Effects of Intermixing in Sb₂Te₃/Ge₁₊Te Multilayers on the Thermoelectric Power Factor

Over the past few decades, telluride-based chalcogenide multilayers, such as PbSeTe/PbTe, Bi₂Te₃/Sb₂Te₃, and Bi₂Te₃/Bi₂Se₃, 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 Sb₂Te₃/Ge₁₊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 Sb₂Te₃ films show p-type semiconducting behavior with superior power factor, while Ge₁₊Te films exhibit n-type semiconducting behavior. The elemental mappings indicate that the as-deposited multilayers have well-defined 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 Sb₂Te₃ sublayers gradually dissolve into Ge₁₊Te. This dissolution depends markedly on the relative Ge₁₊Te film thickness. Rather, full dissolution occurs rapidly at 210 °C when the Ge₁₊Te sublayer is substantially thicker than that of Sb₂Te₃, whereas the dissolution is very limited when the Ge₁₊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.