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

Abstract

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 Eg, was between (3.227 ± 0.010) eV and (3.275 ± 0.010) eV for undoped samples, increasing to (3352 ± 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 EU increased with doping, varying from 30 meV to 90 meV and increasing as Eg increases. This correlation describes the influence of impurity states in the structure and optical properties of the material.