Elucidation of activation layer model by means of measurements of photoelectron energy distribution curves

The comparative study of (Cs,NF3) and (Cs,O2) activation procedures and the measurements of the vacuum level position versus activation layer thickness show that the activation layer for optimally activated GaAs photocathodes with QE of about 20-30% should be treated as a dipole layer. For thick activation layers, the saturation of the vacuum level at the lowest position is observed. The saturation proves, that the activated surface should be treated as a heterojunction