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 [111] 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(3) 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.

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