Structural and thermoelectric properties of TMGa(3) (TM = Fe, Co) thin films

Based on chemically synthesized powders of FeGa(3), CoGa(3), as well as of a Fe(0.75)Co(0.25)Ga(3) solid solution, thin films (typical thickness 40 nm) were fabricated by flash evaporation onto various substrates held at ambient temperature. In this way, the chemical composition of the powders could be transferred one-to-one to the films as demonstrated by Rutherford backscattering experiments. The relatively low deposition temperature necessary for conserving the composition leads, however, to ‘X-ray amorphous’ film structures with immediate consequences on their transport properties: A practically temperature-independent electrical resistivity of ρ = 200 μΩ·cm for CoGa(3) and an electrical resistivity of about 600 μΩ·cm with a small negative temperature dependence for FeGa(3). The observed values and temperature dependencies are typical of high-resistivity metallic glasses. This is especially surprising in the case of FeGa(3), which as crystalline bulk material exhibits a semiconducting behavior, though with a small gap of 0.3 eV. Also the thermoelectric performance complies with that of metallic glasses: Small negative Seebeck coefficients of the order of −6 μV/K at 300 K with almost linear temperature dependence in the range 10 K ≤ T ≤ 300 K.