Spin dynamics and magnetic-field-induced polarization of excitons in ultrathin GaAs/AlAs quantum wells with indirect band gap and type-II band alignment

摘要

The exciton spin dynamics are investigated both experimentally andtheoretically in two-monolayer-thick GaAs/AlAs quantum wells with an indirectband gap and a type-II band alignment. The magnetic-field-induced circularpolarization of photoluminescence, $P_c$, is studied as function of themagnetic field strength and direction as well as sample temperature. Theobserved nonmonotonic behaviour of these functions is provided by the interplayof bright and dark exciton states contributing to the emission. To interpretthe experiment, we have developed a kinetic master equation model whichaccounts for the dynamics of the spin states in this exciton quartet, radiativeand nonradiative recombination processes, and redistribution of excitonsbetween these states as result of spin relaxation. The model offersquantitative agreement with experiment and allows us to evaluate, for thestudied structure, the heavy-hole $g$ factor, $g_{hh}=+3.5$, and the spinrelaxation times of electron, $\tau_{se} = 33~\mu$s, and hole, $\tau_{sh} =3~\mu$s, bound in the exciton.