Proximity Engineered Tunable Phase Transitions in Ferroelectric Hafnia and Steep Switching Phase Change FETs

Ferroelectric hafnia has emerged as a transformative material in advanced electronics, with Complementary Metal Oxide Semiconductor (CMOS) compatibility, robust ferroelectricity, and scalability for applications including memory and ferroelectric field effect transistors (FEFETs). We demonstrate wake-up-free, robust ferroelectricity in solution-processed La-doped hafnia (LHO, 40 nm thick). By leveraging a metal-insulator transition (MIT) in the bottom electrode, TiO_x(N_y), a Mott Insulator, we engineer a depolarization-field-controlled reversible polar-to-non-polar phase transitions via the proximity effect in the ferroelectric hafnia layer. We further show that capping the ferroelectric layer with Al₂O₃ of various thicknesses, allows for subtle oxidation of the bottom electrode, resulting in tunable phase transitions between 140 and 260 K. Using the idea of proximity-coupled phase-transitioning ferroelectric layers, we demonstrate phase-change ferroelectric field-effect transistor (PCFE-FET) integrated onto a 2D MoS₂ channel. Near the induced phase transition temperatures, our devices show steep subthreshold slopes surpassing the Boltzmann limit. The temperature tunability of the phase transitions renders our PC-FEFETs highly promising for low-power computing solutions for both cryogenic and near-room temperature applications.