The effect of working gas on the properties of Al2O3 films and SiOx interlayers was investigated in direct-type plasma-enhanced atomic layer deposition. The density of the Al2O3 film was higher for Ar/O-2 plasma than for He/O-2 plasma, whereas the thicknesses of the Al2O3 film and SiOx interlayer were greater for He/O-2 plasma than for Ar/O-2 plasma. For understanding these phenomena, the amounts of C-and H-containing impurities in the deposited Al2O3 film were evaluated by using X-ray photoelectron spectroscopy (XPS). Further, differences between the plasma properties in He/O-2 and Ar/O-2 were analyzed using optical emission spectroscopy (OES); the consumption rate of O radicals and the production rate of H radicals were estimated from the time-resolved emission intensities for the O I and Ha lines, respectively. The mechanisms underlying the working gas effect on the density of the Al2O3 film, as well as the thicknesses of the Al2O3 film and SiOx interlayer have also been discussed. (C) 2016 Elsevier B.V. All rights reserved.