Superconducting properties of two-step in situ annealed MgB 2 thin films

Abstract

We have investigated the effects of in situ annealing on the superconducting properties of MgB 2 thin films for the purpose of obtaining high critical temperature close to bulk value. MgB 2 thin films were fabricated by rf magnetron sputtering on C-plane sapphire (Al 2O 3) substrate. Thin films were produced by simultaneously sputtering pure B and a Mg metal target. Sputtering deposition was followed by in situ annealing in a high vacuum. To prevent the evaporation of Mg from the film surface, two-step annealing was adopted: the first step is the crystallization stage at low-temperature annealing and the next is an improvement of the film's superconducting properties by annealing at a high temperature. In the optimal annealing process (a thin film is first heated at 600 °C for 2 h and then 670 °C for 2 h) we have consistently obtained thin films with a zero resistivity temperature of 33 K. We have discussed the cause of improvement of the superconducting properties of thin films by two-step annealing.