Determination of free carrier bipolar diffusion coefficient and surface recombination velocity of undoped GaN epilayers

Time-resolved nondegenerate four-wave mixing experiments were performed on 2.6-μm-thick GaN epilayers grown by metalorganic chemical-vapor deposition using picosecond pulses at 355 and 1064 nm for grating recording and probing, respectively. Measurements of free carrier grating kinetics at various grating periods Λ in range from 3 to 10 μm allowed determination of nonequilibrium carrier bipolar diffusion coefficient D=1.7 cm2/s at 300 K. Substitution of the D value into two-dimensional carrier transport model allowed fitting of the whole set of grating kinetics at various Λ varying surface recombination velocity S and linear recombination time τR. This procedure provided us a value of S=5×104 cm/s as well as carrier lifetime of ∼1 ns.