Impact of the electroforming process on the device stability of epitaxial Fe-doped SrTiO3 resistive switching cells

In this work, the results of our detailed investigations on the electroforming procedure in Pt/SrTi0.99Fe0.01O3/SrTi0.99Nb0.01O3 [Pt/STO(Fe)/Nb:STO] metal-insulator-metal (MIM)-devices and its impact on the performance of resistive switching memory devices are presented. Questions about the exact location of the modifications triggered by the electroforming procedure within the investigated MIM-devices will be addressed. From a technological point of view, the thermal stability of formed devices becomes important. An increase in the device resistances during retention measurements has been observed indicating the presence of internal redistribution effects. These may result from an oxygen vacancy gradient induced by the forming process. However, these internal relaxation effects will not end up in the unformed state. Annealing experiments under defined atmospheric conditions allowed distinguishing between internal and external rediffusion effects. We found that SrTiO3 starts to interact with the surrounding atmosphere at moderate temperatures. The occurring external reoxidation effect set the device back to its initial (unformed) state. As a result, the investigated MIM-structures can no longer be regarded as closed systems and presented the large implication on the retention of such devices. The experimental findings are supported by calculations of the penetration depth of oxygen ions/vacancies in SrTiO3.