An electron-beam evaporation technique was used to prepare MgB2 thin films on polished sapphire C-plane single-crystal substrates with and without post-annealing. We maintained the background pressure to less than 1.3×10-7 Pa and the substrate temperature at 247-284°C. The Mg evaporation rate was 2 nm/s and B 0.5 nm/s. The best critical temperature (Tc) for the as-grown films was 34 K and the critical-current density (Jc) was 7.1×106 A/cm2 (4.2 K, self field.). In order to improve the crystallinity of the as-grown MgB2 thin films, we annealed them at 580-620°C for 1 hr under vacuum conditions. The Tc's of the post-annealed films increased about 4 K; however, Jc in the magnetic field decreased proportionate to the as-grown MgB2 thin film. From X-ray diffraction measurements, it was shown that the post-annealing process improved the crystallinity of the post-annealed film. The crystallinity of MgB2 grains may affect the Jc of MgB2 superconductors.
