Table 1 Four different multilayer stacks are fabricated (S1–S4). Only the trilayer with a sputtered TiO_x layer (S4) shows stable bulk switching characteristics without filament formation

S1

ALD Al₂O₃/TiO₂ (3 nm/20 nm)

Filamentary

S2

ALD Al₂O₃/TiO₂ (3 nm/40 nm)

Filamentary

S3

ALD Al₂O₃/TiO₂ (3 nm/3 nm)

/ Sputter TiO₂ (6.5 nm)

Filamentary/Bulk

S4

ALD Al₂O₃/TiO₂ (3 nm/3 nm)

/ Sputter TiO_x (40 nm)

Bulk