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

Abstract

Using first-principles calculations, we identify a robust R30° reconstruction of a Si₃ trimer bilayer on the Si(111) surface with a La coverage of 2/3 monolayer. Each surface unit cell contains two Si₃ trimers and two La atoms, where the upper Si₃ trimer is located just above the lower one with a rotation of about 60°, while two La atoms with different heights are distributed between the Si₃ trimers and located on the T₄ 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 Si₃ trimer, while the higher La atom compensates two electrons to the lower Si₃ 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.