On Optical Characterization of Carrier Lifetimes in GaN Layers by Time-Resolved Four-Wave Mixing and Photoluminescence Techniques

We provide numerical analysis of nonequilibrium carrier dynamics in GaN layers at interband photoexcitation by a picosecond light pulse. By solving the continuity equation for bipolar carrier plasma, we analyze spatial and temporal evolution of carrier density. We show that fast carrier diffusion to the bulk determines the carrier in-depth profile in GaN epilayers with a thickness larger than the carrier diffusion length. By integrating the carrier spatial profiles at experimental conditions, corresponding to time-resolved four-wave mixing and time-resolved photoluminescense we simulate the four-wave mixing and time-resolved photoluminescense kinetics in subnanosecond time domain. The modeling data using parameters of the studied GaN epilayers (their thickness, diffusion coefficient, carrier lifetime, and absorption coefficients at emission wavelengths) were compared with the experimental results. The analysis provided conditions at which the discrepancy between the measured carrier lifetime by time-resolved photoluminescense and time-resolved four-wave mixing may occur. For hydride-vapor phase epitaxy GaN layers with a large diffusion length, the fast photoluminescense kinetics are confirmed by modeling and experiments that they are due to diffusion governed carrier in-depth redistribution, while four-wave mixing kinetics remain insensitive for carrier in-depth redistribution.