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 couldn’t 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 x 1019 cm-3 for undoped α-SrSi2 to 4.49 x 1020 cm-3 for a 1.0 at. % Ga doped sample at 300K. 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, whilst the sample with 0.3 at. % Ga had a value of 1.1 mW/mK2 at room temperature.