Recent Progress at SLAC Extracting High Charge from Highly-Polarized   Photocathodes for Future-Collider Applications
  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. 
J.E. Clendenin (1), A. Brachmann (1), E.L. Garwin (1), S. Harvey (1),   J. Jiang (1), R.E. Kirby (1), D.-A. Luh (1), T. Maru...