Physical mechanisms in ionic conductive bridging devices as studied by nanoscopic observation and manipulation.

The ionic movement in thin films can induce structural changes involving the local conductivity of the material. Filamentary-based resistive switching cells represent an intriguing emerging class of electronic devices, using ion migration to create or dissolve nanoscale conductive filaments in insulating materials. The change in the resistance state of such materials can be used to store a logical bit or to compute non-conventional logic in ultra-scaled devices.

However, tuning the non-volatile internal state of a sub-20 nm device by displacing ions in the space, can turn into a difficult task. In such conditions, the transport of ions lies at the boundary between classical physics and quantum mechanics. The whole switching mechanism is based on solid state electrochemical reactions happening in confined volumes (tens of nm³). This has created, a gap between physical understating and devices experimental results.

This PhD work targets to relate resistive switching with the related nanoscopic fundamental mechanisms. Establishing a correlation between the device and the observed physical effects, is the first step toward the understanding and the engineering of new devices. By using scanning probe microscopy (SPM) methodologies we create, observe and manipulate conductive filaments (CFs). We characterize the fundamental switching mechanisms at the nanoscale for cation- and anion-based resistive switching memory devices. We leverage the probe-sample interaction in order to investigate the RS phenomena in the sub-10 nm regime. The generated fundamental understanding on the filament formation and rupture, is transferred to the integrated memory cells that we characterize in three-dimensions under real operative conditions. This is enabled by an AFM-based tomographic technique named scalpel SPM, that is developed for the characterization of ultra-confined volumes. The extreme high resolution and the precise force control of the AFM is combined to a sub-nm material removal method. The technique has a spatial resolution below 5 nm for the electrical characterization of ultra-confined volume (  ~ 100 nm³).