Influence of Surface Cleaning on Quantum Efficiency, Lifetime and Surface Morphology of p-GaN:Cs Photocathodes

Abstract

Accelerator scientists have high demands on photocathodes possessing high quantum efficiency (QE) and long operational lifetime. p-GaN, as a new photocathode type, has recently gained more and more interest because of its ability to form a negative electron affinity (NEA) surface. Being activated with a thin layer of cesium, p-GaN:Cs photocathodes promise higher QE and better stability than the known photocathodes. In our study, p-GaN samples grown on sapphire or silicon were wet chemically cleaned and transferred into an ultra-high vacuum (UHV) chamber, where they underwent a subsequent thermal cleaning. The cleaned p-GaN samples were activated with cesium to obtain p-GaN:Cs photocathodes, and their performance was monitored with respect to their quality, especially their QE and storage lifetime. The surface topography and morphology were examined by atomic force microscopy (AFM) and scanning electron microscopy (SEM) in combination with energy dispersive X-ray (EDX) spectroscopy. We have shown that p-GaN could be efficiently reactivated with cesium several times. This paper systematically compares the influence of wet chemical cleaning as well as thermal cleaning at various temperatures on the QE, storage lifetime and surface morphology of p-GaN. As expected, the cleaning strongly influences the cathodes’ quality. We show that high QE and long storage lifetime are achievable at lower cleaning temperatures in our UHV chamber.

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

“…The QE dropped significantly during the hours following activation, especially after the sample was analyzed by XPS. Typically, the QE decays exponentially for p-GaN:Cs photocathodes, as we indicated in another publication 12 . The QE stabilized when the sample was returned to the UHV preparation chamber.…”

“…After the new thermal cleaning, the photocurrent was partially restored even before the subsequent Cs activation was conducted. In a previous publication, we noted that this phenomenon occurred during reactivation, but it could only occur if Cs remained on the p-GaN's surface 12 . As shown in Fig.…”

“…Recently, p-GaN has become increasingly popular as a potential electron source because it can be more robust than other semiconductor photocathodes. Further advantages of p-GaN are its simple activation and its possible rejuvenation because the required NEA surface can be achieved by activating commercially available p-doped GaN solely with Cs 12 . Indeed, p-GaN does not require molecular oxygen (O 2 ) for its activation, making the activation process less complex than that of gallium arsenide (GaAs) photocathodes 13 .…”

“…Therefore, we assumed that the C was located at the surface and the O species might was located in the near sub-surface of p-GaN because the Ar + were able to remove those contaminants. However, temperatures above 500 °C and Ar + irradiation were not considered for the following experiments because these treatments resulted in significant surface damage of p-GaN 12,24,25 .…”

Review of photocathodes for electron beam sources in particle accelerators

“…Therefore, O 2 has no obvious benefit and thus is not necessary to obtain a NEA surface for GaN semiconductors. 163 In fact, this means that the activation for GaN with only Cs is more practicable and easier, compared to other photocathodes, such as GaAs or Cs 2 Te. reported on optimal temperatures between 150-300 1C that led to the desorption of surface molecules such as CO and CO 2 .…”