Analysis of the Urbach tails in absorption spectra of undoped ZnO thin films

We report the analysis of the Urbach effect in the absorption spectra of the undoped ZnO thin films. The absorption coefficients of the ZnO thin films show the exponential rise, also known as the Urbach tails, just below the free exciton peak. Fitting of the steepness parameter of the Urbach tails yields the phonon energy to be ℏωp=76 ± 4 meV, consistent with ℏωp=72 meV measured from the photoluminescence spectra of ZnO. The temperature dependence of the Urbach energy, the steepness parameter, and the energy gap strongly suggests that the observed Urbach effect is a result of the cumulative effect of impurities, structural disorders, and electron-phonon interaction in the absorption processes.