Formation of charged excitonic complexes in shallow quantum wells of undoped GaAs/AlGaAs structures under below-barrier and above-barrier photoexcitation

摘要

Low-temperature (T approx=2 K) photoluminescence (PL) and photoluminescence excitation (PLE) spectra of GaAs/AlGaAs (x =0.05) structures with shallow quantum wells (QWs) were investigated. It was found that the PLE spectra exhibit a number of broad bands in the above-barrier energy region; these bands alternate 'in opposite phases' in the spectra of free excitons and excitonic complexes (trions) (i.e. an increase in the exciton luminescence intensity is accompanied by a decrease in the luminescence intensity of the complexes). Effects originating from simultaneous irradiation of the sample by two laser beams of different wavelengths were studied. In the case where the photon energy of the Ti-sapphire laser is tuned to excite only the QW states, additional pumping by a He-Ne or Ar-ion laser results in the shift of the equilibrium in the exciton-trion system towards an increase in the concentration of the latter species. On the other hand, upon excitation into certain barrier states with energies both below and above the barrier bandgap, additional pump shifts the equilibrium in the opposite direction.