Characterization of differently grown GaN epilayers by time‐resolved four‐wave mixing technique

Room temperature time‐resolved four‐wave mixing has been performed in MOCVD GaN epilayers, grown on sapphire substrates by using either conventional or a 3D‐growth mode with Si/N treatment technique (micro‐ELO). Picosecond pulses at 355 nm were used to create light interference pattern and record a spatially modulated carrier distribution by interband transitions, while a delayed probe beam at 1064 nm monitored the carrier dynamics far from the bandgap. The determined values of the bipolar diffusion coefficient and the carrier lifetime were found equal to D a = 1.7–1.8 cm 2 /s and τ R ≈ 1.1 ns for standard grown epilayers, while in the samples with ultralow dislocation density they were found equal to 2.4 cm 2 /s and 2.7 ns. In the interface region of the epilayers with high dislocation density, a dependence of D and τ R values on excitation intensity was found. The latter effect was attributed to a screening of potencial barriers around the charged dislocations by free carriers and an ability of the carriers to become less localized and thus avoid the nonradiative recombination at dislocations. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)