Epitaxial Alkali Antimonide Photocathode Growth and Characterization

Small improvements in cathode performance drive remarkable improvements in FEL x-ray light, towards higher x-ray energies and higher x-ray fluences. Electron beams for future light sources thus must have minimal mean transverse energy MTE). And the single largest contribution to MTE in polycrystalline materials is surface roughness. Therefore, single crystal alkali antimonides will drive MTE to levels lower than ever seen from the antimonide family. Despite all the recent progress, the ultimate goal of a single crystal photocathode with high QE and a truly atomically flat surface remains tantalizingly out of reach. All alkali antimonides to date have been grown in polycrystalline or amorphous form. We believe we have identified a viable step toward that goal which has been missing from all efforts thus far. When seeking to grow alkali antimonides epitaxially, a catch-22 arises. Good epitaxial growth requires excellent surface mobility, which is typically achieved with high substrate temperatures, however the alkali antimonides cannot be grown at tempratures well above the dissociation limit of the crystal well under 200ºC). The antimony atom mobility in particular is severely limited by this constraint. Surface mobility has been highlighted as a key issue in other systems such as Cs on various GaAs crystal faces. We have identified a method of improving surface mobility and reconstruction without raising the substrate temperature, known as ion beam assisted molecular beam epitaxy IBA-MBE). Euclid will deliver a special hyperthermal ion gun for use during growth which will provide the energy needed for antimony mobility, without the limits posed by substrate heating and subsequent dissociation of the antimonide crystal. The substrates will be lattice-matched for the given crystal face after calculation of electronic structure and hybridization of orbitals at the substrate-film interface. Substrates having 10Å or better roughness will be synthesized and prepared by Euclid. Additional substrate selection constraints include UHV compatibility and bakeability, conductivity, and chemical stability. At the end of Phase I, we will quantitatively conclude which lattice-matched substrates and IBA-MBE growth parameters provide the highest QE and the smoothest cathode film, thus resulting in the lowest MTE. The potential market for low MTE cathodes is significant. Such sources are of broad interest to light sources and accelerators, including future THz Dielectric Wakefield Accelerators, Electron Cooling applications for ion accelerators, SRF facilities such as SLAC's LCLS-II and Fermilab's IARC, Ultrafast Electron Diffraction and Microscopy UED/UEM) and RF linac based FELs for industrial applications.