IN-SITU GROWTH-STUDIES OF SPUTTERED YBCO THIN-FILMS BY SPECTROSCOPIC ELLIPSOMETRY

Using spectroscopic ellipsometry we studied in-situ the growth of off- axis sputtered YBa2Cu3O6+x thin films on (001) SrTiO3 as a function of the deposition parameters. Especially in the very first growth stage (<5 nm) we observed that the optical properties of the grown layer dif fers from the ''bulk'' optical properties of YBCO and strongly depends on, both, the deposition temperature and the oxygen partial pressure. Both properties are well established as influencing the superconducti ng properties of thin YBCO films. YBCO thin film growth under optimal deposition conditions (T-c similar to 90 K; j(c)>10(6) A cm(-2) @ 77 K ) is smooth and homogeneous, except for the first unit cell layer (ini tial stage regime). The smoothness of the response is indicative for a step-mode like growth mechanism. In contrast, the initial stage regim e is governed by a 2D nucleation mechanism. This behaviour changes whe n the deposition temperature is lowered. Due to increased disorder, th e initial stage regime is extended to larger thicknesses and a true 2D growth mode is no longer apparent. Similar behaviour is observed with increasing oxygen partial pressure, where the optical response is shi fted from a step-flow mode like mechanism to an island-growth mode.