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Influence of surface carbon on the performance of cesiated p-GaN photocathodes with high quantum efficiency

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


This study shows residual surface-C’s influence on photocathodes’ quantum efficiency based on p-GaN grown on sapphire by metal organic chemical vapor deposition. An X-ray photoelectron spectrometer (XPS) built in an ultrahigh vacuum system allowed the in-situ monitoring of the photocathode surface beginning immediately after their cleaning and throughout the activation and degradation processes. An atomically clean surface is necessary to achieve a negative electron affinity, which is the main prerequisite for high quantum efficiency. The p-GaN samples were cleaned with ethanol and underwent a sub-sequential thermal vacuum cleaning. Although carbon and oxygen contaminations, undesired impurities from the metal organic chemical vapor deposition, remained on the surface, p-GaN could still form a negative electron affinity surface when exclusively activated with cesium. After the activation with cesium, a shift to a higher binding energy of the photoemission peaks was observed, and a new species, a so-called cesium carbide, was formed, growing over time. The XPS data allowed elucidating the critical role of these cesium carbide species in photocathode degradation.
The X-ray damage to the p-GaN:Cs photocathodes, especially the influence on the cesium, was additionally discussed.

Keywords: p-GaN; negative electron affinity surface; surface cleaning; cesium carbide; XPS; quantum efficiency

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