We have studied the evolution of optical absorption spectra in eight samples of n type ZnO:Al as a function of excess electron concentration up to 6.7x10(20) cm(-3) at 300 K. The spectral assignment is examined by taking the many-body effect due to the presence of high-density electron gas into account. We propose that absorption-edge singularity induced by many-body Coulomb effect is observed for the concentration up to 4.8x10(19) cm(-3) at room temperature. Such edge singularity has been so far observed at temperatures less than 50 K in other semiconductors. The strong Coulomb interaction between electrons and holes inherent to ZnO may be related to the persistence of the edge singularity up to room-temperature.
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