Mechanisms of the adsorption of oxygen molecules and the subsequent oxidation of the reconstructed dimers on Si(001) surfaces.

Abstract

The adsorption of ${\mathrm{O}}_{2}$ molecules on 2\ifmmode\times\else\texttimes\fi{}1 reconstructed dimers on Si(001) surfaces and the subsequent oxidation have been investigated by ab initio quantum-chemical calculations. Detailed analyses of the potential-energy hypersurfaces in the spin triplet and quintet states have revealed that the triplet state has the lowest-energy reaction path of the oxidation process. On this lowest-energy reaction path, the electronic state as well as the atomic-level configuration of the molecularly adsorbed metastable precursor of ${\mathrm{O}}_{2}$ on Si(001) surfaces was clarified. The molecular adsorbate is converted into the atomically adsorbed stable state by the dissociation of the ${\mathrm{O}}_{2}$ adsorbate to oxygen atoms. This is just the insertion process of an oxygen atom into a Si dimer bond to produce silicon oxide. The activation energy required for this conversion has been calculated to be 60.4 kcal/mol, which is in accordance with the value 60 kcal/mol obtained by experiments at high temperatures. By the inspection of the temperature dependence of the reaction-rate constants, it has been concluded that the reconstructed dimer is hardly oxidized at room temperature and that the origin of the natural oxide of Si(001) surfaces might be defects of the surface reacting with ${\mathrm{O}}_{2}$ molecules, i.e., the defect-free Si(001) surface is stable against ${\mathrm{O}}_{2}$ molecules and is not oxidized at room temperature. This conclusion is consistent with recent experimental results that reconstructed dimers on the terraces of Si(001) surfaces were inactive for an exposure of ${\mathrm{O}}_{2}$ molecules and only defect sites on the same surfaces have reacted with ${\mathrm{O}}_{2}$ molecules.