The localization of the donor electron wave function can be of key importance in various silicon application, since it determines the interactions between neighboring donors and influences the charge density close to the donor atom. This is important in light of applications like nuclear spin qubits or for determining the critical density of metal-insulator transitions. In particular the delocalization is a critical feature when dealing with nanostructures, where the confinement induces a squeezing of the donor wave function. Using ab-initio calculations, we have studied the delocalization of the donor electron wave function along the axis of a nanowire with different orientations for P and Se donors. We show that the shape and delocalization is greatly influenced by the orientation of the nanowire and that it considerably larger for [011] oriented nanowires, compared to [001] and [111] orientations. We also demonstrate that its …