Structural and analytical characterization of as-grown MgB₂ film sputtered on differently oriented α-Al₂O₃ substrate

Superconducting MgB₂ films were prepared on differently oriented α-Al₂O₃ substrates, C- and R-planes, which yielded superconducting transition temperatures of about 28 K. The electric resistivity of the MgB₂ film deposited on an R-plane substrate is 300 μΩ cm, six times larger than that on a C-plane substrate, 50 μΩ cm. To understand these differences in the electrical properties, various transmission electron microscopes were used to carry out a structural and the compositional analysis. It was shown from selected-area electron diffraction patterns that the microstructure consists of a mixture of columnar MgB₂ grains and amorphous phases in the case of the R-plane specimen, while no amorphous phase was present in the case of the C-plane specimen.