Sputter Growth of Alkali Antimonide Photocathodes: an in Operando Materials Analysis*

Alkali antimonide photocathodes are a strong contender for the cathode of choice for next-generation photon sources such as LCLS II or the XFEL. These materials have already found extensive use in photodetectors and image intensifiers. However, only recently have modern synchrotron techniques enabled a systematic study of the formation chemistry of these materials. Such analysis has led to the understanding that these materials are inherently rough when grown through traditional sequential deposition; this roughness has a detrimental impact on the intrinsic emittance of the emitted beam. Sputter deposition may provide a path to achieving a far smoother photocathode, while maintaining reasonable quantum efficiency. We report on the creation and vacuum transport of a K2CsSb sputter target, and its use to create an ultra-smooth (sub nm roughness) cathode with a 2% quantum efficiency at 532 nm. ROUGHNESS AND EMITTANCE The Alkali Antimonides are a class of materials (I3V semiconductors) often used as photocathodes, both in detectors [1] and accelerators [2]. Many of these compounds are capable of achieving a quantum efficiency (QE) of several percent for green light, making them attractive for high average current accelerator applications. The typical sequential deposition process used to form these materials results in a cathode which is very rough (25 nm RMS for a 50 nm thick cathode, with a 100 nm spatial period) [3]. This results in an undesirable field dependence of the intrinsic emittance [4,5]; an emission field of 6 MV/m is sufficient to double the intrinsic emittance of a K2CsSb cathode illuminated with green light (the expected value should be 0.5 μm/mm [6]). Assuming a simple sinusoidal modulation of the surface height, the field-induced emittance growth can be shown to be given by Eq. (1)