THE MORPHOLOGY OF YBA2CU3O7-DELTA THIN-FILMS INVESTIGATED BY STM

Scanning tunneling microscopy (STM) was employed to study the surface of c-axis oriented YBCO thin films. The films with a thickness of typi cally about 300 nm were grown onto single crystal (100) MgO by pulsed laser deposition. In addition to the well known growth spirals, ''froz en 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 s erve as pinning sites. Sub-unit cell steps were found in regions betwe en neighboring growth spirals. In samples exposed to air for several w eeks, a change in surface morphology was found. Samples that were patt erned by wet-chemical etching did not show the characteristic growth s teps. Irradiation with fast neutrons slightly increased the density of nm sized holes.