Structure of the ba-induced Si(111)-(3 × 2) reconstruction

The Ba/Si(111) surface, previously known as a 3 × 1 phase, is found to have a 3 × 2 periodicity and a semiconducting band gap. The substrate reconstructs into the honeycomb chain-channel (HCC) structure with Ba atoms in the channel, as in the alkali-metal-induced Si(111)-(3 × 1). However, the metal coverage is determined to be 1/6 monolayers, half the alkali-metal coverage. We propose that the structure and the metal coverage determined for the Ba adsorbate is universal for other alkaline-earth-metal adsorbates. With the alkali-metal-induced 3 × 1 case, our results lead to a rule that one donated electron per 3 × 1 surface unit is necessary to stabilize the HCC reconstruction of Si.