STM OBSERVATIONS OF AG ADSORPTION ON THE SI(111)-ROOT-3X-ROOT-3-AG SURFACE AT LOW-TEMPERATURES

We have systematically studied the structural evolutions during adsorp tion of additional Ag atoms on the Si(111)-root 3 x root 3-Ag surface at 70 K by scanning tunneling microscopy. In the coverages less than 0 .02 ML (monolayer), the Ag adatoms distribute randomly as monomers on the root 3 x root 3-Ag surface. With coverage increase up to 0.1 ML, t wo-dimensional (2D) nuclei consisting of four Ag adatoms appear, the d ensity of which is much higher at surface steps than on terraces. With bring deposited further, the root 21 x root 21-Ag domains appear by c oalescing the 2D nuclei with each other. In the coverages from 0.14 to 0.20 ML, a well-ordered root 21 x root 21-Ag superstructure is formed with +/- 10.89 degrees orientations with respect to [11 (2) over bar] directions. The out-of-phase domain boundaries of the root 21 x root 21 phase an usually straight and along the direction of [11 (2) over b ar] 10.89 degrees. An atomic structural model for the root 21 x root 2 1 phase has been proposed in which its unit cell contains four Ag adat oms adsorbed on the Ag trimers of the unaltered root 3 x root 3-Ag fra mework. This model seems to be consistent with the 2D nuclei created a t the initial stage of adsorption and also with the domain boundary st ructure. This model also seems to be applicable to the Au-induced root 21 x root 21 phase on the root 3 x root 3-Ag surface. (C) 1998 Elsevi er Science B.V. All rights reserved.