N-type amorphous silicon-germanium thin films with embedded nanocrystals as a novel thermoelectric material of elevated ZT

Abstract

The state-of-art thermoelectric (TE) materials with high efficiency (i.e., ZT ~ 1) at room temperature require meeting essential features, such as being environmentally friendly (i.e., avoiding the use of rare and toxic elements), cheap to produce, and high ZT. However, the enhancement of ZT has always been a challenge because the Seebeck coefficient, electrical conductivity, thermal conductivity, and absolute temperature are interdependent. In this work, we report the fabrication of amorphous silicon-germanium (a-SiGe) thin films deposited by low-frequency plasma-enhanced chemical vapor deposition (LF-PECVD) with embedded nanocrystals that have a ZT of 2.61 at room temperature. The above is achieved by enhancing the thermoelectric power factor and, simultaneously, keeping the lowest thermal conductivity; both are in concordance with the phonon-glass electron-crystal (PGEC) approach. The power factor enhancement is accomplished by increasing the electrical conductivity through N-type doping and an annealing process at 500 °C to which thin films were subjected to improve their transport properties while avoiding crystallization. Therefore, the room-temperature electrical conductivity increased two orders of magnitude from 1.11E-01 up to 42.79 S/cm. In addition, the growth of nanocrystals (5–15 nm) embedded into the amorphous matrix contribute to the transport of charge carriers. The measured thickness and Seebeck coefficient of thin films were 200 nm and −1.038 mV/K, respectively. On the other side, the lowest thermal conductivity is reached because the material’s amorphous phase is kept despite the applied post-deposition thermal annealing. The experimental value for thermal conductivity was 0.53 W/m·K, almost half of the minimum thermal conductivity proposed by Slack. In summary, the structural characterization (developed by X-ray diffraction, Fourier transform infrared spectroscopy, and Raman spectroscopy) shows that a-SiGe material not only has an amorphous phase despite the applied thermal annealing, but it also possesses nanocrystals, which is demonstrated by transmission electron microscopy in high-resolution mode. Therefore, the a-SiGe material obtained in this research is proposed as a novel and potential thermoelectric material because it has optimized TE properties and meets desirable features, such as being inexpensive, non-hazardous, and environmentally friendly for energy harvesting applications.