Ultraviolet photoelectron spectroscopic study of boron adsorption and surface segregation on Si(111)

The adsorption and surface segregation behavior of elemental boron (B) deposited on Si(111) have been studied by ultraviolet photoelectron spectroscopy and accompanying reflection high-energy electron diffraction as a function of the B coverage (cB) and annealing temperature (T). Our results clearly demonstrate an effective incorporation of B into subsurface sites at T > 800 K and formation of a well-ordered (√3×√3)R30^^∘ surface superstructure. Thereby, a critical cB of about 0.6 monolayers (ML) was determined for different conditions to prevent surface defects resulting from Si dangling bonds, which appear as surface states at 0.4 eV below the Fermi level. Annealing of the defect-free (√3×√3)R30^^∘ superstructure covered by several MLs Si at T {} 1040 K results in a renewal of the perfect B-induced Si surface structure, even after several deposition and annealing cycles. This indicates a dominance of B surface segregation over bulk diffusion. Significant B bulk diffusion commences only above 1100 K. Differences were found for spectra obtained for the B-induced surface structures formed after deposition and surface segregation, respectively. An additional surface state appeared at 2.1 eV below the Fermi level after deposition of 0.6 ML B. The state could be attributed to boron-boron interaction due to the presence of small clusters at the surface. This surface state did not disappear after high-T annealing and was not observed for lower cB and after B surface segregation, respectively.