High-Performancee Planar-Type Nanophononic Si Thermoelectric Generator with a Double Cavity Structure

Thermoelectrics energy harvesting is one of the key technologies for carbon neutral. We demonstrate the planar-type silicon thermoelectric generator (TEG) with phononic nanostructures based on phonon-engineered design. The planar-type device was fabricated in an SOI wafer with CMOS and MEMS processes so that the device can be largely integrated. By using phononic nanostructures and three-dimensional thermal design, we achieved 200 μWcm^-2 output power density at a temperature difference of 12.5 K, which is the highest-performing planar-type Si thermoelectric generator reported to date.<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 poly-Si layer was doped with phosphorous at 2.0×10²⁰ cm^-3 carrier density, and the Seebeck coefficient is 100 μVK^-1. The phononic crystal (PnC) nanostructures were formed by electron beam lithography and reactive ion etching of poly-Si. The PnC structures are an array of circular holes with a period of 300 nm with 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, which was designed for efficient cooling of the device, was fabricated and bonded on the TEG by flip-chip bonding. This double-cavity thermal design leads to a high-power density of the device. The single unit of the TEG has an area of 44 μm by 73 μm, and 2,400 units are integrated into a 2.64 mm by 2.92 mm area with 120 series and 20 times parallel connections.<br/>The TEG performance was measured in the air around room temperature. 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. Since the power density of the TEG is proportional to the square of the temperature difference, 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 about 200 μWcm^-2 at a 12.5 K temperature difference between the device (about 4 K in thermoelectric material). The obtained normalized performance of 1.3 μWcm^-2K^-2 is, to our knowledge, the highest-performing planar-type Si thermoelectric generator reported to date.