Substrate-dependent valency of Yb chemisorbed onto Si(111)7×7, Si(100)2×1, and a-Si surfaces

Yb chemisorption at monolayer coverages onto Si substrates is the precursor stage of Yb/Si interface formation. Ytterbium is divalent as a metal but forms mixed-valent intermetallic compounds with silicon. L-edge x-ray-absorption resonance measurements show that at the early stage of the Yb/Si interface formation the Yb-Si chemisorption on different Si substrates induces mixed valency in the Yb atoms chemisorbed on the a-Si surface sites and a lower mixed valency in those on the Si(100) sites, but leaves purely divalent Yb ions on the Si(111) surface. This has direct consequences on the kinetics of the interface formation that develops for higher Yb coverages. Delocalization of Yb 4f electrons appears to be a unique electron probe of the bonding at semiconductor surfaces, and the Yb/Si interface can be viewed as a prototype case for understanding the kinetics of metal-semiconductor interface formation.