The effect of hydrogen chemisorption on the conductivity of evaporated aluminum films

Abstract

Very thin Al films (<10 nm) were prepared by thermal evaporation on electroded glass substrates in an ultrahigh vacuum system. TEM studies revealed that the films had a discontinuous structure consisting of imperfectly touching crystallites. Since hydrogen adsorption should not decrease electron specular reflection, entirely diffuse scattering of the conduction electrons at the film surface was assumed. At room temperature, hydrogen adsorption caused a decrease in film conductivity and equilibrium was reached rapidly. At temperatures above 80 °C, hydrogen adsorption caused an increase in film conductivity with a slow (16 min at 80 °C) attainment of equilibrium. The results indicate an activated dissociative process which occurs via a precursor state at the higher temperatures. At lower temperatures, an electronegative layer is formed: electrons are withdrawn from the Al film and the conductivity decreases. These results are in agreement with the observation that adsorbates often undergo both dissociative and nondissociated chemisorption on the same metal surface with the former being favored at higher temperatures.