Theoretical studies of the passivants' effect on the Si(x)Ge(1-x) nanowires: composition profiles, diameter, shape, and electronic properties

Theoretically, we have performed a systematic investigation on the passivants' effect on the geometrical and electronic properties of Si(x)Ge(1-x) nanowires. First-principles calculations revealed that, in the nanowires passivated by fluorine (F)∕chlorine (Cl)∕hydrogen (H) atoms, Si atoms preferred to segregate towards the surface due to the stronger Si-X bonds than that of Ge-X bonds (X = F, Cl, H). The energy barriers of X atoms' desorption is higher than that of the Si∕Ge atoms' exchanging, inducing a feasible and strong surface segregation of Si atoms at proper temperature. Considering the Si∕Ge interactions and mixing entropy, the composition profiles of Si∕Ge distributions are obtained by minimizing the Gibbs free energy, which indicates the outmost layer of surface should be mostly occupied by Si. With total Si surface segregation, the diameter and shape of most stable Si(x)Ge(1-x) nanowires are found to be determined by the composition x and the passivants' chemical potential. In addition, charge distribution of near-gap levels can be modulated through the surface passivants. Our finding provides a practical avenue to tune the electronic properties of Si(x)Ge(1-x) nanowires, by modulating the morphologies of nanowires with the composition control of Si∕Ge and the chemical potential of passivants.