Chemical Doping Effect on the Thermoelectric Properties of $T$Ga$_{3 }(T$ = Fe, Ru, Os)

Thermoelectric properties of chemically-doped intermetallic narrow-band semiconductors: $T$Ga$_{3 }(T$ = Fe, Ru, Os) are reported. The parent compounds show semiconductor-like behavior ($E_{g} \quad \sim $ 0.2 eV, $n_{290K} \quad \sim $ 10$^{18 }$cm$^{3})$ with large $n-$type Seebeck coefficients at room temperature ($S_{290K }\sim $ -300 $\mu $V/K). The semiconductor-like FeGa$_{3}$ becomes metallic upon chemical doping (adding electron carriers), but RuGa$_{3}$ and OsGa$_{3}$ remain semiconducting. While the electrical resistivity and the Seebeck coefficients of all the compounds decrease with electron doping, the Seebeck coefficients remain fairly large and $n-$type, which leads to larger power factors than those of the pure samples. The thermal conductivity (\textit{$\kappa $}$_{290K }$= 1.6 W/m K) of electron-doped FeGa$_{3}$ decreases, which increases the room temperature power factor by a large percentage ($S^{2}$\textit{/$\rho $}$_{290K}$ = 60 $\mu $W/m K$^{2})$ over that of pure FeGa$_{3}$. This improvement in the power factor leads to a corresponding enhancement in the thermoelectric figure of merit (\textit{ZT}) -- a factor of 5 increases above undoped polycrystalline FeGa$_{3}$ and two orders of magnitude improvement over that of pure single crystalline FeGa$_{3}$.