Local alkali-metal-promoted oxidation of Si(100)-(2 x 1) surfaces: A generalized-Hubbard-model calculation.

Abstract

In order to study the local regime in the alkali-metal-promoted oxidation of silicon, we consider the coadsorption of an oxygen molecule and a potassium atom on the dimerized Si(100)-(2\ifmmode\times\else\texttimes\fi{}1) surface. Antiferromagnetic (singlet) spin correlations within the Si dimers are taken into account from the outset by working with a generalized Hubbard Hamiltonian. In this background, the adsorption of a K atom strongly polarizes the medium creating a local charge-spin bag, around the ${\mathrm{K}}^{+}$ ion, which sets the scenario for promoted oxidation. When an ${\mathrm{O}}_{2}$ molecule in its ground state ${(}^{3}$${\mathrm{\ensuremath{\Sigma}}}_{\mathit{g}}$) approaches this region of the surface, it is influenced by the attractive electrostatic field of the ${\mathrm{K}}^{+}$ ion, with the corresonding lowering of its affinity level. This eventually crosses the Fermi level and captures the excess electron charge in the bag. A superoxide ${\mathrm{O}}_{2}$ ion, the $^{2}\mathrm{\ensuremath{\Pi}}_{\mathit{g}}$ state, is thereby formed, ionicly bonded to ${\mathrm{K}}^{+}$ and covalently bonded to the Si dimer. In the absence of potassium, the oxygen molecule simply physisorbs in a state adiabatically connected to its gas-phase ground state.