COMPARISON OF THE FIRST STAGES OF THE ADSORPTION OF CS AND GE ON THE SI(111)7X7 SURFACE

Experimental results show that Cs adsorbs preferentially on the threef old hollow (H-3) and threefold filled (T-4) sites of the Si(111)7 X 7 reconstructed surface. Following our recent calculations, adsorption o f Ge on H-3 or T-4 sites of Si(111)7 x 7 is unfavourable. We present a comparative study of various adsoption mechanisms for Ge and Cs on Si (111)7 X 7, in the framework of the crystalline extension of the exten ded Huckel theory. We consider the possibility of Cs and Ge adsorption on T-4 and H-3 sites, comer and center adatoms, as well as restatoms. The faulted and unfaulted parts of the 7 X 7 unit cell are distinguis hed. Adsorption energies and reduced overlap population calculations e ffectively favour adsorption on the Si dangling bonds for Ge atoms and addition on H-3 and T-4 sites for Cs atoms. The cesium atom has diffu se orbitals, so that, at these positions, it can significantly interac t with the dangling bonds despite the long distance. We discuss the di fferences of the behaviour between germanium and cesium interacting wi th Si(111)7 X 7.