Ion beam assisted deposition of superconducting MgB2 thin films

We studied the structural and superconducting properties of MgB2 thin films, deposited by co-evaporation of magnesium and boron in the presence and absence of an ion beam followed by ‘in-situ’ and ‘ex-situ’ annealing. The MgB2 films were capped with a Mg and a B layer to provide an additional Mg source in the following annealing step and to prevent diffusion of Mg out of the films. Films deposited with an ion beam showed lower transition temperature (Tc 21 K), whereas MgB2 films deposited without an ion beam showed a Tc as high as 30.6 K. Third order susceptibility measurements demonstrated homogeneous MgB2 films. Rutherford backscattering (RBS) measurements showed low oxygen concentration throughout the film. For ‘ex-situ’ annealed MgB2 film transition temperatures up to 33.5 K are obtained. X-Ray diffraction suggests some degree of c-axis alignment, but omega scans showed random orientation. MgB2 film on sapphire substrates always revealed a higher Tc than the ones on silicon substrates. However, Tc on silicon substrates was as high as 22.5 K.