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Atomic structure of semiconductor surfaces may show various reconstructions with essentially lower symmetry than the corresponding bulk-cut cleavage surface. The compound semiconductors, with the presence of impurity adsorbates in particular, introduce most variants. Furthermore, the ambient environment with variable chemical potentials of pertinent elements effect on the stoichiometry of surfacemost layers, that is being used in controlled surface growth, for example.
With scanning tunneling microscopy (STM) we recently observed a new bismuth (Bi) induced (2x1) reconstruction at the InP(100) and GaAs(100) surfaces. Interpretation of STM images suggests an interesting structure, which does not saturate the dangling bonds according to the conventional “electron counting model” of semiconductor surfaces. The XPS measurement of the surface-core-level shifts is another experimental tool, which was therefore used to monitor the surfaces, in more details.
In order to elucidate the properties of these peculiar surfaces, we have modeled the possible reconstructions and carried out density-functional theory (DFT) calculations for their electronic structures. The surface-core-level shifts as a sensitive indicator of the local chemical environment and coordination of the atoms in question, were evaluated together with the local densities-of-states. Furthermore, related to the XPS experiments the initial state and final state contributions to the shifts are considered separately.
Calculations were performed both with a plane-wave-PAW (PW-PAW) approach and LCAO all-electron (LCAO-AE) approach to find complementary aspects of the electronic structure. Initial state shifts were calculated from the electrostatic potential in the atom core (PW-PAW) and directly from the one-electron eigen energies (LCAO-AE). Estimation of the XPS final state effects calls for calculations of excited states.
The surface-core-level shifts allow us to compare Bi dimerization: Bi-Bi, Bi-In or Bi-P, and the local densities-of-states allow us to suggest, which surface reconstructions bear metallic or semiconducting behavior. Finally, the "ambient environment dependent total energy calculations" let us comment on the relative stabilities of alternative reconstructions. |