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An energetic condensation technique, cathodic arc discharge deposition, is used to grow epitaxial Niobium (Nb) thin films on a-plane sapphire (hexagonal-closed-packed Al2O3) at moderate substrate heating temperature (<400 °C). The epitaxial Nb(110)/Al2O3(1,1,-2,0) thin films reached a maximum residual resistance ratio (RRR) value 214, despite using a reactor-grade Nb cathode source whose RRR was only 30. The measurements suggest that the film's density of impurities and structural defects are lower when compared to Nb films produced by other techniques, such as magnetron sputtering, e-beam evaporation or molecular-beam-epitaxy. At lower substrate temperature, textured polycrystalline Nb thin films were created, and the films might have twin symmetry grains with {110} orientations in-plane. The texture was revealed by x-ray diffraction pole figures. The twin symmetry might be caused by a combination effect of the Nb/Al2O3 three-dimensional epitaxial relationship ('3D-Registry' Claassen's nomenclature) and the 'Volmer-Weber' (Island) growth model. However, pole figures obtained by electron backscattering diffraction (EBSD) found no twin symmetry on the thin films' topmost surface (∼50 nm in depth). The EBSD pole figures showed only one Nb{110} crystal plane orientation. A possible mechanism is suggested to explain the differences between the bulk (XRD) and surface (EBSD) pole figures. [ABSTRACT FROM AUTHOR]

Two calorimeters, with stainless steel and Cu as the thermal path material for high precision and high power versions, respectively, have been designed and commissioned for the 7.5 GHz surface impedance characterization system at Jefferson Lab to provide low temperature control and measurement for CW power up to 22 W on a 5 cm diameter disk sample which is thermally isolated from the radiofrequency (RF) portion of the system. A power compensation method has been developed to measure the RF induced power on the sample. Simulation and experimental results show that with these two calorimeters, the whole thermal range of interest for superconducting radiofrequency materials has been covered. The power measurement error in the interested power range is within 1.2% and 2.7% for the high precision and high power versions, respectively. Temperature distributions on the sample surface for both versions have been simulated and the accuracy of sample temperature measurements have been analyzed. Both versions have the ability to accept bulk superconductors and thin film superconducting samples with a variety of substrate materials such as Al, Al2O3, Cu, MgO, Nb, and Si. [ABSTRACT FROM AUTHOR]

We report microstructure analyses and superconducting radiofrequency (SRF) measurements of macroscopic scale epitaxial MgB2 films. MgB2 films on 5 cm diameter sapphire disks were fabricated by a hybrid physical chemical vapor deposition (HPCVD) technique. The electron-beam backscattering diffraction (EBSD) results suggest that the film is a single crystal complying with a MgB2(0001) &par; Al2O3(0001) epitaxial relationship. The SRF properties of different film thicknesses (200 and 350 nm) were evaluated under different temperatures and applied fields at 7.4 GHz. A surface resistance of 9 ± 2 &mgr;Ω has been observed at 2.2 K. [ABSTRACT FROM AUTHOR]

This article presents evidence for unprecedented, large crystal grain size in ∼1 μm thick Nb films that were grown on sapphire and copper substrates using a vacuum arc process called coaxial energetic deposition CED™. Most other deposition techniques with low adatom energy produce amorphous or small crystal-grain films. Typically, high substrate temperatures and annealing steps are required to form the large, highly connected grains. The CED™ technique deposits from plasma consisting of a nonequilibrium, high energy (50–150 eV) ion population produced from the ionized source material. At the substrate these fast ions break up columnar structures, intermix with the first few atomic layers of the substrate to improve adhesion, and form dense films at lower substrate temperatures than are typical for low adatom energy techniques, such as physical vapor deposition (PVD). Nanoscale features of the thin films were examined using electron backscatter diffraction (EBSD). The films’ cryogenic state electrical properties were characterized by their residual resistivity ratio (RRR) and superconducting transition temperature (Tc). RRR of ∼77 and Tc∼9.2 K were measured on a Nb thin film deposited on a sapphire substrate. EBSD and x-ray diffraction measurements indicated that the sapphire substrate thin films have single crystal structure, with a Nb {110} crystal plane monolithically aligned and parallel to the sample surface. Nb thin films on an ∼400 °C Cu substrate had average crystal grain size of 50 μm, which is an order of magnitude larger than that which is typical of films grown by PVD. The crystal structure of CED™ thin films is comparable to that of polycrystalline bulk Nb material, which is the state of the art for superconducting radio frequency (SRF) particle accelerators such as at the Thomas Jefferson National Accelerator Facility (JLab). The films’ novel crystal features suggest that CED™ is a promising technique to coat Nb thin films for lower cost SRF particle accelerators. Further studies of the nanoscale grain boundary features would shed light on the role played by these features in determining the performance of SRF cavities using such thin films on Cu. [ABSTRACT FROM AUTHOR]

Superconducting ultrathin films grown epitaxially ontocrystalline substrates exhibit strained epitaxial growth due to latticemismatch, which can have a significant effect on their superconductingproperties. We present a complete correlation of the surface morphology,crystal growth, strain, microstructure, and superconducting propertiesin single-crystal Nb(110) thin films sputter deposited on a-plane sapphire substrates. Notably, we observe that thelattice mismatch between Nb and sapphire induces the formation ofa hexagonal surface structure during the first three atomic layers.This is followed by a strained bccNb(110) phasewhose in-plane lattice parameter progressively relaxes to bulk value.Similar lattice relaxation was also observed in the direction perpendicularto the interface using X-ray diffraction (XRD) and transmission electronmicroscopy (TEM). Significant perpendicular strain in films up to30 nm thick was found to ultimately affect the superconducting propertiesof the Nb thin films as demonstrated with AC susceptibility measurements,where dissipative effects in the lattice associated with the presenceof strain and associated defects were identified. [ABSTRACT FROM AUTHOR]