Stress-induced spontaneous dewetting of heteroepitaxial YBa2Cu3O7 thin films

Experimental evidence is given for the transformation of atomically flat (001) heteroepitaxial YBa2Cu3O7 films, grown from trifluoroacetate metallorganic precursors, into spontaneously dewetted films. We show that a complete migration from the substrate occurs, even in films with thickness up to 150 nm, as a result of the interfacial stress generated by the lattice mismatch of YBa2Cu3O7 with single crystalline substrates (LaAlO3, SrTiO3) or buffer layers grown on them (BaZrO3). The observed structures have a strong tendency to form parallepipediclike hole or island geometries, which is indicative of the relatively low surface free energy of the {100} planes. The generated microstructures have been found to result from a kinetically limited atomic diffusion process and hence very different micro- or nanostructures are found after long post-processing thermal annealing treatments. TEM analysis has demonstrated that the relief of the elastic energy associated with the interfacial strain of the heterostructure proceeds through the formation of misfit dislocations additionally to the dewetting process. A free energy analysis of the different interfacial structures has established a rationale where it can be understood that below a certain film thickness the discontinuous dewetted film structure is more stable and that this instability is promoted by enhanced interfacial energies associated to higher lattice misfits. This scenario establishes the basis for the generation of ordered YBa2Cu3O7 nanostructures.