  We utilize the kinetic spin Bloch equation approach to investigate the
steady-state spin diffusion in $n$-type (111) GaAs quantum wells, where the
in-plane components of the Dresselhaus spin-orbit coupling term and the Rashba
term can be partially canceled by each other. A peak of the spin diffusion
length due to the cancellation is predicted in the perpendicular electric field
dependence. It is shown that the spin diffusion length around the peak can be
markedly controlled via temperature and doping. When the electron gas enters
into the degenerate regime, the electron density also leads to observable
influence on the spin diffusion in the strong cancellation regime. Furthermore,
we find that the spin diffusion always presents strong anisotropy with respect
to the direction of the injected spin polarization. The anisotropic spin
diffusion depends on whether the electric field is far away from or in the
strong cancellation regime.
