The behavior of Cs on S-covered Si(100)-(2 x 1) and Si(100)-(1 x 1) surfaces

In this work we study the adsorption of Cs on (a) clean Si(100)-(2 x 1), (b) 0.5 ML of S-covered Si(100)-(2 x 1) and (c) 1 ML of S-covered Si(100)-(1 x 1) in ultrahigh vacuum (UHV). LEED and AES measurements suggest that the array of a Cs monolayer on clean and S-covered Si(100)-(2 x 1) surfaces was that of the double layer model, according to which, half of the Cs atoms reside on the raised sites of the dimers and the other half in the troughs. However, Cs on 1 ML of S-covered Si(100)-(1 x 1) forms initially a coplanar monolayer with the Cs atoms residing only on the same kind of sites. The presence of S on the Si surfaces increases the subsequently deposited (at RT) coverage of Cs to more than 1 ML. Structural models of Cs on clean and S-covered Si(100) surfaces are proposed.