Epitaxial Growth of Sodium Potassium Antimonide Photocathodes for Electron Cooling at Brookhaven National Laboratory

The success of electron cooling for the Electron-Ion Collider (EIC) relies on the development of high-performance photocathodes (PCs) for photoinjectors. Ideal PC exhibits high quantum efficiency (QE), low emittance, long-lifetime etc. Alkali antimonides are among the most promising candidates, offering high QE and low emittance. In particular, Na-K-Sb photocathodes exhibit superior thermal stability and long-term QE retention under high-power laser illumination. However, surface disorder in polycrystalline films often limits beam brightness. Epitaxial growth provides a viable route to overcome this limitation. We report the epitaxial synthesis of Na-K-Sb thin films on single-crystal substrates. Reflection high-energy electron diffraction (RHEED) reveals streaky patterns and azimuthal angular dependence, confirming smooth epitaxial growth with well-defined in-plane ordering. X-ray reflectivity (XRR) verifies the formation of smooth, uniform films, while X-ray diffraction (XRD) and fluorescence (XRF) confirm crystallinity and stoichiometry. QE measurements demonstrate promising performance. These results establish epitaxial Na-K-Sb as a strong candidate for next-generation high-current, high-brightness electron sources, supporting the advancement of electron cooling systems for the EIC.