Epitaxial growth of cesium potassium antimonide photocathode

The performance of electron accelerators and photon detectors heavily depends on the efficiency and stability of the photocathode. The emitted beam current and brightness can be limited by surface and bulk disorder in polycrystalline photocathode materials. Epitaxial growth offers a promising route to mitigate these limitations and enable high-current, high-brightness electron beams. In this work, we present the first demonstration of the epitaxial growth of a bi-alkali antimonide photocathode, K2CsSb, utilizing both pulsed laser deposition-assisted and thermal evaporation techniques. In our study, the growth of the epitaxial layer with a flat surface was confirmed by reflection high-energy electron diffraction for the K2CsSb thin film. In situ and real-time x-ray characterization were utilized to confirm the film stoichiometry, surface roughness, and crystallinity. Quantum efficiency (QE) of 3% at 530 nm and 9% at 450 nm was obtained from an epitaxial K2CsSb thin film with a film thickness as thin as 6 nm with an ultrasmooth surface of 0.4 nm roughness on a Gr/4H–SiC substrate. Increasing the thickness to 20 nm enhanced the QE to 4.5% and 14.5% at 530 and 450 nm, respectively, with the surface roughness remaining under 1 nm. In a follow-up experiment, QE of over 9% (530 nm) and 21% (450 nm) were achieved from an epitaxial K2CsSb thin film grown on a 4H–SiC single-crystal substrate.