Investigation of group 13 elements as potential candidates for p-type dopants in the narrow-gap thermoelectric semiconductor α-SrSi₂

To investigate the possibility of p-type doping of α-SrSi₂, a promising as an eco-friendly thermoelectric material, the energy changes of substitutions of the Si site of α-SrSi₂ by group 13 elements were evaluated using first-principles calculations. It is found that Ga doping was the most energetically favorable dopant while In is the most unfavorable. We examined the synthesis of Ga- and In-doped α-SrSi₂ using the vertical Bridgeman method and investigated their thermoelectric properties. The Ga atoms were doped to α-SrSi₂ successfully up to 1.0 at. %, while In atoms could not be doped as suggested by calculations. For experimental prepared Ga-doped samples, the carrier density was observed to increase with Ga doping, from 3.58 × 10¹⁹ cm⁻³ for undoped α-SrSi₂ to 4.49 × 10²⁰ cm⁻³ for a 1.0 at. % Ga-doped sample at 300 K. The temperature dependence of carrier concentrations was observed to change from negative to positive with increasing Ga content. In addition, the temperature dependence of the Seebeck coefficient was also observed to change from negative to positive with increasing Ga content. The results indicate that α-SrSi₂ undergoes a semiconductor–metal transition with Ga doping. The power factor for the undoped sample was quite high, at 2.5 mW/mK², while the sample with 0.3 at. % Ga had a value of 1.1 mW/mK² at room temperature.