FREQUENCY AND DENSITY DEPENDENT RADIATIVE RECOMBINATION PROCESSES IN III-V SEMICONDUCTOR QUANTUM-WELLS AND SUPERLATTICES

In this paper we review the radiative recombination processes occurring in semiconductor quantum wells and superlattices under different excitation conditions. We consider processes whose radiative efficiency depends on the photogenerated density of elementary excitations and on the frequency of the exciting field, including luminescence induced by multiphoton absorption, exciton and biexciton radiative decay, luminescence arising from inelastic excitonic scattering, and electron-hole plasma recombination. Semiconductor quantum wells are ideal systems for the investigation of radiative recombination processes at different carrier densities owing to the peculiar wavefunction confinement which enhances the optical non-linearities and the bistable behaviour of the crystal. Radiative recombination processes induced by multi-photon absorption processes can be studied by exciting the crystal in the transparency region under an intense photon flux. The application of this non-linear spectroscopy gives direct access to the excited excitonic states in the quantum wells owing to the symmetry properties and the selection rules for artificially layered semiconductor heterostructures. Different radiative recombination processes can be selectively tuned at exciting photon energies resonant with real states or in the continuum of the conduction band depending on the actual density of photogenerated carriers. We define three density regimes in which different quasi-particles are responsible for the dominant radiative recombination mechanisms of the crystal: (i) The dilute boson gas regime, in which exciton density is lower than 10(10) cm-2. Under this condition the decay of free and bound excitons is the main radiative recombination channel in the crystal. (ii) The intermediate density range (n < 10(11) cm-2) at which excitonic molecules (biexcitons) and inelastic excitonic scattering processes contribute with additional decay mechanisms to the characteristic luminescence spectra. (iii) The high density range (n congruent-to 10(12) cm-2) where screening of the Coulomb interaction leads to exciton ionization. The optical transitions hence originate from the radiative decay of free-carriers in a dense electron-hole plasma. The fundamental theoretical and experimental aspects of the radiative recombination processes are discussed with special attention to the GaAs/Al(x)Ga(1-x)As and Ga(x)In(1-x)As/Al(y)In(1-y)As materials systems. The experimental investigations of these effects are performed in the limit of intense exciting fields by tuning the density of photogenerated quasi-particles and the frequency of the exciting photons. Under these conditions the optical response of the quantum well strongly deviates from the well-known linear excitonic behaviour. The optical properties of the crystal are then no longer controlled by the transverse dielectric constant or by the first-order dielectric susceptibility. They are strongly affected by many-body interactions between the different species of photogenerated quasi-particles, resulting in dramatic changes of the emission properties of the semiconductor. The systematic investigation of these radiative recombination processes allows us to selectively monitor the many-body induced changes in the linear and non-linear optical transitions involving quantized states of the quantum wells. The importance of these effects, belonging to the physics of highly excited semi-conductors, lies in the possibility of achieving population inversion of states associated with different radiative recombination channels and strong optical non-linearities causing laser action and bistable behaviour of two-dimensional heterostructures, respectively.