Diffusion of zirconium on the surface of Schottky electron sources

Abstract

The current study investigates the diffusion of a Zr–O complex on a W substrate, which is one of the surface components of the Schottky electron source. To enable the thermal diffusion of Zr atoms with the assistance of oxygen, the source was heated to approximately 1800 K; this enabled the Zr–O complex to reach the electron emission area at the source apex, which was several hundred micrometers away from the Zr reservoir. At such a high temperature, the evaporation rate of surface components also increased, making it more difficult for the atoms to diffuse farther. To better understand the mechanisms underlying diffusion and evaporation, the amount of Zr on the surface was estimated using Auger electron spectroscopy. The surface coverage of the Zr–O complex was found to be almost constant up to 600 μm without any significant loss. Furthermore, the diffusion of the Zr–O complex was found to be more dominant than evaporation, resulting in coverage saturation. However, when coverage of the Zr–O complex decreased, the observed trends changed drastically, which suggests that the balance of diffusion and evaporation cannot be described by a simple material balance equation. Additionally, the range with a fully covered area of the Zr–O complex became narrower at temperatures higher than 1900 K. These features, however, varied depending on the vacuum level surrounding the sample. Based on these findings, the diffusion and evaporation model of Zr and O on the surface of the Schottky electron source is proposed.