Structure and bonding of small semiconductor clusters.

We calculate the geometrical and electronic structure of small Si sub n, Si sub n sup + and Si sub n sup - clusters up to sites n = 14 within a combined tight-binding/density functional theory scheme. Especially stable structures for n = 6 and 10 coincide with observed abundancies in the experimental mass spectra. All equilibrium structures are found to be close-packed, with a different bonding than in the bulk fragments. A transition to bulk-like open structures is estimated to occur at cluster sizes n ~ 10 sup 2 - 10 sup 3.