Luminescence of stepped quantum wells in GaAs/GaAlAs and InGaAs/GaAs/GaAlAs structures

Luminescence spectra of doped and undoped GaAs/GaAlAs and InGaAs/GaAs/GaAlAs structures containing several tens of stepped quantum wells (QW) are investigated. The emission bands related to free and bound excitons and impurity states are observed in QW spectra. The luminescence excitation spectra indicate that the relaxation of free excitons to the e1hh1 state proceeds via the exciton mechanism, whereas an independent relaxation of electrons and holes is specific to bound excitons and impurity states. The energy levels for electrons and holes in stepped QWs, calculated in terms of Kane’s model, are compared with the data obtained from the luminescence excitation spectra. The analysis of the relative intensities of emission bands related to e1hh1 excitons and exciton states of higher energy shows that, as the optical excitation intensity increases, the e1hh1 transition is more readily saturated at higher temperature, because the lifetime of excitons increases. Under stronger excitation, the emission band of electron-hole plasma arises and increases in intensity superlinearly. At an excitation level of ～10⁵ W/cm², excitons are screened and the plasma emission band dominates in the QW emission. Nonequilibrium luminescence spectra obtained in a picosecond excitation and recording mode show that the e1hh1 and e2hh2 radiative transitions are 100% polarized in the plane of QWs.