Investigation of diffusion mechanisms during nickel oxide film growth by an isotope separation technique

Abstract

Nickel oxide (NiO) films grown at 400, 600 and 800 °C under atmospheric conditions were studied using secondary ion mass spectroscopy to determine the distribution of the stable isotopes of Ni (58 and 60 a.m.u.) in the oxide. A theoretical basis for the separation of the isotopes of the mobile species during film growth is utilized to ascertain whether grain boundary or lattice diffusion of Ni occurs during growth. This is based on the condition that a reverse isotope separation can only occur by grain boundary diffusion, and was seen in the oxides grown at 400 and 600 °C. Oxidation at 800 °C resulted in two layers in the films grown on polished substrates. The upper layer showed no isotope separation, while reverse separation existed in the lower layer. Transmission electron microscopy revealed a distribution of fine oxide particles in the lower layer of the oxide grown at 800 °C. The mechanism proposed for the growth of NiO films at 800 °C involves a combination of both grain boundary and lattice diffusion, with lattice diffusion becoming the dominant mode above about 800 °C. Finally, the lateral growth of NiO grains during oxidation showed a transition at 800 °C, with the activation energy measured as 2.1 ± 0.3 eV below 800 °C and 0.3 ± 0.1 eV above 800 °C.