Surface Reconstruction Puzzle

We study the two lowest-energy isomers of the Ge(111)-(2x1) surface, by a state-of-the-art first-principles calculation of their optical spectra, including the electron-hole interaction effects. A comparison of our results with the available experimental data suggests that, at difference with the Silicon case, the stablest isomer differs from the standard buckled Pandey chains reconstruction. This conclusion is supported by accurate total-energy results.

surface reconstruction
Fig.1 Two possible reconstructions

surface reconstruction
Fig.2 Experiment compared with the BSE results of both reconstructions.

  1. We have studied the two lowest-energy isomers of the Ge(111)-(2x1) surface, by a state-of-the-art parameter-free calculation of their total energy and optical spectra, including the electron-hole interaction effects.
  2. The two isomers yield a surface geometry which differs only starting from the third atomic layer below the uppermost one, hence experimental probes like STM can hardly be employed to discriminate between the two isomers.
  3. Optical properties deduced from electronic structure results obtained at the one-particle level - i.e. neglecting the electron-hole interaction effects - cannot be used to discriminate between the two isomers.
  4. A comparison of our results with the available experimental data suggests that, at difference with the Silicon case, the stablest isomer differs from the standard buckled Pandey chains reconstruction.
  5. The upper panel in Fig.2 is for the positively buckled (i.e., the traditional) Pandey chain while the lower panel is for the negatively buckled chain. In both panels, the full (dashed) curves include (neglect) excitonic effects.
  6. In conclusion, the ground-state geometry of Ge(111)(2x1) is found to correspond to Pandey-like chains with a buckling angle in the opposite direction with respect to that of the commonly assumed geometry. This work has been published in Phys. Rev. Lett. 85, 5440 (2000)
  7. Our conclusions have been later confirmed by low-temperature STM experiments: see R.M. Feeenstra, G. Meyer, F. Moresco and K.H. Rieder, Phys. Rev. B 64, 081306 (2001).