Growth and structural characterisation of Sr-doped Bi ₂Se ₃ thin films

Abstract

We grew Sr-doped Bi ₂Se ₃ thin films using molecular beam epitaxy, and their high quality was verified using transmission electron microscopy. The thin films exhibited weak antilocalisation behaviours in magneto-resistance measurements, a typical transport signature of topological insulators, but were not superconducting. In addition, the carrier densities of the non-superconducting thin-film samples were similar to those of their superconducting bulk counterparts. Atom-by-atom energy-dispersive X-ray mapping also revealed similar Sr doping structures in the bulk and thin-film samples. Because no qualitative distinction between non-superconducting thin-film and superconducting bulk samples had been found, we turned to a quantitative statistical analysis, which uncovered a key structural difference between the bulk and thin-film samples. The separation between Bi layers in the same quintuple layer was compressed whereas that between the closest Bi layers in two neighbouring quintuple layers was expanded in the thin-film samples compared with the separations in pristine bulk Bi ₂Se ₃. In marked contrast, the corresponding changes in the bulk doped samples showed opposite trends. These differences may provide insight into the absence of superconductivity in doped topological insulator thin films.