Introduction

Bi₂Sr₂CaCu₂O₈, called 2212, is the prototypical compound of a class of layered copper-oxide superconductors which have been studied extensively due to their high superconducting critical temperatures T_c. Other known compounds in the BiSrCaCuO family, with the general formula Bi₂Sr₂Ca_n-1Cu_nO _2n+4, include Bi₂Sr₂CuO₆, called 2201, and Bi₂Sr₂Ca₂Cu₃O ₁₀, called 2223. Like other cuprate superconductors, 2212 is a highly anisotropic layered compound, with lattice parameters a = 3.818Å and c = 30.66Å.[1,2] Despite the complexity of the large unit cell, layer-by-layer growth of films has been achieved, as has been demonstrated by the observation of oscillations in the intensity of reflection high-energy electron diffraction (RHEED) features during deposition.[3] Many cubic materials, notably GaAs, Si and various transition metals, can also be grown in a layer-by-layer fashion, so in itself the occurrence of RHEED oscillations is not remarkable. The unusual aspect of the 2212 oscillations is that each molecular unit is composed of 14 layers so that the cyclical growth pattern involves the formation of ordered layers within the unit cell as well as the accumulation of completed unit cells. Putting 2212 films in a multilayer along with other BiSrCaCuO compounds adds a further level of complexity. Because of the three different levels of ordering and the intrinsically strong anisotropy, the dynamics of growth in a multilayer made up of 2212 and its analogs can therefore be expected to be quite different from the case of semiconductor or metallic multilayers.

Cross-sectional TEM is a useful tool for studying the microstructure of multilayers and for classifying stacking faults in layered structures. Here TEM images have been used to study the morphology of single films and heterostructures of BiSrCaCuO compounds which have been grown using ALL-MBE. This is a recently developed technique involving sequential deposition of materials from metal vapor sources (thermal effusion cells) in a highly reactive ozone atmosphere.[4] The ALL-MBE technique has made possible growth of high-quality multilayers and thereby fabrication of Josephson junctions with novel tunnel barriers.[4,5,6] Described below are TEM observations of a sampling of the heterostructures which can be grown by ALL-MBE, including 2212/2201 multilayers and heterostructures with 2278 barrier layers. In addition, the occurence of planar and line defects in 2212 and 2201 will be described and discussed in relation to the unusual growth modes of these materials. The TEM micrographs will be interpreted in comparison to image simulations produced by a sophisticated electron-ray-tracing software package.