Effects of Intermixing in Sb₂Te₃/Ge_1+xTe Multilayers on the Thermoelectric Power Factor

Abstract

Over the past few decades, telluride-based chalcogenide multilayers, such as PbSeTe/PbTe, Bi 2 Te 3 /Sb 2 Te 3 , and Bi 2 Te 3 /Bi 2 Se 3 , were shown to be promising high-performance thermoelectric films. However, the stability of performance in operating environments, in particular, influenced by intermixing of the sublayers, has been studied rarely. In the present work, the nanostructure, thermal stability, and thermoelectric power factor of Sb 2 Te 3 /Ge 1+x Te multilayers prepared by pulsed laser deposition are investigated by transmission electron microscopy and Seebeck coefficient/electrical conductivity measurements performed during thermal cycling. Highly textured Sb 2 Te 3 films show p-type semiconducting behavior with superior power factor, while Ge 1+x Te films exhibit n-type semiconducting behavior. The elemental mappings indicate that the as-deposited multilayers have welldefined layered structures. Upon heating to 210 °C, these layer structures are unstable against intermixing of sublayers; nanostructural changes occur on initial heating, even though the highest temperature is close to the deposition temperature. Furthermore, the diffusion is more extensive at domain boundaries leading to locally inclined structures there. The Sb 2 Te 3 sublayers gradually dissolve into Ge 1+x Te. This dissolution depends markedly on the relative Ge 1+x Te film thickness. Rather, full dissolution occurs rapidly at 210 °C when the Ge 1+x Te sublayer is substantially thicker than that of Sb 2 Te 3 , whereas the dissolution is very limited when the Ge 1+x Te sublayer is substantially thinner. The resulting variations of the nanostructure influence the Seebeck coefficient and electrical conductivity and thus the power factor in a systematic manner. Our results shed light on a previously unreported correlation of the power factor with the nanostructural evolution of unstable telluride multilayers.