Hydrogen‐induced defects in Pd films

Abstract

Abstract Hydrogen absorbed in crystalline solids causes a lattice expansion and the formation of hydride phases. Contrary to free standing bulk samples, thin films are fixed at substrates, which prevent their in‐plane expansion. This makes hydrogen‐induced expansion of thin films highly anisotropic and leads to the formation of high stresses in hydrogen loaded thin films. As a consequence, lattice defects may be created in thin films loaded with hydrogen. This work reports about defects created by hydrogen loading in epitaxial Pd films deposited on Al 2 O 3 substrates by cold cathode beam sputtering. Hydrogen‐induced defects are characterized by positron annihilation spectroscopy performed with variable energy slow positron beams. Extended studies of defect depth profile and its development with increasing concentration of hydrogen are performed by measurement of Doppler broadening of annihilation profile using a continuous positron beam. Selected states are investigated also by positron lifetime spectroscopy on an intense pulsed positron beam. Firstly, the microstructure of virgin films is characterized. Subsequently, the hydrogen concentration in the films is increased step‐by‐step by electrochemical charging. The development of the film microstructure and the evolution of defects are investigated. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)