Large stable ferroelectricity in hafnium zirconium oxide (HZO) solid solution ultrathin films (including pure zirconia (ZrO 2 ) and hafnia (HfO 2 )) and ZrO 2 /HfO 2 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 ZrO 2 ultrathin film with high-pressure orthorhombic ( o ) space group Pbc 2 1 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 ZrO 2 /HfO 2 bilayer ultrathin film, due to the high crystallization temperature of HfO 2 , post-annealing was needed to achieve sufficient confinement of the sandwiched HfO 2 layer by the ZrO 2 top layer and Si bottom substrate to promote the high-pressure ferroelectric o -phase in HfO 2 . 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 HfO 2 has been found to be detrimental to the ferroelectricity, mainly due to the high crystallization temperature of HfO 2 . 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.