Layer thickness dependent carrier recombination rate in HVPE GaN

We report on nonequilibrium carrier dynamics in a set of hydride vapor phase epitaxy (HVPE) GaN wafers of different thickness (11, 17, 41, 90, and 145 μm) grown on the (0001) c-plane sapphire substrates. Carrier lifetime τ R and diffusion length L D were determined by picosecond transient grating and free carrier absorption (FCA) techniques. The nonradiative recombination lifetime increased from 400 ps in the thinnest layer up to 25 ns in the thickest one, and L D varied from 0.24 to 1.9 μm, respectively. The τ R and L D values in the 145 μm-thick HVPE layer are the largest ones reported up to now in bulk GaN at room temperature. The data provided a relationship τ R ∝ d 3/2 between the carrier lifetime and wafer thickness in ∼ 10-150 μm thickness range. The latter dependence indicated the dramatic decrease of threading dislocation density in the probed subsurface area of the studied wafers from 4 × 10 9 to ∼ 10 6 cm -2 .