Two-dimensional versus three-dimensional excitons in wide GaAs quantum wells

Excitons confined in high-quality GaAs/Al0.19Ga0.81As double heterostructures have been studied experimentally and theoretically with emphasis on phenomena associated with the transition from a two-dimensional exciton in a quantum well to a three-dimensional exciton in a thin film. Exciton luminescence and reflectance spectra are obtained for GaAs film widths between 750 and 5200 Å. Exciton polariton luminescence is observed for the 5200 Å sample. Highly resolved exciton peaks are obtained in the free-exciton luminescence spectra for both the 990 and 2010 Å samples. Magneto-exciton spectra of the 990 Å sample reveal an enhanced spin splitting of the ground-level exciton. Experimental results of both the 750 and the 990 Å samples are qualitatively explained by an effective-mass theory which considers mixing of a large number of quasi-two-dimensional excitons. The large spin-splitting of the 990 Å sample is a distinctive feature of a wide quantum well which can be explained by the well width dependence of exciton mixing. The comparison between the discrete luminescence spectrum of the 2010 Å sample and the reflectance spectrum with an exciton polariton interference pattern is presented. The quantization of the spatial dispersion contributes to the luminescence phenomena of the quasi-three-dimensional exciton which means that the exciton coherence length must be larger than 4020 Å.