Selective band engineering of Bi/Si(111) by boron segregation

Atomically thin layers of metals deposited on semiconductors display a variety of physical properties such as superconductivity, charge density waves, topological phases, strong spin-orbit (Rashba) splitting, among others. To access these exotic phases and induce new ones, it is necessary to control and tune the energies of the electronic states of those heterostructures. In this work we investigate the engineering of the band structure of the two-dimensional Bi/Si(111) $\ensuremath{\beta}$ phase using a modulation doping approach based on boron segregation at the surface. We demonstrate that the Bi-induced Rashba bands can be displaced in energy by up to 200 meV without altering their strong Rashba parameter. Importantly, while the Bi states shift upward, the underlying Si valence states remain essentially fixed, which rules out a simple band-bending scenario. Our density functional theory calculations reveal that the displacement originates from changes in the hybridization of Si states near the surface, induced by the presence of B atoms. This selective mechanism halves the distance of the Rashba-split states from the Fermi level, opening the way to their exploitation in transport and spintronic devices and highlighting the broader potential of modulation doping as a band-engineering strategy for two-dimensional metals on semiconductors.