Formation and properties of the Bi (√3×√3⁢)R⁢30∘ wetting layer on Ge(111)

Owing to the broken inversion symmetry at the surface, thin Bi films are expected to exhibit interesting electronic and topological properties stemming from its large spin-orbit coupling. Furthermore, the properties of Bi thin films are also governed by the coupling to the underlying substrate. In this work, we scrutinize the formation and properties of the (√3×√3)⁢R⁢30∘ Bi wetting layer on Ge(111) using scanning tunneling microscopy and first-principles simulations. The deposition of Bi on Ge(111) leads to the de-reconstruction of the c⁢(2×8) adatom structure of Ge(111) and the formation of Ge islands. The unreconstructed Ge terraces and islands are covered by a semiconducting (√3×√3)⁢R⁢30∘ Bi wetting layer. The elementary building block of the Bi wetting layer is a Bi trimer, which is located on a T4 site of the underlying Ge(111) substrate. This Bi trimer has a well-defined electronic state at −0.4e⁢V. The edges of the small Bi covered Ge islands, which are slightly metallic, are mainly of the armchair edge configuration. The most abundant defects in the Bi wetting layer are antiphase boundaries and single Bi trimer vacancies. Our work lays the foundation to investigate the topological nature of the Bi trimer wetting layer and any layers grown on top.