Oxygen self-diffusion in HfO2 studied by electron spectroscopy

High-resolution measurement of the energy of electrons backscattered from oxygen atoms makes it possible to distinguish between O18 and O16 isotopes as the energy of elastically scattered electrons depends on the mass of the scattering atom. Here we show that this approach is suitable for measuring oxygen self-diffusion in HfO2 using a HfO162(20nm)/HfO182 bilayers (60 nm). The mean depth probed (for which the total path length equals the inelastic mean free path) is either 5 or 15 nm in our experiment, depending on the geometry used. Before annealing, the elastic peak from O is thus mainly due to electrons scattered from O16 in the outer layer, while after annealing the signal from O18 increases due to diffusion from the underlying HfO182 layer. For high annealing temperatures the observed interdiffusion is consistent with an activation energy of 1 eV, but at lower temperatures interdiffusion decreases with increasing annealing time. We interpret this to be a consequence of defects, present in the layers early on and enhancing the oxygen diffusivity, disappearing during the annealing process.