  The carrier emission efficiency of light emitting diodes is of fundamental
importance for many technological applications, including the performance of
GaN and other semiconductor photocathodes. We have measured the evolution of
the emitted carriers and the associated transient electric field after
femtosecond laser excitation of n-type GaN single crystals. These processes
were studied using subpicosecond, ultrashort, electron pulses and explained by
means of a three-layer analytical model. We find that for pump laser
intensities on the order of 10^11 W/cm^2, the electrons that escaped from the
crystal surface have a charge of about 2.7 pc and a velocity of about 1.8
um/ps. The associated transient electrical field evolves at intervals ranging
from picoseconds to nanoseconds. These results provide a dynamic perspective on
the photoemission properties of semiconductor photocathodes.
