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

As-grown MgB₂ thin films were deposited on r-plane (112) and c-plane (001) sapphire substrates by using a carrousel-type magnetron sputtering system. Even though the respective critical temperature and residual-resistivity ratio of the MgB₂ 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 μΩcm, were lower than that on the r-plane sapphire substrates, ρ(40 K) about 300 μΩcm. Standard θ/2θ X-ray diffraction measurements showed that the MgB₂ thin films deposited on the c-plane sapphire substrates have c-axis orientation. Cross-sectional transmission electron microscope images showed that the MgB₂ 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 MgB₂ thin films.