The morphology and chemistry of epitaxial MgB2 thin films grown using reactive Mg evaporation on different substrates were characterized by transmission electron microscopy methods. For polycrystalline alumina and sapphire substrates with different surface planes, an MgO transition layer was found at the interface region. No such layer was present for films grown on MgO and 4H SiC substrates, and none of the MgB2 films had any detectable O incorporation nor MgO inclusions. High-resolution electron microscopy revealed that the growth orientation of the MgB2 thin films was closely related to the substrate orientation and the nature of the intermediary layer. Electrical measurements showed that very low resistivities (several μΩcm at 300K) and high superconducting transition temperatures (38 to 40K) could be achieved.

Electron Microscopy Studies of Epitaxial MgB2 Superconducting Thin Films Grown by in situ Reactive Evaporation. L.Gu, B.H.Moeckly, D.J.Smith: Journal of Crystal Growth, 2005, 280[3-4], 602-11