Linear dichroic soft X-ray microscopy of ferroelectric stripe domains in epitaxial K$_\mathbf{0.6}$Na$_\mathbf{0.4}$NbO$_\mathbf{3}$

Abstract

Functional properties of ferroelectric thin films are governed by domains that can be engineered by epitaxial strain. Soft X-ray microscopy can image domain structures with elemental and electronic sensitivity, but hitherto its application to strain-stabilized domains has been hindered by the absorption of soft X-rays in epitaxial substrates. Here, it is demonstrated how this limitation can be overcome by locally back-thinning the (110) TbScO$_3$ substrate of epitaxial K$_{0.6}$Na$_{0.4}$NbO$_3$ ferroelectric thin films to achieve soft X-ray transparency at the O K-edge around 530 eV. Strain-induced ferroelectric stripe domains with periods down to 44 nm were resolved by scanning transmission X-ray microscopy and coherent diffractive imaging by exploiting the X-ray linear dichroism of hybridized O 2p-Nb 4d states, providing sensitivity to in-plane polarization components under normal incidence. The results establish soft X-ray microscopy for nanoscale imaging of epitaxial ferroelectric domains structures and open perspectives for time-resolved studies thereof.

Figures (4)

• Figure 1: (a) The pseudocubic (pc) perovskite unit cell of KNN shows the octahedral coordination of Nb$^{5+}$ by O$^{2-}$. (b) Sketch of the electronic orbitals in the ground (left) and excited hybridized state (right). The anisotropy in the O 2p orbitals causes strong XLD at the O K-edge. (c) X-ray transmission spectrum of 37 nm KNN on TSO at the O K-edge. The peaks assigned to hybridization with 4d t$_{2\mathrm{g}}$ and e$_\mathrm{g}$ are shaded. The broken vertical line indicates the photon energy of maximum XLD. (d) X-ray transmission spectrum of 37 nm KNN on TSO at the Nb M$_{3,2}$-edges.
• Figure 2: (a) STXM image of ferroelectric domains in KNN in an 18 µ m $\times$ 18 µ m area. Horizontally polarized X-rays at the O K-edge tuned to the energy of 2p-4d t$_\mathrm{2g}$ hybridization were used. The in-plane components of the ferroelectric polarization $\textbf{P}$ are in [001]$_{\mathrm{TSO}}$ and [1$\overline{1}$0]$_{\mathrm{TSO}}$ directions. An SEM image of two X-ray transparent windows in the substrate is shown in the top right with the imaged region framed in yellow. (b--d) Higher resolution images of the framed area in (a) with linear horizontally/vertically polarized X-rays and the corresponding XLD image, emphasizing the ferroelectric domains and suppressing topography contrast.
• Figure 3: Resonant X-ray scattering from ferroelectric domains in 37 nm KNN on (110) TSO. (a) RXS at the O K-edge (527.7 eV) shows the presence of diffraction peaks corresponding to the ferroelectric superdomains with ferroelectric stripes of 50 nm mean period in two directions. (b) The normalized X-ray transmission (XAS) and RXS signal from the diffraction peaks are shown. The maximum intensity of diffraction from the superdomains is at the t$_\mathrm{2g}$ hybridization of the O 2p and Nb 4d states. (c) RXS signal at the Nb M$_3$-edge (364 eV) also shows the ferroelectric domains with lower intensity. (d) Intensity of the RXS signal is detectable at the crystal-field split transitions of the Nb M$_{3,2}$-edges.
• Figure 4: Holography-assisted CDI of ferroelectric domains in KNN on TSO. (a) X-ray transparent window for the RXS measurements in Fig. \ref{['fig:fig3']}. (b) Square Au/Cr multilayer FTH mask during transfer to the KNN sample. (c) Enlarged view of the FTH mask with object and reference holes after fastening to the sample surface, corresponding to the red framed area in (b). (d) FTH sample from the KNN side. The transparent circular window for the object is circled. (e) Difference in the diffraction patterns with LH and LV polarization obtained at the maximum ferroelectric RXS at the O K-edge (see Figs \ref{['fig:fig3']}(a--b)). The superdomain diffraction peaks appear with interference fringes. (f) Reconstructed holographic XLD difference image of the ferroelectric superdomains with 57 nm and 44 nm in-plane stripe periods.