  We construct a spin-drift-diffusion model to describe spin-polarized electron
transport in zincblende semiconductors in the presence of magnetic fields,
electric fields, and off-diagonal strain. We present predictions of the model
for geometries that correspond to optical spin injection from the absorption of
circularly polarized light, and for geometries that correspond to electrical
spin injection from ferromagnetic contacts. Starting with the Keldysh Green's
function description for a system driven out of equilibrium, we construct a
semiclassical kinetic theory of electron spin transport in strained
semiconductors in the presence of electric and magnetic fields. From this
kinetic theory we derive spin-drift-diffusion equations for the components of
the spin density matrix for the specific case of spatially uniform fields and
uniform electron density. We solve the spin-drift-diffusion equations
numerically and compare the resulting images with scanning Kerr microscopy data
of spin-polarized conduction electrons flowing laterally in bulk epilayers of
n-type GaAs. The spin-drift-diffusion model accurately describes the
experimental observations. We contrast the properties of electron spin
precession resulting from magnetic and strain fields. Spin-strain coupling
depends linearly on electron wave vector and spin-magnetic field coupling is
independent of electron wave vector. As a result, spatial coherence of
precessing spin flows is better maintained with strain than with magnetic
fields, and the spatial period of spin precession is independent of the applied
electrical bias in strained structures whereas it is strongly bias dependent
for the case of applied magnetic fields.
