Mapping the evolution of Bi/Ge(111) empty states: From the wetting layer to pseudo-cubic islands

Semiconductors interfaced with heavy elements possessing a strong atomic spin-orbit coupling are important building blocks for the development of new spintronic devices. Here, we present a microscopic and spin-resolved spectroscopic investigation of ultrathin Bi films grown onto a Ge(111) substrate. At monolayer coverage, a Bi wetting layer is formed, characterized by a semiconducting behavior and a ( 3 x 3 ) R 30 degrees superstructure. The wetting layer supports the subsequent growth of Bi islands with a pseudo-cubic structure similar to that of Bi(110), showing a well-defined orientation with respect to the substrate high-symmetry directions. We performed photoemission and spin-resolved inverse photoemission experiments at off-normal electron emission and incidence, respectively, along the substrate Gamma K direction. Inverse photoemission, in particular, highlights the presence of a spin-polarized empty Bi state, not reported so far, due to the strong spin-orbit effects characteristic of the Bi surface and thin layers. Finally, scanning tunneling spectroscopy is employed to link the observed spectroscopic features to either the wetting layer or the Bi islands.