Structural properties of ultrathin SrO film deposited on SrTiO₃

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.