Thickness-dependent thermoelectric properties of Si1- xGe xfilms formed by Al-induced layer exchange

Abstract

Highly reliable, thin-film thermoelectric generators are strongly desired for future sensor advancements. Al-induced layer exchange is a unique method for producing thermoelectric SiGe layers on a flexible plastic substrate at low temperatures. In this study, we investigated the thickening of the Si1-xGex (x = 0, 0.6, and 1) layers to improve the thermoelectric output power. The upper limit of the film thickness was approximately 1000 nm, while it influenced the crystal and electrical properties of the resulting Si1-xGex layers. The Si0.4Ge0.6 layer formed at 400 °C exhibited a high power factor of up to 850 μW m-1 K-2 at room temperature, which is the record-high value among p-type SiGe obtained at low temperature (<900 °C). The dimensionless figure of merit was determined to be 0.12 from the power factor and the thermal conductivity of 2.2 W m-1 K-1. The sequential layer exchange allowed us to form a 2000-nm-thick SiGe layer. Furthermore, the output power density was almost proportional to the film thickness and reached 37 nW cm-2 at room temperature under a temperature difference of 10 K. These achievements will present practical applications for next-generation thin-film thermoelectric generators based on highly reliable, human-friendly materials.