Effects of Ge addition on thermoelectric properties in a nanocomposite of MnSiγ and SiGe thin films

In this work, we fabricated nanocomposite thin films of MnSi gamma and SiGe, both of which are promising thermoelectric materials, and studied the dependence of their crystal structures and thermoelectric properties on the Ge content. The addition of Si1-xGex caused a reduction in the grain size of the MnSi gamma matrix to the scale of the phonon mean free path of MnSi gamma, which is essential for phonon scattering to reduce the lattice thermal conductivity. In addition, increasing the Ge ratio in the additive SiGe of the nanocomposite caused both a reduction at x = 0.2 ratio in the thermal conductivity and a monotonical reduction of the electrical resistivity with negligible change in the thermopower. The former of these two phenomena may be attributed to both a reduction in the thermal conductivity of SiGe as a result of heavier element (Ge) substitutions than Si and an increase in the thermal boundary resistance between MnSi gamma and SiGe. In the case of the latter, introduction of carriers caused by the decrease in gamma, which is the atomic ratio between Si and Mn, would be responsible. As a result, the estimated thermoelectric dimensionless figure-of-merit ZT increased to reach a maximum of 0.43 at 459 degrees C, which is larger than that of the single phase MnSi gamma film, without any doping into the MnSi gamma matrix with heavy elements.