The Ferroelectricity and Crystallinity of Zirconia, Hafnia and Hafnium Zirconium Oxide (HZO) Ultrathin Films Prepared by Atomic Layer Deposition With and Without Post-Annealing

Large stable ferroelectricity in hafnium zirconium oxide (HZO) solid solution ultrathin films (including pure zirconia (ZrO₂) and hafnia (HfO₂)) and ZrO₂/HfO₂ bilayer ultrathin films of thickness ranging from 5–12 nm, prepared by thermal atomic layer deposition or remote plasma atomic layer deposition (RP-ALD) has been demonstrated. Ferroelectric crystallization of the ZrO₂ ultrathin film with high-pressure orthorhombic (o) space group Pbc2₁ could be achieved without post-annealing due to the plasma-induced thermal stresses experienced by the film during the RP-ALD process. In contrast, for the ZrO₂/HfO₂ bilayer ultrathin film, due to the high crystallization temperature of HfO₂, post-annealing was needed to achieve sufficient confinement of the sandwiched HfO₂ layer by the ZrO₂ top layer and Si bottom substrate to promote the high-pressure ferroelectric o-phase in HfO₂. The ferroelectric properties of the HZO ultrathin films prepared by RP-ALD were highly dependent on the Hf-to-Zr ratio — an increasing amount of HfO₂ has been found to be detrimental to the ferroelectricity, mainly due to the high crystallization temperature of HfO₂. Without post-annealing, the ferroelectricity of the HZO ultrathin films was governed by the relative amounts of the amorphous phase and the ferroelectric o-phase induced by the plasma treatment. While with post-annealing, the ferroelectricity was governed by the relative amounts of the ferroelectric o-phase and the non-ferroelectric monoclinic (m) phase.