Superconducting FeSe0.5Te0.5 thin films: a morphological and structural investigation with scanning tunnelling microscopy and x-ray diffraction

We used scanning tunnelling microscopy to study the morphology of superconducting FeSe0.5Te0.5 thin films epitaxially grown by pulsed laser deposition. Samples with critical temperature Tc above the bulk value (>16 K) show large atomic terraces, and a square lattice of periodicity 3.8 Å associated with the Se/Te surface termination. Differences in the height coordinate of the chalcogenide atoms are clearly visible at the atomic level. On the contrary, samples with lower Tc (11 K) show hillocks generated by a spiral surface growth driven by threading dislocations of screw character. A comparative x-ray diffraction analysis reveals differences of compressive strain for the two classes of specimens. Variations in the deposition rate are found to affect film growth and inner strain, which ultimately tune Tc.