XRD and TEM studies of as-grown MgB2 thin films deposited on r- and c-plane sapphire substrates

Abstract As-grown MgB 2 thin films were deposited on r -plane (1 1 2) and c -plane (0 0 1) sapphire substrates by using a carrousel-type magnetron sputtering system. Even though the respective critical temperature and residual-resistivity ratio of the MgB 2 films deposited on r - and c -plane sapphire substrates were nearly the same in both cases, the resistivity of the films on the c -plane sapphire substrates, ρ (40 K) about 50 μΩ  cm, were lower than that on the r -plane sapphire substrates, ρ (40 K) about 300 μΩ  cm. Standard θ /2 θ X-ray diffraction measurements showed that the MgB 2 thin films deposited on the c -plane sapphire substrates have c -axis orientation. Cross-sectional transmission electron microscope images showed that the MgB 2 thin films deposited on both the r - and c -plane sapphire substrates contain columnar structures. According to the results of selected-area electron-diffraction patterns, the films deposited on the c -plane sapphire substrates had c -axis orientation and the films on the r -plane sapphire substrates including the amorphous MgB 2 also had c -axis orientation. These results indicate that selection of the cut-plane of the sapphire substrates is very important to grow the high-quality as-grown MgB 2 thin films.