Tri-Level Resistive Switching in Metal-Nanocrystal-Based $\hbox{Al}_{2}\hbox{O}_{3}/\hbox{SiO}_{2}$ Gate Stack

Tri-level resistive switching behavior was observed in an Al
₂
O
₃
/SiO
₂
gate stack with Ru metal nanocrystals embedded in the Al
₂
O
₃
layer. The device was successfully switched among three resistance states (high, medium, and low) after a forming process using a simple electrical method. The resistance ratio of the high-resistance state to the low-resistance state is more than 10
³
. The insulator-to-conductor (and vice versa) transition of the Al
₂
O
₃
and SiO
₂
dielectric layers is elucidated by a physical model, which invokes oxygen ion (O
^2-
) trapping/detrapping at the metal-oxide interfaces, as well as O
^2-
transport and annihilation with the oxygen vacancies in the breakdown percolation path. The switching transition of each individual dielectric layer is found to be dependent on the polarity of the gate bias. This new understanding opens the prospect of metal-nanocrystal-based Al
₂
O
₃
/SiO
₂
gate stacks for a resistive switching memory application.