after cryogenic reforming from 4.2 K to 300 K revealed
different behaviors above 100 K, indicating a potential change in the
conductive filament stoichiometry. The reformed devices exhibit a conductance
level that is 50 times higher than ambient-formed memristor, and the conduction
drop between 300 K and 4.2 K is 100 times smaller, indicating the effectiveness
of the reforming process. More importantly, CR enables analog programming at
4.2 K with typical read voltages. Suppressing the MIT improved the analog
switching dynamics of the memristor leading to approximately 250% larger on/off
ratios during long-term depression (LTD)/long-term potentiation (LTP)
resistance tuning. This enhancement opens up the possibility of using
TiO$_{\textrm{2-x}}$-based memristors to be used as synapses in neuromorphic
computing at cryogenic temperatures.
