14 - Controlling the structure and properties of high T_c thin-film devices

Summary

Introduction

The highly anisotropic crystal structures of the layered high T_c superconducting cuprates induce the necessity of using epitaxial films in most devices based on high T_c thin films. Each individual application demands a specific crystallographic orientation of the film as well as a certain combination of substrate, high T_c superconducting film and non-superconducting layer materials. The intention of this chapter is to provide examples of what aspects need to be considered when designing the device and predicting its behavior. The behavior depends on the detailed microstructure of the thin films. The examples are therefore discussed in terms of microstructure and how it can be controlled and manipulated.

As in all epitaxial structures, the interfacial interactions are crucial for the resulting microstructures. The direct interaction between the substrate and a single-layer high T_c thin film is illustrated in the following section. The description is followed by a discussion of different aspects of the use of buffer layers. The text is restricted to high T_c YBa₂Cu₃O_7–x (YBCO), mainly owing to the fact that the vast majority of published data concern this superconductor. However, there are common characteristics of epitaxial film growth between different high T_c superconductors. Results from the YBCO films can thus be used when considering the other high T_c superconducting thin films which is pointed out in Section 14.10.

The mechanical interaction between the different epitaxial layers may result in the formation of misfit dislocations. Nucleation and propagation of cracks can ensue if the mismatch in thermal expansion coefficient is relatively large.