Phase transitions in thermoelectric Mg-Ag-Sb thin films

alpha-MgAgSb thin films are expected to be compatible with the complementary metal oxide semiconductor (CMOS) technology, and could be used to develop thermoelectric modules integrated to microelectronic devices for energy harvesting. However, bulk studies showed that the presence of secondary phases in addition to alpha-MgAgSb, such as the metallic phase Ag3Sb, deteriorates the thermoelectric properties. Consequently, understanding phase transformations in Mg-Ag-Sb thin films produced by CMOS-compatible magnetron sputtering is capital for the production of homogeneous alpha-MgAgSb thin films to be used in energy harvesting thermoelectric modules integrated into microelectronic devices. In this work, different Mg-Ag-Sb films were deposited by magnetron sputtering using either a single alloyed target with the average composition Mg1/3Ag1/3Sb1/3 or using Mg, Ag, Sb co-deposition from three elementary targets. In situ X-ray diffraction measurements were used to investigate the phase transitions in the films, aiming to determine the thermal stability of alpha-MgAgSb and to investigate the possibility of producing alpha-MgAgSb films without secondary phases. The results show that the use of the single alloyed target does not allow the production of homogenous alpha-MgAgSb films. The phase formation sequence observed in the films is different from bulk samples in this case. Co-sputtering of the three elements Mg, Ag, and Sb, allowing the composition of the as-deposited film to be better controlled, confirmed that the Mg1/3Ag1/3Sb1/3 stoichiometry does not allow the production of homogenous alpha-MgAgSb films. Contrasting with usual belief, the results show that the Ag and Sb compositions of the phases alpha-MgAgSb and gamma-MgAgSb vary during annealing, and the phase transitions alpha-to-beta, beta-to-gamma, and alpha-to-gamma are not allotropic. These finding are of major importance for the production of the thermoelectric compound alpha-MgAgSb, and explain the large variations of the alpha-MgAgSb Seebeck coefficient reported in the literature. (C) 2022 Elsevier B.V. All rights reserved.

Automated summary

This study investigates the phase transitions in Mg-Ag-Sb thin films produced by magnetron sputtering. The results show that using a single alloyed target or co-sputtering three elements cannot produce homogeneous alpha-MgAgSb films. It is also found that the composition of the phases alpha-MgAgSb and gamma-MgAgSb vary during annealing, contrary to previous beliefs.