Two-dimensional bismuth-silver structures on Si(111)

Using scanning tunneling microscopy (STM) observations, it has been found that deposition of 0.8-1.3 monolayer of Ag onto the mixed Si(111)α − √ 3 × √ 3 / β − √ 3 × √ 3-Bi surfaces followed by annealing at 150-250°C induces formation of new ordered and quasi-ordered (Bi,Ag)/Si(111) metastable structures, √ 19 × √ 19, 4 × 4, 2 √ 3 × 2 √ 3, and '3 √ 3 × 3√3'. Scanning tunneling spectroscopy has demonstrated that the 2√3 × 2√3 structure is semiconducting, while the √19 × √19 and 4 × 4 structures are metallic. Structural models of the √19 × √19 and 4 × 4 have been proposed based on placing a single Ag(111)1 × 1 layer with selected Ag atoms being substituted for Bi atoms onto the bulk-like Si(111)1 × 1 surface. The models have been proved with DFT calculations and comparison of simulated and experimental STM images. Calculated band structure of the Si(111)4 × 4 structure displays a spin-split metallic surface-state band with splitting of ∆k ≈ 0.002 Å− 1 and ∆E ≈ 10 meV in the vicinity of the Fermi level.