Thermoelectric Properties of Ag-doped Mg2Ge Thin Films Prepared by Magnetron Sputtering

Silver doped p-type Mg2Ge thin films were grown in situ at 773 K using magnetron co-sputtering from individual high-purity Mg and Ge targets. A sacrificial base layer of silver of various thicknesses from 4 nm to 20 nm was initially deposited onto the substrate to supply Ag atoms, which entered the growing Mg2Ge films by thermal diffusion. The addition of silver during film growth led to increased grain size and surface microroughness. The carrier concentration increased from 1.9 x 10(18) cm(-3) for undoped films to 8.8 x 10(18) cm(-3) for the most heavily doped films, but it did not reach saturation. Measurements in the temperature range of T = 200-650 K showed a positive Seebeck coefficient for all the films, with maximum values at temperatures between 400 K and 500 K. The highest Seebeck coefficient of the undoped film was 400 mu V K-1, while it was 280 mu V K-1 for the most heavily doped film at similar to 400 K. The electrical conductivity increased with silver doping by a factor of approximately 10. The temperature effects on power factors for the undoped and lightly doped films were very limited, while the effects for the heavily doped films were substantial. The power factor of the heavily doped films reached a non-optimum value of similar to 10(-5) W cm(-1) K-2 at 700 K.