  Resistive switching in thin films has been widely studied in a broad range of
materials. Yet the mechanisms behind electroresistive switching have been
persistently difficult to decipher and control, in part due to their
non-equilibrium nature. Here, we demonstrate new experimental approaches that
can probe resistive switching phenomena, utilizing amorphous TaO$_x$ as a model
material system. Specifically, we apply Scanning Microwave Impedance Microscopy
(sMIM) and cathodoluminescence (CL) microscopy as direct probes of conductance
and electronic structure, respectively. These methods provide direct evidence
of the electronic state of TaO$_x$ despite its amorphous nature. For example CL
identifies characteristic impurity levels in TaO$_x$, in agreement with first
principles calculations. We applied these methods to investigate He-ion-beam
irradiation as a path to activate conductivity of materials and enable
electroforming-free control over resistive switching. However, we find that
even though He-ions begin to modify the nature of bonds even at the lowest
doses, the films conductive properties exhibit remarkable stability with large
displacement damage and they are driven to metallic states only at the limit of
structural decomposition. Finally, we show that electroforming in a nanoscale
junction can be carried out with a dissipated power of < 20 nW, a much smaller
value compared to earlier studies and one that minimizes irreversible
structural modifications of the films. The multimodal approach described here
provides a new framework toward the theory/experiment guided design and
optimization of electroresistive materials.
