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

Abstract

Si x Ge1-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 Si0.8Ge0.2 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 · S (2) = 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 Si0.8Ge0.2 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.

Advances in Scanning Thermal Microscopy Measurements for Thin Films

“…One way of reducing the thermal conductivity of a bulk material without affecting the transport properties is through reducing its dimensionality [36], such as preparing thin films. This thermal conductivity reduction has been reported for several materials, such as SiGe [37][38][39]…”

Novel Measurement Methods for Thermoelectric Power Generator Materials and Devices

“…This new method also allows for measuring intrinsic ZT with reduced experimental error by accounting for losses through nonideal contacts and geometry. [16][17][18][19][20]. Reproduced from [11] with permission from The Royal Society of Chemistry.…”

“…Heat transfer rate through the probe is calculated using a fin model with ambient temperature at free end and constant temperature T w ' at its base [16]:…”

“…Measured versus calculated probe temperature. Reprinted from [16] with the permission of AIP Publishing.…”

A comparative experimental study on the cross-plane thermal conductivities of nano-constructed Sb2Te3/(Cu, Ag, Au, Pt) thermoelectric multilayer thin films

“…Nanostructured thermoelectric materials offer a new way of improving thermoelectric conversion performance by tailoring electron and phonon transport properties [14, 15]. Two-dimensional (2D) multilayer thin films or superlattice structures can significantly enhance the ZT values of thermoelectric materials since electrons are confined to move in two dimensions and phonons can be effectively scattered by the high density of interfaces, which results in a decrease in the lattice thermal conductivity [16, 17].…”