Radiative recombination processes of the many-body states in multiple quantum wells

The radiative recombination processes of the electron-hole plasma in a series of GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As multiple-quantum-well (MQW) heterostructures have been studied by means of space-resolved high-excitation-intensity luminescence and optical-gain spectroscopy. The spontaneous electron-hole plasma emission dramatically changes depending on the actual MQW heterostructure configuration, which determines the degree of optical confinement in the epilayer. MQW heterostructures consisting of 100 quantum wells or grown on thick barrier layers, exhibit a saturation of the spontaneous emission and high stimulated-emission efficiency. Heterostructures consisting of few quantum wells exhibit the usual high-energy broadening of the luminescence due to the progressive filling of the subbands in the well. Statistical arguments on the photon-mode distribution inside the optical cavity of a semiconductor laser qualitatively account for the observed spectral features. The results of space-resolved luminescence show that the emission spectra of highly excited quantum wells are mostly given by the spectral superposition of the electron-hole plasma emission from the center of the excited region and of excitonic luminescence originating from the lateral region of the excited spot, where the carrier density is lower. These findings are explained by a simple diffusion model taking into account the drift of carriers in the plasma. The optical-gain spectra of suitably designed heterostructures allow us to determine the band-gap renormalization as a function of density of the photogenerated carriers. Additional important information on the ground-level parameters of the electron-hole plasma in GaAs quantum wells is obtained by a line-shape analysis of the optical-gain spectra using an interband recombination model.