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 Mg1-xAlxB2 phase formed at a thickness of 190 nm is a promising technique for thin-film fabrication. The upper critical field (Hc2) of Mg1-xAlxB2 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 Hc2 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 Tc suppression in the Al-buffered samples. A Meissner force curve reveal larger London penetration depth (lambda L) in Al-buffered films than in pure MgB2 samples. Particularly, the Mg1-xAlxB2 sample, exhibits a lambda L value at 0 K of 1009.68 nm, resulting in a thermodynamic critical field (Hc) of 0.026 T. Higher-thickness Al-buffered film samples, which have a MgB2 layer on the top surface, displays lambda L and Hc values in close agreement with those of a pure MgB2 film.