Untersuchung der elektronischen Struktur der Li induzierten Si(111)-(3x1)-Rekonstruktion mittels Photoemission und ^8Li-beta-NMR

The electronic structure of the Si(111)-(3x1)-Li surface has been investigated by angular resolved photoemission spectroscopy (ARPES) and ^8Li beta-detected nuclear magnetic resonance experiments. The (3x1) reconstruction is well described by the HCC- (honeycomb-chain-channel) model and possesses one Li atom per surface unit cell corresponding to a Li coverage of 1/3 of a monolayer (ML). Single domain reconstructed (3x1) surfaces were used for the ARPES experiments. The usage of this surfaces in connection with an improved energy resolution and an optimized photon energy led to a significant improvement of previous photoemission data. Parallel to the quasi one-dimensional Li chains a very pronounced dispersionless surface state at E-E_{VBM} ~ -0.9 eV was observed. It is shifted by -0.31 eV compared to the prediction of LDA (local density approximation) calculations based on the HCC-model. This shift can probably be related to disregarded electron correlations in the LDA band structure calculations. A previous explanation by a combination of two experimentally unresolved states can be ruled out due to the improved energy resolution of our experiment. In LEED- (low energy electron diffraction) experiments a fully (3x1) reconstructed surface was already observed at a Li coverage of 0.29 ± 0.02 ML. At this coverage and at a surface temperature of 100 K a nuclear spin relaxation rate compatible with zero (alpha = 0.007 ± 0.017 s^-1) was measured. At higher temperatures exponentially rising relaxation rates were observed. Both facts can be explained by a model based on a two dimensional donor-level about 100 meV below the conduction band. This donor level may be induced by the Li additionally adsorbed during the relaxation experiments. Such a relaxation mechanism was already observed on the semiconducting Si(111)-(1x1):H surface. At a Li coverage of 0.47 ML (i.e. (3x1) reconstruction plus 0.14 ML additional Li) a "Korringa"-like nuclear spin relaxation was observed pointing to a metallic surface due to the additional Li coverage. At higher temperatures an additional relaxation process induced by diffusion of the Li on the surface seems to be present.