  X-ray photoemission spectroscopy has been used to investigate the depth
dependent crystal structures and chemical compositions of sequentially
chemical-etched YBa2Cu3O7-d (YBCO) ultrathin film superconductors. In the
near-interface region the crystal structure is severely oxygen deficient and of
tetragonal symmetry. We consider this a revelation of retarded oxygen diffusion
into the O(1) sites during the post-deposition annealing in the presence of
interface mismatch strain. Near the free surface, the oxygen-deficiency was
much alleviated because of the partial strain relaxation and the crystal
symmetry becomes orthorhombic. Compared with as-deposited films of equivalent
thickness, which is less oxygen deficient and shows superconducting transition,
the stripped-down near-interface layer exhibits no sign of superconductivity.
