  We present a complete description of spin injection and detection in
Fe/Al_xGa_{1-x}As/GaAs heterostructures for temperatures from 2 to 295 K.
Measurements of the steady-state spin polarization in the semiconductor
indicate three temperature regimes for spin transport and relaxation. At
temperatures below 70 K, spin-polarized electrons injected into quantum well
structures form excitons, and the spin polarization in the quantum well depends
strongly on the electrical bias conditions. At intermediate temperatures, the
spin polarization is determined primarily by the spin relaxation rate for free
electrons in the quantum well. This process is slow relative to the excitonic
spin relaxation rate at lower temperatures and is responsible for a broad
maximum in the spin polarization between 100 and 200 K. The spin injection
efficiency of the Fe/Al_xGa_{1-x}As Schottky barrier decreases at higher
temperatures, although a steady-state spin polarization of at least 6 % is
observed at 295 K.
