The morphology of YBa₂Cu₃O_7-α thin films investigated by STM

Scanning tunnelling microscopy (STM) was employed to study the surface of c-axis oriented YBCO thin films. The films with a thickness of typically about 300 nm were grown onto single crystal (100) MgO by pulsed laser deposition. In addition to the well known growth spirals, "frozen in" clusters were observed. The latter result in a high density of holes with sizes ranging from a few to about 50 nm. These holes may serve as pinning sites. Sub-unit cell steps were found in regions between neighboring growth spirals. In samples exposed to air for several weeks, a change in surface morphology was found. Samples that were patterned by wet-chemical etching did not show the characteristic growth steps. Irradiation with fast neutrons slightly increased the density of nm sized holes.