Thermoelectrics have always been attractive for power generation and cooling because of power reliability and environmentally friendly issues. However, this concept remains non-competitive due to the limitation in the efficiency of available thermoelectric materials and device designs [1]. In the 1990s, Hicks and Dresselhaus predicted the possibility of a dramatic enhancement in thermoelectric performance based on the special behavior of low dimensional materials [2, 3]. This enhancement is in part due to the increase in quantum confinement effects, the increase in electronic density of states at specified energies, and the increase in the phonon interface scattering for low dimensional structures.

Nanowires and core-shell nanowires can be considered to be model systems to illustrate representative behavior in low dimensional thermoelectric materials. It is expected that a system made out of nanowires or core-shell nanowires would have a higher thermoelectric performance than its bulk counterpart due to an increase in the number of interfaces. The interfaces that are introduced must be such that phonons are scattered more strongly than are electrons. Theoretical studies have been carried out to better understand the transport properties of Si-Si1−xGex ordered nanowire composites. The composite is modeled as having Si wires embedded in a Si1−xGex host matrix. Thus, core-shell Si/Si1−xGex nanowires can be considered as a building block of the composite. The effect of the wire diameter and the shell alloy composition on ZT is presented.