Strained single-crystal Al2 O3 grown layer by layer on Nb (110) thin films

The authors report on the growth of single-crystal Al2 O3 thin films on Nb (110) surfaces. Niobium is grown on α- Al2 O3 (11 2- 0), followed by the evaporation of Al in an O2 background. Initially, Al2 O3 grows layer by layer with hexagonal symmetry indicating either α- Al2 O3 (0001) or γ- Al2 O3 (111). Diffraction measurements show that the Al2 O3 initially grows clamped to the Nb with tensile strain near 10%. This strain relaxes with further deposition and beyond about 50 Å, the authors observe island growth. Despite the asymmetric misfit between Al2 O3 and Nb, the strain is surprisingly isotropic. Josephson junctions employing epitaxial Al2 O3 show low effective tunnel barriers and high leakage currents.