Formation and interface characterization of Bi ₂ Se ₃ /YBa ₂ Cu ₃ O ₇ thin films via two‐step growth using cracked‐Se technique

Achieving a high‐quality interface between the topological insulator Bi 2 Se 3 and the high‐critical temperature superconductors YBa 2 Cu 3 O 7 (YBCO) remains challenging because of the rough surface morphology and elemental interdiffusion originating from YBCO. In previous studies, the Bi 2 Se 3 thin films directly deposited on YBCO exhibited poor crystallinity, which was significantly improved after a subsequent cracked‐Se treatment, although substantial elemental diffusion from YBCO still persisted. Here, we employ a two‐step growth technique consisting of an initial Bi thin‐film deposition followed by a cracked‐Se treatment and additionally introduce a thin Se buffer layer before the Bi deposition to effectively mitigate elemental diffusion. Structural and chemical characterizations demonstrate that the Se buffer layer effectively suppresses Cu diffusion, facilitates the formation of a uniform quintuple‐layer structure, and improves the crystallinity of the Bi 2 Se 3 thin films. Electrical measurements showed that YBCO with the Se buffer exhibits a higher superconducting transition temperature (~10 K) than the sample without the buffer (~4 K).