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 approxim ate to 0.1) films on SrTiO3(100) single-crystal substrates have been studie d by means of electron diffraction and high-resolution transmission electro n microscopy. The film-substrate interfaces are found to be perfectly coher ent and in particular to be free of interfacial dislocations, the small mis fit being accommodated either elastically or by the formation of periodic m icrotwins. Cross-section as well as plan-view specimens of films of four di fferent thicknesses (9, 50, 100 and 220 nm) were studied in detail. The thi nnest films were found to be rhombohedral when present as coherent epitaxia l films in cross-section specimens, the misfit being accommodated by period ic microtwinning. When released from their substrate, as in plan-view speci mens, films of the same thickness are domain-fragmented orthorhombic. It is suggested that the misfit stresses tend to favour a microtwinned rhombohed ral structure, whereas unstressed films of the same thickness, released fro m the substrate, tend to become domain-fragmented orthorhombic. The observa tions illustrate in a striking manner the effect of misfit stresses on the microstructure of thin epitaxial films, explaining also the dependence of t he physical properties on the film thickness and on the misfit.