Surface and grain boundary energy as the key enabler of ferroelectricity in nanoscale hafnia-zirconia: a comparison of model and experiment

Abstract

The unexpected ferroelectric properties of nanoscale hafnia-zirconia are considered to be promising for a wealth of applications including ferroelectric memory, field effect transistors, and energy-related applications. However, the reason why the unexpected ferroelectric Pca2 phase can be stabilized has not been clearly understood although numerous extensive theoretical and experimental results have been reported recently. The ferroelectric orthorhombic phase is not a stable phase under processing conditions from the viewpoint of bulk free energy. Although the possibility of stabilization of the ferroelectric phase due to the surface energy effect has been theoretically suggested, such a theoretical model has not been systematically compared with actual experimental results. In this study, the experimental observations on polymorphism in nanoscale HfO-ZrO solid solution thin films of a wide range of film compositions and thicknesses are comprehensively related to the theoretical predictions based on a thermodynamic surface energy model. The theoretical model can semi-quantitatively explain the experimental results on the phase-evolution, but there were non-negligible discrepancies between the two results. To understand these discrepancies, various factors such as the film stress, the role of a TiN capping layer, and the kinetics of crystallization are systematically studied. This work also reports on the evolution of electrical properties of the film, i.e. dielectric, ferroelectric, anti-ferroelectric, and morphotropic phase changes, as a function of the film composition and thickness. The in-depth analyses of the phase change are expected to provide an important guideline for subsequent studies.

Nucleation‐Limited Ferroelectric Orthorhombic Phase Formation in Hf0.5Zr0.5O2 Thin Films

“…−1 , for PMA and PDA, respectively, by multiplying the Boltzmann's constant. [17] Already in a polycrystalline form with the t-phase being dominant after the first RTA at 600 °C for 20 s, pristine HZO sample would only obtain the m-phase from the adequate distortion and phase transformation from the already present t-phase. As a result, higher amount of the t-/o-phase remained which resulted in the enhanced ferroelectric properties produced in several reports.…”

Effect of Annealing Ferroelectric HfO2 Thin Films: In Situ, High Temperature X‐Ray Diffraction

“…[63] Instead, the Pmn2 1 polar o-phase was energetically more favorable compared to the experimentally observed Pca2 1 o-phase. [23] Thus, Batra et al [26] suggested that various factors including compressive strain and electric field need to be combined to stabilize the Pca2 1 o-phase. [23] Thus, Batra et al [26] suggested that various factors including compressive strain and electric field need to be combined to stabilize the Pca2 1 o-phase.…”

“…[21] Therefore, multiple factors were suggested as origin of the stabilization of the ferroelectric phase. [23] In the latter study, it was confirmed that the o-phase can be stabilized within the nuclei formed during the atomic layer deposition (ALD) process, but the interface/grain boundary effect is not sufficient to stabilize the o-phase within the final grain size after annealing. [23] In the latter study, it was confirmed that the o-phase can be stabilized within the nuclei formed during the atomic layer deposition (ALD) process, but the interface/grain boundary effect is not sufficient to stabilize the o-phase within the final grain size after annealing.…”

“…[23] In the latter study, it was confirmed that the o-phase can be stabilized within the nuclei formed during the atomic layer deposition (ALD) process, but the interface/grain boundary effect is not sufficient to stabilize the o-phase within the final grain size after annealing. [23,24] Stress in thin films was also suggested as a possible origin, [1,25,26] and Shiraishi et al experimentally proved that the polymorphism of Hf 0.5 Zr 0.5 O 2 thin films is strongly affected by the thermal expansion coefficient (TEC) of the used substrate. [23,24] Stress in thin films was also suggested as a possible origin, [1,25,26] and Shiraishi et al experimentally proved that the polymorphism of Hf 0.5 Zr 0.5 O 2 thin films is strongly affected by the thermal expansion coefficient (TEC) of the used substrate.…”

“…Materlik et al suggested that the o-phase can be stabilized due to surface energy effects, [22] and Park et al comprehensively examined the surface energy model and their experimental observations. [23] Therefore, it was suggested that the crystalline phase of the initial nuclei can remain even after crystallization, implying that the phase transformation to the monoclinic phase (m-phase, space group: P2 1 /c) can be kinetically suppressed. [23] Therefore, it was suggested that the crystalline phase of the initial nuclei can remain even after crystallization, implying that the phase transformation to the monoclinic phase (m-phase, space group: P2 1 /c) can be kinetically suppressed.…”