Oxygen octahedron reconstruction in the ${\text{SrTiO}}_3/{\text{LaAlO}}_3$ heterointerfaces investigated using aberration-corrected ultrahigh-resolution transmission electron microscopy

We investigate the ${\text{LaAlO}}_3/{\text{SrTiO}}_3$ interface by means of aberration-corrected ultrahigh-resolution transmission electron microscopy allowing us to measure the individual atomic shifts in the interface at a precision of a few picometers. We find that the oxygen octahedron rotation typical for rhombohedral ${\text{LaAlO}}_3$ is across the interface and is also induced in the originally cubic ${\text{SrTiO}}_3$ layer. Octahedra distortion leads to ferroelectricitylike dipole formation in the interface which is in addition modified by cation intermixing.

Figure 1

(Color online) (a) Cubic structure of STO in a projection along the crystallographic [100] direction. A unit cell is lined out. (b) The structure of STO in [110] projection. (c) Rhombohedral LAO in pseudocubic ${\left[100\right]}_{\text{p}}$ projection. (d) Projection of the LAO structure along pseudocubic ${\left[110\right]}_{\text{p}}$. Rotation of two corner-sharing octahedra around the same ${\left[11\overline{1}\right]}_{\text{p}}$ axis but in opposite directions is indicated. Arrows indicate the corresponding shifts of the oxygen atoms out of the ${\left(001\right)}_{\text{p}}$ Al plane.

Figure 2

(Color online) (a) Scanning transmission electron microscopy (STEM) high-angle annular dark field (HAADF) image of a 2.5 nm LAO layer sandwiched between two STO layers taken along the crystallographic [100] direction. The two dotted lines indicate the (001) interfaces. The cubic unit cell in STO and pseudocubic in LAO are lined out: Sr and La atoms are given by large circles and Ti atoms and Al atoms by small circles. (b) Intensity profile of the image averaged over an area along the interfaces. Arrows denote the intensity which changes in comparison with that away from the interfaces. (c) Image taken in a transmission electron microscope (TEM) under NCSI conditions along the [110] direction. Image simulation leads to a defocus value of $+5\text{nm}$ and a sample thickness of 6.6 nm. In LAO the projected shift of the oxygen atoms out of the respective Al-atom rows as a result of oxygen octahedron rotation is clearly visible (arrows).

Figure 3

(Color online) (a) $c$-lattice parameter in STO and $c_{\text{p}}$-lattice parameter in LAO as functions of distance (in units of $c$ and $c_{\text{p}}$, respectively) from a reference plane in the lower STO layer. The vertical shade lines mark the interfaces. (b) The value of the shift parameter ${\delta }_{\text{O}}$ of the oxygen atoms as a function of distance from the reference plane. Squares denote upward shifts and circles denote downward shifts. (c) The value of the shift parameter ${\delta }_c$ of the center of the oxygen octahedra with respect to the appertaining cations. The error bars indicate a 95% statistical confidence level of a Gaussian regression analysis.

Figure 4

(Color online) Schematic of octahedron reconstruction at the interface of LAO/STO. (a) Rotation (green/light gray arrows) of rigid octahedra in STO as a result of elastic constraints resulting from the rotation of the oxygen octahedra in LAO. (b) Shift of the oxygen atoms in the ${\text{TiO}}_2$ planes of STO in the interface area toward the first LaO layer in LAO. The reduced length of the shift arrows increasing distance indicates a reduction in the attraction of the oxygen atoms toward the interface. An equivalent effect is expected from cation intermixing. (c) When the action due to the forces indicated in (a) and (b) is added the shift is enhanced for every second vertical oxygen position while it is diminished in every other.