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-mum-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 Lambda in range from 3 to 10 mum allowed determination of nonequilibrium carrier bipolar diffusion coefficient D=1.7 cm(2)/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 Lambda varying surface recombination velocity S and linear recombination time tau(R). This procedure provided us a value of S=5x10(4) cm/s as well as carrier lifetime of similar to1 ns. (C) 2003 American Institute of Physics.