The precise and rapid changes in stoichiometry that are possible with ALL-MBE allow growth of 2201/2212 superlattices with alternating molecular units of the two constituents. Each molecular unit is a half-unit-cell of the full crystal structure. A cross-sectional TEM image of a 2201/2212 superlattice grown on SrTiO3 is shown in Figure 2. Superimposed on the micrograph are image simulations for 2201 and 2212 that were produced with the same focus and thickness parameters. Qualitative agreement between the calculated images and the micrograph is quite good. Image simulations with a variety of focus and thickness values convincingly demonstrate that the bright lines bounded with dark regions are the BiO double layers of 2212.
The high crystal quality of the 2201/2212 superlattice is not surprising given the good lattice match between the two phases. What is striking in Figure 2 is the layering of the 2201 and 2212, as is evident from the alternation of their respective 12.3Å and 15.4 Å molecular layer thicknesses. X-ray diffraction studies have previously given evidence for highly ordered growth of 2201/2223 multilayers with alternating half unit cells of the two constituents. The low frequency of incomplete layers or pinhole-type defects supports the previously published interpretation of transport data on 2201/2212 superlattices, which showed that the superconducting transition temperature Tc was not strongly dependent on 2201 layer thickness. The long lateral continuity of the single molecular layers in this image confirms the assertion that the 2212 layers in the superlattices are well isolated from one another by the intervening layers of 2201. The weak dependence of the 2201/2212 multilayer Tc on the thickness of the lower-Tc 2201 phase is therefore strong evidence of the two-dimensional nature of superconductivity in 2212. The two-dimensional character of the superconductivity mirrors the anisotropic nature of the crystal structure.