Atomic-resolution study on the interface structure and strain state reversion of the Bi₂Sr₂CuO_6+δ/MgO heterostructure

Due to the crucial influence of interface structure and strain on the performance of heterojunctions, they have received extensive attention in recent years. In this article, the interface structure and strain of the Bi₂Sr₂CuO_6+δd(Bi-2201)/MgO superconducting heterojunction prepared by molecular beam epitaxy were investigated by aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), energy-dispersive X-ray spectroscopy (EDX), and geometric phase analysis (GPA). The interfacial atomic sequence is determined as MgO-(CuO-CuO-Cu/BiO)-(Bi-2201)_n, where a 0.53 nm thick CuO interlayer accompanied by Bi/Cu atomic co-occupation is observed between the Bi-2201 film and the MgO substrate. In addition, there is a typical coherent lattice-matching epitaxial interface between CuO/MgO with no defects and a semi-coherent domain-matching epitaxial interface between Bi-2201/CuO accompanied by an ideal misfit dislocation network. Because misfit dislocations almost compensate for the strain caused by lattice mismatch, the final Bi-2201 film undergoes in-plane compressive strain (ε_xx ~ -0.573%) rather than expected tensile strain relative to bulk Bi-2201, which is attributed to the thermal expansion mismatch. The compressive strain gradually releases as the distance from the heterointerface increases.