Epitaxial Yttrium Doped Hafnia with Giant Remnant Polarization for Ferroelectric Tunnel Junction Artificial Synapses...

Hafnia-based ferroelectric tunnel junctions (FTJs) emerge as one of the most promising solutions for memristor artificial synapse devices for neuromorphic computing. The device performance enhancement and the application are, however, hindered by the limited ferroelectricity of hafnia films and the unoptimized electrical pulses strategy. We employed yttrium doping engineering and (011)-oriented SrTiO₃ substrates in this work to achieve ultrathin epitaxial hafnia films with a giant remnant polarization of 54.7 μC/cm². The crystallographic properties of (Hf,Y)O_2-δ (HYO) films were thoroughly clarified, and they exhibited a single ferroelectric orthorhombic phase with a certain degree of rhombohedral distortion. Based on such films, high-performance FTJs were implemented, displaying 8 distinct resistance states (3 bits) with excellent endurance of over 10⁵ switching cycles. Combining with the designed optimal staircase step electrical pulses, our HYO FTJs display high-quality artificial synapse behavior, including highly linear potentiation and depression, and thus ultimately permit a recognition accuracy of 97.7% (88.4%) for the MNIST (Fashion-MNIST) data set. These results not only highlight the great potential of epitaxial HYO FTJs for artificial synapses but also provide an in-depth understanding of the physics of structure-ferroelectrics correlation of the hafnia-related fluorite ferroelectric materials.