We studied defect-density governed photoelectric parameters in GaN heterostructures and bulk crystals by using the time-resolved techniques of picosecond four-wave mixing (FWM) and photoluminescence (PL). A few mm-thick GaN layers were grown on 6H-SiC or 4H-SiC substrates by hot-wall MOCVD with varied growth parameters, while the 270-mm-thick free-standing bulk crystal was grown by HVPE on a two-step epitaxial lateral overgrown GaN template on sapphire. FWM was realized by recording transient free carrier gratings at interband excitation at 355 nm wavelength and probing their kinetics by the delayed probe beam at 1064 nm. For spectrally and temporally resolved PL, the samples were excited at 266 nm and luminescence was measured in backward geometry. The excitation density varied up to a few mJ/cm², what allowed us to study effects at nonequilibrium carrier density from 10¹⁸ up to 5x10¹⁹ cm^-3.
Simple correlation between photoelectrical and nonlinear optical properties of FWM allowed us to determine carrier lifetimes τ_R and diffusion coefficients D in differently grown samples. The τ_R increased value from 100-300 ps in the epilayers up to 3 ns in the bulk crystal, while the D value varied from 1.5-1.7 cm²/s up to 2-2.8 cm²/s, correspondingly. We attribute the enhanced values of D and τ_R to screening of potential barriers around dislocations and ability of carriers to avoid nonradiative recombination at dislocations.
We observed a novel feature of FWM in GaN, namely a complete saturation of its diffraction efficiency at the excitation energy 0.5 - 2 mJ/cm², dependent on sample. This peculiarity of FWM correlated with the spectra and intensity of stimulated PL, and pointed out that the origin of the FWM saturation is the favorable conditions for ultrafast stimulated recombination. The kinetics and spectra of PL are compared with the features of FWM to develop complementary characterization of the highly-excited nitrides.