Si Nanoparticles as a Model for Porous Si

Abstract

I describe here our investigation of the structural and optical properties of small hydrogenated Si nanoparticles, including the effect of weak surface oxidation. The investigation was conducted through self-consistent semi-empirical Hartree-Fock techniques, using the Configuration Interaction extension to include correlation effects, and allowing full relaxation of all atoms. Two techniques were specially reparametrized to treat semiconductor systems, MNDO/AM1 and ZINDO/CI. For clean Si:H particles, with diameters of up to ≈15 A, we found that the first optical transition is size dependent, confirming the effect of quantum confinement, and is strongly blue-shifted with respect to bulk Si. The actual value of the transition energy is also affected by structural relaxation. The character of the transition can be described as excitonic, in that it develops in the crystalline-like core of the particle, and involves an almost pure one-electron excitation. Decay however can occur through a highly localised surface state, a transient Si–H–Si bridge defect, that is pinned in energy in the red. Initial oxidation of the surface, as Si–OH units, or as backbond Si–O–Si incorporation, does not affect the energy of the first absorption transition, or the decay through the bridge defect. However, more complete oxidation should kill the red-luminescence though hardening of the surface. Incorporation of a Si=O silanone unit perturbs both absorption and emission properties, introducing a first transition in the red-orange, localised over the defect. We propose that the bridge defect can explain the emission properties of freshly prepared porous Si. We also gained insight in the effect of oxidation of these systems, and propose that is not a simple process, and that the emission properties cannot be understood through one single mechanism. This picture could only be obtained through techniques that allow for investigation of excited states.