First-principles study of atomic structures and electronic properties of Bi ultrathin films on Ge(111)

The atomic and electronic structures of ultrathin bismuth films on Ge(111) surface were investigated using first-principles calculations at Bi coverages ranging from 1/3 ML to 5 ML. Morphology of the surfaces varied as the coverage of Bi was increased. We found that the first layer of bismuth atoms follows the well-known trimer model exhibiting large Rashba spin-splittings. At 2 ML, bismuth atoms of the second monolayer form the second stacking layer of trimers, whereas at 3 ML and 5 ML, bismuth atoms of the topmost two monolayers form a buckled honeycomb structure. While the electronic structures of the two topmost layers exhibit two-dimensional nontrivial topological insulating phase, the bismuth atoms lying under these layers play an important role in p-type doping of the system.