The roles of charged and neutral oxidising species in silicon oxidation from ab initio calculations

We examine the roles of charged oxidising species based on extensive ab ini tio density functional theory calculations. Six species are considered: int erstitial 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 o f electrons is possible, and hence, our results are also relevant to electr on-beam assisted oxidation and plasma oxidation. The calculations yield inf ormation about the relative stability of oxidising species, and the possibl e 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) the molecular species ar e energetically preferable over alo,nic ones, (2) the charged species are e nergetically more favourable than neutral ones, (3) diffusion of atomic spe cies (O, O-, O2-) will result in oxygen exchange, whereas the diffusion of nzoleculai species (O-2, O-2(-), O-2(2-)) is not likely to lead to a signif icant exchange with the lattice. On the basis of our calculation, we predic t that charging of oxidising species may play a key role in silicon oxidati on process. (C) 2000 Elsevier Science Ltd. All rights reserved.