Multilevel Switching in Forming-Free Resistive Memory Devices With Atomic Layer Deposited ${\rm HfTiO}_{x}$ Nanolaminate

We report multilevel switching in forming-free resistive random access memories (RRAMs) with atomic layer deposited HfTiO_x nanolaminate (5 nm) as the dielectric. The devices are fabricated using materials and processes compatible with complementary metal-oxide-semiconductor fabrication, including atomic layer deposition for the mixed dielectric structure. The devices switch between ON/OFF states at ~ ±1 V without any separate forming step. At least four distinct logic levels can be obtained for the HfTiO_x RRAMs by changing the current compliance in the ON state. The importance of the capability to switch from lower to higher as well as from higher to lower compliances in multilevel operation is demonstrated by comparing multilevel operation of HfO_x and HfTiO_x devices. The logic levels for the HfTiO_x devices show reliable cycling endurance and stable retention for all the levels.