Extended Data Fig. 4: Emergence of highly-textured films in the ultrathin regime.
From: Enhanced ferroelectricity in ultrathin films grown directly on silicon
a, Synchrotron GI-XRD scans (λ = 0.775 Å) of HZO thickness series endmembers: 10-cycle and 100-cycle. The 100-cycle HZO film is indexed according to the polar orthorhombic phase Pca21. Many of the polycrystalline reflections, most notably the (111), are no longer present at an appreciable intensity in the ultrathin limit owing to the geometric constraints of one-dimensional spectra (unable to probe all reflections present in highly oriented films) (Methods). Instead tilted-geometry diffraction (pole figures) are used to access the oriented reflections. b, Pole figure of ten-cycle HZO, taken at a Qx slice corresponding to the film (111) lattice spacing. The radial direction represents χ, while the azimuthal direction represents φ (0°–360° range). The presence of four intense peaks corresponding to the four (111)-projections indicate the highly textured nature of the ultrathin HZO film. The four Si (111)-projections would be expected at φ = 45° off from the Qx,y principal axes at a smaller value of Qz. c, Schematic of the (311) (left) and (111) (right) close-packed planes in the fluorite-structure structure. All the cation sites lie on such planes, which minimize surface energy effects because only metal-oxygen dangling bonds are present out-of-plane. We note that all schematics reflect stacking of the respective planes to a total thickness of 1 nm, although ultrathin HZO films may not exhibit such stacking throughout the film. For ten-cycle films, {311} indexing is consistent with the relevant intensity (about 30°) observed in the out-of-plane one-dimensional GI-XRD pattern (a), and the (111) reflections are present from the two-dimensional pole figure pattern (b).
