Interface effects in PbTiO₃ thin films and anostructures : a transmission electron microscopy study

Abstract

In the present work we have investigated ferroelectric PbTiO3 thin films grown on planar substrates and stepped surface templates. In order to probe the structural, electronic and chemical properties of the PbTiO3 thin films and nanostructures at the atomic scale, we have relied on transmission electron microscopy (TEM) techniques. High angle annular dark field scanning TEM provide directly interpretable Z-contrast images at an atomic scale. The electronic and chemical properties are probed by electron energy loss spectroscopy (EELS) at an atomic scale.

TEM-foils of high quality are a prerequisite for performing such studies. The effects of TEM-foil preparation techniques suitable for oxides and perovskites are investigated. A polishing scheme for cross-section TEM foils by applying mechanical tripod polishing and low-angle, low-energy Ar-ion milling is compared to conventional TEM-foil preparation which relies on more heavy Ar-ion milling. For the latter method, structural changes and preferential milling are observed. By applying tripod polishing and gentle low-angle, low-energy ion milling while cooling the sample, effects from specimen preparation are reduced.

In PbTiO3 thin film epitaxially grown on SrTiO3 substrates there is a region with decreased hybridization of cation electron states with oxygen electron states in the PbTiO3 adjacent to the PbTiO3 /SrTiO3 interface. This is commensurate with a decrease in tetragonality and spontaneous polarization in the interface region of the PbTiO3 thin film. At the interface the spontaneous polarization is deduced to be nonzero. Hence there is surface charge present setting up an oppositely directed and energetically unfavorable depolarization field. At the interface to the SrTiO3 substrates it is found that the depolarization field is compensated by an increase in the concentration of oxygen vacancies, which are effectively electron donors in SrTiO3, adjacent to the ferroelectric-dielectric interface. The region of oxygen vacancies reach approximately 5-6 unit cells into the SrTiO3 substrate. A region of tetragonal strain is also found in the SrTiO3 interface, reaching approximately 10 unit cells into the SrTiO3 substrate. A similar experiment on PbTiO3 thin films deposited on conducting Nb-doped SrTiO3 (Nb:SrTiO3 ) substrates shows that the free charges in Nb:SrTiO3 provides screening of the depolarization field to a higher degree. These findings are interpreted as the result of a diffusion of oxygen vacancies from bulk of SrTiO3 and towards the interface region in order to compensate for the depolarization field.

Furthermore, we have investigated PbTiO3 thin films deposited on stepped surface templates based on Nb:SrTiO3 substrates, where the pattern comprised a periodic grating of stripes and grooves. A Pb-deficient layer in the PbTiO3 thin film was observed, adjacent to the Nb:SrTiO3 interface on the stripes of the grating. The variation in stoichiometry is ascribed to differences in chemical potential due to variations in the surface curvature. The effect appears to be largest in the initial stages in the growth. This can have a general impact on the epitaxial growth of perovskite nanoscale structures, where properties are sensitive to composition and structure in the material