Enhanced gas-particle adsorption on strained pseudomorphic Fe films on Mo(110)

Ultra-thin epitaxial iron films grown on a stepped Mo(110) surface in an ultra-high vacuum were studied using scanning tunneling microscopy and low-energy electron diffraction. Films grown at room temperature developed pseudomorphically by forced layer-by-layer growth. These films exhibited high populations of defects, attributed to the adsorption of hydrogen from the residual gases in the vacuum system. The density of these defects increased noticeably with increasing film thickness and exposure time to residual gases. This phenomenon was absent for films grown at elevated temperatures by Stranski–Krastanov (SK) growth. However, networks of strain-relaxing dislocations were observed on the wedge-shaped islands formed in these latter films. It is suggested that residual gas particles, in particular hydrogen, have a higher sticking probability on the strained pseudomorphic films than on the relaxed SK films. The proposed origin of this enhancement is a strain–relaxation process, i.e. the formation of FeHFe bonds allows some rearrangement of surface Fe positions by weakening FeFe bond strengths, thereby lowering the surface tensile stress.