Defects modification in thermoelectric Mg2Sn (Ge) epitaxial thin films through modulation of Mg flux rate in MBE

Abstract

Precise defect control is crucial for optimizing thermoelectric (TE) materials. However, thin film processes differ from bulk synthesis, necessitating distinct approaches to defect management. This study investigates the impact of varying Mg flux rates in the molecular beam epitaxy (MBE) growth of epitaxial Mg _2 Sn (Ge) thin films, with Mg: Sn (Ge) ratios from 3.9 to 9.1 while maintaining constant Sn and Ge flux rates. Our results indicate that while the films mainly consisted of the Mg _2 Sn phase due to excess Mg compensating evaporation at the growth temperature, the Mg flux rate significantly influenced film growth dynamics. X-ray diffraction analysis showed that higher Mg flux rates increased microstrain and decreased vertical grain sizes, suggesting increased planar defect density. However, the full-width half maximums of rocking curves tend to be reduced at higher flux rates, attributed to enhanced in-plane grain alignment and reduction of point defect density. Positron annihilation experiments revealed lower vacancy-type defects at higher Mg flux rates, aligning with the rocking curve measurements. The higher Mg flux rates enhanced surface migration and promoted larger horizontal grain growth. As these grains coalesce, slight misalignments between them introduce strain within the crystal lattice. To accommodate this strain, planar defects such as stacking faults form, as indicated by the x-ray pole figure measurements. Despite the higher crystal quality and reduction in vacancy-type defects, the total thermal conductivity of the films decreased with increasing Mg flux rates. This suggests that modulating Mg flux rates in MBE-grown Mg _2 Sn thin films, it is possible to achieve enhanced crystalline alignment and controlled formation of beneficial higher-dimensionality defects, which together contribute to the reduction in thermal conductivity and improve the film’s overall TE performance.

Keywords

• thin film thermoelectric
• Mg2Sn (Ge)
• epitaxial growth
• defects