Deteriorated superconductivity of MgB2 films due to Al diffusion from Al2O3 substrates: Thermodynamic perspective

• The intrinsic problem of interdiffusion between MgB2 films and Al2O3 substrates was pointed out.
• Mg is a stronger reductant than Al and takes out oxygen from Al2O3.
• Thermodynamics predicts appropriate choice of substrate and barrier materials for MgB2 films.

Recently we reported the growth of MgB2 films by pyrolysis of decaborane (B10H14) in Mg vapor. In this article, we discuss the intrinsic problem at the MgB2/Al2O3 interface, which was encountered during our optimization of the growth temperature. Mg is a stronger reductant than Al, and takes out oxygen from Al2O3. Increasing the growth temperature, this reaction becomes noticeable and leads to the formation of MgO and Mg1−xAlxB2, which deteriorates the properties of resultant MgB2 films. Our observations present a general issue in the choice of substrate and barrier materials in thin film growth and/or junction fabrication of compounds containing reactive cations such as Mg.