Defect passivation by annealing enables stable transport in Li-doped Mg2Sn epitaxial films for microfabricated thermoelectric devices

Abstract

Li-doped Mg₂Sn thin films are promising p-type thermoelectrics, as Li is among the most effective acceptors, yet their impact on defect chemistry, phase stability, and transport remains poorly understood. Here, low-temperature annealing is shown to passivate Li-induced defects and stabilize electrical transport in epitaxial Mg_2−xLi_xSn (0 ≤ x ≤ 0.10) thin films grown by molecular beam epitaxy. X-ray diffraction and electron microscopy reveal that Li incorporation produces Sn-rich precipitates from deviation from the 2 : 1 of Mg : Sn stoichiometry, which are partially dissolved after annealing. Depth-resolved positron annihilation spectroscopy indicates a reduction of Mg-vacancy-type defects at moderate Li content, while Hall measurements show decreased hole concentrations and enhanced mobilities, consistent with reduced ionized-impurity scattering. As-grown films exhibit Seebeck coefficients of 40–70 µV K⁻¹ at room temperature, which increase to ∼200–250 µV K⁻¹ after annealing, accompanied by suppression of cycle-to-cycle drift. The optimized films achieve an exceptional peak power factor of ∼2.4 × 10⁻³ W m⁻¹ K⁻² at the relatively low temperature of 350 K. Thermal conductivity, measured at room temperature, confirms that defect-engineered films retain strong phonon scattering after annealing, yielding zT ≈ 0.25, surpassing prior p-type Mg₂Sn epitaxial thin films. A microfabricated π-type thermoelectric generator using Li-doped Mg₂Sn p-legs delivers higher open-circuit voltage than a Mg₂Sn(Ge) benchmark with comparable output power, demonstrating the practical viability of the processed films.