On the direction of the conductive filament growth in valence change memory devices during electroforming

We discuss theoretically the filament growth direction in valence change memory (VCM) devices and show that this direction is determined by kinetic considerations. It is being fixed by the relation between the kinetics of ion transfer at the electrodes and that of ion transfer through the bulk. Considering a filament consisting of donors (oxygen vacancies) the direction of growth is from the cathode when the anode ion kinetics is limited in comparison to fast ion kinetics in the bulk of the oxide. In the opposite case the filament grows from the anode side. The filament may in some cases extend between the two electrodes while in others leave a narrow insulating gap under the anode.