Characterization of nanometer-thick polycrystalline silicon with phonon-boundary scattering enhanced thermoelectric properties and its application in infrared sensors

Abstract

Although significantly reducing the thermal conductivity of silicon nanowires has been reported, it remains a challenge to integrate silicon nanowires with structure materials and electrodes in the complementary metal–oxide–semiconductor (CMOS) process. In this paper, we investigated the thermal conductivity of nanometer-thick polycrystalline silicon (poly-Si) theoretically and experimentally. By leveraging the phonon-boundary scattering, the thermal conductivity of 52 nm thick poly-Si was measured as low as around 12 W mK⁻¹ which is only about 10% of the value of bulk single crystalline silicon. The ZT of n-doped and p-doped 52 nm thick poly-Si was measured as 0.067 and 0.024, respectively, while most previously reported data had values of about 0.02 and 0.01 for a poly-Si layer with a thickness of 0.5 μm and above. Thermopile infrared sensors comprising 128 pairs of thermocouples made of either n-doped or p-doped nanometer-thick poly-Si strips in a series connected by an aluminium (Al) metal interconnect layer are fabricated using microelectromechanical system (MEMS) technology. The measured vacuum specific detectivity (D*) of the n-doped and p-doped thermopile infrared (IR) sensors are 3.00 × 10⁸ and 1.83 × 10⁸ cm Hz^1/2 W⁻¹ for sensors of 52 nm thick poly-Si, and 5.75 × 10⁷ and 3.95 × 10⁷ cm Hz^1/2 W⁻¹ for sensors of 300 nm thick poly-Si, respectively. The outstanding thermoelectric properties indicate our approach is promising for diverse applications using ultrathin poly-Si technology.