Microwave surface resistance of thick MgB2 films on c-plane sapphire: a study on the depth profile of the surface resistance

A depth profile of the surface resistance (RS) is obtained at 8.6 GHz for a thick MgB2 film both in the superconducting state and in the normal state, for which 1 µm thick MgB2 films are grown in situ on a c-plane sapphire substrate using the hybrid physical–chemical vapor deposition (HPCVD). A critical temperature TC of 40.4 K, a value higher than that of 39 K for MgB2 single crystals, is observed for the pristine 1 µm thick MgB2 film due to the thermal strain caused by its epitaxial nature. The depth profile of the effective RS for the 1 μm thick MgB2 film shows a drastic increase in the effective RS at low temperatures for films having thicknesses of 600 nm or less. The results for the x-ray diffraction and the normal-state resistivity provide evidence for the existence of Mg-rich phases in the lower part of the pristine MgB2 film. Our results show that thick MgB2 films grown using HPCVD could have very low RS despite the existence of Mg-rich phases in the lower part of the MgB2 films and that microwave technique could provide a sensitive method for investigating the homogeneity of thick MgB2 films. (paper)