Microwave surface resistance of thick MgB sub(2) films on c-plane sapphire: a study on the depth profile of the surface resistance

A depth profile of the surface resistance (R sub(s)) is obtained at 8.6 GHz for a thick MgB sub(2) film both in the superconducting state and in the normal state, for which 1 mu m thick MgB sub(2) films are grown in situ on a c-plane sapphire substrate using the hybrid physical-chemical vapor deposition (HPCVD). A critical temperature T sub(c) of 40.4 K, a value higher than that of 39 K for MgB sub(2) single crystals, is observed for the pristine 1 mu m thick MgB sub(2) film due to the thermal strain caused by its epitaxial nature. The depth profile of the effective R sub(s) for the 1 mu m thick MgB sub(2) film shows a drastic increase in the effective R sub(s) 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 MgB sub(2) film. Our results show that thick MgB sub(2) films grown using HPCVD could have very low R sub(s) despite the existence of Mg-rich phases in the lower part of the MgB sub(2) films and that microwave technique could provide a sensitive method for investigating the homogeneity of thick MgB sub(2) films.