Abrupt orthorhombic relaxation in compressively strained ultrathin $\mathrm{SrRu}{\mathrm{O}}_3$ films

Lattice structure can dictate electronic and magnetic properties of a material. Especially, reconstruction at a surface or heterointerface can create properties that are fundamentally different from those of the corresponding bulk material. We have investigated the lattice structure on the surface and in the thin films of epitaxial $\mathrm{SrRu}{\mathrm{O}}_3$ with the film thickness up to 22 pseudocubic unit cells (u.c.), using the combination of surface sensitive low energy electron diffraction and bulk sensitive scanning transmission electron microscopy. Our analysis indicates that, in contrast to many perovskite oxides, the $\mathrm{Ru}{\mathrm{O}}_6$ tilt and rotational distortions appear even in single unit cell $\mathrm{SrRu}{\mathrm{O}}_3$ thin films on cubic $\mathrm{SrTi}{\mathrm{O}}_3$, while the full relaxation to the bulklike orthorhombic structures takes 3–4 u.c. from the interface for thicker films. Yet the $\mathrm{Ti}{\mathrm{O}}_6$ octahedra of the substrate near the interface with $\mathrm{SrRu}{\mathrm{O}}_3$ films show no sign of distortion, unlike those near the interface with $\mathrm{CaRu}{\mathrm{O}}_3$ films. Two orthogonal in-plane rotated structural domains are identified. These octahedral distortions are essential for the understanding of the thickness dependent transport and magnetism in ultrathin films.

Figure 1

(a) RHEED oscillations of the initial growth of SRO films on STO(001) surface. The inset shows the diffraction pattern before and after film growth. (b) Coupled x-ray diffraction scan of a ∼47-unit-cell thick SRO film. The Laue oscillations are fitted and presented in the red curve. Inset shows a schematic orthorhombic structure of bulk SRO. Panels (c) and (d) show rocking curves of the substrate and film peak, respectively. (e) Reciprocal space mapping about the (−103) Bragg diffraction spot. The substrate peak and the film peak are marked.

Figure 2

LEED images taken at room temperature with a beam energy of 74 eV: (a) (1 × 1 ) Bulk truncated STO (001) surface overlapped with a red square for surface unit cell. (b) A 5-u.c. SRO film on STO (001) showing $\left(\surd 2\times \surd 2\right)R{45}^{\circ }$ reconstructed unit cell (blue square) with respect to the substrate one. (c)–(e) The same sample under various annealing conditions. (f) A 1-u.c. SRO film.

Figure 3

Simulated LEED diffraction patterns for different surface symmetry. (a) A schematic showing $\mathrm{Ru}{\mathrm{O}}_6$ tilt and rotations. (b), (c) Top view of a square lattice with no $\mathrm{Ru}{\mathrm{O}}_6$ rotation or tilt and the corresponding diffraction pattern. The integer spots due to a square lattice are shown as a dot with a concentric circle. (d), (e) Square lattice where $\mathrm{Ru}{\mathrm{O}}_6$ octahedra are rotated but not tilted and the corresponding diffraction pattern. Fractional spots are shown as solid dots. Glide lines are shown with dashed lines. Arrows indicate the position of missing fractional spots. (f), (g) Lattice with both $\mathrm{Ru}{\mathrm{O}}_6$ tilts and rotation as well as the corresponding diffraction pattern. Only one glide line survives as shown with dashed lines.

Figure 4

Quantitative analysis of a 22-u.c. SRO film with STEM: (a) HAADF and (b) ABF images taken along the [100] direction of the substrate. (c) A zoom-in of a region of ABF image in (b) overlapped with schematic octahedral rotations. (d) The out-of-plane (OOP) and (e) in-plane (IP) lattice parameters extracted from the HAADF images. Both the substrate STO-film interface and the capping STO-film interface are shown as orange dashed lines. Red and blue lines mark the pseudo-cubic lattice parameters of the corresponding bulk structures, respectively. (f) The oxygen octahedral tilt angles extracted from the ABF image in (b) is plotted vs film thickness. The Ru-O-Ru bond angle of 180° for STO (red line) and 166° for bulk orthorhombic SRO (blue line) as well as the emphasized interface regions (shaded) are marked. (g) A comparison of octahedron tilts near interface of SRO/STO to CRO/STO.

Figure 5

(a) Distorted orthorhombic film on cubic substrate under compressive strain. “o” and “p” index refer to the orthorhombic and pseudocubic lattice parameters, respectively. (b) Four 90° rotated domains are expected as shown. Red arrow points to the direction of in-phase rotation of the neighboring octahedra in Glazer notation. Two distinctly different domains are observed in STEM taken along [100] beam direction. (c) FFT of domain $A$ and simulated diffraction obtained with the beam direction the same as the direction of in-phase rotation of the model structure (inset). (d) FFT of domain $B$ and simulated diffraction with the beam direction perpendicular to the in-phase rotation axis.