Direct observation of Ag filament growth and unconventional SET-RESET operation in GeTe amorphous films

We report on the direct observation of Ag filament growth and a peculiar resistance switching in amorphous GeTe films with a lateral electrode geometry. The Ag filament growth was monitored by in-situ optical microscopy. The resistance switching was studied in three electrode pairs, Ag–Ag, Pt–Ag, and Pt–Ag/Pt (Ag electrode covered with Pt). In all the three electrode pairs, similar dendritic Ag filaments were clearly observed growing along both directions from one electrode to the other, according to the applied bias polarity. However, the SET and RESET processes are quite different. The Ag-Ag pair produces a unipolar clockwise switching. The Pt–Ag pair shows a bipolar counter-clockwise switching, as predicted in the basic electrochemical metallization theory, but the observed switching polarity is exactly opposite to the basic theory prediction. The Pt–Ag/Pt pair produces a unipolar counter-clockwise switching. The peculiar SET/RESET processes are explained on the basis of strong Ag diffusion into GeTe matrix resulting in an asymmetric effective electrode pair. The findings suggest that the SET/RESET processes are controlled by the amount of Ag and the electrode geometry.