Introduction

Thin films of the high-temperature superconductor, YBa2Cu3O7, may be synthesized as highly oriented structures having a high degree of homogeneity as compared with bulk processed materials. These thin films exhibit some of the best transport properties of high-temperature superconductors (HTSC), particularly the highest superconducting critical currents as a function of temperature and magnetic field. The microstructure of films of different orientation is composed of arrays of specific crystallographic defects which may include, for example, low-angle grain boundaries, (110) twin boundaries, 90° domain boundaries, other misoriented regions or grains, stacking faults, antiphase boundaries, second phases, etc. The surface and interface structure may vary considerably, a smooth surface being essential for device fabrication. Thin-film synthesis also provides methods for fabricating or isolating localized crystallographic interfaces, such as grain boundaries, for transport studies. Therefore, in addition to their importance for technological applications, HTSC thin films may also serve as model systems for studying fundamental behaviors of these materials. In understanding properties it is essential to characterize the microstructure in as much detail as possible. In this chapter we will discuss the microstructure of YBa2Cu3O7 (YBCO) thin films as characterized by transmission electron microscopy (TEM), with emphasis on the interface structure of films having a-axis and (103) orientation, and of individually fabricated boundaries synthesized using a-axis oriented films.

Grain boundaries

YBa2Cu3O7 (YBCO) thin films are grown in situ by a variety of vapor phase deposition techniques usually with either the c-axis or a(b)-axis normal to the plane of the film [12.1, 12.2]. We will refer to the films by their normal orientation.