Structural properties of ultrathin SrO film deposited on SrTiO3

Abstract

The role of epitaxial strain and chemical termination in selected interfaces of perovskite oxide heterostructures is under intensive investigation because of emerging novel electronic properties. SrTiO ${}_3$ (STO) is one of the most used substrates for these compounds, and along its $<001>$ direction allows for two nonpolar chemical terminations: TiO₂ and SrO. In this paper, we investigate the surface morphology and crystal structure of SrO epitaxial ultrathin films: from 1 to about 25 layers grown onto TiO ${}_2$ -terminated STO substrates. X-ray diffraction and transmission electron microscopy analysis reveal that SrO grows along its $\left[111\right]$ direction with a 4% out-of-plane elongation. This large strain may underlay the mechanism of the formation of self-organized pattern of stripes that we observed in the initial growth. We found that the distance between the TiO ${}_2$ plane and the first deposited SrO layer is $0.27\left(3\right)$ nm, a value which is about 40% bigger than in the STO bulk. We demonstrate that a single SrO-deposited layer has a different morphology compared to an ideal atomically flat chemical termination.

Keywords: 10 Engineering and Structural materials; 101 Self-assembly / Self-organized materials; 202 Dielectrics / Piezoelectrics / Insulators; 212 Surface and interfaces; 306 Thin film / Coatings; Pulsed laser deposition; epitaxial film; perovskite; surface termination; ultrathin film.

Graphical abstract

Figure 1.

Schematic view of SrTiO${ }_3$ substrate with TiO${ }_2$ (a) and SrO (b) terminations. (c) Most stable phase of SrO rock salt with cubic lattice parameter of 0.51 nm [29,30].

Figure 3.

AFM surface topographies (a–c) and representative line profiles (d–f) of TiO${ }_2$-terminated STO substrate (a), as-grown (b) and post-annealed (c) SrO film. The substrate shows typical step-like morphology with the height of each step of about 0.39 nm. While the pristine substrate has atomically flat terraces, the surface of the as-grown film shows a granular topology. After the annealing, the surface roughness reduces from $0.12(1)$ to $0.06(1)$ nm.

Figure 2.

Typical RHEED intensity during the deposition of about two layers of SrO onto TiO${ }_2$-terminated STO. The inset shows the initial RHEED pattern of the substrate.

Figure 4.

AFM characterization of two different cases of substrate surface partially covered by SrO ($\sim$80%). (a) and (d) show AFM surface morphology of the two films, (b) and (e) show AFM phase contrast, and (c) and (f) show AFM profile in the region marked by the lines. Schematics of the surface cross-section are shown on top of the line profiles.

Figure 5.

(a) XRD $\theta$-2$\theta$ scan of SrO film with a thickness of about 8 nm deposited onto TiO${ }_2$-terminated substrate. The peak present at 28.7° corresponds to the (111) reflection line of SrO. (b) Schematic representation of SrO along its $[111]$ direction.

Figure 6.

(a) Schematic representation of the Sr plane orthogonal to the [111] direction, and (b) TiO${ }_2$ plane in $(001)$-oriented STO. (c) Theoretical pole figure. (d) Experimental $\varphi$-scan acquired around the STO$\{111\}$ reflection (in blue at a tilt angle of 54.3°) and around the SrO$\{111\}$ reflection (in red at a tilt angle of 70.53°).

Figure 7.

(a) TEM image of SrO film grown on STO. Fast Fourier transform of TEM patterns of SrTiO${ }_3$ (b) and SrO (c) selected regions. Here (111)-oriented SrO domain is clearly visible with the following orientation relations: out-of-plane $[$0 0 1${]}_{\mathrm{S}\mathrm{T}\mathrm{O}}\parallel [$1 1 1${]}_{\mathrm{S}\mathrm{r}\mathrm{O}}$ and in-plane $[$ 1 0 0 ${]}_{\mathrm{S}\mathrm{T}\mathrm{O}}\parallel [$1 0 −1${]}_{\mathrm{S}\mathrm{r}\mathrm{O}}$.