  We studied metallic Ag formation inside and on the surface of Ag2S thin
films, induced by the electric field created with a STM tip. Two clear regimes
were observed: cluster formation on the surface at low bias voltages, and full
conductance switching at higher bias voltages (V > 70mV). The bias voltage at
which this transition is observed is in agreement with the known threshold
voltage for conductance switching at room temperature. We propose a model for
the cluster formation at low bias voltage. Scaling of the measured data with
the proposed model indicates that the process takes place near steady state,
but depends on the STM tip geometry. The growth of the clusters is confirmed by
tip retraction measurements and topography scans. This study provides improved
understanding of the physical mechanisms that drive conductance switching in
solid electrolyte memristive devices.
