From the electronic point of view, the electronic gap in Si is larger than in Ge, so it would have been suitable to use Ge as the nanostructure
(wire or dot) material and Si as the matrix one. However as mentioned in the previous study, the heavy element of Ge behaves as a barrier to the Si phonon modes, whereas the Si cannot completely prohibit the Ge phonon modes propagation. Therefore, nanostructures of Si in Ge matrix are addressed here. In this section, simulation results of Si multiple quantum wires in Ge matrix (Si:Ge MQWR) are presented.
The Phonon Density Of States (PDOS) along the confined direction is presented in Figure 15 for a range of Si wire size from 3.51nm to 0.87nm, with a fixed Ge matrix size. The discrete energy levels in the density of states are visible for wire sizes lower than ~2nm, fir the Si modes > 40meV, as expected according to the mass contrast between the two elements.
Figure 16 shows the calculated AVAS (a), MVAS (b), and IMVAS (c) for Si:Ge MQWR. The phonon confinement in this structure are similar to that in SiGe superlattices. The acoustic phonons can propagate though the whole lattice, while optical phonon of both Si and Ge phonons show strong confinement in their own lattices.
However, compared to the SiGe superlattice, the energy levels for interface modes in Si:Ge quantum wires are mostly located around 49 meV.