Carbon-Doped<tex>$rm MgB_2$</tex>Thin Films Grown by Hybrid Physical-Chemical Vapor Deposition

Abstract

Carbon-doped MgB/sub 2/ thin films have been produced with hybrid physical-chemical vapor deposition (HPCVD) by adding a carbon-containing metalorganic magnesium precursor, bis(methylcyclopentadienyl)magnesium, to the carrier gas. The amount of the carbon added, thus the carbon content in the films, was controlled by the flow rate of a secondary hydrogen gas flow through the precursor bubbler. X-ray diffraction and electron microscopy showed that the carbon-doped MgB/sub 2/ films are textured with c-axis oriented columnar nano-grains and highly resistive amorphous areas at the grain boundaries. When the amount of carbon in the films increases, the resistivity increases dramatically while T/sub c/ decreases much more slowly as the current-carrying cross section is reduced by the grain boundaries. The temperature-dependent part of the resistivity, /spl Delta//spl rho//spl equiv//spl rho/(300 K)-/spl rho/(50 K), increases only modestly until the highly resistive grain boundaries completely cut off the conducting path. The impact of the reduced cross section on critical current density J/sub c/ is discussed.