Effect of oxygen on the optical properties of small silicon pyramidal clusters

Optical absorption and light emission of oxygen-incorporated small silicon (Si₃₀H₄₀O_i) pyramidal clusters as a function of oxygen content were theoretically studied using the self-consistent semiempirical molecular orbital method (modified neglect of diatomic overlap-parametric method 3). In the absolute majority of the cluster configurations with low oxygen content (i<4) the excitations have a strong localized character and result in a significant shift from its equilibrium ground state position of one of the silicon atoms inside the cluster volume. The optical transition energies in those cases range from 2.05 to 2.35 eV. The typical Stokes shift for these structures is of the order of 100 meV. However, for some particular cluster configurations the excitations are localized at silicon sites directly adjacent to embedded oxygen atoms and this results in a considerable reduction of the emission energy down to approximately 1.40-1.60 eV and in an increase of the Stokes shift values to 600-800 meV. The same behavior was traced out for the case where the presence of a silanone (Si=O) bond at the surface of the Si₃₀H₃₈O cluster is considered. For intermediate and high oxygen content (i>4) structures, a wide spread of the optical transition energies ranging from 1.60 to 3.00 eV is observed, due to the competition between two opposite tendencies. According to the first one, connected with the increasing of the quantum confinement effects due to oxidation, the optical transition energies tend to increase, whereas the enhanced possibility of involvement of oxygen or oxygen-adjacent silicon atoms in the process tends to decrease the energy transitions.