Thermodynamics and thin film deposition of MgB2 superconductors

Abstract

The recently discovered superconductor MgB2 with Tc at 39 K has great potential in superconducting microelectronics. Thermodynamics studies with the calculation of phase diagrams (CALPHAD) modelling technique show that due to the high volatility of Mg, MgB2 is only thermodynamically stable under fairly high Mg overpressures for likely in situ growth temperatures. This provides a helpful insight into the appropriate processing conditions for MgB2 thin films, including the identification of the pressure–temperature region for adsorption-controlled growth. The initial MgB2 thin films were made by pulsed laser deposition followed by in situ annealing. The cross-sectional transmission electron microscopy reveals a nanocrystalline mixture of textured MgO and MgB2 with very small grain sizes. A zero-resistance transition temperature of 34 K and a zero-field critical current density of 1.3 × 106 A cm−2 were obtained. The qualities of these films are limited by the thermodynamic stability conditions, which favour deposition techniques that can maintain a high flux of Mg.