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 photoex cited electrons in p-doped quantum wells with the spin flip due to the electron-hole exchange interaction. A comparision with the same proce ss taking place in bulk semiconductors is presented, as well as a comp arision with other spin-flip channels that compete with the exchange c hannel. Photoexcitation of electrons in p-doped samples populates elec tronic states with momentum larger than the hole Fermi momentum. For t hese electrons, the calculated spin-relaxation times are of the order of hundreds of picoseconds, as observed experimentally. However, we ha ve found a strong increase in these times with decreasing electronic k inetic energy, which makes this spin-flip process ineffective for elec trons approaching the conduction-band edge. The reason is that the exc lusion 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 ban d provides an additional reduction in hole scattering, lengthening the electron spin-relaxation times.