Humidity-driven modulation of ferroelectricity in hafnia-zirconia membranes

Ferroelectric hafnia-based compounds, known for exhibiting strong ferroelectricity in films of sub-5 nm thickness, hold significant potential for being integrated into complementary metal-oxide-semiconductor devices. Due to the polymorphic nature of hafnia, their ferroelectric properties can be modulated through various mechanisms, including defects, strain, and electrochemical states. In this study, we fabricated ultrathin freestanding hafnia membranes, free from substrate and electrode-capping effects, to explore the relationship between their intrinsic ferroelectricity and surface electrochemical state by modulating humidity conditions during scanning probe microscopy measurements. Our results demonstrate enhanced ferroelectricity in hafnia under low-humidity conditions without requiring a wake-up process. This enhancement is attributed to reduced adsorption of water molecules on the membrane surface, which helps preserve oxygen vacancies that stabilize the ferroelectric phase in hafnia under an applied electric field. These findings suggest that beyond electrical control via field-cycling-induced phase transitions, electrochemical modulation through humidity provides an effective approach for tuning the ferroelectric properties of hafnia-based compounds, optimizing their performance in flexible nanoelectronics applications.