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 unintentionally n-doped GaN epilayer at room temperature under high excitation densities (i.e., N ₀ = 1.0× 10¹⁹- 1.1× 10 ²⁰ cm^-3) have been investigated with nondegenerate femtosecond pump-probe (267/400 nm) reflectance (Δ R/R ₀). All Δ R/R ₀ 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/R ₀ initial (first 10 ps) recovery rate γ_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. As N ₀ is further increased, γ_i accelerates due to the onset of Auger recombination as the trap-bottleneck becomes effective. The best fit by the Auger model for N ₀ in the range of the mid-10¹⁹-10 ²⁰ cm^-3 yields an Auger coefficient of C _a?∼ 5.0× 10^-30 cm⁶ s^-1.