Stoichiometrically optimized, epitaxial SmBa2Cu3O7-delta thin films with high T-c,T-R=0 and high critical current densities j(c) have been prepared for the first time in a tightly controlled molecular beam epitaxy process in non-reactive molecular oxygen, followed by an in situ loading process with molecular oxygen. The surface roughness (on a submicrometre scale) of single-crystal films with their c axes perpendicular to the surface depends markedly on the surface temperature of the substrate during the deposition of the epitaxial films, within a range of only a few degrees centigrade. The calibrated optimal temperature for the preparation of epitaxial films 200 nm thick of this single orientation is found to be 680 +/- 5 degrees C. In scanning tunnelling microscopy investigations, they show a surface roughness of less than 6 nm (five SmBa2Cu3O7-delta unit cells) on a 2 mu m x 2 mu m scale, At deposition temperatures below this optimal deposition temperature, the well-known a-axis growth increases rapidly, whereas higher temperatures give a significantly higher surface roughness, which can be observed by scanning electron microscopy.