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

Abstract

Picosecond carrier dynamics of deep bandtail states (3.1 eV) in an unintentionally n-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 (Δ R/R0). All Δ 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 Δ R/R0 initial (first 10 ps) recovery rate γi 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. As N0 is further increased, γi accelerates due to the onset of Auger recombination as the trap-bottleneck becomes effective. The best fit by the Auger model for N0 in the range of the mid-1019–1020 cm−3 yields an Auger coefficient of Ca∼ 5.0× 10−30 cm6 s−1.