ELASTIC PROPERTIES OF HYDROGEN-LOADED EPITAXIAL-FILMS

We have investigated the elastic effects of hydrogen loading on thin films of niobium. it has been shown recently [1] that epitaxial Nb films grown with MBE on sapphire substrates exhibit a one-dimensional expansion, perpendicular to the film surface, on loading with hydrogen, up to a critical hydrogen concentration. This can be understood as an epitaxial clamping of the metal film by the substrate. We report the first-direct measurement of the relation between out-of-plane expansion and hydrogen concentration in these films. Nb [110] films were loaded with H from the gas phase below the critical concentration. The lattice parameters were measured absolutely with a three-crystal high-resolution X-ray diffractometer. H concentrations were measured and depth-profiled directly using the nuclear reaction H-1(N-15,alphagamma)C-12. The resulting lattice expansion as a function of H concentration is found to be nearly four times larger than expected from bulk values of the elastic constants. The hydrogen depth profiles also show a H concentration gradient extending well beyond the immediate surface region.