In-situ annealing effects on MgB2 thin films fabricated by electron beam deposition

Although the as-grown method is becoming the main current for fabrication of MgB₂ thin films, the in-situ annealing method is still indispensable and effective for obtaining thin films with a high zero resistivity temperature and current density. We have investigated the effects of in-situ annealing on the superconducting properties of MgB₂ thin films. MgB₂ thin films were fabricated by electron beam epitaxy on C-plane sapphire (Al₂O₃) substrate. Thin films were produced by simultaneously evaporating B and Mg metal in a high vacuum of 3×10^-7 Pa. Substrate temperature was 265°C. The zero resistivity temperature of the as-grown thin films was 31 K. Electron beam deposition was followed by in-situ annealing in a high vacuum. In the optimal annealing process (a thin film is heated at 600°C for one hour) we have consistently obtained thin films with a zero resistivity temperature of 35 K. In contrast, the critical current density of MgB₂ thin films was decreased by annealing. This suggests that the crystal grain boundaries in the MgB₂ thin films are working as a flux pinning site, and crystal growth from annealing brought about the reduction of current density.