Controlling Defects in Epitaxial Thin Film Growth of Mg₂Sn_1-xGe_x by Modulating Mg Flux Rate in MBE

Defects manipulation in thermoelectrics is emerging as a key in optimizing their performance. However, because thin film processes differ from bulk synthesis methods, the manipulation of defects in thin films vary significantly from their bulk counterparts. We explore in this study how Mg flux rate modulation in molecular beam epitaxy (MBE) growth affects the defects formation of Mg₂Sn_1-xGe_x (x = 0.05 and 0.12) epitaxial films. Mg flux rates were varied in the range of Mg : Sn(Ge) = 3.9-9.1, while Sn and Ge flux rates were fixed at 1.65 atoms×s^-1nm^-2. Although the relative Mg supply rate largely exceeded 2, the obtained films consisted of Mg₂Sn phase as a main component, because the excess Mg can easily evaporate at the growth temperature of 380°C. However, the Mg flux rate can impact the film growth dynamics.<br/><br/>Higher Mg flux rates substantially enhance carrier mobility, Seebeck coefficient and eventually the power factor, which is linked to the reduction of vacancy-type defects detected in the positron annihilation spectroscopy measurements. Simultaneously, the films’ X-ray diffraction (XRD) indicated higher crystallinity at higher Mg flux rates. Cross-sectional transmission electron micrographs of films reveal Moiré patterns. EDS measurements at the Moiré patterns for the low Mg flux rate film show Mg-poor regions, suggesting Mg vacancies (V_Mg). On the other hand, at the high Mg rate, the presence of Mg interstitials (I_Mg) is hinted within the Moiré patterns. The formation of Moiré patterns can be associated with strain relaxation, and their increased occurrence at higher Mg flux rates could be attributed to the role of Mg in altering strain distributions and controlling the defects. Structural analysis through XRD pole figures also unveiled the presence of stacking faults in the films. The total thermal conductivity measured at room temperature of the thin films tends to decrease with an increasing Mg flux rate. These findings suggest a delicate balance between improved overall crystal structure and the presence of localized structural variations that may contribute to the decrease in thermal conductivity and total improvement of the thermoelectric properties of Mg-based thin films.