Phase Transitions on Silicon and Germanium {111} Surfaces

Abstract

The 2 × 1 reconstructions observed with cleaved {111} surfaces of silicon and germanium are not stable but convert irreversibly to the Si((111))-7 × 7 and Ge((111))-c(2 × 8) structures, respectively, at elevated temperatures. At even higher temperatures, a now reversible phase transition to a “1 × 1” structure occurs. For both silicon and germanium {111} surfaces, the “1 × 1” structures consist of a quarter monolayer of disordered adatoms. During conversions of Ge(111)-c(2 × 8) surfaces, the adatoms already present only become disordered while on Si(111) surfaces corner holes as well as stacking faults have to be removed during 7×7 → “1 × 1” transitions. The existence of adatoms on Si(111)-“1 × 1” surfaces is attributed only to the decreased elastic constants at high temperatures. Then the lowering of the band-structure energy by a reduction of the density of adatoms by a factor of 2 compared with a factor of 2.6 for the 7 × 7 reconstruction obviously suffices to overcompensate the strain energy associated with the existence of adatoms only.