  The properties of photoemission electron sources determine the ultimate
performance of a wide class of electron accelerators and photon detectors. To
date, all high-efficiency visible-light photocathode materials are either
polycrystalline or exhibit intrinsic surface disorder, both of which limit
emitted electron beam brightness. In this letter we demonstrate the synthesis
of epitaxial thin films of Cs$_3$Sb on 3C-SiC (001) using molecular-beam
epitaxy. Films as thin as 4 nm have quantum efficiencies exceeding 2\% at 532
nm. We also find that epitaxial films have an order of magnitude larger quantum
efficiency at 650 nm than comparable polycrystalline films on Si. Additionally,
these films permit angle-resolved photoemission spectroscopy measurements of
the electronic structure, which are found to be in good agreement with theory.
Epitaxial films open the door to dramatic brightness enhancements via increased
efficiency near threshold, reduced surface disorder, and the possibility of
engineering new photoemission functionality at the level of single atomic
layers.
