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GaN (Cs)

In 2014 Hiroshi Amano won the nobel price for blue light emitting diodes (LEDs) based on gallium nitride (GaN). A new idea, besides the usage in LEDs, is to consider GaN as a potenital photocathode in SRF injectors. Only activated with a thin layer of caesium, GaN offers:

- high quantum efficiency (QE) in the UV range

- better stability and a longer lifetime than GaAs or other photocathodes

- negative electron affinity (NEA) surface can be achieved only with cesium

- efficient ability of re-activation by simple vacuum bakeout

P-type (magnesium doped) GaN on different substrates (silicon, sapphire, copper and SiC) can be considered as potential photocathodes. For the first experiments commercially available GaN on sapphire was transferred into a UHV chamber, heat-treated and for the activation only a thin layer of caesium was deposited on the surface. The activation process starts with heating a caesium dispenser up to release caesium atoms which deposit on the GaN surface. Consequently the photocurrent increases depending on the amount of deposited caesium. When reaching a maximum in the photocurrent the caesium depositon is stopped.

Three cycles of reactivation were tested for the same sample with a maximum QE of 11% @ 310 nm UV-light was achieved. The QE decays then over time but even after more than 600 h the GaN:Cs shows 1,5% QE. Additionally the GaN:Cs surface is studied with modern surface analysis devices such as SEM (EDX), AFM and XPS.
More studies are necessary in order to explore the possiblity to apply this material in SRF gun.

GaN:Cs photocathode ©Copyright: Jana Schaber

left: GaN photocurrent curves during activation process; middle: SEM Image of used GaN:Cs photocathode; right: QE decay curves for each activation cycle

Foto: Jana Schaber

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Project:

BMBF-Verbundprojekt 05K2016: HOPE-II

Reference:

Jana Schaber, The attempt of using GaN as a photocathode in SRF Gun II, presented in the 5th MT Meeting, Jena.

R. Xiang, et al., Research Activities on Photocathodes for HZDR SRF Gun, Proceedings of IPAC 2012