First-principles Study Of The Rare-earth Metal Induced Reconstruction And Thin Film On Si(111) Surface

Crystalline silicon has broad application prospects in the fields of semiconductors,microelectronics,and spintronics.Studying the adsorption,reconfiguration and thin film structure of metal atoms on the Si surface will help us to further understand its microscopic mechanism and physics properties.In this dissertation,based on firstprinciples calculations of density functional theory,we have investigated the reconstruction mechanism of rare earth metal La atom induces the Si(111)surface and electronic properties of earth metal silicide(EuSi2)thin films on the Si(111)surface.We study the structural stability and electronic properties of La induced quasi twodimensional Si3 trimer reconstructions on Si(111)-((?))R30° surface using the first-principle calculations.At La coverage of 1/3 monolayer,we propose the Si3 trimer monolayer reconstruction on Si(111)surface.Its surface unit cell(?)has one La atom and one Si3 trimer and Si3 trimer satisfies the electron counting rule with a transfer of valence electrons from La atom,formally as La3+[Si-]3.Electronic band structure and band-decomposed charge density distributions calculations show that this Si3 trimer monolayer structure has a semiconducting character with an indirect band gap of 0.76 eV,and the lowest conduction band and the highest valence band are mainly contributed by surface La atoms and Si3 trimers.Thus,the band gap is surface gap.Moreover,the climbing-image nudged elastic band method dynamic simulations show that it is a two-stage conversion process between the Si3 trimer and SC(Seiwatz-chain)structure.On the other band,at La coverage of 2/3 monolayer,we identify a robust(?)(?)R30° reconstruction of Si3 trimer bilayer reconstruction on Si(111)surface using first-principles calculations.In each surface unit cell,it contains two La atoms and two Si3 trimers,where the upper Si3 trimer is located just above the lower one with a rotation of about 60°,meanwhile,two La atoms with different heights are distributed between the Si3 trimers and located on the T4 top site of Si(111)surface,forming a honeycomb-like network structure.We find that two La atoms have different valence states,La2+and La3+,respectively.Electronic band structure shows a semiconducting characteristic with a small surface band gap of 42 meV.Moreover,Si3 trimer bilayer structure simulated STM images show a good structure match with the recent experimental observation.These results expand our understandings for the rare earth metal induced reconstruction on Si(111)surface.Searching for magnetic silicide thin films has long been a hot topic in condensed matter physics and materials science.We find that the EuSi2 thin films in AA stacking is the most stable using first-principles calculations.The Eu2+ions in EuSi2 thin films have a large local spin moment of 6.96-7.00 μB derived from the Eu-4f orbital electrons in half filling state.It is worth noting that the Eu2+ions are coupled ferromagnetically within each layer and antiferromagnetically across the adjacent silicene layers.Electronic band calculations indicate that the monolayer EuSi2 thin film is a semiconductor with an indirect surface band gap of 0.45 eV,while the multilayer EuSi2 thin films have metallic behavior,contributed by Eu-5d and Si-3p electrons in the EuSi2 layers.These findings provide a systematical understanding for rare-earth metal silicides on the Si surface.