Stable Structure of the Sb Monolayer on a Strained Ge(111) Substrate

Using an ab initio total energy and force method, we have investigated the stability of different structures of Ge(111):Sb(1 ML) as a function of the lateral lattice constant. We find that the (2 x 1)-reconstruction of Ge(111):Sb experimentally found to be stable at the equilibrium lattice constant of Ge is also the stable structure for slightly dilated Ge films (< 1%), while for larger dilatations the (1 x 1)-structure becomes stable. For compressed Ge films the (√3 x √3)T₄-structure (found experimentally on Si(111):Sb) becomes competitive and it is stable for the lattice constants compressed larger than 5\%. Furthermore, we find that for each structure, the equilibrium lattice constant is different from the bulk Ge crystal. Our results are helpful for understanding of surfactant mediated island growth on strained films.