Surface Reconstruction of (123) and (111) Ge(Sn) Solid Solutions Induced by Tin Segregation

We present a comparative study of tin segregation onto the (123) and (111) surfaces of a Ge(Sn)- 0.5 at. % bicrystal using the AES technique. Between 350°C and 450°C, the maximum segregated tin amount, about 1.25 monolayer, does not vary with the orientation of the surface or the temperature. This indicates that attractive Sn-Sn forces are involved in the segregated phase which is in fair agreement with the Ge-Sn phase diagram. The shape of the kinetic curves depends on the orientation. On the (111) surface, the formation of various structures in equilibrium with the bulk is correlated with surface reconstructions. First, Sn atoms take the place of Ge adatoms to form a 2D phase; then, Sn segregates in other superficial sites, which leads to the nucleation of denser 2D phases, such as (7 × 7) structure. On the (123) surface, Sn atoms take the place of Ge atoms in quasisubstitutional sites of the superficial layers; the kinetics of surface segregation is not sensitive to sub monolayer structures.