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.

Automated summary

Through first-principles calculations, a stable R30 degree reconstruction of a Si-3 trimer bilayer on the Si(111) surface with a La coverage of 2/3 monolayer was identified. The unique honeycomb-like network structure formed by two Si-3 trimers and two La atoms on the surface unit cell is attributed to high structural stability, with distinct valence states of La2+ and La3+ present. Additionally, the system exhibits a semiconducting characteristic with a small surface band gap of 42 meV and shows good structural agreement with recent experimental observations in simulated STM images.