Picosecond carrier dynamics near the gallium arsenide surface

Analytical models for ambipolar diffusion and recombination successfully describe picosecond reflectivity signals for flat n-type GaAs, providing quantitative evaluation of surface recombination velocities. The simple models fail to describe the signals observed in the presence of band bending for typical pinned GaAs. Results of numerical modeling calculations developed in order to describe experiments characterized by band bending due to surface electric fields are reported. The results provide an excellent description of carrier dynamics for (100) surfaces of intrinsic, n-doped, and Cr-doped material. The numerical results converge to those provided by the ambipolar diffusion-recombination model for zero surface charge. The investigations suggest that the band-filled processes principally account for the measured reflectivity signals. Whereas the modeling studies successfully described the observations made on the (100) surface of n-type GaAs, radically different types of signals were observed for p-type samples, and for the (111) surface [positive signal amplitude for (1,1,1), negative amplitude (1̅,1̅,1̅)]. These results are not understood at present.