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.
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The Ferroelectricity and Crystallinity of Zirconia, Hafnia and Hafnium Zirconium Oxide (HZO) Ultrathin Films Prepared by Atomic Layer Deposition With and Without Post-Annealing
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Hsu, T, Lin, B, Shieh, J, & Chen, M. "The Ferroelectricity and Crystallinity of Zirconia, Hafnia and Hafnium Zirconium Oxide (HZO) Ultrathin Films Prepared by Atomic Layer Deposition With and Without Post-Annealing." Proceedings of the ASME 2018 Conference on Smart Materials, Adaptive Structures and Intelligent Systems. Volume 1: Development and Characterization of Multifunctional Materials; Modeling, Simulation, and Control of Adaptive Systems; Integrated System Design and Implementation. San Antonio, Texas, USA. September 10–12, 2018. V001T01A004. ASME. https://doi.org/10.1115/SMASIS2018-7946
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