Superconducting properties and microstructures of MgB2 thin films prepared by the ex situ annealing process

Superconducting MgB2 thin films were prepared on Al2O3(0001) substrates using the two-step method. Boron thin films were deposited by electron-beam evaporation followed by ex situ annealing in a magnesium vapour environment. With proper annealing conditions, the superconducting transition temperature (Tc) of MgB2 thin films was ∼ K and their critical current densities (Jc) were above 107 A cm-2 at 15 K. Furthermore, Jc was increased on reducing the thickness of as-grown MgB2 film by the Ar ion milling below a certain temperature although Tc and its transition width were slightly degraded. Through detailed study of the microstructural evolution of the MgB2 layer during the ex situ annealing with scanning electron microscopy and transmission electron microscopy, MgB2 was found to nucleate at the amorphous boron (a-B) layer and then grow into and out of the a-B film simultaneously, resulting in two different MgB2 layers with different grain morphologies