Periodic microtwinning as a possible mechanism for the accommodation of the epitaxial film-substrate mismatch in the La1-xSrxMnO3/SrTiO3 system

The structure and microstructure of laser-deposited La1-xSrxMnO3(x approximate to 0.1) films on SrTiO3(100) single-crystal substrates have been studied by means of electron diffraction and high-resolution transmission electron microscopy. The film-substrate interfaces are found to be perfectly coherent and in particular to be free of interfacial dislocations, the small misfit being accommodated either elastically or by the formation of periodic microtwins. Cross-section as well as plan-view specimens of films of four different thicknesses (9, 50, 100 and 220 nm) were studied in detail. The thinnest films were found to be rhombohedral when present as coherent epitaxial films in cross-section specimens, the misfit being accommodated by periodic microtwinning. When released from their substrate, as in plan-view specimens, films of the same thickness are domain-fragmented orthorhombic. It is suggested that the misfit stresses tend to favour a microtwinned rhombohedral structure, whereas unstressed films of the same thickness, released from the substrate, tend to become domain-fragmented orthorhombic. The observations illustrate in a striking manner the effect of misfit stresses on the microstructure of thin epitaxial films, explaining also the dependence of the physical properties on the film thickness and on the misfit.