Co/Bi2Se3 interfacial reactions and Bi-Co-Se phase equilibria

Background: Bi2Se3 holds significant importance in thermoelectric applications, and Co is considered a promising candidate for use as a barrier layer. This study investigates the interfacial reactions between Co and Bi2Se3, along with the phase equilibria within the Bi-Co-Se system, to provide fundamental insights. Methods: The Bi-Co-Se alloys and Bi2Se3 substrates were prepared with pure constituent elements. The Co/Bi2Se3 couples were formed by electroplating Co onto the Bi2Se3 substrates, followed by subjecting them to reactions at 400 degrees C and 500 degrees C. Significant findings: Isothermal sections of the Bi-Co-Se phase equilibria at 400 degrees C and 500 degrees C have been proposed based on key ternary experimental results and phase diagrams of its three constituent binary systems. No ternary compounds were observed. The stable phases and phase relationships are similar at both 400 degrees C and 500 degrees C, except for the instability of Co9Se8 at 500 degrees C. Significant interfacial reactions were observed in the Co/Bi2Se3 couples reacted at 400 degrees C and 500 degrees C. These results suggest that Co alone may not serve as a suitable barrier layer for Bi2Se3 at temperatures exceeding 400 degrees C. The reaction path at 400 degrees C is Co/Co1-chi Se/(Bi2)m(Bi2Se3)n/Bi2Se3, with Se as the dominant diffusion species. Se atoms migrate towards Co to form the Co1-chi Se phase and leave a Sedepleted Bi2Se3 behind to form (Bi2)m(Bi2Se3)n. Unexpected liquation occurred in the Co/Bi2Se3 couple reacted at 500 degrees C. This liquation resulted from the rapid outward diffusion of Se from the Bi2Se3, leading to the emergence of bismuth from the Se-depleted Bi2Se3 and its transformation into a molten state.