Structure and energetics of segregated and non-segregated H:Ge(001)/Si and Cl:Ge(001)/Si

The equilibrium atomic geometry, energetic stability and chemical bonding for H and Cl passivation of the Si/Ge(001)-(2×1) surface have been studied by applying the ab initio pseudopotential method. In a recent theoretical work, a segregated structure in which Si occupies the second layer while Ge floats to the surface was found to be energetically favourable by 0.38 eV per dimer compared with the non-segregated Si-capped structure. Upon hydrogen passivation, the situation is reversed: the non-segregated structure becomes energetically favourable by 0.08 eV per dimer compared with the segregated structure. For chlorine passivation, this energy difference is 0.3 eV per dimer. For the non-segregated H(Cl):Ge(001)/Si system, the Si dimer becomes symmetric with the bond length elongated from 2.26 إ to 2.39 إ (2.42 إ). For the segregated H(Cl):Ge(001)/Si system, the Ge dimer also becomes symmetric with the bond length elongated from 2.39 إ to 2.45 إ (2.46 إ).