Structural Evolutionand Cu Diffusion Mechanism inBi₂Se₃ Thin Films on YBa₂Cu₃O₇ as a Function of Cracked-Se Processing Time

This study provides a comprehensive investigation into the fabrication and interface chemistry of a heterojunction between yttrium barium copper oxide (YBa₂Cu₃O₇, YBCO), a high-temperature superconductor (HTS), and bismuth selenide (Bi₂Se₃), a prototypical topological insulator (TI). Bi₂Se₃ thin films were deposited on YBCO using a controlled cracked-Se process with varying durations to optimize crystallographic and chemical properties at the HTS/TI interface. Various analytical techniques were employed to systematically characterize the morphological evolution and phase transitions of Bi₂Se₃ on YBCO. Notably, extended cracked-Se process time substantially enhanced the crystallinity of Bi₂Se₃, as evidenced by the emergence of distinct Raman-active vibrational modes and a well-defined <i>c</i>-axis orientation. In addition, the cracked-Se process facilitated the diffusion of Cu atoms from YBCO into Bi₂Se₃, resulting in the formation of interstitial Cu atoms in the van der Waals gaps and Cu–Se bonds. These Cu-related chemical interactions were confirmed via depth-resolved X-ray photoemission spectroscopy, which revealed an expanded mixed interfacial layer with increasing cracked-Se process time. This study offers crucial insights into the complex interfacial dynamics between HTS and TI materials, emphasizing the pivotal role of Se crackers in modulating the structural and chemical characteristics of Bi₂Se₃ thin films. These findings are significant in advancing the integration of HTS/TI heterostructures into next-generation quantum devices.