Ultrafast carrier dynamics in a GaN/Al0.18Ga0.82N superlattice

Relaxation processes of photoexcited carriers in a $\mathrm{GaN}/{\mathrm{Al}}_{0.18}{\mathrm{Ga}}_{0.82}\mathrm{N}$ superlattice are studied in femtosecond spectrally resolved reflectivity measurements at ambient temperature. The transient reflectivity reveals electron trapping into defect states close to the conduction-band minimum with a 150--200 fs time constant, followed by few-picosecond carrier cooling. A second slower trapping process into a different manifold of defect states is observed on a time scale of approximately 10 ps. Our results establish the prominent role of structural defects and disorder for ultrafast carrier dynamics in nitride semiconductor structures.