Study of surface cleaning and Cs-activation on GaN photocathodes


Study of surface cleaning and Cs-activation on GaN photocathodes

Schaber, J.; Xiang, R.; Teichert, J.; Arnold, A.; Murcek, P.; Zwartek, P.; Ma, S.

Free-electron lasers (FEL), synchrotron- and THz radiation sources require injector systems with high brightness electron beams. With such a high intensity and short wavelengths, amorphous materials and chemical reaction steps nowadays could be studied.
GaN is a semi-conductive material that is 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 only 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 [1].
It has also a wide wavelength range from 100 to 380 nm.
The equilibrium phase of GaN is wutzite, which means gallium atoms are tetrahedrically surrounded by nitride atoms in a hexagonal closed crystal structure. The tetrahedrons build alternating bilayers of Ga and N in c-direction.
Doping elements for n-type is silicon (Si) and for p-type magnesium (Mg). Mostly p-doped GaN promises better conditions because magnesium atoms increase the minority carrier diffusion length (about 200 nm). MOVPE is the most used technique to produce p-type GaN. Low temperatures are required in comparison to undoped or n-type GaN. Afterwards an annealing process is necessary to remove magnesium-bonded hydrogen. In p-type GaN electron are the minority carriers whereas holes are the majority carriers. The doping is assumed to lower the band bending around the surface. Therefore, the vacuum level is shifted to lower energy than the conductive band minimum in the flat band region.
Activated with a thin alkali metal layer, like caesium, GaN has the ability to lower the surface work function to produce a negative electron affinity (NEA). This effect originates from the surface band bending. Electrons excite over the band gap and can easily enter into the vacuum.
Generally, the stability has also a great influence on the potential application in high brightness guns. GaN shows the promise of more significant stability and robustness against vacuum contaminations than alternate photocathodes.
Like caesium telluride cathode it is possible to recover GaN(Cs) about 50% of the original QE with a simple bake out of 200°C and doing a Cs-reactivation to recover the degraded
cathode [2].
[1] Uchiyama, Shoichi et al. 2011. “GaN-Based Photocathodes with Extremely High Quantum Efficiency GaN-Based Photocathodes with Extremely High Quantum Efficiency.” 103511(2005):1–4.
[2] Siegmund, O. et al. 2006. “Development of GaN Photocathodes for UV Detectors.” 567:89–92.

Keywords: photocathode; GaN; NEA activation

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