Absorption edge shift and broadening in nanostructured Al doped ZnO thin films

Samples of ZnO were prepared by sol-gel and deposited by dip-coating. The influence of synthesis conditions on the optical properties were studied in different sets of ZnO films, synthesized by alternately varying the following parameters: addition of additives to the precursor solution, Al doping percentage and number of layers. The optical properties of the obtained films were studied by transmittance, being typically >90 % at 600 nm. The absorption edge for undoped samples showed a structure due to exciton formation at room temperature. The bandgap energy E_g, was between (3.227 ± 0.010) eV and (3.275 ± 0.010) eV for undoped samples, increasing to (3.352 ± 0.010) eV for Al doped ones (10 % Al/Zn in solution with additives). For intermediate 5 % doping the mean bandgap energy was (3.315 ± 0.015) eV. A similar value (3.320 eV ± 0.010) eV was obtained for 10 % Al/Zn when no additives were included. Doped samples showed a smoother absorption edge. This edge shape evolution was studied by Urbach band tail analysis. The Urbach band tail parameter E_U increased with doping, varying from 30 meV to 90 meV and increasing as E_g increases. This correlation describes the influence of impurity states in the structure and optical properties of the material.