The Optics Laboratory

Surface Oxygen Profiles in Aged YBCO Samples

Before undertaking electromigration studies, we characterized the as-grown samples. The NSOM-based technique measures the concentration of near-surface oxygen because of the surface sensitivity of the NSOM method. For the films we use, the superconducting transition is sharp, indicating that oxygen concentration in the bulk o the film is uniform. We find that there are significant variations in the surface oxygen content, however:

On the left is the topography, and on the right is the optical image, which is related to the oxygen content (bright is higher oxygen). The x&y scales are in nanometers, and the z-scales in nanometers for the topography and arbitrary units on the optical data. Some correlations between the two can be seen. In particular, the oxygen content is lower when the topography is high, and the oxygen content does not vary sharply except at grain edges.

This can be seen more clearly by overlaying a contour map of the oxygen content on a color scale of the topography:

These observations suggest that the diffusion within the grains is fast, but that the oxygen does not readily diffuse through a grain boundary. This statement should not be confused with diffusion along a grain boundary, which we do not measure here, but has been found to be rapid in other studies. The fact that grains sticking above their neighbors have low surface oxygen content implies that not only is motion within the ab-plane fast in the grain, but also out of a grain into the air.

The last statement has implications for a new growth method for YBCO films that produces very flat surfaces, but at the cost of tilting the ab-planes:

These samples may have trouble retaining their oxygen in storage at room temperature.