Impacts of Cu-Doping and Mg-Deficiency on Mg2Sn Thin Films Thermoelectric Properties

Two groups of Mg-Sn thin films were deposited by magnetron co-sputtering of Mg, Sn, and Cu targets. Group I was Mg-Sn films with [Mg]/[Sn] = 2 and group II with [Mg]/[Sn] = 1.75. The effect of Mg-deficiency and Cu-doping on the structure and thermoelectric properties was investigated. Both groups were doped by different Cu concentrations (0 at.%, 0.5 at.%, and 1.5 at.%). The structure and morphology of the thin films were investigated. The stable face-centered cubic structure was only observed for undoped film of group I, while a mixture of face-centered cubic and metastable orthorhombic structures coexisted for other samples whatever the Cu content. Carrier concentration and mobility were measured to explain the electronic transport behaviors of the films. The bandgap energy for both groups was estimated by the Kubelka-Munk's method. The influence of the Mg deficiency and Cu doping on the thermoelectric properties was discussed for a range of temperatures: 30-200 degrees C. For both groups, the best figure-of-merit, ZT, was obtained at 200 degrees C for samples doped with 0.5 at.% Cu with ZT = 0.07 for group I and ZT = 0.27 for group II.