Geometric and electronic structure of the cs-doped Bi₂Se₃(0001) surface

Using surface x-ray diffraction and scanning tunneling microscopy in combination with first-principles calculations, we have studied the geometric and electronic structure of Cs-deposited Bi₂Se₃(0001) surface kept at room temperature. Two samples were investigated: a single Bi₂Se₃ crystal, whose surface was Ar sputtered and then annealed at ∼500^◦C for several minutes prior to Cs deposition, and a 13-nm-thick epitaxial Bi₂Se₃ film that was not subject to sputtering and was annealed only at ∼350^◦C. In the first case, a considerable fraction of Cs atoms occupy top layer Se atoms sites both on the terraces and along the upper step edges where they form one-dimensional-like structures parallel to the step. In the second case, Cs atoms occupy the fcc hollow site positions. First-principles calculations reveal that Cs atoms prefer to occupy Se positions on the Bi₂Se₃(0001) surface only if vacancies are present, which might be created during the crystal growth or during the surface preparation process. Otherwise, Cs atoms prefer to be located in fcc hollow sites in agreement with the experimental finding for the MBE-grown sample.