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Title:Hydrogen in thin film niobium
Author(s):Reimer, Paul Meyer
Doctoral Committee Chair(s):Zabel, Hartmut
Department / Program:Physics
Thin films
x-ray diffraction
hydrogen profiling
Abstract:An experimental study of hydrogen dissolved in thin films of niobium is presented. The chief techniques used were x-ray diffraction and ion-beam hydrogen profiling. Niobium was grown on sapphire substrates by molecular beam epitaxy, resulting in high quality films. It has previously been shown that such films are clamped by the sapphire substrate: On hydrogen uptake they expand only in the out-of-plane direction. Transverse scans of these Nb films show evidence for a novel ordering, long-range orientational ordering (LROO): For a portion of the film, the crystallographic planes are oriented over in-plane length scales larger than the in-plane atomic correlation length. The well-oriented planes are exactly parallel to the crystallographic planes of the sapphire substrate. The concept of lattice roughness is introduced-this is the root mean square deviation in the out-of-plane direction, modulo the lattice parameter, of the atoms in each crystallographic plane. It is shown that lattice roughness leads to a Q dependent damping factor, similar to a Debye-Waller factor, which affects LROO peaks. This explains, qualitatively, the radial dependence seen in the transverse scans. A method is illustrated to quantitatively measure the lattice roughness and the fraction of the sample which is well-oriented (in the sense of LROO). A Nb sample with an initially broad LROO peak showed marked narrowing of the peak on loading with hydrogen. This effect is ascribed to hydrogen-enhanced defect mobility. This leads to a new means of improving metal film quality: "cold-annealing" by introducing hydrogen to sweep defects out to the sample boundary. A promising preliminary trial of hydrogen defect annealing in the growth chamber resulted in a sample with a larger portion in the LROO state. X-ray reflectivity measurements which support this picture are shown. A direct determination of the hydrogen-induced volume expansion as a function of hydrogen concentration in Nb films, and gold-capped Nb films has been made. Hydrogen concentration was measured by using 15N ion bombardment and lattice parameter were measured with x-ray diffraction. The volume change in a clamped film per unit hydrogen concentration, is found to be approximately three times that . of bulk Nb. It is shown that the hydrogen-induced volume depends on the derivative of of the hydrogen heat of solution, !1H, with respect to the logarithm of the volume. It is argued that the factor three enhancement may be largely explained by the strong dependence of !1H on band parameters which have the necessary volume dependence to reproduce the effect found. A supporting measurement on a partially-clamped polycrystalline film is shown. Ion bombardment yields a profile of hydrogen concentration as a function of depth below the film surface. The H profiles indicate significant hydrogen gradients in thin Nb films, with H concentration increasing with increasing depth. A concentration gradient should lead to a lattice parameter gradient, observable with X-ray diffraction. While the best thin films fail to show lattice parameter gradients, thicker and poorer quality (polycrystalline) films do show gradients. These observations are reconciled by assuming that there is a metastable, non-homogeneous, hydrogen concentration mode (or resonance) in thin films. Supporting evidence is shown, and the conditions for instability of the mode are discussed.
Issue Date:1993
Genre:Dissertation / Thesis
Rights Information:Copyright 1993 Paul Meyer Reimer
Date Available in IDEALS:2011-04-26
Identifier in Online Catalog:3644334

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