Wafer scale production of superconducting magnesium diboride thin films with high transition temperature

A wafer, comprising:

• a superconducting MgB2 film, wherein the MgB2 film:
• has an area greater than or equal to a circular area having a diameter of at least 4 inches;
• a resistivity ρ 25 μΩ*cm≤ρ≤50 μΩ*cm across an entirety of the area
• a surface roughness of 2 nm or less across an entirety of the area,
• a sheet resistance and a thickness varying by less than 10% across the entirety of the area;
• and
• a critical temperature greater than 15K across the entirety of the area.

A method of making a film comprising magnesium diboride, comprising:

• depositing magnesium and boron on a substrate so as to form an Mg—B composite;
• depositing a capping layer to form a capped film, wherein the capping layer has a first melting temperature higher than a second melting temperature of the magnesium;
• thermally annealing the capped film at a temperature;
• and
• cooling the capped film so that a MgB2 film is made.