  Bismuth Selenide (Bi2Se3) is a topological insulator with a two-dimensional
layered structure that enables clean and well-ordered surfaces to be prepared
by cleaving. Although some studies have demonstrated that the cleaved surface
is terminated with Se, as expected from the bulk crystal structure, other
reports have indicated either a Bi- or mixed-termination. Low energy ion
scattering (LEIS), low energy electron diffraction (LEED) and x-ray
photoelectron spectroscopy (XPS) are used here to compare surfaces prepared by
ex situ cleaving, in situ cleaving, and ion bombardment and annealing (IBA) in
ultra-high vacuum (UHV). Surfaces prepared by in situ cleaving and IBA are well
ordered and Se-terminated. Ex situ cleaved samples could be either
Se-terminated or Bi-rich, are less well ordered and have adsorbed contaminants.
This suggests that a chemical reaction involving atmospheric contaminants,
which may preferentially adsorb at surface defects, could contribute to the
non-reproducibility of the termination.
