  Future colliders such as NLC and JLC will require a highly-polarized
macropulse with charge that is more than an order of magnitude beyond that
which could be produced for the SLC. The maximum charge from the SLC
uniformly-doped GaAs photocathode was limited by the surface charge limit
(SCL). The SCL effect can be overcome by using an extremely high (>1019 cm-3)
surface dopant concentration. When combined with a medium dopant concentration
in the majority of the active layer (to avoid depolarization), the surface
concentration has been found to degrade during normal heat cleaning (1 hour at
600 C). The Be dopant as typically used in an MBE-grown superlattice cathode is
especially susceptible to this effect compared to Zn or C dopant. Some relief
can be found by lowering the cleaning temperature, but the long-term general
solution appears to be atomic hydrogen cleaning.
