Influence of aluminum diffusion on MgB₂ films grown by hybrid physical–chemical vapor deposition using amorphous aluminum buffers

This study explores the effect of Al-buffered layers on the crystal structure and superconductivity of MgB₂. The unique Mg_1-xAl_xB₂ phase formed at a thickness of 190 nm is a promising technique for thin-film fabrication. The upper critical field (H_c2) of Mg_1-xAl_xB₂ is suppressed, which corresponds to a change in interband scattering, as confirmed by X-ray diffraction and extended X-ray absorption fine structure results. By contrast, the thicker samples with the Al-buffer layer, which contain MgB₂ on the top surface, show an improvement in H_c2 compared to the pure sample because the grain boundaries act as flux pinning sources. The observed reduction in the electron–phonon coupling constants correlates with T_c suppression in the Al-buffered samples. A Meissner force curve reveal larger London penetration depth (λ_L) in Al-buffered films than in pure MgB₂ samples. Particularly, the Mg_1-xAl_xB₂ sample, exhibits a λ_L value at 0 K of 1009.68 nm, resulting in a thermodynamic critical field (H_c) of 0.026 T. Higher-thickness Al-buffered film samples, which have a MgB₂ layer on the top surface, displays λ_L and H_c values in close agreement with those of a pure MgB₂ film.