Bipolar Ni/TiO_2/HfO₂/Ni RRAM with multilevel states and self-rectifying characteristics

To be compatible with 3-D vertical crossbar arrays, a TiO ₂HfO₂ 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: 1) stable bipolar resistive switching; 2) forming free; 3) self-compliance; 4) self-rectification; 5) multiple resistance states; and 6) room-temperature process. The resistive switching and current rectification are attributed to oxygen vacancy migration in HfO₂ and potential barrier modulation of the asymmetric TiO₂HfO₂ 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.