Symmetric Linear Rise and Fall of Conductance in a Trilayer Stack Engineered ReRAM-Based Synapse

Tapered conductive filament is crucial to safeguard the ideal symmetric linear variation in conductance during learning (potentiation) and forgetting (depression) phases in a neuromorphic synapse, electrically realized with resistive random access memory (ReRAM) cell. Here, we have demonstrated that, by engineering the spatial location of an AlOx intermediate layer in a trilayer ReRAM stack having ZrOx/AlOx/HfOx in a fixed total stack thickness, oxygen vacancies can be nonuniformly distributed, and as a result, an appropriate taper structure of the filament can be realized. Furthermore, spatial location of the AlOx layer in the trilayer stack determines the amount of leakage in the off state of the cell and hence its read current margin between the different conducting states. A remarkable conductance linearity with symmetry was obtained by input pulsing to a favorable ReRAM cell, while improvement in its characteristics was found because of its tapered filament structure and low leakage current resulted due to the farthest AlOx layer in stack with respect to the top electrode than the remaining cells.