Stabilization of the orthorhombic phase of HfO2 with La allows very high polarization and endurance. However, these properties have not been confirmed yet in films having thickness of less than 10 nm. We have grown (111)-oriented La (2 at%) doped epitaxial HfO2 films on SrTiO3(001) and Si(001) substrates, and we report on the thickness dependence of their ferroelectric properties.
Films of less than 7 nm have a high remanent polarization of about 30 µC cm-2, show slight wake-up, endurance of at least 1010 cycles and retention of more than 10 years, both latest properties measured at the same poling voltage. La-doped HfO2 films even as thin as 4.5 nm also show robust ferroelectric properties.