Investigation of optical effects in silicon quantum dots by using an empirical pseudopotential method

Abstract

A computer simulation using a pseudopotential approach has been carried out to investigate the band gap as a function of the size and the shape of small silicon (Si) dots having 3 to 44 atoms per dot with and without surface passivation. We used an empirical pseudo-potential Hamiltonian, a plane-wave basis expansion and a basic tetrahedral structure with undistorted local bonding configurations. In our simulation, the structures of the quantum dots were relaxed and optimized before and after passivation. We found that the gap increased more for an oxygenated surface than a hydrogenated one. Thus, both quantum confinement and surface passivation determined the optical and the electronic properties of Si quantum dots. Visible luminescence was probably due to radiative recombination of electrons and holes in the quantum-confined nanostructures. The effect of passivation of the surface dangling bonds by hydrogen and oxygen atoms and the role of surface states on the gap energy was also examined. We investigated the entire energy spectrum starting from the very low-lying ground state to the very high-lying excited states. The results for the sizes of the gap, the density of states, the oscillator strength and the absorption coefficient as functions of the size are presented. The importance of the confinement and the role of surface passivation on the optical effects are also discussed.