Drift-diffusion crossover and the intrinsic spin diffusion lengths in semiconductors

We study the propagation of electron spin density polarization and spin currents in n-doped semiconductors within the two-component drift-diffusion model in an applied electric field (E). The drift and diffusion contributions to the spin currents are examined, which shows how the spin current could be enhanced. We find that there is a crossover field (E(x)), where the drift and diffusion contribute equally to the spin current in the downstream direction. This suggests a possible way to identify whether the process for a given E would be in the drift or diffusion regime. We derive the expression for E(x) and show that the intrinsic spin diffusion length in a semiconductor can be calculated directly from E(x). The results will be useful in obtaining transport properties of the carriers' spin in semiconductors. This investigation, however, highlights the need for further experiments to be conducted to measure E(x) in semiconductors. (C) 2008 American Institute of Physics.