We have studied the dynamics of photoexcited carriers near the n-type
GaAs (110) surface by time-resolved second harmonics generation (SHG).
A 20 fs laser pulse (840 nm, 88 MHz) generated carriers in the GaAs, and
the SHG from a second time-delayed probe beam was detected. By properly
choosing the GaAs crystal orientation, we are able to measure the DC-field
induced SH susceptibility and reject the bulk dipole-allowed SH. A 90 percent
SHG signal decrease was observed due to the field screening by the injected
carriers. Due to its short probing depth, SHG provides a highly sensitive
tool to study the carrier dynamics near the surface,^[1,2] compared with
previous studies using visible^ or IR^ reflection and transmission.
A diffusion model was used to provide quantitative evaluation of carrier
recombination time and surface recombination velocity. \$  J. Qi \textitet
al., Phys. Rev. Lett., vol. 71, 633 (1993) \$  A. Nahata and T. F. Heinz,
Appl. Phys. Lett., vol. 69, 746 (1996) \$  T. Dekorsy, T. Pfeifer, W.
Kutt, and H. Kurz, Phys. Rev. B, vol. 47, 3842 (1992) \$  R. S. Miranda,
H. W. K. Tom, A. M. Johnson, T. J. Bridges, and G. D. Aumiller, Appl. Phys.
Lett., vol. 60, 1105 (1992)