Influence of defect states on the nonlinear optical properties of GaN

We study the influence of defect states (shallow donors and deep acceptors) on the carrier relaxation dynamics of gallium nitride in the picosecond re gime for different excitation intensities and different lattice temperature s. Time-resolved luminescence, degenerate, and nondegenerate four-wave mixi ng experiments show a saturation threshold in the blue and yellow spectral region, which is found to disappear for lattice temperatures below 200 K. W hen analyzing all these results in the frame of a rate-equation model, we g ive a relaxation scenario for the carriers, the lifetimes of the population of the different states, and identify radiative and nonradiative transitio ns. After filling defect states by an optical excitation, the ambipolar dif fusion coefficient of GaN is measured through degenerate four-wave mixing e xperiments. A law value of 0.16 cm(2)/s at room temperature is determined, indicating that defect states still influence the diffusion. Nondegenerate four-wave mixing experiments exhibit a competition between an electronical and a thermal contribution to the nonlinear susceptibility in GaN. [S0163-1 829(99)00340-9].