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 ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$(0001) surface kept at room temperature. Two samples were investigated: a single ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ crystal, whose surface was Ar sputtered and then annealed at $\ensuremath{\sim}500{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$ for several minutes prior to Cs deposition, and a 13-nm-thick epitaxial ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ film that was not subject to sputtering and was annealed only at $\ensuremath{\sim}350{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$. 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 ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$(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.