  The spin relaxation of a two-dimensional electron system (2DES) formed in a
symmetric quantum well is studied theoretically when the quantum well is
parallel to the (110) plane of the zinc-blende structure, the spin polarization
is perpendicular to the well, and electrons occupy only the ground subband. The
spin relaxation rate is calculated as a function of the distribution of donor
impurities which are placed in the well layer. Considered processes of the spin
relaxation are (1) intrasubband process by impurity-potential-induced
spin-orbit interaction (SOI), which is the Elliott-Yafet mechanism in the 2DES,
and (2) virtual intersubband processes consisting of a spin flip by (2a)
well-potential-induced SOI or (2b) the Dresselhaus SOI, and a scattering from
an impurity. It is shown that all of the above processes disappear, when all
impurities are located on the center plane of the well. Even if impurities are
distributed over three (110) atomic layers, the spin relaxation rate is two
orders of magnitude lower than that for the uniform distribution over the well
width of 7.5\ nm. In GaAs/AlGaAs type-I quantum wells the processes (1) and
(2a) interfere constructively, being dominant over (2b) for the well width of
$\sim$10\ nm, while in some type-II quantum wells they can interfere
destructively.
