Electronic mechanism for alkali-metal-promoted oxidation of semiconductors

An electronic mechanism for the experimentally observed alkali-metal-promoted oxidation of semiconductor surfaces is proposed. The mechanism is based on the assumption that the rate-limiting step for oxidation is dissociation of the oxygen molecule. The role of the preadsorbed alkali-metal atoms is to supply the empty surface-dangling-bond bands with electrons, which will lead to metallization of the surface, reduction of the work function due to the formation of a surface a dipole layer, and a positive shift of the surface Fermi energy. Eventually, as the oxygen molecule approaches the surface, transfer of charge from the dangling-bond bands to the partly occupied antibonding 2${\pi }^{\mathrm{*}}$ orbital of the oxygen molecule takes place. We assume that the activation energy for dissociation of the oxygen molecule, chemically interacting with a Si surface, is given by the energy, 0.9 eV, corresponding to the excitation from the ground triplet state to the excited singlet state in the gas phase, 1 eV. The rate of dissociation is estimated by the probability for this excitation by hot electron-hole pairs, weighted by the amount of electron charge transferred to the molecule. It is found that this electronic mechanism for thermally activated dissociation of an oxygen molecule impinging on the semiconductor surface is substantially increased when an alkali-metal overlayer is preadsorbed on the surface. The proposed electron-hole–pair mechanism introduces a strong density of states and temperature dependence, which could be tested experimentally.