Layer Exchange Synthesis of SiGe for Flexible Thermoelectric Generators: A Comprehensive Review

Flexible thermoelectric generators are leading candidates for next-generation energy-harvesting devices. Although SiGe, an environmentally-friendly semiconductor, is the most reliable and widely tested thermoelectric material, it is difficult to form a SiGe layer with high thermoelectric performance at temperatures lower than the heat-proof temperature of flexible plastic films. In this article, the synthesis of SiGe thermoelectric thin films via the metal-induced layer exchange phenomenon is reviewed, from its mechanism to device performance. The selection of metal species allows low-temperature formation (≤500 °C) of p- and n-type SiGe on insulating substrates. Currently, the maximum power factors near room temperature are 850 µW m⁻¹ K⁻² for p-type Si_0.4Ge_0.6 and 1000 µW m⁻¹ K⁻² for n-type Si_0.85Ge_0.15. These values are the highest among those of Group IV semiconductor thin films formed at low temperatures. The flexible thermoelectric generator consisting of these p- and n-type SiGe exhibits cross-sectional and planar power densities of ≈3.0 mW cm⁻² and 0.50 µW cm⁻², respectively, at a temperature difference of 30 K. Finally, the future challenges of layer exchange for improving the performance of flexible thermoelectric generators based on Group IV semiconductors are discussed.