Electron microscopy and spectroscopy for characterization of surface and film properties of high temperature superconductors

Scanning electron microscopy (SEM), Auger electron spectroscopy (AES), transmission electron microscopy (TEM), and x-ray photoelectron spectroscopy (XPS) are providing a detailed understanding of the oxide superconductors. The SEM has revealed the causes of surface morphologies such as excessive roughness, graininess, and cracking, as well as the mechanisms of particle deposition and the charges on these particles; at low temperatures, it has revealed the reasons for the non-linear superconductive-resistive transitions in current carrying films. AES has been used to study interfacial layers and multi-layer superstructures, such as the atomically abrupt junctions between differently substituted Y-Ba-Cu-O hetero-epitaxial layers. TEM has revealed the defect structure of “epitaxial” films, as well as the nature of the atomic accommodation at film-substrate interfaces, and the surprising complexities of the epitaxial growth processes. TEM measurements of the correct microstructure and local stoichiometry of Bi-Sr-Ca-Cu-O phases have focused attention on the doping mechanisms in these apparently “valence balanced” superconductors, that may be different from the majority of oxide superconductors that are doped by the conventional valence imbalance mechanism. XPS measurements of non-superconducting surface layers on superconducting films can detect layers only 0.1 nm thick and laser deposited Y-Ba-Cu-O films can have surface layers only 1 nm thick after exposure to air. XPS has been used to show that extremely thin superconducting films (< 10 nm thick), having a negligible area of pin-holes, can be made, demonstrating that laser deposition can produce extremely thin films of high integrity. The valencies of all the elements in all major families of oxide superconductors have been measured, which leads to a better understanding of the doping mechanism, the electronic structure, and the nature of the carriers responsible for superconductivity. © 1990 SPIE.