Bipolar ${\rm Ni}/{\rm TiO}_{2}/{\rm HfO}_{2}/{\rm Ni}$ RRAM With Multilevel States and Self-Rectifying Characteristics

To be compatible with 3-D vertical crossbar arrays, a TiO₂/HfO₂ bilayer resistive-switching memory (RRAM) cell sandwiched between Ni electrodes is developed. The proposed device has numerous highly desired features for the implementation of 3-D vertical RRAM, including: stable bipolar resistive switching; forming free; self-compliance; self-rectification; multiple resistance states; and room-temperature process. The resistive switching and current rectification are attributed to oxygen vacancy migration in HfO₂ and potential barrier modulation of the asymmetric TiO₂/HfO₂ tunnel barrier. The rectification ratio up to 10³ is capable of realizing a single-crossbar array up to 16 Mb for future high-density storage class memory applications.