Si-Based Planar-Type Thermoelectric Generators over 1 μWcm^-2K^-2

Thermoelectrics energy harvesting is one of the key technologies for carbon neutrality. This talk introduces recent advances in Si-based thermoelectric generators (TEGs) and we demonstrate a planar-type double-cavity Si TEG with phononic nanostructures based on a phonon-engineered design. The planar-type device was fabricated in an SOI wafer with CMOS and MEMS processes to largely integrate the device. By using phononic nanostructures and three-dimensional thermal design, we achieved over 100 mWcm^-2 output power density at a temperature difference of lower than 10 K.<br/>We fabricated the Si TEG in an SOI wafer with a 1.1-μm-thick n-type poly-Si device layer and a 1.5-μm-thick BOX layer. The phononic crystal (PnC) structures are an array of circular holes with a period of 300 nm and a neck size between 8 and 100 nm. It was designed to reduce the thermal conductivity of poly-Si by considering the phonon mean free path spectrum. The cap wafer is designed for efficient cooling of the device. This double-cavity thermal design leads to a high-power density of the device. We found that more than 30% of the temperature difference between the upper and bottom of the device was applied in the thermoelectric material thanks to the careful thermal design of the double cavity structure. This thermal design is an important key factor, as well as the ZT of the thermoelectric material. Our TEG showed the output power density of more than 100 μWcm^-2 at a lower than 10 K temperature difference between the devices (a few Kelvin in thermoelectric material). The obtained normalized performance of over 1 μWcm^-2K^-2 is, to our knowledge, the highest-performing planar-type Si thermoelectric generator reported to date.