Carrier Localization in Nanocrystalline Silicon

The localization of electronic energy orbitals is computationally examined for silicon condensed matter composed of crystallites encapsulated within continuous random networks. Density functional theory is used to elucidate the relationship between the orbital character, energy, and crystallite size for diameters up to 4 nm. The difference in long-range order alone is sufficient to induce hole localization within crystalline regions provided they exceed a critical size (1.7 nm), and the confinement power of the matrix is found to be the same as that associated with planar boundaries: 0.68. The spatial distribution of confined valence edge states can vary from nearly cylindrical to narrow ribbons. Conduction edge electrons, on the other hand, tend to be localized within satellite states at the interface between regions of local and extended order due to the presence of a shallow (56 meV) energy well surrounding the crystallites.