Thin-film thermoelectric generator based on polycrystalline SiGe formed by Ag-induced layer exchange

Abstract

SiGe alloys are a promising material for highly reliable, human-friendly thin-film thermoelectric generators for micro-energy harvesting. However, it is difficult to obtain high performances at low thermal budgets in SiGe layers, especially in n-type materials. Ag-induced layer exchange enables the synthesis of Si1−xGex (x: 0–0.3) layers at 500 °C and dynamically controls the Fermi level owing to the self-organizing manner of impurity doping during the layer exchange. Intrinsic, p-type (hole concentration >1019 cm−3), and highly n-type (electron concentration >1020 cm−3) SiGe layers are obtained using pure Ag, B-doped Ag, and As-doped Ag, respectively. Owing to the high carrier concentrations, the thermoelectric power factor at room temperature exhibits high values: 230 μW m−1 K−2 for the p-type and 1000 μW m−1 K−2 for the n-type. The latter value is the highest reported power factor at room temperature for SiGe formed below 1000 °C. The dimensionless figure of merit is determined to be 0.19 from the power factor and the thermal conductivity of 1.6 W m−1 K−1. A thermoelectric generator fabricated with the low-temperature SiGe layers demonstrates a relatively large output for thin films (50 nm): 1.4 nW at room temperature with a temperature difference of 15 °C.