The different roles of charged and neutral atomic and molecular oxidising species in silicon oxidation from Ab initio calculations

We examine the roles of charged and neutral oxidising species based on extensive ab initio, DFT calculations. Six species are considered: interstitial atomic O, O-, O2- and Molecular species: O-2,O-2(-),O-2(2-). We calculate their incorporation energies into bulk silicon dioxide, vertical electron affinities and diffusion barriers. In our calculations, we assume that the electrons responsible for the change of charge state come from the silicon conduction band, however the generalisation to any other source of electrons is possible and hence our results are also relevant to electron-beam assisted and plasma oxidation. The calculations yield information about the relative stability of oxidising species, and the possible transformations between them and their charging patterns. We discuss the ability to exchange O atoms between the mobile species and the host lattice during diffusion, since this determines whether or not isotope exchange is expected. Our results show very clear trends: (1) molecular species are energetically preferable over atomic ones, (2) charged species are energetically more favourable than neutral ones, (3) diffusion of atomic species (O, O-, O2-) will result in oxygen exchange, whereas the diffusion of molecular species (O-2, O-2(-),O-2(2-)) is not likely to lead to significant exchange with the lattice. Our results show thermodynamic trends for oxidising species to capture electrons from Si during oxidation. We identify very different roles for atomic and molecular species and also for different charge states of those species. This points out to opportunities, usually not considered, for optimising thin oxide layers and interface properties for use in electronics devices. (C) 2001 Elsevier Science Ltd. All rights reserved.