Stability Model of Silicon Nanowire Polymorphs and First-Principle Conductivity of Bulk Silicon

We present a combined study of the morphology and electronic transport of silicon nanowires (SiNWs). On the one hand, we used a model for the Gibbs free energy of the nanowires in order to assess the importance of the contribution of volume, surface, edge, and planar defects in the different diameter ranges and polymorphs found experimentally, namely, cubic diamond (“cubic”) and hexagonal diamond (“hexagonal”). Surface contribution to the Gibbs free energy explains the appearance of the hexagonal phase in intermediate diameters. Structural relaxations via planar defects explain the abundance of the cubic phase in thinner NWs. On the other hand, in order to approximate the transport in the range of thick SiNWs, we studied the electron transport properties of bulk silicon in the main crystallographic growth directions of SiNWs, namely, (001)d, (110)d, and (111)d for the cubic phase and (001)h for the hexagonal phase, with a density functional theory-non-equilibrium Green functions formalism (DFT-NEGFF) code. We found the ⟨001⟩d direction the one with the greatest conductivity, related to the fact that the absolute minimum of the bulk silicon band diagram is located in this direction, it has the lowest effective mass among all the studied directions, and the transport is also favorable due to the shape of the conduction band minimum orbital (i.e., LUMO, low unoccupied molecular orbital).