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. Absorption threshold energy is located in the vicinity of the resonance energy of the unscreened excitons even when the ns is considerably above this critical density. As ns increases, the sharp excitonic feature should broaden asymmetrically to higher energies: a fast rise at the low-energy side and a slow fall at high energies. The critical density is significantly lower than the Mott critical density ( n M) above which the degeneracy of the conduction band takes place. It is reported to be n M @10 19 cm 23 in ZnO. 17 The injection of a photocre- ated hole into an electron gas creates a bound state below the conduction band edge. This kind of knowledge enlightened the authors. It lead to reexamination of spectral assignment for n type ZnO films along this line although this material has a long research history. 17-20