Rate limited filament formation in Al-ZnO-Al bipolar ReRAM cells and its impact on early current window closure during cycling

Early current window closure effect has been investigated in ZnO bipolar resistive random access memory cell having reactive contacts (aluminum). From spatial oxygen vacancy mapping through low frequency noise (LFN) measurements, a gradual spatial mismatch between the oxygen vacancy profiles of HRS (high resistance state) and LRS (low resistance state) has been found to develop in the cycled cells. A rate limited release of O2- ions from reactive contacts has been mainly attributed to this phenomenon, which suggests that in addition to the field and temperature as the expected natural impetus for ion migration, chemical reaction at the reactive contacts also impacts the complete dissolution of the conductive filament, which leads to the population of a Zn rich donor state (Zn-i(center dot center dot)/Zn-i(center dot)) in the HRS of post-cycled cells. Zn-i(center dot center dot)/Zn-i(center dot) is located relatively far from the ZnO conduction band edge; thus, when an LFN probe was fixed to the oxygen vacancy level (V-0(center dot)/V-0(x)), it was found to decrease particularly in the HRS of post-cycled cells, which justifies the increase in the HRS current level over the uncycled ZnO cells. We also examined the complementary behavior of the (V-0(center dot)/V-0(x)) profile in the LRS of cycled cells, and finally, we validated, through a different nature of current conduction in pre- and post-cycled cells, the current window narrowing effect due to dissimilarities between the spatial profiles of oxygen vacancies.