**5. Conclusions**

The resistive switching mechanism of a bilayer system of Cu2O and Al2O3 was investigated. The main objective of this work was to discriminate between the contributions of each individual layer and to discuss how they affect each other. The outcome features valuable indications for future bilayer device design. The observed multi-level operation was controlled by the current compliance. A transition from a semiconducting filament to a metallic filament with increasing current compliance was observed. For the filament growth in Cu2O, a dual mechanism is proposed, which involves a valence change in the copper oxide at the anode and a metallization reaction at the cathode. The likely preferential filament rupture during reset in the Al2O3 layer confines the switching event to Al2O3. Consequently, the supply of the active metal is conditioned by the CC-dependent filament growth in the copper oxide during forming, which directly impacts cycling stability.

**Author Contributions:** Conceptualization, J.D. and A.K. (Asal Kiazadeh); methodology, A.K. (Asal Kiazadeh); validation, J.D. and A.K. (Asal Kiazadeh); formal analysis, A.K. (Asal Kiazadeh); investigation, J.D. and A.K. (Asal Kiazadeh); resources, A.K. (Andreas Klein), E.F. and R.M.; data curation, J.D. and A.K. (Asal Kiazadeh); writing—original draft preparation, J.D.; writing—review and editing, A.K. (Asal Kiazadeh), J.D. and E.F.; visualization, J.D and A.K. (Asal Kiazadeh); supervision, A.K. (Asal Kiazadeh) and E.F.; project administration, R.M. and E.F.; funding acquisition, A.K. (Andreas Klein), E.F. and R.M.

**Funding:** This research was funded by FEDER funds through the COMPETE 2020 Programme and National Funds through FCT—Portuguese Foundation for Science and Technology under project number POCI-01-0145-FEDER-007688, Reference UID/CTM/50025. J.D. acknowledges funding received from the European Union's Horizon 2020 Research and Innovation Programme through the project HERACLES (Project No. 700395) and the German Science Foundation through the collaborative research center SFB 595 (Electrical Fatigue of Functional Materials). A. Kiazadeh acknowledges FCT for the postdoctoral gran<sup>t</sup> SFRH/BPD/99136/2013 and for funding received through the project NeurOxide (PTDC/NAN-MAT/30812/2017).

**Conflicts of Interest:** The authors declare no conflict of interest.
