THE EFFECT OF SPUTTER-DEPOSITION CONDITIONS ON THE GROWTH-MECHANISM OF YBA2CU3O7-DELTA THIN-FILMS STUDIED BY SCANNING-TUNNELING-MICROSCOPY

Surface topography of as-grown films of YBa2Cu3O7-delta (YBCO) deposit ed by off-axis RF magnetron sputtering on MgO and SrTiO3 single-crysta l (100) substrates, has been investigated by scanning tunnelling micro scopy (STM)/scanning tunnelling spectroscopy (STS) operated in air at ambient temperature. Both c-axis- and a-axis-oriented YBCO films have been investigated. In the case of c axis thin films, we have directly observed spiral-shaped growth terraces, which emanate from screw dislo cations of this layered superconducting oxide. The growth steps are ge nerally seen to possess a step height close to or multiples of the uni t cell height of the YBCO crystalline structure. In the case of a axis growth, the substrate as well as the deposition temperature determine s the grain orientation. Furthermore, our STS data revealed that the s urface layer of the film is semiconducting and the tunnelling spectrum varies its shape depending on the tip-to-sample distance. A supercond ucting gap appears clearly in the spectra when the STM tip is placed c loser to the surface than the normal position of the scanning mode. Th is suggests that the semiconducting layer is confined in the topmost s urface region of the as-prepared film, while the layer beneath it is s uperconducting in character.