‘Stable to unstable’ transition in the (Cs, O) activation layer on GaAs (100) surfaces with negative electron affinity in extremely high vacuum

The degradation of GaAs photocathodes with negative electron affinity, activated by (Cs, O) and (Cs, F) layers, was studied in a vacuum setup with a basic pressure below 10−12 mbar. Activation and degradation are similar for (Cs, O) and (Cs, F) activation. A new type of degradation was observed during the operation of a (Cs, O)-activated cathode, showing up as an instability after exposure to oxygen and subsequent reactivation with Cs. The same amounts of oxygen and Cs added to a (Cs, F)-activated surface did not influence its stability. A rearrangement of oxygen atoms in the activation layer is suggested as an explanation of these observations.