Low-coverage, low-temperature phase of Al overlayers on the Si(111) α-7×7 structure observed by scanning tunneling microscopy

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Abstract
The surface of the α-7×7 phase of Al/Si(111), which appears at low temperatures with low coverage, is examined by scanning tunneling microscopy. The surface is of 7×7 periodicity based on the DAS (dimer–adatom–stacking fault) structure. Al atoms adsorb as clusters preferentially on the center part of the half unit cells in the DAS model, producing characteristic triangular protrusions. The cluster consists of three Al atoms with each Al atom being bonded to the nearest center adatom. Consequently, the number of the dangling bonds is reduced, which lowers the surface energy. Bias-dependent images reveal that the electronic structure of the center adatoms is modified to be nonmetalliclike because of the saturation of the dangling bonds.