3. Amorphous and microcrystalline silicon sectors

Amorphous materials, unlike crystals, do not have a regular atomic organization, at least over long distances. In fact, a local order is preserved. In amorphous silicon, the chemical bonds between a Si atom and its nearest neighbors satisfy the tetrahedral configuration characteristic of sp ³ hybridization, where each atom is linked to its four neighbors (0.23 nm apart) by a covalent Si-Si bond. In covalent compounds and amorphous chalcogenides, interatomic bond lengths and bond angles vary slightly around average crystal values. This dispersion increases with distance, so that as soon as the distance to a given atom exceeds four to five interatomic distances, the positions are randomly distributed and a significant fraction of covalent bonds are cut, so electrons are unpaired. Such atoms are said to have a dangling bond. This dangling bond...