Pii: S0968-4328(99)00045-1

A scanning transmission electron microscope (STEM) is used to examine the structural and chemical quality of the interface between thin films of YBa2Cu3O72d (YBCO) and BaF2. The STEM images indicate that it is possible to grow a uniform thin film of BaF2 that is relatively free of gross defects via a thermal evaporation method on top of a YBCO thin film. The diffraction contrast results also suggest that a threedimensional or “island” growth mechanism is involved in BaF2 films grown by this method. An oxygen map across the YBCO/BaF2 interface produced using core loss energy filtered imaging is used to address a concern regarding oxygen diffusion from YBCO into BaF2. It was found that a good quality BaF2 film can play a significant role in reducing outdiffusion of oxygen from YBCO. Quantitative evaluation of the specimen oxygenation suggests that beam spreading effects may be important. The effect of an ultra-high vacuum (UHV) STEM environment on the specimen oxygenation is also considered. q 1999 Published by Elsevier Science Ltd. All rights reserved.