ANOMALOUS MISFIT STRAIN RELAXATION IN ULTRATHIN YBA2CU3O7-DELTA EPITAXIAL-FILMS

Ultrathin YBa2Cu3O7-delta epitaxial films were successfully grown in situ on (001) SrTiO3 and MgO substrates by means of ozone-incorporating activated reactive evaporation. The x-ray-diffraction study was carefully examined to determine the structural properties of the grown films. Excellent crystallinity with no interfacial disorders was revealed by the appearance of the Laue oscillations. It was found that in a well lattice-matched YBa2Cu3O7-delta/SrTiO3 system, the crystallinity was deteriorated due to defect introduction at the critical layer thickness h(c) (approximately 130 angstrom). Interestingly, also in a poorly lattice-matched YBa2Cu3O7-delta/MgO system, excellent crystallinity was revealed even at above h(c) (< 24 angstrom). This implies that an anomalous misfit relaxation process exists in the YBa2Cu3O7-delta/MgO system. In such a system, no crystal imperfection of the MgO substrate caused by defect introduction was elucidated by the grazing incidence x-ray scattering, which indicated that the MgO substrate did not contribute to the anomalous misfit relaxation. The anomalous growth manner was also found in YBa2Cu3O7-delta/MgO according to surface morphology investigations. Below 40 angstrom (> h(c)), island nucleation growth was found. Above 40 angstrom, it was observed that an atomically smooth surface was obtained and the crystallinity was simultaneously improved. It is suggested that YBa2Cu3O7-delta possesses an anomalous misfit relaxation mechanism, and that especially in the growth on MgO, it couples with the characteristic growth behavior at the initial stage.