Electronic structures of Ga-induced incommensurate and commensurate overlayers on the Si(111) surface

Electronic band structures of Ga-induced dense overlayers on the Si(111) surface have been investigated using angle-resolved photoelectron spectroscopy and first-principle density-functional theory calculations. The well-known incommensurate 6.3 × 6.3 phase formed by the growth on the Si(111)7×7 surface and the newly found 1 × 1 phase grown on the preformed Si(111)3×3-Ga surface are characterized in detail. A highly dispersive surface state (S) is observed for the incommensurate phase but only a weakly dispersing one (S') for the 1 × 1 surface. Both surfaces are found to be nonmetallic, with their surface states fully occupied. The theoretical calculation reproduces well the S band of the 6.3 × 6.3 phase on the basis of a simple 1 × 1 Ga-Si bilayer structure formed with substitutional Ga atoms, which was proposed previously. No explicit sign of the incommensurate periodicity is found in the measured band dispersions, indicating a very weak incommensurate potential. The S band is shown to originate in the sp2-like planar bonds within the Ga-Si bilayer. The width of the S band is sensitive to the surface strain in the calculation and about 8% expansion of the Ga-Si bilayer lattice was needed to reproduce the measured dispersions, which is in good agreement with the previous X-ray study. On the other hand, the surface state S' of the commensurate 1 × 1 phase cannot be explained by any simple model of a substitutional or adsorbed Ga layer. Further structural studies are thus requested. The second Ga layer, which is metallic, grows two-dimensionally over this 1 × 1 layer up to a total coverage of about 5 ML.