Low thermal conductivity and improved thermoelectric performance of nanocrystalline silicon germanium films by sputtering

Si_xGe_1-x alloys are well-known thermoelectric materials with a high figure of merit at high temperatures. In this work, metal-induced crystallization (MIC) has been used to grow Si_0.8Ge_0.2 films that present improved thermoelectric performance (zT =5.6 10^-4 at room temperature) - according to previously reported values on films - with a relatively large power factor (σ • S² = 16 μW • m^-1• K^-2). More importantly, a reduction in the thermal conductivity at room temperature (κ = 1.13 0.12 W • m^-1 • K^-1) compared to other Si-Ge films (∼3 W • m^-1 • K^-1) has been found. Whereas the usual crystallization of amorphous SiGe (a-SiGe) is achieved at high temperatures and for long times, which triggers dopant loss, MIC reduces the crystallization temperature and the heating time. The associated dopant loss is thus avoided, resulting in a nanostructuration of the film. Using this method, we obtained Si_0.8Ge_0.2 films (grown by DC plasma sputtering) with appropriate compositional and structural properties. Different thermal treatments were tested in situ (by heating the sample inside the deposition chamber) and ex situ (annealed in an external furnace with controlled conditions). From the studies of the films by: x-ray diffraction (XRD), synchrotron radiation grazing incidence x-ray diffraction (SR-GIXRD), micro Raman, scanning electron microscopy (SEM), x-ray photoemission spectroscopy (XPS), Hall effect, Seebeck coefficient, electrical and thermal conductivity measurements, we observed that the in situ films at 500 °C presented the best zT values with no gold contamination.