Suppression of oxygen diffusion by thin Al2O3 films grown on SrTiO3 studied using a monoenergetic positron beam

The annealing behaviors of oxygen vacancies introduced by the epitaxial growth of thin SrTiO3 and Al2O3 films on SrTiO3 substrates were studied by using a monoenergetic positron beam. The films were grown by molecular-beam epitaxy without using an oxidant. The Doppler broadening spectra of the annihilation radiation were measured as a function of the incident positron energy for samples fabricated under various growth conditions. The line-shape parameter S, corresponding to the annihilation of positrons in the substrate, was increased by the film growth, suggesting diffusion of oxygen from the substrate into the film and a resultant introduction of vacancies (mainly oxygen vacancies). A clear correlation between the value of S and the substrate conductivity was obtained. From isochronal annealing experiments, the Al2O3 thin film was found to suppress the penetration of oxygen from the atmosphere for annealing temperatures below 600°C. Degradation of the film’s oxygen blocking property occurred due to the annealing at 700°C, and this was attributed to the oxidation of the Al2O3 by the atmosphere and the resultant introduction of vacancy-type defects.