Goux et al.  have studied the effect of a Ta scavenging layer in HfO2 devices. They found that Potassium Canrenoate the resistance window between HRS and LRS is reduced for a Ta scavenging layer. Zhuo et al.  found that a Ge scavenger layer increased the endurance. These results are consistent with our calculations.
The long endurance and retention time of Ta2O5 RRAM  and  may be due to various factors; the ability of Ta2O5 to remain amorphous to higher temperatures than HfO2, its adaptive lattice for easy defect migrations in any crystalline inclusions , and that the vacancy is in V2+ state for typical electrode materials such as TiN.
In summary, we used first principle methods to calculate the energetics of oxygen vacancy processes in resistive random access memories (RRAM) for four typical oxides, HfO2, TiO2, Ta2O5 and Al2O3 as a guide to materials selection. The O vacancy formation energy, charge states and migration barriers are calculated. A band diagram defines the operating Fermi energy and O chemical potential range. We have shown how a scavenger metal can be used to tune the O chemical potential and thus the O vacancy formation energy for higher endurance. The metal electrodes and scavenger metal can also be used to vary the O vacancy charge state, to ensure a 2+ state, to maximize drift. The high endurance of Ta2O5 RRAM is related to its experimentally more stable amorphous phase.