Oxygen effect on optical properties of nanosize silicon clusters

Optical absorption and light emission of oxygen-incorporated small silicon $\left({\mathrm{Si}}_{14}{\mathrm{H}}_{24}\right)$ clusters as a function of oxygen content were theoretically studied using a self-consistent semiempirical molecular orbital method (modified neglect of diatomic overlap–parametric method 3). Two different models of oxygen incorporation were considered. Changes in the appropriate energies were traced when oxygen atoms were introduced either to the cluster surface (OH-group passivation) or inside the cluster. An increase of the number of oxygen atoms both at the surface and inside the cluster results in the decrease of the absorption energy with respect to the values typical for the cluster passivated only by hydrogen, while the emission energy appears to be influenced mainly by the incorporated oxygen content. There is a Stokes shift reaching the significant value of about 0.7 eV. It reduces with the increase of the oxygen content down to a value of 0.2 eV in the case of OH-group-terminated clusters, while it is almost constant for the oxygen-incorporated ones.