Effect of Al Concentration on Ferroelectric Properties in HfAlO_x-Based Ferroelectric Tunnel Junction Devices for Neuroinspired Applications

Since HfO_x-based ferroelectric tunnel junctions (FTJs) are attractive compared to perovskite-based FTJs and other emerging memory devices, they are being actively studied recently. They have advantages such as a simple metal–insulator–metal structure, complementary metal oxide semiconductor (CMOS) compatibility, non-destructive operation, and low power consumption. Moreover, doped HfO_x-based FTJs are in the spotlight in terms of neuromorphic engineering as a way of advancing from the von Neumann structure. In particular, Al dopant is effective for inducing ferroelectric properties due to its smaller radius than that of Hf. The optimal concentration of Al varies depending on the device materials and the annealing conditions during deposition. Therefore, in-depth research is required for neuromorphic applications. Herein, the properties of FTJ devices according to Al doping concentrations are analyzed. Subsequently, using the device with the highest remanent polarization, neuromorphic applications are implemented, including spike-timing-dependent plasticity (STDP), paired-pulse facilitation (PPF), long-term potentiation, and depression. The characteristics in different frequency regions are also studied to satisfy the demand for fast switching. Finally, the FTJ device is used as a physical reservoir in reservoir computing for efficient processing of time-dependent inputs.