Clean Mgb2 Thin Films On Different Types Single-Crystal Substrate Fabricated By Hybrid Physical-Chemical Vapor Deposition

Phase pure and clean MgB2 films have been fabricated on different types of single-crystal substrate, including SiC, sapphire, MgO, LaAlO3, SrTiO3, and yttrium-stabilized ZrO2 (YSZ), via the hybrid physical-chemical vapor deposition (HPCVD) technique. Pure MgB2 films with different structural textures are obtained by selecting the substrate and its orientation, such as off-axis epitaxial MgB2 on MgO(211) or textured MgB2 on YSZ(110) substrates. Films show T-c(0) in the range 37-41 K, and residual resistivity rho(42 K) between 0.3 and 5 mu Omega cm as the substrate and film thickness are varied. The crystal structures and film morphologies are presented and discussed. Further investigations are focused on c-axis epitaxial films. Epitaxial MgB2 films with thickness about 300 nm manifest T-c(0) = 41.2 K and residual resistivity .(42 K) = 0.35 mu Omega cm on SiC(0001) while T-c(0) = 39.5 K and rho(42 K) = 0.27 mu Omega cm on c-cut sapphire, reflecting that the films are of high quality and in the 'clean limit'. Low upper critical field H-c2(parallel to ab)(0) and large anisotropy of the upper critical field gamma comparable to the results from MgB2 single crystals are also revealed. The smooth surface of the epitaxial films, with root-mean-square (RMS) roughness less than 5 nm, makes them well qualified for device applications. Microwave performance measurements are carried out using a coplanar resonator on a 300 nm thick film on sapphire substrate and no power dependence of unloaded quality factor in the output power range of -10-25 dB is found.