  Resonant tunnelling is a quantum mechanical process that has long been
attracting both scientific and technological attention owing to its intriguing
underlying physics and unique applications for high-speed electronics. The
materials system exhibiting resonant tunnelling, however, has been largely
limited to the conventional semiconductors, partially due to their excellent
crystalline quality. Here we show that a deliberately designed transition metal
oxide superlattice exhibits a resonant tunnelling behaviour with a clear
negative differential resistance. The tunnelling occurred through an atomically
thin, lanthanum {\delta}-doped SrTiO3 layer, and the negative differential
resistance was realized on top of the bipolar resistance switching typically
observed for perovskite oxide junctions. This combined process resulted in an
extremely large resistance ratio (~10^5) between the high and low-resistance
states. The unprecedentedly large control found in atomically thin
{\delta}-doped oxide superlattices can open a door to novel oxide-based
high-frequency logic devices.
