The attempt of using (Cs:O)GaAs and (Cs)GaN as photocathodes in SRF photoinjectors

The photocathodes determine the beam quality in linear accelerators and represent a key component for many accelerator projects. High quantum efficiency, a long lifetime and good vacuum stability, fast response time and low thermal emittance are desirable parameters for photocathodes used in accelerators. Semiconductors such as GaN and GaAs as novel materials are showing an enormous potential for an application as photocathodes. GaAs is a well-known material for photocathodes. After activation with caesium and oxygen, it has a high QE for visible light. An advantage of GaAs is the opportunity of the layers to emit spin-polarized electrons. GaN is a semi-conductive material and well known for its high QE when lighted with UV light. For improving the QE only caesium for activation is required. It is very new for application in SRF Guns. It seems to be more robust and achieves higher QE than other photocathodes. Crystallinity and surface parameters define the photoemission properties. For identification impurities, dislocations and characterization of the crystallinity but also finding the perfect cleaning process and caesium rating modern analytical methods are used.