Properties of MgB2 thin films made by radio frequency magnetron co-sputtering

Abstract

We have studied properties of the superconducting MgB2 thin films made on sapphire substrates by radio frequency (rf) magnetron co-sputtering of Mg and B. The effects of annealing temperature and Mg:B ratio on the superconducting properties of the film were investigated. Films were prepared by room-temperature co-deposition of Mg and B using two guns and annealed in situ to form the superconducting MgB2 phase. Ar sputtering pressure was 20 mtorr with 5% of hydrogen gas to trap remnant oxygen gas in the deposition chamber. Average rf power density of Mg target was varied from 2 to 4 W cm−2 with that of B fixed at 12 W cm−2. An additional Mg layer was deposited on top to make up for Mg loss during in situ annealing. Films were heated up to 500–750 °C at the rate of 60 °C min−1, annealed for 20 min, and naturally cooled down by turning off the heater. The best results were obtained for films deposited with the rf power of Mg target at 50 W and annealed at 700 °C. The mechanism for MgB2 formation in low-pressure annealing is believed to be that during annealing the local Mg vapour pressure inside the film falls within the thermodynamic window.