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 setu p with a basic pressure below 10(-12) mbar. Activation and degradation are similar for (Cs, O) and (Cs, F) activation. A new type of degrada tion 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 rearrangemen t of oxygen atoms in the activation layer is suggested as an explanati on of these observations.