Adsorption and diffusion mechanisms of Pb on Si(111)-(7x7) in the initial stages of Pb chemisorption

We present a theoretical study of the adsorption and diffusion mechanisms of Pb on the Si(111)-(7 x 7) surface in the initial stages of Pb chemisorption within the DAS model. In the framework of the crystalline extension of the extended Huckel theory, two adsorption mechanisms are studied: the adsorption of Pb atoms on Si dangling bonds and Pb adsorption with the formation of Pb atom clusters. The second mechanism is found to be more favorable than the first, but the two mechanisms could possibly coexist. Trimers and tetramers of Pb atoms are more likely than pairs. This tendency to agglomeration, in agreement with STM observations for room-temperature deposition, is explained by the strong interaction between Pb atoms which is emphasized in our calculations. The faulted half of the unit cell is preferred by about 0.1 eV. The diffusion paths for Pb on Si(111)-(7 x 7) are then investigated: jumps of Pb atoms between unfaulted and faulted half unit cells are more difficult than jumps inside the same half unit cell. Considering the diffusion of a Pb pair between an adatom and a restatom, we show that the interaction between Pb atoms leads to a significant decrease in the barrier energy as compared to that for the diffusion of a single Pb atom. An interpretation of experimental data obtained for room-temperature deposition at low coverage is proposed. (C) 1998 Elsevier Science B.V. All rights reserved.