Photoemission study of Cs-NF3 activated GaAs(100) negative electron affinity photocathodes

GaAs based negative electron affinity photocathodes activated with Cs and NF3 are used as polarized electron sources for linear accelerators. It is generally believed that the activation layer consists of CsF. The activation layers of Cs-NF3 on GaAs photocathodes are herein investigated using synchrotron radiation photoelectron spectroscopy. F1s, N1s, and other core levels are recorded at photon energies ranging from 70 to 820 eV. Surprisingly, a significant amount of nitrogen is observed in the activation layers. Two distinct species of nitrogen are observed, one of which decreases along with the fluorine signal as the yield of the photocathode decays with time.