Excess carrier lifetime and ambipolar diffusion anisotropy in a nipi-doped In_0.2Ga_0.8As/GaAs multiple-quantum-well structure

The effects of strain-induced structural defects in a nipi-doped In_0.2Ga_0.8As/GaAs multiple-quantum well sample were studied with time-resolved electron-beam-induced absorption modulation, in which carrier recombination lifetimes and ambipolar diffusion constants are measured with high spatial, spectral, and temporal resolution. Based on a phenomenological model, carrier lifetimes in the limit of weak excitation at room temperature were determined. The lifetime is found to be reduced by a factor of ∼10¹³ compared to a theoretically calculated value, owing to the presence of strain-induced defects and alternate recombination channels. By using a two-dimensional diffusion model, the ambipolar diffusion coefficients D_a along high-symmetry [110], [110], and [100] directions were determined and resulted in an anisotropic behavior such that D^[110]_a>D^[110]_a >D^[100]_a. The anisotropy in diffusion is attributed to corresponding asymmetries in the misfit dislocation density.