La induced Si3 trimer bilayer on the Si(111) surface

Using first-principles calculations, we identify a robust R30 degrees reconstruction of a Si-3 trimer bilayer on the Si(111) surface with a La coverage of 2/3 monolayer. Each surface unit cell contains two Si-3 trimers and two La atoms, where the upper Si-3 trimer is located just above the lower one with a rotation of about 60 degrees, while two La atoms with different heights are distributed between the Si-3 trimers and located on the T-4 top site of the Si(111) surface, forming a honeycomb-like network structure. We find that the two La atoms have different valence states, La2+ and La3+, respectively. The high structural stability is attributed to the lower La atom saturating all the three dangling bonds of the upper Si-3 trimer, while the higher La atom compensates two electrons to the lower Si-3 trimer. The electronic band structure and band-decomposed charge density distribution show a semiconducting characteristic with a small surface band gap of 42 meV. Moreover, simulated STM images show a good structural match with the recent experimental observations.