An investigation was made of the crystal structures and superconducting properties of as-grown MgB2 superconducting thin films deposited onto Al2O3 substrates having different crystal planes (C- and R-). The MgB2 thin films were prepared by using a multiple-target sputtering method, without performing post-annealing. The MgB2 films deposited onto the C-plane Al2O3 substrates were found, from the results of 2θ/θ X-ray diffraction measurements, to have a c-axis orientation. However, there were no MgB2 peaks for films grown onto the R-plane Al2O3 substrates. Although the maximum TC values of both films deposited onto the C- and R-plane Al2O3 substrates were almost the same, TC,onset ~ 28K and residual resistivity ratio ~1.2, it was noted that the resistivity of the films on C-plane Al2O3 (~70μΩcm at 40K) was lower than that on R-plane Al2O3 (~300μΩcm at 40K). The results indicated that the grain size of MgB2 in the films deposited onto C-plane Al2O3 substrates was larger than that on R-plane Al2O3; indicating good agreement with the XRD measurements. The surface morphology of the as-grown MgB2 thin films was investigated by using scanning electron microscopy. The surfaces of the films on the C- and R-plane Al2O3 substrates were found to be very flat and smooth. This indicated the high potential for fabricating Josephson tunnel junctions, using as-grown MgB2 thin films.

As-Grown MgB2 Thin Films Deposited on Al2O3 Substrates with Different Crystal Planes. A.Saito, A.Kawakami, H.Shimakage, Z.Wang: Superconductor Science and Technology, 2002, 15, 1325-9