Spin and recombination dynamics of excitons and free electrons in p-type GaAs: Effect of carrier density

Carrier and spin recombination are investigated in p-type GaAs of acceptor concentration N_A = 1.5 × 10¹⁷ cm⁻³ using time-resolved photoluminescence spectroscopy at 15 K. At low photocarrier concentration, acceptors are mostly neutral and photoelectrons can either recombine with holes bound to acceptors (e-A⁰ line) or form excitons which are mostly trapped on neutral acceptors forming the (A⁰X) complex. It is found that the spin relaxation is faster for free electrons that recombine through the e-A⁰ transition due to exchange scattering with either trapped or free holes, whereas spin flip processes are less likely to occur once the electron forms with a free hole an exciton bound to a neutral acceptor. An increase in the photocarrier concentration induces a cross-over to a regime where the bimolecular band-to-band (b-b) emission becomes more favorable due to screening of the electron-hole Coulomb interaction and ionization of excitonic complexes and free excitons. Then, the formation of excitons is no longer possible, the carrier recombination lifetime increases and the spin lifetime is found to decrease dramatically with a concentration due to fast spin relaxation with free photoholes. In this high density regime, both the electrons that recombine through the e-A⁰ transition and through the b-b transition have the same spin relaxation time.