In order to clarify the origin of the previously reported reduction of sub-band gap absorption of sputtered ZnO:Al films upon thermal annealing and raising deposition temperature, structural characterization using Raman spectroscopy was carried out on various films. Correlation of the Urbach energy to the FWHM of the E(high)2 mode was found. Oxygen addition to the film growth did not result in changes of the Urbach energy, despite pronounced changes of the electrical properties. The results suggest that extended defects, rather than intrinsic point defects, give rise to the sub-band gap absorption.