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The crystalline and carrier trapping properties of 1.55 μm emitting beryllium doped GaInAs/AlInAs multiple quantum wells, grown by MBE at low temperatures, were investigated with respect to their application to ultrahigh-speed optical switching devices. As-grown and in-situ annealed materials were compared. While the former material shows only limited substitutional incorporation of Be acceptors due to excess As incorporation blocking Ga-sites, annealed material shows cluster formation of excess As. Femtosecond pump-probe experiments indicate a nonlinear transmission change, which decays due to carrier trapping with a time constant of 230 fs. Experiments with pairs of ultrashort pulses separated by 1.5 ps demonstrate the capability of fast modulation of transmission associated with very small accumulation effects.