Microstructural Investigation of La(1.9)Sr(0.1)CuO(4) Thin Films Grown by MBE

The microstructure of La(1.9)Sr(0.1)CuO(4) thin films grown by molecular beam epitaxy on SrLaAlO(4) and SrTiO(3) is investigated by transmission electron microscopy. Using SrLaAlO(4) as substrate material, compressive strain is induced, which leads to a drastic increase of Tc. In contrast, SrTiO(3) yields a tensile strain and a decrease of Tc. The film on SrLaAlO(4) has a low defect density. Only misfit dislocations with an average spacing of 200 nm are found, which are more or less irregularly distributed. For SrTiO(3), a periodic array of misfit dislocations is present with an average distance of 16 nm. At the interface, as derived from the comparison with simulated images, (LaSr)(2)CuAlO(6-x) and (LaSr)TiO(3-x) are formed on SrLaAlO(4) and SrTiO(3), respectively. These intermediate layers are found to increase the corresponding compressive and tensile strain further. The lattice deformation is determined based on lattice-image analysis. Here, the distance of CuO(2) planes can be measured locally. It turns out that applying a compressive or tensile strain increases or decreases the distance between CuO(2) planes. Accordingly, direct evidence is presented that a decoupling of CuO(2) planes leads to an increase of Tc, which is in contradiction to recent theoretical predictions.