Engineering phase transition and ferroelectric properties in acceptor-doped HfO₂/SrRuO₃ thin-film heterostructures by a charge-balance synergistic strategy

In HfO₂, the stabilization of orthorhombic phase is crucial for hafnia-based ferroelectric devices. Here, we propose a charge-balance synergistic strategy to modulate phase transition and optimize ferroelectric properties in acceptor-doped HfO₂ thin-film heterostructures. Sm-doped HfO₂/SrRuO₃ heterostructures are adopted as the platform, in which the acceptor Sm_Hf^′ introduces extra holes into the HfO₂ controlled by doping concentration, while the SrRuO₃ electrode injects electrons depended on termination-controlled surface work function. Transition from monoclinic to orthorhombic and then to tetragonal phase is observed with increasing Sm concentration. The Sm-doping region for improved ferroelectricity is found to be depended on SrRuO₃ termination. These behaviors are ascribed to the charge-balance effect that combines the acceptor doping and the interface injection in the heterostructures. The holes in HfO₂ lattices are thus modulated to dominantly distribute on specific oxygen sublattices, lowering the relative energy between monoclinic and orthorhombic phases. We also extend the study into other acceptor-doped HfO₂, such as La³⁺ and Eu³⁺, and observe almost identical phase transition and ferroelectric behaviors. Our findings provide more physical insights into the stabilization of orthorhombic phase and open a new gateway for designing high-performance ferroelectric HfO₂ devices by harnessing both the electrode structures and the HfO₂-based layers in heterostructure systems.