Ge substitutional defects and the √ 3 × √ 3 ↔ 3 × 3 transition in α - Sn / Ge ( 111 )


The structure and energetics of Ge substitutional defects on the α-Sn/Ge(111) surface are analysed using density functional theory molecular dynamics simulations. An isolated Ge defect induces a very local distortion of the 3 × 3 reconstruction, confined to a significant downwards displacement (−0.31 Å) at the defect site and a modest upward displacement (0.05 Å) of the three Sn nearest neighbours with partially occupied dangling bonds. Dynamical fluctuations between the two degenerate ground states yield the sixfold symmetry observed around a defect in the experiments at room temperature. Defect–defect interactions are controlled by the energetics of the deformation of the 3 × 3 structure: they are negligible for defects on the honeycomb lattice and quite large for a third defect on the hexagonal lattice, explaining the low-temperature defect ordering. (Some figures in this article are in colour only in the electronic version) The driving force behind the T -induced reversible √ 3×√3 ↔ 3×3 phase transition observed for 3 ML coverage of Sn or Pb on Ge(111) has been under intense debate since its discovery in Pb/Ge(111) [1]. Following the early suggestions of surface Fermi wavevector nesting or electron correlations, leading to the formation of a charge density wave [1, 2] at low T , recent theoretical work indicates that the softening of a 3 × 3 surface phonon [3], associated with the electronic energy gain due to the Sn dangling bond (DB) rehybridization, plays a major role on this transition. The dynamical fluctuations model [4] associated with this soft mode provides a consistent account of the main experimental facts, in particular the similarities of and differences between the isovalent Sn/Ge and Sn/Si(111) systems [3]. Density functional theory (DFT) calculations for the α-Sn/Ge(111) surface predict that the ground state is a 3 × 3 reconstruction in which a third of the Sn adatoms are placed ∼0.3 Å higher than the other Sn adatoms [3–6]. In other words, out of the three 3 × 3 0953-8984/02/307147+08$30.00 © 2002 IOP Publishing Ltd Printed in the UK 7147

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@inproceedings{Ortega2002GeSD, title={Ge substitutional defects and the √ 3 × √ 3 ↔ 3 × 3 transition in α - Sn / Ge ( 111 )}, author={Jos{\'e} Ortega and Rub{\'e}n P{\'e}rez and Leszek Jurczyszyn and Fernando Flores}, year={2002} }