Defects in TiO2 films on p+-Si studied by positron annihilation spectroscopy

Variable-energy positron annihilation spectroscopy has been applied to the study of defects in TiO2/p+-Si structures, in the as-grown state and after annealing in vacuum and in hydrogen, to investigate whether annealing (and film thickness) resulted in an increase of vacancy-type defects in the oxide films. It was found that the concentration of such defects remained unchanged after vacuum annealing for all films studied, but after H2 annealing more than doubled for 150 nm-thick films, and increased by an order of magnitude for 100 nm-thick films. The nature of the vacancies was examined further by measuring high-precision annihilation lines and comparing them with a reference Si spectrum. The changes observed in the ratio spectra associated with oxygen electrons suggest that the defects are oxygen vacancies, which have been shown to enhance electroluminescence from TiO2/p+-Si heterostructure-based devices.

► Positron spectroscopy of TiO2/p+Si films reveals O vacancies at 1017–1018 cm−3.
► Oxygen vacancy concentration increases with film thickness from 100 to 220 nm.
► No significant change in vacancies is seen after annealing at 500 °C in vacuum.
► After annealing at 500 °C in H the vacancies increase by up to an order of magnitude.
► The observations largely support linking O vacancies with electro-luminescence.