High Thermoelectric Performance in Polycrystalline GeSiSn Ternary Alloy Thin Films

Group IV materials are promising candidates for highly reliable and human-friendly thin-film thermoelectric generators, used for micro-energy harvesting. In this study, we investigated the synthesis and thermoelectric applications of a Ge-based ternary alloy thin film, Ge_1-x-ySi_xSn_y. The solid-phase crystallization of the highly densified amorphous precursors allowed the formation of high-quality polycrystalline Ge_1-x-ySi_xSn_y layers on an insulating substrate. The small compositions of Si and Sn in Ge_1-x-ySi_xSn_y (x < 0.15 and y < 0.05) lowered the thermal conductivity (3.1 W m^-1 K^-1) owing to the alloy scattering of phonons, while maintaining a high carrier mobility (approximately 200 cm² V^-1 s^-1). The solid-phase diffusion of Ga and P allowed us to control the carrier concentration to the order of 10¹⁹ cm^-3 for holes and 10¹⁸ cm^-3 for electrons. For both p- and n-type Ge_1-x-ySi_xSn_y, the power factor peaked at x = 0.06 and y = 0.02, reaching 1160 μW m^-1 K^-2 for p-type and 2040 μW m^-1 K^-2 for n-type. The resulting dimensionless figure of merits (0.12 for p-type and 0.20 for n-type) are higher than those of most environmentally friendly thermoelectric thin films. These results indicate that group IV alloys are promising candidates for high-performance, reliable thin-film thermoelectric generators.