Spin relaxation of electrons in p-doped quantum wells via the electron-hole exchange interaction

We have performed a calculation of the spin-relaxation time of photoexcited electrons in p-doped quantum wells with the spin flip due to the electron-hole exchange interaction. A comparision with the same process taking place in bulk semiconductors is presented, as well as a comparision with other spin-flip channels that compete with the exchange channel. Photoexcitation of electrons in p-doped samples populates electronic states with momentum larger than the hole Fermi momentum. For these electrons, the calculated spin-relaxation times are of the order of hundreds of picoseconds, as observed experimentally. However, we have found a strong increase in these times with decreasing electronic kinetic energy, which makes this spin-flip process ineffective for electrons approaching the conduction-band edge. The reason is that the exclusion principle reduces the phase space available for the degenerate holes to be scattered by slow electrons. In quantum wells, the lifting of the heavy-hole-light-hole degeneracy at the top of the valence band provides an additional reduction in hole scattering, lengthening the electron spin-relaxation times.