HREM STUDY OF THE YBCO MGO INTERFACE ON AN ATOMIC-SCALE

The film-substrate interface of c oriented YBCO thin films grown by sp uttering or laser ablation on (001) MgO substrate has been investigate d with high-resolution electron microscopy. The first atomic plane of the YBCO lattice is a CuO chain layer. Two interface configurations oc cur: (1) the YBCO lattice and the MgO lattice continue up to the inter face (this configuration is occasionally associated with some periodic strain in the MgO lattice; (2) the YBCO lattice and the MgO lattice a re separated by an (almost) amorphous layer with a thickness of the or der of two atomic layers. This amorphous layer is found to lead to the absence of strain. In some cases the surface roughness coincided with misoriented grains but most of the steps in the MgO substrate were ac commodated by steps in the YBCO of one or more complete unit cells in height and some lattice bending in the YBCO film.