Thickness effect on the ferroelectric properties of La-doped HfO2 epitaxial films down to 4.5 nm

Abstract

Stabilization of the orthorhombic phase of HfO₂ with La allows very high polarization and endurance to be achieved. However, these properties have not been confirmed yet in films having a 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 report on the thickness dependence of their ferroelectric properties. Films of less than 7 nm thickness show a high remanent polarization of about 30 μC cm⁻², slight wake-up, an endurance of at least 10¹⁰ cycles and a retention of more than 10 years, with the endurance and retention measured at the same poling voltage. La-doped HfO₂ films even as thin as 4.5 nm also show robust ferroelectric properties.