OBSERVATION OF SPATIALLY-INDIRECT TRANSITION AND ACCURATE DETERMINATION OF BAND-OFFSET RATIO BY EXCITATION SPECTROSCOPY ON GAAS/ALGAAS QUANTUM-WELLS LIGHTLY DOPED WITH BE ACCEPTORS

We report on the observation of a spatially-indirect transition in the photoluminescence (PL) excitation spectra of Be-doped GaAs/AlGaAs quantum wells (QWs). Using this spatially-indirect transition we determine the band offset ratio accurately. With Be accepters in the QW, free-to-bound and bound-exciton transitions show up in PL in addition to free-exciton transition. As we vary the excitation photon energy, we find a sharp intensity changeover between the free-to-bound and the excitonic transitions at a certain photon energy. A systematic investigation of this energy as a function of the well width shows that this feature corresponds to the onset of a spatially-indirect transition from the valence-band top of the barrier to the n = 1 conduction subband in the QW. Based on the fact that the energy of this barrier-to-well transition critically depends on the valence-band offset, we have determined the conduction-band offset ratio as Q(c) = 0.62 with accuracy better than Delta Q(c) = +/- 0.01.