Effects of coverage on the geometry and electronic structure of Al overlayers on Si(111)

Scanning tunneling microscopy (STM) has been used to study the coverage dependence of the atomic geometry and electronic structure of ordered overlayers of aluminum on Si(111). STM results for the Si(111)-(3×3)Al, Si(111)-(7×7)Al, and Si(111)-(7×7)Al overlayers, at (1/3, (3/7, and 1 monolayer (ML) Al coverage, identify changes in the local Al bonding sites associated with the different surface symmetry and local Al coverage. A hitherto unreported (2×1) structure is also observed coexisting with both 3 × 3 and 7 × 7 structures. The results confirm a threefold adatom geometry for the 3× 3 surface, and reveal clusters of threefold Al adatoms adsorbed on twofold sites for the 7 × 7 surface and the substitution of Al for Si in the outer double layer for the (7×7)Al surface. The transition from semiconducting to metallic behavior is directly correlated with changes in the local bonding geometry as a function of coverage. © 1989 The American Physical Society.