Bonding of Noble Metals to Semiconductor Surfaces: First-Principles Calculations of Cu and Ag on Si(111).

Understanding the bonding between noble metals and semiconductor surfaces is of fundamental importance in a wide range of technological applications including electronic devices and catalytic processes. Although it has been known for more than 40 years that the synthesis of methylchlorosilanes (direct process) is catalyzed by Cu atoms, a complete understanding of the role of the metal atoms in this process is still lacking. Cu has unique properties in contrast with its congener Ag, which exhibits significantly lower catalytic activity. Their first-principles quantum-mechanical approach to the electronic structure and energetics of Cu and Ag atoms on a Si(111)surface shows that both Cu and Ag atoms are chemisorbed in 3-fold hollow sites at equilibrium heights of 0.74 and 1.48 A above the Si(111) surface with binding energies of 92 and 72 kcal/mol, respectively. Cu has a much lower penetration barrier than Ag (4 kcal/mol vs 52 kcal/mol). Therefore, at elevated temperatures, Cu is expected to diffuse into the Si lattice whereas Ag should desorb into the gas phase. New Auger and thermal desorption spectroscopy measurements confirm this picture.