Controlling thermoelectric properties of epitaxial GeSn film/Si by tuning strain and composition

We develop compressively-strained epitaxial Ge1−xSnx films with large x (>0.1) on Si substrates. Therein, large x and compressive biaxial strain cause a band convergence by movement of Γ valley down to L valleys near the Fermi level, which leads to high Seebeck coefficient while increasing alloy phonon scattering rate related to a low thermal conductivity. Furthermore, a higher electron mobility is also realized due to the reduction of the conductivity effective mass coming from the large contribution of small effective mass of Γ valley. Further increase in x of Ge1−xSnx films decreases thermal conductivities. As a result, the doped compressively-strained epitaxial Ge0.88Sn0.12 films on Si substrates exhibit two times higher thermoelectric power factor (∼15 μW cm−1 K−2) at room temperature than the previously-reported Ge1−xSnx films at almost the same thermal conductivity. This highlights that the application of compressive in-plane strain in Ge1−xSnx films with large x is a promising approach for enhancing a thermoelectric performance.