Surface morphology of high-temperature superconductor thin films using scanning tunnelling microscopy

Abstract

The surface morphology of laser-deposited superconducting YBa₂Cu₃O_7−x thin films has been studied using scanning tunnelling microscopy (STM). Very high resolution of the surface morphology has been obtained showing well-textured surfaces with very distinct growth patterns. The thin films produced byin situ laser deposition are highly oriented with typicalJ _c values > 1× 10⁶A cm⁻² at 77 K. The morphology of these surfaces as revealed by STM is a terraced, layer-like structure with a discrete rise of each terrace level approximately equal to thec-axis lattice spacing. These terraces are stacked upon each other in an ordered fashion. These terraces can also be viewed as a series of chip-like morphologies which increase in size at the lower levels. The surface structure (amount of texturing and size of terraces near the surface) shows a small but observable dependence upon the deposition conditions, with lower deposition rates producing larger grain sizes. We have also studied the chemical etching of these surfaces and the resultant morphologies have been followed using atomic force microscopy. Upon chemical etching the individual grains are revealed and due to the epitaxial growth of these films, most likely represent mutually aligned but separate grains. These etching studies provide valuable information about substructures. A model for film growth based upon nucleation and growth will be described.