The depth-resolved luminescence of the ZnO epilayer has been studied by using cathodoluminescence (CL) at room temperature. Other than the usual excitonic and deep level emissions, two peaks at 3.13 and 2.57 eV have been observed and are attributed to the defects. The variation of all the emission peaks has been examined as a function of accelerating voltage. The decrease of near-band edge emissions with depth is due to the internal absorption caused by the pronounced band tail. The deep level emission, however, is shown to increase with increasing depth. We have modeled the CL spectra with the consideration of internal absorption and determine the profile of the Urbach parameter, EUrbach, to study the structural imperfection at different depths. A strong dependence between the intensity ratios of the defect emissions to the excitonic emission and the imperfection of material has been found.

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