Variable energy positron beam analysis of vacancy defects in laser ablated SrTiO3 thin films on SrTiO3

Abstract

Vacancies are known often to be the dominant type of point defects in perovskite oxides, but their presence can rarely be directly determined. Positron annihilation methods have unique sensitivity to vacancy-related defects and the variable energy positron beam (VEPB) techniques allow near-surface depth profiling. The characterization of strontium titanate (SrTiO3) thin films, deposited on SrTiO3 substrates by pulsed laser deposition, using Doppler broadening of the annihilation radiation detected VEPB measurements, is reported. The laser repetition rate and the postgrowth anneal conditions for the samples were varied. VEPB measurements clearly detected the deposited SrTiO3 layer (the increase in the Doppler broadened line shape parameter S compared to the substrate was ∼3.5%) and showed the presence of a high concentration of open volume defects. The magnitude of the change in S and the lack of dependence on anneal oxygen partial pressure provided evidence that strontium vacancies and VSr-oxygen vacancy defects were responsible for positron trapping. Three of the films studied have also been carefully studied by transmission electron microscopy (TEM). Fitting of the VEPB profiles using thicknesses obtained from TEM provided evidence that the homogeneous growth region, observed by TEM, has a lower vacancy defect content compared to the surface columnar region.