Electron correlation effects at semiconductor surfaces and interfaces: Si(111)-5x5, Si(111)-7x7 and Sn/Ge(111)

Electron correlation effects associated with the dangling bond surface stat es of Si(111)-5x5, Si(111)-7x7 and Sn/Ge(111)-3x3 are analyzed. In all the cases, ex tensive LDA-calculations are performed and effective two-dimensio nal Hamiltonians are deduced. Our analysis of these Hamiltonians shows that : (a) the Si(lll)-5x5 surface states exhibits a metal-insulator transition; (b) the Si(lll)-7x7 surface shows important similarities with,the Si(111)- 5x5 case, but it has a dangling bond surface band having a metallic charact er; (c) finally, the Sn/Ge(lll)-3x3 dangling bond surface bands also shows important correlation effects that are found, however, not to affect the me tallic character of the surface bands.