Surface morphology of YBa2Cu3O7-δ thin films on SrTiO3(100) studied by scanning tunneling microscopy

Thin films of YBa2Cu3O7-δ (YBCO) have been prepared by pulsed laser deposition on single-crystalline SrTiO3(100) substrates at different substrate temperatures Ts. Depending on Ts, different orientations of the films are revealed by X-ray diffraction: (001) orientation between 660 to 800°C, (100) orientation between 610 and 660°C, and no YBCO reflections for films prepared at lower Ts. In-plane orientation (Φ-scan) and out-of-plane orientation data (mosaic spread) of the grains are compared to scanning tunneling microscopy (STM) images. The images show rectangular-shaped growth hills. Their growth steps are aligned along the [100] and [010] directions of YBCO and are one c-axis distance in height (1.2 nm). Occasionally screw dislocations are observed. Highest critical transition temperatures are reached at about Ts = 750°C. The relationship between film thickness and surface morphology has been studied using YBCO films with a thickness gradient prepared by a half-shadow technique. Thus, films of different thicknesses can be obtained simultaneously under identical conditions. STM images give evidence of different growth stages such as nucleation and coalescence of two-dimensional islands, their growth and, finally, the formation of growth hills.