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 mechani sms of Pb on the Si(111)-(7 x 7) surface in the initial stages of Pb c hemisorption within the DAS model. In the framework of the crystalline extension of the extended Huckel theory, two adsorption mechanisms ar e studied: the adsorption of Pb atoms on Si dangling bonds and Pb adso rption with the formation of Pb atom clusters. The second mechanism is found to be more favorable than the first, but the two mechanisms cou ld possibly coexist. Trimers and tetramers of Pb atoms are more likely than pairs. This tendency to agglomeration, in agreement with STM obs ervations 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 di ffusion paths for Pb on Si(111)-(7 x 7) are then investigated: jumps o f Pb atoms between unfaulted and faulted half unit cells are more diff icult than jumps inside the same half unit cell. Considering the diffu sion of a Pb pair between an adatom and a restatom, we show that the i nteraction between Pb atoms leads to a significant decrease in the bar rier 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.