Extended Data Fig. 9: Ultrathin-enhanced distortions and polar signatures from spectroscopy.
From: Enhanced ferroelectricity in ultrathin films grown directly on silicon
a, Crystal field splitting diagram for the fluorite-structure structural polymorphs; symmetry-induced e-splitting provides a spectroscopic signature for the polar O-phase (Methods). b, Delineating symmetry-split energy regimes in oxygen K-edge XAS. Just as convergent beam electron diffraction provides signatures to demonstrate inversion symmetry breaking49, XAS provides spectroscopic signatures to distinguish between the nonpolar tetragonal and polar orthorhombic polymorphs (difficult to resolve from GI-XRD). Left, simulated XAS spectrum for tetragonal ZrO2 (P42/nmc) and right, polar orthorhombic ZrO2 (Pca21), both courtesy of the Materials Project56,57. The background colour shading denotes the symmetry-split regimes explained in the crystal field splitting diagram. c, Experimental XAS data on ultrathin HZO displays similar spectroscopic XAS features as the simulated polar O-phase (Pca21)—namely, relative e/t2 spectral weight and splittings corresponding to tetrahedral (∆T) and rhombic (∆R) distortions. Left, XAS of the HZO thickness series at the O K-edge, zooming in on the e- and t2-regimes. Right, O K-edge spectral weight trends as a function of HZO thickness. The relative spectral weights from the t2/e and e-split regimes indicate enhanced tetrahedral (∆T) and rhombic distortions (∆R) in ultrathin films, respectively, consistent with C2v symmetry of the polar O-phase. d, Schematic representation of the cation nearest-neighbour coordination dropping from NN = 8 (T-phase) to NN = 7 (polar O-phase) as the crystal symmetry is lowered. The disorder in oxygen polyhedral coordination (note the different oxygen atoms denoted by the blue and cyan atoms in the polar O-phase) manifests as spectral weight in the pre-edge regime62. e, The experimental pre-edge spectral weight as a function of thickness, indicating ultrathin-enhanced polyhedral disorder. f, Top: PEEM-XLD images of ten-cycle (1 nm) HZO at the O K-edge. Pre-edge images (left) exhibit no XLD contrast, while on-edge images (right)—at the energy corresponding to the polar-distortion split e-regime—demonstrate XLD contrast. This suggests that XLD is indeed sensitive to polar features in ultrathin highly textured HZO. Bottom, line profile of the XLD intensity, demonstrating substantial variations in on-edge XLD data compared to noise for pre-edge XLD. g, Crystal field splitting energies in HZO-related transition metal oxide systems. The material system, primary crystal electric field (∆1), secondary crystal electric field (∆2), and structure for various systems related to HZO and perovskite ferroelectrics are shown, where ∆O, ∆t, ∆T and ∆R corresponds to octahedral, tetragonal, tetrahedral, and rhombic crystal electric field (CEF), respectively. The reference crystal electric field values are taken from the Materials Project database57 (reference codes denoted by ‘mp’), and the experimental values are extracted via XAS energy-split features (b). The large tetrahedral (∆T) and rhombic (∆R) crystal field splitting energies present in ten-cycle HZO films are much larger than expected values for the polar fluorite-structure ZrO2 (b), which highlights the enhanced distortion present in ultrathin films subject to confinement strain, and is consistent with anomalously large structural distortions extracted from diffraction (Fig. 3g).
