UC Irvine

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 sub-picosecond, ultrashort, electron pulses and explained by means of a "three-layer" analytical model. We find that for pump laser intensities on the order of 1011 W/cm2, the electrons that escaped from the crystal surface have a charge of ∼2.7 pC and a velocity of ∼1.8 μm/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.