Structural and thermoelectric properties of TMGa3 (TM = Fe, Co) thin films

Based on chemically synthesized powders of FeGa3, CoGa3, as well as of a Fe0.75Co0.25Ga3 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 rho = 200 mu Omega.cm for CoGa3 and an electrical resistivity of about 600 mu Omega.cm with a small negative temperature dependence for FeGa3. The observed values and temperature dependencies are typical of high-resistivity metallic glasses. This is especially surprising in the case of FeGa3, 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 mu V/K at 300 K with almost linear temperature dependence in the range 10 K <= T <= 300 K.