Strained single-crystal Al2O3 grown layer by layer on Nb (110) thin films

The authors report on the growth of single-crystal Al2O3 thin films on Nb (110) surfaces. Niobium is grown on alpha-Al2O3 (11 (2) over bar0), followed by the evaporation of Al in an O-2 background. Initially, Al2O3 grows layer by layer with hexagonal symmetry indicating either alpha-Al2O3 (0001) or gamma-Al2O3 (111). Diffraction measurements show that the Al2O3 initially grows clamped to the Nb with tensile strain near 10%. This strain relaxes with further deposition and beyond about 50 angstrom, the authors observe island growth. Despite the asymmetric misfit between Al2O3 and Nb, the strain is surprisingly isotropic. Josephson junctions employing epitaxial Al2O3 show low effective tunnel barriers and high leakage currents. (c) 2007 American Institute of Physics.