THE FERROELECTRICITY AND CRYSTALLINITY OF ZIRCONIA, HAFNIAAND HAFNIUM ZIRCONIUM OXIDE (HZO) ULTRATHIN FILMS PREPARED BY ATOMIC LAYER DEPOSITION WITH AND WITHOUT POST-ANNEALING

摘要

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 Pbc2_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.