The attempt of using GaAs (Cs:O) and GaN (Cs) as photocathodes in SRF photoinjector
The photocathodes determine the beam quality in linear accelerators and represent a key component for many accelerator projects. Free-electron lasers (FEL), synchrotron- and THz radiation sources require injector systems with high brightness electron beams.
High quantum efficiency, a long lifetime and good vacuum stability, fast response time and low thermal emittance are desirable parameters for a perfect photocathode used in accelerators. Semiconductors such as GaN and GaAs as novel materials for photocathodes are showing enormous potential.
GaAs is a well-known material for photocathodes. After activation with caesium and oxygen, it has a high QE for visible light (red or green). 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.
At the moment GaN is used for photocathode-based detectors such as photomultipliers or phototubes and for LEDs. They have characteristics of low dark current, high-speed response and high sensitivity. 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. Modern analytical methods are used for identification of impurities, dislocations and characterization of the crystallinity of the semiconductors and the right cleaning treatment as well as the right caesium rating.  Uchiyama, Shoichi et al. 2011. “GaN-Based Photocathodes with Extremely High Quantum Efficiency” 103511(2005):1–4.