BELOW-GAP SPECTROSCOPY OF UNDOPED GAAS ALGAAS QUANTUM-WELLS BY 2-WAVELENGTH EXCITED PHOTOLUMINESCENCE/

By using an improved method of two-wavelength excited photoluminescenc e, we observed a considerable decrease in the band-to-band photolumine scence intensity of an undoped GaAs/AlGaAs quantum well structure when a below-gap excitation was superposed on an above-gap excitation. Thi s intensity quenching was attributed to the enhanced nonradiative reco mbination through below-gap states which were activated by the below-g ap excitation. By changing the energy of above-gap excitation, it was shown that these below-gap states were formed neither inside GaAs well layers nor at the GaAs/AlGaAs heterointerface, but inside AlGaAs barr ier layers. Tuning the photon energy of below-gap excitation revealed that the activation energy of the nonradiative recombination process v ia these below-gap states is around 1.2 eV. Lowering the above-gap exc itation density to a single-photon-counting region enabled us to chara cterize below-gap states spectroscopically without the need of a high- power tunable laser source for the below-gap excitation.