Interaction between photocreated carriers and background excess-electrons in single-crystalline Al-doped ZnO epilayers (1.9 x 10(19) less than or equal to n less than or equal to 6.7 x 10(20) cm(-3)) are studied by using optical absorption spectroscopy. The Burstein Moss shift of the absorption edge energy over the carrier concentration range of n greater than or equal to 5.9 x 10(20) cm(-3) was confirmed. We propose that the Fermi-edge singularity as a result of many-body Coulomb effect could be observed in the samples of 1.9 x 10(19) less than or equal to n less than or equal to 4.8 x 10(19) cm(-3) even at room temperature. In the second part, we report the optical properties of a ZnO epilayer capped with a MgZnO layer. The luminescence spectrum taken at 5 K encompassed free exciton emissions from the ZnO layer. These free excitons have been hardly recognized in the uncapped epilayers grown under the identical condition. This may indicate the suppression of the exciton-trapping center formation attained by the capping.
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