Trends and strain in the structures of two-dimensional rare-earth silicides studied using medium-energy ion scattering

The surface structures of the two-dimensional (2D) rare-earth (RE) silicides formed by Gd and Tm on Si(111) have been determined using medium-energy ion scattering (MEIS). These data have been taken together with those from earlier MEIS studies of the 2D silicides formed by Dy, Ho, Er, and Y to investigate the existence of trends in the structural parameters of this class of surfaces. It was found that the Si-RE bond length associated with the surface bilayer in the 2D silicides followed the same trend as the bond length in the bulk silicide compounds which is simply related to the size of the RE atom. In contrast, changes in the Si-RE layer spacing in the 2D silicides were found to be too large to be accounted for by a size effect alone. It was found that the strain that results from the expansion or compression of the overlayer to fit the Si(111) surface is compensated by changes in the Si-RE layer spacing. Finally, there is a systematic variation in the rumple of the surface bilayer in the 2D RE silicides which appears to increase with the magnitude of the lattice mismatch suggesting that strain in the overlayer results in a weakening of the Si-Si bond in the surface bilayer.