Abstract
Strain effect on the growth mode of high-temperature-superconductor (HTSC) (RBa2Cu3O7-d, R= Y and most rare earth elements) thin films has been investigated thoroughly since the discovery of this material. However the relationship between strain and the precipitation of off-stoichiometric phases has not been fully studied. Through a series of experiments, the author elucidates the interplay between the dislocations induced from the release of strain and the off-stoichiometric phases segregated out from the single crystal grains. Despite of many attempts to optimize the deposition conditions, we have found that there is always a substantial resistive tail below the bulk Tc of EuBa2Cu3O7-d and SmBa2Cu3O7-d films deposited on nearly perfect lattice matched SrTiO3 (STO) substrates. This is in contrast to the same films deposited on YSZ, LaAlO3 (LAO) and NdGaO3 (NGO) substrates under similar conditions. The films on YSZ, LaAlO3 and NdGaO3 substrates have a sharp resistive transition without any tails although the lattice match is poorer. The evolution of the precipitate density and intergranular coupling with increasing film thickness has been investigated for the films deposited on both nearly perfect matched and less matched substrates. It reveals that dislocations induced from the release of strain affect the precipitation of off-stoichiometric phases. We have developed a self-consistent scenario to explain this phenomenon. In this scenario, dislocations attract the off-stoichiometric phases and thus lead to tower-like precipitates. These tower-like precipitates further segregate the potential impurities out from the grain boundaries. Therefore the intergranular coupling of RBCO film deposited on the less lattice matched substrate is getting better with the increasing thickness. On the other hand, in the film grown on the nearly perfect matched substrate, the off-stoichiometric materials homogeneously coat each columnar grain in absence of these tower-like precipitates. A network model of Josephson junctions, which is composed of well-oriented (along all three axes) crystalline grains coated with non-superconducting materials of thickness of a few nm, can explain our experimental observation. This is a novel granular system different from the conventional one where the grains are misoriented with each other. This adverse effect for the transition temperature and intergranular coupling may find application in the fabrication of Josephson junctions.