Deep bandtail states picosecond intensity-dependent carrier dynamics of GaN epilayer under high excitation.

Picosecond carrier dynamics of deep bandtail states (3.1 eV) in an unintentionallyn-doped GaN epilayer at room temperature under high excitation densities (i.e.,N0 = 1.0× 1019- 1.1× 1020 cm-3) have been investigated with nondegenerate femtosecond pump-probe (267/400 nm) reflectance (?R/R0). All ?R/R0 traces possess a ~2 ps buildup time that represents an overall time for the initial non-thermal carrier population to relax towards the continuum extremes and then into the probed tail states. We observe a saturation of ?R/R0 initial (first 10 ps) recovery rate ?i at a density of 5- 6×1019 cm-3 close to the Mott transition threshold obtained from time-integrated PL measurements. Such a saturation phenomenon has been identified as the trap-bottleneck due to the bandtail states and deep traps. AsN0 is further increased, ?i accelerates due to the onset of Auger recombination as the trap-bottleneck becomes effective. The best fit by the Auger model forN0 in the range of the mid-1019-1020 cm-3 yields an Auger coefficient ofCa~ 5.0× 10-30 cm6 s-1. [ABSTRACT FROM AUTHOR]