Insights into the atomic structure of the interface of ferroelectric ${\mathrm{Hf}}_{0.5}{\mathrm{Zr}}_{0.5}{\mathrm{O}}_2$ grown epitaxially on ${\mathrm{La}}_{2/3}{\mathrm{Sr}}_{1/3}\mathrm{Mn}{\mathrm{O}}_3$

Epitaxial growth of ${\mathrm{Hf}}_{0.5}{\mathrm{Zr}}_{0.5}{\mathrm{O}}_2$ (HZO) thin films allows for the stabilization of the metastable orthorhombic phase with robust ferroelectric properties. So far, the ferroelectric phase is most commonly stabilized on perovskite substrates upon insertion of a buffer layer of ${\mathrm{La}}_{2/3}{\mathrm{Sr}}_{1/3}\mathrm{Mn}{\mathrm{O}}_3$ (LSMO); however, little is known about the role played by the LSMO buffer layer and the interface between HZO and LSMO. Inspection of a $\mathrm{HZO}/\mathrm{LSMO}/\mathrm{SrTi}{\mathrm{O}}_3$ heterostructure by scanning transmission electron microscope imaging and electron energy loss spectroscopy shows that, despite the substantial structural mismatch between HZO and LSMO, the interface between them is relatively sharp spanning ∼2 atomic layers. The LSMO surface, expected to be mostly $\mathrm{Mn}{\mathrm{O}}_2$ terminated, undergoes a chemical reconstruction consisting of the substitution of the Mn cations by a mixture of Hf/Zr cations. Density functional theory calculations show that the substitution of Mn by Hf on the $\mathrm{Mn}{\mathrm{O}}_2$-terminated surface of LSMO is energetically favorable, as the higher electronegativity and valence of Hf with respect to Mn balances the surface charge of the $\mathrm{Mn}{\mathrm{O}}_2$ layer.

Figure 1

(a) $Z$-contrast image of the HZO/LSMO interface along the [100] LSMO zone axis. (b) and (c) Images along the [110] LSMO zone axis, displaying the two possible zone axes (according to the in-plane HZO twins) for the o-HZO grains. No HZO zone axis is close to the substrate [100] zone axis in (a), and as a result, the HZO atomic columns are visible as bright horizontal stripes. The interface monolayer is denoted as $x$, and it is indicated by a yellow arrow in the three images, while the first atomic plane of the o-HZO layer is marked with an $i$. The row-averaged intensity profiles across the interface (along the out-of-plane direction) are shown on the right side of (a)–(c). Scale bar is 1 nm. (d) In-plane and out-of-plane spacings between planes as seen along [110] of LSMO in the HZO twin variant shown in (c). The distance between consecutive ${\mathrm{La}}_{2/3}{\mathrm{Sr}}_{1/3}\mathrm{O}$ planes is around 3.9 ± 0.1 Å, and that between plane x and 1 is around 2.3 ± 0.1 Å.

Figure 2

(a) High-angle annular dark field (HAADF) image acquired simultaneously with the electron energy loss spectrum (EELS) image dataset along the [100] zone axis. (b)–(f) Elemental maps of La-$M$, Sr-$L$, Mn-$L$, Hf-$M$, and Zr-$L$ edges, respectively. (g) Sketch of the uppermost region of the LSMO layer illustrating the substitution of the terminating Mn plane by Hf/Zr cations. Mn (purple), Hf/Zr (blue), La (dark green), and Sr (light green) atoms are shown.

Figure 3

Change of electronic fine structure across the LSMO/o-HZO heterointerface tracked using row-averaged (a) Mn-$L$ edge and (e) O-$K$ edge. (b) and (f) Superimposed spectra of the uppermost (2) and lowermost (8) $\mathrm{Mn}{\mathrm{O}}_2$ planes of Mn-$L$ and O-$K$ edges, respectively. (c) The high-angle annular dark field (HAADF) image (acquired along the [110] LSMO zone axis) from where spectra in (a) and (e) have been extracted, with the cation model structure superimposed [Mn (purple), Hf/Zr (blue), La (dark green), and Sr (light green)]. The colors of the spectra in (a), (b), (e) and (f) and of the arrows in (c) indicate the element analyzed in each atomic plane, with Hf/Zr in blue, O in red, and Mn in purple. The intensity ratio of the O prepeak (at 527 eV) and the main peak (at 535 eV) is ∼0.3 in the layers further from the interface, while this ratio becomes smaller (∼0.2) at the interface, as shown in (f). (d) O elemental map across the LSMO/o-HZO heterointerface, and the corresponding intensity profile (yellow background) shows a smaller O signal at the interface, indicating that a smaller quantity of O is present. The O map in (d) was obtained by integrating the O-$K$ in each pixel from ∼527 to ∼540 eV.

Figure 4

(a)–(c) Top view of ${\mathrm{La}}_{0.75}{\mathrm{Sr}}_{0.25}\mathrm{Mn}{\mathrm{O}}_3$ slabs with Hf (Hf-A), HfO (HfO-A), or $\mathrm{Hf}{\mathrm{O}}_2$ ($\mathrm{Hf}{\mathrm{O}}_2\text{−}\mathrm{A}$) clusters adsorbing on the $\mathrm{Mn}{\mathrm{O}}_2$-terminated surface, left panel, and the Hf atom exchanging position with a surface Mn atom and getting embedded in the LSMO matrix (Hf-E, HfO-E, and $\mathrm{Hf}{\mathrm{O}}_2\text{−}\mathrm{E}$), right panel. The conformations are represented by ball-and-stick models. Oxygen atoms adsorbing on the LSMO surface are labeled as O(s). The relative energies with reference to the $\mathrm{Hf}{\mathrm{O}}_x\text{−}\mathrm{A}$ configurations are also shown. (d)–(i) Spin-polarized, atom-projected density of states (PDOS) of surface atoms in Hf-A, Hf-E, HfO-A, HfO-E, $\mathrm{Hf}{\mathrm{O}}_2\text{−}\mathrm{A}$, and $\mathrm{Hf}{\mathrm{O}}_2\text{−}\mathrm{E}$ configurations.